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A few "failed" spectra of symbiotic stars

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#1 Organic Astrochemist

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Posted 15 December 2019 - 11:27 PM

ISIS is a great program to process spectra but the learning curve is pretty steep.

 

I seem to still be having some problems in the bluest part of the spectra. These spectra are still fun for me and show me a lot about the systems I'm looking at. But they're not quite ready for prime time. I just seem to always get too much flux in the blue. Especially with symbiotic stars.

 

EG Andromedae

 

I used 37 And as my reference star. In ISIS it looked like a good match with A6V but I couldn't export the comparison. Looks good with an A7V star here.

37 And.png

 

Here's EG And. Not so noticeable, but the extreme blue is above the reference

EG And.png

 

V694 Mon

 

I used Delta Mon as a reference star. Pretty good match to A0IV. But maybe not good enough?

22 Mon'.png

 

Maybe this spectrum of V694 Mon is all right but it makes me nervous when I see the baseline going up in the blue.

V694 Mon.png

 

BX Mon

It was late so I also used Delta Mon as the reference star for this system and maybe that was a no-no. I think I caught the system not far out of eclipse (H-alpha is small), but the blue end doesn't look right at all for the cool star.

BX Mon.png

Something weird is going on with ISIS because often I have to crop my spectra to remove parts of the extreme blue that go off scale. 

 

After I process my reference, I think I'm supposed to just hit GO to process the target but sometimes I have to change the binning settings and sometimes I have to change the location of H-alpha and the SMILE values. I'm sure I haven't figured this all out yet.

 

I also always have a hard time subtracting away the light pollution lines, especially with the symbiotic systems.

 

Spectroscopy -- lots of fun but always lots to learn.


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#2 robin_astro

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Posted 18 December 2019 - 05:25 AM

 

After I process my reference, I think I'm supposed to just hit GO to process the target but sometimes I have to change the binning settings and sometimes I have to change the location of H-alpha and the SMILE values. I'm sure I haven't figured this all out yet.

 

I also always have a hard time subtracting away the light pollution lines, especially with the symbiotic systems.

 

Definitely something is going wrong here. Provided everything is screwed up tight and the reference and target are placed in the same location along the slit, there should be no need to change any settings. If you are seeing any movement,  as well as the usual screws, check that the grub screws holding the grism in place in the core module are tight as there have been instances of these coming lose and the grism rotating.  

 

What wavelength calibration procedure are you using?  If you are using the "predefined mode" "ALPY600 (Balmer lines)" Once you have done the calibration on the reference star, you change the calibration mode to "predefined dispersion equation" and set the reference line wavelength and pixel position to zero.  ISIS then uses the same dispersion equation, including the offset on the target spectrum.

 

To set the smile accurately a good tip is to  make the height of the measuring box only just tall enough to cover the sky background subtraction regions. If you have problems nulling the sky lines, I recommend ticking the no box in "settings"  "erase intermediate files" and examining the files, in particular the _objectname file which is the preprocessed,stacked,background subtracted, geometric corrected image the spectrum profile is created from. 

 

Cheers

Robin


Edited by robin_astro, 18 December 2019 - 05:35 AM.


#3 Organic Astrochemist

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Posted 19 December 2019 - 12:45 PM

Thanks Robin.
I think the ALPY is very stable physically (a design plus).
I also think that ISIS automatically changes the calibration mode for the target star once it has calculated the dispersion equation, but I will take a look.

I didn’t realize it was strictly necessary to have the reference and the target at exactly the same place on the slit. I think I can do this with Phd2 and will adopt this as a best practice.Do you have a preferred wavelength to do the smile measurement? I think I need to be very consistent with that too. I have found it better to use a box smaller than the sky subtraction zones, but I will also try this.

Thanks for all your help.

#4 robin_astro

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Posted 19 December 2019 - 02:24 PM

Ah, I guess you are using the calibration assistants. (I don't use these) If you are having issues, it might be worth dropping back a step and setting  things up manually using the individual functions. I use just the lamp for wavelength calibration for example using method 2 here

http://www.astrosurf...calibration.htm

 

It is not essential to put the reference and target at the same position but if not, the binning zones have to be shifted to a different position and the smile correction and reference line location at that position might be slightly different so it all helps with consistency to use the same location every time, storing it in PhD

 

For the smile I just chose a lamp line which is around the middle of the spectrum. If you have a particularly troublesome sky line eg NaD it might help actually using that line to measure the smile rather than a lamp line. Provided the sky lines are not saturated and you are not undersampled (ie at least 2-3 pixels slit width, the sky lines should cancel cleanly with the right smile setting 

 

Another thing to watch for is the cosmetic removal function if you are using it which needs to be used with caution if you are close to undersampled or the spectrum is narrow or has narrow lines as it can pick these up as cosmetic defects. A look at the @map files will show what pixels are being "corrected"

 

Cheers

Robin



#5 robin_astro

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Posted 19 December 2019 - 03:42 PM

An example of how the _objectname.fit file should look (a rather faint object for the ALPY 600, dwarf nova AL Com in outburst at ~mag 15) if the geometric corrections are set correctly

(a) with the binning zones shown

(b) with "sky not subtracted" ticked

© with sky subtracted

Note how the smile is exactly correct to remove the sky lines in the sky background binning zones but is not perfect outside these regions

 

Cheers

Robin

 

ISIS_skybackgroundsub.jpg


Edited by robin_astro, 19 December 2019 - 03:55 PM.



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