Collimation, and the star test....

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

On 22 Jan 2006 08:32:46 -0800, "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
===========================
Rough Collimation will show the donut ...
what you want to see at this point is the black center hole perfectly
in the center of the donut .. NOT a little pinched to one side...!

The next step is to up the magnifacation and look for the again look
for the refraction rings...around the center... and in this case you
(or at least I) always go slightly in and out of focus ....should be a
very small "dot" in the center surrounded by a few concentric rings
both before and after focus...

By flatting I think you are refering to what I discribe as a comets
tail... with the fat part towards the center ...not really sure but in
either case I have never see (and what do I really know) the tail go
horizontal then vertical as I pass thru focus...

My scope is a SCT with only one mirror that is adjustable..Thats
enough for me... playing with a Priimary and Secondary mirrors may
make that comet tail go verticla then horizontal I just do not
know...BUT in either case I think your collimation is NOT quite on the
money...

Bob

..

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?

I found this helpful:
http://www.astunit.com/tutorials/startest.htm

Shawn

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?

I found this helpful:
http://www.astunit.com/tutorials/startest.htm

Shawn

"Starboard" wrote in message
ups.com...

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?

Could be astigmatism.

It could be in your eye. With the star flattened horizontally, rotate your
viewing position 90 degrees. If the flattening follows, that is, if it
remains horizontal to your perspective, then it's your eye. If the
flattening remains in its original position, that is, it's now vertical to
your perspective, then its in the optics.

The Asian Dobs have a tendency toward astigmatism at the secondary, due to
pressure from the secondary holder. (It has a remedy.)

Check out the Skyquest group on yahoo. One remedy is to remove the secondary
assembly, pull the secondary mirror, shave down the wall thickness on the
inside of the holder, and reassemble. As always, be careful not to touch the
mirror surface with your fingers.

It's my belief that they do this (make it too tight) to prevent the mirror
from coming loose in shipping.

"Starboard" wrote in message
ups.com...
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?


What you are seeing is astigmatism.
If you are using a diagonal, try without it. If the problem goes away, throw
away your diagonal.
Rotate your eyepiece, If the flatening follows the eyepiece rotation, throw
the eyepiece away.
Rotate your head, if the flattening follows your direction of view see an
optician.
If it is still present, careful collimation of all the components may
improve it or you may have an astigmatic lens/mirror somewhere (possibly due
to pinched optics)

Robin

you may have an astigmatic lens/mirror somewhere (possibly due
to pinched optics)

Pinched optics? Do you mean stressed by the collimation screws/mirror
fasteners?

Errol
NOLA

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

What you are seeing is astigmatism, i.e. more than one focal point. If you
see this only at low power, then I don't think it is a big deal. However,
if you see it 30x/inch or higher power, then you probably do want to
research what is causing it and correct it. It is caused by one or more
elements in the optical path, from your eye all the way to the primary
mirror or objective lens. You really should seek out and join your local
astro club, where you will find folks who can help you understand what is
going on and maybe fix it.

Dennis

"Starboard" wrote in message
oups.com...
you may have an astigmatic lens/mirror somewhere (possibly due
to pinched optics)

Pinched optics? Do you mean stressed by the collimation screws/mirror
fasteners?


Not usually the collimation screws, but the mirror (primary or secondary)
/objective can be distorted by the clips that hold it in the cell. They
should just hold the mirror in position, not clamp it tight. eg I had a
problem with my scope where over tightening 3 clips produced triangular
stars.

Robin

You never did tell us what kind of telescope you are referring to ???
....

Starboard wrote:

you may have an astigmatic lens/mirror somewhere (possibly due
to pinched optics)

Pinched optics? Do you mean stressed by the collimation screws/mirror
fasteners?

Errol
NOLA