Star testing for collimation using a webcam?

[Kaptain Klevtsov]

There has been discussion elsewhere in this section about collimating on a star and how much fun it is to not use a diagonal and point the 'scope up. These two take away any error caused by the diagonal (by taking it out) or by atmospheric distortion caused by looking through more air than necessary.

It occured to me just now that poking a webcam up the drawtube wouldn't hurt nearly as much, so I pose the question, will it be good enough for a rough collimate, a medium one or would it let you get it spot on?

What do you think guys?

Kaptain Klevtsov

[David Galvin]

Much easier on the neck and patience, K.K.

The old text book method was to start with a low to medium magnification on a 2nd to 3rd mag star, 45 degrees or higher. The scope would then be 'tuned' to the best that you could. A higher power was then used and the collimation repeated and improved, the magnification was then pushed as high as seeing would allow and the collimation checked again. That is how it was done before the ccd or webcam came about.

Using the webcam is much more convenient but getting the high magnification needs to be done with a barlow in the train which then, may, introduce some error due the Barlow being slightly loose etc.

[SteveL]

Metaguide helps with webcam collimation. Might be worth looking at.

http://www.astrogeeks.com/Bliss/MetaGuide/index.html

[Kaptain Klevtsov]

That software looks very cool Steve, thanks for the link. I might have a bash at that, if the clouds ever clear off.

Kaptain Klevtsov

[jcm]

Thanks for the link , Steve. The software looks as though it could be very useful. Have you ever used it.?

John

[SteveL]

Used it a couple of times last year before I really started to get a clue. Might try it again with the picostar now the evenings are getting longer.

[ngc2403]

Hello

just thought i would ask if you guy's had a preferred way of collimating your telescope?

i have a 10'' f/4 and i plan to image with it but i don't yeah have a laser collimator or eyepeice and wondered what you would suggest??

[Astronut]

Hello

just thought i would ask if you guy's had a preferred way of collimating your telescope?

i have a 10'' f/4 and i plan to image with it but i don't yeah have a laser collimator or eyepeice and wondered what you would suggest??

I used a Baader laser collimator and a cheshire but have recently given up using the laser (gave it to Keagle as it happens) as I prefer using a cheshire/sight tube for more consistent collimation. Lasers are OK but they don't tell you if your secondary is aligned correctly with the focuser tube - you need a collimation cap or a cheshire for that.

I've recently ordered a Catseye kit (should be here this week sometime) as it gets rave reviews. It's a bit pricey but what the hell.

http://stargazerslounge.com/index.php/topic,25258.0.html

For an F/4 'scope such as yours (I'm assuming it's a newt.), decent collimation becomes more critical and sensitive than a slower 'scope.

[George]

There's actually a program for specifically using a webcam for collimation......it works too

http://www.astrogeeks.com/Bliss/MetaGuide/

[Ajohn]

You can tell if the diagonal is causing problems by rotating it. Any distortion will move with the rotation. It can be sort of ignored that way. There's no difference in the length of the light path with or without the diagonal. There is also the point that some scopes will not focus unless a diagonal or an extension is used.

My view is that an artificial star is the best bet but has the problem that the scope will be collimated horizontally so it depends if things bend or move around. The "star" also has to be a longish way away which can also be a problem for many people.

John

[ngc2403]

thank you,

i was under the impression that the best way to do collimation was to do it on a star on the night that you plan to image/observe. i was thinking that i could make an eye piece for rough collimation then fine tune on a star?

what do you think would be better that or the Cheshire.

other possibility is that i get both a laser for alignment when it's dark and am not to fussed plus a Cheshire for the serious nights of imaging, though at the moment it's not important

ally

[peteri]

Apologies for the late contribution to this thread...

I would highly recommend MetaGuide for assisting with collimation. Not only does the software auto-guide on the star you are using for collimation, but it also provide a time averaged display of the Airy pattern supplemented by an intensity graph showing both the measured and the theoretical profiles for the scope. There are many other useful features and the software is under continual development by the author.

Also of interest is the excellent article by Andre Paquette on the influence of atmospheric dispersion on collimation. The article can be view here http://www.paquettefamily.ca/astro/star_study/ and basically advises against using stars significantly away from the zenith for collimation, otherwise you'll end up chasing the distorting effects of the atmosphere. BTW, the author used MetaGuide to obtain many of the images used in the article.

Regards

Peter