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Betelguese - non-linear calibration?

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#1 descott12

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Posted 15 December 2019 - 11:19 AM

Hello all,

I was capturing some spectra from some M-class stars last night. I think I was doing pretty well with focus as I was getting some nice, deep absorptions. However, as you can see in the attached of betelgeuse,

my absorptions are shifted to the left at the red end and shifted to the right at the blue end. This also happened with Mira.

 

I am using my Evolution 8 + 178MM + SA-100 at 3.65 A/pix

 

Any idea why this was happening?

Thanks in advance.

 

Attached Thumbnails

  • Betelgeuse 100 frames - offset2 .png


#2 Organic Astrochemist

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Posted 15 December 2019 - 05:24 PM

I think the Star Analyzer is slightly nonlinear and you can use the nonlinear calibration function in RSpec WITH THE BALMER LINES.

What you have shown here in grey are mostly TiO absorptions and you can’t use these for very accurate calibration. First, because this is a molecule and has very wide absorption over a wide range of wavelengths and second, the deepest part of your absorptions don’t necessarily correspond to the TiO absorptions. For Betelgeuse I think this is true, for example, for The Mg I triplet at 516, 517 and 518 nm and the Na I doublet at 589 and 590 nm.

Here’s my latest attempt at Betelgeuse

Edited by Organic Astrochemist, 15 December 2019 - 05:56 PM.


#3 descott12

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Posted 15 December 2019 - 06:22 PM

I think the Star Analyzer is slightly nonlinear and you can use the nonlinear calibration function in RSpec WITH THE BALMER LINES.

What you have shown here in grey are mostly TiO absorptions and you can’t use these for very accurate calibration. First, because this is a molecule and has very wide absorption over a wide range of wavelengths and second, the deepest part of your absorptions don’t necessarily correspond to the TiO absorptions. For Betelgeuse I think this is true, for example, for The Mg I triplet at 516, 517 and 518 nm and the Na I doublet at 589 and 590 nm.

Here’s my latest attempt at Betelgeuse

Thank you for the clarification. I was hoping it was something like that. Your spectrum looks very similar...I think I am getting the hang of this!  

I didn't actually use the TiO absorptions for calibration. I used just the zero order and my dispersion so I think this is actually pretty close. It would be really hard or impossible to see any balmer lines with such wide and deep TiO absorptions??



#4 robin_astro

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Posted 15 December 2019 - 07:15 PM

I suspect the grey lines may not in the right place. (They will be molecular band head wavelengths which may not be exactly the same as the minima.) Have you tried overlaying an actual spectrum of an M2i star from the Pickles library? 

 

Cheers

Robin



#5 descott12

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Posted 15 December 2019 - 08:05 PM

I suspect the grey lines may not in the right place. (They will be molecular band head wavelengths which may not be exactly the same as the minima.) Have you tried overlaying an actual spectrum of an M2i star from the Pickles library? 

 

Cheers

Robin

Hi Robin, That was it. It is a very good match. I noticed that the TiO absorptions have an asymmetric profile. They drop off rapidly (almost a vertical line) on the left and are wider on the right side.

 

The TiO bands at the red end that were very off were marked as 6250, 6780, 7210 in RSpec

Attached Thumbnails

  • Betelgeuse 100 frames with M2i .png



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