Focusers & Reducers, & Calculating f/ratios

MAPUG-Astronomy Topical Archive     AstroDesigns


Subject: Focal Reducer Adapter by Ted Agos

It has options that work with a Barlow as well.

PRICING: (October, 2002)
UNIVERSAL ADAPTER TUBE with sliding f/6.3 insert ----- $149.00
with Celestron Optics installed ----- $275.00
Extra insert for use with f/3.3 optics ----- $69.00
1.25" I.D. Insert to hold a Barlow or Eyepiece for projection ----- $35.00
AO-7 Short 6.3 Adapter tube ----- $98.00
AO-7 version with Celestron Optics installed ----- $225.00
Shipping in Cont. U.S. included in all the above

Note: For those desiring optics installation we suggest careful wrapping of the optics before shipping to us. Please include payment.

Make checks payable to Ted Agos
  Any questions, e-mail: <>
  Phone: 508-865-5613
  Mail to: Ted Agos, 85 Town Farm Rd., Sutton, Mass. 01590


Subject: Optec Maxfield f/3.3 Considerations   Top

From: Michael Hart

In earlier posts, I have briefly mentioned that this reducer is not a panacea for CCD imaging. The reasons become more apparent with experience, which I've outlined below. There are definite advantages when using this reducer and limitations as well, of interest to a prospective buyer. A typical ST-7 or 416 owner will be making a healthy $325 investment for the focal reducer, CCD camera adapter (camera specific) and Wrattan filter. He or she may also opt for a f/10 scope rather than a native f/6.3 scope to use it the Optec. Add $70 for SCT adapter, if needed, and your just shy of $400. If you need a more expensive camera adapter, such as for the Pictor 216, the price is now $460 for the complete outfit. Is the Optec worth it? Let's explore this a bit....

The Optec Maxfield can play an important role in optimizing a CCD imaging system. By adjusting the telescope's focal length to match pixel size we can optimize the CCD camera resolution to signal/noise ratio. The Optec can also play a role in reducing image scale to fit a particular object on a CCD chip, but this should be a secondary consideration. The most important consideration is the former. I have detailed reasons for this in another post titled "Optimizing a CCD Imaging System- Pixels and Focal Length"

First, Doc G is absolutely correct as stated in his post, ".... thus the spacing between the back of the reducer and the chip is quite critical. (plus or minus a few mm)" In fact, the distance to the chip is VERY critical and not forgiving. As little as 1 mm will introduce what I consider VERY objectionable coma, to the order several arc seconds. Not just in the very corners but noticeably extended well into the image, 40% by area. So, I recommend the proper adapter be purchased or fabricated or you won't like the results.

Second, do not fail to use a #12 Wrattan filter (at least purchase it). I believe use of this filter is quite important, though many won't or don't use one. Chromatic aberration is quite pronounced with this focal reducer which is optimized for wavelengths between 5500 and 8500 angstroms. The blue end takes a big hit, which is not an accident by Optec. If you must have chromatic aberrations, the best place to have them is the area the CCD chip is least responsive too, the blue end for SBIG ST-7 and Pictor 416 (0400) chips. Without the filter, the point spread increases dramatically in the blue because this wavelength is not brought to focus. The #12 Wrattan (yellow) has 74% transmission, which at first appears as a major light loss, but actual transmission is effectively higher for the 0400 chip. Actual exposure increases over non filtered images for an equivalent signal/noise ratio will be less than the 74% implies.

Third, order the telescope mount (2" barrel) outfitted with a large registration stud to fit the JMI-NGF-S (2") opening slot for the 1-1/4" adapter thumbscrew. You will have to ask for this. This will aid in duplicating exact imager position that will be needed for taking and using a master flat field, desirable because the extreme focal reduction comes at a price, significant vignetting. The vignetting will be most apparent with images that are displayed with a narrow background and range. The telescope adapter (2" to SCT threads) includes a registration pin to align the reducer to the SCT adapter. I would not and do not use the telescope adapter with the JMI NGF-S, rather I insert the Optec directly into the 2" focuser.

Fourth, expect to loose resolution with this reducer. I have measured a point spread increase of over 2 arc seconds (measured at 1/2 the stars maximum intensity point) from f/10 to f3.3 with the Wrattan filter. Stars that are just separated at f/10 will appear a little bloated and will touch at f/3.3.

Fifth, this reducer is for f/10 and slower SCTs, correcting for SCT coma not found in Maksutovs. It needs at least 195 mm of back focus and CCD chips whose size is less than the light cone produced by this reducer- around 10-1/2 mm.

In summary, the Optec Maxfield does a good job at reducing the focal length of a larger aperture f/10 and slower scope's to optimize pixel size to signal/noise ratio. A secondary consideration is the reduction in image scale allowing larger objects and a larger field of view on the CCD chip. However, there are considerations for gains made, which I've outlined above. For many, the Optec Maxfield may be an important accessory, others may want to consider a native f/6.3 scope with a standard SCT focal reducer working at f/4.


Subject: Focal Reducers vs. Low Power Eyepieces    Top

From: Nigel Puttick <Nigel_Putticka_tCompuServe.COM>

I have done some research and asked some questions about (f/6.3) reducers recently, and have been interested to see the same questions coming up (yet?) again. My conclusions were:

Visual: why bother? Use a longer f.l. eyepiece instead if a wider f.o.v. is what is desired. An 8" scope gathers the same light whether at f/10 or f/6.3, no amount of extra optics will make it brighter. A smaller visual image of an extended object will appear brighter regardless of how it is achieved, as the same amount of light will occupy a smaller area.

Film: at f/6.3, film exposures will (theoretically) be approximately 1.25 stops faster, i.e. about 60% shorter. With reciprocity failure taken into account, the saving in exposure time may be even greater. As Ric Ecker says, you will get more exposures in a night. However, this is not the only effect. The image size will be smaller due to the shorter f.l. of the focal reduced scope: if an 8" f/10 SCT has a 35mm field (angular) of approx. one degree, then at f/6.3 the field will be 2000/1260 times larger, i.e. approx. 1.6 degrees.

No magic here. The image will need enlarging more, by a linear factor of 1.6, and show much more grain. Bearing in mind that many deep sky objects are very small (e.g. M51, 10' x 5': M27, 8' x 4'), you are only using a tiny portion of the 35mm frame anyway and pushing the acceptability of an enlargement to or beyond the limits of emulsions, so why make things worse? With the bigger objects (e.g. Rosette Nebula, 1.2 x 1.0 deg) you will fit them on the frame with the reducer, but suffer the inevitable vignetting described by Philip Perkins and Graham Warellow among others. With intermediate sized objects you may get acceptable results as the peripheral parts of the final print will be stars on a dark background. Basically there is no substitute for the right f.l. (and hence field) for a given subject. This is what you would normally do with a terrestrial camera ... use different lenses.

CCD: in this case the vignetting is irrelevant as you are using only a small central portion of the light cone onto a tiny chip. The effect of shortening exposure time is much less important as the exposure is so short anyway, measured in minutes only. The main point is that some objects are simply too large to fit on the chip at a (to the viewer) acceptable resolution, and that for the best images it is necessary to optimize the field size per pixel, the exact value being controversial. It seems that f/3.3 reducers are most popular for this application, but are much more expensive so the f/6.3 could be a good start for a minimal investment.

Now, I am still very new to this and have taken few astrophotos and have no CCD imager (but am playing with an ST4 for guiding). I was initially interested in a reducer, and had one (Meade) on trial briefly. On my OM1 focusing screen, in daylight, there was appreciable darkening at the edges, and as you looked away from the center of the field, the image became noticeably soft. I had been told it would "flatten the field", the curvature of which I understand is also an inevitable optical consequence of the SCT design: to me it appeared worse with the reducer in place. I thought if it looked like that in daylight it would have no chance of producing acceptable images on film of point sources! I sent it back.

Most of this is predictable because you are trying to cover a wider field with an optical system that is only just able to cover 35mm in the first place, and so vignetting and peripheral aberrations are inevitable. It is likely that most of the vignetting is due to the exit pupil of the primary baffle tube rather than the optical characteristics of the reducer itself: the effect would be similar if instead you tried to use a larger film format with no reducer. Conventional photographic lenses don't use focal reducers, for the same good reasons, though I understand it is possible to design a fully corrected reducer for some scopes, usually refractors that can cover larger formats.

I now believe that for good results on film, it will be better if I choose subjects appropriate to my native f/10 8" LX200, and perfect my technique with that. I dread long periods of guiding so have bought an autoguider. A piggyback lens or small scope (300mm f/4.5, 480mm f/6.8) will give wider fields. Only when I have all these pegged shall I think about instruments of other focal lengths. These are my conclusions, and my logic, with help from this group ... others may differ!


Subject: Apogee vs. JMI vs. Van Slyke Focusers    Top

From: Jerry Stephenson

I have owned both and also now own a Van Slyke Monster. Here is my quick review:

The Apogee is smoother than the JMI and has a longer travel (3/4" vs. 1/2" although I've heard that new Apogees have even longer travel). I sold the Apogee and went to the JMI because the JMI was short enough to clear the fork on my 12". With the Apogee, I couldn't look at the zenith. The JMI is a good focuser and, although I have a fetish for quality, I was very happy with it. In other words, although I felt the Apogee was superior, the JMI met my high standards for everyday use.

The Van Slyke is another story. It is expertly machined and over designed in every way. Huge bearings give extremely smooth movement and a full inch of travel. It is short enough to clear the fork if it is not rolled out all the way. Additionally, it has a nice internal compartment for the focal reducer. It does weigh more but with my heavy Meade dew shield, that's actually a bonus. Currently the JMI is on the shelf and after a few more outings with the Van Slyke, it will be sold too.

You can't go wrong with any of these three fine focusers, but if you want the best, consider the Van Slyke.


Subject: Meade 1209 Focuser?  Top

From: Richard Bennion <> Date: Sep 2003

-----Original Message-----
From: Mike Schriber <>
Okay, now that I'm doing more CCD work, I'm fed up with the image
shift on the standard focuser (LX200 classic).
I'd like to get a zero image shift focuser. The NGF-S comes to mind,
but I'm also curious if anyone has used Meade's 1209 unit.
-----End of Original Message-----

The Meade 1209 focuser is a huge improvement over the standard manual focuser because you can lock down the mirror and then make very fine adjustments using the micro focuser. I am not sure if the classic has a focuser port (I have a GPS), but if it does, you can control the focuser with software to do automated focus runs that really zero in on the critical focus zone.

The problem with the 1209 is that it uses electronic pulses to move in and out that are not very precise. In other words, it is not an absolute micro focuser, it is relative in its positioning. What does this mean? If you are using software to control the focuser such as the Sky or Maxim or FocusMax to try to find perfect focus, then you have to deal with backlash issues with the 1209.

Automated focus routines start out of focus, and then move the focuser in steps, take a picture, test sharpness and brightness, and then repeat all this until it finds the critical focus zone. Once it finds the CFZ, it moves past it until it is out of focus again. Once it has done this, it looks back at the focus curve and moves the focus back to the middle of the CFZ to achieve perfect focus. The 1209 does a OK job of returning to the CFZ although it can miss often because of the backlash.

I decided to upgrade my micro focuser to the OPTEC TCF-S which is the best microfocuser on the market. It uses a stepper motor, where each step of the motor is EXACTLY 2 microns. The Optec is also an absolute focuser (from 0 to 7000). So you can turn it off, and back on again and just punch in the number you used the night before to reach perfect focus everytime (temperature permitting). Also when using automated focus routines, the Optec has zero backlash and returns back to ideal focus 100% of the time. In my opinion the Optec reaches perfect focus with software assisted focusing 100% of the time, while the 1209 reaches it 30-50% of the time.

There are also some focusers from JMI that are supposed to be pretty good at half the price, but I have no first hand knowledge of these. The Optec also has temperature compensation, so as the temp changes throughout a long night if imaging the focus will change based on a probe from the unit attached to the telescope so you stay in perfect focus from 70 F to 30 F.

F/10 scopes have about a 250 micron critical focus zone (1/4 of a millimeter) while fast scopes (f/6) have only a 60 micron CFZ. So finding best focus for CCD imaging is very challenging with out the software. If you are just playing around with CCD imaging, then start out with 1209. You will get a 10X improvement over manual focusing just using the hand panel because you can lock the mirror and avoid mirror flop and get much better control.

If you are serious, then get a better focuser like the Optec for perfect results every time.


Subject: Field Flattener Without Focal Reduction Available? --part 1 of 2   Top

From: Roger Hamlett <> Date: Dec 2003

> I am looking for a field flattener only. I have an LX200
> 10" F/6.3 scope and the edges are out of focus when the
> center is in focus. There are lots of focal reducer/field
> flatteners, but I don't need further focal reduction, and I
> don't suppose these combo items do any more flattening beyond
> that created by their focal reduction anyway (?). Can anyone
> point me to a source of just field flatters?

Focal reduction, and field flattening, are separate functions in a sense (though there is a slight tendency to reduce/increase from a normal flattener - doing a 'unity gain' device would require yet another lens to compensate for the gain/reduction introduced by the flattener). Field flattening, is achieved by having a meniscus lens, that is a bit like the corrector on a Mak (concave on one side, and convex on the other), but with a slight +ve or -ve magnification as well. Focal reduction, just requires a normal +ve lens (this will also increase the effective field curvature). The combo units do contain separate lenses to perform both functions, but the flattener lens, also provides part of the color correction. Borg do a flattener, that has a very small +ve effect (nominally 1.04*). It is designed for the field curvature from a refractor, rather than an SCT.

However it is possible to use this, by spacing it to the right distance from the target. A similar mildly -ve flattener, is available from Takahashi. With both these, it'd take some calculation/experiment, to get the right spacing for the best effect on an SCT. To calculate this, the 'easiest' way (assuming you have Excel), is to download the telescope design spreadsheet from Oldham optical (, and put together an imaginary SCT, based on the dimensions of your scope (remember the primary is about f/2). This package outputs the radius of the field produced by the scope. The Borg unit, has the separation needed for different refractors specified (and the radius these generate can be predicted the same way), allowing a very close figure to be calculated. For your scope, the spacing should be close to that needed for about an 1m uncorrected refractor.


Subject: Field Flattener Without Focal Reduction Available? --part 2 of 2   Top

From: Bostjan <>

1. A focal reducer ALONE has the tendency to increase EXISTING field curvature, as is expressed in the excellent book "Telescope Optics" written by Harrie Rutten and Martin van Venrooij:

"A focal reducer is a positive lens system and works in the opposite sense from a focal extender. This means there is a tendency to produce a focal surface more strongly curved than it was initially. Unless care is taken, off axis image sharpness may suffer."

BUT, if the telescope has NO field curvature (e.g. the fairly popular flat-field f/4 Schmidt Cassegrains) a normal focal reducer could work just fine.

2. Meade and Celestron reducers are SCT-dedicated REDUCING + FIELD FLATTENING systems and thus IMPROVE off-axis performance on a SCT (but would DETERIORATE off-axis performance on flat-field telescope).


Subject: Robo-Focus (Remote Focusing) --part 1 of 5   Top

From: John Teel <> Date: Nov 2001

I checked into the Robo-Focus (RF) and it sounds like a very good product and may be the way to go. Their website is <>. It costs $375. They also offer a serial pass thru port that allows you to use one serial cable to control the telescope and the focuser. This RF can be used either on the SCT main focus knob or on any Crayford focuser. The thing I find interesting is that they claim that when using the RF there is no advantage to using it on a JMI NGF Crayford focuser over using it on the SCT main focus knob. They did a fairly detailed test (they have a paper on their website) where they had one SCT with a RF on a JMI focuser and one with the RF on the SCT focus knob. This test showed the focus quality (lack of backlash and image shift) to be nearly identical. They claim the RF compensates for focus backlash AND image shift. I can understand how it compensates for backlash but I don't understand how a focuser can compensate for image shift. Is that possible? If this is true I think it would be highly advantageous to be able to focus via the main knob (less weight, less money, more focus range, etc.).

Boy its amazing at how many pieces (both software and hardware) there are to setting up a remote imaging telescope! Money, money... I'm really interested how others focus remotely?


Subject: Robo-Focus (Remote Focusing) --part 2   Top

From: Roger Hamlett <>

Data about 'Robo-focus' is at: <>

If you include the hyphen, a web search will find it. Robo-Focus, can 'share' it's port with a telescope (don't know if you can do this in Maxim, but in AstroArt, it works). All you do is deselect the telescope application, and run the focus application. When you have got your focus 'right', turn off the focus application, and reselect the telescope. Works for me with a LX200.

Robo-Focus, can be fitted to just about any focus knob, with a bit of metalwork. I have ended up removing my NGF-S, and running Robo-Focus on the scopes own knob. Though this re-introduces the mirror-shift problems (Robo-Focus can correct for backlash), I was finding that the total 'optical' length was so great, when wanting to use a focal compressor, that spherical aberration was starting to get unacceptable, so removing the extra length of the NGF-S, was worthwhile... I hope the new LX200-GPS, has the 'optimal' correction distance set for a longer optical length at the rear of the scope, to allow for it's extra focuser there.

The LX200 focus port is 'dumb', only comprising a motor forward/reverse signal. You can fit a simple motor to this, if you only want a simple 'movement' control, but something like the Robo-Focus, adds more, allowing you to 'GOTO' a particular focus setting for various setups, and even if required to compensate for temperature variations.


Subject: Robo-Focus (Remote Focusing) --part 3

From: Blair MacDonald

You might want to check out APT. It allows focus goto's with a decent precision with the Meade focus control built into the LX200. It works with any analog focuser that can connect to the LX200 control panel. Its focus control is not a closed loop system so it won't quite match products like Robo-Focus, but then again it is only $45.00. With the aid of a diffraction focus tool (made from some coat hanger) I can get perfect focus in under 5 minutes. Best of all it allows remote telescope control as well.


Subject: Robo-Focus (Remote Focusing) --part 4

From: Charles Baker

I am using RF3 on the Meade primary focus knob and it works beautifully. Suggest you ask John Menke about how the backlash corrector works. The motor takes the mirror a little beyond the point you are aiming for then backs up so the motion change is always from the same direction.


Subject: Robo-Focus (Remote Focusing) --part 5 of 5   Top

From: Roger Hamlett <>

Important to understand that there are different types of image shift. Robo-Focus almost totally eliminates the image shift associated with focussing - watching on a focus window, you see the star move sideways as the focuser operates, then return to almost exactly it's starting point - better than 10 arc seconds, I see only perhaps one or two pixels, running scaled at about 1.6 arc seconds/pixel. However it has no effect on the other type of shift, which is where the main mirror 'rocks' as you pass the meridian. This is avoidable by being careful where you are imaging, but cannot be removed on a simple 'tube over tube' main mirror mount. With the NGF-S, you can remove this too, by using a locking bolt in the shipping hole, and applying some torque to the mount, which reduces this shift to practically zero.

I like the Robo-Focus. If is remarkably accurate (when you consider how small the main mirror movements are for focussing), and seems very repeatable.

One 'caveat' I have about it, is that I have a variety of devices using these power connectors, and some are US/English designs (tip +ve), while others are Japanese units (tip -ve). Of all the units, the Robo-Focus, is the _only_ one I have that does not have a protection diode if you accidentally reverse the power connection. I have modified the unit, and added a 3A Schottky barrier diode (low voltage drop), in the wire between the power connector and the PCB, after having a 'scare' of this type with another piece of equipment borrowed from a friend. I think this is a modification that the units should have....


Subject: RoboFocus Metal Bracket    Top

From: Frank Mollica <>

Here are 3 pics of the Robofocus mounted to the course focuser of my 10" LX200. I used it last night in the field for the first time and it worked great.

I made this prototype bracket out of brass because that was the material I had at my house that was best suited for the fabrication. If I made more of them I would probably use a structural Grade 80 galvanized steel, it would be lighter, stronger, and rustproof. The bracket itself couldn't weigh more than a couple of ounces. You can still use the locking bolt hole also. My scope it is perfectly balanced with the RF attached as shown with the diagonal and eyepiece on the scope. I think it could be configured to work on any SC scope.

I don't know how much interest there is in this bracket, but if there was enough I could produce them. (see below)

Robo Bracket-1 Robo Bracket-2
Robo Bracket-3

Here's some photos of the "production" model, contact me directly at my email address above if you are interested in purchasing one:

Robo Bracket-4 Robo Bracket-5
Robo Bracket-6


Subject: Lubrication for Focuser and Other Items

From: Eric Ebner <>

Just one miracle product works... Radio Shack's, Archer "Lube Gel."

Exceptional adhesion.
Does not attract dust.
Stays where you put it.
Non toxic, non corrosive.
Won't break down or decompose.
Works in temps from -45 to +450F.
Contains a patented formula of (PTFE).

I've found the above to be true. My dad and I have used this stuff on telescopes and other items for ~20 years or so. Great for crayford focusers (gives that resistive buttery feel) and any machine threads like the Dob wheelbarrow bolts and the like. I guess you can see I'm a fan! No other product seems to work as well, and is as clean to use. It's also inexpensive.


Subject: Motor Focusers     Top

From: Doc G

Jon Brewster wrote:
> >I have a 10" LX200 and have a lot of trouble with focusing. The stars shake
> >violently when I try to turn the knob (partly because I was too cheap to buy
> >the more stable SuperWedge) making it hard to get tight focus. Is the
> >motorized focusing attachment worth buying and is there still image shift
> >when using it? Any comments about the usefulness of this attachment from
> >those who own one would be greatly appreciated as would any other tips for
> >achieving good focus.
> With mine I get no vibration, still get image shift, its slow and un-encoded.
> I do use it to focus my CCD but look forward to going to a JMI system
> with no image shift and just using the hand knob for coarse corrections.
> As much as I like everything else about my equipment, I could wish I had
> not spent this particular money.
> BTW, can someone tell us if the JMI stuff gets controlled from the LX200 panel?

I have two JMI focussers with their Digital Read Out (DRO) on two LX200s. The focusser works at two speeds from the LX200 panel through the keypad. But the JMI controller has a greater range of speed control and the DRO enables you to return to a focus point and to know just where you are. It is also easier to push the buttons. With the Meade keypad you have to set the mode and push two buttons at once. I strongly recommend the JMI with the DRO.


Subject: Motor Focuser Test Circuit    Top

From: Michael Hart


This circuit provides variable high speed isolation, reversing and braking functions to small DC motors. I can give you a list of materials and wiring procedure from memory. JMI uses a similar circuit, I believe. I give no authorization to use this circuit for commercial perposes, only for personal use as a guide in troubleshooting. It is up to the builder to determine the fitness of this circuit for their application and if any existing patents exist. Those whom wish to control focus motors should contact JMI (Jim's Mobile Industries) for pricing.

(2) push buttons, SPDT
(1) 330 ohm resistor
(1) 500 ohm variable resistor (sets the speed range)
(1) miniature audio jack (attach your minature plug from a JMI or Meade focuser plug to this)
(2) 9 volt lithium batteries in series (long life, works at -23+ degrees F.)
(1) case (I used a small beeper sized case)
(1) hook or velcro to secure to tripod legs or scope

a) Wire the left common SPDT contact to the ground of the phone jack.
b) Wire the left normally open SPDT contact to the (+) of the battery.
c) Wire the left normally closed SPDT contact to the right normall closed SPDT contact.
d) Wire the right common SPDT center conductor of the phone jack.
e) Wire the right normally open SPDT contact to the left normally open SPDT contact.
f) Wire the right normally closed SPDT contact to the left normally closed SPDT contact.
g) Wire the right normally closed SPDT contact to the 330 ohm resistor AND center tap of the pot.
h) Wire the other end of the 330 ohm resistor to the end tap of the variable resistor that results in a resistance DECREASE to 0 ohms when the the pot is turned to the right (increases speed). One tap on the pot is not used.
i) Wire the end tap the resistor is hooked on to the (-) from the battery.

>It seems to make sense except for one thing: why the two
>resisters in parallel? Would it not be equivalent to just use a smaller pot?

I took my written instructions and went ahead and drew the circuit

The parallel resistors allow precise selection of the maximum resistance which determines the slowest speed. In addition, the parallel resistors enable spreading the desirable speed range across the entire pot range. The maximum resistance is set to reflect the amount of torque needed to turn the gears fully loaded. In this way, you have no dead zone in the speed control.

>And as to the theory, I don't see how the braking effect is accomplished.
>It would seem that when the button is released the motor is simply shorted out.

You are controlling a DC motor that consists of a permanent magnet, commutator and armature. The commutator acts as a switching mechanism to apply DC current to the a winding at an optimal armature position that will produce a like polarity with the permanent magnet. The like polarities repel, which is converted to a circular motion (rotation).

When you stop applying power to a DC motor, the coils cutting the magnetic fields turn the motor into a generator. Now, if I short the generator leads during rotation, a field builds up in the armature that is of opposite polarity to the permanent magnet. The opposite polarities attract which result in breaking.

>My intention is to get the Meade focusing motor and control it with this
>circuit instead of the LX200's keypad. Does that sound reasonable?

Very reasonable, that's exactly why I built one. And it's simple enough to do without drawing a schematic.

By the way, I manually guide with a little circuit I put together that installs on the LX200 keypad that is 1/3" tall. See picture) I needed to assure 100% reliable guiding corrections (membrane pads inherently have inconsistent pressure requirements that result in lost corrections or excessive pressure and fatigue). Pressure required to activate a correction: 6 oz. I won't divulge details do to potential control damage and deference to Rob Roy.

  Test Circuit   Joystick


Subject: LX200/f6.3 Focal Reducer/NGF-S Focuser --part 1 of 2     Top

From: Doc G, Visit my website for more info.

> I just ordered the Optec Maxfield reducer .33 for the NGF-S and the
> 416 adapter. The Maxfield barrel for the NGF-S is $225 and
> the 416 adapter is $60, the adapter for the 216 is $125. I have a
> 216 but was told not to buy that adapter because of cost. Do you
> know the difference of the two adapters ??????

Yes, I have studied this thoroughly and even better, I actually have had several of the adapters. (as you say practice makes it truer) The reasoning goes like this. The Optec 0.33 focal reducer is a very strong reducer and thus the spacing between the back of the reducer and the chip is quite critical. (plus or minus a few mm) To get the correct reduction and the best optical performance you need to have the distance at 29mm. Optec thus makes adapters which fit each camera exactly. When I had my 416 I got the focal reducer and they sent the correct adapter with it. When I changed over to the ST-7, I called them and they exchanged the ST-7 adapter for the 416 adapter. The ST-7 adapter is a few mm shorter. If you use the reducer with the 416 adapter on the 216 you will find that the reducer to chip distance in not quite correct. But the difference will only be a few millimeters. I believe they would rather not make so many different adapters and have found that the back focus difference is small enough work just fine. The difference between the 416 and the 216 is I believe about 6 mm. The difference is because the 416 has a mechanical shutter and the chip is thus a few mm further from the T thread seat. These numbers are listed in one of the articles on my web page. Under Optical tubes I think.

In the case of the 0.33 reducer you must screw the adapter directly into the T thread on the imager. The Optec can be purchased without a front adapter since it will fit the JMI 2" tube directly. They also sell adapters for other imagers and for attachment to optical tubes with the standard Schmidt thread.

By the way, I think you will find that even with the Meade 0.63 reducer you will want to put the JMI directly on the telescope using JMI's adapter plate. It has a much larger opening that the Meade plate and will reduce vignetting, especially if you go with 35mm at any future time. The reducer then goes on the rear of the JMI with an appropriate adapter and you use a Meade Schmidt to T tube. They make one just the right length, 95mm.

Since the JMI NGFS quite large and heavy you will find it unwise to place the reducer directly on the back of the optical tube and then add the JMI. The other order is much more stable mechanically. Additionally, you will, for the best mounting solution purchase the JMI adapter which fits directly on the back of the OTA and has a much larger opening than the standard Schmidt screw thread adapter. It is mechanically much stronger and more stable as well as eliminating any possible vignetting.


Subject: LX200/f6.3 Focal Reducer/NGF-S Focuser --part 2 of 2   Top

From: Michael Hart
Doc G wrote:

> > John W Downs wrote:
> > Yes, the focus reducer is designed to butt up next to the prime focus
> > with no other devices first, this will insure a f/6.3 ratio.
> > > Jim Wilson < writes:
> > >My question to the group. If I am going to use The f6.3
> > >focal reducer with a NGF-S focuser what order of attachment
> > >does one use? Does it matter what goes on first?
> The focal reduction you get depends only upon the focal length of the
> focal reducer and the distance between the focal reducer and the final
> image plane. This is an optical fact.
> No amount of discussion will change the optical equations.--Doc G


I believe both Doc G and John Downs are correct.

Doc G is correct that the amount of reduction after a focal reducer is a function of the reducer's focal length and the reducer's distance to the image plane.

However, without violating any optical facts or equations, John Downs is correct as well when we consider that a focal reducer is only part of a complete optical system, the customary use of such devices.

The final focal ratio of a complete SCT will vary with the position of the primary mirror before magnification of the secondary. Depending on where the primary mirror ends up to reach focus, the focal length is changed and therefor, the effective focal ratio of the SCT. So, the true focal ratio of a SCT rated f/10 with a f/6.3 reducer can be something other than f/6.3 when we change the position of the focal reducer as stated by John Downs while the focal reducer reduction remains at f/6.3 as stated by Doc G. I have obtained something on the order of f/11 using the GEG without a focal reducer due to the extreme back focus requirements of this off-axis guider.

In addition, commercial SCT focal reducers are optimized for a specific primary to secondary distance for minimal aberrations, so some image quality is sacrificed when the primary mirror is moved from the optimal position to achieve focus when we move the focal reducer far from it's optimized distance from the secondary. Here, the focal reducer position does matter if you desire to maintain optimal image quality and/or a true f/6.3 effective focal ratio.

Having said that, the original question was where best to put the focal reducer. In practice, I have found one can make prudent position changes of the focal reducer (behind or in front of the NGF-S) that are quite tolerable near the center of the light cone (CCD), but less tolerable as we move to the edges of the light cone (medium format and 35 mm emulsion). With the NGF-S, the use of the optional anti-vignetting adapter (10" & 12" only) at the scope back, will leave you no choice but to mount the reducer on the 2" to SCT adapter that is inserted in the 2" NGF-S.

If you use the Meade SCT thread adapter on the scope back, the focal reducer can be used in either location. The best location depends on what type of imaging your doing (if at all) or the type of visual work and other accessories used as well as results expected/desired.


Subject: Calculating  f/ratios    Top

From: Del Stanton <>

In general the f/ number is the focal length divided by the clear aperture of the objective (be it a mirror or a lens). You could also use the expression "focal ratio" for f/ number. This applies directly to a simple telescope like a newtonian reflector or a refractor that has an objective lens and not other optical elements between the primary mirror or lens and the focal plane.

When you are dealing with a Cassegrain telescope, a Schmidt Cassegrainor any telescope with a Barlow lens, a focal reducer or with eyepiece projection then it is different. The f/ number becomes the "effective focal length" divided by the aperture.

f/ number = (effective focal length) / (clear aperture)

For example the primary mirror of an 8" aperture SCT (Schmidt Cassegrain Telescope) has perhaps a focal length of 18 inches (a wild guess!! estimated from the diameter of the secondary). That would mean the primary mirror has an f/ number of about f/2.25. The convex secondary mirror of my Model 2080B Meade 8 inch aperture f/10 SCT changes the effective focal lengthfrom my guestimated 18 inches to 78.75 inches (converted from thee 2000 mm focal length figure on the telescope), a factor of 4.38 times. And theoriginal f/2.25 times 4.38 = 9.84, or f/ number of f/9.84. (I wish that had come out to F/10 !! I suppose that either the corrector plate has a clear apertureless than 8 inches or that Meade fudges a bit somewhere.)

The f/6.3 field reducer is a positive lens that makes the cone of lightcoming tothe focal plane converge more rapidly. The original effective focal length of your 10 inch aperture f/10 scope was 100 inches. The "field reducer" reduced that to 63 inches. It is actually increasing the field of view of thetelescope by creating a smaller image at the focal plane than the original f/10 system. And that smaller image will include more of the sky. (Just as the smaller image(meaningany given object appears smaller in the image) produced by a 35 mm wide angle lens in a 35 mm camera will include more of a scene than the larger imageproduced by the standard 50 mm lens on the same camera.)

Is it called a "field reducer" or a "focal reducer". It seems that thefirst is incorrect and the second would be correct.

Suppose you have a 6" f/8 reflector, the focal length of the primary is then 48inches. And you use a 2x barlow. Now the effective focal length is 96 inches and the telescope now has a f/number equal to the

(effective focal length)/aperture = 96 / 6 = f/16.

When you use eyepiece projection the eyepiece is re-imaging the original focussed image and it is possible to change the magnification by moving the eyepiece and the final focal plane. Perhaps the best way to determine the effective focal length is by the image drift method, determining the length of time it takes for a star near the celestial equator to drift a given distance across the focal plane (with the clock drive tirmed off), recorded either by a piece of film or a CCD. This will allow you to calibrate the currentconfiguration in terms of inches at the focal plane per degree of field of view. Theeffective focal length is then calculated from the following formula ;

Effective Focal length x tangent (angle) = (length of image)
Effective Focal length = (length of image) / <(tangent (angle)>

The drift of the star near the celestial equator will be 0.25 degrees perminute. Starting 360 degrees / (24 hours) and converting that to per minute by the following calculation:

360 degrees / (24 hours) x (hour / (60 minutes) =
<360 / (24 x 60) > degrees x (hours/hours) x (1/ minutes) =
0.25 degrees / (minute)

Thus if your trace for a 5 minute exposure measures 1.5 inches long the effective focal length of your instrument can be calculated as follows:

5 minute exposure of a star on the celestial equator. It will have moved:
5 minutes x (0.25 degrees/(minute) = 1.25 degrees (minute/minute) 1.25 degrees
Tangent (1.25 degrees) = .02182

Plugging that into:

Effective Focal length = (length of image) / <(tangent (angle)>
Effective Focal length = (1.5) / <(tangent (1.25 degrees)>
Effective Focal length = (1.5) / (0.02182) = 68.74 inches

If you scope had a 4" clear aperture then the effective f/number would be

f/ number = (effective focal length) /aperture = 68.75 / 4 = f/17.19

Also see:   Calculating Focal Length for a SCT


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