Building An Inexpensive Artificial Star

Good collimation is important for getting the best performance from a telescope. I recently installed Bob’s Knobs on my Celestron SCT to help with collimation – mostly to reduce the risk of damaging the corrector lens when fumbling at night with a screwdriver. Unfortunately, installing the knobs put the telescope way out of collimation – it wasn’t just a minor mis-alignment, in-focus stars were comet shaped and the out-of-focus stars appeared as crescents rather than the desired donut pattern. I spent a couple of hours, over two nights, trying to improve the collimation to no avail but some Google searches and building an incredibly simple artificial star helped.

Flashlight with aluminum foil
An Inexpensive Artificial Star from a bike light and aluminum foil.

An artificial star is a small point of light that is used for collimation or optics testing. With an artificial star, collimation can be done during the day and doesn’t require clear skies or good seeing conditions. Artificial stars are available commercially from $25 USD but a precision artificial star is not required for rough collimation so I “built” one for free using a bike light and aluminium foil.

I started by poking four small holes in a piece of aluminium foil with a sewing needle while varying the pressure to produce holes of different sizes. The foil was then wrapped around a bright MEC bike light and secured with a rubber band.

startest_collimation_in_tweaked
The donut pattern on a out-of-focus real star after tweaking collimation.

I set up the telescope on its mount in my yard and aimed it at the artificial star which was placed on on barrel about 20 meters away. The second largest hole produced the best image when viewed through the telescope so I put a bit of tape over the other holes to block them out then followed these instructions to restore rough collimation in about ½ hour. A few minutes of tweaking at night produced a much improved view and the expected donut pattern on an out-of-focus star.

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