question about star-testing in S&T article

[Mizar]

Hi, just joined the forums. I'm just trying to read up on doing a star test to check if there's any inaccuracy to the collimation on my Mak. Part of what they said is confusing me in this Sky & Telescope article: http://www.skyandtel...iy/3306876.html

Where they describe "Star-Testing Your Collimation", it says:

At high magnification and in good seeing conditions, stars at focus should appear in the eyepiece as tight, symmetric diffraction disks.

Stars should always appear as pinpoints, no matter how high the magnification. Unless they are trying to say the optics should have some kind of aberration. I'm thinking this is an error, and should say they should appear as diffraction disks when the stars are not at focus. This article is contradictory, because they also mention a star test is done with stars out of focus. Does this sound correct that it's an error? Or if not, what are they talking about?

[ollypenrice]

I'm afraid they are absolutely right. They are talking about the Airy Disk, after George Biddell Airy, an Astronomer Royal of the C19th. This disk's size is proportional to the diameter of the objective and the wavelength of light observed. It represents the absolute limit of resolution for a given aperture irrespective of optical quality. An optical system cannot resolve stars to pinpoints but only to Airy disks.

Actually seeing an AIry disk is not always dead easy! You need good optics and good seeing. It is a very tiny , faint and subtle effect, at least for me.

It should not be confused with diffraction circles which, provided you have the seeing, are easy to observe and are far bigger and brighter. Again, though, there are nights when the seeing just won't allow you to generate diff circles. Try a moderate star, Mag 2 or 3, on the zenith at about 100x, though doubtless your article has covered this.

You can do it in the daytime very easily with some Boys' Own Paper bits and bobs and a long building like a warehouse. Glue a small ballbearing onto a matt black card and illuminate with a decent beam like a good modern bike lamp from just off to one side. Observe this from well back behind the light source. 30 metres is supposed to be ideal but I've got away with far less than that. Only light from the point of the ball will find its way to the scope, making a near point-source. It should give you a great star test. You can do it outside on your drive as well. Total darkness isn't needed.

Olly

[Mizar]

Thanks for the great info. So my other suspicion that they were describing an aberration was correct, sort of... more like an optical limitation.

As it happens, I observed the double double in Lyra when it was near the Zenith. With the most precise focus I could get, the stars were small disks. This was only at high power (167 x), not at medium power (63 x). I'd been meaning to check if there is any issue with my collimation, so that got me wondering. I don't think I saw an Airy Disk then, since they're so tiny and faint. I didn't see what looked like a diffraction disk either, but it sounds like I should use a star two mag brighter.

Actually, you gave a bunch more details on doing a star test than the article (or other ones I could find so far). Thanks. I read somewhere recently a collimation eyepiece can be used with a Cassegrain with the diagonal removed. I have a simple collimation cap with peephole, so perhaps I'll try that too.

I'm afraid they are absolutely right. They are talking about the Airy Disk, after George Biddell Airy, an Astronomer Royal of the C19th. This disk's size is proportional to the diameter of the objective and the wavelength of light observed. It represents the absolute limit of resolution for a given aperture irrespective of optical quality. An optical system cannot resolve stars to pinpoints but only to Airy disks.

Actually seeing an AIry disk is not always dead easy! You need good optics and good seeing. It is a very tiny , faint and subtle effect, at least for me.

It should not be confused with diffraction circles which, provided you have the seeing, are easy to observe and are far bigger and brighter. Again, though, there are nights when the seeing just won't allow you to generate diff circles. Try a moderate star, Mag 2 or 3, on the zenith at about 100x, though doubtless your article has covered this.

You can do it in the daytime very easily with some Boys' Own Paper bits and bobs and a long building like a warehouse. Glue a small ballbearing onto a matt black card and illuminate with a decent beam like a good modern bike lamp from just off to one side. Observe this from well back behind the light source. 30 metres is supposed to be ideal but I've got away with far less than that. Only light from the point of the ball will find its way to the scope, making a near point-source. It should give you a great star test. You can do it outside on your drive as well. Total darkness isn't needed.

Olly

[ian_bird]

You can do it in the daytime very easily with some Boys' Own Paper bits and bobs and a long building like a warehouse. Glue a small ballbearing onto a matt black card and illuminate with a decent beam like a good modern bike lamp from just off to one side. Observe this from well back behind the light source. 30 metres is supposed to be ideal but I've got away with far less than that. Only light from the point of the ball will find its way to the scope, making a near point-source. It should give you a great star test. You can do it outside on your drive as well. Total darkness isn't needed.

Olly

Olly

Seriously? Could you check collimation with this? Why have I never heard of this before.

I know it's asking a lot - but could it be possible to do a bunch of ball bearings and check collimation of a ccd?

Yes - before you ask - I am p*ssed!

Cheers

Ian

[ollypenrice]

Olly

Seriously? Could you check collimation with this? Why have I never heard of this before.

I know it's asking a lot - but could it be possible to do a bunch of ball bearings and check collimation of a ccd?

Yes - before you ask - I am p*ssed!

Cheers

Ian

You mean have several BBs on a card so that the centre and, say, four corners of the chip had a false star? I don't see why not. I was thinking about this while totally failing to tune out tilt on an OS Veloce about a week ago.

I've used the BB on card method several times. I got the idea from TeleVue so it's serious. They were helping me collimate an injured Genesis over the phone! It worked, too.

Olly