Coma Correction for visual at f5

[John]

Having invested in eyepieces that are corrected well enough to give sharp star images right across the field even with fast scopes, it must be nice to actually get that now Richard 

[Scooot]

Having invested in eyepieces that are corrected well enough to give sharp star images right across the field even with fast scopes, it must be nice to actually get that now Richard 

It certainly is john. I'm becoming a great fan of the "whole view" and I find myself looking for other interesting aspects of the scene when viewing.

[estwing]

Fantastic scoot,now get a F3.6 20" dob!

[Scooot]

Fantastic scoot,now get a F3.6 20" dob!

One day maybe

[SimonfromSussex]

I bit the bullet, and decided to upgrade to a paracorr at kelling this weekend, encouraged by a 15pc discount. I was lucky to get a good 2 or 3 hours of observing with it Saturday night and the views in the fantastic sky were stunning. I didn't do a with/without comparison, I just made a point of looking at the whole FOV to see if there was any coma.

However, when I realised it was clear, I also scrambled out of my tent this morning at around 05.45, to have my first look at Jupiter and orion this year. I had a beautiful view of the planet and its moons, Europa and Ganeymede looked like a double star. What surprised me was Wasat on the edge of the FOV which was split nicely with my 10mm (120 mag). I'm very doubtful I would have seen this without the paracorr so improving the views with this eyepiece is an added bonus.

Would you say the paracorr is better than the GSO then Richard?

I've found a spacer in a box of old bits this weekend so I'm going to give my GSO a go with my ethos.

[Scooot]

Would you say the paracorr is better than the GSO then Richard?

I've found a spacer in a box of old bits this weekend so I'm going to give my GSO a go with my ethos.

It certainly is from a user friendly and flexibility viewpoint, I can just leave it in the focuser for all eyepieces. It's extremely well made and just smells quality. I originally wanted a corrector mainly for my nagler. The GSO corrected it but when my eye shifted in the eyepiece I used to get a little bit of shift in the focus of some stars, a bit like coming in and out of focus slightly. I used to wonder if it was just my eyes but I don't seem to get that with the paracorr, the view is equally good and stable across the field. It's a lot of money though so difficult to advise. I definitely had coma with the nagler, and everything now seems good with all the eyepieces, but I now have excellent optics throughout the optical chain, a tenth wave mirror etc so others might not see the same benefits. Best advice is to try one but I realise this might not be possible.

Next time its clear I'll see if I can do a side by side comparison, I was too wrapped up in the dark skies to do it over the weekend.

[mitchelln]

I just bought  a Baader MPCC MKIII to go with my Hyperion MKIII 8-24 Zoom, Hyperion 31mm Aspheric and SW 300P.

Tested it last night against M11, M13, M103 and NGC869. The results are fantastic. Coma is completely eliminated with no loss of field size or vignetting.

I discovered that the MPCC comes completely apart, so it then just screws straight onto the Hyperions with no adapter. There are always so many combinations possible with Baader gear.

[cs1cjc]

Spacing of a coma corrector for optical use is not very critical, but it does need to be roughly correct.  Televue in the past has said +/-4mm on the nominal 55mm (so +/-7%).  Roger Ceragioli, the designer of the GSO coma corrector optics has said that the spots look good from 65mm to 85mm (so +/-13% on the nominal 75mm back focus).  I have no idea whether the GSO design is actually more forgiving or whether its designer is more forgiving of good performance...

[SimonfromSussex]

It certainly is from a user friendly and flexibility viewpoint, I can just leave it in the focuser for all eyepieces. It's extremely well made and just smells quality. I originally wanted a corrector mainly for my nagler. The GSO corrected it but when my eye shifted in the eyepiece I used to get a little bit of shift in the focus of some stars, a bit like coming in and out of focus slightly. I used to wonder if it was just my eyes but I don't seem to get that with the paracorr, the view is equally good and stable across the field. It's a lot of money though so difficult to advise. I definitely had coma with the nagler, and everything now seems good with all the eyepieces, but I now have excellent optics throughout the optical chain, a tenth wave mirror etc so others might not see the same benefits. Best advice is to try one but I realise this might not be possible.

Next time its clear I'll see if I can do a side by side comparison, I was too wrapped up in the dark skies to do it over the weekend.

Thanks again Richard, I'd be very interested to hear how they compare side by side.

I'm going to have a play with the GSO and hopefully get it to work if the sky ever clears. We've had 3 teasing nights hear with patches of clear so you think it might be possible only for the mist to roll in!!!

My Lightbridge is f4.5 but I don't really notice any coma in my Delos eyepieces, it's really only for the Ethos, so a parracor is a big investment at the moment for just one eyepiece.

Having said this if the GSO isn't as good then it's something to save up for to perfect my chain too!!

[Scooot]

Thanks again Richard, I'd be very interested to hear how they compare side by side.

I'm going to have a play with the GSO and hopefully get it to work if the sky ever clears. We've had 3 teasing nights hear with patches of clear so you think it might be possible only for the mist to roll in!!!

My Lightbridge is f4.5 but I don't really notice any coma in my Delos eyepieces, it's really only for the Ethos, so a parracor is a big investment at the moment for just one eyepiece.

Having said this if the GSO isn't as good then it's something to save up for to perfect my chain too!!

Just had a good 2or 3 hours this evening to try and compare the two. The conditions weren't ideal. Only 3 nights from full moon, so the skies were quite bright. I'm not sure the seeing was that good either, I couldn't split the double double before I came in, even when I barlowed the 10mm. However I suppose it's the same for both the GSO coma corrector and the Paracorr. I only used the Nagler 26mm as this is the most prone to Coma and the spacing for the GSO is only set up for this eyepiece.

I first looked at the double cluster. I swapped the correctors backwards and forwards several times. It took me a little while to get the ideal focus each time as I also had to adjust the dioptrx. I couldn't really detect a noticeable difference in the views. The stars were just as clear without distortion in both correctors.

I then had a look at the ring nebula placed below centre on the right of the view, and Sheliak with its dart shaped cluster around it, above centre on the left. I looked hard at Sheliak and its stars, again there was no noticeable difference in the views. NU1 Lyra above and slightly right of sheliak in the eyepiece was clear with no coma in both. Again I switched back and forth a few times. Sometimes I thought maybe the paracorr was slightly better but there wasn't much in it.

One thing I did notice was that I had to move my head slightly to the side to see the edge of the FOV. When I did this it altered my eye sight angle and this would cause the star shape to distort. It did this with both correctors but it was definitely worse with the GSO. If I held my head steady and just turned my eyes to look at the edge there was no distortion.

So overall I was fairly impressed with the GSO. As mentioned I didn't have a dark sky and the seeing wasn't that good, whether someone could discern some differences in better conditions I don't know, but to my eyes tonight, there wasn't anything in it from a visual viewpoint.

It took me a lot of trial and error before tonight to get the spacing correct on the GSO. The paracorr is more flexible in that it can be used with all eyepieces.

Hope this helps Simon, as you already have the GSO and you only intend to use it with one eyepiece Its probably worth trying to get the spacing right and sticking with that. Under those circumstances I think I'd view the paracorr as a "nice to have luxury".

[SimonfromSussex]

Many thanks for the comparison Richard, very interesting read & could well save me some money, thank you!!!

I had a go with my GSO and 21 Ethos the other night and found it worked really well. Stars were nice and sharp across the field (I managed to find an old meade adapter like you were using to space the eyepiece further away from the corrector).

The only thing I did notice was it appeared to shrink the field of view slightly which was annoying. I'm wondering if this is part of the spacing issue? I.e. my spacing might be too big and hence the FOV is getting smaller?

[Scooot]

Many thanks for the comparison Richard, very interesting read & could well save me some money, thank you!!!

I had a go with my GSO and 21 Ethos the other night and found it worked really well. Stars were nice and sharp across the field (I managed to find an old meade adapter like you were using to space the eyepiece further away from the corrector).

The only thing I did notice was it appeared to shrink the field of view slightly which was annoying. I'm wondering if this is part of the spacing issue? I.e. my spacing might be too big and hence the FOV is getting smaller?

https://www.altairastro.com/product.php?productid=16363

I think this corrector is essentially the same one. The last question mentions it increases the focal ratio by 10pc so your observation is probably correct.

[SimonfromSussex]

Ah yes, thanks Richard, "In practise this is undetectable visually" lol, I thought it was very noticeable :-(

Does the paracorr affect the fov? Did you notice any change?

[Rustysplit]

I think I read that the paracorr acts as a 1.15X extender. The SW acts as a .9 reducer, very handy.

[Rustysplit]

Here you go..... televue.com/engine/TV3b_page.asp?id=61&Tab=_spec

[SimonfromSussex]

Thanks Alan, so, much the same as the GSO.

Please can I have a play with your SW next observing sesh?

[Rustysplit]

Of course chap! You supply the clear skies and I will supply the beers

[SimonfromSussex]

Thanks matey, looking at the forecast my job might be a bit harder!!!

[Scooot]

Ah yes, thanks Richard, "In practise this is undetectable visually" lol, I thought it was very noticeable :-(

Does the paracorr affect the fov? Did you notice any change?

Yes I think I did, but being only 5pc between the two correctors it wasn't perhaps as noticeable as a comparison with no corrector. Its a bit of a catch 22, a wider field with coma, or slightly narrower without! I like the wide field view but I don't like the coma so I'm going to stick with the Paracorr.

If I get a darker sky with decent seeing I might have another go at comparing them but I did have a brief comparison a week or so ago and couldn't tell the difference then so I don't really expect different results.