GPS timestamps
Siril can manage GPS timestamps and metadata coming from QHY PRO cameras. Two completely different systems exist:
the all-in-one global shutter cameras, like the famous QHY174GPS and the infrared 990 and 991 PRO versions,
the rolling shutter cameras with the external GPSBOX, like the QHY268PRO, QHY600PRO and other high end models.
Their data is managed quite differently, refer to the sections below. But in both cases, the camera writes GPS metadata in the form of 44 bytes at the start of the image pixel data, generally appearing much brighter than the rest of the image and displayed at the bottom because FITS images are displayed bottom-up. Siril can either read this data or use the already extracted data from an acquisition software like NINA that would have saved it in the agreed-upon FITS headers. This code has been contributed to both software by the OurSky company.
Warning
Make sure your GPS antenna is not moving with the telescope, mounted in a safe place with a clear view of the sky. Otherwise, data will be erroneous, which is often indicated by a bad lock status value (it must be 3) or a bad PPS value (it must be slightly below 10000000).
Many cameras have problems and produce erroneous data once in a while, for unknown reasons. Using firmware and SDK versions more recent than 2022 is a good idea for this and metadata bug fixes.
To cover the various use cases, data can be extracted or displayed using the
gps command. For rolling shutter cameras, it is expected that in the near
future the GPS timestamps will automatically be used in photometric extraction,
this is not currently the case and requires manual use of this command.
Siril command line
Global shutter cameras
These cameras require extensive calibration and set-up, but once working, all
needed data is in the so-called metadata at the start of image pixel data. This
data is quite simple and can be summarized as GPS lock status, an exposure
start date and an exposure length. If the lock is good and all data looks
valid, extracting it in Siril will replace DATE-OBS and EXPTIME, so any
image timestamp usage will use the GPS time. To indicate that this replacement
has been done, another key is added: DATE-GPS, taking a boolean value.
To extract the metadata, simply use the gps command with no argument on a
raw loaded image. Data will be read and if looks valid, it will be used and
removed from the image by cropping the first row of pixels, to avoid disturbing
image statistics with it (also because after image processing they will not
mean anything anymore).
To verify the extracted metadata from previous uses or from the acquisition
software, use the gps -header command.
Rolling shutter cameras
In addition to the metadata found in the start of image pixel data, rolling
shutter cameras require QHY SDK calls to obtain rolling shutter information for
the camera and the gain mode being used. Siril cannot know that from a random
image unless the capture software captured this information and stored it in
the FITS header. NINA does that. While the keys representing the GPS metadata
are named with the GPS_ prefix, those keys are named with the QHY_
prefix.
Rolling shutter information is important if one wants to compute accurate timestamp of a pixel of the image. It is very significant for short exposures as the rolling shutter can take in the order of 250 ms to capture the entire image, meaning there would be a difference in timestamps of 250 ms between first and last row of pixels in this example.
For this reason, DATE-OBS is not overwritten with GPS data. Also, because
the metadata of global and rolling shutter cameras are in the same format, some
of it is meaningless with rolling shutter cameras: the start and end capture
times are the same, so computed exposure is always 0. This is normal.
Siril can compute the timestamp for a pixel row (1000 in this example) of a
rolling shutter image using the command gps 1000.
Flipping, cropping and re-binning the image is supported, Siril keeps track of these operations to keep timestamps accurate.