I am throwing together an artificial star. I have drilled and aluminium case and fitted a 1000mcd blue LED with a 2.7k dropper and a switch. I have fitted a slightly opaque mask to flatten the response of the led and am working out a way of making a foil 100 micron (.004") round hole.
I tried it last night but found the minimum focus distance is more than 8 meters.
Any ideas on making a 100 micron hole? I was going to enquire about the sizes of acupuncture needles?
cheers
Last edited by DavidU; 17-09-2009 at 01:24 PM.
Reason: spelling as usual
I'd need to dig out my notes, but i made a hole punch using an acupuncture needle, and it works brilliantly.
You can fit the foil over the hole first, then punch it easily
IIRC the minimum sized acupuncture needle was 0.12mm
( and they are really bendy if not supported, hence my punch guide )
Thats great Andrew. I found a .007" guitar steel string I will make a punch out of and give it a go tonight.
I may make it switchable from Blue to green LED's as I'm using it to adjust the air space in my 6" achro to fine tune spherical aberation and collimation.
I think the blue may throw an out of focus airy disk.
Just be careful with the guitar wire
as the end may not be a point, and wont give a round hole.
The acupuncture needles have a sharpened "round" point
vs the triangular point on some other sharp pointy items.
I have a microscope view of the differences in some of the points somewhere.
By only just piercing the foil and rotating the needle,
vs pushing all the way through, you can actually get a smaller circular hole,
but you need steady hands
Any ideas on making a 100 micron hole? I was going to enquire about the sizes of acupuncture needles?
cheers
Fold up a piece of foil until you have many layers. Sharpen up a needle on a grinding stone until you have a really fine tip. Stab the wad of foil, but not right through, and then unravel it until you can find the smallest hole. It's not hard to make a 50 micron pinhole this way.
The smaller the hole , the better definition you'll see in the light and dark Fresnel rings you see when you defocus a star, which will allow you to collimate to a better accuracy.
Another option, if you can't get a small hole, is to use a 10mm eyepiece in front of it.
I've lost the formula, but a 10mm eyepiece about 35cm from a 5mm LED produces a small enough source for the collimation of a 250mm SCT about 60m away. I used irrigation tubing, an LED torch (an old Fenix L2T in this case), and a few homemade baffles in the tube to build mine, along with a cheapo Meade plossl. Baffles were made from irrigation tube caps with their outer edges trimmed off and a small hole drilled through the middle.
I'm also willing to bet that Ozstockman could make some mighty fine holes with his laser
i use some kind of thing from a paint sprayer attachment maybe? the hole is slightly less then 1mm but seems as true as can be- i mount to an led flashight with putty, then put some putty on the side to stick it anywhere- i usually use a distant telescope pole- it works fine for me
Thanks Mark ! Is a ball bearing still the go during sunny days?
A ball is certainly easier than building an illuminated pinhole.
Monochromatic light from a suitable LED will give clearer fresnel ring structure for collimating.
The distance of the source determines how much residual spherical aberration will be inherent in the test set up. The size of the source will determine whether or not you can observe any genuine Airy pattern structure or not, and is really only important if you want to study the optics for finer defects rather than just collimate it.
Hubble Optics produce a LED torch with five precise holes, from 50 to 250 micron diameter. Around us$12...
I removed the steel plate with the holes and put in on my trusty ol' foucault tester. Now I can use a neon, fluoro or LED light at any artificial star size for testing.
Well worth the money!!
Suiter " Star testing astronomical telescopes" has all the mathematics for the size and distance of ball bearings; everything from Xmas decorations to 6mm ball bearings.
The blue 130 micron star I made does a great job for collimation. I studied the IF & OF rings and found the definition lacking. I will toy with the 1500MCD white LED and try again.
The blue 130 micron star I made does a great job for collimation. I studied the IF & OF rings and found the definition lacking. I will toy with the 1500MCD white LED and try again.
You need a much smaller star to see good definition in the fresnel rings .
David, here is Peter Langsford's site which discusses the aluminium foil method which Mark recommends. I built one successfully, and I know that BinTel Melbourne also use one based on the same principles.
I went down the 10mm ball bearing rout today.Dodging clouds every 15 min!
I spent a few hours re building and adjusting the focuser that was .032" off center. I removed the slider blocks from inside the focuser and fabricated some thicker polythene units.
Removed all the grease and measured the eccsentricity with a dial indicator. All centered. put it all back together and recollimated it. The IF &OF fresnal rings look perfect.
The factory element alignment marks were off about 2 deg.
The spherical aberration seems very good.may be about .1RMS.
I will re test the spherical aberration with green light.
but then again I am old, 
well old' ish. 
