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How to capture a decent Spectrum and the Process


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So I have been capturing spectrums for a while now,  I use an SA100 on a QHY5 and use QG Video all on my Skywatcher 250, I typically take a 2 minute video and then use Registax to process


So I think my process is ok ?


So I typically start with an A Type Star as a reference, so get the spectrum in the QG Video window and focus and adjust the gain until I can clearly see the HB lines


A typical raw image out of Registax is attached, this was a tif but I converted to a JPG just for posting, you can however clearly see the lines and it will process in RSpec with no issues.


So then If I go to another target where the HB lines are not obvious I get issues, so I leave the focus and simply adjust the gain so its not over saturated so Attached is a spectrum of Capella, I can make out the telluric band but nothing else, and once processed and loaded into Rspec im seeing litte or no features.


So I guess Im asking if


A - Is my capture process ok

B - How can one be sure that you have a good base spectrum and apart visually looking on the capture screen is there any other method


Thanks for the support and apologies for the continuous questions


Regards


John Berman


Sirius - Raw Capture

post-4419-0-62901600-1391542255_thumb.jp


Capella - Raw Capture

post-4419-0-81561300-1391542252_thumb.jp

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Hi John

Thought someone might have chipped in by now so I`ll add my twopenuth if thats ok?. I can`t see any problems with your method although I would say you need to refocus on each object as I woudn`t rely on the mirror staying put all night long,Its different I know with your reflector but I had endless problems with mirror flop with my old C8!. No problems at all asking questions, questions and making mistakes (which I do daily!) is all part of it and its purely through guys on here helping me that I`m a bit better at taking spectra than I used to be. As you know the seeing is never perfect in the UK so a star showing nice lines one night might not be so good the next night when the air not so clear. I assume you use the `one click` option in Rspec which helps if you have a previously calibrated good base spectrum?..Keep asking John and hopefully the more experienced guys will be able to answer your queries!

Cheers

Steve

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Hi John,

your technique is ok and the fact that you are seeing several Balmer lines in Sirius confirms this.  The difficulty  is the target which is a tough one to see anything in at the resolution of the Star Analyser.

The problem is Capella is a G8III spectral type.  These have hundreds of  lines which are unresolved at the Star Analyser resolution so appear as small broad humps and bumps in the spectrum and additionally being a luminosity class III giant star means the lines are narrower than for example in main sequence class V stars. With care you could probably tease out the small humps and bumps in RSpec though

As an experiment I suggest taking the reference spectrum in Rspec, filter it to make the Balmer lines to make it look like your Sirius spectrum ( in terms of line width and intensity) and note the filter setting. Then take the G8 III reference spectrum and filter it in the same way. This is what you should expect your Capella spectrum to look like.  

Very low resolution spectrographs like the Star Analyser work best on stars with bold broad obvious features (Try Betelgeuse or Be star Gamma Cas for example) and are not so good for teasing out fine detail.

If you have a DSLR, you can get more resolution on bright targets like this with the Star Analyser mounted in front of the camera lens.  Here is epsilon Aurigae for example which is also a difficult target to see much with the Star Analyser between telescope and camera but which shows good detail when used this way

http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm

and here using the SA200

http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm

Robin

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As an experiment I suggest taking the reference spectrum in Rspec, filter it to make the Balmer lines to make it look like your Sirius spectrum ( in terms of line width and intensity) and note the filter setting. 

That should read 

.......suggest taking the A1V reference spectrum in Rspec, filter it to make the Balmer lines.........

Robin

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Attached (using visual spec) is the simulated Capella spectrum at the resolution of the SA (used between telescope and camera) using an A0v (27 Lyn) spectrum I took the other day  (which looks similar to your Sirius spectrum I think).  You should be able to get something approximating this.

Cheers

robin

post-522-0-18560800-1391606989_thumb.png

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John - QGVideo software has a feature whereby you can see the profile in real time. Press the Measure button (ruler icon) on the bottom of the screen. Then draw a line on your spectrum. You might need to redo this if your images moves, but being able to see a profile in real time will help you focus.

Robin - thanks for posting the setup of using the Sa100 in front of the objective. Been meaning to try that for ages.

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