The random points are drawn with a density of 100 points per sq. arcmin, for each patch and each filter that have been processed by the pipeline. We draw the points and probe the mask information at each random position using a python module running within the HSC pipeline environment. The source code can be accessed on gitHub here: https://github.com/jcoupon/maskUtils.
The random points can be used for several purposes; (1) a random sample for clustering analysis, (2) to identify problematic areas, (3) to compute the survey area and the fraction of masked areas, etc. Available information carried by the random points, per filter and per patch, is:
- patch and tract number
- patch and tract geometry (non-overlapping regions: patch_inner and tract_inner)
- pixel mask bits such as flags_pixel_interpolated_any
- local PSF size: recorded as the second moment matrix (see FAQ: how to measure the size of an object, to convert into FWHM)
sky_mean: mean of sky value in 2″ diameter apertures (
merge.footprint.sky == Trueobjects). Constant within patch.
sky_std: standard deviation of sky value in 2″ diameter apertures (
merge.footprint.sky == Trueobjects). Constant within patch. To compute the limiting magnitude at 5 sigma, do:
lim_mag = -2.5 log10(5*sky_std) + 27.0
adjust_densityparameter: a random number between 0 and 1 to adjust the sky density. A cut like
adjust_density < 0.5will decrease the sky density by a factor of two
- number of visits contributed to coadds (countinputs)
pix_variance: pixel variance at random point position (note that the variance given for the object is the sum of the pixel variance over the source footprint).
- Milky Way extinction value (Schlegel map).
- centroid_sdss_flags and flags_badcentroid flags must be ignored because they are meaningless for randoms. If set to False, 85% of randoms are lost.
- The bright-star masks are too large for a few very bright stars and include about 10% of bright galaxies. See also the Known Problems page.
- Some functions of the object database are not available in the randoms database.