Calibrating a LowSpec3.0 Spectrum of Alpha UMa

[SteveBz]

Hi Fellow Astronomers,

I'm trying to get the hang of a LowSpec3.0 spectrometer.  The other night, everything fell into place and guiding on the slit worked (after a bit of star-hopping). The slit is set to 20 um and I took 10 x 20 second exposures, stacking in Bass.

Here is a spectrum of Alpha UMa in the H-alpha region:

I hope the big dark line is the H-alpha line, but I don't know the names of any of the other lines so that I can calibrate it.  Any ideas?  Here is a rough chart from Bass, after removing hot pixels except one at about 540 px:

Thanks for your thoughts.

Kind regards,

Steve.

[drjolo]

Hi Steve,

What is the dispersion of your LowSpec? What grating do you use?

Here is the Dubhe spectrum that I have obtained with 600lpmm grating, so maybe you will be able to compare 

https://astrojolo.com/spectroscopy/big-dipper-cluster-collinder-285-spectra/

Unfortunately Dubhe is a G9 type giant, so there is a plenty of lines in the spectrum  

 

[SteveBz]

1 hour ago, drjolo said:

Hi Steve,

What is the dispersion of your LowSpec? What grating do you use?

It's 1,800. 

Thanks for the Dubhe spectrum. I'm trying to calibrate now

Steve

[SteveBz]

The issue I'm having is matching up lines when the resolutions are different as close together spikes can become joined up and mislead the eye.

I think I need an alternative source for calibration.  I may try to calibrate with the Fraunhofer lines from the Sun, or maybe a luminescent bulb from Wickes.  Stars don't come out until nearly midnight now, so that's a bit painful.

Steve.

[drjolo]

Hm, if you do high res spectra with 1,800 grating, you may try to calibrate on atmospheric telluric lines around Ha band - they should be already present in the spectra, but identification is required. If you have calculated dispersion of your system in A/px or nm/px it would definitely help.

I did it once https://astrojolo.com/spectroscopy/menkalinan-spectroscopic-binary-star/

[robin_astro]

If you are looking specifically around H alpha at high resolution a simple neon lamp will do the job. Point the telescope at a bright A or B star and look for H alpha which will be obvious as there will be no other strong lines near it. Then when you have it centred take a neon lamp as a refence and identify the lines from for example Christian Buil's website here

http://www.astrosurf.com/buil/us/spe2/hresol4.htm

Cheers

Robin

 

[SteveBz]

On 25/05/2023 at 23:52, robin_astro said:

If you are looking specifically around H alpha at high resolution a simple neon lamp will do the job. Point the telescope at a bright A or B star and look for H alpha which will be obvious as there will be no other strong lines near it. Then when you have it centred take a neon lamp as a refence and identify the lines from for example Christian Buil's website here

http://www.astrosurf.com/buil/us/spe2/hresol4.htm

Cheers

Robin

 

Hi Robin,

I just saw that.  It's pretty much what I did.  I found an old Philips low energy fluoscent bulb and got this:

Which resulted in this:

I calculated the R number. The half height width for Ha seems to be about 0.16 so R=656/0.16 or about 4000. Is that right?

Now I need to improve the process with flats, darks and biases etc.

I've also bought soem neon lights, so the calibration will get better.

Tx

Steve

 

 

[robin_astro]

1 hour ago, SteveBz said:

I calculated the R number. The half height width for Ha seems to be about 0.16 so R=656/0.16 or about 4000. Is that right?

Provided the actual width of the line is significantly narrower than the resolution then yes this works but Ha alpha can be wide particularly in some stars eg like in Vega so you really need a line which is known to be very narrow. Try measuring the width in some of the narrow metal lines in your spectrum and see if this gives a higher resolution or  sky lines (natural or from light pollution) or from narrow lines in the calibration lamp provided it illuminates the spectrograph well (Note a single small lamp at the telescope aperture will not work for this as it like  a very high focal ratio pinhole camera and gives a perfect in focus image even when you are out of focus! If doing this using neons arrange 3 or 4 of them round the aperture edge)

Cheers

Robin

[SteveBz]

9 minutes ago, robin_astro said:

Provided the actual width of the line is significantly narrower than the resolution then yes this works but Ha alpha can be wide particularly in some stars eg like in Vega so you really need a line which is known to be very narrow. Try measuring the width in some of the narrow metal lines in your spectrum and see if this gives a higher resolution or  sky lines (natural or from light pollution) or from narrow lines in the calibration lamp provided it illuminates the spectrograph well (Note a single small lamp at the telescope aperture will not work for this as it like  a very high focal ratio pinhole camera and gives a perfect in focus image even when you are out of focus! If doing this using neons arrange 3 or 4 of them round the aperture edge)

Cheers

Robin

Great advice.  The lines at 639 are much narrower and I get nearer 9k.

I bought 5 neon lights.  I thought I'd 3d print a kind of ring round the rim to embed them in it.