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Astro-Tech Coma Corrector in a 12" f/4.9 Scope

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#1 rtomw77

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Posted 31 May 2010 - 06:20 AM

There have been many threads on CN about exactly when does a Paracorr become a necessity. Most posters have felt that this occurs at f/4.5 or faster. They also have indicated that at f/4.9 a coma corrector was nice to have but that it was not a necessity. This has made me reluctant to spend $330 for a Paracorr for my Orion XT12i f/4.9 scope.

The new Astro-Tech coma corrector at the lower price of $135 renewed my interest in a coma corrector. I purchased one from Astronomics a month ago.

The instructions that come with the ATCC state that the optimum spacing from the lens assembly/shoulder to the image plane of the EP is 73.5mm. The distance from the top of the 2" eyepiece holder of the unit is 26.5mm to the image plane of the EP. I used this 26.5mm length as my goal for spacers.

The lack of an adjustable top for the ATCC limits its usefulness. You can add spacers between the bottom lens assembly and the top EP holder of the ATCC, or you can add spacers to the bottom of each eyepiece. I chose to add spacers between the lens assembly and the EP holder.

The two parts of the ATCC are connected by standard 2"/48mm filter threads. The 14mm and 28mm Baadder Hyperion fine tuning rings and a 2" color filter were picked up from OPT. Removing the glass from the filter left me with a 6mm tuning ring.

For 1.25" EP I use the 14mm and 6mm ring between the 2 parts of the ATCC. The 2"/1.25" adapter in the top of the EP holder adds another 6mm for a total of 26mm. For 2" EP I use the 28mm ring.

This 'one size fits all and use it like a barlow' approach works fairly well. Visually judging the amount of coma correction and field flattening from one combination to another is harder than I thought it would be. My subjective and unscientific estimation is that for most of the eyepieces in my collection about 1/2 to 2/3 of the edge coma effects are removed. The area of the flattest portion of the FOV increases as well.

Improvement varies from one EP design to another. Premium EP most clearly showed improvement. The exact position of the focal plane of each EP also has an effect.

2" EP used: 28mm WO UWAN; 30mm AT Titan II ED; Meade 34mm SWA 5000; and Orion 40mm Optiluxe

1.25" EP used: 9 & 11mm t6 Naglers; 15, 19, & 24mm Panoptics: Orion Ultrascopics, UO HD orthoscopics, and BO/TMB 9mm Planetary

Now for the big question: Is it worth it? I have mixed feelings about the ATCC. I think it is great for wide field low to medium magnification views. It needs more testing on a night of good seeing before I will decide to use it for high power viewing. The bottom line is that at f/4.9 the ATCC is nice to have, but it is not a necessity. For the relatively low price of the ATCC compared to a Paracorr I am happy with it.

Random thoughts in no particular order:

This is not intended as a scientific report. Someone else will have to do that. To put it another way, "Dammit Jim! I'm a Pharmacist, not an Optician!" ;)

This thing NEEDS an adjustable top. Are you listening Atro-Tech?

You will need at least 8 to 12mm of in focus (lower focuser tube top). This distance will vary with different spacers and different EP. I ran out of in focus several times with various combinations.

Increased spacer distance has some effects. The barlow effect increases. Eye placement becomes more critical.

The ATCC dims faint objects slightly. Some of this might be due to the weak barlow effect. There is some extra glow around bright objects.

The 10% increase in focal length/barlow effect was usually only noticeable if I specifically looked for it.

At low to medium magnifications I did not notice any image deterioration. Seeing was not good enough for 200x plus testing.

So far I have not found a good combination for a 1.25"/2" Meade 4K 14mm UWA.

The Orion Helical focuser screwed all the way down just barely works in my scope with the 14mm and 28mm spacers.

Changes in spacer length of 6mm or less produced little visual change.

I will be happy and eager to hear if someone has a practical way to improve my setup.

Your mileage may/will vary.

Tom

#2 csa/montana

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Posted 31 May 2010 - 10:21 AM

Tom, thank you for your thoughts on the AT coma corrector! I love reports such as yours, as I can relate your experience to what I would face with my scope.

Excellent! :bow:

#3 Bill Weir

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Posted 01 June 2010 - 12:29 PM

Perhaps you could add one of these to it.
http://www.televue.c...emDesc=Tunable%
It's an addon turntable built for the origional Paracorr. Probably by then you will be up to the cost of just buying a Paracorr in the first place. I think this show that you often get what you pay for.

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#4 Sky Captain

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Posted 01 June 2010 - 05:24 PM

Perhaps you could add one of these to it.
http://www.televue.c...emDesc=Tunable%
It's an addon turntable built for the origional Paracorr. Probably by then you will be up to the cost of just buying a Paracorr in the first place. I think this show that you often get what you pay for.

Bill



Just happens that I have a Para and ATCC in my hot little hands right now just waiting for the sky to clear.

It so happens that the tunable top from the Para does indeed fit onto the ATCC. Problem is that the screw is a different thread on the Para and the screw on the AT body just above the threads is too big. I could see the screw thing being worked out by an industrious soul though.

I have used my ATCC in the 12" f5 Lightbridge once during a short break in the clouds. The 22 Pan was used and the stars were sharp across the field.

#5 Sky Captain

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Posted 01 June 2010 - 05:28 PM

The two parts of the ATCC are connected by standard 2"/48mm filter threads. The 14mm and 28mm Baadder Hyperion fine tuning rings and a 2" color filter were picked up from OPT. Removing the glass from the filter left me with a 6mm tuning ring.



Nice report Tom.
Also thanks for mentioning the Baader FT rings fit. As noted in my ATCC thread, there are certain brands that won't thread on or only one end. I'll have to pick up a few for my personal use along with the adjustable top I made.

#6 sailor70623

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Posted 01 June 2010 - 07:08 PM

I have a paracorr and a Baader MPCC, and am still thinking about getting the AT coma corrector. Sky Captain makes a nice top for it. But even at F4.5 I'm finding it hard to justify another coma corrector. Two I can, because on is in Mi. and the other in Ok., but 3??? Only because it might make a better CC than the Baader for visual.

#7 rtomw77

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Posted 02 June 2010 - 04:56 AM

Sky Captain:

The Baader rings do fit, but the male threads on the rings are a tight fit into the female threads on the ATCC EP holder top. I get about 2 turns before it stops. This does feel safe to use though. Continued use is improving this. The Baader rings are smooth, and it is difficult to get a firm enough grip to unscrew them if they are on too tight.

The filter ring without the filter is flexible under hand pressure and is tricky to thread on. It is also easier to get stuck and harder to get off. Its male threads are also tight on the ATCC top. It fits the Baader rings and the ATCC bottom better than the ATCC top. It is a close out Antares 2" color filter from OPT. I have since found that Surplus Shed has empty 2" filter rings (M2801D) for $6.50. How well these fit is an unknown.

If you decide to make an adjustable top available to others please let me know. When I ordered the ATCC I asked if AT was planning to make an adjustable top. The salesman said "there has been some talk" about it. What this means is hard to say.

Thanks also for the info on the difficulty of using the Paracorr top.

The Orion helical focuser plus the 14mm ring shows some promise as an adjustable top for 1.25" EP. The main problem is that my scope focuser runs out of in focus quickly with this arrangement. Moving the mirror forward would cause problems with some 2" EP that require almost all of the focuser's out travel. Even with an extension tube I am reluctant to make a major forward movement of the mirror. The Orion mirror cell design has the mirror cell attached to the end ring, not to the optical tube wall.

Tom

#8 Sky Captain

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Posted 03 June 2010 - 02:16 AM

Sky Captain:

The Baader rings do fit, but the male threads on the rings are a tight fit into the female threads on the ATCC EP holder top. I get about 2 turns before it stops. This does feel safe to use though.



Good to know Tom, gotta find one that screws on all the way. Would be nice if AT would make some accy's that fit instead of the hit an miss that we have been going through. I still can't tell if its the threads on the CC or the tubes that's the problem.

#9 csa/montana

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Posted 03 June 2010 - 08:06 AM

Would be nice if AT would make some accy's that fit instead of the hit an miss that we have been going through. I still can't tell if its the threads on the CC or the tubes that's the problem.



Kerry, in case Astronomics doesn't see this thread; why not give Mike a call; I'm sure he's very interested in this.

#10 rtomw77

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Posted 04 June 2010 - 03:22 AM

Carol

The two parts of the ATCC fit together just fine. I think the problem Kerry is referring to is the mismatch of the 'standard' screw threads on 2" accessories from one supplier to the next. You never know for sure if a new 2" filter or accessory will fit your other 2" items until you try fitting them together. If Astro-Tech would make some tuning rings or an adjustable top for the ATCC this problem would be unlikely to occur with all AT parts.

Sometimes the problem is too much paint in the threads and all you need to do is screw and unscrew the two items until the paint wears off. Sometimes it involves the metal not being machined to standard thread dimensions. I have an Orion 2" moon filter (marked Korea) that would not fit an Orion 40mm Optiluxe 2" EP. I had to use a triangular needle file to open up the male threads on the filter before it would fit.

In my case the best fit is: ATCC lens assembly to Antares filter ring to Baader ring to ATCC EP holder. This arrangement has all the screw joints fully engaged except for the Baader ring to EP holder joint which is on only ~2 revolutions.

Tom

#11 csa/montana

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Posted 04 June 2010 - 08:09 AM

Thanks Tom.

#12 rtomw77

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Posted 27 June 2010 - 04:00 AM

Update

I have been trying to improve the performance of the ATCC in my scope. Checking previous coma corrector threads on CN produced an interesting item: Someone stated that a coma corrector helped to reduce the coma to a point to where your eye's own accommodation of focus would help to obtain a sharp focus at the edge of the FOV.

Uh-oh. I am very near sighted and wear trifocals. The only good things about my eyesight is that there is no astigmatism or floaters. I always flip my glasses up onto my forehead to look through an EP.

I tried viewing with a set of single vision eyeglasses that I use for binoculars with my long eye relief 2" eyepieces. Bingo! Edge correction through the ATCC improved to the proverbial "pin point stars from edge to edge" level.

My new plan is to use the ATCC with 2" EP and glasses for low power wide field views. 1.25" EP will continued to be used "bare eyed" with the ATCC since few of the 1.25" EP in my collection have eye relief long enough for use with glasses.

Tom

#13 JayinUT

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Posted 27 June 2010 - 12:43 PM

Tom,

Excellent thread. I am about ready to pull the trigger on this but may just wait for a little bit more. My 14" f4.6 may need it (haven't had it out yet) and not sure on the AT or the paracorr (it's the price thing that stops me on the paracorr).

#14 MessiToM

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Posted 10 October 2010 - 07:16 PM

any updates here? Id like to buy one of these but now am worried Id run out of travel for the focuser

#15 Sarkikos

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Posted 10 October 2010 - 10:52 PM

I pulled the trigger on the AT Coma Corrector last Tuesday. I haven't received a shipping confirmation e-mail. The coma corrector is on back-order. Well, it is an early Christmas present for myself, but I hope it arrives well before Christmas. Halloween would be nice. :grin:

Mike

#16 rtomw77

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Posted 11 October 2010 - 10:57 AM

The only new thing I have to add is that I had to use a 3mm Allen wrench to increase the tension of my scope's Crayford focuser in order to handle the heavy weight of the ATCC with a 2" EP.

So far my favorite 2" EP in my collection for use with the ATCC is a Meade 5000 34mm SWA. The eye guard all the way down gives plenty of room for use with my glasses.

Tom

#17 MessiToM

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Posted 11 October 2010 - 07:51 PM

^ Intresting, I plan on using this mostly with my Meade 4000 QX 30mm with a 70* FOV

#18 Sarkikos

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Posted 11 October 2010 - 08:27 PM

I want to use it with my ES 14 100 Degree eyepiece for wide-field views of DSO with my 10" f/4.8 Newt Dob. But I would also like to try it with my binoviewer, ideally so I can have a wide flat coma-free field for observing planets without tracking.

Mike

#19 MessiToM

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Posted 12 October 2010 - 12:35 AM

That does make sense^ ill put jupiter at the edge of my fov and watch it drift through the ep but it only looks good toward the center anyway....

#20 Jon Isaacs

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Posted 12 October 2010 - 05:27 AM

I want to use it with my ES 14 100 Degree eyepiece for wide-field views of DSO with my 10" f/4.8 Newt Dob. But I would also like to try it with my binoviewer, ideally so I can have a wide flat coma-free field for observing planets without tracking.

Mike


I am not sure how well coma correctors work with binoviewers. The spacing between the focal plane of the eyepiece and the coma corrector is important/critical in optimizing the correction of coma and it seems to be a matter of a few millimeters. The MPCC with it's spacing needs and the Paracorr with it's included tunable top clearly demonstrate this.

But with a Binoviewer, the distance from the coma corrector to the focal plane of the eyepiece is several inches, this seems like a real problem. Ideally, you could have a coma corrector for each eyepiece but obviously that is impractical.

Something to think about before diving in...

Jon

#21 Sarkikos

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Posted 12 October 2010 - 07:44 AM

MessiToM,

That does make sense^ ill put jupiter at the edge of my fov and watch it drift through the ep but it only looks good toward the center anyway....


That would depend on how well-corrected the eyepiece is, also. But in general, with the coma corrector the planet's image should be sharper beyond the usual diffraction delimited area for the telescope.

Mike

#22 Sarkikos

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Posted 12 October 2010 - 07:56 AM

Jon,

I am not sure how well coma correctors work with binoviewers. The spacing between the focal plane of the eyepiece and the coma corrector is important/critical in optimizing the correction of coma and it seems to be a matter of a few millimeters. The MPCC with it's spacing needs and the Paracorr with it's included tunable top clearly demonstrate this.

But with a Binoviewer, the distance from the coma corrector to the focal plane of the eyepiece is several inches, this seems like a real problem. Ideally, you could have a coma corrector for each eyepiece but obviously that is impractical.

Something to think about before diving in...


Well, I've already dived in, because I ordered the AT Coma Corrector last week. But it's on backorder, so I still have a chance to come out of the water. :grin:

My intended use of the ATCC is not solely dependent on binoviewing. I'm flexible. If binoviewing is not possible with the corrector, I'll go back to cyclops mode. I'll just try some of my wide-field EPs, maybe with a Barlow or lens cell screwed on for higher power. Binoviewing planets is great, but monoviewing with a wide flat field in a non-tracking Newt should also be very nice.

But I will definitely try the corrector with my binoviewer to see if I can get it to work. Of course, dual CCs is out of the question for several reasons: (1) cost, (2) weight, (3) my binoviewer is not a 2".

Clear & Steady Skies,
Mike

#23 audioaficionado

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Posted 24 June 2012 - 05:40 PM

Any updates on this? Christmas 2010 was 18 months ago :grin:

#24 Raginar

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Posted 25 June 2012 - 03:45 PM

It took 6 months to get mine :) To be honest, it's not a great CC. I end up with flat stars but they're not perfectly round.

#25 audioaficionado

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Posted 25 June 2012 - 06:25 PM

Thanx for getting back. Guess I'll cross this one off the list. Flat is the most important to me, but CC is close second.






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