So just how difficult is spectroscopy?

[higginsdj]

I'm considering the L200 for asteroids and eclipsing binaries but its a reasonable sized investment for what will be a second string activity. The aim will be to have my ST-8 permanently affixed to it (I'm in the process of buying a new CCD camera but have to wait for a grant submission result before I decide which) so I can just attach it to the scope and start imaging. Guiding will be by my current external guider which is good for +/- < 2 pixels at 1.3"/px. After centering using zeroth order I just start guiding adjust for a different order and away I go - that simple?

So now I have my image, I load it into VSpec and....... What else do I need to calibrate the images? How difficult is it to take a sky spectra and remove that etc etc etc. You see Astrometry and Differential Photometry is a breeze for me, and I imagine that spectroscopy will just be something that needs to be learned and experienced along the way but it's never going to get my full attention (at least in the short term).

Cheers

[theodog (Jeff)]

Hi David,
Greetings from Observatory E08.

Spectroscopy is as time consuming and as accurate as your time and budget allows.
Look through Ken's excellent forum @
http://tech.groups.yahoo.com/group/a...guid=322612425
it has many and varied levels of experience and all seem willing to answer questions.
Like most good things -have a dable and your hooked.

[sheeny (Al)]

Originally Posted by theodog

Like most good things -have a dable and your hooked.

Yep...

I don't think spectroscopy is any harder than astrophotography. There may be a little more cerebral exercise involved in identifying lines, etc rather than aesthetic appreciation, but I think if you're into astrometry and photometry then using your head isn't a problem.

Its like everything else though... there is a learning curve, but between IIS and a couple of Yahoo lists (especially Ken's) there's not much you can't do! Its so easy in these days of the internet!

Have you given any thought to a Staranalyser? Less cost than the L200, but it will let you learn the basics of spectroscopy. The only thing that would go against it for asteroid work would be the interference from background stars and limited resolution, so I think the L200 would probably be better for what you particularly want to do.

But like Jeff says... consider yourself warned... it might start out as a second interest, but I wouldn't guarntee it'll stay that way!

Al.

[Terry B]

Is it easy. NO
I just spent the evening playing with my new spectrascope.
Spectra of asteroids would be a big ask.
I have succeeded with may 5 stars so far.
I tried NGC6752 (mag 5.4 GC)
I could see it easily on the slit with a 5 sec exposure. A 300 sec exposure aimed at the middle of the spectrum didn't reveal anything through my 200mm scope.

[sheeny (Al)]

Originally Posted by Terry B

Is it easy. NO
I just spent the evening playing with my new spectrascope.
Spectra of asteroids would be a big ask.
I have succeeded with may 5 stars so far.
I tried NGC6752 (mag 5.4 GC)
I could see it easily on the slit with a 5 sec exposure. A 300 sec exposure aimed at the middle of the spectrum didn't reveal anything through my 200mm scope.

I stand corrected... once you start playing with a slit I suppose you start rejecting a lot of light you would otherwise use. I haven't started that yet, though I'm about to. At least with an SA you're still using most of the light from a star...

Al.

[theodog (Jeff)]

Originally Posted by Terry B

Is it easy. NO
I have succeeded with may 5 stars so far.
I tried NGC6752 (mag 5.4 GC)
I could see it easily on the slit with a 5 sec exposure. A 300 sec exposure aimed at the middle of the spectrum didn't reveal anything through my 200mm scope.

Spectroscopy is possibly a little more difficult, but I only think it’s to do with the exposure length.
For bright point objects (stars & asteroids) most of the light (magnitude) will pass through the slit. However for extended objects, only the portion that passes through the slit can be counted. Given that galaxy magnitudes are usually integrated magnitudes –all of the light as if it’s in one point, the magnitude at any pixel would be much lower than a star at the published value for the galaxy. This much lower light level is then spread over the spectrum. Given only a small portion of the light is at each frequency, then the time needed for an integration and signal to noise ratio is much increased. It is not a simple, straight multiplication. The exposure time will increase out of proportion with an increase in magnitude value and size of the extended object.
Does this make sense I know what I’m trying to say, but failing to say it I fear.
Maybe like taking the spectra of a much out-of-focus star through a slit would demonstrate the increase in exposure times needed.

[CraigS]

Hi Guys;
I'm not in a position to be anything other than interested in Spectroscopy, but I'm very interested to learn more.

So I have a question that perhaps you could help me with .. does the software you're using run any algorithms to improve the signal-noise ? For instance, if the spectrum of a particular element is known, this spectrum can be applied using a mathematical function called 'auto-correlation' to extract that spectrum from an observed, noisy spectrum. Does the post processing software you're using make use of such techniques?

Thanks for your comments.

Cheers & Rgds.

[bojan]

In my attempts earlier, to improve S/N, I was using standard AP techniques: stacking many frames, darks, flats..

[CraigS]

Originally Posted by bojan

In my attempts earlier, to improve S/N, I was using standard AP techniques: stacking many frames, darks, flats..

Hi Bojan;

Hmm .. so that's applying known palettes directly on the optical image ?
(Ie: pre-digital domain processing) ?

Cheers (& thanks for the feedback).

[bojan]

Errr.. not so. It is completely digital (hope I understood you properly..).
The idea is to take the standard astro image of the star through the diffraction grating (or prism), with all standard techniques to improve S/N, using DSS
http://deepskystacker.free.fr/english/index.html

Then, the spectrum post processing was done using Iris:
http://www.astrosurf.com/buil/us/iris/iris.htm

[sheeny (Al)]

Originally Posted by CraigS

Hi Guys;
I'm not in a position to be anything other than interested in Spectroscopy, but I'm very interested to learn more.

So I have a question that perhaps you could help me with .. does the software you're using run any algorithms to improve the signal-noise ? For instance, if the spectrum of a particular element is known, this spectrum can be applied using a mathematical function called 'auto-correlation' to extract that spectrum from an observed, noisy spectrum. Does the post processing software you're using make use of such techniques?

Thanks for your comments.

Cheers & Rgds.

G'Day Craig,

I think there's a couple of things your talking about Craig.

Bojan is right as far as improving signal to noise... it's the same as any other AP. Long exposures and/or stacks of images.

As for identifying spectral lines, there are plenty of aids but not really any automatic lines identifiers.

For example, it doesn't take long to learn to that there is a dominant adborption band about 7600A (1 Angstrom = 1x10^-10m) which is caused by atmospheric oxygen. A lot of stars also have either Hydrogen absorption lines or emission lines, and these are very quickly learned. Once you know two lines, the software lets you use these two lines to calibrate your spectrum. If you don't change the setup of your spectroscope, all your spectra will have the same dispersion (angstroms per pixel) so then you really only need to identify one line to be in business.

Once your spectrum is calibrated, there are tools that allow you to overlay the lines for most of the elements.

Al.

[bojan]

Craig, OK, now after Al explained it to you, I've got your question .. I don't know what I was thinking
Next time I will be more careful and slower in replies

[CraigS]

Hi Al & Bojan;

Thanks kindly for both of your answers .. after reading thru Al's response and those links Bojan forwarded, I now realise how much I don't know about processing the spectra.

Interesting to see where they deduce from a slight dip in the peak of the Ha line of a Be star that a cloud of gas rotating around it probably caused the dip. Thats amazing to think that a setup like this can detect such things. Great stuff ! I'd love to know what other bits you guys find out using these techniques !

Looks like I've got lots of reading up to before I can even understand my own questions, too !!



I'll have to get a copy of Merlin66's new book !! Can't wait for that, though !

Cheers & rgds ... & thanks again.

[Terry B]

Originally Posted by sheeny

I stand corrected... once you start playing with a slit I suppose you start rejecting a lot of light you would otherwise use. I haven't started that yet, though I'm about to. At least with an SA you're still using most of the light from a star...

Al.

The big difference between the SA and the new spectroscope is the dispersion. With my setup the spectrum produced by the SA will fit actoss 1 CCD frame at ~31A/pixel. With the spectra L200 the same spectrum covers about 5 or 6 frames at ~1.6A/pixel.
This makes the entire thing much dimmer needing longer exposure time.

[Merlin66 (Ken)]

David,
In your original post you mention guiding after centering the Zero Order star image. Yes, this will work on spectroscopes like the Spectra-L200 but not all slit spectroscopes allow access to the Zero order star image.

All the commercial instruments use fixed width reflective slits, so the image of the focused star on the front of the slit can, with additional optics, be used to position and guide on the target.

An adjustable slit has the advantage of allowing the spectroscope to be used in "Slitless" mode; the slit being much wider than the target star/ asteroid. The resolution will then depend on the linear size of the seeing disk. A 20 to 50 micron slit if set to 80% of the seeing disk still allows >80% of the light to contribute to the spectrum but at significantly improved spectral resolution.
Using a guide scope and guide CCD works well with the Spectra-L200.
Ken

[higginsdj]

The L-200 sounds like a great deal but I also figure that using it will eat well into my observing time. If I had a second scope it would be a different story.

Cheers

[Merlin66 (Ken)]

What other observing time
When you get into spectroscopy there IS nothing else!!

[sheeny (Al)]

Originally Posted by Merlin66

What other observing time
When you get into spectroscopy there IS nothing else!!

Oh come on Ken! There's solar! It fills it the part of the day when you're not doing spectroscopy...

Al.

[Merlin66 (Ken)]

True
But the weather over here has been so bad I'd forgotten.

[theodog (Jeff)]

Solar?
Isn't that when we sleep?