Coma

[thing]

I tried a new eyepiece on my 8" Newtonian this evening, a 7.5 mil giving me x133. The old 'comet tails' were evident even in the centre of vision, double stars were hard to split etc. If I put say a x50 (20 mil) eyepiece in it looks better. Does coma show itself more at high mags or could it be the eyepiece? I've ordered a Baader laser collimator, does anyone have experience of these? By the way, it's an F5 scope.

[barkis]

I can understand a bit of coma at the edge of the field Thing, but to have it in the central region as well sounds ominous to me.

I would assume collimation is spot on, ie, all the optical elements are properly adjusted .

If you have another eyepiece of similar focal length to the 7.5mm, and the coma is non existent in that, then You may have a duff eyepiece for whatever reason. Coma across the entire field sounds odd to me. I would hang on for some of the other guys giving their opinion before you take any action.

Ron.

[GazOC]

"Coma" across the field of view can mean the secondary is pinched in its holder.

[thing]

I just took the scope inside to check it out and the primary looks absolutely manky. It has craze lines across it, it's really dull, I can hardly see my reflection in it. I thought it might just be dew but it's been inside an hour or so now an it looks exactly the same. I've only had the scope a fortnight, what do you think this is? The mirror was perfectly clear when I bought it. I'm worried now... :shock:

[supernova]

I have a Baader and it is spot on for colli but do a star check after it will not be far out.

[James]

I've only had the scope a fortnight, what do you think this is? The mirror was perfectly clear when I bought it. I'm worried now... :shock:

I'd guess its dew - which can take a long time to dry out. When you bought your scope inside to a warm house it would have just added more dew. Its unlikely that your mirror has gone to pot in just too weeks (unless you've left it exposed to lots of dust!) so I'd not worry about it, get some sleep and see how it looks tomorrow.

James

[thing]

I gave it some gentle air with a hair dryer an it went after about 15 minutes, so it must have been dew. I suppose the mirror holds onto the cold more than the tube does. By the way, the mirror has some small black specs on it, it's had those from new, will they make much of a difference or do you think it's a not very good mirror? I'm also wondering if the coma I saw earlier was caused by the dew on the mirror.

[James]

Your mirror will have to look pretty grim for it to make any noticeable effect on the views you're getting. It can be offputting to see things on your mirror (especially a new one) but I doubt its anything to worry about - mine had the same. Whatever you do, don't go shining a toch at your main mirror when you are out as this will make the mirror look really bad when it isn't. Of course now that I've told you not to you're going to do it anyway?

Dew on the mirror (or on the secondary which is what I have ocassionally had) can lead to all kinds of wierd visual issues but sometimes you can observe away for ages and only realise afterwards you were looking through dew... :?

James

[barkis]

If you have a protective cover for your scope, be careful where you put it when you take it off to start your observing. Make sure it does not pick anything up when you come to place it back on the scope. Any dirt sticking to it, can easily drop off and land on the mirror.

If the mirror has condenstion on it at the end of your nights work, don't put the cover on until the condensation has cleared from the mirror.

Ron.

[thing]

Thanks for the advice guys.