P. Clay Sherrod Jun 21, 2012
----- Original Message -----
From: "Bill J Gray" <pluto@...>
Sent: Thursday, June 21, 2012 10:41 AM
Subject: Re: [guide-user] Another in favor of Charon!
> Hi Christian,
>> as I had to do some testing on my data anyway, I gave it a try comparing
>> charon an astronmetry.net on the same machine, charon in a dosbox... The
>> results are not compareable. charon took more than 1 hour and was not
>> able to get me any astrometry data from the same field as astrometry.net
>> did within 12 seconds.
> I'd expect astrometry.net to be mostly limited by connection speed and
> server demand. At least as described in their paper, the algorithm ought
> to be very fast. If astrometry.net can accept a list of point sources
> and return the alignment (i.e., you don't have to send the image back
> and forth, just the much smaller data about what sources are where and
> the astrometric reduction results), it should be _really_ fast... it
> sounds as if that's what you're doing?
> Charon can be insanely slow, as you found, until you have it set up
> with the correct focal length and a suitable tolerance for focal length
> error (that is, you may tell it you have a 1.23-meter focal length, but
> will still have to allow a percent or two of error on this). The speed
> is also very sensitive to the number of stars used for pattern matching.
> Of course, if the pixel sizes are incorrectly set, you're almost
> assuredly out of luck. But once you've got Charon set up, it shouldn't
> take more than a couple of seconds per image.
> Getting it set up, though, can be a pain. The focal length given
> by the manufacturer is often not very close; don't just enter that and
> tell Charon the scale tolerance is 1%. Charon is also picky about
> inverted vs. uninverted. When people send me images and ask about how
> to get them to work in Charon, I usually try the example image both
> ways (inverted and uninverted) with a large scale tolerance.
> You may want to try sending me an example FITS image, and I'll see
> what I can do. I'd need to know what the target object was. I can then
> send you the 'charon.dat' file, and you'd be able to run images without
> trouble. (Keeping in mind that Charon still won't update the FITS header
> with WCS data and will require an approximate RA/dec, and will still
> be running in DOS. Astrometry.net will probably remain your preferred
> -- Bill
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