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cs1cjc

Astro-Tech/Altair Astro/GSO Coma Corrector and User Guide

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The Newtonian telescope design is both simple and remarkable. It is capable of producing a perfect image on axis, but off axis, the image quality degrades mainly due to an optical aberration called coma. Modern fast Newtonians and Donsonians of F/5 and below have a surprisingly small diffraction limited spot (just 2mm across in an F/4.5), where the image is not disturbed by coma. The Astro-Tech (also sold under the Altair Astro and GSO brand labels) coma corrector has been designed to cancel out this aberration to give a flat, wide field with high resolution from edge to edge. It is manufactured by Guan Sheng Optical (GSO) and was developed by Astro-Tech from a high quality, modern optical design by Roger Ceragioli

My corrector came in a nice box and consists of two parts, the coma corrector itself and a 2" eyepiece adaptor which screw together with a 48mm (2" filter) thread. The eyepiece adaptor has two screws and a brass compression ring and is marked ALTAIR ASTRO 2", Coma Corrector, Made in Taiwan. At least I knew I had the right part, but no other documentation was supplied and I had to search the web for information on how to use it.

Unfortunately the corrector is not ready for visual use as supplied, because of inadequate eyepiece spacing. The proper spacing is not critical and a compromise spacing to cover your eyepieces can made up with 2" extender tubes, such Hyperion fine tuning rings or empty 2" filters. You do not need a turntable like that of the Tele Vue Paracorr. With the spacers installed, the assembly which is now about 70mm long just slides into the focuser tube like a barlow. In this arrangement the focal point is moved in by a small distance of about 10mm (see photographs below). The corrector acts as a very slight barlow, enlarging the image by just about 10%. The lenses are nicely coated and reflect pale green. The aluminium housing is cleanly finished in satin black and the combined unit weighs about 350 grams.

Once set up properly in a collimated telescope, the corrector works just as you would expect to give a clean, flat image. The view feels quite different, much more like a refractor, with pin point stars from edge to edge, but no chromatic aberration. Objects can be allowed to drift across the view of wide angle eyepieces with little or no visible loss of sharpness. The removal of coma can be clearly demonstrated by doing a star test on and off axis without the corrector installed and then with it. Any loss of contrast due to the extra corrector glass (two doublet lenses) in the light path is undetectable, I think. The coma corrector is now a permanent fixture in my focuser except on occasion when viewing planets with my 200mm Newtonian which now has a motor drive. It seems to me that a coma corrector should be a standard accessory for all fast Newtonian telescopes and particularly for larger Dobsonians with no tracking. This model is an effective, affordable example and I strongly recommend it.

The first issue is actually finding one in stock. Supply has been patchy over the years and at the time of writing, it is listed by Astronomics (Astro-Tech brand at $135, including T-mount, but out of stock), Agena (GSO brand at $130, including T-mount, but out of stock), Ian King (Altair Astro brand at £88) and Telescope Service (GSO brand without visual adaptor at 61 Euro). There is then the issue of setting it up properly and most of the remainder of this review is devoted to showing how this can be done, but first there is a little information about Newtonian telescopes and coma.

Newtonian telescopes are all designed with a single figured mirror in the shape of a parabola rotated on its axis, a paraboloid. All mirrors of a given focal length are the same shape. If you have a fast mirror, it is easy to to create a slow one of the same focal length, just by blanking off the outer part of the mirror. It is the outer part of the mirror that generates coma, which is zero on axis but which increases linearly the further from the axis you get. At the focal surface, the amount of coma is independent of the mirror focal length so a single corrector will work for any Newtonian. In practice, a perfect corrector is not attainable so the designer will aim to produce the best result he can for a specific F/ ratio, F/4.5 for this model I understand. However, the corrector will give good results for mirrors that are somewhat faster than this and for all slower mirrors.

Coma correctors would actually be better called Newtonian correctors, because the designer is looking to produce the smallest attainable spot size for a point source, so will also be looking to reduce the other lesser Newtonian aberrations, field curvature and astigmatism. To do this, he will have in mind a particular focal length, around the longest that is commonly used (so about 2000mm or slightly less), because these aberrations are less in longer telescopes and it is wise not to over correct significantly.

Newtonian telescopes are perfect on axis, but coma damages image quality at even a modest distance off axis. At the focal plane, about 1mm off axis, in an uncorrected F/4.5 Newtonian, the image is just at the diffraction limit and the strehl of even a perfect mirror has fallen to 0.8. In a 250mm scope, this gives a coma free, sharp field of about 6 arc minutes across, about 1/5 of the apparent diameter of the moon. For comparison, the field stop of a 9mm orthoscopic eyepiece is about 6mm so only the central 1/3 (1/9 of the area) of the view is free of coma in an F/4.5 scope. Coma increases sharply with the speed of the telescope, at the focal surface inversely with the cube of the F ratio.

Collimation is the business of lining up the coma free sweet spot with the centre of the eyepiece axis. The tolerance for collimation is perhaps 1/4 (though some would say 1/6) the size of the sweet spot so that it covers the centre of the eyepiece. So far as I can tell, this tolerance also looks good for a telescope fitted with a coma corrector.

To set up the GSO coma corrector properly, the total back focus (distance from the last lens to the focal plane) has to be about 75mm. The designer says that it is not critical and from 65mm to 85mm will produce a good spot size. This distance will be made up somthing like mine below, added to the height of the eyepiece focal point height above the eyepiece shoulder (or subtracting the height below the shoulder).

1.25" My 2"

2mm 2mm Spacing from last coma corrector lens to the shoulder

45mm 45mm 2" adaptor spacing

11mm .... 2" to 1.25" adaptor (if any)

19mm 19mm Spacers (Hyperion 14mm ring + empty 2" filter)

77mm 66mm Total (excluding eyepiece distance)

My one 2" eyepiece has a focal point above the shoulder, and my 1.25" eyepieces are all within -12mm/+8mm of nominal, so are all fine.

Tele Vue is unique in publishing the height below the shoulder of the focal point for all their eyepieces. For other users, you are going to have assume the focal point is close to the shoulder or measure the position. First, locate the prime focus by taping a piece of tracing paper to the top of the focuser and focusing on something. This does not have to be at night and can anything sufficiently distant so that it comes into focus, such as a church spire or distant tree. It does not depend on the telescope so using a refractor with a graduated focus scale is very convenient. You then measure how far in (plus) or out (minus) you have to move the focuser for each of your eyepieces in turn.

For users only intending to use 2" eyepieces, a single 28mm Hyperion tuning ring might be fine. If you do not like the idea of finding empty filter rings, or more likely buying cheap ones on eBay and removing the glass, some suppliers (in particular Telescope Service) have spacing rings with the right 48mm thread, in a few sizes such as 10mm and 20mm, but these are generally expensive. Variable spacers are also available but these are not going to sink into your focuser tube.

When I first set this up I had to remove a 2" to 2" adapter to allow the unit to go all the way into the focus tube. This left too little out focus so I made a plastic washer (from a yoghurt tub, see photo below) to prevent the corrector slipping all the way into the focuser and providing the necesssary out focus.

One correspondent who uses only 2" eyepieces has done away with the eyepiece adaptor and has simply added enough extender rings to screw the corrector to each eyepiece as he uses it.

I hope that this will is enough information to set up this corrector properly but I would welcome questions, and of course comments and correction.

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Edited by cs1cjc
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Great write up Chris, thanks for taking the time and effort to post. I am very tempted to try the Baader MPCC when they become available again early September. I haven't had the opportunity to test my 21mm Ethos to see how badly its affected by coma, but I suspect at f5, it would benefit from the addition of a coma corrector. I like the fact that the Baader unit doesn't alter the focal length as my 21mm is my lowest power eyepiece and don't really want to lose any tfov, however slight.

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I understand that the Baader MPCC, being a simpler design with only two lenses, adds a degree of spherical aberration which leaves the resolution suitable for photography but perhaps not for high powered viewing. Also the spacing is lower at 55mm and the tolerance tighter so setting it up for multiple eyepieces might be more of a challenge. See, for example this thread:

http://www.cloudynig...&Number=3056605

However, I gather the new Baader MPCC will be an improved design, which will be interesting to see.

Edited by cs1cjc

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If I can just add, that having one of these and having used it on my 16" LB, I can report good things.

Just be aware the back focus for astrophotography is not as advertised I don't think. Mine is more like 70 than 75 but then, that's within the tolerances allowable I guess.

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A thought...

The problem for photography is illustrated in the last photograph of your excellent write up. Some correctors for Newtonians hang the camera a remarkable distance outboard from the tube and this is an invitation to sag. Fast systems, as Newts usually are, have shallow depths of field and need very highly orthogonal chips. I've seen this problem at first hand. I doubt it would have much effect visually, though.

Olly

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Having read all that the best I can say is I will stick to ED and APO refractors. :grin: :grin:

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Having read all that the best I can say is I will stick to ED and APO refractors. :grin: :grin:

You and me both! (Yves' 14 inch ODK excepted, of course...)

Olly

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A thought...

The problem for photography is illustrated in the last photograph of your excellent write up. Some correctors for Newtonians hang the camera a remarkable distance outboard from the tube and this is an invitation to sag. Fast systems, as Newts usually are, have shallow depths of field and need very highly orthogonal chips. I've seen this problem at first hand. I doubt it would have much effect visually, though.

Olly

Very true. Currently this is my biggest headache with this corrector. I will be trying a different one later in the year.

Typed by me, using fumms...

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Received my GSO corrector - sent from Germany in the middle of the night - excellent service from Telekop-Express!

I checked it out this afternoon on some trees about 200m away.

I thought I was going to be making up an ultra low profile mount to accomodate this CC - a Crayford with half the bearings inside the telescope tube.

But the final focus is not inwards - towards the scope - 'by about 10mm' as read on forums.

It was measured as moving outward - by about 35mm.

Double checked I had the unit the right way around and spacing to eyepiece field stop was 75mm!

This isn't mentioned anywhere on reviews.

The issue hinted at most often refers to 'infocus' travel not being sufficient on focusers and having to move the primary mirror up.

I will say there was poorer contrast and definition with the CC compared to same view with just the eyepiece - but this is quite likely the result of not being adequately collimated with a temporary rig on the side of the tube.

Is this particular unit behaving normally?

Looking very carefully into the CC with a bright light I can only see a single lens element at one end and a cemented double at the other. The single element may be two which have been so cleverly cemented and afterward edges ground so well that the edges appear as one - but I have never experienced this in a life-time of taking optics apart.

Anyone comment on the outward focus shift? It would be a helpful addition to the GSO user guide if this change in final focus were resolved - I'm reluctant to make a focuser until I know what's to be expected and whether this is ok.

Many thanks for previous comments.

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I have looked carefully at the photos posted here with before/after cc and eyepiece.

The movement of the focuser 'inwards' towards telescope is given as about 10mm - after 45mm 2" spacer is in place. So about 35mm movement of original, non-CC, focus relative to telescope tube/focuser mount, outwards.

Which is what I observed when testing on distant trees.

The photo is neccesarily at lo-res - but the 2" spacer looks the same diameter as the focuser drawtube...so it is a 2" drawtube adapter being used as a "spacer" with the CC sitting inside it.

Is this understanding correct?

I have only 20mm left to move the secondary right up to the spider vanes - so will have to move the primary down too.

Why not just hang the camera out on extension rings/adapters like everyone else? Because it must be squared on exactly and I have to make another low profile crayford micro-focuser to accept the 2" CC anyway. My secondary is also only just big enough at 50mm minor axis for DSLR sensor. I will re-fit the OAG pickoff mirror/tube into the CC-to-DSLR adaptor I must now make using a lathe.

I hope this helps anyone who is in a similar position, wanting to swap in the CC to a newtonian astro-imaging setup that has already been optimised/customised for stability with little room to just re-focus by as much as 35mm.

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No, I have got it wrong!

The 45mm 2" adapter referred to in the photos and the spacing distances list by Chris is the GSO 'eyepiece 2" adapter' which is not supplied with the cc from Teleskopexpress?

Is that correct?

The 2" adapter extender (2" long?)on the focuser drawtube is in place in both photos.

The eyepiece shown in the vertical setup thumbnails is a 2"? There seems to be an approx 5mm shoulder sitting on the eyepiece holder WO cc in place and in last pic with cc in place it is also just sitting on the top of the 'eyepiece adaptor'. Can I ignore this as just being part of the eyepiece?

Is the GSO 'eyepiece adaptor' itself sliding into the 2" holder at all, or just the filter ring and Hyperion spacer/youghurt pot ring?

Had all the things set out ready to machine, now I'm not so sure...

Many thanks if you can see what I mean and assist.

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No, I have got it wrong!

The 45mm 2" adapter referred to in the photos and the spacing distances list by Chris is the GSO 'eyepiece 2" adapter' which is not supplied with the cc from Teleskopexpress?

Is that correct?

It is not supplied by TS. It is the length of 45mm that means the focuser is moved inwards by 10mm.

The 2" adapter extender (2" long?)on the focuser drawtube is in place in both photos.

Yes the GSO 2" eyepiece adapter is in place.

The eyepiece shown in the vertical setup thumbnails is a 2"? There seems to be an approx 5mm shoulder sitting on the eyepiece holder WO cc in place and in last pic with cc in place it is also just sitting on the top of the 'eyepiece adaptor'. Can I ignore this as just being part of the eyepiece?

The eyepiece in this case is a 1.25". For the purposes of the pictures it is in a "zero height" adaptor, not my normal one which is 11mm high, which is why the apparent 5mm lip is there. It can be ignored because it is the same in both photos. The spacing was 75mm (+/- a couple of mm) and was focused on a distant object in both photographs.

Is the GSO 'eyepiece adaptor' itself sliding into the 2" holder at all, or just the filter ring and Hyperion spacer/youghurt pot ring?

Had all the things set out ready to machine, now I'm not so sure...

Many thanks if you can see what I mean and assist.

The GSO eyepiece adaptor does not go into the 2" holder.

Hope this helps. I am sorry that the photos are confusing and am happy to answer any other questions you have. Good luck!

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Thanks Chris.

I meant is the GSO coma corrector inside the focuser drawtube and then the 2" spacer is added by threading it on? The 'eyepiece adaptor' I can see is after that - just ignore the mis-typed wrong label.

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Chris - is that a drawtube extension piece - it's difficult to see any change in diameter of the focuser tube and whats added to it? So, the CC is inside that extension?

That's what you called a 'holder'. And hence it has the thumbscrews to hold the eyepiece/2" to 1.25" adaptor etc.

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Having spent time getting the telescope optics and GSO coma corrector aligned - it may be useful to anyone contemplating a Newtonian plus this corrector for astrophotography to note how it performed in practice.

firstly - with the GSO CC in the optical train you have to provide an extra 35mm to reach the new focal plane. The CC itself also shifts the focal plane by its insertion length, about 50mm. I was able to fit longer bolts on the mirror cell to take care of this extra distance. If you just hang extensions on the focuser the secondary mirror size needs checking.

secondly - don't expect 'razor-sharp' 'tack-sharp' (or any of the other phrases that get used subjectively) image quality from edge-to-edge of an APC sensor. Forget about using this with full 35mm frame sensors. The focus is softer in the centre of frame compared to an image without any CC. I estimate a star is 30 to 40 microns at best. I mean when done properly, measured 50% CCD Full Well, geometric diameter of blob. By the time you get to the corners of the frame it is a smear image of a star.

I should have checked the few images on the web 'before-after GSO cc' with a forensic eye before buying this!

My GSO CC also has a chromatic error due to one element being non concentric or with 'wedge'. I can only minimise the effect by trading off against other image faults that arise when trying to tilt the cc independently of the optical train axis.

Roger Ceraglio, the designer of this coma corrector according to other blogs, only claims that it reduces coma in fast Newtonians. Maybe the GSO is only loosely based on his design 'type' and as I suggested earlier may not even have two groups of four elements. Poor Quality Control in manufacturing may be an issue. Shame he didn't feel able to just post his spot diagrams or indicate likely results empirically when asked on other forums a couple of years ago. Whether he fudged the answers because of manufacturing licence issues, only he can say.

I wouldn't recommend a Newtonian with coma corrector for astrophotos that will be enlarged more than 10 times or so, say a screen image width of 1000 pixels. I say 'coma corrector' because so many contributors have subjectively praised or derided any one unit over others, whatever the cost. Lots of major errors go unnoticed or overlooked during visual use. Technically speaking, the camera does not lie - I took hundreds of test images, analysed star images and tweaked everything in the alignment as best as I could over several days.

I have a perfect imaging scope DIY Crocheted (Corrected) Dall-Kirkiham 20" aperture f8, with a corrector which gives images 100 times better than this GSO. But its useless in my skyglow-polluted backyard with an unmodded DSLR. Over-sampled, too slow, and now an even more restricted sky view from the permanent dome now a neighbours tree has been allowed to grow!

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Sorry top resurrect an old thread.

Do you mean that the eyepiece drawtube needs to be 10mm further in towards the secondary to acheive focus?

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