Error map of a CCD image
lilirayhk wrote on Aug 11, 2008
Hi all,
I got some calibrated images from the HST (ACS and WFPC2) and I would like to produce a *.fits format image to represent the noise of each pixel of those CCD images. Just want to ask how to do that? Can I just use the formula noise = sqrt [no. of signal + readnoise ** 2] to estimate the error. THANKS for answering!!
RAy
I got some calibrated images from the HST (ACS and WFPC2) and I would like to produce a *.fits format image to represent the noise of each pixel of those CCD images. Just want to ask how to do that? Can I just use the formula noise = sqrt [no. of signal + readnoise ** 2] to estimate the error. THANKS for answering!!
RAy
Francisco Valdes wrote on Aug 11, 2008
The task that would probably of most use is imexpr. The actual expression you use might be based on the equation you noted. My main concern would be what does "calibrated" mean. If any scaling is done, say normalizing to unit exposure time or gain or dividing by an unnormalized flat field) or the units are change from adu to something else then you need to figure out how to convert the detection gain and readnoise.
I notice you did not have the gain in your equation. What you show would only be correct if the gain is one (i.e. the signal is in terms of photons). The readout noise would also need to be in photons though typically it is in measure electrons.
I hope this helps,
Frank Valdes
I notice you did not have the gain in your equation. What you show would only be correct if the gain is one (i.e. the signal is in terms of photons). The readout noise would also need to be in photons though typically it is in measure electrons.
I hope this helps,
Frank Valdes
Last post on Aug 11, 2008