# The Pipeline Steps¶

The three main steps of the pipeline are

• identifying all sources in all images using Source Extractor
• cross-matching all sources between all images using SCAMP
• separating the SSOs from other sources such as stars, galaxies, artifacts, using a filter chain

## SExtractor¶

SExtractor identifies sources in CCD images using the pixel values and outputs catalogue data. Parameters like the pixel positions, sky coordinates, instrumental magnitudes and more are retrieved for each source. Refer to the official documentation and the Guide to SExtractor for much better explanations.

SExtractor is highly configurable using a configuration file. The config file ssos.sex that is provided with this survey differs slightly from the default version default.sex, specifically the deblending and photometry parameters were adjusted to better deblend SSOs close to other sources. The path to this config file has to be set in the default.ssos file using the SEX_CONFIG parameter. Likewise, SEX_PARAMS has to point to the output parameter file, by default semp/ssos.param.

The SExtractor configuration as set in ssos.sex requires two more files, the convolution filter set via the SEX_FILTER parameter and the neural network for star-galaxy differentiation, set via the SEX_NNW keyword, located in semp/gauss_2.5_5x5.conv and semp/default.nnw respectively.

Unless the path parameters above are set using absolute paths, the pipeline will look for the files in the directory it is executed in.

Before running SExtractor, all input images are checked for valid WCS keywords in their headers. As pre-existing projection parameters interfer with running SCAMP, they are removed by creating a temporary copy of each image and editing the header inplace. The original images are not edited.

Finally, using the SCI_EXTENSION, the user has to provided the index of the science extension of the FITS image. It is common case that data is provided in multi-extension FITS format, where besides the science data also weight images and other supplementary data is stored in the FITS file. Therefore, the user has to specify the science extension. Multiple extensions are allowed as well, if for example different CCD images were stored in the same file. The valid values are integers, separated by commas if multiple extensions should be analysed, e.g. 0, 1, or 1,2.

If you are unsure which extension contains your image, you can trial run SExtractor with the following syntax and check the output catalogues:

sex -c semp/ssos.sex image_file.fits[SCI_EXTENSION]


Note

Searching multiple extensions at the same time only makes sense if the field-of-views overlap. Otherwise, running the pipeline on the extensions separately will yield better results.



## Flags¶

The FLAGS_SSOS parameter is used to highlight sources which pass the filter but might be artifacts. An example are sources with jumps (outliers) in their observation epochs, which fools the linear motion filter. The flag values are represented by powers of 2 and added together, allowing for multiple flags to be set at the same time. The flag values are:

 Integer Value Meaning 1 Source detection is an outlier in EPOCH 2 Source detection used as reference for fixed aperture magnitude measurement
 [1] Appending the [SCI_EXTENSION] bit after .cat confuses the popular TOPCAT tool, so consistency in naming was neglected here.