Artificial Star Collimation

[Jupiter1]

Hello All,  Waiting Forever for clear STEADY Skies I have bought a Artificial Star to Collimate my TAL 200K to Hopefully achieve the Ultimate.    However I realised as soon as I had ordered the Device that the TAL does not like near Focussing ,in fact in needs quite a Far object , Which I do not have where I live.!

So , can anybody suggest any tricks that I can employ to shorten the Focal Distance just for the Collimation use. I have about 90 Foot Max Garden length ( way too short! )  

 

Many Thanks in anticipation , Clears skies ?   Jupiter 1

[spillage]

I have always wondered about this as most people do not really have the distance required for artificial stars. Would a reflection in a mirror not double the distance though?

[Dave1]

Do you not know a local farmer or land lord? Which you could ask to do astronomy on there land? That way, you could do a star test outside in a field, where there is plenty of space. Failing that some dark sky sites may provide you with enough distance to do the test.

[JOC]

How far away do you need to be?  Depending on where you live could you do it in your front street, or maybe drive some distance and put the 'star' into the car and then go away and work on the telescope?  Maybe sit the star on a top floor angled down and work at the end of the garden you'd pick up a little diagonal distance that way.   Maybe find an astronomy friend with that sort of distance and go and play telescopes together!

[Peter Drew]

With a 90ft potential I would have thought an extension tube or two would bring even a Tal to focus. Don't forget to try with and without the star diagonal, this will show whether the diagonal is collimated.  

[spaceman_spiff]

I have been star testing my scope recently, and distance is an issue for me. I found I need about 54m (about 30 focal lengths) between the scope and the light source to do a fair star test! The issue is that my scope  (and yours) use a spherical primary mirror which will generate different amounts of spherical abortion depending on the distance to the artificial star.

If you have a short distance then the primary mirror will produce a smaller amount of SA and the corrector will seem to over-correct it.

I think that's right...

Dan

[mikeyj1]

umm surely you want the 'star' to be out of focus for the collimation check?

[Sunshine]

i would just take the scope to a local park and do it there, you need a good 80ft or so.

[Ben the Ignorant]

Have you tried not inserting the eyepiece fully inside the focuser? Or try extension tubes like Peter Drew suggested; if you don't own one, make one, it does not have to be well finished.