How do i know if my scope needs collimating?

[alowen]

I have a skyliner 150 dob, and after using my dads meade ls6 last night side by side I noticed mine was a little glarey and smeared almost. I checked mirrors for dirt and allowed a good cool down time.

Does it need collimating?

Al

[themos]

Let it settle down outside for half an hour and then look at Polaris with your highest power eyepiece. Observe carefully until you can make out the faint diffraction rings around the star. If the air is not steady or your scope has not cooled down sufficiently you will not see them or they will be dancing all over the place. If you do see them steady, see if they seem to be nicely symmetric all around the star. If they are not, then you need to collimate.

[Helen]

The classical way is to defocus a star and see whether you get a regular donut with the 'hole' central, then as you get close to focus whether the rings are concentric and there is no flaring to one side.

Astro Baby has written an excellent guide which should allow you to check and get it pretty close in daylight:

Astro Babys Guide to Collimation

HTH

Helen

[themos]

I checked mirrors for dirt

You won't see the effect of that easily. Make sure your eyepiece surfaces are clean, instead.

[brianb]

Observe carefully until you can make out the faint diffraction rings around the star.

Yeah ... as if the seeing was ever good enough ... (well it sometimes is, but it's a very rare event, especially with larger apertures!)

A more sensitive test is to examine the out of focus disk/donut at insanely high power (I use about x800 on my CPC1100 - more than twice what is ever worth using for "proper" observing) as you go from slightly inside to slightly outside focus. If the collimation is slightly out, the circular image will distort to a "V" with the arms being much fainter than the bright star at the join of the legs. Don't be distracted by diffraction spikes which may appear as you get close to accurate focus if your scope has a "spider" supporting the secondary mirror; these will result in the star image remaining perfectly symmetrical (though with four or six spikes) as focus is passed through, assuming the collimation is perfect.

Very slight collimation errors are nothing to worry about unless you're a world class planetary imager. Don't waste more valuable observing time than necessary; steady air is a rare commodity & needs to be used to observe real targets, not fiddling with collimation. Gross collimation errors will smear the image even at moderate magnification and are definitely worth getting rid of.

[acey]

after using my dads meade ls6 last night side by side I noticed mine was a little glarey and smeared almost.

Compare the two scopes by using the same eyepiece in each. "Smearey" could be dirt on the eyepiece, "glarey" could be internal reflections, which you're apt to get in low-cost eyepieces. Flared star images could be an indication that the scope needs collimating, but could also be caused by tube currents. Do the star test to check (the donut and rings inside and outside of focus should ideally look identical).

[The Warthog]

Get a collimating tool, which you should have anyway, put it in the focuser and check against the instructions to see how well lined up everything is. You will have to collimate your scope at least a few times a year. Astro Baby's guide is probably the best I have ever seen for a beginner.

If the stars are not focusing to points, you probably need to collimate. Poor optics can cause problems with focus too, but Skyliner is usually a decent scope.

[Moonshane]

I collimate (or at least check collimation) every single time I observe. it becomes a habit after a while and takes a few minutes at worst and a few seconds at best.

I don't do star tests usually but might try this sometime, Brian makes it sound so easy!

as far as I'm concerned if the Cheshire dot is in the primary donut and the primary is centred on the secondary then that's close enough for me and visual work.