[ATM] Ghost image

[Mike Lockwood]

David,

vorblesnak at peak.org wrote:
> 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?

How is the primary mirror mounted?  How thick is it?  Is it Pyrex?

A number of Coulters I have seen were duct-taped to a slab of 
chipboard, and this could cause strange distortions very close to a 
star image, planet, etc.

Also, the secondary is typically attached to the "spider" with 
double-sided tape, and you might check the condition of it.  I had a 
friend with one that was incredibly difficult to separate from its 
mounting, but I tested it on and off of the tape and it appeared to be 
mounted with relatively little warping of the mirror.

I've seen some excellent and some bad diagonals in Coulters.  Most 
primaries I have come across have needed some type of refiguring.

	Mike Lockwood