NSFCam Observing Cookbook
Introduction
There are many ways that NSFCam can be used to suit a wide range of observing programs. The array can be read out in 3 modes: raw, single, and double. The gain and well depth can be set to 4 different values. This document will help you choose the correct settings for your program.
Readout Mode
Raw mode: This most is primarily used for engineering purposes. The output counts in ADU is the output form the A/D electronics. Since the voltage of a pixel decreases from the reset value as more photons hit the pixels, the ADUs decrease with increasing flux. Thus, an image looks like a negative. Every read (NDR and coadds) is output to a separate FITS file.
Single read mode: Similar to raw mode in that the images look like a negative. NDRs and coadds are added together in memory and written to one FITS file. This mode is used for cases where the background is high, or a target is too bright to be observed in double mode.
Double read mode: In this mode, we do a correlated double sample. The NDR number of 'pedestal' reads are taken, and are added together in memory. Then the same number of 'singal' reads are taken, and are added together in memory. Then the sum of the pedestal reads is subtracted from the sum of the singal reads, and the difference is the ADU written to the FITS file. The advantage of this method is that the resulting image is less noisy since we essentially measure the reset voltage of each pixel with the pedestal reads. The disadvantage is the higher overhead to do two sets of reads. For example, a 1s exposure takes 2 seconds of readout time.
Gain and Well Depth
High gain, low well depth: This mode sets the gain to 1.83 e-/ADU. This mode is recommended for users observing faint targets and who need maximum sensitivity. The lower gain yields a finer resolution of photons per ADU than low gain mode.
Low gain, low well depth: Not used much
Low gain, medium well depth: Not used much
Low gain, high well depth: This mode sets the gain to 2.55 e-/ADU. This mode is recommended for general purpose observing, and especially for users who will observe bright targets and/or observe at thermal IR wavelengths.
Examples
JHK imaging of a faint target: Double read mode, high gain low well depth
JHK imaging of a bright target: Double read mode, low gain high well depth
L'M' imaging of a target: Single read mode, low gain high well depth
Imaging w/ ND filter or narrow band filters: Double read mode, low gain high well depth
Imaging w/ CVF filter: Low gain high well depth. Double read mode for wavelengths less than X. For wavelengths longer than X, use single read mode.
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Last modified 23 September 2013