Artificial Star

[AlistairW]

Hello,

I have read about using a ball bearing, a piece if black card and a light source to create an artificial star. Could someone explain what an artificial star is used for, and how this method works - I can't quite picture the configuration.

Thanks

Alistair

[cloudsweeper]

I use a "star simulator" to collimate my SCT in daylight.

It consists of a piece of thickish black plastic with a very small hole drilled through it, the hole then being filled with white DIY filler.

I suspend this as far away as possible and then perform the collimation using the image of the small, round white dot.

It obviously works well since a star test with the 'scope reveals nice concentric circles, and - in good conditions - all images are sharp and detailed.

Simple, effective, and costs nothing!

Doug.

[Stu]

This site, which sells ready made artificial stars, has some explanation of what is involved.

http://www.hubbleoptics.com/artificial-stars.html

Basically you put the scope horizontal and position the star a suitable distance away (I believe a few multiples of the focal length but can't recall exactly). You then centre the 'star', refocus and star test as you would on a real star.

[ollypenrice]

The ballbearing is widely used by professionals in optical workshops.  Dany Cardoen uses them at Puimichel.

You can simply glue a BB onto black flocking or other non reflective surface and set it up a good distance away - about 30 metres in my case. You then observe the BB in your scope. By going just inside and outside focus you can then check your diffraction pattern, hoping to find symmetrical concentric circles inside and outside focus. There is plenty of internet info an star testing to get you started.

The BB may work from sunlight or may need an outside beam aiming at it, depending on conditions.

It works because the tiny bit of BB surface closest to the the telescope is the only part to reflect light towards the scope.

Olly

[Peter Drew]

There are subtle but important differences between using an illuminated ball bearing for collimation purposes and that for determining optical quality. For the former, sunlight or torchlight on to a ball is sufficient to produce a small starlike image for collimation provided the telescope will come to focus at its distance. For the latter, an optical bench is required, the ball, as small as possible to reduce the image, is illuminated by a collimated light source,the ball is at the focus of a paraboloid of good figure and large enough to exceed the diameter on test. The optics on test are aligned with the image seen in the bench mirror which will be seen as being at infinity.  

[ronin]

Not sure of the optics of the Hubble item but the others are a point source and as the distance is not great it is not really a "star", you need the light from the "star" to be collimated.

This can be approximated to by thwe use of a point source and a convex lens, where the seperation is the focal length of the lens:

Then you need to place the axis of this artifical star on the axis of the scope - not overly easy.

[ollypenrice]

5 hours ago, Peter Drew said:

There are subtle but important differences between using an illuminated ball bearing for collimation purposes and that for determining optical quality. For the former, sunlight or torchlight on to a ball is sufficient to produce a small starlike image for collimation provided the telescope will come to focus at its distance. For the latter, an optical bench is required, the ball, as small as possible to reduce the image, is illuminated by a collimated light source,the ball is at the focus of a paraboloid of good figure and large enough to exceed the diameter on test. The optics on test are aligned with the image seen in the bench mirror which will be seen as being at infinity.  

Thanks Peter. That's an important distinction. I've only ever used the BB test for collimating, for which it seemed to work well. The first time I did it I first 'tested the test' by using a good refractor which was giving excellent images. The BB gave concentric circles in and out of focus. I then moved over to a refractor which had been damaged in a fall and which I was trying to repair. It gave a terrible star test but was easily collimated to give concentric circles again so I was happy with the result.

What's your view of the Pico Star artificial star?

Olly

[Peter Drew]

1 hour ago, ollypenrice said:

Thanks Peter. That's an important distinction. I've only ever used the BB test for collimating, for which it seemed to work well. The first time I did it I first 'tested the test' by using a good refractor which was giving excellent images. The BB gave concentric circles in and out of focus. I then moved over to a refractor which had been damaged in a fall and which I was trying to repair. It gave a terrible star test but was easily collimated to give concentric circles again so I was happy with the result.

What's your view of the Pico Star artificial star?

Olly

Fine when the weather isn't.