[ngcles]

Hi Stephen & All,

To echo what Satchmo said, coma is inherent in any optical system that contains a paraboloid. That includes classical Cassegrainians and Schmidt Cassegrainians as well. Though it is true that S/Cs use spherical mirrors, the corrector plate on the front of the 'scope makes the primary mirror "look like" a paraboloid to the secondary mirror -- so in effect it is a paraboloid.

Coma is generally more noticeable in a Newtonian because most Newtonians these days are fast systems (ie f/4 to f/6 rather than f/10 - 15 like most Cassegrainan systems). All proper Newtonian telescopes have coma. The faster (shorter f/ratio) the telescope, the more coma it produces. ie the coma-free field becomes smaller and smaller in angular size. The further out from the optical axis, the more coma intrudes.

As Satchmo says, most of the effect you are seeing in your 10" and its non-symetrical out-of-round stars is caused by lateral astigmatism and some field curvature. You will notice it is much more pronounced the further the star is from the centre of the field -- yes? Once you remove those effects (and you can kill most of it with Nagler-type exotic wide-field eyepieces), most of the coma remains and can be seen nearer the edge of the field with non-round star images (now much less pronounced).

A coma-corrector like a Paracorr (short for parabola-corrector) will (essentially) eliminate the coma then producing round small stars all the way to the edge of the field.

Stephen wrote:

"So a Televue Paracor .... what, fits over the eyepieces I have?
And a Televue Nagler eyepiece is ... an eyepiece?"

A paracorr looks like a barlow lens but works differently. Slip it into the focuser and then put the eyepiece in on top. There are several brands of coma correctors. The Paracorr is marketed by Televue.

Yes a Nagler is an eyepeice design marketed by Televue. It is a wide-field exotic (ie expensive) but you do get what you pay for. There are other brands that also have similar properties and produce excellent results like the Pentaxes.

When you finally do get around to doing that 24" mirror, as Satchmo suggested, keep the f/stop short (ie <f/4.5) get some good eyepieces and a coma corrector. It will be so, so much easier and in the end cheaper than building and lugging around a 24" f/5 - f/7 that will need a 10 foot lader, will suffer image degridation due to tube sag and weigh a ton, be harder to design and fabricate etc etc.

I own an 18" f/4.9. I don't find the coma particularly objectionable, though I know many people do find it so. I do own many (older) Nagler eyepieces and I love them for the above reasons. I can't imagine observing with an f/4.9 'scope without them.

Best of luck with it,

Les D