I'm in the process of designing and building one of my own. I have yet to test it, but it should give me a 0.16mm "star" if I mount a 9.7mm eyepiece on the front of it. At the present it consists of a point to attach a light source (LED torch), a 5mm aperture, 30cm baffled tube, and eyepiece mount. The 300mm tube combined with the 9.7mm eyepiece gives a 31x reduction in the angular size of the 5mm hole. At a distance of 80m, it should provide an angular size of 0.44", which is under the resolving limit of my scope. The real question is if the magnitude will be adequate. My torch has multiple brightness levels, so I'm hoping one of them will work.
I may build a second one using a 5mm high intensity green LED for star testing purposes. The other option would be a green filter on the eyepiece, but that would require me to change the way that I am mounting it on the tube.
The plus side to this design is that it should be usable over the FOV of the eyepiece, in this case a 50 degree Plossl. Some designs I have seen would have a very narrow usable field, or require some specialized parts to build. I don't happen to have any 0.0625mm fibre optics around (or maybe I do?), so I decided to roll my own.
I built a cheap and nasty one, but it works well
I use the std "light shining through hole in alfoil", but made a special punch to ensure i got proper round holes each time.
Basically, i drilled a hole into a piece of wood that i can fit a high intensity white LED.
I then fit the alfoil over the front of the hole and tape it in position.
I then position my punch and make the hole ( in situ ).
The punch is basically a std hypo needle set into a piece of wood.
This acts as a guide, and is very easy to set/hold in position.
I then gently push an acupuncture needle ( 0.12mm ) through the syringe.
Perfect round hole every time, and if you do stuff it up, just load a new alfoil and redo.
Just remember that any closer than infinity for your optics will appear over-corrected. Dick Suiter's star testing book gives minimum distances that indicate and inducement of X amount of spherical aberration.
I've read 24x focal length as the minimum, but am no authority on that. I'll have to look up that book.
Eric
I just did a quick check on Table 5-2 for you.
Artificial star distance is very dependent on the aperture and F ratio. Distance of light source ( in multiples of focal length ) can range from 10X for a 2.4" F5 refractor to 126X for a 24" F4 reflector.
This is the minmum distance to cause 1/4 wave overcorrection at the eyepiece. If you were wanting to evaluate the wavefront probably 1/10 wave or less would be a better figure with coreesponingly longer distances.
If you are just wanting to collimate your scope, or solve problems with say , astigmatism, then distance is not so important.
Thanks Mark. Since I'm mainly using mine for collimation, as you say distance becomes much less relevant, other than just to reduce the angular size. If I decide to actually use it to star test, I'll need to get a bit more precise.
This design offers some advantages over the IIS article design, as it provides a smaller star, which means you don't have to place it as far away for a star test.
Hope you find it as useful as I did. Note, these days, ultra bright white LEDS are easy to find as bright as 20,000 or 30,000 MCD. The article refers to around 5000 MCD, so it's easy to exceed the brightness mentioned in the article,
At MAS we've had good luck with an artificial star made from a single strand of optic fibre poking though a hole in a plastic jiffy box - and lit by a variable LED at the end of the strand. When you stick the strand out of the hole just seal it up on the inside with black (automotive) silicone and when it's set just snap it off clean. Just buy a cheap optic fibre lamp off eBay for about a buck and get a few hundred strands.... just make sure they're glass!
At a distance of about 25 meters, at night, it looked for all intents and purposes like a star. It made collimation a breeze.
