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PhotoGav

Starting out in Spectroscopy?

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Hi, I am interested in starting some simple spectroscopy. This is primarily aimed at workshops for astronomy GCSE students to let them see some of the theoretical elements of the course in practice. I am considering the Star Analyser 1.25” diffraction grating. I have a variety of telescopes at my disposal, the main one being a 10” Thomas Cooke refractor. I am an experienced astrophotographer, but a total newbie to spectroscopy. What can I realistically expect to manage with ‘basic’ equipment? How long an exposure is required to create a spectrum? I have a ZWO ASI 120 MC-S camera or a DSLR available for use. Are either of these going to be suitable?

As you can see from my questions I am clueless at the moment, but would love to get something up and running, so your help will be greatly appreciated. 

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I am sure Robin will be along with lots of good advice and links but try www.rspec-astro.com for some ideas. 

You will need software to analyse the spectra and rspec has a free trial if I remember correctly. 

Also try Rrobin's site www.threehillsobservatory.co.uk

Regards Andrew 

Edited by andrew s
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There is a spread sheet floating around in threads here that is very useful and you should look it up.

It gives basic calculations depending on configuration used - of what you can expect to achieve with SA.

I think that better option is to go for mono sensor, but OSC can be used as well.

Important aspects are F/ratio of beam (too wide and it will introduce aberrations in spectrum), distance to sensor - also SA100 vs SA200 models - different dispersion, and sensor size and pixel scale will be important.

Exposure length will of course depend on strength of source, and you will be limited to point sources - stars and quasars, since you won't be able to use a slit. If you fiddle around with SA and build your own contraption - you can include slit - that should give you very good resolution - SA by it self can go to medium resolutions like R1000 - R2000 if it's properly "configured" (slit, collimation and so on)...

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1 hour ago, andrew s said:

I am sure Robin will be along with lots of good advice and links

Did somebody Call ? ?

Hello Gav,

Yes either camera can be used with the Star Analyser but I would suggest the ASI120 for starters. A mono camera if available would be better though for the various reasons I outline here

 https://groups.yahoo.com/neo/groups/RSpec_Real_Time_Spectroscopy/conversations/messages/6782

To use the SA100 with the ASI120, you just screw it onto the nosepiece and use it pushed into the eyepiece holder as you would for normal imaging.  You might want to add a spacer or two. For best resolution check the calculator and keep increasing distance from the sensor until the calculator starts complaining. 

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

Exposure times depend on how faint the target is (Anything from less than a second for the brightest stars to tens of minutes for  mag 12 quasars or supernovae) but there is no need to guide, just align and stack shorter exposures. (An SA100 spectrum is typically about 6 mag fainter than the equivalent star so if you can record a mag 16 star say in a normal image say, you should be able to record a spectrum of a mag 10 star)  For example there are many examples on my website taken using a mono modified webcam which should give some idea of the performance with the ASI120.

The 10 inch Cooke refractor is a spectacular instrument but may not be the best choice. (What is the focal length? The SA works best with shorter focla lengths provied the focal ratio is higher than ~f5)  What other scopes do you have?  I would say the sweet spot for the SA is something like an f6 6-8 inch newtonian or a 100mm f6-8 apochromat. At larger apertures/longer focal lengths you can go fainter but resolution suffers

Note that the Star Analyser is a very low resolution device, typically around 30-50A resolution so will not show fine details but works best on objects showing bold spectroscopic features. 

Cheers

Robin

Edited by robin_astro
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32 minutes ago, robin_astro said:

Yes either camera can be used with the Star Analyser but I would suggest the ASI120 for starters.

Christian Buil has some nice images and tips for using the SA with a DSLR (The wedge prism it is not needed) 

http://astrosurf.com/buil/staranalyser/obs.htm

It helps extend the wavelength range if the IR blocking filter has been removed as here, also for the ASI120MC if it can be removed.

Robin

Edited by robin_astro

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Brilliant, thank you all for your help so far. I will read the various links and I’m sure will then be back with more questions. 

I do have a mono camera (PG Chameleon3) and a modified DSLR (Canon 60D). Scopes wise: the 10” has a focal length of 3.8m at f15. There is also a Skywatcher 200 pds on an HEQ5 up at the observatory, which might be the better option, though it needs to be set up every time, which is a bit of a hassle, especially when the 10” is ready and waiting!

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Hi Gav,

The focal length of the Cooke will be too long to give decent resolution used with the small sensor ASI camera I think but might possibly work with the modded DSLR where you can spread the spectrum out further. (The maximum resolution of the spectrum is effectively defined by  the size of the star image relative to the length of the spectrum, then there are some additional aberrations due to the simple converging beam arrangement which ultimately limit it) . You could try a focal reducer but  I suspect this setup will show significant chromatism though in any case, losing focus at the blue/UV end, seen as a "fishtail" at the end of the spectrum. The 200mm f5 Newtonian though would be an excellent choice if you can set that up.  (You could almost piggyback it and use it as a finder/guidescope for the Cooke.  The mount  might not notice the small extra weight ? )

I dont know the Chamelion 3 I'm afraid. A quick google suggests it comes with various sensors so you would need to plug the appropriate numbers into the calculator. Does it have any long exposure capability ie a few seconds at least ? If so, it could be a better choice than the ASI 120.  Room for experiment there I suspect

Cheers

Robin

Edited by robin_astro
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Robin,

Thank you for your continuing advice. Hah! Yes, the 200 could happily be a guide scope...!

It looks like I have enough options to play about with and see which gives best results. I will be placing an order for the SA 100 very soon and downloading the RSpec software to have a play. I look forward to it and hopefully it can soon be introduced to the students.

No doubt many questions to come!

Thanks,

Gav.

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Brilliant, thank you Robin & Merlin.

This is all further evidence that SGL is indeed at the centre of the Cosmos; the fabulous help that has come forth after a simple post is outstanding. Thank you all. I now need to convince my education establishment to sign off the total budget requirements and we’re away!

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