bias¶

bias averages zero-exposure frames into a master bias, the electronic offset of the readout.

It is the first step of the calibration sequence. Each pixel is a robust mean, so cosmic-ray hits are rejected. Bias frames are cheap, so take many (more than 10 is better).

Usage¶

munipack bias [options] file(s)

Options¶

Option	Default	Description
`-o`, `--output`	`bias.fits`	Output file.
`-st`, `--saturate`	header `SATURATE`	Upper limit of raw values used [ADU].
`-th`, `--threshold`	1	Lower limit of raw values used [ADU].
`-bitmask`	—	Mask frame (0 = skip pixel).
`-B`, `--bitpix`	−32	Output type: −32, 32, 16 or 8.
`--verbose`	off	Print the mean level, exposure time and temperature.
`-h`, `--help`	—	Show the help.

Output¶

The master bias, with a STDERR extension holding the error of each pixel. Give it to dark, flat and phcorr with -bias.

Examples¶

# master bias from all bias frames in the directory
munipack bias -o bias.fits bias_*.fits

# frames listed in a file, with a bad-column mask
munipack bias -bitmask badcols.fits -o bias.fits @bias.lst

Common to bias, dark and flat¶

Pixel rejection. Raw pixel values outside the range set by -th (threshold) and -st (saturation) are left out of the average. Both are in raw ADU. Saturation defaults to the SATURATE header keyword.

Masks. -bitmask takes a FITS image of the same size. Pixels with value 1 are used and pixels with value 0 are skipped.

Header checks. bias and dark warn when the inputs differ in exposure time or chip temperature, and flat warns when the filters differ. If your files use other keyword names, set the FITS_KEY_* environment variables, for example FITS_KEY_EXPTIME=EXPOSURE. Others are FITS_KEY_FILTER, FITS_KEY_TEMPERATURE, FITS_KEY_DATEOBS, FITS_KEY_GAIN and FITS_KEY_SATURATE. The read noise is taken from READNS (in ADU) or RDNOISE (in electrons).

Input lists. Give files as arguments, as @list.txt (one name per line), or as - to read the names from standard input.

Output. Each master has a STDERR extension with the error of each pixel. Keep the default float output (-B -32); an existing output file is replaced.

Docker. Mount your data on /data and pass environment variables with -e, e.g. -e FITS_KEY_EXPTIME=EXPOSURE. Add -i to docker run when reading file names from standard input.