laser collimator behavior

I recently purchased a Hardin 12" DSE and one of their laser collimators.

I noticed that depending on the rotational position of collimator in the
eyepiece, the position of the laser on the primary changes by as much as an
inch or so.

In other words, if I put the collimator in with the viewing port pointed
towards the back of the scope and adjust the secondary to center the beam on
the primary, then turn the collimator 180 degrees so the view port faces the
front of the scope, the spot on the primary moves about an inch away from
center (I'm guessing from memory that it might have been that much --
perhaps it was less but it was definitely not what one would call
'collimated'.)

Now I'm just guessing, but I assume it shouldn't behave that way, since
there's no way to know a priori how to insert the collimator to get the best
result. I wouldn't think the correct position of the secondary would be a
function of how the collimator is inserted.

Is the problem with the laser alignment within the collimator barrel, the
mechanical collimation of the focuser on the OTA, or with the new telescope
owner / novice astronomer (me!) who has never done a collimation before?
That novice guesses that it's the collimator, since I didn't move the
focuser or the 2" - 1.25" adapter at all, just rotated the laser and
retightened.

I ended up just keeping the collimator in one position and doing both the
secondary and the primary without moving it, which I assumed was the best I
could do under the circumstances. Is that sufficient, or should the
collimator be returned? Or should I pursue walleye fishing?

Forgive me if this is a FAQ...

Thanks in advance,
Duane

On Thu, 22 Jul 2004 10:28:40 -0400, "dgblues" wrote:

I recently purchased a Hardin 12" DSE and one of their laser collimators.

I noticed that depending on the rotational position of collimator in the
eyepiece, the position of the laser on the primary changes by as much as an
inch or so...

The collimator itself needs to be collimated. I'm not familiar with the model
you have, but good laser collimators have adjustment screws for that purpose.

It is also possible that the collimator is fitting loosely in your draw tube,
but since you seem to be describing a problem that is repeatable with collimator
position, that is probably not the case here.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

That novice guesses that it's the collimator, since I didn't move the
focuser or the 2" - 1.25" adapter at all, just rotated the laser and
retightened.

I ended up just keeping the collimator in one position and doing both the
secondary and the primary without moving it, which I assumed was the best I
could do under the circumstances. Is that sufficient, or should the
collimator be returned? Or should I pursue walleye fishing?

It seems from what I read here that as you rotate the collimator in the focuser
draw tube, the spot on the primary mirror moves, most likely in a circle.

This, as you have figured out, means that the collimator is out of collimation
and should be returned to the vendor for a replacement. I understand this is
not uncommon with the Asian collimators.

With a misaligned collimator, your best bet is probably to use a cheshire or a
film cannister and a star test.

Myself, notoriously cheap, I dug some money out from under the bed ($100) and
bought myself a nice Howie Glatter collimator. That was about 5 years ago and
it has worked like a charm. It is still on its first battery and is still in
perfect collimation.

Sometimes the cheapest way to go is to spend the money and get the best, in the
long run you only spend it once.

And besides, Howie's a good guy and rumour has it he was around during the
'60's too.... g


jon

You might first ask the seller for advice on collimating the collimator (I am
NOT kidding), and, failing good advice, return it. Please read the stuff at
http://www.vvm.com/~piscescs/collimat/NoTools2.html
to check on how well the laser collimator did.
(Maximize this note to get the URL complete).

Bill.

Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

"dgblues" wrote in message ...
I recently purchased a Hardin 12" DSE and one of their laser collimators.

I noticed that depending on the rotational position of collimator in the
eyepiece, the position of the laser on the primary changes by as much as an
inch or so.

In other words, if I put the collimator in with the viewing port pointed
towards the back of the scope and adjust the secondary to center the beam on
the primary, then turn the collimator 180 degrees so the view port faces the
front of the scope, the spot on the primary moves about an inch away from
center (I'm guessing from memory that it might have been that much --
perhaps it was less but it was definitely not what one would call
'collimated'.)

Now I'm just guessing, but I assume it shouldn't behave that way, since
there's no way to know a priori how to insert the collimator to get the best
result. I wouldn't think the correct position of the secondary would be a
function of how the collimator is inserted.

Is the problem with the laser alignment within the collimator barrel, the
mechanical collimation of the focuser on the OTA, or with the new telescope
owner / novice astronomer (me!) who has never done a collimation before?
That novice guesses that it's the collimator, since I didn't move the
focuser or the 2" - 1.25" adapter at all, just rotated the laser and
retightened.

I ended up just keeping the collimator in one position and doing both the
secondary and the primary without moving it, which I assumed was the best I
could do under the circumstances. Is that sufficient, or should the
collimator be returned? Or should I pursue walleye fishing?

Forgive me if this is a FAQ...

Thanks in advance,
Duane

Hi Duane
Before getting too excited. First, it is correct that the laser
is most likely not aligned correctly. Still, this offset only
adds a small error angle to the secondary of the telescope.
That angle only effects the exact centering of the 100% illuminated
spot and will not effect the quality of the alignement as long
as it is reasonably centered in the field of view. There are
other errors using a laser that create bigger issues.
Still, you need to read the following. Don't give up early.
That last one will be your salvation but you must first start
with the first one and work your way through things before
you can appreciate the final solution. Take your time and
learn a little.
Dwight


http://w1.411.telia.com/~u41105032/myths/myths.htm

http://www.atmsite.org/contrib/Carlin/collimation/

http://www.atmsite.org/contrib/Carli...tm#barlowlaser