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Here is a rather short exposure (5x45sec.'s) of variable V442 And taken from my dark sky altitude site on 23Sep17:


As seen in the continuum (circled in red), it has two "squares" that are suspiciously symmetric and look like artifact of processing.
I used two different IR stars from the miles catalog and processed with the newest version of IS 1.91, but these "squares" in the
continuum persists. Is the a natural phenomenon (not likely) or some weird artifact that the scope/software produced?

Thanks for looking!

Hi James,

It's difficult to answer just with your graph.
Could you post here a raw image and your instrumental response ?

Hello James,
I think it's possible cosme make this. You use Isis logiciel?
When I take star calibration with short time exposure I don't take cosme option with Isis.

Hi James,

A good tip in this situation is to first check the individual spectra ( @pro_n ) that ISIS generates (using PlotSpectra for example) This immediately shows if there is any unexpected variation between exposures.

Robin

Like Etienne, I have also seen the cosmetic function produce this sort of problem. If the signal is strong and the spectrum is narrow there can be large differences between pixels in adjacent rows which ISIS thinks are defects and "corrects" the whole row !

Robin

Another idea on this problem, if the spectrum is too narrow (in binning 2X2 for example), the cosmic remove function can lead to modifying the information contained in the spectrum, so try not to use this function.

Sorry James, the effect I saw was with the cosmic ray removal function as Olivier has just described, not with the cosmetic file. Running with each disabled in turn though should quickly show if either are the problem.

Robin

To: Lobin and previous,

Here is my 2D Isis V5.91 processed spectra


The larger "square" of brightness is clearly visible. Since I was working at 2500 meters elevation with
very cold temperatures (near 0 deg C) and possibly low voltage power, any number of things might
have resulted in this strange spectral brightening; Note that I did not see this artifact in any of the other 40 or
more spectra set I shot over 5 nights up there.

I will go over V442 And's spectra individually to see if only one of them have the brightness spike in the continuum.

James

HI james,

Have you a raw spectrum (just the fit image from your CCD and your instrumental response ?)

James, your spectrum trace seem to narrow, not well sampled. Deselect the cosme filter removal function if necessary (the possible solution) or adopt a bin 1x1 strategy during observation (better).

Christian