Focusing Thought Experiments

The film we use doesn't see the unfocused light, so it's not a big issue (as far as I know).

I do know that the lodestar autoguider that I use in my bahtinov mask focus confirmation tool sees the near IR.  Thus, I'm considering filtering the light either at the sensor of the digital camera, behind the lens, or in front of the lens.  Each option presents its own challenge.

I found that for the 300 mm lens, the focus pattern didn't seem to have too many problems, but when looking at the 400 mm lens, the star image is sloppy and I can't get a consistent pattern.  I think that the issue is the NIR.  I ended up using no mask for the 400mm images and focused to get the smallest stars when using the digital sensor.  This may have been the biggest issue after sag.

Before the star party, I was doing testing in the back yard where I'd take a picture of a measuring tape at close focus.  I'd develop the film and then set the distance of the lodestar to match the focus spot exhibited on the film.  It required a few days to get it right.  I also was swapping the ground glass back and forth with using shims.  In the end, I found that no shims were required and a ground glass with no fresnel gave the best view.

The testing scene was lit with LED lights which may not have had a NIR component.  Plus, I was testing with the 300mm lens.

The difficulty about introducing an IR filter to the light path has these problems:

1. At the sensor for the autoguider: Easy to do, cheap option ($50 for a screw in filter).  Could GLUE a cheap colored filter in place - yellow would be ideal as it would block blue and should block IR.  Works for all the lenses to be used, adds protection for the sensor when it's not in use.  Disadvantage is that it requires refocusing the system because the sensor and film plane will no longer match. Requires another couple of pictures of a ruler (again).  There may not be enough adjustment room left on the guider body to accomodate an additional 1mm of distance.

2. Behind the lens with a screw in filter.  Expensive but I should have the filter already - just have to find it.  The backfocus adjustment issue should be mitigated because both the film and the digital sensor will be offset the same amount.  Disadvantage is that different sizes of filters would be required for each lens set.  

3. In front of the lens with a screw in filter or gel or square filter.  Expensive as these will be even larger.  Gels don't block IR, either.  I have fabricated a 100mm square filter holder which might work if I can confirm that LEE filters block IR.  I've sent a note off to LEE for information.