Measuring field flatness without stars

[codemonkey]

I imagine there's a near infinite number of ways to do this, some better than others, some requiring specialist equipment, some not.

We've had lots of cloud cover here lately which means no chance to get out under the stars. I'm wanting to confirm spacing for my flattener so that I can order some custom adapters, which can take a few weeks to get made, plus shipping time.

Is there an especially good, yet simple way to test for this indoors? I'm thinking about just printing off a sheet of paper with a bunch of lines that are close together, take some subs, then checking for any apparent distortion.

[Shiraz]

Maybe get (or make) an artificial star and record the focuser position at "in-focus", at various positions across the field. If you know the focuser step size, you should get an idea of curvature. You will need to place the artificial star a fair way from the scope to get reasonable accuracy - maybe 50+m - so part of it isn't indoors. A LED torch and a pinhole made from Al foil and with one or a few a tiny pinprick(s) in it will do as the star(s).

[codemonkey]

Hmm, that's a pretty good idea Ray, thanks. My focuser's a DIY one with a bunch of backlash and I wouldn't call it deterministic though, so I'm not sure I'd trust that in my particular case.

[Shiraz]

perhaps always drive it the same direction as you approach focus. Or use verniers

[codemonkey]

Fair point. Thanks Ray!