Raies d'émission inhabituelles, pollution ou autre ?

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Peter Somogyi
Posts: 420
Joined: Sun Jul 13, 2014 8:56 am

Re: Raies d'émission inhabituelles, pollution ou autre ?

Post by Peter Somogyi »

I've reproduced similar (with RMS=17 as just given 5944.83A = 687 (pixels).
There are 2 cases:
1) with cosmic ray enabled, I get this:
_V250.png
Result is very similar to what we seen above.
Reason is obvious, at undersampled lines filtered (wrongly).
You can repeat checking your _V250.FIT downloading ds9 (I used the windows version for now) selecting "Scale -> Min Max -> Sample" menu.
I think it is mandatory for everyone to look at that file in such a doubt.
This is the critical phase where ISIS processed 2D and right before 1D.

2) cosmic rays disabled (as you mentioned and tried):
_v250_nocosmic.png
Well, here is also an error just a little left from the green line, but the reason is different: you have an unreproduced hotpixel left (ADU=65535).
Note that cosmic ray won't remove it anymore.
Now it is a dark calibration error.
Solution:
1) redo 1 dark, at the same exposure time if needed (lowering the threshold didn't help)
2) add that point to your cosme.lst (a bit guesswork of 2 coords... question how compatible ds9 and ISIS indexes, I tried 1-2 but guessed wrongly...)
3) move away the star on the slit next time from critical / wrong behaviour pixels (by 1st binning zone)
4) this the reason I don't use binning in general.... it bins them with good pixels, too... And cosmetic rays would become usable then (at 20 min exposures I do have a lot of them...).

But with this 2) error, I can see only a less big ripple.

Peter
Peter Somogyi
Posts: 420
Joined: Sun Jul 13, 2014 8:56 am

Re: Raies d'émission inhabituelles, pollution ou autre ?

Post by Peter Somogyi »

One more thing, I may forgot to mention.
When you get over the hotpixel / cosmic ray problem, your green line will still leave a nasty trail (I have such only above 13-14 mag resulting as a ripple):
_V250.png
As you can see, there's a little rotation error, as well as some pixel arithmetic error along the sharp line.
If I'd do it in ISIS, I'd transform it first to a much higher resolution and do the rotation in there, using double float precision at least.
But other practices (re-modeling such line by Gauss fit, ...) may exist.

All in all I suspect room for improvement on this in ISIS (not done probably because of an edge case scenario), until that I suggest avoid binning.
Hubert Boussier
Posts: 156
Joined: Fri Aug 17, 2012 12:55 pm

Re: Raies d'émission inhabituelles, pollution ou autre ?

Post by Hubert Boussier »

Thank you Peter to have passed time on my question. Next time I will try to shift the Spectrum track on an other part of the CCD Chip and use 1x1.
Paolo Berardi
Posts: 578
Joined: Thu Sep 29, 2011 10:51 pm

Re: Raies d'émission inhabituelles, pollution ou autre ?

Post by Paolo Berardi »

Hi Hubert, also consider that, from V5.5, ISIS allows to dose the cosmic rays filtering, useful for those who work under "near-undersampling" conditions (not only for sky lines but also for spectrum image - it might be suffer artifatcs). The parameter is located in the Settings tab: "Coef. cosmic rays filter". 100 is the standard value (used internally in previous versions). You may increase the effectiveness using a lower value (maximum effectiveness at zero value) or decrease it using a larger value.

Trying with your files, I found that a value of 1000 could be a good compromise. The sky line at 5461A totally disappear but the other at 4047A is still present. This narrow residual feature, although weaker, is superimposed to a very low intensity (ADU) continnum which is then enlarged by the instrumental response.

Concerning the saturated hotpixel (projected near 540 nm in the spectrum) I agree with Peter, you should recreate the cosmetic map from a new master dark or add the specific entry in actual cosme.lst: P 555 396.

A possible alternative to the use of no binning camera mode: since you use a relatively long focal lenght (C11 scope), you might want to change to 31 micron slit aperture. I think you'll gain in general efficiency (throughput) reducing the actual strong undersampling of spectral lines.

Let us know!
Ciao
Paolo
Hubert Boussier
Posts: 156
Joined: Fri Aug 17, 2012 12:55 pm

Re: Raies d'émission inhabituelles, pollution ou autre ?

Post by Hubert Boussier »

Grazie Paolo per questi consigli, li mettero in atto per i prossimi spettri.
Hubert
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