12 replies to this topic

[cWR_]

My school has recently purchased a SA grating for its observatory and I plan on using it with a planetary camera to capture the spectra of stars. I have watched this video explaining how to use Rspec: https://www.youtube....h?v=6MrFuNGLsUE

 

It is explained that you should start with an A-star to calibrate using the Balmer lines. The star that was used as an example was Vega. I live in the Northern Hemisphere and as of right now Vega is just more than 20 degrees above the horizon once it gets dark enough to start shooting (or what I think is dark enough to start shooting).

 

My questions are:

Can I still capture Vega to calibrate?

If I can't capture Vega either because it is too low above the horizon or it turns out that the view is blocked (unlikely), what other stars can I capture instead of it?

How do I determine the best settings to capture spectra?

Are there any other things I might want to know about before acquiring data?

[BlueMoon]

Here's an article on CN from some years back that may help you with your questions. While much of the information is general at the bottom of the article are several references with more detailed information. https://www.cloudyni...tronomers-r2907 Cheers and good luck.

[SeymoreStars]

This is a screenshot from Stellarium (free planetarium software). It show Mahasim's position at 6:05PM January 6th. It's an A0V target for you.

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[cWR_]

Thank you both! About Mahasim: Since it is a double star, does it show any doppler shift that could potentially disrupt the calibration?

[SeymoreStars]

Thank you both! About Mahasim: Since it is a double star, does it show any doppler shift that could potentially disrupt the calibration?

Sorry beyond my paygrade.

[david_od]

You can definitely use other A stars to calibrate. Indeed, I try to calibrate using bright A stars as close as I can to the target I'm going for, but sometimes they may be several degrees away. Close but too faint A stars may be difficult to use for calibration, making things more difficult. They don't need to be in the same field of view.
I use Stellarium to find calibration stars, and even their apparent color on the screen may assist you in selecting probable candidates; sometimes they will actually be different classes, it's not directly apparent exactly which ones are ok.

Also, even "cool" B stars (those with a relatively large number, like B6) and hot F stars (like F0 or F1) should display Balmer lines appropriate for wavelength calibration.
See for instance this: https://www.e-educat...mage/starsl.jpg

 

[david_od]

Thank you both! About Mahasim: Since it is a double star, does it show any doppler shift that could potentially disrupt the calibration?

You'd need very particular stars (very massive, close orbit) to even start registering somehow at the typical resolution of the star analyzer. I often get close to 8 Angstroms per pixel, and a shift of half of that, 4 Angstroms, near the ~6563 Angstrom hydrogen line would require a projected radial speed of 1800 km/s. For comparison, the WR 20a binary star in Carina has components close to 80 solar masses and a period of ~3.7 days, resulting in a relative speed of about 700 km/s. All of this is still well under the typical line width of a Balmer line, and will be masked by the apparent stellar size and additional seeing effects as well.

Edited by david_od, 06 January 2025 - 10:38 AM.

[scanner97]

Looks like you're getting some good support - which is definitely more than I can provide.

 

I thought I'd just mention The Astro Imaging Channel (youtube) as one place that often has useful videos.  I was a bit surprised to see how many came up when I search on spectroscopy.  Here's the list. 

[pvdv]

You'd need very particular stars (very massive, close orbit) to even start registering somehow at the typical resolution of the star analyzer. I often get close to 8 Angstroms per pixel, 

8 Angstroms per pixel looks like a typical dispersion value. That value is mostly dependent on the grating to sensor distance and is not really connected to actual resolution 

Since the Star Analyze 100 as a R of 100

R = λ / Δ λ

Δ λ = λ / R 

So resolution should be around 50A at best at 5000A I believe.

[robin_astro]

Thank you both! About Mahasim: Since it is a double star, does it show any doppler shift that could potentially disrupt the calibration?

At the resolution and accuracy of the Star Analyser, you don't need to worry about doppler shifts for the velocities of targets in out own galaxy, though extra-galactic redshifts are measurable

 

Cheers

Robin

Edited by robin_astro, 06 January 2025 - 11:02 AM.

[robin_astro]

I can suggest Gamma Cassiopeiae as a first target. It has a bright hydrogen alpha emission line which is easy to spot and you can use that and the zero order star image to calibrate the spectrum.  Delta Cassiopeiae then has a similar spectrum to Vega with strong Balmer lines in absorption. Here are some measurements of them I made in 2005 using a prototype of the Star Analyser before it went into production later that year

http://www.threehill.../spectra_12.htm

 

Cheers

Robin

[cWR_]

I can suggest Gamma Cassiopeiae as a first target. It has a bright hydrogen alpha emission line which is easy to spot and you can use that and the zero order star image to calibrate the spectrum.  Delta Cassiopeiae then has a similar spectrum to Vega with strong Balmer lines in absorption. Here are some measurements of them I made in 2005 using a prototype of the Star Analyser before it went into production later that year

http://www.threehill.../spectra_12.htm

 

Cheers

Robin

Thanks! Also, love the spectra!

[david_od]

8 Angstroms per pixel looks like a typical dispersion value. That value is mostly dependent on the grating to sensor distance and is not really connected to actual resolution 

Since the Star Analyze 100 as a R of 100

R = λ / Δ λ

Δ λ = λ / R 

So resolution should be around 50A at best at 5000A I believe.

Yes, thank you, I mixed concepts up there.