Collimation discrepancies

[Tomjo59]

Hi all,

Having fairly recently acquired a 10 inch Newtonian, I have, in the past few weeks been attempting to get the collimation as accurate as I can. Firstly, I bought a laser collimator, which is an Altair Astro one, and is itself collimatable - just as well as it was well out of collimation when it arrived. After collimating the collimator, I used it to collimate the newt, which resulted in improved views; so far so good.

Today, I have used a Cheshire collimator to tweak the newt, diligently following Astro Baby's excellent instructions, and believe I have got the collimation spot-on. Here's the thing though - if I now put the laser collimator back in, it indicates that the collimation is some way off!

Now, I am yet to test the scope on stars to see if there is any discernible improvement, but has anybody else experienced such a difference between the two methods for collimating a Newtonian?

Tom

[Dom1961]

I have but not with a cheshire, my laser is collimated but when I collimate the scope with a collimation cap and check with a laser, it's apparently way off. I have been thinking of selling my laser to buy a cheshire. It is too much hassle.

[jetstream]

Believe the Cheshire- no question..... and now I must hide!

Seriously though- believe the Cheshire

[ScottS]

Depends what you mean by "some way off". The discrepancy is probably due to slop in the focuser. A Cheshire is generally regarded as being more reliable but lasers do have their place.

As you've already suggested, a star test is the best way of checking.

[Dom1961]

with a star test it looked fine with the collimation cap. so ive always relied on the cap mostly and using the laser to adjust primary

[Tomjo59]

By 'some way off', I meant the laser spot on the primary was about 25 mm from the mirror centre, and at the very edge of the 'screen' on the laser collimator.

I wanted to try a Cheshire as there is always the nagging suspicion that the view is still not quite as good as it could be! When I next get a clear night with reasonable seeing, I'll try a star test, as suggested.

[ScottS]

You may even see a discrepancy if you collimate with the laser, then remove it from the focuser, put it straight back in without changing anything, and it will still be out.

[niallk]

By 'some way off', I meant the laser spot on the primary was about 25 mm from the mirror centre, and at the very edge of the 'screen' on the laser collimator.

I wanted to try a Cheshire as there is always the nagging suspicion that the view is still not quite as good as it could be! When I next get a clear night with reasonable seeing, I'll try a star test, as suggested.

I could be way wrong here - but if your laser is collimated, could it be the secondary needs retweaking?

I used a Cheshire only for my 10", and did find I needed a couple of iterations between primary and secondary. When secondary was well aligned, mostly the primary only has ever needed adjustment.

I use a laser on my 15". Again the hardest thing was the secondary... Primary is a doddle if the secondary is sorted.

[AndyWB]

Hi Tom,

That's what I get with mine; the Cheshire and laser don't correspond exactly. I've said mine tends to be about 20mm off. I did find that any play in the system accounted for most of that difference; if I gently prod the side of the collimator, the difference drops to about 5mm.

The good news is I've collimated with one, checked the view, and them collimated with the other, and checked the same view - and I couldn't see any difference. I'm sure it'd matter for photography, but visually, I'd simply not panic.

I do trust the Cheshire more than the laser, though. 

[Ally8446]

I only ever use my laser (Baader Mk3) for primary collimation (barlowed). With the Cheshire my secondary is pretty spot on.....well I'm happy with it. My views are pin sharp. Trust your eyes.

Ally.