NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Notes on camera CN
From: Peter Monta
Date: 2013 Sep 26, 12:57 -0700
From: Peter Monta
Date: 2013 Sep 26, 12:57 -0700
A few notes on possible camera issues when using them for CN: - Rolling shutter might be a concern for CMOS sensors. A CCD-based DSLR doesn't have this problem (global shutter). One way to get around it is to shoot a pair of objects always at the same height in the frame (y coordinate) so that the rolling shutter hits both objects approximately simultaneously. For collimation reasons one wants to do this anyway. - Shoot in raw mode. I was pleasantly surprised that CHDK, the third-party Canon firmware, supports the SX160. The raw pictures are a big improvement: background noise is now uncorrelated, and the images are 12-bit linear rather than 8-bit JPEG, so maybe a shade is no longer needed for the Moon. I'm also using just the green channel, since the lens is showing some lateral chromatic aberration near the edges. - Image stabilization considered harmful? After seeing spread of nearly half an arcminute over a long series of shots that should be much better, I'm starting to suspect it. The next run will have it off. I believe this camera moves a lens element, not the sensor, and perhaps this alters the distortion profile or has some other bad effect. - Use the bright sources. I'm leaning back toward short exposures (1/60 sec) and some a priori knowledge of which bright sources are within the frame. A pity. - Use star-star distances as local calibrators. For example, tonight the Pollux-Moon, Betelgeuse-Rigel angles are nearly equal; photographing them one after the other allows the sextant angle to remain unchanged, cancelling all the sextant errors. It's best to set the sextant to the midpoint of the two angles; that way any lens-distortion-model errors would also tend to cancel (I think). Orthogonally to the above, one can also take Capella-Moon, Capella-Aldebaran. With these four photos, one should be able to pin down the Moon in both coordinates. Not there yet, though. I should put everything on a tripod and use a DSLR; that way there would be guaranteed success, and then work back toward the handheld situation. Cheers, Peter