Spectrometer

[JRWASTRO]

Greetings Readers,

I have no experience in the field of stellar spectrometry however I am undertaking some research that necessitates the measurement of the spectra of various light sources, stars etc.

Please tell me about commonly used spectrometers  in the field of amateur astronomy their availability and cost. My budget is modest so professional level instruments are out of the question.

Many thanks in advance,

Jeremy.

[andrew s]

Have a look here https://www.shelyak.com/produits/?lang=en. Some of them  are available in the UK via @FLO .

Regards Andrew 

[JRWASTRO]

Thank you Andrew.

That's a start for me.

What I would like to  do is obtain the spectrum of the light of a star and/or the light of diffuse source (scattered light) and record this spectrum (amplitude vs. wavelength) for analysis etc.

Jeremy.

[andrew s]

49 minutes ago, JRWASTRO said:

Thank you Andrew.

That's a start for me.

What I would like to  do is obtain the spectrum of the light of a star and/or the light of diffuse source (scattered light) and record this spectrum (amplitude vs. wavelength) for analysis etc.

Jeremy.

What resolution are you looking for?  If you want to do diffuse object you will need a slit or fiber spectrograph.  How bright are the objects, size if telescope? 

Regards Andrew 

[JRWASTRO]

Here are 3 pictures, of our southern skies at Sandy Creek, taken the other night.  I need to look at the spectra between  the horizon and at zenith.

What I am looking for is a spectrum analyser with about 10 bands in the visual  each band about 35nm( (750-400)/10) wide . I imagine this resolution will will help to determine a characteristic signature. I will be taking photometric measurements in the John-Cousins R, V, and B bands.

Jeremy

Edited March 13, 2021 by JRWASTRO
Provide more information.

[andrew s]

The resolution of 35nm at 750 nm is about 20 so very low. I am not clear what you want to measure the spectra of. Is it sky brightness, how a stars spectra changes with altitude etc. Can you explain what you will be measuring. If its star then a Star Analyser in slitless mode would do R ~100 but if is the sky the you will need a slit spectrograph.

These links might help  ROBIN'S ASTRONOMY PAGE SPECTROSCOPY (threehillsobservatory.co.uk) &  Photometric slit (astrosurf.com) 

Regards Andrew

 

[JRWASTRO]

Re: I am not clear what you want to measure the spectra of ...

I am interested in measuring the spectrum of the background (only) at various altitudes.  What I would like to measure is sky background brightness (power  per unit solid angle per unit area or watts per steradian per square metre projected area) ) I would like to get an estimate for each band, To that end I need to take  measurements at various azimuths and altitudes.

Re: how a stars spectra changes with altitude ...

I do not think that this will happen (?) but I would take spectra of the stars above 30 degrees elevation. The star spectra on average  will be different than the background and will be used as a discriminator to select the background only.

This is one of the resources I use:  https://voservices.net/filter/

Thank you for the link - Just had a cursory look and it looks excellent. Need to spend much more time looking at the content and that I will do !

In a previous post you asked about my telescopes.

For star measurements I use a Takahashi Mewlon 250CR and for wider FoV measurements a (well corrected) Sigma Lens (8 to 14mm).

Thank you, Andrew, for taking the time to reply,

Jeremy.

 

 

 

[robin_astro]

On 13/03/2021 at 23:16, JRWASTRO said:

 

Re: how a stars spectra changes with altitude ...

I do not think that this will happen (?) but I would take spectra of the stars above 30 degrees elevation.

 

The spectrum of a star varies significantly in both intensity and the shape of the spectrum with altitude depending on the air mass, due to atmospheric extinction. 

A simple slit spectrograph will give you the data you need (The sky background comes for free with any astronomical spectrum and has to be subtracted to give the spectrum of the target) but you will need some way of absolute calibrating the result.  For astronomical objects this is done by measuring a reference star and calibrating against a published spectrum defined as measured at the top of the atmosphere.  To measure the sky brightness at the Earth's surface though you would not want to include the atmospheric extinction contribution in your measurement so you would need to separate out the instrument component and just correct for that.  Christian Buil's webpage here holds some clues to how this could be done

http://www.astrosurf.com/buil/isis/guide_response/method.htm

Cheers

Robin

[robin_astro]

If you are only interested specifically in the sky brightness in each photometric band though why not measure it directly from images taken through each filter using stars as flux references? (Though you would still need to correct the stellar fluxes for the effects of atmospheric extinction)

Cheers

Robin

[JRWASTRO]

Greetings Robin,

Thank you for your response.

Re: If you are only interested specifically in the sky brightness in each photometric band though why not measure it directly from images taken through each filter using stars as flux references? 

Yes exactly that is what I will be doing measurement in the Johnson R,V,and B bands in a hemispherical dome. 

Re: (Though you would still need to correct the stellar fluxes for the effects of atmospheric extinction)

Yes. This is where I will have to find a suitable model.

Re: The spectrum of a star varies significantly in both intensity and the shape of the spectrum with altitude depending on the air mass, due to atmospheric extinction. 

I did not even think about it until now.  But I will heed to incorporate a model as I will be taking data in a dome (0 to 360 degrees (azi) and 0  to 90 degrees. elevation) . I have a feeling that this might not be straightforward as one will need environmental data. 

Initially I am taking data and I am improving my methodology to taking a single "hemisphere worth" of data. When I get this down to a fine art I will take the data in 3 bands (R, V and B). In the mean time I am exploring spectroscopy - to this end I will be getting the Star Analyser 200 - excellent stuff!!

Thank you for your time,

Kind Regards,

Jeremy.

 

[robin_astro]

2 hours ago, JRWASTRO said:

I did not even think about it until now.  But I will heed to incorporate a model as I will be taking data in a dome (0 to 360 degrees (azi) and 0  to 90 degrees. elevation) . I have a feeling that this might not be straightforward as one will need environmental data. 

Hi Jeremy

You can use a model of the atmospheric extinction and depending on the accuracy you are looking it you might find a single set of coefficients (or seasonal ones) good enough

http://astrosurf.com/aras/extinction/calcul.htm

or measure the extinction directly using  stellar measurements at different elevations (either the same star as it rises or different stars with known spectra) You do need stable conditions for the period of the measurements though.. You end up with something like this

http://www.astrosurf.com/buil/atmosphere/transmission.htm

Cheers

Robin

Edited March 16, 2021 by robin_astro

[JRWASTRO]

Greetings Robin,

Regarding the first link : One will always find someone who has done all the hard work !

Now I'm getting somewhere. The first link is exactly what is needed. Those equations are easy to code in Octave. Why, even with my schoolboy French I can read the paper (in parts).

As an aside, L'absorption par la molécule d'ozone reminds me of my youth where I was given the task of measuring the amount of  pricipitable water vapor in the atmosphere, in a given range, for our thermal (8 to 14 micro metres) imagers. But that was many years ago.

Regarding the second link: I will have to struggle through this one but the conclusion is encouraging ie.

quote

On a démontré ici la possibilité de mesurer la transmission atmosphérique absolue avec un équipement simple et un peu de méthode

unquote

Thank you indeed,

Jeremy.