coma correctors?

[Ags]

I get the general principle of coma correctors, but not the practicalities. Is it possible to just insert the coma corrector into the drawtube and leave it there permanently for both visual and imaging? Or do you need to fiddle with it every time you change an eyepiece?

[RikM]

You need to match the spacing between the coma corrector and the focal plane above it. The MPCC needs 55mm (I think) The photo visual kit comes with a bunch of adaptors and whatnot to get the spacing right for eyepieces but you need to tweak it each time unless they are parfocal.

[Ags]

Sounds like a lot of bother, am I right?

[RikM]

I only ever use mine with a camera so I don't know really. Coma doesn't bother me visually all that much now that I have decent eyepieces.

[umadog]

I found a coma corrector really helped to clean up the views at f/5 so that open clusters looked significantly better. The corrector is pretty much obligatory at f/4. What the corrector doesn't do, however, is change the angle of the light cone by very much. This, of course, is what a Barlow does and it's what provides the magnification increase. Eyepieces that aren't designed with a steep light cone in mind (i.e. a fast scope) will tend to exhibit astigmatism and this can be very severe. So the coma corrector may clean up the coma but if the eyepiece you're using has a lot of astigmatism the view won't get much better. Good eyepieces will clean up astigmatism but won't help with coma. Although the Paracorr has a tunable top to set the optimum distance for each eyepiece, this tuning isn't massively important. Simply having the corrector in there will make a big difference. In practice, the coma corrector gets slotted into place and then you forget about it. I don't know about imaging, where you will want a field flattener also. I think some correctors will do both, but check.

[Moonshane]

I am visual only but my Paracorr works superbly and effortlessly on my 16" f4.

[MjrTom]

I am visual only but my Paracorr works superbly and effortlessly on my 16" f4.

This is where the more expensive Paracorr comes into play versus the Baader Coma corrector.

The Televue one in essence has a rotatable top which adjusts that all important distance that's required for the coma corrector to perform correctly. No messing about with adapters to change the spacing when changing eyepieces.

Not cheap but worth it from what I have seen of them.