I'm in the middle, more or less, of my 12.5", F/5.2 (I think) project. Most of the carpentry is done and optical components are in-house. Since this is a solid tube Dob, I want to be particularly careful where I drill the hole in the tube for the focuser. The tube matches the reinforcement rings and flanges that I routed out exactly and if I screw up the location of the hole the odds of getting another cardboard tube from Lowes the exact same diameter is slim and none. So I want to measure the focal length of the mirror exactly and therein lies the problem. I measured it twice several weeks apart and got two different focal lengths. Either I don't know what I'm doing or I'm not being consistent.
The mirror is nominally F/5.2. It was made by Mr. Dodds at Nova Optical sometime in the 90s, weighs 30 lbs and is 2" thick. I used what I thought is the standard method of measuring by reflecting a clear light bulb onto the mirror and viewing the image on a white screen placed alongside the bulb. I used a clear bulb so I could place the screen and filament of the bulb exactly in line with each other. When the filament appeared on the screen in perfect focus I measured the distance to the mirror and divided by 2.
The first time I did this, I measured 130 3/8" (FL=65.2). Yesterday I did it again and came up with 127 1/2" (FL=63 3/4). So something is wrong somewhere. The only thing different is the first time I had the mirror on a test stand. The second time I had the mirror in its cell mounted inside the mirror box. So the mirror is either F/5.2 or 5.1 and I'm not sure how much leeway I will have with the location of the focuser (a GSO from Agena Astro that I already had). I'd hate to mount the spider and focuser and end up with a scope that won't come to focus. I'm thinking I should measure two or three more times and take an average but other than that I'm stumped. My test apparatus is pretty simple. Mirror and cell on the floor and the light bulb and screen opposite with a tape measure in between.
Ideas anyone? Or if anyone has a picture of how they test (or tested) the focal length of their mirror or maybe a different method, I would certainly be interested in seeing.
Edited by xrayvizhen, 16 November 2015 - 01:12 PM.