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Getting started with Spectroscopy - Prismv10

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#1 rottielover

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Posted 24 June 2019 - 07:25 PM

Hi all,  I recently became interested in spectroscopy after a friend of mine from the local club mentioned it.  I have recently (last few months) purchased Prism Advanced v10, and I have also piggy-backed my DSLR onto my scope so that I now have 3 cameras potentially available.  This was suggested as something I could do while waiting for astronomical dark and the start of imagining time, but I think I could potentially be able to have Prism do an automatic observation of my imaging target while I use the DSLR for spectroscopy.

 

I've just done a tiny bit of cursory overview, and started looking at the "Star Analyser 100" grating.  At first I was thinking this might be a no-brainer because I have the ZWO 8 slot filter wheel and I think I may have finally found my 8th slot... then I looked at the calculator and since my ZWO 1600mm-cool is screwed directly into the filter wheel I don't have enough backspacing there for it.   I could potentially shift my backspacing around with different adapter combinations, but also keep in mind that I run 2 different field flatteners, one is the 3" Explore Scientific reducer flatenner (gives me a 667mm FL on my ES 127 scope), or I use Hotech 2" SCA flatenner only, gives me around 957mm FL on my scope.  Both require 55mm backspacing so I can easily swap them out using my moonlight focuser adapter rings with the way I currently have the spacers setup, that is all the spacing between the flatenner and the filter wheel.  Although I just purchased a tip/tilt plate and will be adding that to the mix so now seems as good a time as any to "futz with" my rig again :) 

 

In addition to that I have my Canon 70D DSLR mounted to the 2nd finder scope slot, I have a few different lenses I could use with that.  I also have a ZWO291mono mini for my guide scope camera, on a 60mm Stellaviue guidescope.  I use an IR Cut filter on the front of the camera to help with guiding, I could potentially swap out the filter and grating (since this one is mounted on a filter body) and use it for this?

 

I was wondering if anyone here has used Prism v10 for this, and if so, do you think this hardware combination would work, or should I be looking for something other than this grating?  Barring that, if anyone would be willing to share some raw data that I could load into Prism to see if I can figure out how it works with spectroscopy and see if I'd like doing this, I would appreciate it.  



#2 robin_astro

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Posted 25 June 2019 - 05:49 AM

Welcome to the fascinating world of spectroscopy!

You could use the DSLR but  mono cameras (preferably with good long exposure capability) are much better than  one shot colour for spectroscopy so I would go with the ZWO1600mm.  What distance do you have between  filters in the wheel and the camera sensor? Have you tried the SA200 with the calculator? This gives the same dispersion as the SA100 at half the distance and is designed for filter wheels with a slimmer form. The SA100 would give slightly better results though if you can get enough spacing. More on the SA200 here on my website

http://www.threehill...troscopy_16.htm

 

I have not used Prism for processing spectra. (processing spectra is normally done off line so any software can be used for acquiring the spectra as they are just images)  but there are several other perhaps more commonly used spectrum processing alternatives around eg Visual Spec, Bass project, ISIS (all free) RSpec (which has a free trail and has a real time display function, useful with the Star Analyser)

 

Cheers

Robin


Edited by robin_astro, 25 June 2019 - 05:59 AM.

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#3 rottielover

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Posted 25 June 2019 - 08:16 PM

Thank you Robin,

 

Now that I've added in the tip/tilt plate I have to re-measure from the filter wheel to the sensor.  Off the top of my head the 1600mm says 6.5mm from sensor to the threads, the tip/tilt plate is around 10mm.  The filter wheel itself is 20mm thick I believe (EFW 8 slot), but I'd guesstimate maybe 10mm to the filters (thus where the SA100 or SA200 would sit).

 

I'll need to checkout your website and the calculator and see if I can figure out how to arrange things.  Thank you for the fast reply! 



#4 rottielover

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Posted 25 June 2019 - 09:04 PM

Not sure what I was thinking.  I ended up having to mount the tip/tilt plate behind the flatenner,  the 1600 is screwed directly onto the EFW, so I'm getting maybe 16.5 mm between where the filters are and the sensor max.



#5 rottielover

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Posted 26 June 2019 - 12:36 AM

Robin,

 

I used the calculator here: https://www.rspec-as...om/calculator/ 

 

Let's see if I did this right...

 

I have an explore scientific 127ed   in the calc tool I put 127 for apeture, and F 7.5 scope  seeing I left at 3 arcsec,  selected the SA200,  set the distance as 16.5 (estimate 10mm from filter wheel mount to camera threads and ZWO says 6.5mm from sensor to camera threads).  Pixel size of the 1600mm-cool is 3.8, and for camera width in pixels I used the ZWO spec of 4656 pixels.

 

Output seems to be correct, it estimated 953mm FL (correct), Star image size 3.6 pixels FWHM  (not sure), Dispersion 11.5 and spectrum coverage 0 to 53,615

 

Assuming all things to be true, sounds like I would want to go with the SA200 and place it into my 8th filter wheel slot?? 



#6 robin_astro

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Posted 26 June 2019 - 05:42 AM

Yes that should work, giving a result similar to that shown for P Cygni on my website for example. (To check, if you measure the typical in focus FWHM of an unsaturated star image in one of your images you could compare this with the 3.6 pixels the calculator predicts).  Increasing the distance  even by say 5mm-10mm (eg by transfering space before the wheel to after it but keeping your flattener distance correct) would likely improve resolution(you would need to check that does not give vignetting with your imaging setup) but the Star Analyser is in any case a very low resolution device designed to give a low cost introduction to spectroscopy and works best on objects with bold spectra features. If you catch the bug, the next step  up using commercial equipment (eg an ALPY 600 slit spectrograph that several contributors here use, browsing the past posts will give you an idea) is a big one in terms of cost (5-10x) and complexity, though there are some interesting 3D printed home built alternatives around at potentially significantly lower cost.

 

Cheers

Robin


Edited by robin_astro, 26 June 2019 - 05:43 AM.


#7 robin_astro

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Posted 26 June 2019 - 06:03 AM

Note this source suggests the distance from filter to sensor with this setup may only be 10mm

https://www.rotherva...ed-filters.html

If this is the case the resolution will be lower than that typically achieved with the Star Analyser (The calculator gives a warning) so I would advise increasing the distance if possible.

 

Cheers

Robin



#8 robin_astro

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Posted 26 June 2019 - 06:15 AM

Also check there is enough clearance in the filter wheel. (There should be if it takes standard 1.25 inch filters. The SA200 is 7.2mm thick including the thread, 5.2mm above the thread)



#9 rottielover

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Posted 28 June 2019 - 11:15 AM

The filter wheel I have itself is 20mm thick total, I was just guessing that there would be around 10mm between the threads and where the glass of the filters sit.

 

We FINALLY have a couple of clear nights in the forecast where I don't have work the next day :)   Might finally be able to get that FWHM measurement done.  I don't have a grating yet, but I was thinking I would attempt to do a little photometery with Prism and see if I can't spot any asteroids before I setting in for the night to get "pretty pictures".  



#10 descott12

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Posted 28 June 2019 - 12:36 PM

Hello,

I can't help you solve this specific problem but I just wanted to say that the SA-100/RSpec combo is really a pleasure to use.  Highly recommended.  I'm pretty much a newbie with spectroscopy but I have been able to capture good spectra, adjust for my camera 's absorption profile and correctly classify a few stars. Really an amazing (and simple) gadget.


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#11 Stu Todd

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Posted 30 June 2019 - 07:13 AM

Hi! 

 

I have been using the trial version of Prism and find it to be an excellent software. As far as spectroscopy goes, it does well tracking on a line and slit placement etc but don't process your spectra with it.

As Robin said, use Vspec, ISIS or BASS. My daughter has just used BASS and the SA100 for her school science project on the "Balmer Thermometer".

BASS gives a lovely presentation and is easy to use.

 

+1 for using Prism though 

 

Stu

 

P.S Prism is nicely demonstrated in use on youtube as you will know.


Edited by Stu Todd, 30 June 2019 - 08:30 AM.

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#12 rottielover

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Posted 18 July 2019 - 09:43 PM

My SA200 arrived today, and my license code for RSpec is in my email.  Time to SCIENCE  ;) lol 




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