Ok you Dob people....

If you don't collimate correctly, you'll get an image looking just like Phil's new avatar....perish the thought!

Yeah, thats even more of a central obstruction than an SCT!!

The good news is that lasers work fine if the scope is already close to perfectly collimated and just needs a tweak on the primary screws.
Phil.

You scared me for a minute there Phil, but this last sentence explains why it seems to work for me, phew..

If you don't collimate correctly, you'll get an image looking just like Phil's new avatar....perish the thought!

Yeah, thats even more of a central obstruction than an SCT!!

thats a gas avatar, you da man :lol:

going back on topic, I'd imagine you'd recognise miss collimation if it was there, the stars would be comet like, you say that the scope is giving fuzzy star points, might this be dog biscuit? (slang for rough optical surface, go figure!), if so I hope you have the receipt.

Some interesting stuff to go on there, guys, so thanks.
I've been using laser collimation, which worked well on the 12", and I should have expected similar results with the 16".
I'd a good look at defocusing using the Airy disc (about x250)and it wasn't too bad at all, but I'd no Cheshire at hand to investigate further.
I'm interested in the pinching possibility, as I haven't come across it before, but the image isn't smeared in any particular way that would indicate anything relating to coma. Dog Biscuit'd be about right...it looks a bit lumpy at times!
I'd actually been looking at Polaris on Tuesday night to see how bad it would look with the smearing, and how it would affect the companion star.
It was quite visible, but then it's relatively easy.
How did you dob builders align your secondary mirrors to your complete satisfaction?

I'm not a dob user, so take this with a grain of salt, but you probably want to use much higer powers to collimate and star-test. I use about 320x on my 4" refractor, and I throw everything I have at the 16" SCT when doing critical collimation (about 850x).

I don't think my eyesight is the best so you may not need that much, but I think you'd want at least 40x/inch to collimate on a star. Suiter recommends 25x/inch to do the rest of the star test (such as looking for pinched optics).

Cheers,
-- Jeff.

Collimation is a difficult technique to describe in a few short words, so your best bet is to have a look at the following links:

stellafane.org/misc/links.html - scroll down to the list of collimation links.

Without doubt the best set of collimation eyepieces (there are three of them!) are from Tectron. They come with a short book, "Perspectives in Collimation" that is the definitive reference on the subject. Yes it's an expensive set... but it is *well* worth it! You can order the Tectron set at: www.shop-cabinets.com/collimation/tools.html

Phil.

This might not work for you as your scope is a manual one, but I often use my webcam to make sure the star has good concentric patterns on both sides of the focus.
Given that this is inherently at very high powers, I'm not sure how you'd be able to keep the star in the narrow fov.