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Improving my spectra quality

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#1 descott12

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Posted 13 January 2020 - 07:54 PM

Hello all,

I have been slowly getting better at capturing data and producing spectra however, the quality is not as good as I would like and does not come close to what others are posting with similar equipment.  Specifically, my absorptions are often pretty shallow and sometimes (such as on some A and B-types, they are completely absent.

 

I am using a C8 at f10 with the SA-100 and a 178MM.

My typical spacing gives a dispersion of around 3.65 A/pix and an FWHM of 12.3.

 

I have done a few experiments with a greater distance ( and a dispersion closer to 2.0) but I am not sure that is helping and, from what I have read, that may be pushing things beyond what is useful.

 

I think I have my focus and exposure nailed pretty well so I am wondering what else I can do. And I am beginning to be able to stack multiple exposures to reduce noise (but that also seems to reduce the depth of my absorptions...).

 

Adding a 0.63 FR would reduce my FWHM. Would that be useful? Would an Alpy 600 running in slitless mode (I don't want to add the complexity and cost of guiding in slit mode right now) produce better results or do I simply need to practice and improve my technique some more?

Thanks for your advice.



#2 robin_astro

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Posted 14 January 2020 - 01:34 PM

It sounds like a star image size issue, perhaps from poor seeing or collimation (The calculator assumes 3 arcsec seeing but you can dial in the actual value if  you know it. You can check what it actually is from an unsaturated in focus star image). 

 

The maximum theoretical resolution (mono camera) is the dispersion (A/pix) x FWHM (pix)  which works out at ~45A for your setup, which is OK but it could be better, and it could be worse than this if your in focus star image size is worse than the 12 pixels the Calculator assumes    

 

If it is poor seeing, a focal reducer is a good idea. (This is a common configuration for the Star Analyser with f10 SCT) as it will give you better resolution without having to increase the length of the spectrum.

 

Cheers

Robin



#3 robin_astro

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Posted 14 January 2020 - 01:43 PM

Would an Alpy 600 running in slitless mode (I don't want to add the complexity and cost of guiding in slit mode right now) produce better results?

 

I would not recommend this at this stage, particularly if the star image size is the issue with the Star Analyser as this would be carried over to the ALPY if you run slitless.  (Operating the ALPY slitless is tough in any case) 

 

 Perhaps an independent check of your image quality might help. Is it possible for you to  post one of your raw subs and your aligned and stacked image of a bright main sequence Av star somewhere on line? 

 

Robin


Edited by robin_astro, 14 January 2020 - 01:43 PM.


#4 descott12

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Posted 14 January 2020 - 02:06 PM

 

If it is poor seeing, a focal reducer is a good idea. (This is a common configuration for the Star Analyser with f10 SCT) as it will give you better resolution without having to increase the length of the spectrum.

 

 

Thanks Robin. I will give it a try. I was wanting to get a FR for other reasons as well.



#5 descott12

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Posted 14 January 2020 - 02:07 PM

 

 

 Perhaps an independent check of your image quality might help. Is it possible for you to  post one of your raw subs and your aligned and stacked image of a bright main sequence Av star somewhere on line? 

 

 

That is a great idea. I will do that as soon as the clouds disappear...



#6 descott12

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Posted 15 January 2020 - 11:07 AM

Hey Robin,

Here a few single frame captures. I am still having some trouble with stacking and they are blurring so not worth looking at.

1. Sirius - this one looks pretty good with relatively deep absorptions

2. Rigel - no detail whatsoever, just a smooth curve

3. Alcyone - pretty weak absorptions and I am missing the Ha emission. There seems to be another star in the spectrum so I wonder if there was blending of two spectra??

 

Can you determine anything from these?

Thanks alot
Dave

Attached Thumbnails

  • Sirius_00001_Cropped.png
  • Rigel_00001_cropped.png
  • Alcyone_00001_Cropped.png

Edited by descott12, 15 January 2020 - 11:07 AM.


#7 cardanoc

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Posted 15 January 2020 - 03:06 PM

Hi

 

How do go about and get the best focus for your spectra?

 

For over eight years, I have been using RSpec to focus my spectra.

 

I has a great feature to fine tune your spectra.



#8 descott12

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Posted 15 January 2020 - 03:13 PM

Hi

 

How do go about and get the best focus for your spectra?

 

For over eight years, I have been using RSpec to focus my spectra.

 

I has a great feature to fine tune your spectra.

Well, I tried to use the focus tool in RSpec and to just watch for the deepest notches on the live spectra display but it runs so slowly in live capture mode that I no longer capture the spectra in RSpec.

It was so slow that changes in the scope's position or exposure wouldn't appear on screen for something like 5 or 10 seconds. Unusable performance...and if it had to rotate the image at all it got even worse. I am sure I could use a more powerful machine, but my PC handles SharpCap and Stellarium and even AS3 all at the same time pretty well so I don't really want to invest in a new machine.

 

So, I now just zoom in in SharpCap and adjust the focus until the absorption lines are the sharpest and darkest that I can achieve. I know it is not ideal for sure.



#9 robin_astro

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Posted 15 January 2020 - 03:50 PM

Hey Robin,

Here a few single frame captures. I am still having some trouble with stacking and they are blurring so not worth looking at.

1. Sirius - this one looks pretty good with relatively deep absorptions

2. Rigel - no detail whatsoever, just a smooth curve

3. Alcyone - pretty weak absorptions and I am missing the Ha emission. There seems to be another star in the spectrum so I wonder if there was blending of two spectra??

 

Can you determine anything from these?

Thanks alot
Dave

The difference between Sirius and Rigel is due to the effect of surface gravity on the line width which is much lower in  the blue supergiant Rigel compared with main sequence Sirius.  You can see the effect in the Pickles spectra

 

a0v_b8i.png

 

Hot supergiants  are a tough test for the Star Analyser as the difference becomes exaggerated if we filter the spectra to approximately match your expected ~40A resolution to the point where the Balmer lines in Sirius are only slightly muted but in Rigel they almost disappear.

 

a0v_b8i_filtered.png

 

If you are still not getting good resolution by increasing spacing to give 2A/pix I think I would be inclined to try a focal reducer as a next step

 

Cheers

Robin



#10 robin_astro

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Posted 15 January 2020 - 03:59 PM

Well, I tried to use the focus tool in RSpec and to just watch for the deepest notches on the live spectra display but it runs so slowly in live capture mode that I no longer capture the spectra in RSpec.

 

With your small pixels the spectrum is oversampled so you could bin without losing any resolution in the spectrum (3-5 pixels FWHM is about optimum compared with the estimated 12 pixels from the calculator so you could try binning 2x2 or even 3x3 without losing resolution which might speed up the live view.



#11 descott12

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Posted 15 January 2020 - 04:00 PM

Hey Robin,

I was hoping there was an explanation for Rigel.

I will increase my distance and I already have a FR on order.

Did the Sirius spectrum look ok?  I feel like the exposure and focus look pretty good on that one?

Thanks
Dave



#12 robin_astro

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Posted 16 January 2020 - 04:02 PM

 

Did the Sirius spectrum look ok?  I feel like the exposure and focus look pretty good on that one?

 

Yes that looks pretty good to me. I can see 5 Balmer lines and a hint of a sixth. I did a quick rough and ready calibration on the jpg image and I got FWHM of the Balmer lines of around 45-50A which will be slightly larger than the actual resolution due to the true width of the lines so is in line with your theoretical resolution.  The lines are all similar width too which suggests even focus along the spectrum

 

sirius_profile.png

 

Cheers

Robin



#13 descott12

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Posted 16 January 2020 - 04:18 PM

Awesome. Thanks for taking the time to check  it out




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