Some groups are not currently available in the MegaPipe archive. It may appear that while sufficient data has been taken by CFHT to warrant stacking, no group has been produced with MegaPipe. Possible reasons for this include:
- CADC does not have the Elixir processed version of some images. Currently, the CADC receives from CFHT raw versions of every MegaCam image. Elixir pre-processed versions of every CFHTLS image are also tranferred. For non-CFHTLS data, at present, only some images are transferred from CFHT to CADC. CFHT is currently sending Elixir processed versions of all available MegaCam images, so future MegaPipe generations will contain more groups.
- Photometric calibration may not be possible for a group. If the group does not lie in the SDSS, or if none of the images in a filter was taken on a photometric night (as determined with SkyProbe) that filter of that group will not be processed.
- Astrometric calibration may not be possible for a group. The astrometric calibration pipeline relies identifying compact sources. If there are not enough compact sources visible in the image, it can fail. Alternatively, the astrometric pipeline can be confused if there are too many sources. These two cases are discussed in the following sections.
The astrometric calibration depends on matching observed catalogues to reference catalogues. In crowded fields, the matching algorithm can become confused and misidentify sources in some cases. This causes the astrometric pipeline to fail. When this occurs, the resulting images have conspicuous smeared and doubled stars. These images have been removed from further analysis.
Several of the tools that go into MegaPipe (AstroGwyn, SExtractor, SWarp) were originally developed for use for high galactic latitude fields. These fields contain a moderate number of compact, isolated sources. When the same tools are applied other fields, the results are not always optimal. This particularly true of fields containing extended sources such as nebulae or nearby galaxies. Nebulae which contain few compact sources cannot be astrometrically calibrated. Other fields contain enough sources for astrometric calibration, but problems arise at the SWarp stage. If the extended emission varies smoothly and slightly (as in a large nebula) SWarp's background subtraction removes the extended emission at the same time as it removes the sky background, leaving a blank field. If the extended emission has sharp variations (such as the spiral arm of a nearby galaxy), SWarp's background subtraction can produce peculiar results.
These problems mostly affect images taken as part of the Hawaiian Starlight program, but there are a few science images affected as well. They will be addressed in future versions of MegaPipe. In the meantime, users should be aware that some background subtraction is taking place, which may affect extended sources.
Some of the MegaPipe groups overlap, usually by only a small margin, but sometimes by more. The catalogues from overlapping groups will be doubled up. That is to say, there will be sources that appear in two different catalogues. This fact is exploited to check the photometry (as discussed in the internal checks on photometry section) but may be undesirable for some users.