Calibration frames
The HiPERCAM pipeline reduction software provides convenience routines to easily create master calibration frames from a FITS file containing a series of calibration frames.
The following routines are available (click on the links for more information):
- makebias
create a master bias frame via a clipped mean of all bias images within a FITS file. The routine also produces a plot of the bias level as a function of time, which can be useful for identifying issues with the bias data.
- makeflat
typically flat-fields for HiPERCAM are taken with a strongly time-variable twilight sky as the Sun sets or rises. A typical flat field run may start out bright, or even saturated, but by the end be only a few thousand counts above bias. Moreover, there are very often stars visible in the images, so we usually take them while offsetting the telescope in a spiral pattern. The challenge is to combine these images while rejecting the stars and saturated frames and giving due weight to the better exposed images. This has to be done for each CCD which vary significantly in sensitivity.
makeflat does this using a clever algorithm that groups frames of similar brightness, median comibines them and combines each group after rescaling to similar levels. For full details, see the linked documentation.
Dark frames are typically not used with HiPERCAM, since the dark current in less than 0.1 e-/s/pixel and is always a negligible noise source for HiPERCAM.
The z’-band (CCD 5) shows a strong fringe pattern, also visible to a much lesser extent in the i’-band (CCD 4). All HiPERCAM pipeline reduction and plotting routines can remove the fringe pattern using some fringe calibration frames. Since these fringe patterns are stable over time, it is recommended to use pre-prepared fringe frames. Simply copy these to the current reduction directory with the commands below:
cp /home/observer/reduce/fringes/fmap.hcm /home/observer/reduce/yyyy_mm_ddcp /home/observer/reduce/fringes/fpair.frng /home/observer/reduce/yyyy_mm_dd
where yyyy_mm_dd is the current date. To use these fringe frames in data reduction
or plotting simply procide the following parameters to a pipeline command such as nrtplot
or genred:
fmap=fmapfpair=fpair
Defect frames
IT IS STRONGLY RECOMMENDED THAT YOU PLOT CCD DEFECTS DURING ACQUISITION.
A file with known defects for each camera is available in the reduce directory
- /home/observer/reduce/defect.dft. To plot defects with nrplot,
specify the parameter
defect=/home/observer/reduce/defect
