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2" GSO Coma Corrector a disappointment.

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#1 Daniel Guzas

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Posted 14 October 2013 - 10:07 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?

#2 tonyt

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Posted 14 October 2013 - 10:31 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.

#3 Daniel Guzas

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Posted 14 October 2013 - 10:39 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.


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...

#4 tonyt

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Posted 14 October 2013 - 11:25 PM

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.

#5 kkokkolis

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Posted 15 October 2013 - 02:07 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.

#6 acochran

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Posted 15 October 2013 - 02:30 AM

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

#7 Damo636

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Posted 15 October 2013 - 06:07 AM

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


http://stargazerslou...o-coma-corre...

#8 BSJ

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Posted 15 October 2013 - 06:34 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.


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

#9 BillP

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Posted 15 October 2013 - 06:46 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.

#10 howard929

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Posted 15 October 2013 - 06:57 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?

#11 Jon Isaacs

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Posted 15 October 2013 - 07:02 AM

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

#12 nevy

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Posted 15 October 2013 - 08:43 AM

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

#13 Eddgie

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Posted 15 October 2013 - 08:47 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...

#14 Achernar

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Posted 15 October 2013 - 08:48 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

#15 precaud

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Posted 15 October 2013 - 09:01 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. :)

#16 Daniel Guzas

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Posted 15 October 2013 - 11:33 AM

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...

#17 Scanning4Comets

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Posted 15 October 2013 - 12:04 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,

#18 Jim Romanski

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Posted 15 October 2013 - 03:05 PM

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.

#19 Daniel Guzas

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Posted 15 October 2013 - 03:58 PM

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..

#20 faackanders2

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Posted 16 October 2013 - 08:55 PM

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.

#21 JimMo

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Posted 17 October 2013 - 04:19 PM

Hi Ken,

Hmmm, a PC2 corrects all of my eyepieces and the stars are pinpoint to the edge, even in a 21 Ethos although I do notice a bit of my eye's slight astigmatism. I don't wear my glasses at the eyepiece, but if I put a .75 Dioptrx lens in front the astigmatism vanishes, though then I can't see the whole field of view.

#22 Starman1

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Posted 17 October 2013 - 05:22 PM

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 complete elimination...

Ed,
The spot diagrams show that coma is not zero and they show that spherical aberration is present (otherwise the center of the image (on the left) would read zero). But they also show that, in a 40mm wide field stop, coma is contained inside the Airy Disc. That means that, visually, there is no coma in a 40mm wide field. I'd call that more than improvement--I'd call that elimination. That there is coma inside the Airy disc is irrelevant.

#23 Starman1

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Posted 17 October 2013 - 05:40 PM

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..


Daniel,
If the spacing between the coma-correcting lens and the eyepiece's focal plane is not correct, coma will not be eliminated.
You can pull the eyepiece out of the barrel a couple millimeters at a time, refocus, and examine the edge. If it's getting better with each increase in the distance, then you can add barrel extenders to the eyepiece to allow you to safely pull it out even farther, or add spacers between the correcting lens and the insertion barrel. Whichever way you feel comfortable increasing the distance between the eyepiece and the lens.

You will know when you hit the right point, because coma will become inconsequential.

But let's say that coma grows WORSE as the eyepiece is pulled away, and you need to get closer to the lens. In that case, dispense with the barrel the eyepiece inserts in and use barrel extenders to space out the coma correcting lens from the eyepiece without the top barrel of the coma corrector (this is how the Baader MPCC is attached, for instance).

Barrel spacers are available in 14mm, and 28mm from Baader (called the Fine Tuning rings), TeleVue (their 30.5mm barrel extender) and from several other sellers in other lengths.

Now, let's say you've found the correct distance between the lens and the eyepiece to correct coma. How do you adjust your other eyepieces? Easy. Without touching your focuser, insert the second eyepiece and pull it out of the corrector until it's in focus. THAT will be the correct setting for that eyepiece. And if it's a 1.25" eyepiece? Same thing. Remember than 1.25" to 2" adapters exist that drop the eyepiece closer to the lens by 3/4", 1/2", 1/8" or raise it out of the corrector by 0", 1/4", 3/8", 1/2" So a choice of adapter might allow the 1.25" eyepieces to focus at the same point as a 2" when fully inserted. In fact, when all the adjusting for each eyepiece is done, you will discover that now all your eyepieces are parfocal and you only need a couple millimeters of adjustment on your focuser.

Which is why having a little box filled with parfocalizing rings makes sense. You can "lock" the eyepiece into a particular position so that, when switching eyepieces, you do almost no focusing at all. Orion sells some thick ones, Scope stuff sells some thin ones.

Your 24mm eyepiece does have a trace of astigmatism at the edge, but though it will prevent the star images at the edge from being the tiny little points they are in the center, it will be minor and quite tolerable compared to the amount of coma in an f/4.5 scope.

#24 calypsob

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Posted 18 October 2013 - 11:57 PM

What I did not even know a coma corrector would work with EP's. Field flatteners and coma correctors are usually designed for a specific aperture range, the hotech FF is for F6-F8 refractors I do believe. A coma corrector would be made for faster apertures, I guess you could get lucky on an EP and happen to get flat stars but it is really up to the eyepiece manufacturer to add a field correcting element to their lens array.

#25 Starman1

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Posted 19 October 2013 - 12:50 AM

What I did not even know [is that] a coma corrector would work with EP's. Field flatteners and coma correctors are usually designed for a specific aperture [sic] range, the Hotech FF is for F6-F8 refractors I do believe. A coma corrector would be made for faster apertures [sic], I guess you could get lucky on an EP and happen to get flat stars but it is really up to the eyepiece manufacturer to add a field correcting element to their lens array.


Well, coma is a linear aberration of paraboloidal optics. Not the eyepieces.
So if you don't correct the coma before it gets to the eyepiece, the eyepiece shows it.
If you correct coma before it gets to the eyepiece, the eyepiece doesn't show it.

Eyepieces are typically fully corrected for coma, though they do not correct the coma from the mirror.

You are correct that a coma corrector's correction is related to an f/ratio, but longer f/ratios are also corrected by it. So, though the TeleVue Paracorr II, for example, corrects visible coma completely down to f/3.5 (coma is confined to smaller than the Airy disc out to a field diameter of at least 40mm), it will also do so for slower f/ratios.

Photographic images used to be less sensitive to coma than visual observations because the chip was a lot smaller than the visual field and the star images a lot larger than the visual ones. Now, with large chip cameras and high-resolution (small pixel) cameras, those days are gone. Now, both visual and photographic needs are about the same.






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