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Inaccurate astrometry in SCAMP/SWarp workflow - A challenge?
12-21-2018, 21:48
Post: #1
Inaccurate astrometry in SCAMP/SWarp workflow - A challenge?
Hello to everyone who reads this. I have a problem that has plagued me for a little while and, honestly, am not sure whether it is with SCAMP or SWarp.

The work that I do is typically with largish fields of view (15x15 degrees) for meteor work at video(ish) rates. Normally I have no problems with using SCAMP on my EMCCD imagery which has a limit of about 11th or 12th magnitude. Some of the cameras that I'm working with now, however, are much wider field (~80x50 degrees) and not nearly as sensitive (6th magnitude limit). This complicates my choice of catalogue and presents my first problem.

1. I would like to use Tycho-2, but I'm always faced with a 'No source found in reference catalog(s) for group 1; wrong sky zone?' regardless of the fiddling that I do with my scamp.conf

In the end, I've been using NOMAD-1 as the other catalogs that I've tried don't have the brighter stuff that I need. I should also note my imagery has no pointing information to begin with, so I use astrometry.net installed locally to generate a first-pass solution before moving on to SCAMP. Astrometry.net projects my images in TAN-SIP so I first remap (WCStools) to TAN. Possibly this is one problem.

The next step in my workflow is then use SWarp to create a max pixel mosaic. This works incredibly well with my medium-field EMCCD imagery but not well at all for the wide-field data. So, problem two is,

2. Regardless of the SWarp projection and fiddling that I do with my swarp.conf, I always see the warning, 'Astrometric approximation too inaccurate for this re-projection' and 'Significant inaccuracy likely to occur in projection.' I presume that this means that I need to fix things with SCAMP before expecting SWarp to work its magic.

I'm pretty sure that a lot of it comes down to poor conf files, but I've been struggling on this one. Does anyone care to take a look and try to help me out? I've uploaded some files to Google Drive at,

https://drive.google.com/open?id=1nyL8WM...sxJxWJsM3w

There are a selection of raw (no WCS), WCS (astrometry.net solution), SCAMP (remapped and SCAMPed), conf, and coadd files. There's also an example PNG (from the EMCCD workflow) that shows what I'm trying to achieve.

Thanks in advance for any pointers that you can provide!

Mike
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