Coma Corrector

[MadAlbert]

Still considering myself a novice I have now run into the effect known as coma.

Pretty certain I understand what's going on but not sure which corrector would suit for visual.

Any recommendations?

[garethmob]

theres the televue parracor but it is fairly expensive (its actually more expensive than most of my scopes together:P

theres also this one

http://www.firstlightoptics.com/coma-correctors/baader-mpcc.html

and this one http://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html but the SW seems designed for a specific scope

i have to get one for the coulter, as its F4 so im prob gonna go for the baader

[MadAlbert]

See what you mean about the Paracorr...

Baader could be the one, then. I assume this is a 2" fitting?

[garethmob]

i would assume so but im not 100% but it seems out of stock with FLO only just noticed:(

[steppenwolf]

Yes it is a 2" fitting.

[MadAlbert]

Many thanks

[Stephen]

For visual I would not recommend a coma corrector.

The CC will only work correctly when it is spaced 55mm from your EPs focal point.

Problem is each EP has a different focal point, so you will need some form of variable adjuster in between the CC and EP. You will then need to adjust it each time you change EP ......

A bothersome and time consuming task IMHO : enjoy the centre of the view and don't worry about the coma

[Phaser6]

Is this coma on an F5 200P? Is this normal as I don't see it on mine.

[MadAlbert]

...A bothersome and time consuming task IMHO : enjoy the centre of the view and don't worry about the coma

Hmmm, I did think this would lead to bother, particularly as most products would reference AP (i.e. rather static and specialised). The effect is only on the very wide angle views and very minimal; the effort and expense would not seem worth it.

[cs1cjc]

Coma is a feature of all newtonian telescopes, but faster scopes, with low f/ numbers are much more affected. I use the GSO corrector which is relatively tolerant of spacing, 75mm +/-10mm. It is available from Agena and Telescope-Services and Astronomics (under the Astro-Tech label). I know of no current Uk source. It works well.

[John]

I use a 10" F/4.8 newtonian by Orion Optics. It does show some coma, as all fast newtonians do, but I don't find it distracting for visual use.

With lower cost wide angle eyepieces in fast scopes you can't see the coma anyway as it tends to get hidden by the astigmatism that the eyepiece introduces

[MadAlbert]

Thanks, again

[Phaser6]

Damn I am seeing it now, now why did you have to go and bring that up eh? lol

[sgazer]

yeah, I think the coma correctors are more for imagining where the coma is really obvious. The Baader MPCC worked perfectly on my 150pds, it fits into a 2" focuser, but has a T-thread on the other end for screwing into a camera, so can't be used for visual.

[umadog]

No, coma correctors are not just for imaging. Hence the term visual Paracorr: http://www.optcorp.com/product.aspx?pid=15095

Personally, I use a coma corrector at f/4 and it only leaves the focuser for collimation and storage. The scope would be horrible without the corrector. Prior to that, I started using the corrector with an f/4.9 scope: I didn't realise how much coma was in the lower-power fields until it went away. Again, at f/4.9 the corrector never left the focuser.

It is not true that the Paracorr is a hassle to use. Yes, there's a tunable top for getting the distance right. In practice, however, this just optimises the view. You can just leave it set to the middle position all night and the views are just fine. Zero hassle.

John is right about the astigmatism introduced by eyepieces. Some wider angle eyepieces can have a lot of astigmatism in the outer part of the field and this can look a lot like coma to the untrained eye. Faster focal ratio telescopes exacerbate the effect. So if you want clean fields of view then you should either not stray below f/6, use well corrected Plossls (narrower apparent field so fewer aberrations), or buy good wide-angle eyepieces that are known to play well with fast scopes. Once you've made those choices, you should examine the views and decide if you still want a Paracorr (at >f/6 it would be unlikely).

[sgazer]

No, coma correctors are not just for imaging. Hence the term visual Paracorr: http://www.optcorp.com/product.aspx?pid=15095

Personally, I use a coma corrector at f/4 and it only leaves the focuser for collimation and storage. The scope would be horrible without the corrector. Prior to that, I started using the corrector with an f/4.9 scope: I didn't realise how much coma was in the lower-power fields until it went away. Again, at f/4.9 the corrector never left the focuser.

It is not true that the Paracorr is a hassle to use. Yes, there's a tunable top for getting the distance right. In practice, however, this just optimises the view. You can just leave it set to the middle position all night and the views are just fine. Zero hassle.

John is right about the astigmatism introduced by eyepieces. Some wider angle eyepieces can have a lot of astigmatism in the outer part of the field and this can look a lot like coma to the untrained eye. Faster focal ratio telescopes exacerbate the effect. So if you want clean fields of view then you should either not stray below f/6, use well corrected Plossls (narrower apparent field so fewer aberrations), or buy good wide-angle eyepieces that are known to play well with fast scopes. Once you've made those choices, you should examine the views and decide if you still want a Paracorr (at >f/6 it would be unlikely).

I can't see where anyone stated coma correctors are just used for imaging, nor about the Paracorr being used just for imaging, nor about the Paracorr being a hassle to use.

[Stu]

I seem to remember that Moonshane uses a Paracorr in his 16" dob, he will have some views on them for visual

Stu

[cs1cjc]

A reflector with a coma corrector is much more like a refractor, being sharp across the full field. The view through even cheap eyepieces can be much improved with my 38mm SWA being a good example.

[umadog]

Yes, and don't forget that coma is a problem at low powers. Beyond a certain magnification point, depending on your focal ratio, the coma will vanish anyway. Also, it's worth noting that astigmatic eyepieces perform better when barlowed because the light cone entering them is a lot shallower.