MegaPipe Stacking Procedure

• Image grouping
• Quality control
• Astrometric calibration
• Photometric calibration
• Coaddition
• Photometric catalogues
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The data are retrieved from the CADC archive. The images have already been detrended with the Elixir pipeline. The images come with a fairly accurate (0.5-1.0 arcsecond) astrometric solution and a photometric calibration. One CCD of each exposure is inspected visually. Exposures with obviously asymmetric PSF's (due to loss of tracking) or other major defects such as terrible seeing, bad focus, or poor transparency are discarded. In some of the exposures, one or several of the CCD's in the mosaic are dead. These images are also discarded.

The AstroGwyn astrometric calibration pipeline is run on the images. The first step is to run SExtractor on each image. The parameters are set so as to extract only the most reliable objects (5 sigma detections in at least 5 pixels). This catalog is further cleaned of cosmic rays and extended objects. This leaves only real objects with well defined centres: stars and (to some degree) compact galaxies.

This observed catalogue is matched to the USNO astrometric reference catalogue. The (x,y) coordinates of the observed catalogue are converted to (RA, Dec) using the initial Elixir WCS. The catalogues are shifted in RA and Dec with respect to one another until the best match between the two catalogues is found. If there is no good match for a particular CCD (for example when the initial WCS is unusually erroneous), its WCS is replaced with a default WCS and the matching procedure is restarted. Once the matching is complete, the astrometric fitting can begin. Typically 20 to 50 sources per CCD are found with this initial matching.

Elixir provides a first order solution for the WCS with typical errors on the order of 1 arcsecond. AstroGwyn improves on this to provide a higher order solution with an accuracy of typically 0.2 arcseconds. As the accuracy of the WCS improves, the observed and reference catalogues are compared again to increase the number of matching sources. A larger number of matching sources makes the astrometric solution more robust against possible errors (proper motions, spurious detections, etc.) in either catalogue.

The higher order terms are determined on the scale of the entire mosaic - that is to say, the distortion of the entire focal plane is measured. This distortion is well described by a polynomial with second and fourth order terms in radius measured from the centre of the mosaic. The distortion appears to be stable over time, even when some of the MegaPrime optics are flipped. Determining the distortion in this way means that only 2 parameters need to be determined (the coefficients of r² and r⁴) with typically (20-50 stars per chip) * (36 chips) =~ 1000 observations. If the analysis is done chip-by-chip, a third order solution requires (10 parameters per chip)*(36 chips)= 360 parameters. This is less satisfactory.

From the global distortion, the distortion local to each CCD is determined. The local distortion is translated into a linear part (described by the CD matrix) and a higher order part (described by the PV keywords). The higher order part is 3rd order as well, but the coefficients depend directly and uniquely on the 2 parameter global radial distortion. The error introduced by this translation is less than 0.001 arcseconds.

For the first band to be reduced (the i-band, if it exists, otherwise the order of preference is r, g, z, u), these source catalogues are matched with the an external astrometric catalogue to provide an initial astrometric solution. This external catalogue is either the USNO A2 or the Sloan Digital Sky Survey DR5.

For the other bands, the image catalogues are first matched to the USNO to provide a rough WCS and then matched to a catalogue generated using the first image so as to precisely register the different bands. The final astrometric calibration has an internal uncertainty of about 0.03 arcseconds and an external uncertainty of about 0.2 arcseconds, as discussed here.

The Sloan Digital Sky Survey DR7 serves as the basis of the photometric calibration. The Sloan ugriz filters are not identical to the MegaCam filters (as discussed here). The colour terms between the two filter sets can be described by the following equations:

u_Mega = u_SDSS - 0.241 (u_SDSS - g_SDSS)
g_Mega = g_SDSS - 0.153 (g_SDSS - r_SDSS)
r_Mega = r_SDSS - 0.024 (g_SDSS - r_SDSS)
i_Mega = i_SDSS - 0.085 (r_SDSS - i_SDSS)
z_Mega = z_SDSS + 0.074 (i_SDSS - z_SDSS)

The relations for the griz bands come from the analysis of the SNLS group. The relation for the u band comes from the CFHT web pages.

All images lying in the SDSS can be directly calibrated without referring to other standard stars such as Smith standards. The systematics in the SDSS photometry are about 0.02 magnitudes (Ivezic, et al., 2004). The presence of at least 1000 usable sources in each square degree reduces the random error to effectively zero. It is possible to calibrate the individual CCDs of the mosaic individually with about 30 standards in each. For each MegaCam image, one matches the corresponding catalog to the SDSS catalogue for that patch of sky. The difference between the instrumental MegaCam magnitudes and the SDSS magnitudes (transferred to MegaCam system using the equations above) gives the zero-point for that exposure or that CCD. The zero-point is determined by median, not mean. There are about 10000 SDSS sources per square degree, but when one cuts by stellarity and magnitude this number drops to around 1000. It is best to only use the stars (the above colour terms are more appropriate to stars than galaxies) and to only use the objects with 17<mag<20 (the brighter objects are usually saturated in the MegaCam image and including the fainter objects only increases the noise in the median). This process can used for data from any night. It is not necessary for the night to be photometric.

For objects outside the SDSS, the Elixir photometric keywords are used, with modifications. The Elixir zero-points were compared to those determined from the SDSS using the procedure above for a large number of images. There are systematic offsets between the two sets of zero-points, particularly for the U-band. These offsets show variations with epoch, which are caused by modifications to Elixir pipeline. There also differential offsets between the CCDs of a single image. For MegaPipe, the offsets are applied from the Elixir zero-points to bring them in line with the SDSS zero-points. An analysis of these offsets can be found here.

The Elixir photometric keywords are only valid on photometric nights. Archival data from the SkyProbe real-time sky-transparency monitor is used to determine if a night was photometric or not. Data taken on photometric nights is processed first through the astrometric and photometric pipelines to generate a catalogue of in-field standards. These standards are then used to calibrate any non-photometric data in a group. If none of the exposures in a group was taken on a photometric night, then that group cannot be processed.

AB_magnitude = -2.5 * log10(counts) + 30.000

Sextractor is run on each stack using the weight map. The configuration file is given here. The resulting catalogues only pertain to a single band; no multi-band catalogues have been generated. While this fairly simple approach works well in many cases, it is probably not optimal in some situations. Depending on the application, some users may wish to run their own catalogue generation software on the stacks.