Collimation and optical testing with artificial star

[Giles_B]

I have become the proud owner of a Geoptik artificial star, and got a chance to try it out a few days ago.

Testing the collimation of my 10" dobsonian from a distance of 23m away from the (50 micron) star showed a good pattern of concentric rings, but unexpectedly the circle of the airy disk was not perfectly round, but a bit more like a 20 pence piece.

Somewhere in my mind the cogs began to turn and I eventually remembered (a few days later) that an off-round airy disk of this sort was the result of the mirror being pinched. In the last 9 months I've changed my old 3 point mirror cell to one of the newer Orion Optics 9-point cell.

Now I've been wondering if I pinched the mirror when fitting it to the new cell - although I have generally felt the mirror has some play in the cell, rather than being tightly held (I can hear it shifting slightly when I take the scope out of the dobsonian base and lay it flat - and vice versa)

However the artificial star has two distances quoted for testing for a 250mm aperture - 21m and 1km - the smaller distance is for collimation and the latter for testing optical quality (or so I believe - I'm a bit sketchy about why there are two distances!)

So before I strip the scope down again to test the mirror, I was wondering if what I am seeing is an artefact of using a very close light source, rather than a genuine aberration in the optics?

Wise thoughts and explanations would be very welcome!

[pipnina]

You could just be seeing the effect of the spiders, which could give the airy disk a slightly boxy look compared to what you'd normally expect.

this image from google suggests it could be affected

[vlaiv]

45 minutes ago, Giles_B said:

However the artificial star has two distances quoted for testing for a 250mm aperture - 21m and 1km - the smaller distance is for collimation and the latter for testing optical quality (or so I believe - I'm a bit sketchy about why there are two distances!)

So before I strip the scope down again to test the mirror, I was wondering if what I am seeing is an artefact of using a very close light source, rather than a genuine aberration in the optics?

One of distances is related to resolving the small opening of artificial star - your being 50 micron star.

Other has to do with spherical aberration.

You can calculate first distance quite easily - look into what is resolving power of your aperture and then look at which distance star presents angular size that is smaller than this value.

For spherical aberration - things are not quite as easy as you need to understand quite a bit of optics to calculate that one - but it boils down to this:

Point source "radiates spherical wavefront" around it. When this wavefront hits aperture - it is slightly bent. When we look at objects that are far away - they are effectively at "infinity" as far as numbers are concerned - and that sphere has infinite radius and thus is almost flat - we get flat wavefront. This is what we want, and what the scopes are optimized for.

If our point source is not at infinity - it will introduce some level of spherical aberration to the wave front. When you examine image in telescope - you won't know how much of spherical aberration comes from telescope itself and how much is from the fact that source is close and not at infinity. Closer the source - more spherical aberration there will be (ratio of distance to aperture changes - so part of sphere changes - this is because aperture is fixed but we change the distance).

at some distance this spherical aberration induced by closeness of the target gets very small and can't be detected and won't affect results of optical tests. That is the second distance at which you need to place artificial star if you want to check for spherical aberration of your telescope and want to get accurate results.

[vlaiv]

55 minutes ago, Giles_B said:

Testing the collimation of my 10" dobsonian from a distance of 23m away from the (50 micron) star showed a good pattern of concentric rings, but unexpectedly the circle of the airy disk was not perfectly round, but a bit more like a 20 pence piece.

You could still be seeing effect of resolving the artificial star. If you get the same pattern on airy disk if you move star further - then it is probably related to mirror pinch.

However, it is highly unlikely that you'll see 9 sides in your airy disk even if it is a pinch. That would mean that each point is equally pinching the mirror. Not likely thing to happen.

I'd think that most likely scenario is triangle or similar with pinch.

[Giles_B]

1 hour ago, vlaiv said:

One of distances is related to resolving the small opening of artificial star - your being 50 micron star.

Other has to do with spherical aberration.

You can calculate first distance quite easily - look into what is resolving power of your aperture and then look at which distance star presents angular size that is smaller than this value.

For spherical aberration - things are not quite as easy as you need to understand quite a bit of optics to calculate that one - but it boils down to this:

Point source "radiates spherical wavefront" around it. When this wavefront hits aperture - it is slightly bent. When we look at objects that are far away - they are effectively at "infinity" as far as numbers are concerned - and that sphere has infinite radius and thus is almost flat - we get flat wavefront. This is what we want, and what the scopes are optimized for.

If our point source is not at infinity - it will introduce some level of spherical aberration to the wave front. When you examine image in telescope - you won't know how much of spherical aberration comes from telescope itself and how much is from the fact that source is close and not at infinity. Closer the source - more spherical aberration there will be (ratio of distance to aperture changes - so part of sphere changes - this is because aperture is fixed but we change the distance).

at some distance this spherical aberration induced by closeness of the target gets very small and can't be detected and won't affect results of optical tests. That is the second distance at which you need to place artificial star if you want to check for spherical aberration of your telescope and want to get accurate results.

That is the clearest explanation I could wish for! Amazing

 

certainly the image looked similar to the aberration caused by the spider @pipnina - and i'm reassured by vlav's point that it is unlikely pinching is even across the mirror. However I will take another look with these answers in mind - if nothing else it gives me something constructive to do with scope when the cloud comes in!

[pipnina]

One thing I will add though is that the results at the eyepiece and or camera are the proof that's in the pudding: A flawed star test could help diagnose a problem, but if you are observing or imaging and the views seem good already, then performing a star test is just looking for an itch to scratch!

[Giles_B]

Yep, fully agree I'm just looking for trouble. That's what these short cloudy nights have driven me to!