Low temperature affecting collimation?

[Lee M]

Just had a chilly but thoroughly enjoyable 3 hours.

The scope (200p Dob) was out a couple of hours before that.

I did notice that the collimation looked off though when defocussed on Jupiter. Is that likely simply due to the scope being significantly colder than when I've previously used it?

I didn't try to do anything about it this evening & carried on with reasonable results.

Lee

[Mr Q]

To do an optical test on an object in the sky, you need to do this on a bright star nearly overhead. The object used has to present a pinpoint source of light for the "star test" to work. Planets are not pinpoints of light but appear as tiny to fairly large discs of light so are not of any use in determining if the scope's collimation is off.

The collimation may drift off a bit as the scope cools down but not enough for you to notice. Do a star test the next time out with the scope cooled down to do a difinitive collimation test. The link below shows how to do one and what the results should be:

COLLIMATION

[Lee M]

To do an optical test on an object in the sky, you need to do this on a bright star nearly overhead. The object used has to present a pinpoint source of light for the "star test" to work. Planets are not pinpoints of light but appear as tiny to fairly large discs of light so are not of any use in determining if the scope's collimation is off.

The collimation may drift off a bit as the scope cools down but not enough for you to notice. Do a star test the next time out with the scope cooled down to do a difinitive collimation test. The link below shows how to do one and what the results should be:

COLLIMATION

Cheers MrQ. That's a bit more in depth! I'll give that a try at the next opportunity.

Lee

[Mr Q]

Lee - Once you do the star test and the difraction rings are concentric, the scope's collimation is as close to what you can get. The only catch is finding a night with good seeing - stars not twinkling overhead and make sure the scope is cooled down so the rings can be seen easily. The wait is worth it, knowing your scope is collimated correctly.

The effects you mentioned while observing Jupiter may be from using too high a power for your scope and/or sky conditions. Bad seeing can ruin decent views of the planets and their details. Try the same observation a couple of different nights to see if the problem was sky conditions.

[cotterless45]

I use a collimated laser in a Barlow to check collimation over the course of an evening. I 've found that it'll slightly shift with a good temperature drop in a solid tube and with flex in a truss. With super seeing conditions it will make a difference to very fine detail such as spots and tears in Jupiter's belts.

When viewing dull fuzziness or in poorer seeing, you'd be hard pressed to notice if the collimation is not far off.

Nick.

[Lee M]

Cheers both. I suspect it was something to do with defocussing on Jupiter rather than a star. When the clouds next deign to part for a bit I'll give it a check.

Lee