Dewar 4 for the Hamilton spectrograph have a number of low sensitivity pixels, giving data from the instrument a pocked-marked look. To remove these and other pixel-to-pixel variations, Ryan Cooke developed a procedure which documented below.
Above, are two images, one raw pixel flat and one combination from images obtained using the script below.
The process involves moving the dewar out of focus as much as possible, and using a larger aperture on the decker. This will result in an image that has light almost everywhere, but will will still preserve the mapping between pixel and wavelength. To sets of 10 flats are recommended, one set with the BG12 and one with the BG13 filter. The BG12 filter have longer exposure times to provide more light in the blue part of the detector. The whole process takes about an hour. Because the dewar is defocused, this should be done first, before taking any other data, or can be done at the end of the night when all other calibrations have been acquired.
The procedure is outlined below, and a shell script to execute it is attached. This will take about half an hour if using the fast readout mode.
- Set the Aperture plate to 800:6.0
- Set the dewar focus to 1850 stepper counts
- Turn on the Polar Quartz
modify -s hammotor LAMPPOS=PolarQuartz - Set the filter to BG13
modify -s hammotor DEWARFILTNAM=BG13 - Set the exposure time to 3s
modify -s hamucam exposure=3 - Take 5 exposures
igetham 5 - Reset the lamp timer
modify -s hammotor LAMPREM=1800 - Take 5 exposures
igetham 5 - Set the filter to BG12
modify -s hammotor DEWARFILTNAM=BG12 - Set the exposure time to 22s
modify -s hamucam exposure=3 - Take 5 exposures
igetham 5 - Reset the lamp timer
modify -s hammotor LAMPREM=1800 - Take 5 exposures
igetham 5 - Turn off the Polar Quartz
modify -s hammotor LAMPPOS=Off