[fitsbits] Appropriate BUNIT for ADUs vs electrons?

[James Turner]

> 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.

Thanks. Hmmm. Our group leader has quite a strong preference for
retaining the original value of GAIN unchanged throughout the
processing and tracking whether the data have already been scaled
to electrons or not, which is how this question came up. If anyone
does use BUNIT, it would be helpful to know what they do.

Cheers,

James.