imwcs(1)

✨ NAME ✨

imwcs — set WCS in FITS and IRAF image files 💫

(hey bestie, quick vibe check: this page lives in the General Commands Manual, and the command itself is imwcs. ok love u, continue 🫶)

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🎀 SYNOPSIS 🎀

imwcs [options] FITS or IRAF filename

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💅 DESCRIPTION 💅

imwcs (and yes, you say it im-W-C-S, not "im-wicks", ok we've all been there 😭) is a utility for setting the world coordinate system — the WCS — of a FITS or IRAF image. The WCS is literally the relationship between sky coordinates and image pixels, and it can be described in a standard way right there in the header of an image. Not me being obsessed with headers. 🙈

The program was originally written at the University of Iowa, but has been adapted and amplified by Jessica Mink at the Smithsonian Astrophysical Observatory. glow up fr. 👑

🚨 important reality check, bestie: imwcs currently works only on images where the rotation about the center is known to within a few degrees. It has been tested extensively only on images with near-zero rotation. no cap. 💀

So how does it actually do the thing? A plane-tangent projection (center right ascension and declination, plate scale, and rotation angle) is fit based on matches between stars in the image and stars in a reference catalog. And which catalogs, you ask? The HST Guide Star Catalog, the USNO A 1.0, SA 1.0, or UJ Catalog, or a user-supplied, tab-delimited Starbase catalog. pick your fighter ✨

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🔥 OPTIONS 🔥

ok so here's the full menu, no skips, we're eating everything 🍓

• -a <angle> — Initial rotation angle in degrees (default 0).
• -b <RA> <Dec> — Initial center in B1950 (FK4) RA and Dec (default=RA,DEC from file).
• -c <catalog name> — Reference catalog. Options include gsc (=HST GSC), ua2 (=USNO-A2.0), TMC (=2MASS Point Source Catalog), gsc2 (=GSC II), ub1 (=USNO-B1.0), local file, etc.
• -d <catalog name> — Use this DAOFIND output catalog of X, Y, and magnitude instead of searching for stars in the image. The format is simply white-space-separated X, Y, and magnitue on a line, with # at the beginning of comment lines. (yes "magnitue", we keep her exactly as she is, iconic 💅)
• -e — Erase image WCS keywords, getting initial WCS from command line.
• -f — Write FITS output no matter what input.
• -g <class> — Guide Star Catalog class (-1=all,0,3) (default -1).
• -h <number> — Change the maximum number of reference stars to match from 25 to… and that's literally where the sentence ends on the original page 😭 not me editing it, fr fr, that's all it says.
• -i <counts> — Minimum peak value for star in image. If negative, use -<counts> standard deviations as minimum.
• -j <RA> <Dec> — Initial center in J2000 (FK5) RA and Dec (default=RA,DEC from file).
• -k <magnum> — Use this magnitude to sort brightest catalog stars.
• -l — Reflect left<->right before rotating and fitting.
• -m [<bright magnitude>] <faint magnitude> — Limiting catalog magnitude(s) (default none, bright -2 if only faint is given).
• -n <number> — Number of parameters to fit (1-8, or digit string). The default is 2 (RA+Dec) if <3 stars are matched, 3 (RA+Dec+scale) if <5 stars are matched, or 4 (RA+Dec+scale+rot) if 5 or more stars are matched. Alternatively this may be a list of all of the parameters to fit, in the form nnnnnn.., where 1=RA, 2=Dec, 3=RA scale, 4=Dec scale, 5=rotation angle, 6=Y axis rotation (CD matrix), 7=RA reference pixel, and 8=Dec reference pixel.
• -o <filename> — Write output to this file; may be the same as input file. This option automatically sets the -w option to write the file.
• -p <scale> — Initial plate scale in arcsec per pixel (default 0).
• -q <option list> — <i>terate, <r>ecenter, <s>igma clip, <p>olynomial, <t>olerance reduce (half for each iteration). A number following an option repeats the option that many times.
• -r <angle> — Rotation angle in degrees before fitting (0, 90, 180, 270) (default 0).
• -s <frac> — Use this fraction extra stars (default 1.0).
• -t <num> — Offset tolerance in pixels (default 20).
• -u <file> — File of X Y RA Dec assignments for initial WCS.
• -v — Verbose listing of processing intermediate results.
• -w — Write WCS to a new file (default is read-only).
• -x — X and Y coordinates of reference pixel (default is image center).
• -y — Multiply image dimensions by this for search (default is 1).
• -z — Use AIPS classic projections instead of WCSLIB; use CD matrix or CDELT instead of polynomial solution.

heads up bestie: on the original page, -w, -x, -y, and -z are all squished together into one run-on line. I gave each one its own bullet so your eyes can breathe, but every single word is still there. 🫶

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🌈 WEB PAGE 🌈

http://tdc-www.harvard.edu/software/wcstools/imwcs/

go look, she's got lore ✨

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👑 AUTHOR 👑

Jessica Mink, SAO (jmink@cfa.harvard.edu) — main character energy, we stan 💜

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and that's the whole page, bestie!! 🎀 this absolute banger of a manual page comes to you from the WCSTools crew, dated 30 January 2003, and it's page imwcs(1) of the manual. go set some WCS, u got this 💖✨🦋