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Equipment Discussions >> Eyepieces

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Daniel Guzas
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Reged: 11/20/10

Loc: Bethlehem NH/ Boston MA
2" GSO Coma Corrector a disappointment. new
      #6138045 - 10/14/13 11:07 PM

I recently purchased a GSO 2" coma corrector to help with the comet likes stars that are evident in my 2" Explore Scientific 24mm 82 deg eyepiece. I use this eyepiece in a 15" f4.5 reflector dob. However I am disappointed in the results... I used Pleiades as my target because this has a number of bright stars right across the field of view. I noticed the coma in this eyepiece right away when using it before adding the corrector. But after placing it in the coma corrector I have to say it really didn't make all that much of a difference. I was expecting to see sharp pin point stars all the way to the field stop.

Am I expecting too much from the corrector? Granted I did notice that it does help achieve focus on stars about half way to the field stop but not enough to make a real difference in my opinion. I did notice that due to the wide field of this eyepiece I have to move my head around to look at all the stars. And when looking at different areas by slightly shifting my head I could get some of the stars about half way to the edge to remain more point like than without the corrector.

To say I am disappointed in its performance would be an understatment. I have heard all sorts of rave reviews about coma correctors and how people get sharp stars all the way to the field stop they weren't able to get before. I just don't get it.... Am I doing something wrong here? I can't even imagine how I would have felt if I spent $500-$600 on a Televue paracorr if it won't work on a wide field eyepiece.

Can you all enlighten me? Is this due to the fact that I have an 82deg and a 24 mm eyepiece that makes it very difficult to correct?


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tonyt
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Reged: 09/02/09

Loc: Australia
Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6138094 - 10/14/13 11:31 PM

You could try pulling the eyepiece out of the coma corrector by 5 or 10mm and refocus to see if there is a particular position that gives a better result, since different eyepieces need to be different distances from the correcting lenses to get best results.

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Daniel Guzas
professor emeritus
*****

Reged: 11/20/10

Loc: Bethlehem NH/ Boston MA
Re: 2" GSO Coma Corrector a disappointment. new [Re: tonyt]
      #6138104 - 10/14/13 11:39 PM

Quote:

You could try pulling the eyepiece out of the coma corrector by 5 or 10mm and refocus to see if there is a particular position that gives a better result, since different eyepieces need to be different distances from the correcting lenses to get best results.




I did try this and it didn't seem to make any difference... This is exactly what I thought, pull the eyepiece out a little and try to get it to perform better... But to no avail... Maybe I need to fool around with it more...? One thing I did find was there was a limit to how much I could pull the eyepiece out as I would run out of in focus travel. I wonder if I were to move the eyepiece out even further, and have the focus travel I needed, maybe that would have made things better...


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tonyt
professor emeritus


Reged: 09/02/09

Loc: Australia
Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6138175 - 10/15/13 12:25 AM

Sonds like a barrel extender would be worth a try if you have something laying around. Probably someone on the forum has used that eyepiece with the GSO corrector and will chime in. I have the same corrector which I'm planning to mount directly to the barrel of an ES30mm (so it goes into the focuser instead of sticking out) and I have various spacers to play around with but haven't had the opportunity yet. The lens set is attached to the body of the coma corrector with filter threads.

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kkokkolisModerator
Carpal Tunnel
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Reged: 09/23/09

Loc: Piraeus, Greece
Re: 2" GSO Coma Corrector a disappointment. new [Re: tonyt]
      #6138291 - 10/15/13 03:07 AM

You have to find the proper distance for every eyepiece you have (if that's just one eyepiece, the better). That's why Paracor has a tunable top. I have a SW version of CR and, although I keep it for the future, I decided that a little coma is better than the effort to use it right. But my Newtonians are f/6 and f/4.9. You have a more demanding telescope, so it might worth the try.

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acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6138303 - 10/15/13 03:30 AM

I saw a used Paracorr for sale in the CN classifieds, maybe in the Eyepiece section? Not sure.
Andy


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Damo636
sage


Reged: 08/16/11

Loc: Co Fermanagh, Northern Ireland
Re: 2" GSO Coma Corrector a disappointment. new [Re: acochran]
      #6138414 - 10/15/13 07:07 AM

Check out this user guide, it may be just a case of setting it up right....


http://stargazerslounge.com/topic/158529-astro-techaltair-astrogso-coma-corre...


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BSJ
Pooh-Bah
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Reged: 12/22/08

Loc: Grand Isle, VT
Re: 2" GSO Coma Corrector a disappointment. new [Re: tonyt]
      #6138444 - 10/15/13 07:34 AM

Quote:

Sonds like a barrel extender would be worth a try if you have something laying around. Probably someone on the forum has used that eyepiece with the GSO corrector and will chime in. I have the same corrector which I'm planning to mount directly to the barrel of an ES30mm (so it goes into the focuser instead of sticking out) and I have various spacers to play around with but haven't had the opportunity yet. The lens set is attached to the body of the coma corrector with filter threads.




I look forward to hearing about it on the ES 30! I've been wondering how one would work with it in my Z10...


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6138459 - 10/15/13 07:46 AM

FWIW, I have an older tunable top Paracorr, and in my 10" f/4.7 Dob it does *not* give sharp star points to the edge for all eyepieces. There are many eyepieces where it does not, so a hit or miss thing, usually it's FC that it is unable to flatten sufficiently.

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howard929
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Reged: 01/02/11

Loc: Low End of High Ground
Re: 2" GSO Coma Corrector a disappointment. new [Re: BillP]
      #6138469 - 10/15/13 07:57 AM

Of the hits and misses, have you noticed a brand or type of eyepieces that has more hits, more misses or is it all over the place?

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6138475 - 10/15/13 08:02 AM

Quote:

I used Pleiades as my target because this has a number of bright stars right across the field of view.




I would try something a bit less demanding. When viewing the Pleiades in a 24mm eyepiece in a 15 inch F/4.5, there are a number of things going on, not all of it is coma and eyepiece astigmatism.

I don't know how well the GSO Coma Corrector works but I do use a Paracorr in my faster Newtonians. The 20mm Nagler Type 2 + Paracorr provides near perfect views in my 16 inch F/4.5 but the Pleiades are one target generally skip in a larger scope... they are so much better in a smaller scope that is able to frame them.. The stars are so bright in a larger scope, at the 4-5mm exit pupil, the human eye does not resolve the disk structure...

Rather than trying a difficult target right out of the box, I suggest that since you don't know the proper spacing, you systematically determine the optimal spacing between the corrector and the eyepiece.

First I would pick a magnitude 5 star and observe the coma without the corrector, get a good handle on what is coma and what is astigmatism. Then I would, add the coma corrector and spend some time making small changes and observing the differences, again trying to isolate the coma from the astigmatism, the mixture does make it more difficult. This way you should be able to determine the optimal spacing. I would focus in the center and then move the star to about 80% of the distance to the edge.

Once you have the spacing figured out, that's the time to just use it in your nightly observing and see what you think...

Jon


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nevy
professor emeritus
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Reged: 02/07/12

Loc: UK
Re: 2" GSO Coma Corrector a disappointment. new [Re: Jon Isaacs]
      #6138647 - 10/15/13 09:43 AM

Lesson learnt , only buy once , it's cheaper that way , get the proper parracor ( televue).

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Eddgie
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Reged: 02/01/06

Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6138660 - 10/15/13 09:47 AM

This is a hard target.

Even the Paracorr is not listed as being able to completely correcto coma.

If you read the literature for the Paracorr, it says that it increased the diffraction limited true field by something like 36 times in size. That sounds great, but since the diffraction lmited size of the field of a fast dob is only four or five arc minutes in diameter, this constitutes a diffraction free field only about 1/3rd the diameter of the largerst true field the instrument can provide.

Now that is not at all bad.

But, it is not complete coma correction.

And the target you chose has so many really bright stars that you were seeing just would you should have seen, which is a field that is not perfect, just as Televue says for their corrector.

So, you are expecting to much if you expect to see the Pliedes refractor sharp at the edge. Stars are to bright.

Dimmer targets will look great though.

For a decade I have maintained thet the real advantage to refractors is not their on axis performance, but rather their coma free off axis perfromance. It is very easy to beat the contrast transfer of a big refractor with a bigger reflector.

But good luck beating them for there outstanding off axis performance. Noting does a 2 degree field like a big refractor.

That is the only reason I keep my 6" APO. It is the most amazing wide field instrument I have ever owned.

Reflectors struggle to get this even with a Paracorr/Coma corrector.

Here are the diagrams from Televue. They suggest that getting coma free performance to the edge of the field stop of a modern wide feild with this king of approach is not that easy with this approach.

Televue Spot size shows improvement, but not coplete elimination...


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Achernar
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Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6138664 - 10/15/13 09:48 AM

I'm mystified here, I use the exact same eyepiece you have with an original Televue Paracorr with the tunable top with my own 15-inch with very good results. However I also have astigmatism in my eyes, and therefore I base my conclusions on how eyepieces work with my eyeglasses on. If you must wear contacts or eyeglasses to see well, stars are going to look funny without them. Yes, sometimes your eyes do play tricks on you.

Taras


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precaud
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Reged: 12/05/12

Loc: north central New Mexico
Re: 2" GSO Coma Corrector a disappointment. new [Re: Jon Isaacs]
      #6138688 - 10/15/13 10:01 AM

I have the GSO CC and am very happy with it. Not all eyepieces are equally well corrected by it, but they are all significantly improved by it.

If you want the freedom to put any old eyepiece into it and be able to adjust it on the fly, then a Paracorr is probably the way to go.

The GSO requires some planning and adjustments prior to use. As explained in the post Damo gave the link to, it needs additional spacing between the CC lens assembly and EP holder. How much spacing depends on your eyepieces. You basically want to set it up to accomodate the EP's that requires the most in-focus, and then use spacers for all the others, which also parfocalizes them. Starman1 has given excellent instructions on how to parfocalize your eyepieces.

In my case, I saw that my 1.25" eyepieces generally fell into two groupings: those requiring about 10mm in-focus, and the bulk were approx neutral. It so happens that the Glatter Parallizer doesn't move the focal plane, and the GSO 1.25" adapter has a 10mmm thick top plate. So there's the 10mm needed to accommodate all EP's. Adding a .75" (19mm) spacer to the CC put everything into the right position, and I only needed to use parfocalizing rings for my 2" EP's.

It sounds more complicated than it is. It's just a statistical/organizational exercise. Once done, life is easy and good.


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Daniel Guzas
professor emeritus
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Reged: 11/20/10

Loc: Bethlehem NH/ Boston MA
Re: 2" GSO Coma Corrector a disappointment. new [Re: precaud]
      #6138943 - 10/15/13 12:33 PM

It seems that I have a little planning and testing to do before I give up I'm this coma corrector... I appreciate all your tips and look forward to reporting my findings.....

I really want to have a couple of really wide view eyepieces that work well in my scope...


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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: 2" GSO Coma Corrector a disappointment. new [Re: Daniel Guzas]
      #6139025 - 10/15/13 01:04 PM

Bummer!

I just use eyepieces that don't show much coma from the mirror. Some will show coma more than others. I'd rather do that than fiddle with something else in between my eyepieces, but that's just how I do things!

I hope it works out for you.

Cheers,


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Jim Romanski
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Reged: 01/02/05

Loc: Guilford, Connecticut
Re: 2" GSO Coma Corrector a disappointment. new [Re: Jon Isaacs]
      #6139368 - 10/15/13 04:05 PM

Quote:

Quote:

I used Pleiades as my target because this has a number of bright stars right across the field of view.




I would try something a bit less demanding. When viewing the Pleiades in a 24mm eyepiece in a 15 inch F/4.5, there are a number of things going on, not all of it is coma and eyepiece astigmatism.




I agree with Jon. But I'd also like to remind you that the Pleiades is surrounded by nebulosity. You can see this in a 15" scope. So that's another reason why you should check a different object.

Some other thoughts. Be sure that you have the proper spacing between the corrector and the focal plane. Try a different eyepiece if possible. Do you know someone or a club where you can try a Nagler or an Ethos? Do you wear glasses and if so are you wearing them at the eyepiece. Astigmatism in your eyes could be greater than the coma.


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Daniel Guzas
professor emeritus
*****

Reged: 11/20/10

Loc: Bethlehem NH/ Boston MA
Re: 2" GSO Coma Corrector a disappointment. new [Re: Jim Romanski]
      #6139451 - 10/15/13 04:58 PM

Quote:

Quote:

Quote:

I used Pleiades as my target because this has a number of bright stars right across the field of view.




I would try something a bit less demanding. When viewing the Pleiades in a 24mm eyepiece in a 15 inch F/4.5, there are a number of things going on, not all of it is coma and eyepiece astigmatism.




I agree with Jon. But I'd also like to remind you that the Pleiades is surrounded by nebulosity. You can see this in a 15" scope. So that's another reason why you should check a different object.

Some other thoughts. Be sure that you have the proper spacing between the corrector and the focal plane. Try a different eyepiece if possible. Do you know someone or a club where you can try a Nagler or an Ethos? Do you wear glasses and if so are you wearing them at the eyepiece. Astigmatism in your eyes could be greater than the coma.





I do wear glasses but. It at the scope. You all have given me lots to try out and I am grateful. I will give it another go with a not so demanding object.... It seems that I need to do a couple of things... First find an alternate object, second, find the ideal spacing between the coma corrector and the eyepiece, and third, try additional eyepieces with the corrector...

I will keep at it... Thanks for all the helpful replies..


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faackanders2
Carpal Tunnel


Reged: 03/28/11

Re: 2" GSO Coma Corrector a disappointment. [Re: Eddgie]
      #6141892 - 10/16/13 09:55 PM

Quote:

This is a hard target.

Even the Paracorr is not listed as being able to completely correcto coma.

If you read the literature for the Paracorr, it says that it increased the diffraction limited true field by something like 36 times in size. That sounds great, but since the diffraction lmited size of the field of a fast dob is only four or five arc minutes in diameter, this constitutes a diffraction free field only about 1/3rd the diameter of the largerst true field the instrument can provide.

Now that is not at all bad.

But, it is not complete coma correction.

And the target you chose has so many really bright stars that you were seeing just would you should have seen, which is a field that is not perfect, just as Televue says for their corrector.

So, you are expecting to much if you expect to see the Pliedes refractor sharp at the edge. Stars are to bright.

Dimmer targets will look great though.

For a decade I have maintained thet the real advantage to refractors is not their on axis performance, but rather their coma free off axis perfromance. It is very easy to beat the contrast transfer of a big refractor with a bigger reflector.

But good luck beating them for there outstanding off axis performance. Noting does a 2 degree field like a big refractor.

That is the only reason I keep my 6" APO. It is the most amazing wide field instrument I have ever owned.

Reflectors struggle to get this even with a Paracorr/Coma corrector.

Here are the diagrams from Televue. They suggest that getting coma free performance to the edge of the field stop of a modern wide feild with this king of approach is not that easy with this approach.

Televue Spot size shows improvement, but not coplete elimination...




Paracorr II does not fully correct coma with 20mm ES100 in my 17.5" f4.1 dob; but it is much better and tolerable with it (this eyepiece was the one that made me require a Paracorr). I hoped it to be perfect, but didn't expect guranteed perfection.


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