Eyepiece Topics

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Subject: Building an Adjustable Illuminated Reticle Eyepiece

By: Walt Dutchak <walt_dutchaka_tyahoo.ca>

For those of you who do not wish to invest in a Meade 909 APM but wish to use an illuminated reticle eyepiece, are not shy about making your own things, and would also like to be able to control the brightness of the illumination in your eyepiece, then my design (below) may be just right for you.

Reticle Eypiece Schemetic

On the Volume Control/Pot. only connections (1) O--- | and (2) -----> O are used.There is no connection at terminal (3) ---O
Volume OFF (terminal 1) would be the position at which the LED is off. As the volume control is moved towards terminal 3 the volume would get louder on a radio, for example, or the LED in the eyepiece would get brighter. If you have it wired backwards (3) swapped for (1) then when the switch is turned on, the LED will be at its brightest and become dimmer as you turn the volume control towards terminal 1. If you salvage the volume control from a radio or other source, then you will probably be able to identify terminals 1 and 3. (Terminal 2 is always the middle one). If you buy a new control you will have to experiment by measuring the resistance with a meter or test it on the actual circuit with the LED connected, or test it with a 12 Volt flashlight bulb connected and see which connection arrangement makes the light go from off to brightest. Actually a volume control potentiometer is not linear due to how we perceive sound, and therefore the brightness is not controlled in equal turns of the knob. A linear potentiometer would be the best to use. However,if only an audio pot. is available it will still work. I actually used a miniature pot., in fact a whole miniature FM radio ( made in China and purchased at a "DOLLARAMA" store in Canada for $1! ). I left the guts inside and simply severed the existing connections to the pot and the earphone jack, and then wired in my own setup. The pot has an integrated "ON/OFF" switch, so that when the volume knob is turned fully counter-clockwise it will 'click' the switch off. (The switch, of course, is not necessary). This radio measures only 2 1/4" x 2 3/4" x 1/2" ! Very small and quite convenient with the right size of jack to boot! Maybe you can cannabalize something similar. This APM design for an illuminated reticle eyepiece with cord works very well ( especially the ability to control the illumination brightness ). If you have no need of Meade's APM for CCD connections or focuser control, then you can save a lot of money by building a unit based on this design to provide power and brighness control for your corded, illuminated reticle eyepiece. ( You can also get the 12 Volts from an AUX port on the LX90's power pannel by connecting to pins 1 and 4 via an appropriate plug for the AUX port - also available for a very small cost at electronics surplus places. I got mine already connected to a cord for 99 cents ). I hope that my diagram displays correctly on this posting. Good Luck on your project. - Walt


Subject: Wireless Meade 9mm Plossl: Building a Wired Illuminator

By: Bob Leichner <bobhomea_tpacbell.net> Date: Feb., 2000

The wired fitting described below replaces the wireless illuminator on a Meade 4000 series 9mm Plossl illuminated eyepiece giving the option of wired or wireless operation. The fitting makes use of the threaded portion of a plastic Johnson Components banana jack which has the matching thread for the Meade eyepiece illuminator port. As a friend observed, a wired illuminator with plastic fitting prevents his nose from freezing to the metal wireless illuminator on cold nights. Your use may vary.

PARTS LIST----------

I've used Digi-Key (1-800-344-4539 - parts cost plus $5 handling on small orders the last time I looked.) as they carry the critical Johnson Components jack. Radio Shack does not carry this part. The two pieces that really matter are marked with a "*". You might order a couple of extra banana jacks and LEDs... just in case.:

Specific Components

  • Description Digikey # Cost Manufacturer Mfg #
  • *banana jack - black J152-ND 0.45/ea. Johnson Components 108-0903-001
  • *2.5mm power plug SC1051-ND 2.82/ea. Switchcraft 760
  • ultrabright red LED T1 case LT1034-ND 0.35/ea. Liteon n/a
  • 1/8" miniature phone plug SC1055-ND 2.79/ea. Switchcraft 750
  • 4.7K 5% 1/8W carbon film 4.7EBK-ND 0.28/5 Yageo n/a

Generic Parts

  • 4' to 6' two conductor wire. Light and flexible coax such as RG-174U works well and is a good compromise between handling and durability.
  • 5 minute clear epoxy (available from most hardware stores)
  • clear RTV sealant (available from most hardware stores.)


The illuminator of the reticle or reticule consists of two sub-assemblies, an LED fitting an a power connector, connected by a length of two-conductor cable. Construction of these parts is detailed below. A few hand tools and a small soldering iron are necessary for assembly.

CAUTION: Use eye protection while you are working. Material can go flying when cut, solder can splash off components during soldering, etc.

1) LED fitting

You must first remove the metal insert from the banana jack. Flex off the solder lug which extends from the back end of the jack and round the small metal stub that is left protruding by squeezing with pliers. Force the metal flush with the plastic part by LIGHTLY squeezing the jack end to end in a small vise or pliers. Once flush with the end, push the metal part free with the tip of a needle nose pliers or pen.

Cut the large plastic end off of the banana jack using wire cutters or a fine saw. Be careful not to damage the threads on the un-cut portion of the jack. Clean up the cut end of the threaded plastic part with sandpaper or a fine file.

--- ---
     | |
     | |
     | |
     --- --- 
     ---> z z --- cut here
     z z
     z z
     z z
     z z
     z z
     z z
     z z
     --- --- --- back end 

Take the plastic cover from the 2.5mm power connector and thread it onto the cut end of the plastic jack. The plastic cover is a tight fit as the threads are not a perfect match to the jack. Hold the cut plastic jack by the flat sides with a crescent wrench or small smooth face vise to get a better grip. When assembled, about 0.2" of thread should remain exposed. Remove the cover and trim back the cut end of the jack if necessary. Separate the cover and threaded body once the length has been verified as 0.2".

To identify the LED anode, solder one end of the 4.7K resistor to the long lead of the LED. Solder at the ends of the two leads to avoid problems later on. Hold the short lead of the LED to the negative terminal of a 9V transistor battery and touch the free end of the 4.7K resistor to the positive terminal. If the LED lights, the long LED lead is the anode. If the LED does not light, reverse the polarity of the battery. The LED should now light indicating that the short LED lead is the anode. Trim the LED anode, as determined above, to 1/4" and the LED cathode lead to 3/8". Solder the coax center conductor to the 1/4" anode lead and the coax shield to the 3/8" cathode lead. Use a minimum of heat as insulation on the coax center conductor tends to melt.

Mix a bit of 5 minute epoxy and put a few drops into the cut end of the modified jack. Push the LED through the cut end and wet epoxy until the LED body protrudes from the opening on the other end. Clean the front of the LED and pull it back by the wired end until the tip of the LED is flush with the back opening of the plastic jack. Make sure no epoxy is on the front of the LED, the plastic threads or the exposed cable, then let the epoxy set with the LED pointing straight out the end of the jack.


___ coax soldered to LED
-- | -- --- cut end
z | z
z | z
z | z
z | z
z L z
z E z
z D z
--- U --- --- back end
\______ tip of LED flush with end

Install the plastic cover over the wire and LED assembly when the epoxy has set. You will need to seal the LED assembly by filling the cover with RTV when you install it. Once again, keep the LED and exposed threads clean.

2) Power Connector

I'd suggest trying the illuminator prior to final sealing of the 1/8" plug and resistor as described below. My LX200 illuminator port supplies 5 VDC pulse modulated to control intensity. At lowest intensity continuous illumination, 3.3K was a bit brighter than needed, 6.2K was a bit too dim. These are probably good values to try if you want to adjust the 4.7K resistor.

Assemble the 1/8" miniature phone plug end of the cable: Attach one end of the 4.7K resistor to the tip connection of the 1/8" plug. Solder the coax shield to the metal body of the plug and the center conductor to the free end of the 4.7K resistor. There is room for the resistor inside the screw-on cover of the plug if you are careful. Be sure the center pin, resistor and coax center conductor do not short to any other parts of the circuit. Filling the plastic cover of the plug with RTV after testing it is a good way to prevent any motion of the parts inside the cover... messy to assemble, but secure. As an alternative, heat shrink tubing or electrical tape will make resistor changes easier if you want to adjust the value.

An alternative is to use a pre-assembled cable with a 1/8" plug molded to a length of wire. The 4.7K resistor can go into the LED end of the assembly if you take this approach. My preference is to put the resistor in the 1/8" plug where it protects the LX200 from damage by limiting current if the illuminator cable develops a short.

Test the illuminator prior to final assembly of the 1/8" plug if you are going to seal the plug with RTV and then again prior to trying it on your scope. The simplest way is to apply 9V from a transistor radio battery (do not use a larger battery!) to the 1/8" miniature phone plug. The LED should light when the connector is placed across the battery terminals with the tip of the plug touching the positive terminal of the battery.


Subject: 9mm Reticle Eyepiece Recommendation --Part 1 of 2     Top

From: Clark Williams

Stephen, I own the 9mm corded version. Everyone I talked to who used the battery version complained about the cost of batteries. It doesn't take long to eat the cost difference in the two just in cheap batteries. If the cord worries you then stop worrying. I've never had the cord be a problem. It is a nice little (relatively) inexpensive eyepiece.


Subject: 9mm Reticle Eyepiece Recommendation --Part 2 of 2

From: Chris Margaritis

I use the Meade corded version, and it is excellent. It is true that the LED is brighter than expected, but I've been told that a Radio Shack mini-headphone volume control will solve this (blinking works well as well, or nail polish on magic marker on the LED). The cord hangs and can get tangled with other cords, such as power, CCD, RS232 and Dew system, but the LX is smart enough to remember that is has cords hanging all over it, and never slews to the detriment of the wiring. I'm sure the battery version is a cleaner setup, but leave it on by forgetting once, and you'll always wonder... Besides, the LX is reticle ready, why not use it and save a buck or two on batteries.


Subject: Wired Reticle EP or Battery      Top

From: Doug Carroll <voxra_tattbi.com>

I've used Both. When it comes down to it for me I use the wired more. I'm always forgetting about turning off the cordless and have replaced the batteries several times already. At about $4 each time it adds up. I converted one of the illuminators in a wired one with some hot glue and phono cable from Radio Shack details are on my website (though just an overview)



Subject: Astrometric Eyepiece Instruction Sheet -- part 1 of 2     Top

From: Doug Carroll <voxra_tattbi.com> Date: Sept 2002

I uploaded a PDF copy to my site and you can get it at:


From: Ells Dutton <eduttona_tinfi.net>

Three pages of instructions are posted at:


Editor's note: there is a PDF copy here on the MAPUG-Astronomy.net site for download. Click here.


Subject: Astrometric Eyepiece Instruction Sheet -- part 2 of 2     Top

From: Eugene Lanning <ealannia_talltel.net>

Three items to be aware regarding the Meade instructions:

  1. Just be aware that in the "Determining Image Scale" in Step 4 should instruct the user to aim at declination 0 degrees, not towards zenith.
  2. Be aware that in the Section "Determining Position Angle" that a similar misstatement occurs.
  3. There is a very good alternative to turning the telescope drive off during the calibration or determining east-west, as instructed in the manual. The problem with turning the power off is that one must the go back through the telescope alignment, work. Better than turning the telescope off is to simply to be in the Guide slew speed and press East ("E") on the keypad. If your timing gets interrupted, simply press "W" on the keypad to move the star back to its starting position.

Other info that was not requested:

  1. I spent some time making sure that the center of the scale (scale unit 25) was really center. In my particular eyepiece I found the true center was nearer to 24.75 units ( about 5 arc-seconds to the left of the marked center)
  2. On my native 8" LX200, with Meade diagonal in use, the scale calibration is 19.2 (standard deviation 0.12") arc-seconds per division. That is based on my calibration efforts. Without the diagonal I measured 21.2" (standard deviation 0.12") per scale unit.
  3. I checked for curvature of the field in the scale range (1-50) and found it was less than 1% at any point (my methods were good to around 1%).

I've created a 25 page PDF file that describes the testing and use of the astrometric eypiece. Here's the introduction and a list of activities that can be done with it:

Testing and Uses of Meade’s Astrometric Eyepiece
by Eugene Lanning

"A standard Meade brand Astrometric eyepiece was acquired for use with an 8" f/6.3 SCT LX200 (pre-GPS) telescope. This document is intended to describe the various tests that I made on the eyepiece, and some past and intended uses of the eyepiece. The eyepiece refereed to herein is the modified achromatic eyepiece. It is a 12mm focal length uncorded (battery driven) device. Generally, eyepieces have several functions, all of which need not be simultaneously present in each eyepiece. The most common function is that of magnification. Another common function is to provide a quality image, eye relief, etc. A lesser used function is to provide a wide field of view. A fourth function is to provide the capability to measure the angular size or measure angles. They have primarily intended the Meade Astrometric eyepiece for the latter function, measuring."
Some of the activities that are described that can be done with the Meade (or other brand) Astrometric Eypiece are:

  1. compute the distance to the Moon
  2. compute the angular size of the Moon
  3. determine a double star separation and position angles
  4. make quick estimates of size of moonscape features
  5. determine the masses of Jupiter & Saturn
  6. determine physical size of solar system planets
  7. assist public star party guests find the object of interest

    Click here to download a 1.1mb zipped file of the PDF.
    Click here for an Excel spreadsheet of test data.

You'll need a zip ulitity to unzip it and Acrobat Reader to view it.


Subject: Re-centering Adjustable Reticle Eyepiece --part 1 of 2    Top

From: Don Tabbutt <dona_ttabbutt.com>

The reason some reticle eyepieces have an adjustable reticle is so you can guide with them on a star that is offset from the center of the field during astrophotography. If you're not doing astrophotography and don't plan to, buy a fixed reticle eyepiece.

For scope alignment purposes, the reticle does not have to be perfectly centered, it just has to remain fixed during the alignment procedure. The easiest way to get the Meade 9mm adjustable reticle eyepiece's reticle close to center is to simply turn it upside down and look at it. You will see the reticle mount within the cylinder walls of the eyepiece. Simply move the mount with the thumbscrews to center it within the cylinder walls. Use your eyes for this. The result will be close enough for aligning your scope and virtually anything else that requires a reticle. Even for precision polar alignment the errors are relative, such as drift, and not related to absolute center.

About the only reason to have a reticle perfectly centered is for collimating your scope. But how do you align an adjustable reticle on a perfectly centered star that wasn't centered with a perfectly centered reticle? For this, you'll be in a tweaking loop: center the star, rotate & tweak the reticle, tweak the star again, tweak the reticle again, on and on. You'd be much better off with a fixed reticle for this.

So eyeball your reticle to center, leave it there, and align your scope.


Subject: Recentering Adjustable Reticle Eyepiece --part 2 of 2

From: Bostjan

You can simply adjust the screws in such a way that the center BOX of the crosshairs stays centered when rotating the EP in the EP holder.


Subject: Making a Parfocal Eyepiece --part 1 of 2     Top

From: John Rostoni

A source of homemade rings is a suitable diameter of PVC pipe. If you use a 1" diameter piece, you can cut a ring from the pipe and use sandpaper on a flat surface to sand the ring to the desired thickness. Cut a slot in the ring (making a "C") and snap it onto the ep. No need for a screw or anything.

If you can't find a piece large enough for a simple "C" shape, cut one from a 3/4" pipe and put the ring in the oven for a few minutes...it'll soften and let you slide it over the ep barrel.

Leave them white, and you'll be able to tell at night whether they're on or off (as well as help you find the eyepiece you dropped into the grass).


Subject: Making a Parfocal Eyepiece --part 2 of 2

From: Jason Chan

If you are really looking to buy one and don't have the patience to make your own, Spectra Astro Systems sells them. They are supposed to be for SBIG CCD cameras, but they'll do. They call it a 'focus collar lock ring' and it sells for $16 shipping not included.

Spectra: 1-800-735-1352. Call them and ask for Dan.


Subject: Using Secondary Shadow to Calculate Lowest Power Eyepiece     Top

From: Michael Richmann

Well, you have to decide what the largest apparent central obstruction you can tolerate is and work back from there. Let's take 2 mm as an example. Assuming your actual secondary obstruction is 3.7" out of a total aperture of 10", it means your total pupil size at the eyepiece needs to be 2*(10/3.7) or 5.405 mm. Since you have an f/6.3 system and the total pupil size is the focal length of the eyepiece divided by the f/value, then this would correspond to an eyepiece focal length of 5.405*6.3 or 34 mm.

One more example:
my 8" f/10 LX200 and assumed 1.5 mm is the maximum acceptable secondary obstruction size at the eyepiece. Total pupil size is 1.5*(8/3) = 4.00 mm . Focal length of the corresponding eyepiece is 4.00*10 or 40 mm. Substitute whatever maximum central obstruction size in at the top and take it from there.


Subject: Eyepiece & Telescope Reviews URLs --part 1 of 2     Top

From: Todd Gross

For more information on about 50 eyepieces, and 35 scopes see my web page

<http://www.newxspotters.bizland.com/todd/weatherman/wxastrob1.htm> Astro Product Review section.
Clicking this link should open a new browser window over this page.


Subject: Eyepiece Review URLs --part 2 of 2

From: Doug S. <Albireo54a_taol.com>

There are many sources you can go to get excellent advice on the design and use of eyepieces. One of the best I've seen is Jay Freeman's Astronomical Telescope Eyepieces - a Discussion for the Beginner. It has answers to just about every eyepiece question you'll ever have, and it can be found at:

    Note: should open a new browser window over this one.

With eyepieces you generally get what you pay for. The top brands such as TeleVue, Pentax and others are truly outstanding, but watch out! As good as they are, their prices will bankrupt you if you're not careful. Reading the pages of MAPUG-Astronomy.net, sct-user, etc., will reveal good experiences we've all had with eyepieces in less than the "Mercedes" class.


Subject: Lumicon Filters Detoriate Over Time? --part 1 of 3     Top

From: Doc G

----- Original Message -----
From: Bill VanOrden
I was wondering if anyone out there has had any problems with the coating deteriorating on a Lumicon O-III or UHC filter? Holding my O-III filter up to a bright light I can see an area around the perimeter of the glass where the coating is a distinct different color.

I called Lumicon to find out about the problem and discovered several things. The coating can do this if it is subjected to a hot, wet or just plain wet climate. I live in Phoenix and our weather here is sort of like the old dodge about "It's a DRY heat..." all but two months of the year. I store the filters in one of those aluminum briefcases and keep a can of desiccant in the case. My other filters and eyepieces show no signs of problems, but I am kind of concerned right now. I have large format view camera lenses that have resided here for almost 16 years under the same conditions, no signs of fungus or coating issues, ditto on my 35mm equipment. All of these optics are kept in the house, which during the "monsoon season" is air conditioned, which should reduce or alleviate any fungus and moisture issues.

Secondly Lumicon will not stand behind anything sold prior to the purchase of the "old" Lumicon, even if the purchase occurred in the last year. I am now left holding the bag on this filter. Unsurprisingly I have decided to put on hold the purchase of the 2" O-III that I was planning on getting next month.

Thirdly, the filter can be "repaired" for $69, apparently they replace the glass part in your filter housing. Anyway back to the present, keeping my two remaining UHC filters from crumbling into dust....
----- End of Original Message -----

I had a few 2" and 1-1/4" Lumicon filters about 5 years or so ago. Several of them went bad. I still have one 2" and one 1-1/4" which do not look uniform in their coatings. I no longer purchase anything from them. Sometimes it is best to cut and run. No good news from my experience.


Subject: Lumicon Filters Detoriate Over Time? --part 2

From: Randy Marsden <jmarsdena_tsan.rr.com>

I used to apply coatings for optical and other purposes years ago. Interference coatings are very thin since they are a fraction of a wavelength of light. So, it does not take much etching or other chemical reaction to severely affect the optical or mechanical integrity of the coating. Dew or even high humidity combined with atmospheric pollutants will created acidic or basic films on the filter which may slowly, over time, react with the deposited layers. So, to best protect any filter or other optical coating, you should make sure your equipment is very dry before storing it and if possible store it in a container which also has a desiccant pack to absorb the residual moisture. Coatings typically degrade from the edge first since the edge provides a location for etchants or chemicals to get directly at the film/substrate interface.


Subject: Lumicon Filters Detoriate Over Time? --part 3 of 3   Top

From: Bill Keicher <wekeichera_tcomcast.net>

Bill, You might be interested in checking what Barr Associates, one of the better manufacturers of premium specialty optical filters has to say about filter specifications:

Coating hardness (the ability to resist scratches):

Coating environmental stability:

Coating operating temperature:

Coating pinholes:

It is possible to produce durable optical filters. Unfortunately, they may be expensive.


Subject: Meade 2" Eyepiece Filter Threads    Top

From: John Hopper <JohnLX200a_taol.com>

> At a star party this last weekend, I was using a Lumicon 2" UHC on my Meade
> 40mm Super wide angle eyepiece. A friend asked if he could try the filter
> on his Meade 56mm ep. The filter just slipped into the opening loosely
> with no threads engaging! Anybody know what gives here?

Meade has really crappy quality control on those threads. I guess they just made a version of them for "48mm drop-in filters."

My 32mm SWA threads are horrible, I can slip the filters in easily. Luckily mine is just good enough to allow me to screw it down a tiny amount once it's in place. I have a UV haze filter permanently mounted there (now that I got it into a stable position) and screw any additional filters to that. I'm tempted to epoxy the UV in place as it's "hanging by a thread". I suppose I could take out the glass to keep from adding one more glass element, but keeping the eyepiece clean is important to me, too. I figure that SWA, UWA, Panoptic, and Nagler eyepieces already have so many elements that if transmission is a major concern, you should be using something else anyway.

Briefly, though, the consensus on s.a.a. that the very light blue 82A is better than the 80A for Jupiter is overly pessimistic, at least with 10" of aperture to make up for the light loss of the 80A. The 82A works, but the 80A works even better, due to the fact that Jupiter is a pretty bright object. The 80A does turn the planet a distastefully tinted blue, which (and here's my biggest "discovery" of that night's observing) can be negated by stacking an 85A (or any other 85 series) with it to get the bright parts of the planet the right color again while keeping most or all of the gains in visible detail. A similar trick can be done by combining the 82A with either an FL-D (my preference) or 81 series, if you don't want anyone to realize they're looking through a filter. They'll just think your scope is better than it is.

The green filter and yellow (with a hint of Mountain-Dew fluorescent green) filter were also outstanding for helping planetary detail on both Jupiter and Saturn, but there was no mistaking the fact that you are looking through a filter with those. As to mismatched threads, again, it's almost certainly the Meade eyepiece barrel rather than the Lumicon filter which is at fault, although a combination of the two is also possible. I just know that every one of my filters fits everything but my Meade eyepiece just fine.


Subject: Eyepiece Filter Thread Specs   Top

From: Roger Hamlett <ttelmaha_tntlworld.com>

From: Darren Carlisle
> Does anybody know the thread size on a 1.25" eyepiece, where the filters screw in?

Filter sizes are a metric standard. Some manufacturers use a letter after the diameter to specify the pitch, so (potentially) you could have 28E, and 28ES filters. If no letter is given, the 'E' is assumed. Normally the 'E' pitch for sizes under about 40mm, is 0.5mm, and over this it goes up to 0.75mm. So a 28mm filter with no letter, should have a 28mm * 0.5mm thread. The ES pitch is coarser, but hardly ever met. Just to confuse, most telescope filters are a little oversize (normally about 10 thou). 0.5mm pitch, is all most exactly 50TPI, and like with the larger threads, over the short lengths involved, this will differ from the 48TPI mentioned by another poster, by such a small amount it won't matter at all.

So if you were being really accurate, you'd probably have to cut a thread with an outside diameter of about 28.3mm, with a 0.5mm pitch for a nice fit on the 28mm filters. However if you were stuck with imperial, one could cut to about 1.125", at 50TPI, and be pretty close...

It is interesting that this 'metric' (nominally) thread, appears to be a sort of 'hybrid.' I have never seen this on other camera threads (a 50mm thread, is as near as practical 50mm OD). It probably suggests that the 28mm thread, has been sort of 'bent to fit' the metric progression. However it is then odd, that they don't use 28.5mm (since higher up the range, there are several 0.5mm steps - 30.5, and 35.5, are 'standard' filter sizes, for some old Leica models), and 28.5, would then be very close to the 1.125 size. I wonder if anybody knows the 'history' of this particular thread?. 28.6, as a 'internal' diameter, would tally very nicely with the sizes I have seen, since something like 28.3 as an 'OD,' would give a really 'nice' fit clearance of about 10thou.


Subject: How Much More Space Can You See with a Wide FOV Ep Top

From: Doc G

> Before making a decision on which eyepiece to purchase, I'd like to ask
> those with super wide or ultra wide EP's how much more 'space' do you see
> with these types.
> You may also want to factor in the following if you live in or near an urban
> area like I do. The wider the FOV the more light pollution becomes a factor.
> Normal (50-52 degree) EPs in the higher powers darken the sky very nicely
> whereas the wide field EPs do not.
> Taking this into account I found I am better off with a normal Plossl although
> I do have a couple of wide EP but in the low power range.

I will say this again since there still seems to be confusion between actual "real" field of view and apparent field of view.

The actual field of view is the actual angular size of the sky you see. This is only and entirely given by the focal length of the telescope divided into the diameter of the field stop in the eyepiece.

The apparent field of view is the angular field of view that the eye sees when looking into the eyepiece. Apparent fields of view are numbers like 55 degrees, 63 degrees or even 85 degrees for some extreme eyepieces.

The real fields of view are for long focal length eyepieces numbers like 26 mm divided by 2500 and then times 57 to convert from radians to degrees. In this example the real field of view is 0.6 degrees. For a two inch eyepiece the field stop might be 48 mm and the same focal length telescope will give 48 divided by 2500 times 57, which is 1.1 degrees.

I think it is so very important to be specific about stating the field of view in accurate terms. One is real or actual field of view and the other is apparent field of view.

A more complete discussion in on my website.


Subject: Choosing a Wide Field Eyepiece   Top

From: Leroy Guatney <lwlguatneya_tusa.net>

Jerry wrote:
>I've been on a crusade to find the widest field possible and my
>eyepiece budget. An easy way to compare eyepieces
>is to multiply the apparent field by the focal length of the
>eyepiece. I call this the focal product (FP). If you can get your
>FP up to the focal length of your scope (3000mm in this case) then
>you get 1 degree of actual field.

>The Meade 56 or TeleVue 55 has the highest FP possible in a 2"
>eyepiece at about 2750 (actually some eyepieces calculate to 2800
>but this is probably within the accuracy of manufacturers claims).
>The 31 Nagler, is actually quite a bit narrower at 2540. I used the
>TeleVue 55 for a while and then sold it because the narrow apparent
>field was so different than all my other eyepieces that have at least
>a 68 degree apparent field. Right now my favorites are the UO 40mm
>Mk70 and the Celestron axiom 40mm, both at 2800 which is about the
>same field as the long Plossls but the field looks much closer because
>of the wider apparent field. I highly recommend both of these
>eyepieces but recognize that they are not perfect at the edges. And I
>can't discuss wide field eyepieces without a plug for my favorite
>eyepiece of all times, the 35mm Panoptic. At 2380, it is quite a bit
>narrower but if the object fits in that field, that s still my favorite eyepiece.

>The next step is to install the .63 focal reducer. I tried that last
>month and didn't enjoy it. After discussing it with friends who say
>it works for them, I'm planning to give it another try.

>Then there's the other option: buy a wider field telescope. I have a
>10" Discovery PDHQ Dob that has phenomenal views but no tracking and
>it harder to transport than the 12" LX200 (Tube type Dobs are BIG).
>But it's half the focal length, thus twice the field. I just
>bought a 4" short focus refractor to use for wide field views and am
>making a dovetail to put it on the back of my Dob. With my wide
>field eyepieces, this scope will have a field similar to binoculars.

I, too, have been down the path you mention. I have decided, after acquiring three eyepiece cases plus (see my Astronomy link below, then click on my Schmidt-Cassegrain page), that as much an important factor to the original question of widest field, which was accurately answered, is also the question of useful magnification.

Doc G. set me to thinking about this sometime back when he pointed out that the personal preference of the Apparent Field of View of an eyepiece is key as well. As I recall, he liked the 60-deg.-and-under "generation" of eyepieces. At that time, I liked the neighborhood of 70 degrees myself, having the predecessor to the UO 40mm Mk70, the classic no. 7/70.

I still like the no. 7/70, but recently acquired the 31mm Nagler Type 5. My indulgence of 82 degree field eyepieces has been slow and gradual, starting with the classic 13mm (sometimes called a type 1), and then the 17mm Type 4, followed by the 31mm Type 5, and the 12mm Type 4.

All of these longer focal length eyepieces are up against the 2" barrel diameter for maximal true (or actual) fields of view. What I find significant is the higher magnification that comes with the longer focal lengths (40mm/70, 31mm/82) over my Meade 56mm SP or the TV 55mm Plossl. Also, at the lower magnifications of the latter two, the extra field achieved tends to be lost in the small apparent image size. Of course, if you don't like the larger apparent field angle, this may have to be the trade-off you accept.

My advice is to look (if you can) through examples of these oculars, and see for yourself, and also pay attention to what you like best.

Also, a couple of comments on Jerry's interesting FP value. Being a compound factor, the final product tends to exaggerate the actual comparison, as I made above between the 31mm (for example), and the 55mm or 56mm. Perhaps the product could be smoothed out by dividing by a constant. The 2350 of the 31mm vs. 2750 of the 55mm/56mm seems to really skew the differences of the eyepieces, but at the same time does not take into account for the ease of viewing through a 55/56, yet swinging back in favor of the 31mm providing a higher magnification of objects observed, which translates into perception of detail.

The other comment was that I had seen this product used before, as I recall a Michael H. (?) on sct-user (haven't seen a post from him in some time). He was quite knowledgeable w.r.t. optics.


Subject: Individual Screw-Bolt Eyepiece Cases  Top

From: Ed Stewart<stargazera_tskymtn.com>

Source for the individual eyepiece cases that are two pieces that screw together. Meade includes them with their scopes. They are sometimes referred to as screw bolt cases since they have a hex base. In searching around this source was found:
<http://www.optiguy.com/> Scroll part way down the page to the screw-bolt cases.
Note should open a new browser window over this one.
Subject: Individual EP Cases--Caution
From: John Downs <docdba_tjuno.com>
Those are really neat cases to keep your EP's clean, but remember never put one away in the case damp, or you can say good-bye to your EP--MILDEW!


Subject: Vixen Zoom Eyepiece Review URL    Top

From: Gilles Grosgurin <gillesa_tmagnitude-electronics.com>

I have the zoom ep by Vixen and although the "experts" will tell you that they are not worth talking about I disagree. I find my zoom eyepiece very convenient and very useful specially when trying to locate very faint objects. I do realize that with a GoTo type scope locating object is not very difficult however.... I use it all the time, and for about $190.00 I think it's a very good deal. Here is a link to a popular web site by Ed Ting where you will find reviews on astro product by an expert.

   Zoom review: <http://www.scopereviews.com/page3c.html>
   Note: should open a new browser window over this one.


Subject: Calulating Eyepiece Actual Field of View (FOV)     Top

From: Chris Frye <cfrye44a_tbellatlantic.net>

The method discussed below will get you close to the actual field of view but because of design variations it can be as much as 10% off.

The best method is to measure it directly. Do the following:

  1. Position a star on the celestial equator at the 3 o'clock position at the edge of your FOV.
  2. Press the EAST button to stop the RA motor.
  3. Note the time (in seconds) it takes the star to drift to the 9 o'clock position at the edge of your FOV.
  4. Your actual FOV (in minutes of arc) is: Divide the results step 3 by 4.

Another way to express this is:
   FOV = .25*t
   Where "t" is the results of step 3, above.


Subject: LX200 + Eyeopener + Eyepiece --part 1 of 5    Top

Editor's Note: the Eypopener I (the original version) is no longer manufactured.

From: Leroy Guatney <lwlguatneya_tusa.net> Date: Mar 2001

Larry Owens <ccda_tastrophotographer.net> wrote:
>I have a question about eyepiece selection. I have an LX200 f/10, 10".
>If I add the Eyeopener adapter with a 2" diagonal, what eyepiece would
>give me the full field of view of my 10", assuming an eyepiece of a
>conservative 60° +/-5° apparent field of view.

>My guess would be somewhere between 45 and 55mm at 60° FOV. Does
>anybody have a more accurate answer?

The answer is irrespective of which telescope you have. Magnifications are used to determine the answer to your questions, and because field is directly related to aperture, the sizes cancel in the outcome.

It boils down to maximum field for a given eyepiece, and several combinations allow for your solution. The shorter the focal length of your 2" eyepiece, the wider field it may have in a 2" barrel. So, you will find a 40mm with a 70 degree field will be comparable to a 56mm and a 52 degree field, both of which are (roughly) maximal.

It is just a matter of what magnification and FOV within those ranges are comfortable to you, as well as type of ocular (Plossl, Erfle, Konig etc). Beyond the 56mm, the FOV must decrease with longer focal length eyepieces, so at that point, you are just trading off magnification for same FOV. But in all of these cases, you will find the barrel limit. Personally, I like them all.


Subject: LX200 + Eyeopener + Eyepiece --part 2   

From: Doc G

Because of the lax use of terms, it seems that this matter has to be clarified one more time.

The fundamental optical formulas are as follows:

The actual angular field of view is the diameter of the field stop in the eyepiece divided by the focal length of the telescope. The apparent angular field as seen by the eye is the diameter of the field stop in the eyepiece divided by the focal length of the eyepiece. The magnification of the combination of the eyepiece and the telescope is the focal length of the telescope divided by the focal length of the eyepiece.

There is another fundamental relationship which gives the f number of the telescope. It is the diameter of the telescope divided by the front aperture of the telescope. This number has nothing directly to do with the issue of the field of view of the telescope and eyepiece used.

>From the first three relationships, it can be seen that the actual field of view (the amount of sky encompassed) is equal to the apparent field of view of the eyepiece alone divided by the magnification of the telescope eyepiece combination.

This last relationship is derived from the two fundamental relationships.

When asking the question about the maximum actual field of view of the system, it is apparent that one needs to know only the focal length of the telescope and the maximum field stop possible. For a 2" eyepiece, the maximum field stop diameter is about 48 mm. That is because the field stop cannot be larger than the eyepiece tube.

Thus if you want to get the largest possible actual field of view, you need to select an eyepiece that has the largest possible field stop diameter.

For an eyepiece of 48 mm focal length and the largest field stop possible the eyepiece will have an apparent field of view of about 60 degrees.

For so called super wide eyepieces it is possible to find an eyepiece as short as 30 mm with the largest possible actual field of view. For a simple wide or Plossl it is more common to find an eyepiece of focal length 40 or 50 mm.

If you get the shortest possible eyepiece with a super wide field of view, you will get slightly more magnification for the system. You need to take into account what apparent field of view you feel comfortable with. I have never like an apparent field of view much more that 60 degrees or so since I do not feel I can see the entire field anyway. But some prefer the widest apparent field of view possible. If so you need to get a super wide eyepiece which will be complex optically, have a lot of glass in it and generally be very expensive.

I had a 31 mm TeleVue at one time, but was not comfortable with the large apparent field of view. Others love this eyepiece. So, in the final analysis, the choice is personal. But generally eyepieces of greater than 35 mm will give the largest possible actual field of view but slightly different apparent fields of view.


Subject: LX200 + Eyeopener + Eyepiece --part 3    Top

From: Leroy Guatney <lwlguatneya_tusa.net>

>When asking the question about the maximum actual field of view of the
>system, it is apparent that one needs to know only the focal length of
>the telescope and the maximum field stop possible. For a 2" eyepiece,
>the maximum field stop diameter is about 48 mm. That is because the
>field stop cannot be larger than the eyepiece tube.

I agree that maximum (actual) FOV depends upon f.l. Awhile back, when I had first learned how field stop played in such computations, I did some calculations to evaluate a couple of eyepieces and quickly found a generalization.

I have three SCTs: 2110mm, 2500mm, and 3048mm. When one is computing near the limit of maximum field of view, the focal length does not matter. This was the point the original poster was making, or guessing at and asking for confirmation of?

My repetitive calculations proved to me that 52 degrees is the maximum Apparent FOV that one can get at 56mm in a 2" barrel, independently from the telescope's focal length (ignoring possible baffle constrictions and other imperfect limits of "real-world" telescopes).

As I recall, the same was true of a 65 degree apparent FOV 1.25" eyepiece. There the maximum focal length for that AFOV is 28mm. I even confirmed this one for my daughter's 910mm telescope.

I *may* have stumbled across a computational idiosyncrasy, but I think I had done the calculations enough times to realize that the results were independent of focal length of the telescope.

In my last post on this topic, I attempted to contort my findings more to the direct questions of the original post. I should have just presented this post.

So, with a 2", 56mm @ 52 degrees, you can go with a longer f.l., but the FOV of that eyepiece must be smaller than 52 degrees. Moving upwards, my 40mm Konig No.7/70 (70 FOV) has the same true field as my 56mm Meade Plossl.(1) And this result is independent of which telescope I put these eyepieces in. Higher power, wider apparent FOV.

Anyway, call it the "Guatney limit" if you would like and if this effect is not already named for someone else.


(1) - this is only approximately equal; the differences may be due to round-off of statistics, or perhaps the Guatney limit for my 40mm is actually 71 degrees.


Subject: LX200 + Eyeopener + Eyepiece --part 4     Top

From: Doc G

Your analysis and conclusions are correct as I see it. I really am trying to teach everyone to say "apparent field of view" when they look into the eyepiece and "actual field of view" when they are thinking about the section of the sky that is encompassed.


Subject: LX200 + Eyeopener + Eyepiece --part 5 fo 5   Top

From: Larry Owens <ccda_tastrophotographer.net>

The answer I was looking for: "For an eyepiece of 48 mm focal length and the largest field stop possible the eyepiece will have an apparent field of view of about 60 degrees."

Understand the relationships of eyepiece FL and power, and eyepiece apparent field of view to the scope's actual FOV, and that even a higher power eyepiece can take advantage of you scope's maximum actual FOV (determined by focal length of the telescopes optical system and the maximum field stop) if you're willing to endure a superwide Apparent FOV. Just didn't know exactly how to calculate Actual FOV - until now. Thanks...

Like you, I've never liked super wides, around 60 degrees is my personal preference.


Subject: Eyeopener Benefit Wide-Field Eyepieces? --part 1 of 3    Top

From: Scott Kephart <scott_kepharta_thotmail.com> Date: Dec 2003

From: <Morejoy41a_taol.com>
What is your opinion on the Eye-Opener or similar? We use the 31mm Nagler on
the LX200 12", as well as a 56mm Plossel. Do you think it will really
make a difference?

Sadly, it is discontinued. I talked to the folks at Petersen Engineering in October, because I wanted to buy one for my 12" LX200 classic. They don't make them anymore. The EyeOpener II - the current model, will work on an LX200 Classic, but there's no way to connect a diagonal to it. You can only use it if you also use an zero image shift focuser, like the JMI NGF.

Lumicon makes something similar. It's really intended for use with their giant easy guider / giant richfield viewer, but it seems like an adapter is available for it to allow use with a diagonal. They don't really advertise this as a product as such - it's really an accessory for their giant easy guider.


Subject: Eyeopener Benefit Wide-Field Eyepieces? --part 2

From: Richard Winter <rcwintera_tntlworld.com>

I think AP do a version of it. 2" Adapter for 10" and larger SCTs, with Brass Locking Ring (ADASCTLC and ADASCTLM)
Have a look: <http://www.astro-physics.com/>


Subject: Eyeopener Benefit Wide-Field Eyepieces? --part 3 of 3    Top

From: John Mahony <jmmahonya_thotmail.com>

It helps on very wide true field EPs. Those with field stops wider than 1.5" will have some vignetting around the edge of the field if used with the standard rear port. The field stop of the 56 Plossl is close to the maximum possible 2". The 31 Nagler is around 1.8". If the EyeOpener is not available from Peterson, AP makes the same thing. It has a compression ring to hold 2" accessories, which is more solid than simple set screws.

Note that if you're currently using the standard 2" screw-on diagonal (which also limits the field to 1.5"), you'll need a "refractor style" 2" diagonal that attaches with a 2" barrel.

>From: Morejoy41a_taol.com
>It is my understanding that the GPS EyeOpener will fit the Classic, but you
>cannot screw anything onto the rear such as a filter or diagonal.

You wouldn't want to screw the diagonal onto it as that would defeat the purpose- the screw-on diagonal has only 1.5" clear aperture, just like the standard rear port.

>Astro-physics is said to have one but you can only slip
>things in the rear and they will be held by screws.

It uses a compression ring system, very solid.

Here's a link for Lumicon's large skylight filter: <http://lumicon.com/uv.htm>.

Televue's diagonals are threaded for 2" filters. Most SCT-thread filters or other accessories have only 1.5" clear aperture, so you can just remove the EyeOpener and put the old part on. Or there are 2" to SCT thread adapters available. I think JMI sells them.


Subject: LX200, Eyeopener II, and 6.3 Reducer   Top

From: Rick Woods <Rick.Woodsa_tAD.STATE.AZ.US> Date: Jan 2004

When I got my LX200, I was dismayed to find that I couldn't use my focal reducer (FR) while the microfocuser (MF) was attached. This wasn't a big problem, since I didn't use the MF anyway and so just removed it. A few days ago I received an Eyeopener II, which requires the MF to be used. I thought "Bummer! I can't use my FR!", and I called Pete Peterson yesterday and indicated I wanted to send it back. (Pete was very gracious and said that would be no problem).

However, now I think I can make it all work. The MF comes with a 2" adapter that has a threaded end, I guess so you can screw on the 1.25" visual back. This end also screws into the female end of the FR. In addition, I have a 2" female adapter, which allows you to use 2" diagonal on the normal visual back, and thus, also screws on to the male end of the FR. (I got this adapter at a local camera store). Presto! I now have a 6.3 FR "barlow" that I can place into the 2" diagonal before the eyepiece, on the eyepiece side.

I've done this before and it works fine. I'll eventually be wanting to do imaging, and I'll need Pete's Eyeopener then. So I'm going to make sure all of this works like I think it will, and then probably keep the Eyeopener.


Subject: SCT Vignetting -- part 1 of 2    Top

From: Leroy Guatney <lwlguatneya_tusa.net>

R. A. Greiner wrote:
> The information contained in these speculations is not correct. This
> is a relatively easy optical problem.

You gave me enough information (confirming what I thought) that perhaps I can get to Mr. Alexander's question. Removing that doubt, I can say we are talking the field stop. Think of the field stop as the minimum opening in the tube along the light path. If you have an ideal system, this becomes the barrel of your eyepiece. If your eyepiece itself has a field stop, then even the barrel's inside diameter is too big to be the field stop.

Maximum *true* field of view of any optical configuration is determined by dividing the field stop diameter by the focal length of the telescope and multiplying by 57.3 degrees which is a radian.

Within this constraint (Max true FOV), you have the eyepiece FOV, true and apparent. The AFOV is usually specified by the maker of the eyepiece. The (true) FOV of an eyepiece is determined by dividing the AFOV by the magnification of that eyepiece.

Here is where I am less sure. As vignetting relates to all this, I *think* it is just a matter of the eyepiece FOV being greater than the maximum FOV of the telescope/eyepiece combination. So, if the eyepiece's FOV is greater than the telescopes, you see a darkening of the image because you are still getting some light, but it is not fully illuminated in the eyepiece.


Subject: SCT Vignetting --part 2 of 2 Top

From: Doc G, Date: Apr 2001

I have finally finished a small article, with some illustrations, on the problem of vignetting caused when too small tubing is used on the backside of an SCT.

For those interested this can be seen on my website.
   Note: should open a new browser window over this one.


Subject: Apparent vs. True Field of View --part 1 of 2    Top

From: Paul Markov <pmarkova_tica.net>

On June 9, I posted the email below about a discrepancy between apparent field of view and true field of view. I received some replies saying that my results were quite unusual and the I may have made a mistake somewhere in my calculations / measurements.

Well, I think I got to the bottom of it - it's called "pincushion distortion".

I have received a couple of good explanations from the "telescope-eyepieces" email list on Yahoo, and if you want you can read them at:

   (you'll have to join the group to read them).

The bottom line is that the commonly used formula True FOV = Apparent FOV / Magnification is not that accurate!! (due to pincushion distortion).

For the eyepieces noted below, the delta between Apparent FOV and True FOV is:

  • 12 mm Meade MA illuminated reticle - 8.9%
  • 15 mm TeleVue Plossl - 15.3%
  • 26 mm Meade Super Plossl - 16.0%
  • 32 mm University Optics Koenig - 19.9%
  • 40 mm Meade Super Wide Angle - 13.7%

This discrepancy is large enough not to be ignored, especially when testing the pointing accuracy of your LX200. In conclusion, my understanding is that the formula is not reliable "as is" (it would have to be tweaked for pincushion distortion somehow) and that manufacturers are not mis-representing the apparent field of view of eyepieces.


Earlier Paul Markov wrote:   Top

Before you can accurately test the centering accuracy of your LX200 you must know the actual field of view (FOV) of your eyepieces. Calculating the actual FOV based on the apparent field of view is a good approximation, but for more precise results follow this method:

Point your scope at a star near the celestial equator (i.e. Dec. = 0 deg) and near the meridian, turn off the telescope drive and time how long it takes the star to cross the eyepiece's field of view with a stop watch. The actual field of view in degrees is the time you measured in seconds divided by 240 seconds. Just multiply by 60 to convert that number to arc-minutes.

Here are my results on the following eyepiece on my 10-inch f/10 (f.l. 2500mm)

  • 12 mm Meade MA illuminated reticle - 10.5 arc-min
  • 15 mm TeleVue Plossl - 15.25 arc-min
  • 26 mm Meade Super Plossl - 27.25 arc-min
  • 32 mm University Optics Koenig - 30.75 arc-min
  • 40 mm Meade Super Wide Angle - 55.5 arc-min (2-inch)

Now I can work the math backward and compare the advertised apparent fields of view with the real apparent field of view:

  • 12 mm Meade MA illuminated reticle: advertised 40 deg., real 36 deg.
  • 15 mm TeleVue Plossl: advertised 50 deg., real 41 deg.
  • 26 mm Meade Super Plossl: advertised 52 deg., real 43 deg.
  • 32 mm University Optics Koenig: advertised 50 deg., real 40 deg. (15-year old eyepiece)
  • 40 mm Meade Super Wide Angle: advertised 67 deg., real 58 deg.

This was quite a revelation for me! It looks like that generally speaking the advertised apparent field of view is about 10 degrees more than the real apparent field of view!! Let us know what your experiences have been!


Subject: Apparent vs. True Field of View --part 2 of 2     Top

From: Bob Nanz <cathynanza_tnctimes.net>

Also a 2" diagonal introduces some back focus which does increase the focal length more than one might think. There was a article about the back focus and increase in focal length in Sky and Telescope a while back. If you get a chance try your measurements with no diagonal. The nominal focal length of the 10" is 2540 mm but I do not know where the eyepiece needs to be to get that focal length.


Subject: 8mm-24mm Zoom Eyepiece     Top

From: Jack Estes <jackestesa_thotmail.com> Date: Apr 2001

I work for a camera store that sells Celestron & Meade. The Meade 8mm-24mm zoom ($220) and the Celestron Vixen Lanthanum 8mm-24mm zoom ($185) and the TeleVue Lanthanum 8mm-24mm zoom ($210) are all exactly the same eyepieces made by Vixen of Japan. Vixen just puts the different names on them. The only exception is the TeleVue click stop version which is about $25.00 more expensive. I took home the Celestron version, tried it for a weekend, and fell in love with it as a high power eyepiece. Yes the FOV is only about 55° but when you're using medium to high power, you're only looking at the center of the eyepiece anyway. You only need a large FOV with your low power 2" eyepieces.

BTW, I bought the TeleVue click stop zoom as I think it's a real help to know in the dark what power you're using. I replaced three high power Pentax eyepieces with this one zoom and haven't looked back. To be viewing the moon and zoom in on a crater is a sight to behold. Same with a double star. There is very little focus touch up you have to do from 24mm to 8mm. I also use this eyepiece exclusively on my little red Edmund Astroscan. On that scope, it provides powers of: 28, 26, 35, & 55 at the click stops. On my 10" LX200, the powers are: 105, 155, 210 & 310.


Subject: Magnification -- What Does It Really Mean? -- part 1 of 4    Top

From: Peter Dietrich <dimex.enterprisesa_tt-online.de>

What is the magnification of a telescope (or SLR lens) when used without any eyepiece, for example in prime focus photography? Is there an equation to calculate this for a given focal length?

I talked to someone, who is at the moment attempting to get his PH.D., in my local astronomy club about this. He said one could not speak of "magnification" in that case, as it was similar to a slide projector, that also has no certain magnification factor.

Is it possible to implicitly compare the ray traces of a telescope and a slide projector with each other?

From a layman's point of view I doubt that explanation. After all, the objects photographed in prime focus appear greater in diameter than with the unaided eye. So there has to be some kind of "magnification".


Subject: Magnification -- What Does It Really Mean? -- part 2     Top

From: Doc G

There are two major types of optical systems. In which a real object is projected through the system and generates a real image. Such a system generally has a value for the magnification. In many cases the magnification value can be near infinity or one over infinity, but usually that is an easily calculated value.

In the second case, the optical system starts with a real object and projects a virtual image, generally in focus at infinity and generally intercepted by the eye. Then a magnification can be calculated according to some rule, but there will be involved in this calculation a reference relating to how the eye normally sees.

This may sound complicated, but a few examples should make it clear. In the case of a slide projector there is an object, the slide, that is projected on a screen, the image. If one knows the distances between the lens, object and image, the magnification can be calculated and is independent of any "eye" related criterion. This is a simple case of real object to real image.

In the case of a microscope there is a real object, but the image is projected through the eyepiece and focuses at infinity. In this case one cannot calculate a real magnification. But it is common agreement that the object will appear larger to the eye than it does when the same object is viewed at a distance of 10 inches. In this case, the objective and eyepiece are designed to have focal lengths such that the markings on the objective and eyepiece when multiplied together yield this number. It is the apparent "magnification" of the microscope. Notice that the way the eye sees the object is part of this calculation.

In the telescope, we have the very same situation. The telescope projects a virtual image of an object in the sky as a virtual image. The agreed upon definition of magnification is the apparent size of the object compared to how the unaided eye sees the object. This value is calculated by dividing the focal length of the objective lens by the focal length of the eyepiece.

A magnifying glass has an agreed upon magnification that is the focal length of the lens divided into 250 mm. These three latter cases clearly are somewhat arbitrary in that they involve a "definition" of magnification that depends on the characteristics of the eye.

Only in an optical system that generates a real image of an object can a true optical magnification be calculated. This is the case for the slide projector and also for a camera lens. If we image a meter stick, we can measure the length of the image on the film. The ratio of these lengths is the true optical magnification.

But the case of the camera designation has been distorted over the years. In photography, a lens is often designated to have a magnification, not the true optical magnification, which is designated by the focal length of the lens divided by the focal length of what is considered a normal lens. What is the focal length of a normal lens?? It has historically been equal to the diagonal of the format of the film. In 35 mm cameras, it is considered to be 50 mm. (45 mm in some cases) Thus a telephoto lens of focal length 200 mm is considered to have a magnification of 4 times. This has, in fact, nothing to do with the true optical magnification of the lens.

But then there is the case of a telescope objective which generates a real image of a real object. Does such an optical system have a true optical magnification. The answer is yes. But is difficult to calculate since we have to know the distance of the object and its size. For example what is the distance and size of Andromeda. If you have those figures, you can calculate the size of the image you will get and thus calculate the real optical magnification of the objective lens.

We do not do this calculation for obvious reason. Thus we revert to the earlier definition of magnification which is the focal length of the telescope objective divided by that of the eyepiece. Also, at this point we often go to angular definitions of the object and the image. This is very convenient since if we know the angle subtended by the object. Andromeda for example, we can calculate the object size by multiplying this angle in radians by the focal length of the objective.

An example is the angular diameter of the moon is 1/2 degree. This is about 0.009 radians. Thus a 200 mm lens will form a real image of the moon that is 200 times 0.008, or about 1.8 mm in diameter. This way we can calculate the expected image size without directly knowing the diameter of the moon and its distance. We usually know the angular sizes of galaxies and nebulas and the like. So we can easily calculate the size of the image we will get with a given telescope focal length.

Technically we could talk about the actual optical magnification of the object and the image. This would be 1.8 mm divided by about 2000 miles. I let this silly number for the reader to determine. (G)

Sorry to go on so long, but the term "magnification" is a hard to define and elusive number in some cases. It depends on convention as much as calculation. I hope this discussion has been of some value.


Subject: Magnification -- What Does It Really Mean? -- part 3     Top

From: Peter Erdman <erdmanpa_terau.edu>

Partly right, partly nonsense. The magnification of a slide projector is obvious to any observer. One definition of magnification is the ratio of the size of the image to the size of the object. For a slide projector this is easy to calculate (or observe) the image is significantly larger than the object (the slide). Hence it is "magnified".

This simple definition of magnification has meaning for finite object-image distances (such as a slide projector). At infinite object-image distances we run into trouble. After all, the star is physically huge, yet its image is a tiny dot on our detectors.

For telescopes with an eyepiece (and similarly for microscopes) we then shift to the concept of "angular magnification"--or how much the ANGLE between two chief "rays" exiting from the object is "magnified" as it enters our eye. This definition of magnification is useful because it relates the size of the final image on our retina with optical aid to that without the optical aid. In other words, our perceived "magnification".

For an image at the focus of our telescope we have a problem in deciding what to use for our definition of magnification. Using the ratio of image/object size, we get zero for the magnification (since the object is huge and the image is small). Using "angular magnification" we get unity since the angles are not changed by the telescope at its focal point. "Prime focus" has a particular meaning that we must be careful in applying, but being sloppy we can just call it the focal plane without the use of an eyepiece. Since the chief ray angles are unchanged, we say that "magnification" has no meaning here.

However, using the focal length of the optical system, we can easily calculate the "image scale" or "plate scale". This is the commonly used "arc seconds/pixel" number given for an optical system, and lets us know the physical separation (on the detector) of two rays emitted from the object differing in angle by an arc second. The best our unaided eye can achieve is a resolution of about 1 arc minute. Even modest telescopes and CCD cameras can achieve a few arc seconds/pixel. We usually refer to this as increased resolution, but not as "magnification".

The "magnification" of an SLR lens is another nonsense number commonly used. It actually relates the angular field of view with one lens to that with a "normal" lens which is roughly taken as about 50 degrees. Hence the size of the film detector (35mm, 120, etc.) changes what is "normal". For 35mm film, a 50mm lens is roughly "normal". For 120 film, it would be wide angle simply because the detector size is so much larger. It's like comparing an ST7 camera with an ST8 on the same optical system.

Just words, don't let them confuse. Remember that "magnification" has meaning when it tells us how much the image on our retina will be enlarged by the optical system. When our eye (and its own optical system) is not (or cannot) be part of the optical train, then "magnification" loses meaning. Quantifiable parameters such as "plate scale" always have meaning for film or CCD detectors at any focal plane, but their use requires some explanation and that is always a difficulty.


Subject: Magnification -- What Does It Really Mean? --part 4 of 4    Top

From: Chris Frye <cfrye44a_tbellatlantic.net>

I've always considered the approximate magnification at prime focus to be the focal length of the system (in mm) divided by 254mm. The reason for this is that 254mm is considered to be the normal (1 X) reading distance for the human eye. If you measure the Moon's apparent diameter at 254mm from your eye it will measure 2.217mm. If you measure the Moon's diameter on the photographic negative taken at the prime focus of a 10" f/10 telescope it will measure about 22mm. Therefore the magnification appears to be about 10. To reinforce this viewpoint, if you observe through a 25mm EP using this same 10" f/10 telescope, you will be observing at about 100X. If you hold a sheet of paper 254mm from your eye (like you're going to sketch what you are observing) you will note that the Moon measures 220mm in diameter on the paper. Therefore a 22mm image on the negative held at 254mm from the eye represents a magnification of 10X. This same logic is used in calculating microscopy magnification, except they use 250mm, not 254mm.


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