Collimation, and the star test....

Lets start again. Tell us what your scope is? It helps us keep from
guessing.





Starboard wrote:

I have read that there is a test of collimation in with one compares
the views of a star just before and after focus. The result, if the
scope is collimated, is that the image of the star will appear to take
the shape of a donut and that it will look the same in both out of
focus positions.

My scope seem to be pretty well collimated when I use the manufactures
collimation *cap* and procedures.

However, when I perform the star test, I see the star flatten
horizontally when I focus up and flatten vertically when I focus down.

Is this poor collimation, poor primary mirror quality, poor eyepiece,
or something else?

TIA,
Errol
NOLA

Sorry. SkyQuest XT-8 Dobsonian.

I drew a picture (in AutoCad - converted to .jpeg) of what I see in the
collimation cap peep hole. I could mail it to anyone willing to take a
look at it. I don't currently have a web site to post it to.

Errol
NOLA

"Starboard"
However, when I perform the star test, I see the star flatten
horizontally when I focus up and flatten vertically when I focus down.

Errol
NOLA

I had a similar effect with a clipped secondary on a single arm spider. It
too moved through 90 degrees intra or extra focally. A 4 arm spider and
silicone instead of clips cured it. Whether the effect was due to pinching
or diffraction I never found out.
jc

Starboard wrote:
Sorry. SkyQuest XT-8 Dobsonian.

I drew a picture (in AutoCad - converted to .jpeg) of what I see in the
collimation cap peep hole. I could mail it to anyone willing to take a
look at it. I don't currently have a web site to post it to.

Errol
NOLA

If you want, mail it to me. I'll put it in a temp directory in my site
and will post the link.

- Alex

Will do. Thank you Alex..

Errol
NOLA

On 2006-01-22, Starboard wrote:

However, when I perform the star test, I see the star flatten
horizontally when I focus up and flatten vertically when I focus down.
Is this poor collimation, poor primary mirror quality, poor eyepiece,
or something else?

This is the result of astigmatism in the telescope or in your eye.
If it appears at low power and turns when you turn your head it is in
your eye. If it appears at high power and doesn't turn when you turn
your head it is in your telescope. With bad eyes you keep your glasses
on or get new glasses. With a bad telescope you complain to the maker.

--
The night is just the shadow of the Earth.

Thanks. Waiting for a break in the clouds (been raining here in
southern Louisiana for a few days) to do some testing. It may be my
eyes. They're not the best....

Starboard wrote:
Will do. Thank you Alex..

Errol
NOLA

Here it is:

http://www.avtanski.com/images/var/Collimation.gif

Sorry about the delay - the stupid spam filter gobbled
your message and it was some time before I figured out
what happened. :-)

- Alex

just as I thought. The secondary may be pinched.


Starboard wrote:

Sorry. SkyQuest XT-8 Dobsonian.

I drew a picture (in AutoCad - converted to .jpeg) of what I see in the
collimation cap peep hole. I could mail it to anyone willing to take a
look at it. I don't currently have a web site to post it to.

Errol
NOLA