Disposable SCT Artificial Star

[michael8554]

My SCT is permanently mounted, and collimation remains good, with only a small tweak necessary "on the day" on a nearby star.

So when I recently cleaned both sides of my SCT corrector plate, I thought about buying an artificial star to get collimation in the ball-park, until such time as a real star becomes available.

But I reasoned this artificial star would then remain in its box until the next clean in a couple of years time, so I decided to make a disposable artificial star. This was made from readily available household items, so in my case was free.

 I made several calculations of the size of the hole using formulae from a number of sites, which gave a range between 79 and 84microns for my 200mm F6.3 scope. 
I notice that the Hubble unit offers 50, 100, 150, 200, and 250 microns, so my figure isn't outrageous.  For comparison, a 1/16" hole is 1600 microns, so I realised that making an 80micron hole was going to be "suck it and see".

I flattened an aluminium foil milk bottletop and glued it onto a cardboard sheet that had a 10mm x 10mm hole in it. I oversprayed the face of the bottletop black on the telescope side, to increase the contrast, making the
hole more like a star in a black sky. 

 

I laid the cardboard sheet bottletop side down and made the smallest hole I could, the lightest of impressions with a needle - no science here. This sheets fits into one end of a cardboard box behind a larger hole cut into it. 

 

The other end of the box has a hole cut into it to take a standard pendant light fitting - you could borrow one from your spare bedroom if you haven't a spare one - with a 30W energy saving bulb inserted.
The box was loosely closed to allow hot air to escape. 

I put the box on top of a shed and framed up the hole at 200X magnification, adjusted collimation, then fined tuned at 400X. The telescope wasn't pointing much above zero dec, so the final adjustment on a real star at much higher dec may reveal some mirror flop has taken place. 

If you setup every night there are more permanent diy stars that use a battery driven LED andreverse eyepiece projection of the hole, so that eg a 1/16" hole is reduced to an accurately sized smaller image of the hole. 

http://observatory.mvastro.org/library/Star_Test/ArtStar.html

Also I'm not sure if the Daylight bulb I used has a suitable wavelength or not.

Michael

[Ajohn]

Opticians have been fond of using microscope objectives to provide small pin holes also a wide beam for testing very fast optics such as Schmidts.

Might be of interest to some.

Most of the newer ones are designed for what is called an infinite tube others and older ones are used with a fixed tube length. Apart from Leitz most are marked with a tube length of 160mm. This mean that if a pin hole is positioned 150mm from the shoulder of the objective the size of the "hole" that comes out will be reduced by the magnification of the objective. 10x and 20x seem to have been popular but the higher the magnification the wider the beam that comes out. The objectives are corrected for working at this distance so it's a high quality beam. It's possible to buy drills easily down to 0.3mm / 0.012" dia. It's also possible to make neat pin holes in foil. Place it on a piece of lightly ground glass. Press a sowing needle down on it vertically and rotate it. The ground glass will stop it from skidding. People have been known to strop the needle on news paper to make it extremely sharp and obtain pin holes down near to 1 thou diameter.  If some one has a microscope they can have a look and see. With a bit of care it seems to be fool proof anyway.

It might be best to use a high powered green led for illumination as most optics are fully corrected for green. On the other hand if something else is used and there aren't any funny chromatic effects other than any due to the scope so what.

John

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[badnit12]

I just had a go at producing a small pin hole in some self adhesive foil, I'm at work so only had a needle that I reserve for removing splinters  

Anyway, I was able to measure the hole on our Micro Vickers hardness tester, which is  normally a machine for accurately measuring the size of a diamond impression to determine the hardness of heat-treated metals. My effort was difficult to measure as under high magnification the hole looks jagged, but the average size would be about 25 microns.

So if size is important ? I wound be able to measure for you, as long as you enclose a pre paid envelope.

Andy

[badnit12]

I forgot to add notify any replies - oops!

[Ken82]

That's a cool idea its almost like a back to front pin hole camera