Artificial Star Testing

[Moonhawk]

I have been building a prototype fibre optic/LED artificial star. Here are some quick tests I have conducted using my Celestron C90 3.5" Mak-Cas. Both images were captured using a Philips Toucam pro and stacked in registax. Apart from some minor histogram adjustment and cropping - there has been no other processing.

The artificial star I have constructed has interchangeable fibre optics and can accomodate multiple fibre optic core diameters (allowing longer or shorter telescope to star distances). The fibre optics are also flexible allowing the atificial star itself to be placed well away (up to several meters) from the LED lightsource. The artificial star has a mounting that is compatible with standard tripod adapters (although this can be detached if neccesary).

This image was captured using a 50 micron multi mode fibre optic at a distance of 2.5m from the telescope:

http://www.jupiterrising.co.uk/photo/Astronomy/collimate50_COL.jpg

and this image was captured using a 9 micron single mode fibre optic at a distance of 2.5m from the telescope:

http://www.jupiterrising.co.uk/photo/Astronomy/collimate09_COL.jpg

Overall I am quite happy with the results. The next set of tests will be conducted on my 10" LX90.

[JAICOA]

Hi MoonHawk, Very interesting project. Looking forward of the outcome with your sct. Thanks for the info.

[Moonhawk]

Here are some pics of the prototype - attached to a compact camera mini tripod. This version has a 1m fibre optic - but it can be built with a fibre optic up to 20m in length.

http://www.jupiterrising.co.uk/photo/Astronomy/ArtificialStar1.jpg

http://www.jupiterrising.co.uk/photo/Astronomy/ArtificialStar2.jpg

[andyschlei]

Did you get good enough results for accurate collimation?

[Moonhawk]

I havent had the chance to test that yet, however looking around the web for DIY as well as commercial artificial stars - I dont expect mine to perform to a lesser degree.

The image that you see visually through the scope is very similar to the photos I have attached above - the diffraction rings are clearly visible and the lack of atmospheric disturbance really makes it easy to judge how symmetrical they are.

The real test will come when I can try it out on my LX90 (which has been moved around quite a lot recently and I expect has lost collimation). I guess a good test would be to compare the diffraction parttern from the artificial star to that obtained using a real star.

[andyschlei]

When I was looking at artificial stars several years ago the real problem was that they had to be 60 to 100 feet from the OTA for a C-11. This really limits the usefulness in collimation.

2.5 meters would be useful.

[Moonhawk]

When I was looking at artificial stars several years ago the real problem was that they had to be 60 to 100 feet from the OTA for a C-11. This really limits the usefulness in collimation.

2.5 meters would be useful.

This is true, since the artificial star should have an angular diameter less than the resolution of the telescope to be an effective point source.

To calculate the angular diameter of the star - use the equation:

Angular Diameter (arcsec) = 206265*(d/D)

where d = diameter of the fibre optic in mm and D is the distance in mm.

The dawes resolution of a telescope is calculated using the equation:

Resolution (arcsec) = 116/Telescope Mirror Diameter(mm)

Therefore for a C11 - you have a resolution of about 0.41 arcsec - and so would be looking at a minimum distance for the artificial star of about 25m for the 50 micron and about 4.5m for the 9 micron.

Of course there is the problem of focusing - can a C11 focus on something as close as 4.5m?

One point of interest however. The tests I ran above using my C90 at 2.5m distance. At this distance - the 9 micron star's angular size is 0.74 arcsec which is below the 1.30 arcsec resolution of the telescope, however the 50 micron star's angular size at this distance is 4.12 arcsec. I would say that the diffraction rings are slightly clearer on the 9 micron test shot (especially the inner ones), but I still got a clear diffraction pattern for the 50 micron test shot despite it not being an effective point source.

[Moonhawk]

I have done a few more tests on my C90 (still havent managed to get my LX90 out yet).

I tried several focus points using the 50 micron @ 4.5m distance (I also used the 9 micron, but apart from being fainter, I could see no real difference in the diffraction patterns). As before these were captured on a Toucam pro (approx 100 frames) and stacked in registax 4. Levels were adjusted in Photoshop elements.

http://www.jupiterrising.co.uk/photo/Astronomy/50_Test1c.jpg

http://www.jupiterrising.co.uk/photo/Astronomy/50_Test1.jpg

http://www.jupiterrising.co.uk/photo/Astronomy/50_Test1b.jpg

[publiusr]

Thanks for the link.