Getting started in spectroscopy

Hi Gianluca,

I too an new to spectroscopy now for the past year or so. I use a Star Analyzer 200 in a filter wheel with my 12-inch LX-200 and QHY 174GPS CMOS camera. I have been told the SA200 works better in filter wheel applications due to the typically shorter distance to imaging sensor. It spreads the spectra out relatively motre for a given distance than the SA100. The downside is the field is more curved, so focussing on the zero order star image is not recommended. You will want to focus on the spectra itself-or even better, on features within the spectra image like the H alpha absorption line or emmission features. I use the SA200 for simultaneous differential photometyry and low res spectroscopy, and plan on submitting a proposal soon to the AAVSO Journal for publication on my results observing SS Cyg in September, 2019.

I dont see why you couldnt continue to use your OAG for your photometry work, but as I dont use that method cant be sure. IYour question about analyzing the spectra is a good one as there are several programs I have heard about. The one I use is RSpec from Field Tested Systems. Tom Field is the developer and overall a really nice and helpful guy.

Good luck with your adventure in Spectroscopy. I know for me it has been very rewarding, and I cant observe anything anymore without wanting to also get a spectra of it! Please post your results when you get them. I think there are alt of people like us that have an interest but not alot of experience yet. On this forum you will find some of the most skilled and helpful amateur astronomer spectroscopist in the world.

Myron Wasiuta

Mark Slade Remote Observatory

Does anyone know if an f/4 system is too fast to get decent star spectra? I have heard of possible problems with the Star Analyzer with fast systems.

Hi,

I have used an SA200 with an ML16803 CCD. The vingetting is huge in my case but as long as your target is inside the field of view of the SA it is not a problem.

You just use only part of your CCD, that is all. My system is a 40 cm f/6.8 scope with large focal length of 2700mm.

If you have a f/4 system you probably will have lots of stars in the FOV and hence lots of spectra which might overlap. That could be the biggest problem with a fast system.

Hope that helps.

Josch

Thank you Josch. Is there anything to do to minimize the overlapping spectra in crowded star fields? I seem to recall that changing orientation of the CCD/filter or something could help (I may be wrong however)

Because the beam is converging through the grating with the usual simple Star Analyser setup you do get aberrations which degrade the spectrum resolution and these are worse with lower focal ratios where the beam is converging steeper. The aberrations are also worse the finer the grating so fast systems with the SA200 show the problem the worst.  I developed a calculator which roughly suggests if a particular setup will work ok. It is implemented on line by Tom Field on his RSpec website. This can give you an idea of what might and might not work.

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

There is also some information comparing the SA100 and 200 on my website here

http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm

f4 may be too fast to get good results with the SA200 so  you should use the SA100 if you can but you will need a bigger gap between the sensor and the filter wheel and you would need to check if you have enough clearance in the wheel for the thicker SA100 compared with the slimmer SA200

Interfering stars are always a hazard with slitless setups,particularly if going after faint objects. By planning your observation and rotating the camera plus grating (The spectrum should always be kept aligned horizontally in the camera field) It is usually possible to avoid overlaps. 

Cheers

Robin

With a 6 inch f4 scope you do have a key parameter on your size in that your star size will be very small.  The resolution of a spectrograph depends on the size of the star image relative to the length of the spectrum. Because your star image will be small you dont need a very long spectrum to get good resolution which means you can place  the grating closer to the camera sensor. Having a short spectrum also helps when going for faint targets. This all means that the Star Analsyer works better in general with modest apertures

Cheers

Robin

Robin,

thank you for your remarks.

If I have understood properly I could do spectroscopy with my 6" f/4 and the SA 100 providing I increase the spacing. Can you tell me what the optimal spacing between SA100 and the sensor is in my system?

I would like to point out that as I am using the Paracorr Type II the actual focal length of my scope increases by 15%, so the system is actually f/4.6. Is it any better?

Can you also give me some hints about focusing? Shall I attampt to focus directly on the spectrum rather than the star?

Cheers

Gianluca

Gianluca,

Definitely focusing on the spectrum is the way to go. Look for the clarity of the dark absorption lines and practise with brighter A type stars.

I use BASS project for all my spectral processing. Works well for me.

https://groups.io/g/BassSpectro

We have a lot of good information and useful web links on our website:

http://www.astronomicalspectroscopy.com/

I see you have added some revised information about your setup on the RSpec forum.  I have replied there but for completeness here are my recommendations 

At just 25mm spacing you will get better resolution with the SA200 even with the warning about running at f 4.6.  With this spacing and the marginal clearance in the filter wheel I would now recommend the SA200 for your setup. The length of the spectrum (from the zero order star image to 7500A) will be ~700 pixels so will easily fit in the camera field

You will get to mag 10 easily but mag 13-14 will be near the limit with a slitless system like the Star Analyser depending on your light pollution as the spectrum is superimposed on the sky background. As a rule of thumb, you will be able to record spectra of stars to approximately 6 magnitudes brighter than you can image so you to reach mag 14, you need to be able to image mag 20 stars in normal imaging.

Cheers
Robin

I've read this forum thread with interest, but I've been at the AAS meeting in Hawaii and haven't had a chance to post.

Some of the AAVSOnet telescopes do have SA200 diffraction gratings that can be requested, if you submit a proposal.  Right now, BSM_NM in the north and BSM_Berry in the south are so equipped.  This may change in a few months, as we are upgrading our cameras, and the new filter wheels have fewer slots!  The only requirement is that you need to be an AAVSO member to submit a proposal.  We haven't seen much interest in using the grating capability of AAVSOnet, but if there are requests, we'll make sure the SA200 gets into the new filter wheels.  We do have a spare SA200 that could be put into use at BSM_NH2.  Using AAVSOnet is a hice way to try out the diffraction grating concept before getting one for yourself.

AAVSOnet also has a Shelyak eShel spectrograph mounted on the Mt. John 61cm telescope (OC61) that can be used in a shared-risk program.

I have 3 spectrographs at my personal observatory and will be putting one on the 80cm telescope in a few months (after I finish automating it) for shared-risk observations (not connected with AAVSOnet quite yet).

Arne