Tuesday, January 19, 2021

Telescope Focuser

I have mentioned my work on rebuilding Baby, a 3" f/4.5 Newtonian reflector I started making in the late 1980s, before I realized that I had a more important hobby: preventing fascism in America.  

I have slowly been rebuilding all mechanical aspects of it for several years (probably about one hour a week on average).  I am very proud of most of it. Once I had the mirror cell and diagonal spider properly rebuilt,  i discovered that the mirror that I ground,  polished, and figured to a paraboloid wad actually a pretty decent mirror!  

Of late I have replacing a cheap and crummy Edmunds rack and pinion eyepiece focuser.  I really did not need an enormous focal range, and the work of making a Crayford focuser seemed unreasonable.  So I machined a base of acetal,  then an aluminum focuser tube held in position by a set screw.   This is intrinsically not a precision way of getting a sharp image, so I have put a helical focuser in it.   The drawtube gives a coarse focus and the helical  focuser allows very precise focus.

I used it on the Moon this evening with very satisfactory results.  25mm and 18mm eyepieces produced sharp images.  Unfortunately,  there is not enough in focus travel for my shorter eyepieces or the 35mm SuperPlossl.  Solution: take another 1/4" off that acetal base so the drawtube goes a little deeper.

1 comment:

  1. You mentioned Edmunds as in Edmunds Scientific. I hadn’t thought of them for decades. They still sell the Fresnel lens that I bought from them in the 1950s and made the illustrated solar furnace.