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Nigella Bryant

Calibrating in BASS project?

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Hi all,

Thought I'd start a new topic after gaining some first spectra. I've read the helpful comments on my first post under, "First Spectra" and am at the stage of trying to calibrate the image and spectra. I've attached the chart of Vega and an AOV reference star. My work flow was first to load the image, crop the image to exclude background but include the star and spectrum (Vega). Next on the produced chart I calibrated by setting the left hand spike to zero and the main spike on the spectrum to HBeta. It seems to have calibrated right but not sure. I've attached the chart. Appreciate all your input, many thanks. My last attempt after refining my work flow seems to be more on target after I cropped nearer to zero. 

Am I on the right track? 

vega-spectrum chart.png

vega-spectrum chart-eb.png

vega-spectrum chart-eb1.png

Edited by Nigella Bryant
Added third chart.

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Hi Nigella,

The 3rd image is correct but not because of your cropping. On the 1st and 2nd image, only the 0 Order (obviously) and the Hb line at 486.1 are correct. The other points are incorrect which has thrown the calibration out, and therefore why the absorbtion features at the red end don't line up correctly. 

Calibration point 04 on the first and 06 on the second is the response of the camera and not a spectral feature. As you learn to see the Hydrogen lines in A type stars, this becomes more obvious though so keep it up, you're doing great.

Also, when you do your calibrating in BASS, the number in the box which says "RMS error" should be as small as possible.

Stu 

Edited by Stu Todd

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27 minutes ago, Stu Todd said:

Hi Nigella,

The 3rd image is correct but not because of your cropping. On the 1st and 2nd image, only the 0 Order (obviously) and the Hb line at 486.1 are correct. The other points are incorrect which has thrown the calibration out, and therefore why the absorbtion features at the red end don't line up correctly. 

Calibration point 04 on the first and 06 on the second is the response of the camera and not a spectral feature. As you learn to see the Hydrogen lines in A type stars, this becomes more obvious though so keep it up, you're doing great.

Also, when you do your calibrating in BASS, the number in the box which says "RMS error" should be as small as possible.

Stu 

Thanks Stu, Yeah, I thought my third attempt was correct so I'm getting there, lol. Yes my "RMS" error was in the 0. something on the third attempt so I knew I was getting somewhere. It's a learning curve alright, lol, but I'm enjoying it, especially when I get it right. 

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On my third chart I corrected the lines. Is this correct. As the 06 line on my third chart didn't correspond to any line on the reference spectrum I assume this also is a camera response and not anything to do with the star spectrum. Also I added another calibration point at 397.0nm and this seems to line up with the reference spectrum. Am I correct in assuming this in my forth spectrum chart?

vega-spectrum chart-eb2.jpg

Edited by Nigella Bryant
added comment

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Hi Nigella,

The calibration of the Star Analyser is very close to linear so to start with you can use just the zero order and one other line eg H beta which are easy to identify. If you first calibrate using these two lines and then overlay the Hydrogen lines it becomes obvious which features approximately line up with the other lines and you can then add these to the calibration if you like. (In practise I dont usually bother recalibrating using a non linear calibration and just use a linear calibration as wavelength calibration of slitless spectra is only approximate anyway)

Once you have this calibration, you can use this and the zero order to wavelength calibrate any spectra using just the zero order.

Cheers

Robin

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26 minutes ago, Nigella Bryant said:

On my third chart I corrected the lines. Is this correct. As the 06 line on my third chart didn't correspond to any line on the reference spectrum I assume this also is a camera response and not anything to do with the star spectrum. Also I added another calibration point at 397.0nm and this seems to line up with the reference spectrum. Am I correct in assuming this in my forth spectrum chart?

 

Looking much more like it. I am slightly puzzled as to why your zero order is not at zero.

What order fit are you using? As Robin points out it should be close to linear so I would use a linear fit or a 2nd order at most.

I suspect you line 06 is teluric in origin that is from our atmosphere. 

Regards Andrew 

Edited by andrew s

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32 minutes ago, andrew s said:

Looking much more like it. I am slightly puzzled as to why your zero order is not at zero.

What order fit are you using? As Robin points out it should be close to linear so I would use a linear fit or a 2nd order at most.

Yes it is very important that the calibration passes through zero at the zero order otherwise you cannot use it to calibrate other stars where the wavelength of the lines will not generally be known. This may be at the expense of precise alignment of other lines but this is less important

Cheers

Robin

Edited by robin_astro
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38 minutes ago, robin_astro said:

Yes it is very important that the calibration passes through zero at the zero order otherwise you cannot use it to calibrate other stars where the wavelength of the lines will not generally be known. This may be at the expense of precise alignment of other lines but this is less important

Cheers

Robin

Thanks Robin, it's a learning curve. 

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A bloomin' steep one too. One step at a time. 

You have the 2 best in the realm helping you in Robin and Andrew. You couldn't buy better advice!

Stu

 

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9 minutes ago, Stu Todd said:

A bloomin' steep one too. One step at a time. 

You have the 2 best in the realm helping you in Robin and Andrew. You couldn't buy better advice!

Stu

 

Thanks Stu, just realised that when Jeremy Shears emailed me back today. I'm very humbled. 

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Hi all, 

Onward and upward, please find attached BASS project for Alpheratz and Vega. Hopefully getting to grips with it. Please comment, thanks. 

P. S. Just noticed my Cal02 line on Vega is slightly out, humph. 

alpheratz021019eb.jpg

vega-02-10-19.jpg

Edited by Nigella Bryant
Added Vega Chart
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Hi Nigella

The RSpec video tutorials are very good and I imagine the BASS software isn't vastly different. Same principles anyway. I joined the Astrobodger forum last night but haven't yet downloaded the BASS software but will do soon. 

Here's a link to the videos on youtube:

Workshop:

RSpec link:
https://www.rspec-astro.com/more-videos/

Good luck!

Louise

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39 minutes ago, Thalestris24 said:

Hi Nigella

The RSpec video tutorials are very good and I imagine the BASS software isn't vastly different. Same principles anyway. I joined the Astrobodger forum last night but haven't yet downloaded the BASS software but will do soon. 

Here's a link to the videos on youtube:

Workshop:

RSpec link:
https://www.rspec-astro.com/more-videos/

Good luck!

Louise

Thanks Louise, I'll take a look. Welcome to BASS Project too. 

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