Spectral Type

[PhotoGav]

I am investigating the whole area of spectroscopy on a budget... how ‘easy’ is it to establish a star’s spectral type by using a Star Analyser? Would I get any usable results or is this a step too far for the humble amateur astronomer?

[johnfosteruk]

12 minutes ago, PhotoGav said:

I am investigating the whole area of spectroscopy on a budget... how ‘easy’ is it to establish a star’s spectral type by using a Star Analyser? Would I get any usable results or is this a step too far for the humble amateur astronomer?

I've been looking into this as well Gav, as long as you can produce a reasonable curve, you can determine type and temperature. 

I've been looking at the Star Analyser SA-100.

[PhotoGav]

Thanks John, I’m going to take that as a ‘yes’! I look forward to comparing notes as we progress in this branch of astronomy.

[johnfosteruk]

Just now, PhotoGav said:

Thanks John, I’m going to take that as a ‘yes’! I look forward to comparing notes as we progress in this branch of astronomy.

You'll probably progress further sooner than I Gav!! But yes at some point I'll have notes to compare.

[vlaiv]

It should be fairly easy and even automated process ...

Once you extract the spectrum of star - you can compare it to reference spectrum for each class - one that gives best match is your class.

I think a lot of fun things can be done with just stellar class identification. One of my goals with this, once I embark on this journey as well (SA200 is ready and waiting for suitable time / weather) is determining distance to a star.

Maybe even double star or small cluster. With more samples you get better "precision", but process is fairly easy - get spectral class, get expected absolute magnitude from that, and do a bit of photometry to get relative magnitude and from that - distance.

[PhotoGav]

23 minutes ago, johnfosteruk said:

You'll probably progress further sooner than I Gav!! But yes at some point I'll have notes to compare.

I wouldn’t gamble on that!

13 minutes ago, vlaiv said:

It should be fairly easy and even automated process ...

Once you extract the spectrum of star - you can compare it to reference spectrum for each class - one that gives best match is your class.

I think a lot of fun things can be done with just stellar class identification. One of my goals with this, once I embark on this journey as well (SA200 is ready and waiting for suitable time / weather) is determining distance to a star.

Maybe even double star or small cluster. With more samples you get better "precision", but process is fairly easy - get spectral class, get expected absolute magnitude from that, and do a bit of photometry to get relative magnitude and from that - distance.

Thanks Vlaiv. Fingers crossed. Distance calculation with spectroscopy, photometry and the distance-modulus equation is exactly what I am trying to do. 

[PhotoGav]

16 minutes ago, vlaiv said:

SA200 is ready and waiting for suitable time / weather

Mmm, interesting. Why have you chosen the SA200 rather than SA100? What kit are you using? I am trying to establish which grating I should purchase. I have two telescope options: a Thomas Cooke 10” refractor or a SkyWatcher 200pds. I will use either a DSLR or a ZWO ASI 120-MC. My investigation suggests that the SA100 with the 200pds & asi 120 is probably the sensible option. However that needs to be set up outside each time for use. Meanwhile the 10” refractor is permanently mounted in an observatory - I would much rather use that. The SA200 would be the better choice for that, I think. 

[vlaiv]

At the time I was purchasing - I knew almost nothing about all of it, and since that option was in stock, and it seemed better because higher dispersion - I purchased that one

As for suitability - there is nice spreadsheet floating around (you can find it in some threads here on SGL as well) that does decent job of calculating expected resolution depending on focal length of the scope, field curvature, angle of the beam (unfortunately, using grating in converging beam produces aberrations), expected seeing. Maybe I have it somewhere downloaded and I can attach that for you, let me see.

Yes, of course, have it in my "docs" section, so here it is:

TransSpecV3.1.xls

You can play around with it and see what setup would give you best results. Just pay attention that you will be using color sensor, so sampling rate will be affected, and also, you can stop down your scopes to make slower beam - that can help.

SA200 might be better for slower scopes as resolution depends on how many lines beam intersects (area of the beam at the grating). Larger sensor with large pixels helps with SA200, but you can always bin your data. SNR is not something to be overly concerned about if you are not time limited (like for transient events or real time display of spectrum), guiding will help of course.

I think that SA200 is also better in "advanced" configurations - like making collimated beam with eyepiece - grating - lens - camera configuration. There was a thread recently showing this configuration. I plan to make something similar one day - with addition of "half slit" sort of thing - half of FOV clear and half blocked with slit. Upper part will be used for plate solving / centering, and part with slit will provide added resolution and proper background subtraction.

Btw, here is that thread:

In first go, with just grating and nothing fancy, I plan to use it on RC8" with ASI1600. I did calculations with above spreadsheet and I should be able to get something like R300 in favorable seeing conditions. Of course, I'll be happy with any sort of decent spectrum, but going a bit higher resolution with SA is one of my goals.

[PhotoGav]

Thanks Vlaiv, I will try plugging in some numbers to that spread sheet. 

[Merlin66]

Gav, 

You can certainly use the gratings either the SA100 or SA200 on the 10" - it depends on the available spacing between the grating and the camera chip.

Keep things simple to start with....get the practise in acquiring the spectral image and the gain experience in the spectra processing. "Spectroscopy 101"

I must say that mono cameras give better results than OSC cameras.....

Richard Walker's "Spectral Atlas" https://www.cambridge.org/core/books/spectral-atlas-for-amateur-astronomers/32126CD730EBA49238F9318C16B90523#

Is highly recommended, is gives fully annotated spectra of all the star classifications, as well as other very useful information and data.

François Teyssier 's "Low resolution Spectroscopy" is also well worth studying.

http://www.astronomicalspectroscopy.com/files/SpectroscopieBasseResolution_En.pdf

 

 

[PhotoGav]

Thank you @Merlin66, yes keeping it simple and doing things step by step is key. Thanks for those links. 

More questions to follow!!

[Merlin66]

Gav,

Our webpage ( see signature) has many good links for both techniques and info which gives good info for all amateurs.

Check it out.

 

[robin_astro]

20 hours ago, PhotoGav said:

I am investigating the whole area of spectroscopy on a budget... how ‘easy’ is it to establish a star’s spectral type by using a Star Analyser? Would I get any usable results or is this a step too far for the humble amateur astronomer?

OK time for reality check here I am afraid.  The key point to understand is that spectral classification uses the presence or absence of particular lines, not the shape of the spectrum continuum. The reason is that using the shape of the spectrum continuum can be totally misleading due to interstellar extinction.  (If this could be used it would be easier just to use the colour (UBVRI) photmetry.   I will post an extreme example of this for a star I am currently studying, later today when I have reduced it.

Detailed spectral classification is not the Star Analyser's strength I am afraid because the resolution is not high enough (You really need a resolution of 5-10A or better to resolve the lines sufficiently)   You can get a rough idea (ie hot stars show H and He lines, cool stars show molecular bands but the spectra in between tend to be a mush of overlapping lines which are difficult to distinguish.)   

For me the Star Analyser strength is looking at the more unusual objects with bold features in their spectra. These are much more interesting astrophysically in any case. (eg exploding stars, stars with hot discs around them or with intense stellar winds, planetary nebulae, methane in planet atmospheres, distant quasars and active galaxies

Cheers

Robin

 

[PhotoGav]

Robin.

Thank you for your in depth answer. That is what I feared and expected. How useless will it be?! Would certain brighter stars be easier to classify or is it completely beyond the reach of the Star Analyser? Should I abandon all hope of spectral classification with this level of kit or is their some merit in targeted attempts?

Thanks,

Gav.

[robin_astro]

Hi Gav,

It is not so much the brightness but the ability to resolve the key lines that determine the spectral type. This can be done approximately for types where the features are clear eg A and M but not in general for intermediate types F-K . The resolution of the Star Analyser is also insufficient to determine the luminosity class.  The difficulties typically encountered can be seen in the post "A tale of two stars" where I discuss an attempt to classify two newly discovered variable stars using spectra at Star Analyser resolution. 

BTW if you are interested in a particular star's classification Brian Skiffs huge database of published classifications is a good resource

http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk

A quick browse of that will indicate in practise what an inexact science spectral classification is !

Cheers

Robin

[PhotoGav]

Thank you again Robin. I’ll hunt down your post. 

What level of kit beyond the Star Analyser would be needed for a resolution suitable to get somewhere with this?

[PhotoGav]

Robin,

Found your post (not hard!) and it is very interesting. While it highlights the inaccuracies of the low resolution system, it actually fills me with hope that this is something worth pursuing. I think it will be possible to do something useful, even if not totally accurate. My aim is to introduce spectroscopy to Astronomy GCSE and Physics A level students at the school I work at, gathering data to use the distance-modulus equation for real. Inaccuracy is a useful thing to discuss and makes it ever clearer that accurate astronomy is difficult!!

Thank you for your help so far. You aren’t coming to SGL SP2019 by any chance are you??!

[vlaiv]

Actually, I think that SA can be used to do that level of spectroscopy, although it needs to be "modified" quite a bit. Actually, grating is not modified, but resolution can be improved by following:

1. use of slit

2. putting grating in collimated beam.

SA200 for example, with 200 l/mm and clear aperture more than 20mm can in theory produce resolution of R4000 (and that is about 1.5A in visible). Problem is of course that we tend to use it in converging beam and limiting factor is star FWHM as well. Addition of collimation optics (eyepiece and lens for example as in topic I linked in above post) provides collimated beam. Size of the beam (or rather its diameter) will depend on speed of optics and focal length of eyepiece and is equivalent of exit pupil in observational use - which means we can easily have something like ~6mm (maybe a bit less with slow scopes), and 200 L/mm will give us R1200 theoretical maximum from grating, which is enough for classification at about 5A (if I'm doing my math correctly).

20um slit placed at field stop of eyepiece with reduction factor of about 1/3 - 1/4 will be "pixel wide" (with perfect lens and that lens being about 1/3 to 1/4 in focal length of eyepiece used), so we end up with sampling being limiting factor. 20um slit is both hard to make (as far as I'm aware - no diy there so needs to be purchased) and requires certain focal length to make star large enough so that it is about FWHM wide (you want most of the light from the star to be in slit, but keep slit narrow enough).

@robin_astro - does my rambling above make any sense?

 

[robin_astro]

7 minutes ago, PhotoGav said:

Thank you again Robin. I’ll hunt down your post. 

What level of kit beyond the Star Analyser would be needed for a resolution suitable to get somewhere with this?

Something like a LISA spectrograph with a resolving power of 1000.  To be honest though other than identifying cataclysmic variables (novae, supernovae etc) from their spectra precise spectral classification is a specialist area which needs quite a bit of skill and knowledge and is not an area amateur spectroscopists generally get into really. If you are looking for an up to date book on the subject a good reference is

Gray and Corbally "Stellar Spectral Classification"

 Stars who's spectra vary are more interesting.  For example there have been some recent posts on the AAVSO forum tracking the dwarf nova SS Cyg through its cycle every 2 months where the changes both in temperature and emission lines are clearly seen at Star Analyser resolution

Cheers

Robin

[PhotoGav]

Vlad, I believe you! (No idea...!!) I look forward to following your progress. If you get it to work, I’d be interested in buying one off you... if it’s cheap!

[PhotoGav]

1 minute ago, robin_astro said:

Something like a LISA spectrograph with a resolving power of 1000.  To be honest though other than identifying cataclysmic variables (novae, supernovae etc) from their spectra precise spectral classification is a specialist area which needs quite a bit of skill and knowledge and is not an area amateur spectroscopists generally get into really. If you are looking for an up to date book on the subject a good reference is

Gray and Corbally "Stellar Spectral Classification"

 Stars who's spectra vary are more interesting.  For example there have been some recent posts on the AAVSO forum tracking the dwarf nova SS Cyg through its cycle every 2 months where the changes both in temperature and emission lines are clearly seen at Star Analyser resolution

Cheers

Robin

Ok, thank you. I think I will rein in my expectations a little and look at the diversity of possible projects. Really what I need to do is just purchase an SA100 and get started!

[robin_astro]

If you are into self build then there are a couple of 3D printed designs (lowspec and Uvex) which potentially have enough resolution get you into this area

UVEX in particular would make a very nice spectral classification instrument because of its good UV end capability

http://www.astrosurf.com/buil/UVEX_project_us/

Robin

[Thalestris24]

9 minutes ago, robin_astro said:

Something like a LISA spectrograph

They are over £3k! 

Louise

[robin_astro]

But if all you need is a general impression of how the spectrum changes with temperature then the Star Analyser can do this as Torsten Hansen showed here 

[vlaiv]

5 minutes ago, robin_astro said:

If you are into self build then there are a couple of 3D printed designs (lowspec and Uvex) which potentially have enough resolution get you into this area

UVEX in particular would make a very nice spectral classification instrument because of its good UV end capability

http://www.astrosurf.com/buil/UVEX_project_us/

Robin

I was looking at lowspec and it indeed seems interesting.

In all of these designs, reflective grating can be replaced with transmission grating if layout is slightly altered - so SA can be reused in higher resolution instrument (it does require some way of easily mounting it and removing from spectroscope - like filter drawer or similar).