Artificial Star(s) - Fibre Optic/LED - anyone interested?

[Moonhawk]

I built a prototype artificial star and am quite encouraged by the results. It uses a 20 lumen super bright white LED coupled to a fibreoptic cable with either a 62, 50 or 9 micron core.

http://www.jupiterrising.co.uk/AstroGallery_util.htm

I have had a couple of people show interest in this and am thinking of building a batch of them. Would anyone be interested - the cost should be approximately £30-35+P&P (based on a 50 micron star with a 1m cable and without the mini-tripod - although the star itself will be tripod mountable).

Options at additonal cost.

Cable lengths up to 20m

Multiple Star sizes (e.g. both 50 and 9 micro)

[JohnC]

Yes i would be interested in one.

Best regards

John

[Doc]

You state that the photos were took 4.5m away. Most artificial stars like the Astrozap needs about 30 metres. What makes yours closer?

[Moonhawk]

It depends on the size/resolution of your telescope. The top image was taken using a 3.5 inch celestron Mak-newt which has a theoretical Dawes resolution of about 1.55 arc seconds. For a 50 micron artificial star to appear 1.55 arcseconds across would require it to be at a distance of about 6.5 meters. This would be optimal for the artificial star to be an effective point source.

For a 10" telescope - the resolution would be about 0.54 arcseconds, therefore giving an optimum distance of about 19 meters.

Obviously both of my scopes were closer than optimal - but the diffraction patters are clearly visible non the less.

My figures agree with those quoted on the rigel picostar - which uses similar technology (albeit more nicely packaged).

http://www.pulsar-optical.co.uk/prod/rigel/generalaccessories/picostar.html

[Doc]

So how far away would it need to be for my scope.

Meade Lightbridge 16" F4.5 FL 1829mm

[Moonhawk]

Your scope has a theoretical resolution of 0.34 arc seconds - so a 50 micron artificial star would have to be just over 30 meters away to be optimal.

Equations I used

Telescope Resolution (arcseconds) = 0.1384/D (where D is the diameter of your scope in meters)

Artificial Star Size (radians) = 2 * atan * (d/D) (where d is the diameter of the star and D is the distance in the same units)

Arcseconds = Radians * 206264.8

[Doc]

Thanks for the info but I haven't got that much room spare in my garden.

[Moonhawk]

Those figures were based on a 50 micron star. A 9 micron star could be placed approximately 6 m from the scope (assuming you can focus that close). The brightness of a 9 micron star is a lot lower than the 50 - but with a 16" scope I wouldnt imaging this would be too much of a problem.

[Moonhawk]

Yes i would be interested in one.

Best regards

John

Sorry - missed your post. I'll add you to the "interested parties" list. What options would you be looking at if you did decide to go ahead - cable length, star diameter(s) ?

[Moonhawk]

Sorry multiple post - damn brodband!!

[Moonhawk]

Sorry - duplicate post

[JohnC]

I have an LX200 8" would go for the brighter option.

[Merlin66]

Doc, and others...

To get say 10 or 20 metre distance to the artificial star you can reflect the star image of a small mirror ( temporarily removed from a secondary or star diagonal) placed infront of the scope and illuminated by the artifical star mounted just behind your shoulder directed towards the small mirror. This doubles the effective distance. A laser pointer and some Blu-tack helps with the alignment.

[Moonhawk]

Although I am offering these pre-built - I have posted some instructions in the DIY forum in case you want to have a go at building one yourself.

[RobH]

Hello MH,

What would the distance need to be for a 14" F10 SCT?

My main reason for not getting one in the past has been the distance needed, but it sounds like it can be done much closer.

I would certainly be interested for the right price

Cheers

Rob

[Moonhawk]

You would need about 27 meters for the 50 micron star. For the 9 micron you could in theory use it at less than 5 meters (assuming you can focus that close - I stuggled with my 10" F10 SCT at 10 meters - I had to use two barlow lens barrens to get the additional distance on my focusser just to get the images for my website).

Unfortunately I dont know what the impact of having the star that close to the telescope would be - the light reaching the telescope is going to be far from parallel to yout telescopes tube.

[Merlin66]

Suiter in his book " Star Testing of Astronomical Telescopes", page 84 states that the pinhole should be about 20 times the focal length of the scope being tested otherwise spherical abberations will be significant.....

[Moonhawk]

Suiter in his book " Star Testing of Astronomical Telescopes", page 84 states that the pinhole should be about 20 times the focal length of the scope being tested otherwise spherical abberations will be significant.....

Thanks for that - ok so that changes things quite a bit. For my LX90 that would mean an optimal distance of at least 50 meters.

[ringz]

Suiter's rule of thumb is irrelevant though if you are only using the artificial star for collimation purposes.

The spherical aberration that is induced by having the light source too near to simulate a parallel wave front can be ignored.

--

Martyn

[Moonhawk]

I guess this would make the artificial star useless for testing for the presence of optical abberations - but as you say, collimation only requires that you can get the diffraction rings symetrical. From my test images - i'd say they were certainly clear enough for that.