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Setting up the GSO coma corrector

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#1 precaud

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Posted 22 May 2014 - 03:38 PM

The GSO coma corrector is probably the most misused and misunderstood device I've encountered in the wonderful world of amateur astronomy. And truth be told, we have none other than GSO to thank for this. They provide NO user instructions with it! I know... hard to believe, but its true. Nevertheless, it is actually an excellent coma corrector when set up right, and so I thought it would be worthwhile to go through the setup parameters, with some pictures to make things clear.

If you put a GSO CC in your scope and put an eyepiece in it, you'll probably find that you can not bring the combo to focus. That's because two things are out of whack:
1) The CC has a working distance of about 75mm from the top lens of the CC to the focal plane of the eyepiece, but the eyepiece holder section of the CC is nowhere near that height.
2) The CC lens requires quite a bit of in-focus (inward travel of the focuser) to place the CC lens at the proper place in the light cone.

Because this is a one-piece unit, both of the above parameters interact. Further complicating matters is the fact that the location of the focal plane relative to each eyepiece's shoulder varies widely. I've found that the distribution of the variance is a curve centered roughly around the focal point of the scope (zero offset), but with more of them leaning toward in-focus than out-focus.

When I made a list of all of my eyepieces and their required offset, I found that they all fall within the +1mm to -10mm offset range, with most of them clustered near 0 and -10. So I set out to optimize my setup with those offsets in mind.

The mission, should we decide to accept it, is to achieve the needed 75mm spacing for all of our eyepieces, arranged so they can be brought to focus without modifying the scope.

Here is a photo of the standard CC, showing the 46mm working distance of the stock unit, some 29mm short of what is needed for most eyepieces.

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  • 6540150-gsocc1.jpg

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#2 precaud

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Posted 22 May 2014 - 03:39 PM

If you use a standard 1.25" adapter, it adds some of the needed height. The adapter that came with my GSO dob is 10mm high. 46 + 10 = 56mm, still 19mm short of the 75mm target.

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  • 6540153-gsocc2.jpg


#3 precaud

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Posted 22 May 2014 - 03:39 PM

If you have ooodles (30+mm) of available inward travel on your focuser, you can simply add an extension tube of the right height on top of the CC and you're home. Unfortunately current mass-produced dobs don't have that much inward travel available. The solution is to add the 19mm spacer between the two sections of the CC, as shown in the pic below. This reduces the in-travel requirement by the height of the spacer used, in this case 19mm. This ring does the job perfectly:
http://agenaastro.co...-extension.html

and gives a 65mm spacing without the 1.25" adapter, and 75mm with it.

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  • 6540157-gsocc3.jpg

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#4 precaud

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Posted 22 May 2014 - 03:40 PM

For 1.25" eyepieces with little or no offset needed, simply use the 10mm adapter, and enjoy (see photo). This works quite well with all of the 1.25" Explore Scientific eyepieces I have, which require no offset and are parfocal within a mm or two. Just insert the eyepiece, focus with the focuser knobs, and the coma correction will be somewhere from excellent to quite good. No need to "prime" the CC distance ala Paracorr; just plug and play.

The Sterling plossls, the 35mm Ultrascopic, and some orthos, require 9-10mm of in travel. For those, I use the Glatter Parallizer in place of the standard adapter. The Parallizer does not add any height, which brings those eyepieces right into the proper spacing. For eyepieces with offsets in between say -9mm and -1mm in-focus, add spacers or parfocalizing rings to them and use them with the Parallizer.

I tend to remove the CC when viewing at high magnitudes (generally 120X and up), so I don't bother parfocalizing my shorter FL eyepieces.

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  • 6540159-gsocc4.jpg


#5 precaud

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Posted 22 May 2014 - 03:41 PM

For 2" eyepieces, add a parfocalizing ring to bring them to the proper height. Shown below is my 34mm ES68 with the ring at the optimum position, which you can see is about 10mm, the same as the 1.25" adapter height, showing that it is roughly parfocal with the 1.25" ES68's and ES82's.

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  • 6540162-es6834.jpg

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#6 precaud

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Posted 22 May 2014 - 03:42 PM

The above shows the simplest way to use the GSO coma corrector. For those who want the optimum in accurate coma correction possible with it, there is another way to go about it... a way that fixes the CC lens at the proper position in the light cone (as it is supposed to be) and adds a tunable top to focus each eyepiece. A GSO coma corrector with tunable top? Yep. I'll cover that in the next installment.
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#7 Cames

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Posted 22 May 2014 - 08:26 PM

John
Many thanks for your detailed instructions. The photos are great and very helpful. I’ve really needed this type of discussion and your posted explanation is a welcome event.

I have a question for you:

In the third photo, the one with the three parts exploded, the segment containing the lenses has a ledge adjacent to the male mating thread. Regarding that ledge, is the top of the corrector lens that you mentioned north or south of that ledge? How many millimeters is the top of that lens from the ledge?

Also, I’m eagerly awaiting your tunable top concept.
-------
C

#8 precaud

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Posted 22 May 2014 - 08:44 PM

Hi Cames,

I'm glad you found this useful. If I understand you correctly, that so-called "ledge" on the lens assembly is nothing to worry about, it is just the top lip of the "safety undercut" and has no relation to any of the measurements being discussed.

#9 DaveG

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Posted 22 May 2014 - 08:47 PM

:like:

Thank you!

#10 mcoren

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Posted 22 May 2014 - 09:31 PM

John, thanks for the great write-up! I recently purchased a GSO CC, and I did a lot of reading on CN and other sites beforehand to make sure I really understood how to use it. It's not as easy as just opening the box and popping it in, as you point out.

The optical designer of the GSO/AT CC, Roger Ceragioli, is on CN and has posted a lot of valuable information about it. He just designed the optics, so don't blame him for the mechanics! ;)

In my opinion, one of the best instruction sets as far as giving useful information is for the Baader MPCC. Their information is a little confusing, however, because they recommend a mixture of 48mm (standard 2" eyepiece filter) and 42mm (T thread) spacers. Baader has information on their web site (in both German and English), and there is also good information here.

I have found that with my GSO CC installed, the focal plane of my eyepieces is moved out about 35 mm versus where they focus without it, so the actual in-focus requirement is closer to 40 mm. So, with the appropriate spacers to make up the difference, the supplied 2" eyepiece adapter may not be all that bad.

Rather than use the supplied 2" eyepiece adapter, I stacked a couple of spacers between the 1-1/4" eyepiece adapter on my focuser and the GSO CC, so it all sits down within the focuser drawtube. I can do this because all of my 1-1/4" eyepieces are TeleVue Panoptics, Radians, and T6's in "parfocal group B", so the eyepiece focal planes are all roughly 1/4" below the eyepiece body shoulder, so I need about 81mm from the top of the CC to the top of the 1-1/4" eyepiece adapter.

Agena Astroproducts sells a series of 2" eyepiece barrel spacers in 1/4" increments from 1/4" to 1", plus a 1.5" spacer. Here is a link. You can also get 14mm and 28mm spacers from Baader Planetarium, and an approximately 8mm spacer from ScopeStuff.

The other point I'll make is for people not to stress about getting the spacing perfect to within 1mm. As has been discussed many times on CN, 2" filter threads rarely mate well together, even from the same manufacturer, so there will be some inaccuracy there. For the GSO CC, Roger has indicated that it should produce acceptable performance (my words, not necessarily his) with a +/-10mm spacing around the 75mm nominal value. While some people expressed doubt about that when he said it, the number of obvious errors in the Paracorr settings table here on CN is proof that most people will be perfectly happy with this level of accuracy. Just get out there and enjoy the views!
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#11 precaud

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Posted 22 May 2014 - 09:33 PM

For best coma correction, the CC lens is placed at its optimum position relative to the primary mirror, and an adjustable top is used to tune the eyepiece focus. And, as I later discovered (but can't explain why), the optimum CC-lens-to-eyepiece-focal-point distance for this unit appears to change a little depending on the focal ratio or focal length of the scope. While I had first found that 75mm was best for my F/5 dob; after experimenting with a tunable top I found it to be around 78-79mm, and closer to 80mm when used in my F/4.5 scope. So, although I didn't know it going in, the parameters are a little wider than those stated earlier, with minimum CC-lens-to-top-lip height of about 68-69mm, and an additional 10mm of adjustment. I need to use available parts for this if possible, as I don't have machinist skills or tools to make something.

The eyepiece holder section of the stock unit can not be used for this; its 46mm height has room to install a helical adapter on top but would require more in-focus than my GSO dob has. Lumicon makes a 2" extension tube with 1" setback, threaded for filters, and 51mm total height that might work.

But first thing is to find a "fine focus helical adapter" with sufficient adjustment range that is not too tall. It turns out this is a tall order (pun intended). Nothing is available in 2" format. And the 1.25" offerings from Baader and Hutech/Borg are either too tall or don't have enough adjustment range.

But... thanks to Shane's excellent review of helical focusers:
http://www.cloudynig...rd=telescope...
I learned that, once upon a time, Orion made something called the "Fine Focusing Adapter", a 2" to 1.25" adapter with builtin helical focuser that has plenty of adjustment range. And its 18mm minimum height looks good too. 51mm (Lumicon tube) + 18mm Orion FFA = 69mm, with 25mm of it below the shoulder, which should be enough.

So I found one on the used market (thanks to CN classifieds), bought one of the Lumicon extension tubes mentioned above, and put it together. Here are the three pieces ready to assemble.

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  • 6540744-gsocct1.jpg


#12 precaud

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Posted 22 May 2014 - 09:43 PM

And voila! it works perfectly. The height adjustment range is sufficient for all of my eyepieces, AND my GSO dob focuser has enough in-focus adjustment to optimally position the CC lens (with a couple mm to spare).

In use, once the optimum placement relative to the primary mirror has been found, there is no need to use a "priming eyepiece" for setup. I made a 2mm-thick spacer to position the focuser drawtube height with. Then lock the focuser in place, install the coma corrector, and use the helical top for all eyepiece adjustments. It only takes a few seconds to do. It's very slick and I love it. It is MUCH better than the Paracorr 1's clunky tunable top.

The only possible disadvantage to this setup is that it is only for 1.25" eyepieces. For me, that is no big deal, as I only have two 2" EP's, and I don't use them much. For those I use parfocalizing rings to set the proper distance and remove the Orion adapter. But 95% or more of my observing is with 1.25" eyepieces.

Here is the assembled unit with top tuned out a bit.

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  • 6540767-gsocct2.jpg


#13 precaud

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Posted 22 May 2014 - 10:10 PM

That's some good useful links and info, Mike. By all means post a photo of your setup so others can see it. The best solution may not be the same for all eyepiece collections...

I've found that the Agena spacers work well with the GSO CC, but the Baader ones do not. Though spec'ed the same, their thread pitch is slightly different.

I came to a bit different perspective on the accuracy of the spacing. I found that, with some eyepieces, introducing a 1mm error in the spacing creates as much or more coma toward the edge than the differences between eyepieces we obsess over... fixing the CC lens in place removes that variable, and also reveals more clearly other edge aberrations some eyepieces have for what they are.

#14 mcoren

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Posted 25 May 2014 - 10:30 AM

Sorry it's taken me a couple of days to get back to this with pics.

Here is my 1-1/4" eyepiece setup. As I mentioned, all of my 1-1/4" eyepieces that I use regularly are Televue "parfocal group B", which has the focal plane 1/4" below where the barrel meets the body. As such, I sought to get a spacing between the top of the GSO CC and the top of my focuser's 1-1/4" eyepiece adapter of (75mm + 1/4" = ) 81 or 82mm.

The height of my focuser's adapter is 30mm. Knowing that 2" eyepiece and extender threads rarely fit together well, I went with a 1" extender from ScopeStuff that I had already, and added the 8mm extender from ScopeStuff and a Baader 14mm extender, for a total of a bit more than 77mm, expecting that the threading errors would add a couple of millimeters.

As you can see in this picture, I think I nailed it pretty well. From L-R is GSO CC, SS 1", Baader 14mm, SS 8mm, and 1-1/4" adapter.

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  • 6544828-gsocc_1_25_small.JPG


#15 mcoren

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Posted 25 May 2014 - 10:35 AM

I only have one 2" eyepiece, a 31mm Baader Hyperion Aspheric. According to the technical data from Baader, its focal plane is right were the eyepiece body meets the barrel, so I would want the total spacing from the GSO CC to be 75mm. The barrel is 27.5mm long, so for this I used the same ScopeStuff 1" (25.4mm) adapter and a 3/4" (19.1mm) adapter from Agena. The nominal length of all of these together would then be (25.4 + 19.1 + 27.5 =) 72mm, again allowing for some slop due to the threading to add a couple of millimeters.

In this pic, it looks like I ended up around 2-3mm longer than I wanted, but that seems good enough as views are fine.

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  • 6544840-gsocc_2_small.JPG


#16 mcoren

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Posted 25 May 2014 - 10:44 AM

The argument can rightly be made that I've added four separate spacers to the system, at an additional cost of around $70 ($10 for the SS 8mm, $20 plus or minus for each of the other three). That's a valid point, but I'll counter that my total cost of GSO CC + spacers is still less than $200, or $100 less than anything with a "tunable top". (I'm talking about new here. I haven't looked at the prices of used Paracorr 1's recently, which seem harder to come by these days).

Besides, as I have said, all of my 1-1/4" eyepieces have their focal planes at (ideally) the same place, so I don't have to constantly change the spacing. I only have to do that if I want to switch to my single 2" eyepiece, so this setup works for me. As always, YMMV.

Enjoy the views!

#17 REC

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Posted 25 May 2014 - 12:13 PM

I just came across this post. I recently bought one of these and used it last night. I didn't have any issues with getting this to focused ok. I'm using it in my new AD-10 Dob.

#18 Starman1

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Posted 25 May 2014 - 12:14 PM

This brings up an interesting point: if you parfocalize all your eyepieces in advance (using barrel extenders, and parfocalizing rings), then use of the eyepiece insertion tube provided with the coma corrector would be OK, and all the eyepieces would already be in the proper position for correction when they are inserted.
It's likely you still might move the focuser by a millimeter for the sharpest focus, but it would eliminate any necessity for a tunable top or a lot of different spacers on the coma corrector.

This actually makes all coma correctors easier to use. I parfocalized all my eyepieces (except one), and they all use the coma corrector at the same exact setting.
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#19 precaud

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Posted 25 May 2014 - 12:49 PM

REC, that may be, but unless you're using a spacer or extension tube as described, there is no way the coma correction is anywhere near optimum. I know of no eyepiece whose focal point is some 29mm above the shoulder.

#20 precaud

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Posted 25 May 2014 - 01:07 PM

Yes, Don, and I started out months ago on the "parfocalize everything" path. It came to an end when I found it impossible to attach parfocalizing rings to the Sterlings' tapered barrels (the same must be true of other EP's too). And when I realized I am not settled in the eyepiece department. And when it became obvious that the funky R&P focuser in the DS-16 wasn't going to be replaced until other decisions about that scope had been made.

But if used with a small, stable collection of eyepieces without tapered barrels, on a single scope, with a good fine-focus capability focuser, parfocalization makes good sense. But you'll still need to add some spacing between the standard eyepiece insertion tube and the CC lens to get it in a useable zone. In stock form the GSO CC doesn't work well for any known eyepiece.

And, as I discovered, the other reason to have a tunable top is if you use the CC in different scopes with different focal lengths.
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#21 REC

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Posted 26 May 2014 - 07:51 AM

Well I have a 35mm extension tube that came with my scope. I never use it as all my EP's focus ok.

Should I now be using it when I use the CC now? This is all news to me as you said the GSO does not come with any instructions. I thought you just popped it in the focus tube like you would a barlow lens?
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#22 precaud

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Posted 26 May 2014 - 08:55 AM

Well you have a good piece to experiment with, then. That extension tube is 70mm long, half of which is below the shoulder (as opposed to none on the standard setup), so you can just screw the CC element onto it to get ALOT closer to proper correction than the stock setup, especially with your 2" eyepieces.

So here's the simple experiment: Add a 5mm spacer on top and try it with some of your 2" EP's, especially any whose focal point is known to coincide with the shoulder within a mm or two. Coma correction will be much improved over the stock setup!

#23 REC

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Posted 26 May 2014 - 09:21 AM

John, so if I understand you, remove the element and screw it on the Apetura 35mm extension tube?

The GSO is about 4" long and with the 35mm tube attached it is now about 5" long...so use it this way?

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#24 precaud

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Posted 26 May 2014 - 09:39 AM

Yes, that's correct. We're not concerned about the total length, but primarily:
a) the distance from the top of the CC lens section to the top shoulder of the eyepiece holder. Adjust this for best coma correction.
And secondarily:
b) the distance from the top of the CC lens section to the bottom shoulder of the eyepiece holder. This distance removes the necessity for focuser in-travel by the same amount.

For experimenting purposes, you can make spacers out of anything with a 2" inside diameter. I had a round mailer tube (from Catseye...) that is 2" i.d. I sliced it on a table saw into a variety of lengths from 2mm to 10mm. They're very handy to have around when experimenting...
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#25 REC

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Posted 26 May 2014 - 11:46 AM

John, thanks for the info as we also have the same scope. Are you using the 35mm extender as well like mine?

Also, is the CC mainly for the low power wide field EP's or do you use it as well for say EP's under 20mm and below?

Bob


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