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Star Testing & Optical Quality

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Subject: Artificial Star for Testing --part 1 of 7       Top

From: Tom Whicker <>

To those who continue to have optical problems, and in fact to all Meade owners who are curious about how good their optics are, I strongly urge you to do an ARTIFICIAL STAR TEST! Burt Whicker and I had similar problems with his new 10" but could never really get a definitive look at a star test because the atmosphere was always a problem. You can't get a clean diffraction pattern so you always blame it on poor seeing.....even on the best night, the pattern jumps all around. It could never be photographed for later reference.

So we made an artificial star and in 5 minutes had all the answers we needed! Now, there are published methods in Sky and Tel. and other places that can get complicated.....about making the pinhole small enough, about locating it many hundreds of feet away so as to not induce spherical aberration into the test. But believe me when I say that a simple pinhole in a piece of tinfoil as close as 40 feet will show a good star test with rings and an Airy disk. And it is *Rock Stable* if you set it up inside your house or garage. It is no problem to photograph with an eyepiece projection set-up. It is easy to video-tape with one of the little mini-cam video cards. Then you have a permanent record of your scope's condition. When Meade sends the scope back to you after repairs, you can compare A to B.

Here's how to do it: Get some aluminum foil. Cut several pieces about 4" square or so. Stack the pieces and flatten them nicely together. Place on a flat surface. Take a sewing needle and put a hole through the stack, such that the needle barely pierces the bottom foil. You now have a range of hole sizes. Select a pinhole that looks can start with the larger ones and as you get the process under control, work to the smaller (and dimmer and harder to find with your scope....) as you progress.

Take a 12" square piece of cardboard and put a 3" hole in the middle. Tape your foil pinhole onto the cardboard. Tape the cardboard in front of a 75 watt utility lamp with frosted bulb. Put the lamp at the end of a darkened hall, etc. We only had about 36' for our first set-up and it worked fine.

Use a low power eyepiece to get it centered and then work up to your highest power plus barlow if you have it. You should easily see the intra and extra focal diffraction rings and a good Airy disk at focus. If you get no patterns, go to your next smaller pin hole, etc.

Note: Finding the pin-hole with your scope can be surprisingly difficult. It will be about a Mag. 7 star. It helps to paint or draw a big cross hair guide pattern on the tin-foil. Then you can leave a little ambient room light on to allow you to find the cross hair pattern and follow it to the pin hole.

Now of course at this close range you are inducing a good bit of spherical aberration into the test. Not to worry. Once you have the technique down, you can take the rig outside and set it up with 300 or 400 feet or more range. Even better, find a warehouse or school gym to set up in.

But even at close range, the test will clearly show the kind of astigmatic problems and problems of irregular surface figure that are common in SCTs. Our particular scope produces a Mercedes Hood Ornament at best focus. It is a non-symmetrical three pointed star with several other smaller lobes that add up to a contorted pentagon pattern. It is about 2.5 arc seconds in size at best focus. On either side of the "focus" it changes to rings that have large lumps where the points of the stars were.

With a stable indoor star, you have all the time and repeatability you need to methodically rotate the corrector plate, or the secondary, or tweek the collimation, to try and isolate your problem. For us, nothing helped, so likely a poor surface on the primary or corrector.

I really hope that many of the Mapug will do this test! If you go back through the archives, you will see again and again people reporting star tests that look like "wheels with spokes", "flairs", "dandelions", "flower petals", "Mercedes Hood Ornaments", "Chrysler Hood Ornaments"! But every time, the Meade owner ends up saying...."Well, the seeing was not too good, so maybe that was the problem..."

Some Final Suggestions: If you can, place a second pin hole at a distance of .1" from your first. This will give you a reference scale that will allow you actually calculate the size of your aberration in terms of Arc Seconds. It also means that if you take a photograph or a CCD image of the star test, then you will have a scale mark so that later tests can be accurately compared.

A good way to make two pinholes at .1" spacing is to get a piece of pre-punched printed circuit board from Radio Shack. The holes are in a .1" grid. Also, try set up a second scope with diffraction limited optics as a side-by-side comparison. We used my old Criterion 6" Newtonian. It will immediately show you what you are supposed to be seeing 8^)

Update: Just a quick note on artificial stars. It is a bit of misconception that one needs a really tiny pinhole....especially if you are planning only to use the star for collimation (and not for the more demanding requirements of testing for spherical aberration, etc). A typical pin-prick into aluminum foil that gives you a 100micron (0.1mm) hole is fine for collimation use. In fact, smaller holes become very dim and are hard to use in daylight. Also, you can use the artificial star at a fairly close range (ie 40 feet) to do the collimation....since you are just interested in the centering of the optical axis....and not worried about how your diffraction pattern may or may not show spherical aberration from the unusual closeness of the star.

To really test your scope, you need to move the artificial star at least several hundred feet away. If you want it to be smaller than your "Airy" disk, use a rule of thumb that 1 arc second is equivalent to about a 1 million to 1 ratio of star diameter to star distance.

So if you want a 0.5 arc second test star.....put your 100 micron pinhole at about 700 feet distance. Such a test star is the best (and in the real world....the only practical) way to get a rock-stable view of your diffraction pattern. The sky will never be stable enough....even on the best allow you to photograph (or video...even better) your diffraction pattern as a function of focal travel. But with the artificial star, you can get a repeatable photo-record of your performance to compare with later scope tunings .....or after it returns from a Mead service trip!


Subject: Artificial Star for Testing --part 2     Top

From: Ron Hendrickson <>

Jeff--I also have a 10" LX200 and I have experienced some of the same frustration you seem to be having. I am a relative novice in astronomy (3-4 years) and it has always seemed to me that much of the advice and info from books is contradictory and confusing when it comes to checking optics.

However, my advice (use at your own risk!) is:

1. Use a good quality Plossl eyepiece that will give you a least 300 power for collimating and star testing.

2. Don't use a Barlow for star testing.

3. The size of our scope does make atmospheric turbulence a problem. The books say that you should get a good in-focus star image looking at a mag. 2.0 star. With a 10" scope, this is too bright in my opinion for almost all sky conditions. I think a mag. 4.0 or 5.0 star will give you a much better chance of seeing a good Airy disk with diffraction rings. The double double in Lyra near Vega is a real good choice right now, as it is high in the sky and about the right magnitude. But even so, I have only seen a good Airy disk and rings (partial) just once when the skies were very steady. I talked to a guy who has a 12" LX200 scope who is very experienced and he says the seeing only permits a good Airy disk about 2 or 3 times a year. (We live in San Antonio, TX)

4. Purchase a cheap (about $8) Ronchi plastic tester and check your scope. They come with directions and made me feel much more confident about my scope. The test showed good straight lines, just like they are supposed to be.

5. If possible, compare your planetary views with those from a known telescope of good quality. This was difficult for me until I met a friend who has a 100mm Takahashi refractor. Its star images were beautiful mostly because of its excellent optics, but also because of its relatively small aperture (compared to your SCT). But when observing Mars side by side a few months ago, my LX200 showed more definition than his excellent refractor. He commented that he was impressed with the Meade optics, even though its star images were not that good that night.

6. Wait at least two or three hours for your tube currents to die down before collimating or star testing. The books say one hour, but that isn't long enough for me. Also, most importantly, make sure the ground you are set up on isn't giving out heat waves. I found that any type of asphalt, concrete, hard surface, etc. is going to mess up your view for hours. You will swear that it is your scope until one night you set up on grass and see the difference.

7. I guess big apertures can be a curse, but its still worth it in my opinion.


Subject: Artificial Star for Testing --part 3     Top

From: Frank Loch <>

If you have access to a CCD imaging camera, you can use this to great advantage for collimating.

The procedure is to image your collimation star with your CCD camera on to the monitor. Center the collimation star in the image box To do this, I take a fine tip magic marker and mark the center of the image box with a small cross hail mark .

With the collimation star centered, defocus to make the star image almost fill the image box. Now the image is big and stable and you can easily see the rings and the airy disk and the "out of collimation" positions.

Now tweak the collimation screws and re - image. Note the alignment changes. Repeat this, enough times until you have the airy disk centered. If the star moves off your center marks keep recentering it until the scope is collimated and the star and airy disk are both centered.

This has worked nicely for me and improved my images a lot. You may want to scrub off the marker cross hairs at this point. Mine are so small, I leave them in place permanently as I then also use them for a center reference for all my imaging work, and they don't bother me in my other applications.


Subject: Artificial Star for Testing --part 4     Top

From: Chris Vedeler <>

I also have a 10" LX200. I had it for a year living in Spokane before I ever saw a defraction ring around a star. Seeing makes all the difference in the world when star testing. Some places don't get good enough seeing to star test but a few nights a year. I live in Tucson Arizona now, and frequently see very nice concentric rings in my out of focus star images. When I lived in Spokane the image would dance and shake too much to ever see them. I thought it was my optics because each time I would try and star test, I would get the same result.

Another thing that might be contributing to your poor star test performance is using a Barlow and a wide angle eyepiece. That is a lot of glass the light must pass through. Keep star testing as simple as possible. Sometimes I don't even use a diagonal. Simple eyepieces like a good Plossl work best. 362X is OK with very good seeing, but if the image wavers like you said, it is probably to much for the seeing. Vega and Arcturus are pretty bright to do star testing especially with poor seeing. Polaris is about the right brightness, but it is a double star which can make the out of focus image look strange.

I went though a stage of wondering about the optical quality of my scope too. I've relaxed a lot though. My scope does about as well as the seeing most of the time. There have been a few nights where it might have helped to have better optics, but I can count those on one hand in 2 years. I wanted a large refractor until I looked through one (7" Meade APO) side by side with my scope.


Subject: Artificial Star for Testing --part 5      Top

From: Gary McKenzie <>

Jeff, All of the conditions you describe could be caused by turbulence, however bear the following in mind:

1. Before star testing make *absolutely* sure that your scope is as perfectly collimated as possible. You do not *need* to use a collimation tool with an SCT, although you can use either a Cheshire eyepiece, or an autocollimator eyepiece to come close, the final tweak will *always* have to be done on a star image.

2. There are very, very few nights - and locations - where you can profitably use 360x and not see horrible images. For most times 200x is a reasonable limit, and 250x or so on a fairly good night. 3. Star testing should be performed without a diagonal mirror or prism in place, since these will *possibly* have some effect on the image.

4. A *bad* star test can be the result of a dud eyepiece, so before believing that your scope is bad, check it out with several eyepiece/Barlow combinations.

5. Most experienced testers would not have a dew removal system since it *can* cause some deterioration in images, or cause thermal effects that mimic bad seeing, particularly if the temperature differential between the scope/ ambient air temp, is large. i.e. if the heater is turned up too much.

6. Get someone who is experienced to star test the scope for you, then get someone else to check it out again, then maybe 3 or 4 others, average the opinions and you will probable be *somewhat* close to a true idea of your scopes performance.

An easier method is to make or buy a Ronchi testing eyepiece--see <>

If you don't know how to use one, e-mail me and I'll try to help, but basically if you see straight lines when the Ronchi is used in your scope alone, then your scope is *possibly* OK, if there is ANY curvature of the lines then the scope is BAD and get onto Meade straight away. To confirm that your scope is good, insert the Ronchi eyepiece in a GOOD Barlow and view the lines, if they are still straight then you have a very nice scope even if the star test is not perfect. The star test is so sensitive that any scope will fail it (well at least the 60 or so that I've looked through will)


Subject: Artificial Star for Collimation --part 6    Top

From: Gary McKenzie <>

I was just browsing the Topical Archives and found that a simple method of collimation that I use was not mentioned so I thought it might be helpful. I got sick of trying to do it at night, and could never make a pinhole in aluminum that seemed to work so I do it the following way:

Get a piece of wood, paint it flat black, glue a 1/4 inch ball bearing to it. Hang it from tree, or fence etc. position your scope 10-20 meters away such that the sun is behind you as you look at the ball bearing. The ball bearing will have a nice glint on it that makes a perfect artificial star. Focus on it and collimate. The star doesn't move and you don't get a crick in the neck, you don't have to worry about seeing (provided it isn't too windy and you are on a grass surface.) Note that you can't star test your scope this way as the close range induces spherical aberration, but this doesn't affect the collimation process.


Subject: Artificial Star for Collimation --part 7 of 7       Top

From: Ric Ecker, <>

I use a projector bulb which is very similar to an automotive bulb, mine operates at 6 volts. This is placed in a aluminum housing, the bulb is 4" away form the eyepiece. The eyepiece is an old ortho 6mm which acts as a star projector. The unit can be placed at least one mile from your scope and makes a fine airy disk. Not really much to it, very simple. I use 6vdc nicads to power it. P.S. make sure the eyepiece aligns with the filament of the lamp.


Subject: Spherical Aberration 101   Top Button

From: Roger Hamlett <> Date: Dec 2002

----- Original Message -----
> I am aware that many SCTs will show some spherical aberration on the
> inside/outside focus star test. My question is how much is typical for
> the LX200? I am familiar with Suiter's ratio test and am wondering if
> his limit of 3.0 is applicable to the LX200? Is a factory rematch of
> the corrector to mirror feasible if a scope is outside that limit? I am
> happy with the focused image I get in my LX200 for visual viewing but
> wonder how much better it might, or should, be. And yes, I do my star
> tests with a well equilibrated and collimated scope under rare steady
> skies; and no, I have not removed the corrector but did buy the scope used.
> Is splitting Zeta Ori a good test of a properly aligned and corrected
> LX200 10"/10? Other suggestions?

The SA shown on an SCT, varies according to the spacing of the primary/secondary mirrors. Since this changes to achieve focus, this gap is actually controlled by the position of your eyepiece/camera. There will be just one point with corrected SA. The location required to achieve this, is rarely where the eyepiece normally sits!. Hence you can adjust SA, by changing the length of your optical train, and refocussing. If you experiment, with an eyepiece very close to the scope, and then add extension tubes, you can generate a figure for the point where your scope is corrected. With this figure, you can try to lay out the optics when working to keep close to this length, and minimize the problem.

An SCT can only be properly corrected at one point, so even if the entire optical train was refigured to suit your diagonal/eyepiece, as soon as you changed anything, and had to refocus, the aberration would reappear. As an example, my own LX200, had the 'optimum' point, slightly further out than the spacing produced by a 1.25" diagonal, but slightly closer than the spacing with a 2" diagonal. However in both cases, the error was tiny. However when the focus distance is really pushed by using a Crayford focuser, F/3.3 compressor, filter wheel, and camera, despite the 'field flattening' properties of the compressor, the aberration became unacceptable (to me). Running without the Crayford, shortened the optics enough for reasonable results.


Subject: Optical Quality Testing: Ronchi Lines? Top Button

From: Bill Dougherty <> Date: Jan 2003

Spherical aberration only measures the scope performance at one point in the image plane, exactly in the center which corresponds to the optical axis of the instrument. If you are interested in splitting close double stars or viewing planets, it (along with on-axis chromatic aberration) becomes the primary consideration because you are only interested in the center of the field of view. However, even if the scope is perfectly corrected for spherical aberration, it can still suffer from off-axis aberrations -- coma, astigmatism, lateral color and field curvature to name the usual suspects.

For purely visual use, field curvature can be somewhat compensated for by the accommodation of the eye. You automatically change the focus of your eye lens to adjust to the different focal planes of the center and edges of the field of view. Since all SCTs have some degree of field curvature anyway, the designer will often accept a fair amount of field curvature and concentrate on reducing coma and astigmatism instead.

For photographic use, a flat field becomes very important. CCD sensors are flat, and bending film to match the radius of the field is not practical as a rule. In actual practice, the available adapter tubes and external focuser used will place the camera sensor at a particular point along the optic axis. You will then use the coarse focus knob to move the main mirror to achieve approximate focus at this point. Fine tuning of the focus will be done with the MicroFocuser or other external focuser, or with fine movements of the coarse knob, for example, using the RoboFocus system.

Using focal reducers or expanders will generally require shifting the coarse focus point considerably as well. The point to bear in mind is that you will rarely be adjusting the primary focus of the scope to exactly match the optimum or 'design' point. A secure and stable attachment of the camera, and the ability to achieve and maintain an accurate focus at that point are more important.

By all means continue to test and use your scope in various ways. You will learn a lot and may even find a problem that requires attention. Just bear in mind that the SCT is a complex system, and tests such as the Ronchi require a good deal of care and experience if you need conclusive results. Actual star testing may in fact be more accurate for a beginner, since there are fewer variables to manage. Moreover, as others have pointed out, that is what really matters when the light hits the electronic, chemical or biological sensor.

The "Easy Tester" is listed on the ScopeStuff site. It is a Ronchi grating mounted in a standard 1"1/4 eyepiece mount.


Subject: 10" f/10 LX200 Classic Mirror Measurements   Top

From: Matt Considine <> Date: Apr 2001

I've dug up my notes that I took when I had the OTA in the cellar for flocking. At that point I ran a quick Ronchi test to look at the primary, measure the ROC and take other measurements on the corrector/secondary cell. The Ronchi lines were straight to the edge.

Given the distances involved, I had to use a steel tape measure and some measured dowels to try to get a decent measurement of the distance between the grating and the mirror surface without risking a scratch to the mirror itself. When all was said and done, I ended up measuring the distance from the mirror surface *just outside the baffle tube ring* to the Ronchi screen plane, as well as from the edge of the mirror to the Ronchi screen plane. (I measured to the Ronchi plane so as to keep everything linear).

I got two measurements of the ROC for the primary : 39.89302" and 39.91338" I'll take the average of the two and use 39.90". That would get us an f/2 spherical primary.

However, taking into account the central obstruction (= 3 19/32" diameter of secondary baffle cone) gets you an effective f ratio of 2.72.

For the secondary, I measured the distance from the center of the surface to the top of it's baffle cone (straight up), as well as the distance from the edge to the top of the baffle cone (*along* the cone). Converting this latter measurement to "straight up" (the cone seems to flare out by about 31/32"), taking the difference between as the sagitta and solving for R in sagitta = R - sqrt(R*R - d*d)

(where d = the diameter of the secondary = 2.625) yields a ROC of 22.06 or an f ratio of 4.20.

Multiplying the two f-ratios together to get the approximate t-ratio for the systems yields 11.4 - which, if I recall, meshes well with what has been measured by people from images. Of course, this does not take into account any power imparted by the corrector plate, which I think is 1/2" thick. (For some bizarre reason I never wrote that down...)

>From this info, combined with a spacing measurement when the scope is at focus, one could solve for the ideal corrector profile, enter the design into OSLO (or some other package) and then figure out what the effect would be of various changes to the optical path or additions of other known optical elements.

If anyone has any comments, corrections, suggestions, etc. to the above, I'd love to hear them. This was by no means the most rigorous analysis, as I lack some of the equipment to do such a thing. And there may well be errors in the above analysis/approach. But I hope it is helpful to someone.


Subject: Calculating Focal Length for a SCT-- part 1 of 4  Top

From: Gene Horr <> Date: Aug 2002

Barre Spencer wrote:
> Could someone please explain how the focal length of a schmidt-cassegrain telescope
> is calculated. I have 10" SCT and have always wondered how it is done.

Well, this is not an easy task due to the Cassegrain design. Since the secondary is a "magnifier" (has a negative focal length) the total system focal length is dependent upon the distance between the two mirrors. Since these units move the primary mirror to focus changing EPs or camera adapters will change the focal length. So all you can do is measure the FL of a particular configuration.

For me the easiest way is to take a photograph, measure the distance between two stars, calculate the angular separation then with these two numbers calculate the focal length. Of course this will be limited upon how accurate the star positions are. And how accurate your measurements of the photograph are. And only gives you a number for that configuration.....

If you _know_ the FL and AFOV of an EP you can use this to measure the FOV and from that calculate the FL. Again, just for that configuration and just that particular EP. But the first two variables are rarely known accurately enough as you are stuck with the chicken or the egg problem. To measure the FL and AFOV of an EP you need to _know_ the FL of the instrument.

The focal length for the Cassegrain systems is supposed to be the stated amount when set up for 35mm film use with the standard SCT camera adapter. So in your case it should be 2500mm at that point. If you make that assumption then you can measurer your EPs and from there measure different configurations. But you are starting with a number that may be several percentage points off.

Confused? You should be....


Subject: Calculating Focal Length for a SCT -- part 2   Top

From: Gene Chimahusky <>

I have found the following publication to be helpful in SCT focal length calculations:

Scroll down and the PDF: "Calibrating The Effective Focal Length Of Catadioptric Cassegrains With Moving Primary Focusing" A lot of math up front and reasonable graphs further on in the publication. The paper from Brayebrook Observatory site can look a little daunting from the math end of it, but the graphs they also supply make life much easier.

I am not sure of the design spec Meade used for the SCT as far as where the image plane has to be in order to get the F10 stated focal ratio.

Let's 'assume' it is with the visual back attached to the tailpiece and no diagonals or other things attached. Insert the 25mm eyepiece and focus on infinity. This will define the location of the image plane for 'F10'.

Now if we look at the graphs in the publication and find the line for the 10" SCT, we can read the graph and find if we move the focal plane back 3 inches the scope will now operate as if it is an ~F11.2 or 112 inch focal length.

So if you are using a 25mm eyepiece it is not 100x but 112x. Using a 9mm give 311x versus the native 277x. Now how can you get into the situation of shifting the image plane back 3 inches? Add a Crayford type focuser.

Things get even worse if focal reducers are entered into the picture. If you look at Doc G's page:

Here Doc gives an example of a Meade 0.63 reducer that needs the focal plane 148mm behind the tailpiece. Now 148mm is ~6 inches so reading the graphs we really start at F12.2. The actual focal reduction is ~7.7 versus the 6.3 expected. We can get back to the F6.3 by varying the spacing of the focal reducer to the CCD slightly but the math requires using the rough ~F12 as the basis.

These are not meant to be 'exact' numbers but only illustrate the point. You exact focal length may vary


Subject: Calculating Focal Length for a SCT --part 3

From: Roger Hamlett <>

On the SCT design, the secondary mirror, as well as reflecting the light, also acts like a Barlow, narrowing the angle of the light cone, to give the final effective focal length. The actual result, depends on the spacing of the two mirrors, so with moving mirror focussing, the effect will change according to where you want to focus. Hence if you attach a focal compressor, and have to alter the focus of the main scope to use this, the focal length of the main scope will be changed by this, altering the overall effect.

It is possible to calculate the actual focal length, if you know the focal lengths of the two mirrors, and the spacing, but is not normally done (especially since on a normal SCT, you rarely know the mirror spacing...). The normal practice is instead to measure the effective focal length (rather than to calculate it). You can measure the focal length of any scope, by timing the transit of an object across a known distance. The 'easiest' method normally is to use the field stop of an eyepiece (measurable with a micrometer), and a star near the celestial equator. If you aim at this, with no motor drive, the time taken is related to focal length.

The stars basically move at approximately 15 degrees per hour = 0.00416 degrees per second. Given the field stop _radius_ (half the diameter), the effective viewing 'angle' of this, with a given scope, is theta=2*tan-1(radius/fl). If you rearrange for fl (focal length), you get fl=radius/tan(theta/2). So if you time a star across a 20mm diameter field stop, and it takes 90 seconds, you get theta = 90 * 0.00416 = 0.375 degrees. This then gives fl = 10/tan(0.375/2) = 3050mm. This can be done for any eyepiece or camera combination, to give a good measure of the actual focal length. You can do a similar thing by using a measure of distance at the focus (such as the pixel size of a CCD), and an object of known angular size/separation (a pair of stars), and measuring the number of pixels this separation gives.


Subject: Calculating Focal Length for a SCT-- part 4 of 4  Top

From: Don Tabbutt <>

For most purposes, the stated f/ratio is close enough.


Subject: Optical Defects Simulation Software  Top

From: Gene Chimahusky <> Date: Jan 2003

A nice simulation program that can induce currents, pinch, turbulence and more:


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