Artificial star, alternative method.

[ollypenrice]

I first heard of this from TeleVue when I asked them how to collimate one of their refractors. I've since seen it used in professional optical shops such as Dany Cardoen's at Puimichel. It is insanely simple. Glue a ballbearing onto a dark surface. That's it. Here I quickly lined a pastic box with flocking. It keeps light from getting too much onto the sides of the ball. It works in daylight or in the dark if the BB is lit by a torch beam slightlly off to one side. The angle at which you observe the BB in the scope is of no importance. A BB is spherical. Only light from the tiny surface closest to the scope reaches the scope.

We made it to test an RC which had just been laser collimated. First we tested the BB with a good refractor and it gave perfect diffraction circles. We then tried the RC and found it was miles out, probably due to tilt. In a nutshell the BB technique works superbly. This is the third time I've had success with it. 10 minutes to make and pretty well free!

Olly

 

[Robp]

I think this is one I'm going to try myself.

Thanks for sharing Olly.

[kirkster501]

I have likewise found this method very successful.  Much better than "artificial stars" where one can never get the light aperture hole small enough over a practical distance over which to separate the scope and artificial star. 

The ball bearing method can be used at short distances.

[ollypenrice]

29 minutes ago, kirkster501 said:

I have likewise found this method very successful.  Much better than "artificial stars" where one can never get the light aperture hole small enough over a practical distance over which to separate the scope and artificial star. 

The ball bearing method can be used at short distances.

Yes. Also I have my doubts about holes as point sources. It would be good to hear from a physicist on this but when I think of the double slit experiment and the way light propagates from a slit I wonder how well it might replicate a point source. Where's Richard Feynman when you need him?

Olly

[kerrylewis]

Interesting. I've looked at possible artificial stars before but they always seem to require being placed some distance from the scope which I can't achieve in my garden (too much vegetation!). So I'm interested in the comment that this method can be used over short distances. Can anyone say how short?

Kerry 

[ollypenrice]

On one occasion I used it at maybe 25 feet in a warehouse. The thing is to test it on a good scope before you try the project scope. If you get clear diffraction rings on the good one then the test is good.

Olly

[YKSE]

Yepp, that's a good method. I fine-tuned my C8 some weeks ago with a BB inside, no need to waste observing time when seeing is perfect

A refractor may need extension tube to get in focus inside though.

[laudropb]

What a great idea. Must try it soon. Thanks Olly for the info.

[Peter Drew]

Be aware though, that although good enough for collimation purposes, this setup should not be used to evaluate optical quality. The proximity of the light source can lead to misleading appearances. 

[michael8554]

Does the star need to be located to the north of the telescope to be able to get a reflection from the predominately southerly sun?

Will it still work if located to east or west of the telescope, with a 90 degree reflection from the spherical surface?

(My setup has a neighbour's fence and house immediately to the north)

Michael

[sloz1664]

Cracking idea. I'll have to have a go at this with my TAL 200K. Ball bearings.......must look through my Man Drawer

Steve

[CraigT82]

12 hours ago, michael8554 said:

Does the star need to be located to the north of the telescope to be able to get a reflection from the predominately southerly sun?

Will it still work if located to east or west of the telescope, with a 90 degree reflection from the spherical surface?

(My setup has a neighbour's fence and house immediately to the north)

Michael

I think the best way to do it is to shine a bright torch at the BB. Super bright LED torches and so cheap these days! 

[CraigT82]

I'm guessing that the smaller the BB the smaller the speck of light that is reflected back to the optics? Or is it the case that once the BB is down to a certain size it essentially becomes a point source and hence reducing the size of the BB further had no effect on reducing the size of the speck of light? 

 

Wonder what hat the maximum size of BB you can use to reflect a point source? 

[YKSE]

A common size of BB should work fine. I've BB of size 10mm and 20mm in diameter, essentially no difference.

[ollypenrice]

15 hours ago, michael8554 said:

Does the star need to be located to the north of the telescope to be able to get a reflection from the predominately southerly sun?

Will it still work if located to east or west of the telescope, with a 90 degree reflection from the spherical surface?

(My setup has a neighbour's fence and house immediately to the north)

Michael

My longest handy axis is E/W and that worked.

2 hours ago, CraigT82 said:

I'm guessing that the smaller the BB the smaller the speck of light that is reflected back to the optics? Or is it the case that once the BB is down to a certain size it essentially becomes a point source and hence reducing the size of the BB further had no effect on reducing the size of the speck of light? 

 

Wonder what hat the maximum size of BB you can use to reflect a point source? 

I guess as you do, but the BB in Dany's workshop was a large one. I used a small one and would, on reflection, (groan) go for smaller still. A nice shiny new one would be good!

Olly

[zicklurky]

Ohhh this is handy, cheers Olly!

[N3ptune]

Hemm.. I wish to share the following information, I found an old childhood game inside a box with various BB sizes (has the picture bellow). Depending of the angle I put the flash light on each of them, the speck is bigger or smaller but generally the smallest the bearing the smallest the speck too.. it can be seen in the picture.

Box contain +- these: 2mm, 5mm, 7mm and 10 mm

With full direct exposure, the 10mm has a speck almost twice the size of the 5mm to me. But maybe it's not that important. Personally I would try with a 5mm to build an artificial star following these observations.

Thanks Olly for your idea, I like it too.

[skybadger]

For the detail on this method check out Suiters 'testing astronomical telescopes' . You need a minimum distance for both ball bearing and pin hole sources. It depends on size of source and size of optics (resolution). The size of the bearing dictates the size of the image and Suiter shows that a 2"Christmas bauble is just as suitable. 

Bring the source near introduces spherical aberration and requires focusing extensions to get to focus and finally - I recommend never to collimate with a horizontal scope - it bears no relation to reality in use.

Other than that it probably works very well for very stiff, well constrained , rigid optics where you can't dream of aligning 3 elements anywhere but on a bench.

Cheers

Mike

[N3ptune]

Olly,

Do you point some kind of lamp at it during the night or you simply use whatever light you can see on the ball from the streets or houses around?

Thanks

 

[ollypenrice]

Streets or houses around????   Quelle horreur!!!! We don't have anything like that here, oh the very idea... 

No, just use a torch beam which can be propped up nearby and sufficiently off axis to give a clear view of the BB.

lly

[N3ptune]

Ok i'll try that real soon (:

I have a spot 340 meters away to install my own future BB device. (:

[skybadger]

That's more like it.

 

[N3ptune]

Olly, I did my one similar version of the BB artificial star, you can see it on my previous thread if you want.

[cardconvict]

This is great I will be making one to collimate my C9.25 thanks for the detailed instructions Olly

[joe1950]

I once found a kitchen cabinet knob that had a very nice convex mirrored surface. So I made a small, portable artificial star arrangement on a piece of wood.

The torch is a single LED type, covered by a cigar tube that properly fit and I used a fine drill bit to make a 0.8mm aperture in the tube end. The small hole and the convex knob provided an image smaller than the resolving capabilities of my scope (which is the goal) yet an image bright enough to display an Airy disk and a bright first ring and usually a detectable second ring.

It was such a joy to use without being concerned about seeing conditions, awkward positions and moving stars. Even Polaris moves some and proper collimation requires the star image to be centered as close as possible.

The gizmo could be used during the day by simply placing it in a cardboard box to shield reflections from other sources. Of course, a smaller BB or ball bearing would do well, though too small and the brightness is diminished.

I read an article authored by someone very capable in optics (but the name escapes me) who suggested a minimum distance from the scope under test to the reflective sphere of 20 times the focal length of the objective. This would minimize the chance of proximity causing an out of focus star test to display false spherical aberration, if testing for such or other possible optical characteristics. I generally placed the 'star' further, as space permits.

For my 150mm f/8 reflector, 1200mm focal length, the 'star' should be at least 24 metres distant (about 80 feet, here) My garden allowed me to extend that distance to nearly 40 metres.

However, if used only for collimation purposes, as pointed out by others, the distance is not critical and can be shorter, significantly shorter, than the 20 x FL standard. One must be concerned that the 'star' image is small enough so as not to be resolved as an extended object by the scope, that the shift in focus point not result in vignetting, and that the assessment of collimation be done with the 'star' image centered in the FOV.

I know of some who have successfully collimated scopes using this artificial method within their home, having long hallways or the like.

I hope I've added some to the topic. I thoroughly enjoy dabbling with these sort of gadgets!

Joe