Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Setting up the GSO coma corrector

  • Please log in to reply
180 replies to this topic

#26 precaud

precaud

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 26 May 2014 - 03:25 PM

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


Bob, I now use the two setups detailed earlier in the thread. But I used the 35mm extender to first try out the spacing thing to see if it had merit.

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?


Well, many folks keep the CC in all the time. With the AD10 I keep it in for eyepieces down to 10mm (125x), and sometimes for the 8.8mm ES82. Below that, I prefer to remove it and work an eyepiece directly on the coma-free region, which on the AD10 is about 2.7mm (0.022 x Fratio^3).

#27 swix

swix

    Mariner 2

  • *****
  • Posts: 218
  • Joined: 25 Mar 2012
  • Loc: Maryland

Posted 03 June 2014 - 10:22 AM

John, first of all thanks for the excellent tune-up information on the GSO
Coma Corrector. I purchased mine from High Point and expected it to work the
way it came to me... it did not. After reading your thread many times, I ordered
the Blue Fireball extension 0.75" from Agena. Now the corrector works !! I use
Luminos 19mm, 15mm, and 10mm eyepieces as well as the Baader Aspheric 31mm. In
order to get the 2" eyepieces par-focal with the 1.25" eyepieces, I was off to
the hardware store for 2" pvc pipe. Low and behold, I found a section filled
with slip joint washers !! That is the orange spacer you see between the 31mm
and the corrector. They are 4mm thick and a snug fit around the 31mm 2" barrel.
Perfect spacing that I needed. I am using a Celestron C8N - AVX scope setup. As
you see, the focuser has been changed. This Antares focuser has the necessary
in travel for the corrector to reach focus. It is low profile and has a 2"
extension tube as part of the draw tube.
I have massaged this scope pretty good, new focuser, Scopestuff flocking,
painted secondary flat black, etc...
With the coma corrector now performing the way it should, I am running out
of projects on this scope. Time to just enjoy and use it...

Thanks...

Attached Thumbnails

  • 6560907-COMA COR 010.jpg

  • BLACKDRAGON and wcoastsands like this

#28 swix

swix

    Mariner 2

  • *****
  • Posts: 218
  • Joined: 25 Mar 2012
  • Loc: Maryland

Posted 03 June 2014 - 10:23 AM

Pic #2 1.25" eyepiece..

Attached Thumbnails

  • 6560910-COMA COR 013.jpg


#29 precaud

precaud

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 03 June 2014 - 11:18 AM

Glad to hear of your good results, Pete. I've been unable to find any 2" ID slip joint washers, where did you find yours?

Here's another thing you can try with the GSO. This one's gonna really cost you, though - all of 79 cents! Go to Ace Hardware and get one of the 02-2016P union washers in the plumbing department. Slip it into the field lens end of the CC, and watch contrast improve *significantly* and edge-of-field-brightening practically disappear! This works wonders with all of my 1.25" EP's; I haven't tried it with the 2"ers yet. The 32.5mm ID of the washer may vignette them slightly. But boy, the visual cleanup is tremendous...

#30 swix

swix

    Mariner 2

  • *****
  • Posts: 218
  • Joined: 25 Mar 2012
  • Loc: Maryland

Posted 03 June 2014 - 11:25 AM

I found the slip washers at my local Ace Hardware in the plumbing area. I will go back and get the 02-2016P union washer and give it a try. Thanks...

#31 swix

swix

    Mariner 2

  • *****
  • Posts: 218
  • Joined: 25 Mar 2012
  • Loc: Maryland

Posted 03 June 2014 - 12:38 PM

John,
My Ace store uses Danco Inc. slip joint washers The 2" eyepiece washer is part # 36652B and the 1.25" washer part # 36642B.
They did not have the union washer by part number that you mentioned. Can you send me the ID & OD measurements ??

Thanks..

#32 swix

swix

    Mariner 2

  • *****
  • Posts: 218
  • Joined: 25 Mar 2012
  • Loc: Maryland

Posted 03 June 2014 - 01:57 PM

John,
Forget the last request.. I found the ID & OD... I made one..

Thanks..

#33 REC

REC

    Voyager 1

  • *****
  • Posts: 11,996
  • Joined: 20 Oct 2010
  • Loc: NC

Posted 03 June 2014 - 02:20 PM

John, I have your scope, so question about you $.79 mod. How thick are those washers and exactly where do you place them?

Does than mean I should not use the 35mm extension tube that came with the scope?

I just got the GSO CC a few months ago and have not used it much yet. Looking forward to using it now with the summer Milky Way stuff coming into view with some low power wide angle EP's.

Thanks,

Bob

#34 precaud

precaud

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 03 June 2014 - 02:45 PM

Pete: Thanks for those part #'s. On this washer's packaging, the size is given as 1-9/32" ID x 1-27/32" OD. But on the display drawer in the store, it was listed as a 1-13/16" OD x 1-1/4" ID.

Bob: This gasket is a totally separate matter from the spacing being discussed. It is just a thin rubber washer and is placed on the field lens end of the CC.

#35 MSWcdavis

MSWcdavis

    Viking 1

  • *****
  • Posts: 544
  • Joined: 07 Feb 2012

Posted 11 January 2015 - 06:17 PM

If i buy the type 2 paracorr - do I have to do anything like this?  this seems somewhat inaccurate...

 

that being said the price is right

 

if i want to use two inch eyepieces it seems that i need to add a spacer of 30mm in between the two sections and call it a day?

 

am I making this too simple?



#36 MSWcdavis

MSWcdavis

    Viking 1

  • *****
  • Posts: 544
  • Joined: 07 Feb 2012

Posted 11 January 2015 - 06:37 PM

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?

 

so should I add a spacer of 30mm in between the top and bottom piece...

 

or should i take off the top piece and instead use a 35mm extension tube?  

 

How is anyone doing who does the 35 mm extension tube?  does it work well with two inch eyepieces from explore scientific?



#37 precaud

precaud

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 11 January 2015 - 08:28 PM

If i buy the type 2 paracorr - do I have to do anything like this?

 

No, the P2 has a tunable top, so once the CC is in its correct place you focus using the tunable top.

 

if i want to use two inch eyepieces it seems that i need to add a spacer of 30mm in between the two sections and call it a day?

am I making this too simple?

 

 

It is impossible to answer that without knowing the amount of infocus or outfocus required by each eyepiece. If their focal plane are all at the mounting lip, then yes, a 30mm spacer would do the trick.



#38 MSWcdavis

MSWcdavis

    Viking 1

  • *****
  • Posts: 544
  • Joined: 07 Feb 2012

Posted 11 January 2015 - 09:06 PM

 

If i buy the type 2 paracorr - do I have to do anything like this?

 

No, the P2 has a tunable top, so once the CC is in its correct place you focus using the tunable top.

 

if i want to use two inch eyepieces it seems that i need to add a spacer of 30mm in between the two sections and call it a day?

am I making this too simple?

 

 

It is impossible to answer that without knowing the amount of infocus or outfocus required by each eyepiece. If their focal plane are all at the mounting lip, then yes, a 30mm spacer would do the trick.

 

 

 

thanks for feedback!

 

Perhaps you or somebody else could help me out a little more

 

I have an explore scientific 14mm 100degree and an explore scientific 32mm 82degree that I will be using (both are two inches)

 

i have an xx12i scope orion

 

The focal plane is the first piece of glass that comes into contact with the image from the mirror (and sends it off through the eyepiece)?

 

And so then I would need to adjust each eyepiece so that they have the exact same distance to this point (the bottom / first piece of glass in the eyepiece)?

 

One of my hang ups has been what use is the focuser then...but i'm beginning to realize that the focuser would still merely adjust relative to this crazy apparatus i have to construct


Edited by MSWcdavis, 11 January 2015 - 09:32 PM.


#39 precaud

precaud

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 11 January 2015 - 09:33 PM

I can't help you with either of those eyepieces.

 

As to the focuser... it is no longer used to focus the eyepieces... it is used to set the coma corrector into proper position.



#40 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 48,242
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 15 January 2015 - 02:34 PM

OK, here is the simplest way to set up the GSO coma corrector after reading all the advice on this thread:

1) Insert longest focal length eyepiece into coma corrector.  Focus scope.  Examine stars at edge of field.  Still show a trace of coma?

2) Pull eyepiece 1/8" out of coma corrector and clamp down.  Refocus scope.  Examine stars at edge of field.  Better but not fully corrected?

Then repeat, pulling eyepiece 1/8" farther out of coma corrector.  Repeat until star images at edge look perfect. If you feel this results in having the eyepiece unsafely pulled out of

the CC, then insert the spacer previously mentioned or add a barrel extender to the eyepiece.  Either way should work to safely support the eyepiece.

3) Now that you have optimized one eyepiece, all the rest of your eyepieces will follow.  Do not touch the focuser after optimizing eyepiece 1.  Insert eyepiece 2 and pull it out of the CC until it is in focus.  That will be the perfect position for that eyepiece.  It will help to add a parfocalizing ring to the eyepiece so the next time you insert the eyepiece it returns to exactly the same point..  In this manner, you can set all your eyepieces to their correct positions for best coma correction.

4) After you're done, all your eyepieces will be parfocal (+/- a millimeter or so) and they will all use the CC with the focuser at approximately the same point.

 

So what happens if eyepiece 2, when inserted all the way into the CC, still hasn't reached focus?.

Then start out with that eyepiece in the CC first and go back to step one.  It will be the eyepiece you use to determine the setting of the CC relative to the telescope's focal point.

Whichever of your eyepieces focuses the farthest in will be the eyepiece you use for the "control".  If you already know that, start with that eyepiece.  If you don't already know that, it's commonly the longest focal length (but there are many exceptions).

 

So what happens if coma gets worse as you pull that first eyepiece out of the coma corrector, i.e. its position of best correction would be farther in, closer to the CC?

Well, barring having a custom top machined for the CC, that eyepiece will not be fully corrected by the CC.  But note: You will not use that eyepiece to set the initial position of the CC--you'll use another eyepiece.  This is not too far-fetched.  It applies to the Nagler 31, and Ethos 17 and 21 when used in the original Paracorr, for example.


  • michaeldurban, bsavoie, Nick Salt and 4 others like this

#41 careysub

careysub

    Mercury-Atlas

  • *****
  • Posts: 2,629
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 21 January 2015 - 11:30 AM

I am looking at using the GSO CC in a sliding block focuser design (the position of the CC relative to the secondary is fixed, the whole secondary EP holder assembly moves up and down). How far does the focal plane need to be beyond the entrance light cone for the GSO CC to work?


  • lakland5 likes this

#42 careysub

careysub

    Mercury-Atlas

  • *****
  • Posts: 2,629
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 28 January 2015 - 01:40 AM

I see that there is a Chinese eBay seller now offering 2" helical focusers for $89:
http://www.ebay.com/...sd=321190618702

The seller is 'hunter2099'.

Perhaps someone famililar with GSO coma correctors (and 2" helical focusers) can determine whether these are suitable for a 'tunable top'.

#43 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 48,242
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 28 January 2015 - 02:01 AM

You'd need a way to remove the eyepiece tube on the GSO CC and replace it with the helical focuser.

The helical focuser needs at least 1/2" of movement.

Otherwise, interesting suggestion--make your own tunable top.



#44 precaud

precaud

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 28 January 2015 - 08:30 AM

Yeah, I think parfocalizing rings are the way to go for 2" eyepieces with the GSO CC. The Lumicon 1" extension tube sets the CC lens at about the right distance for it. If GSO were smart, they'd scrap their current setup and supply one like it. Then pick up a used Orion helical adapter for your 1.25" EP's. Or just remove the CC for mags 100X and up. The GSO glass isn't as transparent as the Paracorr at high magnifications, anyway.



#45 jsmoraes

jsmoraes

    Apollo

  • -----
  • Posts: 1,213
  • Joined: 20 Nov 2014
  • Loc: Vilatur, Saquarema, Rio de Janeiro, Brazil

Posted 10 May 2015 - 08:11 PM

Congratulations for this topic and contents. Really adjust the distance of CC GSO is a task. And some times we haven't the best spacer to do it.

 

I would like to add informations to the topic. I use the CC for astrophoto with Canon. And because its sensor is very internal, the measure and maximum distance is important.

 

From mcoren:

 

Roger has indicated that it should produce acceptable performance (my words, not necessarily his) with a +/-10mm spacing around the 75mm nominal value.

Yes, I agree. I am working with 83 mm and I am at the limit. An example of using CC GSO.

 

photoset-measure.jpg

 

note: it was luck to have that distance as it is. A little more and ... lost !

I don't know if with these set of accessories a Baader MPCC will work. It seems that the Baader has 10 mm less for limit distance.

 

The performance of CC GSO in this configuration isn't 100 %. More because of tilt of the camera inside of the focuser, causing more coma at an periphery area of image: right botton in my case. But the OAG take as guide, stars at far periphery where the coma are more strong. And its image is very round shape.

I may say that perhaps it is working around 90% of its full performance.


Edited by jsmoraes, 10 May 2015 - 08:12 PM.


#46 jsmoraes

jsmoraes

    Apollo

  • -----
  • Posts: 1,213
  • Joined: 20 Nov 2014
  • Loc: Vilatur, Saquarema, Rio de Janeiro, Brazil

Posted 23 May 2015 - 10:21 PM

COMA corrector, is it the guilty ?

 

I was having problems with coma. Weird problems, since nothing was modified.

Theses images show what I am saying: Different days and with reposition of camera inside of the focuser.

 

foc_light_10s_3200_20150507-20h05m58s171

 

foc_light_15s_3200_20150509-22h48m28s234

 

Distortion like coma only at unique side of image. Side that change the position if you reposition the camera.

 

Many check and tests were done. Cleaning mirror, collimation, cleaning coma corrector, and all accessories involved with the current optics.

 

In short and direct words:

 

1. position of the camera inside of focuser can tilt the sensor with reference to optic axis of telescope.

2. this tilt can produce a similar image of coma. The difference is that the distortion will be only at a side of image. And can change at each camera mounting.

3. Problem with real coma, from corrector or mirror will affect all the periphery of image.

4. GSO coma corrector can work fine from 65 mm up to 85 mm. With nominal distance in 75 mm

 

A specific issue of my set:
The best performance would be with 75 mm. But because of some reflection the best distance should be 83 mm.

Some filter can have a barlow action. When used with coma corrector we can have some problems. Reflection and spheric distortion are most present with my set of accesories.

 

coma_light_10s_3200_75mrepo_20150523-20h

 

coma_light_10s_3200_83m_20150523-19h35m0

 

coma_light_10s_3200_weelrrepo_20150523-2


Edited by jsmoraes, 23 May 2015 - 11:04 PM.


#47 jsmoraes

jsmoraes

    Apollo

  • -----
  • Posts: 1,213
  • Joined: 20 Nov 2014
  • Loc: Vilatur, Saquarema, Rio de Janeiro, Brazil

Posted 23 May 2015 - 10:27 PM

And the center of FOV

 

star-75mm.jpg

 

star-83mm.jpg

 

star-filterweel.jpg


Edited by jsmoraes, 23 May 2015 - 11:05 PM.


#48 Jack Day

Jack Day

    Ranger 4

  • *****
  • Posts: 337
  • Joined: 04 Mar 2005
  • Loc: Wenatchee, WA

Posted 18 June 2015 - 01:52 AM

So what if....   

 

Let's pretend for a minute that we were building a new upper cage for a 17.5" f/4.5 scope.  What would prevent you from isolating the barrel (optics package) of the coma corrector from the focuser itself?

 

My thought was this: Suspend a 58 to 48mm filter adapter about 1/2 inch below the focuser board using some spacers mounted to the flange of the filter adapter.  To this you would screw on the optics package of the coma corrector just as if you were screwing it onto a 48mm filter.  The optics of the coma corrector are now fixed to the focuser board passing up through the board into the bottom of the focuser.  

 

Now the focuser can move up and down, as if it were a tunable top.  Add the 19mm barrel extension to the top of the coma corrector so it can be inserted into the focuser, this adds spacing needed to achieve the proper working spacing (73.5mm) between the eyepiece and coma corrector optics. 

 

Now put a 31mm Nagler in the coma corrector top.  Now focus the telescope by moving the upper cage closer to or further away from the primary mirror.   Once the best focus is found then truss tubes can be cut to proper length.  

 

Potential issues: Focuser or focuser board not square to optical axis.  Upper cage not truly aligned with optical axis of scope in general.

 

Well, Any thoughts?

 

Thanks!

Attached Thumbnails

  • coma corrector1.jpg

Edited by Jack Day, 18 June 2015 - 01:56 AM.

  • stargazer193857 and RFeaster like this

#49 precaud

precaud

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,010
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 18 June 2015 - 07:59 AM

My thought was this: Suspend a 58 to 48mm filter adapter about 1/2 inch below the focuser board using some spacers mounted to the flange of the filter adapter.  To this you would screw on the optics package of the coma corrector just as if you were screwing it onto a 48mm filter.  The optics of the coma corrector are now fixed to the focuser board passing up through the board into the bottom of the focuser. 

 

OK so it is fixed in place, but in the correct place? The nominal 75mm spacing between CC lens and top shoulder is a proxy for placing the CC lens at the right place relative to the primary mirror. The latter is the actual goal - the former is more convenient to measure, but is actually variable due to eyepieces coming to focus at different heights with respect to the shoulder. Your arrangement needs to allow for the CC lens placement to be adjusted. Other than that, it's a good idea.



#50 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 48,242
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 18 June 2015 - 08:53 AM

Yes, the idea is not bad.  It basically turns your focuser into a SIPS, the FeatherTouch Focuser with the CC in the base of the focuser.

What are the pitfalls?

1) placing the CC lens at the correct place in the optical light cone.

2) getting eyepieces with their focal planes high in the eyepiece close enough to the CC lens to focus without having the focuser drawtube hit the CC lens holder.

3) since this does not parfocalize your eyepieces, the range of focuser movement will likely have to be 1.5" or more.

4) it eliminates the use of barlow lenses, which are typically used below the CC lens.

 

Let's address each issue.

1) I suppose the same technique for setup used by the Paracorr-in-a-focuser would work, assuming you could put some translucent tape over the focuser 75mm from the lens:

http://starlightinst...nstructions.pdf  This wouldn't be too big of a technical hurdle, but you'd need some way of moving the lens in and out relative to the light cone of the scope.

2) Some eyepieces would need to nearly touch the lens to get close enough to focus.  So it's possible one would not add any spacers to the lens.  You would want the eyepiece to be placed in the focuser drawtube, not any accessory, so it could get as close as the barrel of the eyepiece coming close to hitting the CC lens.  Otherwise, those eyepieces with high focal planes would never achieve focus.

3) A typical set of eyepieces that includes some 2" eyepieces will need that much travel, and the drwtube will need to slide past the CC lens and not bottom out on anything past the CC lens in order to get the aforementioned eyepieces close enough to the CC lens to focus.

4) To barlow 2" eyepieces, a barlow is usually placed inside the coma corrector in the light cone, maintaining the coma corrector to eyepiece distance.  Mounting the CC lens as envisioned eliminates this possibility.  Using the barlow in front of the eyepiece precludes coming to focus in all likelihood.

 

So what if you used the coma corrector in its own tube, adjusted the spacing for perfect correction for one eyepiece, froze your focuser, and parfocalized all your eyepieces to the one you had adjusted correctly?

--all your eyepieces would be parfocal

--all your eyepieces could/would be adjusted to an optimum coma correction setting.

--barlows could be used

--finding the correct position of the CC in the scope's light cone is easy

--and adjusting a new eyepiece would be as simple as parfocalizing it to all your other eyepieces.  If you parfocalize it, the CC doesn't even have to be in place 9though it could be), since parfocalizing is a process of bringing all your eyepieces' focal planes to the same position relative to the focuser.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics