Astro

Astronomy and telescope related interests

One of my favorite projects while working for the UCLA Astronomy Department was rehabilitating the UCLA 24″ telescope.

A few years ago, I converted a Meade ETX-90RA into a Goto telescope. It was a great improvement. I’ve since upgraded it to use a Meade Coronado single arm RA and Dec mount.

Back in the mid 70’s, I made (and briefly used) a 6″ aluminum telescope mirror. The aluminum mirror blanks and processing instructions came from a company called Innovative Optical Materiel. My mirror (a standard f8 design) worked well, although the natural microscopic roughness of even polished aluminum produced a slight halo around bright objects.

Innovative Optical Materiel is long out of business. The owner, Tom Settimi expanded the original company into Sky Scientific. Among their products is a range of inexpensive aluminum mirror blanks.
If you decide to make an aluminum mirror, I’d like to hear about your experiences. The blanks from Sky Scientific are ready to use and they include updated grinding and processing instructions. For a larger mirror (6″ – 8″) just about any aluminum should work for the blank, although T6061 is probably the easiest to obtain. NASA Tech Brief 1831, Process for Polishing Bare Aluminum to High Optical Quality describes using India Ink to polish the aluminum surface. This is critical is you want a high quality telescope mirror.

A few years ago, I decided to make a new aluminum mirror. I used the equations below to calculate the curve required, then cut the curve on a lathe:

Calculating the Sagitta of a spherical mirror:

S is the Saggita, ROC is the mirror’s radius of curvature and r is its radius.

S = ROC - SQRT (ROC^2 - r^2)

Sagitta at any radius of a spherical mirror:

Sr = SQRT (ROC^2 - y^2) - SQRT (ROC^2 - r^2)
 where ROC = radius of curvature
r = radius of mirror
y = radius at which sagitta is measured.

updated January 30 2020