First light on Homemade 80mm Achro

From a variety of Surplus shed parts, and some home depot trips, I've
managed to cobble together a little 80mm f/5 scope.

As with any first time project, when I put it under the stars, I had some
issues. The first one was my culmination was awful. I made some changes,
and made the culmination adjustable, and now I have nice circular stars (for
the center stars) when I go unfocused on either side of focus.

What I have now figured out, is that I have some very serious coma, all
those nice seagulls are pretty far from the water when I'm looking through
the scope.

My question to any takers out there, what is the best approach for
correcting coma? You can assume that all the mechanical parts are
adjustable for me.

Also, I was expecting to see the violet fringing around bright objects.
What I was not expecting was a green fringe!?! Have I messed something up,
or are my focal planes off, or, is my objective worth what I paid for it?

Thoughts? Suggestions?

Thanks,

Ed L.

Thoughts? Suggestions?

Thanks,

Ed L.


Sounds like an interesting project, hope you get it sorted out. Makes you
believe in those inexpensive Chinese scopes a bit more...

Coma=collimation....

jon

"Ed L." wrote in message news:zhA_b.111399$jk2.496927@attbi_s53...
From a variety of Surplus shed parts, and some home depot trips, I've
managed to cobble together a little 80mm f/5 scope.

As with any first time project, when I put it under the stars, I had some
issues. The first one was my culmination was awful. I made some changes,
and made the culmination adjustable, and now I have nice circular stars (for
the center stars) when I go unfocused on either side of focus.

What I have now figured out, is that I have some very serious coma, all
those nice seagulls are pretty far from the water when I'm looking through
the scope.

My question to any takers out there, what is the best approach for
correcting coma? You can assume that all the mechanical parts are
adjustable for me.

Also, I was expecting to see the violet fringing around bright objects.
What I was not expecting was a green fringe!?! Have I messed something up,
or are my focal planes off, or, is my objective worth what I paid for it?

Thoughts? Suggestions?

Thanks,

Ed L.

Sorry if this reply is too late, but I am posting through Google.

It sounds like you built the same scope I did. It works great as a
spotting scope, but I have noticed the same coma problem with it.
Lenses are spherical and will tend to have a pronounced coma with fast
scopes like an f/5. And I think that this lens in particular is not
well corrected, but hey, you get what you pay for. I have found that
using a 3x Barlow helps somewhat, simply by increasing the focal
length. They make coma-correcting lenses, which are very thin and
almost flat, but I think they tend to be pricey.

Not sure if this would work, but you might try masking the outer edge
of the lens (or should it be a center dot?). Of course you will lose
some aperture that way.

Good luck and let me know if you get it corrected,
James King