First stab at stellar spectroscopy - Vega SA200

[physicus]

I've been into deep-sky astrophotography for a couple of years now, but am only now putting my toe in the spectroscopy pond.  And for a week's worth of learning I'm happy enough with this spectrum.  The rig is a C9.25 at f10 with the SA200 mounted in the nose-piece of my Atik460.  Processing was in Nebulosity (stacking), GIMP (for rotation) and the BASS Project software for charting/calibration.  My workflow through that lot was a bit of a mess, but the fastest way to get a result without learning BASS inside out; which may be where things end up.   I chose the SA200 over the SA100 as, looking through Robin Leadbetter's material, there seem to be more possibilities down the line re collimation/imaging configurations.  The main thing that struck me at this stage was the ambiguity in focusing on the spectrum.  After assisted FWHM on point sources, this was a bit of a challenge.  Lots more to do, but I can feel the bug biting...

 

[robin_astro]

Hi Tim,

Looking good!    Finding best focus is a bit more of a compromise with the SA200 compared with the SA100 as the larger dispersion angle gives more  field curvature so more focus change along the spectrum. I can see 6 Balmer lines there though which is good going and the ones at blue end will stand out much more clearly after instrument response correction.

Cheers

Robin

[physicus]

51 minutes ago, robin_astro said:

Hi Tim

Looking good!    Finding best focus is a bit more of a compromise with the SA200 compared with the SA100 as the larger dispersion angle gives more  field curvature so more focus change along the spectrum. I can see 6 Balmer lines there though which is good going and the ones at blue end will stand out much more clearly after instrument response correction.

Cheers

Robin

Thanks Robin,

I started out intending to use the SA200 in a collimated configuration with an eyepiece and spare Zuiko 50mm, per one of your own rigs; but on the night my junk box lacked the critical mass to quite make that happen with the right distances/geometry.  Also tried barlow collimation and got severe coma at edge of field.  I'll work it out though. 

Regards

 

Tim

 

[Merlin66]

Tim,

Looks like you're off to a good start.

I use the old Baader 207 l/mm grating. Focusing the spectrum is the key - gets a bit easier with practise.

I'm sure you'll find BASS Project a very useful tool as you progress forward.

Well done, keep it up.

 

[physicus]

On 16/09/2016 at 01:54, physicus said:

I've been into deep-sky astrophotography for a couple of years now, but am only now putting my toe in the spectroscopy pond.  And for a week's worth of learning I'm happy enough with this spectrum.  The rig is a C9.25 at f10 with the SA200 mounted in the nose-piece of my Atik460.  Processing was in Nebulosity (stacking), GIMP (for rotation) and the BASS Project software for charting/calibration.  My workflow through that lot was a bit of a mess, but the fastest way to get a result without learning BASS inside out; which may be where things end up.   I chose the SA200 over the SA100 as, looking through Robin Leadbetter's material, there seem to be more possibilities down the line re collimation/imaging configurations.  The main thing that struck me at this stage was the ambiguity in focusing on the spectrum.  After assisted FWHM on point sources, this was a bit of a challenge.  Lots more to do, but I can feel the bug biting...

 

For good order, here's the above data with instrument responsivity corrected and continuum removed.  Indeed, Balmer lines better defined, and a few other suspects appearing. 

[robin_astro]

Tim,

If you fancy a couple of reasonably bright  "wow" targets for the Star Analyser at this time of year I can recommend  P Cygni (HD: 193237) ~ mag 5 and WR140 (HD193793) ~mag 7   These show really interesting spectra at low resolution and easily within range of your setup. You can use your Vega instrument response to ~ correct them but even the raw image without any processing will show very obvious features.

Robin

[physicus]

27 minutes ago, robin_astro said:

Tim,

If you fancy a couple of reasonably bright  "wow" targets for the Star Analyser at this time of year I can recommend  P Cygni (HD: 193237) ~ mag 5 and WR140 (HD193793) ~mag 7   These show really interesting spectra at low resolution and easily within range of your setup. You can use your Vega instrument response to ~ correct them but even the raw image without any processing will show very obvious features.

Robin

Thanks Robin; I'll check those next outing.  Spooky you suggest WR140; I was just this afternoon reading about it in Richard Walker's 'Atlas'. 

Tim

[ChrisLX200]

I'm considering having a stab at spectroscopy, one advantage I have is that I may well be able to get away without guiding with the 10-Micron mount - it works well enough unguided for imaging so it should keep a star image in the slit (for a slitted spectrometer). As to choosing a spectrometer that is not so clear - so many to choose from. Hi-res, Low-res, slitless, slitted, Alpy, Eshel, LISA, LHires, DADOS, Starlight Xpress... it's worse than choosing a scope. To keep my interest it would need to do something interesting though - not just confirm the spectral characteristics of a few well-known bright stars! So I'll need to read up on what the amateur can achieve using one.

ChrisH

[JeremyS]

Hello ChrisH,

you ask about interesting projects for spectroscopy and I am sure Robin will comment. But I'd like to draw your attention to some important science Robin himself has done very recently. With many supernovae now being discovered by professional and amateur surveys, follow-up spectroscopy is not always timely or even available. Well, Robin obtained the confirming spectrum of supernova at2016gfr. Have a look here: https://britastro.org/node/7976

Jeremy

[Gasman]

Hi guys

Pretty timely choice for WR 140 to do spectro and also variable observations . Just had a `The Astronomer` bulletin saying WR140 is a long period variable composed of a Wolf Rayet star and an O type star in orbit and they are approaching periastron (nearest approach to each other) in December  so well worth monitoring its brightness and  spectrum. I echo what Robin says about P Cyg too. H alpha line very prominent.

cheers

Steve

[robin_astro]

2 hours ago, ChrisLX200 said:

To keep my interest it would need to do something interesting though - not just confirm the spectral characteristics of a few well-known bright stars! So I'll need to read up on what the amateur can achieve using one.

Hi Chris,

The ARAS group website and forum is a good place to get an overview of what type of serious work amateurs can achieve in this area

http://www.astrosurf.com/aras/

http://spectro-aras.com/forum/

It covers a wide "spectrum" from my very low resolution faint supernovae work ( Thanks Jeremy :-)  )  to high resolution monitoring of line profiles of bright Be stars and everything in between

Cheers

Robin

[Merlin66]

Chris,

I'd strongly recommend starting with a slit-less grating set up - call it "Spectroscopy 101"

As Robin has shown there is a lot of potential even with such a basic set-up.

It will allow you to come to terms with the pre and post processing of your data - a skill needed when you move into slit spectroscopy.

The range of slit spectrographs from Shelyak is designed to meet the need of amateurs and covers lo-res (ALPY), med res (LISA) through to hi-res (LhiresIII) as well as semi-professional eschelle instruments. The choice comes down to the type of spectroscopy you wish to follow.

No matter how good your mount performs, you will need the ability to guide the target star on the slit gap. All the modern instruments supply varying slit gap options (to suit your telescope and seeing conditions) and reflective guide slit plates.

I use a Spectra-L200 with an ATiK314 on a C11/ NEQ6pro guided with a Lodestar.

 

[ChrisLX200]

Many thanks for the replies chaps, I'm hacking my way through a couple of books and also some interesting websites. First concern is how practical it would be to work without an observatory, i.e., whether I can operate remotely - well, 15 meters remote because that's how far away the scope and mount are from my warm control room. Appreciate the comment about starting with a Star Analyser or similar grating, most of the work does appear to be processing and interpreting the data rather than simply capturing it

ChrisH

[Merlin66]

Chris,

I assume you already have a set-up between your telescope/camera and the control room??

Imaging and guider set-up is very similar to AP requirements.

Simpler for the grating set-up.

With a slit spectroscope there is the need to acquire a reference lamp spectrum before and at least after (for calibration) and usually a spectrum flat.

This can be automated, but many do the change-over manually.

Robin has his gear located in a small run of shed...works well.

I have mine set-up in a ROR observatory.

 

 

[ChrisLX200]

The mount stays on its permanent pier, all comms (Ethernet and USB cables) and Power lines (12v and 24v) are underground and remain permanently connected. Each session I have to mount the OTA of choice together with whatever camera then just switch on the power. It only takes 10mins to set up and everything is then operated remotely.

I found a source for the Star Analyser 100 online but not the Star Analyser 200 - which might be a better option slotted into my Atik EFW2 filterwheel. Scope would be an ODK12, camera Atik490EX. I do have guide cameras available but have not needed to use them with the 10-Micron, I used a Lodestar with my old Losmandy G11.

ChrisH

[Merlin66]

Chris,

Paton Hawksley, the manufacturer shows the SA200 on their site.

http://www.patonhawksley.co.uk/staranalyser200.html

The f6.8 of the ODK12 should be good.

I'm not aware of any spectroscope users with the 10 Micron mount.

I understand your comments on tracking performance....but even the professionals make use of the reflective slit guiding to ensure maximum transmission. It allows you to verify the target star is well focused on the actual slit gap.

The slit gap would be around 25micron and you want to achieve subs of ten mins, total exposure >60mins.....

[physicus]

On 21/09/2016 at 19:31, robin_astro said:

Tim,

If you fancy a couple of reasonably bright  "wow" targets for the Star Analyser at this time of year I can recommend  P Cygni (HD: 193237) ~ mag 5 and WR140 (HD193793) ~mag 7   These show really interesting spectra at low resolution and easily within range of your setup. You can use your Vega instrument response to ~ correct them but even the raw image without any processing will show very obvious features.

Robin

I had a crack at WR140 and P-Cygni last night, also Altair (below).  Those carbon 'beads' in WR140 really jump off the screen!  The fully collimated set-up to your design is working nicely now (w 25mm eyepiece+50mm zuiko camera lens, so some mag.) and I've reduced the scope to f6.3.  I'll do the maths for this arrangement, but it's giving a good image scale with hardly any vignetting on the 460.  As I'm now collimated, is it fair to say the first order star image should be at the same focus as the rest of the image plane?  It certainly looks that way on my raw images, and I can anticipate with dimmer targets it would be useful to focus (minimise FWHM) on that rather than the dispersed region.  Pleased with having what seems a quite capable collimated spectroscope for a hundred quid.

Tim

   

[robin_astro]

Hi Tim,

Very nice!  I'd not thought about the zero order focus due to field curvature with this sort of setup but you are probably right. This will depend on performance of the objective (camera) lens. Perhaps more significantly it should get rid of the focus shift along the spectrum seen with the conventional simple converging beam Star Analyser setup.   

In this case the magnification you have due to the shorter focal length collimator may be limiting the resolution as the resolution of the setup will most likely be limited by the star image size rather than the pixel size. (The optimum is a star image size of 2-3 pixels) so you might be able to squeeze a bit more resolution using a longer focal length collimator.  (An idle thought but I wonder if the focal reducer could be pressed into this role somehow?)

For perhaps the ultimate in home build of this sort of transmission grating arrangement, take a look at Fulvio Mete's example here.

http://www.lightfrominfinity.org/Spec 600/Spec 600.htm

Cheers

Robin