[fitsbits] Appropriate BUNIT for ADUs vs electrons?

[William Thompson]

The word "counts" implies a quantity which behaves stochastically with a Poisson 
distribution.  In general, I wouldn't have any problem using the term counts at 
low photon energies, where only one photo-electron is created in any photon 
event.  At higher energies however, such as in the extreme-ultraviolet, several 
photo-electrons are typically created for each detected photon, so "counts" 
wouldn't apply there.

Bill Thompson


Paul Hirst wrote:
> It has to be said, GAIN is a somewhat general term to be used for this, 
> there are typically many amplifiers in the system, each having a gain.
> 
> I know of systems that have used DEPERDN (Detector electrons per data 
> numer) or similar to represent this number.
> 
> Cutting more back to the original question - is there any support in the 
> fitsbits community for the thought that we should be using "counts" when 
> we have the data in electrons - on the grounds that counts implies 
> you're counting physical things in some sense (as opposed to ADUs) - ie 
> it's something which you shouldn't have to apply a scale factor to if 
> you're going to do Poisson statistics on it, for example.
> 
> Cheers,
>   Paul.
> 
> 
> 
> On 03/04/2011 07:56 AM, Steve Allen wrote:
>> On Fri 2011-03-04T13:27:10 -0300, James Turner hath writ:
>>> What do other observatories do?
>> Ponder the same question, but for reasons noted at the end it's a bit
>> moot for us.
>>
>> As you note, one common way at the telescope and during intial
>> reduction is to leave the array values as DN/adu/count and to include
>> the GAIN keyword with a value according to the convention used in
>> various IRAF procedures.  If GAIN is reset to 1 when it is applied
>> then the conventional IRAF meaning is retained.  This is all, of
>> course, dependent on keyword conventions for images from CCDs which
>> are not registered with the IAUFWG.
>>
>> Keck data acquisition systems generally do not include a GAIN keyword.
>> Lick data acquisition systems have GAIN with entirely different meaning.
>>
>> --
>> Steve Allen<sla at ucolick.org>                 WGS-84 (GPS)
>> UCO/Lick Observatory--ISB   Natural Sciences II, Room 165    Lat  +36.99855
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-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov