Coma Correction for visual at f5

[A McEwan]

Hi,

I'm looking for opinions on a coma corrector for visual use in a GSO f5 Newtonian. I know some introduce SA and others affect the focal length of the scope.

I'm looking for best overall performance at eliminating coma.

With thanks,

Ant

[AlexB67]

I'd be interested to hear also for future reference. I was told the paracorr is the business, but then having checked, they're not cheap. Something worth watching  out for second hand I think however.  I've seen much cheaper, but I suppose as with all things, not as good  but I wonder if any half decent budget ones exist that work well enough.  Reading around seems to me also that the paracorr is flexible in that it works well over a wide f ratio, that could be handy for the future.

[John]

I've not found that I needed one. My last newt was F/4.8 and my current one is F/5.3. The F/4.8 did show coma but I don't recall finding it unacceptable despite my fondness for 100 degree EP's. It's only visible at F/5.3 right out by the field stop in my 20mm 100 degree Explore Scientific eyepiece.

I'm really not keen on astigmatism, hence my liking of Tele Vue's, but I either don't notice coma so much or I don't find it so bothersome.

Doesn't answer your question though Ant. Sorry about that 

The Paracorr always seems to get rave reviews but I don't seem to read much about the Baader, Skywatcher and other branded coma correctors.

[Scooot]

I'm using a GSO coma corrector set up like this with my nagler. My scopes f4.8, I do notice coma if I use the nagler without it, but don't really notice it much with the Delos, a little bit perhaps with 17.3mm. Getting the correct spacing between the corrector and the focal plane is important for it to work well. To be honest with all the messing around trying to get a spacer, the adapter etc I wish I'd bought a paracorr in the first place. I still might as I've got a nagging feeling the paracorr will give me better views.

[estwing]

I bought that paracorr 2 to use in my F3.9 a must in a scope  this fast but I've yet to use it in the F4.8...didn't notice a great deal of it in this one, it wasn't as bad as you'd think...I wouldn't spend too much on correcting coma at F5,unless you intend to get a faster scope later on

[Scott]

I bought that paracorr 2 to use in my F3.9 a must in a scope  this fast but I've yet to use it in the F4.8...didn't notice a great deal of it in this one, it wasn't as bad as you'd think...I wouldn't spend too much on correcting coma at F5,unless you intend to get a faster scope later on

Dob Snob.......hahahaha, sorry mate, couldn't resist. Have you had much of a chance to get the big gun out since getting/making the shroud? been pretty lousy this side of stoke. only been out imaging for about four or five hrs one night since Hersty. that and a few hrs visual here and there. 

[scarp15]

At around F5, whilst a bit of coma may be present around the edges,  more so at low power, you have to be looking for it to really notice. It just isn't  so bothersome. At least I found that to be the case, with my then F4.9 scope. My current scope at F4.5 may also not be so much of an issue (for myself), although this is most probably the point in which you reach the line, with divided opinion either way.

[Scooot]

Although mostly clear its a bit murky so I thought I'd go out and test how much coma I get. I only tested the 26mm which gives a 1.67 TFOV.

First without the corrector I centred on Altair, then gradually moved it to the edge. I was quite surprised at how quickly the sea gull shape begun to show itself. I'd say by the time I'd got half way it was very apparent and becoming progressively more apparent as I got nearer the edge.

With the corrector it just didn't happen, almost right on the edge it had held its shape. I was obviously looking for it but now I know how improved the corrector view is I would not be happy using this eyepiece without it.

I take others viewpoints, it is a matter of personal preference and how it bothers you or not.

[A McEwan]

Thanks for the replies. I'm actually just in from observing and had a few looks through a 6" f6 Newtonian. We were using Naglers, and at one point my 9mm 100-degree ep. I didn't really notice any coma so I'll maybe just see how I get on without a corrector until I see if I need one or not.

That is, IF I get an f5 Newt...

Ant

[jam1e1]

I did a write up while back on paracorr 2, got mine 2nd hand I use every time in pds 250, I'm keeping it, but as others say if you don't notice or bothered by it don't buy, if you do expect results then paracorr wins for usability and results in my opinion. I spent couple hours comparing with and without on couple of targets, results best seen on open clusters

[John]

Although mostly clear its a bit murky so I thought I'd go out and test how much coma I get. I only tested the 26mm which gives a 1.67 TFOV.

First without the corrector I centred on Altair, then gradually moved it to the edge. I was quite surprised at how quickly the sea gull shape begun to show itself. I'd say by the time I'd got half way it was very apparent and becoming progressively more apparent as I got nearer the edge.

With the corrector it just didn't happen, almost right on the edge it had held its shape. I was obviously looking for it but now I know how improved the corrector view is I would not be happy using this eyepiece without it.

I take others viewpoints, it is a matter of personal preference and how it bothers you or not.

I thought coma revealed itself as comet shaped stars with their "tails" pointing away from the center of the FoV ?. I always associated "seagulls" with astigmatism.

[crashtestdummy]

I have a parracor 2 in my 16" f4.5 and tbh I hardly use it.the coma in my scope isn't that bad really and as I mainly look for galaxies I pretty much just look around the centre anyway.im only actually keeping it because im looking at a 22" f3.6 next and I wont be able to get by without one

[AlexB67]

Now there is a man with aperture fever, already got enough of that 16 inch have we ? did you not get it too long ago  ?? 

[crashtestdummy]

yeah ive had it about 3 or 4 months but it was only intended as a stop gap anyway as I had planned an 18"-20" self build but decided to go all out on a 22" im sure that will be big enough as they start to get really silly above that size.

im my old f5 12" I didn't bother with a coma corrector as I couldn't really tell if there was any or not

[Scooot]

I thought coma revealed itself as comet shaped stars with their "tails" pointing away from the center of the FoV ?. I always associated "seagulls" with astigmatism.

Sorry wrong terminology, they were comets

[AlexB67]

yeah ive had it about 3 or 4 months but it was only intended as a stop gap anyway as I had planned an 18"-20" self build but decided to go all out on a 22" im sure that will be big enough as they start to get really silly above that size.

im my old f5 12" I didn't bother with a coma corrector as I couldn't really tell if there was any or not

No doubt the 22 inch will give some cracking views, I'd like to hear a first light on that

[Moonshane]

a coma corrector will improve the view but it's possibly not worth it at f5, I never felt I needed one. I use it all the time in my f4 scopes though (a paracorr - did I buy this off you?? - it was certainly someone on SGL). paracorrs are not the only option but are certainly the most convenient and user friendly.

[AlexB67]

I thought coma revealed itself as comet shaped stars with their "tails" pointing away from the center of the FoV ?. I always associated "seagulls" with astigmatism.

Quite educational to see the combined effects of astigmatism and coma or just one or the other.  You can have a play around with a star in that app, so you can basically see how the shape is affected 

http://aberrator.astronomy.net/html/mdibeta.html

Old software but runs fine for me on win 64.

[John]

I found this often linked website gives a good explanation:

http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

It's not always easy to interpret what you are actually seeing. Apart from optical issues relating to the scope and eyepiece optics, you have those created by the seeing conditions as well. 

[Moonshane]

plus of course, those aberrations created by the eye

[SimonfromSussex]

I'm using a GSO coma corrector set up like this with my nagler. My scopes f4.8, I do notice coma if I use the nagler without it, but don't really notice it much with the Delos, a little bit perhaps with 17.3mm. Getting the correct spacing between the corrector and the focal plane is important for it to work well. To be honest with all the messing around trying to get a spacer, the adapter etc I wish I'd bought a paracorr in the first place. I still might as I've got a nagging feeling the paracorr will give me better views.

image.jpg

This is really interesting Richard, I've been having a play with a GSO coma corrector recently and struggling to get it to work.

In my f4.5 lightbridge coma hasn't been too much of a problem but a recent acquisition of a 100deg ethos has changed that unfortunately, coma is still not bad but I'd like to sort it if poss to really appreciate the full view.

I tried just screwing the coma corrector onto the bottom of the eyepiece but I got a strange lop sided correction (actually worse than without the corrector) I did wonder if spacing was critical?

Please can I ask what adaptor are you using to get your spacing correct?

[Scooot]

This is really interesting Richard, I've been having a play with a GSO coma corrector recently and struggling to get it to work.

In my f4.5 lightbridge coma hasn't been too much of a problem but a recent acquisition of a 100deg ethos has changed that unfortunately, coma is still not bad but I'd like to sort it if poss to really appreciate the full view.

I tried just screwing the coma corrector onto the bottom of the eyepiece but I got a strange lop sided correction (actually worse than without the corrector) I did wonder if spacing was critical?

Please can I ask what adaptor are you using to get your spacing correct?

This is an interesting thread, it also explains the spacing.

http://stargazerslounge.com/topic/158529-astro-techaltair-astrogso-coma-corrector-and-user-guide/

I m using the Altair Astro 35mm self centre adapter

http://www.altairastro.com/product.php?productid=16412&cat=314&page=1

The total length is 68mm, my nagler sits about 2mm above the barrel,( there's about 2mm of silver visible when the eyepiece is in the adapter). My field stop is a little bit inside the eyepiece. According to the televue spec page the field stop for the Ethos is further in so I don't think you'd need as much spacing as me.

Hope this helps.

[SimonfromSussex]

That's excellent Richard, thank you so much!!

[Luke]

I use a Paracorr 2 with my 10 inch F5 GSO dob. I got the Paracorr mainly for my F4.4 dob and was surprised at how much it tidied up the view on the F5, so that was a very nice bonus.

I find the improvement enough that I always put the Paracorr on, but other people's mileage may vary. I think it's not just that it tidies the outer edge, it also seems to tighten up the detail across a lot of the field of view.

The price is the downside, it cost about 2/3 the price of my dob!

[Scooot]

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.