Primary mirror surface

[george7378]

Hi everyone,

I have just got in after a small observing session - my dark skies at home and the 10" showed me a shadow transit of Io, and (I think) 2 dust lanes in the Andromeda Galaxy, and nebulosity around the Pleiades. When I got in, I had a little look at the primary mirror, and I found that when it was pointing directly at a pretty bright light, I could see a strange 'swirling' pattern across its surface. I'm sorry I can't describe it better, but that's what I think I saw. It obviously has no impact on observations as it was a crystal clear view, and when the mirror is not pointing at the bright light the surface looks perfectly silver, but what do you think - is it a film of some kind, or just how the mirror is made?

Thanks.

[Chris H]

When I shine a light at my mirror it looks exactly the same, too me it looks as if there was some protective film on it which has now been removed and only the swirling marks remain. It has no effect on viewing and I am happy to leave it well alone.

[NGC 1502]

If you had just brought your scope indoors after an observing session,

then what you saw could have been moisture forming on the cold mirror.

Just let it dry naturally.

Regards, Ed.

[barkis]

The mirror has a figure of revolution polished into it's surface.

This figures is designed to bring parallel light from Infinity to a focus If incident light hits it, you may perceive the sort of effect you saw. Certainly not a flaw. If you were to place an artificial starlike point at your mirrors radius of curvature, (Twice it's focal length) and intercept the returning beam with a knife edge, and move it back and forth slowly, you would see some wonderful shadow patterns dancing across the mirror.

If you follow the thread by Glasspusher on mirror making, and any of the guys who are engaged in this, may use a Foucault Tester to test their mirrors, they will no doubt express their thoughts on the shadows that show during testing. The classic one being the shadow of the Parabola itself, or the Doughnut as it is usually called.

Ron.

[perrin6]

Nah - what you have there is a 'star imp' which you've accidentally captured. They get confused by the curvature of the mirror - confuses their navigation rather like moths around a lightbulb. Very rare. Next time you're outside with the scope (at night because they sleep during the day) just shine a red torch at the mirror making circular motions with your wrist and the little chap will fly back out and onto it's interplanetary migration route.

Alan

[Astro_Baby]

I see that in my Nexstar 4SE mirror when its under a powerful tungsten beam - it looks almost like oil on water type effect.

Its probably what John said above or an effect caused by the coatings on the mirror. Either way its nothing to worry about.