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Now that I have a mounting fork for my point-and-shoot camera, taking
photos through the sextant is easy and stable.  As a first step, I
thought I'd take an image of stars with the sextant set to 40 degrees
0.0 minutes, then do some astrometry.

Of course the image is a mixture of two star fields, one from the
horizon-mirror path and one from the index-mirror path, making it more
difficult than usual to identify stars.  Gratifyingly, astrometry.net
seems to be up to the job.  Submitting the original image allows it to
find a first location on the sky, together with many puzzling alien
sources without any catalog matches; but its algorithms are robust and
it soldiers on, using the good matches.  Then I used an image editor
to blot out just the star images it used to find the first location.
Resubmitting the now-altered image, the software finds the other
location on the sky, 39.94 degrees away!  Is that cool or what?

Here are the links:

http://nova.astrometry.net/user_images/80012#redgreen
http://nova.astrometry.net/user_images/80013#redgreen

Red circles are sources detected in the image (some of which are
spurious); green circles are catalog objects.   As you can see, the
first link contains matches (aligned red and green circles) mostly
from the bottom half of the image, and the second link mostly from the
top half.  I think this is from a certain amount of vignetting---the
sextant has a traditional split horizon mirror.

So the upshot is that one simply waves the sextant around the sky,
pushes the images at some software, and back comes a sextant
calibration across its entire arc good to a few arcseconds (once
everything is working right).  Much easier than a long series of
star-star sights.

For this image, index error means that the disparity is only about
0.01 degree (not 0.06 degree), but this is still too large.
Refraction is not yet taken into account, and maybe collimation is an
issue (can be inferred from the image?).  Also the manual step of
erasing star images needs to be automated, which will present no
particular problem.  But in principle, it's looking pretty good.

Cheers,
Peter

   

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