Ad Astra Observatory

So I’m thinking about building an optical bench, and what I’d like to do is something like this:

1) The tester and mirror are on the same rail.
2) The tester can move up and down to keep the ronchi image in view (as opposed to doing this from the mirror stand end)
3) The tester will have a camera (probably a webcam) to be used to view the test images on a computer screen.

Things I’m going to want to add at some point:

– a sighting laser to help line things up initially
– the ability to rotate the mirror while testing (can you say “astigmatism check”?)

– a variable-brightness light source… because eventually, I want to produce a “perfect” image (calculated) superimposed over the “current” image (from the camera), and maybe even have the software compute some analysis ( “you’re 10% undercorrected, with a turned up edge — continue with a 4/5 stroke” ).  So being able to attenuate the light source properly will help the computer evaluate what the camera is seeing.

I don’t know that I want to design in the ability to use the device visually, but if that’s in the plan, I need to figure out how to effect this (*without* moving/removing the camera if possible).

I’m also thinking about motorizing the test head in both axes (up/down and in/out), probably also controlled by the computer. I want to have the axes move relative to each other; that is, as you move closer to the mirror (from outside of RoC), the “spot” on the mirror moves downward, so why not have the vertical axis lower automatically relative to the horizontal axis?

This leads to my questions.

First, why does the spot move downward? Obviously, there’s some problem with angle-of-incidence, but does this still occur if the bench is all locked together? Where could I find some decent equations about this? I’m thinking about trying to put together a software simulator (maybe could use it as a “testing tutorial” program) to look at this, but need to know what I’m trying to simulate (ie; does the spot move because the light source is not on the optical axis? the viewer?)

Second, would you expect that the dy/dx is based on:
– the mirror? (size, figure, …)
– the tester? (distance from the true optical axis of the mirror, … )
– the screen? (number of lines)
– something else?

In other words, *is* the dy/dx calculable? Also, do you think it’ll probably be a linear relationship (spot moves by N mm down for every M mm in), or not (spot moves faster/slower the closer to focus you are)? Also, do you remember if the spot moves down whether you’re inside or outside of RoC (does it move “up” when inside RoC?)?

Basically, I’m trying to figure out if I’m going to need to write “guiding” software, or if I can just lock the gears in at a certain ratio and let ‘er rip.

Food for thought.