Strange artefact in one processed sub
Posted: Fri Mar 19, 2021 1:35 pm
I came to do the routine processing of a reference star for an Alpy target and saw something strange in the final profile.
Thus, there were several well-defined, and equally-spaced 'notches' therein.
I finally tracked the source of this artefact to the processing of a singe sub-exposure (1 out of 5). This led to an artefact in the intermediate '@' file - see attached.
Note the 'notch' just to left of H_beta which gets repeated at regular intervals as you go towads the red end of the spectrum.
This is not present in the original data.
No other subs suffered from this.
Indeed, as a 'control experiment', I reworked the whole reduction on an alternative platform - Demetra. No problems there.
One possibility I considered was that, this was the brightest sub, and internal processing in ISIS may be sensitive to some kind of 'saturation' which is not otherwise apparent.
To test this hypothesis, I scaled the sub using the 'Multiply image by a constant' tool, but this made no change.
As a clue to what might be going on, note the weird uniform grey sector at the top left...
I also notice all files of the '@' form have negative numbers in them ....any cause for concern?
Kevin
Thus, there were several well-defined, and equally-spaced 'notches' therein.
I finally tracked the source of this artefact to the processing of a singe sub-exposure (1 out of 5). This led to an artefact in the intermediate '@' file - see attached.
Note the 'notch' just to left of H_beta which gets repeated at regular intervals as you go towads the red end of the spectrum.
This is not present in the original data.
No other subs suffered from this.
Indeed, as a 'control experiment', I reworked the whole reduction on an alternative platform - Demetra. No problems there.
One possibility I considered was that, this was the brightest sub, and internal processing in ISIS may be sensitive to some kind of 'saturation' which is not otherwise apparent.
To test this hypothesis, I scaled the sub using the 'Multiply image by a constant' tool, but this made no change.
As a clue to what might be going on, note the weird uniform grey sector at the top left...
I also notice all files of the '@' form have negative numbers in them ....any cause for concern?
Kevin