Star Analyser - solar observations?

[Brutha]

Hi All,

Somewhat inspired by the online astrophysics course I've been doing, I'm now tempted to have a go at a bit of spectroscopy.

The "Star Analyser" looks like a good place to start, but I was also looking at the "Solex" 3d printed spectrograph. The Solex would be quite a lot more expensive I think, but if I'm right it can with a bit of additional cost be used both to produce images of the Sun and star spectrographs.

So, question: with Solex, the approach is that you take a single slice of the sun and an image of all the spectra of that slice. Then, you let the slice drift across the sun, creating images as you go. Finally you can take the data and create images using a whole bunch of different wavelengths of light.

Has anyone tried the same thing with the Star Analyser? I guess you need a very thin slit and some kind of neutral density filter to do it?

And re: Solex, when used as a star spectroscope - is it a lot higher resolution than the star analyser?

Thanks!

Brutha,

[vlaiv]

33 minutes ago, Brutha said:

Has anyone tried the same thing with the Star Analyser? I guess you need a very thin slit and some kind of neutral density filter to do it?

Star analyzer is diffraction grating same as one used in Solex - differences being that Star analyzer is transmission grating and one used in Solex is reflection grating. Other difference is dispersion power of these elements.

In theory - SA (depending on model) will have from x12 to x24 less dispersion than grating used in Solex (one that is specified in design).

SA has either 100 or 200 lines per mm, while one in Solex has 2400 lpmm.

36 minutes ago, Brutha said:

And re: Solex, when used as a star spectroscope - is it a lot higher resolution than the star analyser?

Well, this part is tricky to answer, because it is not all down to dispersion of grating.

There are several elements that go into "equation":

1. Size of slit and seeing blur (whichever is smaller)

2. Size of collimated beam that is hitting the grating

3. Quality of collimation optics (this is rarely an issue - but it can be if very low quality items are used)

4. Pixel size of sensor and how it is matched to dispersion of light

For good spectroscope - all these things must be properly matched to achieve optimum resolution that is possible with said elements.

On the other hand, if you want to image Sun using this technique - I don't think you'll get very satisfactory results in say Ha with low dispersion element like SA.

If I'm not mistaken, good solar scopes have filter with band of about 0.5A - that translates into 6568 / 0.5 = R13000 to get very contrasty image of the Sun.

This means that we need to illuminate at least 13000 slits on grating - that means 65mm wide collimated beam (13000/200) - and that is impossible with 28mm filter (max beam width that I'd put on that is about 20mm so R4000 is theoretical max for SA200 - which is more than 1A - not good enough for solar Ha filter).

[Brutha]

Thanks for the detail response! So, I think I’ll start with the star analyser since it’s cheap enough to buy it as something to play with, then see how things go!

[vlaiv]

3 hours ago, Brutha said:

Thanks for the detail response! So, I think I’ll start with the star analyser since it’s cheap enough to buy it as something to play with, then see how things go!

That is a good idea.

If you have 3d printer - then you can make SA into higher resolution spectrograph by adding a slit. That can be intermediate step before going onto the high res items like Solex.

You'd need 2 pairs of lenses for collimation and optical slit and 3d printed housing to make it all work. I also have SA200 and 3d printer and plan on doing something like that myself at some point.

[robin_astro]

The Star Analyser on its own is to produce spectra of objects that appear as points which rules out objects like the sun though you can take low resolution spectra of the sun using the star analyser by catching the glint of sunlight off a needle, for example here

http://www.threehillsobservatory.co.uk/astro/spectroscopy_14.htm

You can make simple slit spectrographs based around the Star Analyser for example my SEPSA design

http://www.threehillsobservatory.co.uk/astro/spectroscopy_18.htm

and a slit could be incorporated into my fully collimated "junk box" spectrograph

http://www.threehillsobservatory.co.uk/astro/spectroscopy_19.htm

These are low resolution designs which can be useful for faint objects, though the length of the spectrograph becomes rather unwieldy if you try to increase the resolution using a longer focal length lenses so most designs use higher dispersion gratings. like the ALPY for example which uses a 600l/mm grating

With any slit spectrographs for stellar spectroscopy you and have to consider how you are going to find the star and keep it in the slit. This is normally done using a mirror slit guider built into the spectrograph. All commercial spectrographs for the amateur include one as does the 3D printed Star'Ex and LowSpec designs, Here is an example of a complete system designed around a grating similar to the SA200

http://www.burwitz-astro.de/spectrographs/tragos/index.html

The Solex spectroheliograph, used to image the sun in specific wavelengths  is a very different instrument though which needs much higher resolution than an instrument based around the Star Analyser could achieve.

If you are interested in the kind of things an image taken through the  Star Analyser on a telescope can show about stars here is an example on my BAA page

https://britastro.org/observations/observation.php?id=20201216_234948_8cabda965bfe692f

Cheers

Robin

Edited April 2, 2023 by robin_astro
typo, added missing link