Collimation question

[weetooner]

Hi, folks, bought myself a laser collimator (sky-watcher), when it arrived I checked the collimation of the laser and it seemed fine.

I then decided to check the collimation on my scope, used a bit of plumbers tape (as advised) round the collimator to ensure it was sitting in the centre of the focuser and collimated the scope. I then checked it with a Cheshire and all seemed well. I then decided to double check with the laser again but when I put the laser back in the laser was way off.  

Can I ask if this is normal? I thought maybe it was off because I had removed and then reinserted it or do you think I have done something wrong?

Many thanks.

[Mr Q]

   The problem may be in the focuser. If there is any play in it from side to side, the focuser will not allow proper collimation - in other words, each time you insert the device, it will not be centered in the optical path that travels from the focuser to the secondary mirror and on to the primary mirror. Only the best, most expensive focusers will be tight enough to allow several tests with similar results. The focusers found on most amateur scopes used today are not that high in quality though they do allow clear, sharp images with fairly good quality EPs.

   The best way to check a scope's collimation is by doing a star test. This procedure is very accurate and easy to perform. The below link shows how to do the test.

http://legault.perso.sfr.fr/collim.html

   Though some of the text may be difficult to understand, the main thing is to center the light and dark rings so that they are concentric, using the adjustment screws on the primary mirror cell.

[weetooner]

Thanks for that Mr Q, star test will have to wait as its been cloudy here for the last 5 days and no sign of it clearing soon.

[moffer]

Have you tried double checking collimation with the cheshire? If a similar thing happens as with the laser, and providing the cheshire is a snug fit in the draw tube, then as Mr Q says - there's probably a bit of slop in the focuser. Not sure about your scope but on the skywatcher 200p focuser there are 2 small screws you can tighten that can reduce the amount of slop. You can only tighten them a little though...too much and the focuser becomes stiff to operate, 

[umadog]

This is the problem with lasers. The important adjustment is the primary mirror tilt and here you're using the laser return beam, which means you've doubled any errors it might create through being tilted in the focuser. For this reason it's recommended you use the barlowed laser technique for adjusting the primary (http://www.astromart.com/articles/article.asp?article_id=827) as it does not require the laser itself to be aligned or positioned accurately in the focuser.

[andyman]

This is the problem with lasers. The important adjustment is the primary mirror tilt and here you're using the laser return beam, which means you've doubled any errors it might create through being tilted in the focuser. For this reason it's recommended you use the barlowed laser technique for adjusting the primary (http://www.astromart.com/articles/article.asp?article_id=827) as it does not require the laser itself to be aligned or positioned accurately in the focuser.

The problem is with the focusser..not the laser.but i agree..using a laser for the secondary..not a good idea in my opinion a simple collimation cap works just fine.+1 for barlowed laser method