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Check on SA100 results from Capella.


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Hi People,

 

I just wanted to ask for your feedback on this spectrum of Capella taken with a borrowed SA100.  I've ditched the SW80 for a Bresser 114mm Newtonian with a focal length of 500mm which piggy-backs my main scope.  It's much easier to focus.  Sadly, the camera turns out to be a DSI III colour, not the mono, so there is some distortion because of the Bayer matrix.  Here I took 50 images of Capella at 2 secs and stacked and cropped with Siril.  The distance to the sensor is about 50 mm (I've used a 20mm extender).  In the hour or so of clear skies last night I popped out to do a bit of spectroscopy.  Here's the result:

Capella2_stacked.png.e494c5cb1f1ad273c63f3aa7a3183ffd.png

You can already see a strong line in the IR, which I take to be Oxygen.  If I calibrate and analyse with RSpec, here is the result:

Capella_20210302_101350.thumb.png.3e00274240b5b7685d27d304d79932e6.png

I'm thinking about how to get a cheap mono camera, I'll have a look in the classifieds, but apart from that, have I got the lines right and what's the next step for me?

Thanks very much.

Steve.

Edited by SteveBz
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That's much better. (No more of that terrible chromatic aberration) Your identification of the O2 telluric band looks ok so your wavelength calibration should be roughly correct. Capella is a G type star (G3iii) though so the spectrum is mostly a forest of blended metal lines at this resolution and will not show obvious  Hydrogen lines.  It is best to use a main sequence  A star to start with so you can use the clear Balmer lines to focus on and produce an accurate wavelength calibration which can then be transferred to any other target. Capella should look like something like this after correcting for instrument and atmospheric response.  

pickles_G5iii_filtered.png.502ad3ba0052f53f14e9d7933b9fa788.png

Try overlaying a G star spectrum from the library in RSpec on your spectrum and see if anything lines up. The colour camera will make this more difficult though because of the humps and bumps in the camera response where the filters overlap. (The big dip at ~5700A in your spectrum for example)

Cheers

Robin

Edited by robin_astro
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18 hours ago, robin_astro said:

That's much better. (No more of that terrible chromatic aberration) Your identification of the O2 telluric band looks ok so your wavelength calibration should be roughly correct. Capella is a G type star (G3iii) though so the spectrum is mostly a forest of blended metal lines at this resolution and will not show obvious  Hydrogen lines.  It is best to use a main sequence  A star to start with so you can use the clear Balmer lines to focus on and produce an accurate wavelength calibration which can then be transferred to any other target. Capella should look like something like this after correcting for instrument and atmospheric response.  

pickles_G5iii_filtered.png.502ad3ba0052f53f14e9d7933b9fa788.png

Try overlaying a G star spectrum from the library in RSpec on your spectrum and see if anything lines up. The colour camera will make this more difficult though because of the humps and bumps in the camera response where the filters overlap. (The big dip at ~5700A in your spectrum for example)

Cheers

Robin

Hi Robin,

Thanks for this.  Here's an update.  I wasn't sure which stars were which so I took a handful of stars from Auriga.  All about 50 exposures, then stacked in Siril and finally, I stretched the spectrum in Gimp.  Here's the result for 4 Aur:

4Aur_stacked2.png.6dd2e7bfa5cdd058b59bbb245a368151.png

You can see three Balmer lines quite clearly here.  Here is the graph from RSpec, which also shows them (I think I've truncated the O-O lines at 762nm).

4Aur20210303_184410.thumb.png.daba2fc04ff1064c72adda2c31991022.png

I haven't quite worked out to keep the scale between two spectra so I can just do the zero degrees line.

Thanks

Steve.

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19 hours ago, SteveBz said:

haven't quite worked out to keep the scale between two spectra so I can just do the zero degrees line.

The dispersion (Angstrom/pixel) will stay the same provided you do not change the distance of the grating from the sensor so you can just use the zero order with  the dispersion you calculated from the Balmer lines. (4 Aur is A1v so perfect for this) See video # 24 here to see how to do it in RSpec 

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

Cheers

Robin

Edited by robin_astro
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4 minutes ago, robin_astro said:

The dispersion (Angstrom/pixel) will stay the same provided you do not change the distance of the grating from the sensor so you can just use the zero order with  the dispersion you calculated from the Balmer lines. See video # 24 here to see how to do it in RSpec 

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

Cheers

Robin

Hi Robin,

Thanks for this.  I found I needed to click the checkbox.  Done.

But I totally can't get ISIS to work!

Tx

Steve.

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1 hour ago, SteveBz said:

But I totally can't get ISIS to work!

ISIS is very powerful and although it is possible to use ISIS to process Star Analyser spectra, only a masochist would probably use it for this. It is really designed for highly automated rigorous processing of spectra from slit (or fibre fed) spectrographs to produce research quality spectra. For the beginner, programs like Visual Spec and RSpec are much easier to use with the Star Analyser.

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