Collimating in the dark !

Hi,

A quick question about collimation. I have an 200mm Newt, that was
unfortunately knocked off the mount a couple of nights ago. As soon as
picked it up I could see that the collimation was now miles off. The
only way I could get it back to approx. collimation was to use a laser
collimation tool. As I put it in and peered (carefully) down the tube
I was amazed to see the beam clearly hitting the main mirror and then
reflecting back towards the secondary (as a more 'fuzzy' beam). I
guess this was to do with the dew...

After a bit a fiddling with the collimation nuts I got it so the laser
beam left the collimation tool, hit the centre of the secondary, then
hit the centre-spot of the main mirror, and then reflected back to the
secondary directly along the path of the beam going from the secondary
to the main.

As far as I could work out, I figure that this must be collimated
since all the optical paths are aligned - is this true ?

Many thanks

Andrew

P.S. I didn't do a star test since I can never make head or tail of
them anyway - they always look fuzzy to me !!!

On 3 Sep 2003 06:09:06 -0700, (Andrew) wrote:

Description of laser collimation snipped

As far as I could work out, I figure that this must be collimated
since all the optical paths are aligned - is this true ?

This will get you in the ball park, but you *must* learn to use the
star test for final collimation. Nothing else will suffice...

Wayne Hoffman
http://home.pacbell.net/w6wlr/
33° 49' 17" N, 117° 56' 40" W
"Don't Look Down"

I was having trouble with my XT10 one night as the collimation screws seemed
to be at their ends and things just wouldn't quite cooperate. With the
laser, I was able to back the screws all the way off, then get into the ball
park. Turns out that if you do it right, the ballpark is tiny, and the
collimation comes out quite good. Know thy laser, and thy focuser, and thou
shalt do well.

--
-Stephen Paul

"WayneH" wrote in message
...
On 3 Sep 2003 06:09:06 -0700, (Andrew) wrote:

Description of laser collimation snipped

As far as I could work out, I figure that this must be collimated
since all the optical paths are aligned - is this true ?

This will get you in the ball park, but you *must* learn to use the
star test for final collimation. Nothing else will suffice...

Wayne Hoffman
http://home.pacbell.net/w6wlr/
33° 49' 17" N, 117° 56' 40" W
"Don't Look Down"

This will get you in the ball park, but you *must* learn to use the
star test for final collimation. Nothing else will suffice...



Amen.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

Collimation will be OK in this case, only if the laser hits the optical
center of the main mirror. This is very critical (very small tolerance) for
fast mirrors.

--
Jan
http://home.wanadoo.nl/jhm.vangastel/


"Andrew" schreef in bericht
om...
Hi,

A quick question about collimation. I have an 200mm Newt, that was
unfortunately knocked off the mount a couple of nights ago. As soon as
picked it up I could see that the collimation was now miles off. The
only way I could get it back to approx. collimation was to use a laser
collimation tool. As I put it in and peered (carefully) down the tube
I was amazed to see the beam clearly hitting the main mirror and then
reflecting back towards the secondary (as a more 'fuzzy' beam). I
guess this was to do with the dew...

After a bit a fiddling with the collimation nuts I got it so the laser
beam left the collimation tool, hit the centre of the secondary, then
hit the centre-spot of the main mirror, and then reflected back to the
secondary directly along the path of the beam going from the secondary
to the main.

As far as I could work out, I figure that this must be collimated
since all the optical paths are aligned - is this true ?

Many thanks

Andrew

P.S. I didn't do a star test since I can never make head or tail of
them anyway - they always look fuzzy to me !!!

As far as I could work out, I figure that this must be collimated
since all the optical paths are aligned - is this true ?

Many thanks


I suggest checking out the Nils Olof Carlin's "Barlowed Laser" technique. It
is useful for the final collimation of the primary mirror and is less sensitive
to play in the focuser. It was written up in Sky and Telescope within the last
year or so.

From: http://zebu.uoregon.edu/~mbartels/ko...tm#barlowlaser

"A Barlowed Laser Collimator

As described above, the collimation with a laser collimator is quite sensitive
to errors in centering the beam on the main mirror. Here is one way that lets
you use the laser to get the accuracy of a Cheshire, as well as its
insensitivity to centering errors. Besides, it will work even if there is no
center hole in the mirror spot!

Put your laser collimator into a Barlow lens, and aim it at a wall, at a
distance about the focal length of your mirror. You will see a much larger
spot, perhaps not quite round, but if it is large enough to cover the main
mirror spot, you are ready to go on ( if it is too small, try a shorter
Barlow!).

You need to put a target over the lens end of the Barlow: Cut a circle of
cardboard, large enough to fit over the lens end of the Barlow, and make a
center hole to let through the laser beam. A circledrawn the size of the main
mirror spot will also help. Attach it to the Barlow (make it a tight fit to the
lens cell, but make a tab to let you lift it off afterwards).

Do step 4 the usual way without the Barlow. For step 5, put the Barlow in the
focuser and the laser in the Barlow. You should now see the wide return spot
illuminate the target, and also the shadow of the main mirror spot. Now
collimate the main mirror to center this shadow."

It works.

jon

I left the 'scope for a couple of hours to cool down, and then pointed
it at Polaris. Upped the magnification to x300 and had a look. I could
make out the faint companion. I then moved the focus slightly in, and
I could make out the diffraction(?) rings etc..., but the seeing was
pretty rotten so it was all a bit wobbly. The outside focus seemd to
missing a bit of the rings (maybe the focusser tune ???)

If you're missing a bit of the rings but the shadow of the secondary
is still centered and concentric with the rings you can see, you
probably have vignetting. If so, one or more of the following needs
to be corrected:

1. focuser is not square to tube (very important!, need to have
adjustment screws on focuser base though)

2. vertical placement of the secondary (use nut in center of secondary
holder to adjust)

3. axial rotation of the secondary (loosen nut and rotate secondary by
hand until outline appears circular through the focuser)

4. offset of the secondary from physical center (makes a tangible
difference in illumination for scopes faster than f/5, use center nut
and spider screws to adjust, don't worry about it if f/5 or slower)

It didn't look all that good so I thought I would tweak the main
mirror nuts to see if it made any difference (just an experiment
really). I moved one of the nuts about 1/8 turn (maybe less) and
looked back into the ep - Polaris had gone - moving the mirror had
moved the image out of the FOV. Is this normal ? It makes sense, but
it doesn't mention this in the stuff about star testing on the web.

Well, if you're using very high power then the star will easily move
out of the field with 1/8 turn! 200x should be enough for a star test
on a Newt. An eyepiece with a wide apparent FOV helps a lot here.

Ritesh