Question regarding data reduction

Hi,

I have an exam (or a 'quiz' as my lecturer so cheerfully calls it) next
week on astronomical techniques. One of the topics is data reduction and
image processing.

On this site - http://www.astrosurf.com/re/process.html - it says that
the dark frame is the sum of the bias frame and the thermal frame. This
makes sense to me, but down further the 'equation' for the process
implies that the dark frame is the thermal frame subtracted from the
bias frame.

Could someone confirm which one it is? i.e.:

Dark = Bias + Thermal
or
Dark = Bias - Thermal

--
Conor
www.TechPhreek.com
.

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