Re: [ATM] Ghost image

["Jerry" <wa4guu@verizon.net>]

I assume that is a Newtonian? Excepting the eyepieces, no optics but the
primary and secondary? It is not absolutely clear to me but I am assuming
you mean the ghost is a focused image like the main image but just dimmer.
You have ruled out eyepiece refections.

Sounds like a second surface reflection. May be the secondary is polished
both sides, maybe cut from plate glass, and a fair sized area of its coating
is missing allowing a reflection from the back surface. Or the diagonal is
put in reversed, coating down. That last one sounds quite possible, doesn't
it? If you got it from a not so "telescope nut" type person something like
that could go un-noticed easily. Could be the reason he sold it.

If you wear glasses when observing you could be getting a reflection either
off the surfaces of the lens (off back surface and then the front surface)
or off your eyeball and the lens. That can happen with the eyeball and
eyepiece too, but I don't think the ghost would be focused. When you say
below and right of the main image, is that for any orientation of the
telescope. If so then one of these reflection scenarios is the likely cause.

And then there can be a warp in the surface of the mirror. Seems unlikely
that the surface would have a warp that just happened to have a slightly
offset focus, but maybe a small wedged zone at the edge of the primary could
have that result. If the secondary has a small area warped but still
reasonably flat I guess that could do it too. But I think that is unlikely
too.

Having looked at a couple of Coulter mirrors (the old Coulter) under
Foucault test, I could believe the most unlikely defects could be on one of
those mirrors.

I really like the upside down diagonal idea.

Jerry

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
vorblesnak@peak.org
Sent: Saturday, April 14, 2007 10:47 AM
To: atm@atmlist.net
Subject: [ATM] Ghost image

I bought an old Coulter blue tube 10 inch.  The images are pretty good
except for a small ghost below the and right of the main image.  Swapping
eyepieces does not affect it.  The scope is well collimated.  Any ideas
what may cause this artifact?

David Davis
Toledo, OR 97391



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