Sirius Spectra

[Lucas Barclay]

Good evening!

I recently got my hands on a star analyzer 200 and took a glance at Sirius. As a result I was able to gather this spectra from the star. Although I am struggling to annotate it and make some observations of the star. I also feel as though some of the absorption lines are due to atmospheric interference. Any advice or tips would be appreciated! The software I'm using is Visual spec for reference!

[robin_astro]

You have a mix of Hydrogen lines from the star, Telluric bands from the atmosphere and ripples from the spectral response from the camera but it is difficult to wavelength calibrate and identify them with any certainty without the zero order star as a reference point. (Is this outside the camera field?) Can you describe your setup  (telescope, camera and the distance of the grating from the sensor) and also post an image of the raw spectrum please ?

Thanks

Robin 

[robin_astro]

Here's my stab at a wavelength calibration and line identification

blue is your raw spectrum, red are the Hydrogen Balmer H alpha, H beta green are the telluric bands (O2 and H2O) The others are ripples in the camera response.

If the calibration is correct, the lack of signal at the blue end is surprising. Normally H beta at 4863 A is near the peak of the curve. Is your spectrum correctly exposed with no saturated pixels in the spectrum? (Sirius is a very bright target  and will need very short exposure.)

Check also that you have to correct spectrum, The brighter one of the two nearest the zero order

Sirius is also not an ideal target from the UK as it is low so the star image is likely to be large, reducing the sharpness of the spectrum and scintillation will distort the spectrum in short exposures. 

Cheers

Robin

 

Edited February 19, 2022 by robin_astro

[robin_astro]

12 minutes ago, robin_astro said:

 

If the calibration is correct, the lack of signal at the blue end is surprising. Normally H beta at 4861 A is near the peak of the curve.

 

This is more like what I would expect to see (in black)  Are you using a mono or colour camera ?

[Lucas Barclay]

9 hours ago, robin_astro said:

This is more like what I would expect to see (in black)  Are you using a mono or colour camera ?

Thank you for all the help so far! I may need a bit of help on producing that calibrated spectrum since I am not quite sure how to do that.. I used a monochrome camera for this spectra (ASI178mm) with my william optics ZN73 and the staranalyser 200 which was about 45mm away from the sensor. I'm not too sure what you mean about the brightest of the two spectra nearest to the zero order though.. Sorry for not knowing much about the process of spectrometry, I have only recently begun! Here is an image of the spectra produced without taking a cut from it : 

21_16_45_lapl5_ap4930_Drizzle15.tif

[robin_astro]

1 hour ago, Lucas Barclay said:

I'm not too sure what you mean about the brightest of the two spectra nearest to the zero order though.. 

 

Have a look at the image in the Star Analyser manual (I wrote it 😀)

https://www.patonhawksley.com/resources

Cheers

Robin

[robin_astro]

Also have a look at the tutorial videos on the RSpec website (It uses RSpec software but that was loosely based on Visuals spec and the steps are the same.)

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

there is a calculator there to help optimise your setup (based on my calculations)

https://www.rspec-astro.com/calculator/

The slides from my BAA tutorial "Low Resolution Slitless Spectroscopy" might also be of use, downloadable from the bottom of this page

https://www.britastro.org/downloads/15701

and Christian Buil's tips for using the Star Analyser

http://www.astrosurf.com/aras/staranalyser/userguide.htm

[robin_astro]

2 hours ago, Lucas Barclay said:

Here is an image of the spectra produced without taking a cut from it : 

OK I recommend rotating the grating so the spectrum is horizontal and moving the spectrum so the zero order (the star image) is visible as this is the 0 reference point we can use for the wavelength scale. You might need to reduce  the distance of the grating to fit  the spectrum and zero order in the frame. Use the RSpec calculator as your guide

Cheers

Robin

[robin_astro]

12 minutes ago, robin_astro said:

there is a calculator there to help optimise your setup (based on my calculations)

https://www.rspec-astro.com/calculator/

Can you post a screen shot of the calculator with details of your current setup entered please

Robin

[Lucas Barclay]

3 minutes ago, robin_astro said:

Can you post a screen shot of the calculator with details of your current setup entered please

Robin

Of course! Thank you for all these links I shall have a read through them now! Here is what the results come out as on the calculator :

[robin_astro]

11 minutes ago, Lucas Barclay said:

Here is what the results come out as on the calculator

OK it looks like you should just be able to fit the zero order and spectrum in the frame. Check the grating is rotated so the brighter blazed spectrum is to the right of the zero order (There is a mark on the Star Analyser filter cell to show  approximately the correct orientation) and place the zero order near the left edge of the frame. To get you started, the blamer lines should then be roughly in the right place. For example H beta at 4861A should be ~4861/2.7 = 1800 pixels along from the zero order. 

It still does not explain the apparent low sensitivity in the blue (Do you have any other filters in the imaging train?) but we can worry about that once we have the full spectrum and zero order in the image

Cheers

Robin

[Lucas Barclay]

1 minute ago, robin_astro said:

OK it looks like you should just be able to fit the zero order and spectrum in the frame. Check the grating is rotated so the brighter blazed spectrum is to the right of the zero order (There is a mark on the Star Analyser filter cell to show  approximately the correct orientation) and place the zero order near the left edge of the frame. To get you started, the blamer lines should then be roughly in the right place. For example H beta at 4861A should be ~4861/2.7 = 1800 pixels along from the zero order. 

It still does not explain the apparent low sensitivity in the blue (Do you have any other filters in the imaging train?) but we can worry about that once we have the full spectrum and zero order in the image

Cheers

Robin

Got it! I do have a few other images I took with my DSLR camera but I have yet to look at them in much detail yet, though with my DSLR I believe the entire spectra should be present including the zero order so I may have a look at that too!

[robin_astro]

OK I think I know what has happened. It looks like you have captured the second order (the next spectrum out) This will be ~2x longer and much weaker than the first order, particularly in the blue. I think You will be surprised by the much shorter exposure you will need when you find the right spectrum !

Cheers

Robin

[Lucas Barclay]

18 minutes ago, robin_astro said:

OK I think I know what has happened. It looks like you have captured the second order (the next spectrum out) This will be ~2x longer and much weaker than the first order, particularly in the blue. I think You will be surprised by the much shorter exposure you will need when you find the right spectrum !

Cheers

Robin

Ahh I see! I will try to get the correct one next time! I am wondering now though, could I have gotten the two ends mixed up? By mirroring the image it somewhat looks more correct than the original.. 

[robin_astro]

Perhaps, but I dont think so. The Balmer lines and Telluric bands line up too well the original way round and the dispersion is about right for the second order (higher than calculated for the first order as expected.) The very deep band at ~7600A from O2 in the atmosphere is also good indicator and a common feature seen in all spectra taken from the ground. It could also explain the bias in the spectrum towards the IR (The blazed grating is optimised to preferentially select the wavelengths in the visible in the first order)

[robin_astro]

The uploaded tiff image seems to be 1.5x larger than the camera sensor (4644x3120 compared with  3096x2080)  has it been resampled somehow?

Cheers

Robin

[Lucas Barclay]

26 minutes ago, robin_astro said:

The uploaded tiff image seems to be 1.5x larger than the camera sensor (4644x3120 compared with  3096x2080)  has it been resampled somehow?

Cheers

Robin

Ah yes, I took multiple images of the spectra and then stacked them up to try and get a bit more detail through!

[Lucas Barclay]

7 hours ago, robin_astro said:

The uploaded tiff image seems to be 1.5x larger than the camera sensor (4644x3120 compared with  3096x2080)  has it been resampled somehow?

Cheers

Robin

Just went out tonight and decided to try do the spectra of one of the stars in Pleiades! I think it has actually turned out pretty well I just wanted to confirm and see your thoughts on how the graph looks!

 

[robin_astro]

The series of absorption lines at the left end look like the Balmer series which suggests you are getting something at the blue end now but we really need the zero order in the same frame to calibrate it properly without guessing. Were you able to find the various spectra I talked about and chose the right one?  This is what a wide field view (of Jupiter) through a grating looks like (From Christian Buil's page)

http://www.astrosurf.com/aras/staranalyser/flat.jpg

Notice how one of the spectra of Jupiter is much brighter than the others. this is the blazed first order and the one you want to get in the frame, together with the zero order

Cheers

Robin

[robin_astro]

4 minutes ago, robin_astro said:

Notice how one of the spectra of Jupiter is much brighter than the others. this is the blazed first order and the one you want to get in the frame, together with the zero order

The region I have marked as the white rectangle.  Based on your setup, it should neatly fit in your field of view with the zero order near the left edge