strange black lines in ISIS image

[Robin Leadbeater]

I am still playing with the ALPY200 but a rare clear night last night was spoilt by the full moon. Instead of going fainter, I decided as a challenge to try measuring 3C273 just 20 deg from the full moon. The test was successful with a good spectrum but when I looked at the resulting 2D image from ISIS I noticed 2 horizontal black lines on the image (see attached. A sum of 16 images dark subtracted, geometric corrected but not flat corrected or background subtracted) Does anyone know where they might have come from? They are not in the original images.

Cheers
Robin

[Ken Harrison]

Robin,
They look symmetrical to the target spectrum....something to do with the automatic crop selection??

[Robin Leadbeater]

It turns out that the lines are at the top and bottom limits of the background subtraction zones, even though I did not select background subtraction. Very strange. It would appear to be a bug

Robin

[Christian Buil]

Do not panic Robin, it is normal (linked to an internal
algorithm procedure). Of course, zone out of sky substraction
area are not valid. Only inner part is valuable
(the dark "indicate" a frontier).

Christian