![]() For example, ACS and WFC3 data have a velocity aberration keyword which reports the specific aberration for that exposure. The updatewcs task, used by AstroDrizzle in the pipeline (and also available to users), reads in base models from the reference files it computes components such as time- and orbit-dependent WCS information that are unique to an exposure, and populates the relevant WCS keywords in the image header. This can include multiple components, some that are time-dependent, and others that are exposure- or orbit-dependent. Types of Information Contained in HeaderletsĪstroDrizzle requires that all WCS and distortion information be present in the input images in order to align them. This information, stored in the headerlets, can then be easily applied to the same images being studied by other users, saving them the work needed to align the images to the same reference image or catalog. ![]() That expertise can now be embodied in headerlets created using the tweakreg task, which was written to simplify image alignment and provide updated WCS information for aligned images. It requires some expertise with source-finding algorithms, computing offset fit solutions, and a knowledge of FITS WCS standards. Why Headerlets are UsefulĪ lot of effort can be expended in determining the alignment between an image and an astrometric field or another image. ![]() For example, the name of a headerlet written out by tweakreg for the WFC3 image iabf01ckq_flt.fits for the WCS solution named TWEAK would be named iabf01ckq_tweak_hlet.fits. The default suffix for headerlet files, as used by all DrizzlePac tasks, is hlet.fits. They can be used to update the WCS information in additional copies of the same image.They contain a summary of all WCS information for a single exposure.This avoids the need to send that entire image containing the new WCS to the collaborator. ![]() It could be utilized as a convenient WCS information package that a user could e-mail to a collaborator working on the same image, so it can be incorporated into the collaborator’s copy of the image. A headerlet is a compact FITS file representing a single WCS solution for a single exposure that has been aligned to a particular image or astrometric catalog, complete with all distortion information. ![]()
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