GSO Coma Corrector anyone?

[Barry Fitz-Gerald]

Has anyone had experience of the GSO Coma Corrector used for visual in a SW 250 F4.7 NEWTONIAN or similar - or maybe even the Stella Lyra one offered by FLO which I guess is the same or similar?

Is a Coma Corrector worth shelling out for - or is the visual improvement marginal? 

I would get a Paracorr but I would have to sell the telescope to pay for it.

 

 

[Sargares]

I’ve got one for the exact same scope. 
 

it needs some simple modifications to make it work (spacer) and then you ideally need to set the coma corrector with your most info is eyepiece and then parfocalise the rest (or just focus them in the coma corrector by sliding them out untill they focus. All easier done than it sounds) 

 

personally I don’t use it full stop with higher magnification and I can take it or leave it on low power. I can see the coma without it in for sure but it doesn’t annoy me enough that I can’t get by. If I’m having a proper session I will use it but if it’s just a spur of the moment thing I usually don’t bother. 
 

personal taste. if the coma really annoys you then it’ll get more use, it’s really not the hard to use once you’ve done the small amount of set up to make it work though. 

[Barry Fitz-Gerald]

Thanks for that info, That is very useful for me - if it is not providing a dramatic improvement I think I can live with the coma as well - especially as I do a lot of spur of the moment viewing and seldom go for tight doubles and so on.

Another factor that I never considered was my own deteriorating eyesight contributing to the less than pin point stars - I should have realized this much earlier but never factored it in.

Last night I experimented with a pair of new specs to correct for astigmatism and the difference was noticeable - less spiky stars and sharper focus - so it looks like I was the weak link in the optical train al along. So maybe a TeleVue Dioptrix might be a wiser investment instead.

 

[Paz]

On 12/10/2021 at 14:26, Barry Fitz-Gerald said:

Thanks for that info, That is very useful for me - if it is not providing a dramatic improvement I think I can live with the coma as well - especially as I do a lot of spur of the moment viewing and seldom go for tight doubles and so on.

Another factor that I never considered was my own deteriorating eyesight contributing to the less than pin point stars - I should have realized this much earlier but never factored it in.

Last night I experimented with a pair of new specs to correct for astigmatism and the difference was noticeable - less spiky stars and sharper focus - so it looks like I was the weak link in the optical train al along. So maybe a TeleVue Dioptrix might be a wiser investment instead.

 

When I got my first ever scope I thought something was wrong and after some research I thought it had astigmatism. But after further research and testing I worked out it was my eyes not the scope. That was the first I realised I had astigmatism!

I have a televue dioptryx, and the views are great and the optical quality is better than my specs but there is a  challenge with it in keeping the dioptryx aligned correctly if you are rotating your head  compared to the eyepiece. This is easy if you are observing a bright star as you can tell when it's right, but if you are looking at something faint or extended it's not so easy.

Putting an alignment marker on the dioptryx can help but then seeing that in the dark becomes the challenge.

Edited October 13, 2021 by Paz

[Mike Q]

I tested mine out this evening.  Just dropped it in the focuser and slipped in an eyepeice.  Yes it is worth it.  No doubt.  I have a 10 inch F5 dob and while my eyepieces are good there is a bit of coma out around the edges....not anymore.  It works as advertised.

[cajen2]

8 hours ago, Mike Q said:

I tested mine out this evening.  Just dropped it in the focuser and slipped in an eyepeice.  Yes it is worth it.  No doubt.  I have a 10 inch F5 dob and while my eyepieces are good there is a bit of coma out around the edges....not anymore.  It works as advertised.

Do you need any form of spacer or extension with it, Mike?

[Mike Q]

2 minutes ago, cajen2 said:

Do you need any form of spacer or extension with it, Mike?

Not at all.  I just dropped it into the focuser and slipped in the eyepiece.  I have heard that you do, but I didn't need any with my 10 in F5 dob.  

[Mike Q]

I picked the Orion Nebula as my target using the coma corrector and tried all three of my Q70s 2 inch eyepieces.  Focusing on the stars in the middle I found that the stars at the edge of the field were much sharper then before.  Then slapped in the big 2 inch 2 x Barlow....great image.  Then the big test.  You know the eyepieces they give you with the scope?  The ones that barely qualify as eyepeices at all....it made them usable.  So for 129 dollars US....it might be the best deal out there for the money.

[Zermelo]

47 minutes ago, Mike Q said:

Then the big test.  You know the eyepieces they give you with the scope?  The ones that barely qualify as eyepeices at all....it made them usable.

This intrigues me Mike. I've not bothered with a coma corrector yet as I'm visual only and I wasn't sure how much difference it would make. I do have an F/5 Newtonian, so I assumed that a degree of coma was inevitably baked into the view, at least towards the edge of field. So I'm wondering how a device for correcting coma in the primary can improve the view in a cheaper eyepiece.
I expect a stock eyepiece to suffer more from defects introduced within the eyepiece, like astigmatism. But if the coma comes from the mirror, isn't it present whichever eyepiece is being used? Or conversely, if it is true that a more expensive eyepiece already corrects well for the mirror's coma, then won't the addition of a coma corrector cause a deterioration?

[Mr Spock]

I have a 12" f5. After viewing with a 22mm LVW, x69 and 0.94°, I decided coma was minimal and acceptable. It may be a problem with 82° - 100° eyepieces, but mine is just fine.

[Zermelo]

7 minutes ago, Mr Spock said:

I have a 12" f5. After viewing with a 22mm LVW, x69 and 0.94°, I decided coma was minimal and acceptable. It may be a problem with 82° - 100° eyepieces, but mine is just fine.

Yes, I've certainly noticed myself that I see more edge defects with 82 degree EPs than my 60 degree Starguiders. That may or may not be coma I'm seeing. But the stock EPs are usually no more than 50 degrees AFOV, so I'd expect even less of an issue from coma.

 

[Mike Q]

32 minutes ago, Zermelo said:

This intrigues me Mike. I've not bothered with a coma corrector yet as I'm visual only and I wasn't sure how much difference it would make. I do have an F/5 Newtonian, so I assumed that a degree of coma was inevitably baked into the view, at least towards the edge of field. So I'm wondering how a device for correcting coma in the primary can improve the view in a cheaper eyepiece.
I expect a stock eyepiece to suffer more from defects introduced within the eyepiece, like astigmatism. But if the coma comes from the mirror, isn't it present whichever eyepiece is being used? Or conversely, if it is true that a more expensive eyepiece already corrects well for the mirror's coma, then won't the addition of a coma corrector cause a deterioration?

I don't know enough to explain it.  All I can do is report what I have seen.  There have been reports, either on here or CN, of the same findings.  At any rate I believe it was 129 USD well spent.  At least for me it was.  I have a friend that is ordering one for his 16 inch Orion and he uses a lot better eyepieces then me and eventually I will here from him and I will report his findings

[Zermelo]

3 minutes ago, Mike Q said:

I don't know enough to explain it.  All I can do is report what I have seen.  There have been reports, either on here or CN, of the same findings.  At any rate I believe it was 129 USD well spent.  At least for me it was.  I have a friend that is ordering one for his 16 inch Orion and he uses a lot better eyepieces then me and eventually I will here from him and I will report his findings

Yes, and thanks for report Mike - my question was directed at anyone who might be able to correct the gap in my knowledge!

 

 

[Elp]

I did a brief comparison here regarding imaging with and without one:

Note this was done with the old style (long) GSO coma corrector which is generally discontinued now. For visual I didn't really feel there was much difference.

 

 

Edited March 17, 2022 by Elp

[Barry Fitz-Gerald]

Hmmmm, sounds like getting one of these for visual use only might be quite a subjective experience, might make a big difference, but might not depending on individual taste/perception. Bit like the CA in the SW Startravel 120, some find it a problem, I barely notice it as I avoid anything bright and shiny when I get it out.

Anyway, all the above comments are very useful, and I may give the Stellalyra one from FLO a punt as it looks like a GSO clone and reasonably priced.

[Mike Q]

I took my brand new GSO coma corrector out last night for the first time.  I used them in my 28, 32 and 38 Q70s.  It seems to work as advertised.  Then I tossed in the cheapo 30mm that came with the scope and it definitely improved it's performance.  For 129 USD it's a steal.

[Jules Tohpipi]

On 17/03/2022 at 21:03, Barry Fitz-Gerald said:

Hmmmm, sounds like getting one of these for visual use only might be quite a subjective experience, might make a big difference, but might not depending on individual taste/perception. Bit like the CA in the SW Startravel 120, some find it a problem, I barely notice it as I avoid anything bright and shiny when I get it out.

Anyway, all the above comments are very useful, and I may give the Stellalyra one from FLO a punt as it looks like a GSO clone and reasonably priced.

 

Was reading the thread with interest and wondered if you had given the coma corrector a go?

I have a SW 250 F4.7 Flextube Dobsonian and am cursed by being a sensitive soul to coma. A Parcorr is above budget but I see FLO sell the StellaLyra for £99 - which appears to be the same as the GSO CC.

Silly question, I assume it's OK to use a 1.25" adapter and 1.25" eyepieces with the GSO? I don't have any 2" EPs.

Anyone using the GSO/StellaLyra with the SW250 F4.7 have to use some spacers or just drop it in?  I don't mind a bit of fiddling around if it works.

 

 

[Mike Q]

3 minutes ago, Jules Tohpipi said:

 

Was reading the thread with interest and wondered if you had given the coma corrector a go?

I have a SW 250 F4.7 Flextube Dobsonian and am cursed by being a sensitive soul to coma. A Parcorr is above budget but I see FLO sell the StellaLyra for £99 - which appears to be the same as the GSO CC.

Silly question, I assume it's OK to use a 1.25" adapter and 1.25" eyepieces with the GSO? I don't have any 2" EPs.

Anyone using the GSO/StellaLyra with the SW250 F4.7 have to use some spacers or just drop it in?  I don't mind a bit of fiddling around if it works.

 

 

I run the GSO coma corrector in a 10 inch F5, which I think is 250mm.  You see a lot of people adding a spacer to the GSO corrector, I personally don't do that and don't notice any ill effects.  I have never tried it with the 1.25 inch stuff as that is all high power planetary and I have never noticed coma with those 

[Barry Fitz-Gerald]

1 hour ago, Jules Tohpipi said:

Was reading the thread with interest and wondered if you had given the coma corrector a go?

I have a SW 250 F4.7 Flextube Dobsonian and am cursed by being a sensitive soul to coma. A Parcorr is above budget but I see FLO sell the StellaLyra for £99 - which appears to be the same as the GSO CC.

Silly question, I assume it's OK to use a 1.25" adapter and 1.25" eyepieces with the GSO? I don't have any 2" EPs.

Anyone using the GSO/StellaLyra with the SW250 F4.7 have to use some spacers or just drop it in?  I don't mind a bit of fiddling around if it works.

 

Well so far - no I have not taken the plunge. I mainly use TV Delos E/P's and to be honest coma is not something that has bothered me sufficiently to shell out on a CC - maybe if I spend more time using the Newtonian I might re-consider. Until then I will just concentrate on the center of the FOV and resist the temptation to look at the edges!

[Jules Tohpipi]

Thanks guys for the replies. I think will give one a go - that is, once the nights darken again and there's chance to try it properly within the returns window. But I've got high hopes for it.

I use the dob for a bit of everything and the coma is not intrusive on high power planetary or faint fuzzies. But it proper gets on my nerves for wide field star gazing. For £99 it would be worth it for that alone and anything it adds on top would be a bonus.

 

Edited July 17, 2022 by Jules Tohpipi

[bosun21]

On 12/10/2021 at 14:26, Barry Fitz-Gerald said:

and seldom go for tight doubles and so on.

You can still go for all the tight doubles you want, just place them in or near the centre field. The coma is only a problem for visual at the edges.

[Louis D]

I use a GSO CC in my f/6 scope with a 25mm spacer ring.  It nicely cleans up the coma in my widest true field eyepieces and in my UWA eyepieces like my ES-92s.

[TheLookingGlass]

I've owned two GSO coma correctors. It def helped clear up coma and some field curvature, but at the expense of very noticeable chromatic aberration on bright stars at higher powers. I also noticed on axis that planets are sharper without one. I prefer to not use one at all now.  I now use a 22mm Ultima LX, 12.5mm, 9mm and 6.5mm Baader Morpheus and all four are fine without the coma corrector. One less piece to add in between for me now.

A good observing friend of mine said his Televue Paracorr, (newer type I believe), does not have any of these aberrations. Very expensive though!

If you don't mind the above aberrations, the GSO coma corrector does fix up the outer edges nicely and the cost isn't too high. It all depends on what you can tolerate.

Edited September 5, 2022 by TheLookingGlass

[Louis D]

On 05/09/2022 at 01:41, TheLookingGlass said:

but at the expense of very noticeable chromatic aberration on bright stars at higher powers

Probably due to spherical aberration.  I remove it for planetary observations as well.