SA200

[Thalestris24]

Hi all

Was just doing some online calculations with a view to using a SA200 with my 80mm F6 APO  Putting it just in front of the t-ring on the FS1100d, gives a sensor distance of ~62.5mm. The calculator returns 4.2A/pixel and a spectrum width of 20268A (2026nm). The only complaint from the calculator was that it might be a bit dim? Can I compensate with longer exposures? Other than that, do the numbers sound ok?

Thanks

Louise

[vlaiv]

I think it's a bit like with deep sky imaging - higher resolution requires more precise tracking and steadier skies (in absence of a slit in system). So yes, I think that the brightness can be compensated with longer exposure, but not sure if you will benefit from this resolution at all. You will probably need very good / excellent seeing, and your tracking error should be really small - otherwise long exposure will smear spectrum - you will get same resolution as with shorter spacing but will need much longer time to get the same result.

[Thalestris24]

Ok, thanks, Vlaiv. I must have entered the sensor width wrong before. The correct value (4272) now gives a lower coverage of 17,772 and 4.2A/pixel but still complains spectrum brightness is on the low side. The seeing value only seems to affect the star FWHM. I put in a value of 5 which gives a fwhm of 2.2 pixels. I'd be guiding with PHD2, as usual

Louise

[vlaiv]

You can't go wrong by trying it out. So yes, to address original question, you will need longer exposure to get decent brightness / SNR for spectrum.

You gave me food for thought with this. I also have SA200 and still have not got the time to do a bit more serious work then just try it out. But hopefully that is about to change, weather is getting better, it's not that cold any more, and fiddling with spectroscopy is certainly plausible on nights when Moon is preventing serious work in other fields (like imaging / observing).

Back on a subject of resolution, from a brief read on topic (some info is even present on the page of distance calculator), one should aim for spread that allows max resolution of the system (R100-200) not to be blurred by star image / psf. This really means that I was wrong in the first post - one wants more spread to overcome seeing / guiding issues to a certain point after which there is no purpose to spread more - it will just increase exposure time (hence of course green OK-s in calculator )

[Merlin66]

Louise,

What the calculator is telling you is that 4A/pixel is starting to push the limits.....

The spread of the spectrum (dispersion) increases with grating spacing and obviously the spectrum appears fainter, requiring long exposures to get good SNR.

I use a 200 l/mm grating with a CCD and around 30mm spacing.....

The seeing condition and aberrations will limit you to a resolution of around 40A, so such a high dispersion doesn't transfer to higher resolution.

I use the TransSpec spreadsheet for analysis.....

 

 

TransSpecV3.1.xls

[Thalestris24]

18 minutes ago, Merlin66 said:

Louise,

What the calculator is telling you is that 4A/pixel is starting to push the limits.....

The spread of the spectrum (dispersion) increases with grating spacing and obviously the spectrum appears fainter, requiring long exposures to get good SNR.

I use a 200 l/mm grating with a CCD and around 30mm spacing.....

The seeing condition and aberrations will limit you to a resolution of around 40A, so such a high dispersion doesn't transfer to higher resolution.

I

Ok - I have to multiply the calculator dispersion value by 10 then! Is 40-ish an ok value? If external limitations hold me to that sort of figure, I don't see any way to improve things other than ditching the dslr? I have a mono minicam5s which might be better in some ways (small chip, high qe) but then I'd lose the spectral colours.

Thanks

Louise

[Merlin66]

The 40A resolution comes from the seeing conditions and the aberrations....you can "play" with the spreadsheet inputs and look for any improvements (if any). It's not just a multiplier of the dispersion....

The 200 l/mm certainly works better (brighter spectra, shorter exposures) when the spacing is around 40mm or so. (The results from the 100 l/mm would be identical at twice the distance, so your 60mm available spacing would be more suited to a 100 l/mm grating.)

You don't want to worry about recording a colourful spectral image! After processing - calibrating to wavelength - you can always display the result in full technicolor!!!

In fact recording the spectrum in mono is much better - no Bayer Matrix effects to worry about.

 

[Thalestris24]

17 minutes ago, Merlin66 said:

The 40A resolution comes from the seeing conditions and the aberrations....you can "play" with the spreadsheet inputs and look for any improvements (if any). It's not just a multiplier of the dispersion....

The 200 l/mm certainly works better (brighter spectra, shorter exposures) when the spacing is around 40mm or so. (The results from the 100 l/mm would be identical at twice the distance, so your 60mm available spacing would be more suited to a 100 l/mm grating.)

You don't want to worry about recording a colourful spectral image! After processing - calibrating to wavelength - you can always display the result in full technicolor!!!

In fact recording the spectrum in mono is much better - no Bayer Matrix effects to worry about.

 

Ah ok. Maybe it'll be better with the mono then. In that case the sa100 might be better also. I'll check out the sa100 mounting options on the minicam and the sensor distance tomorrow

Thanks again

Louise

[robin_astro]

Hi Louise,

Using the Star Analyser at too large a distance is a bit like using too high a magnification eyepiece. You get to a point where you dont see any more detail, things just get fainter. The calculator is just warning you that you have reached that limit.   

Colour cameras are not ideal for spectroscopy because in some regions of the spectrum (red and blue) only 1 in 4 of the pixels are sensitive due to the Bayer pattern. Also the red green and blue  filter responses dont match perfectly so you get humps and bumps in the overlapping regions in the spectrum which should not be there. 

As a beginner you will find the mono minicam5 easier to get to grips with using the Star Analyser 100 (It was designed exactly for this kind of camera) Focusing is critical to getting good resolution but is tricky  to learn and a fast updating webcam type camera is ideal for this. Being monochrome is an extra bonus too.  (Software like RSpec can even give you a live view of the spectrum profile)  

With cameras like the minicam5 it is usual to just screw the grating onto the end of the 1.25 inch nosepiece (with spacers if needed to get the distance optimum) and put it directly into an eyepiece holder.  The same is usually done with DSLR, using a T2 converter and a 1.25 inch nosepiece

I know it sounds a bit counter intuitive but when used behind a telescope, the SA100 used at twice the distance works better than the SA200. This is because the angle the beam is deflected is less which gives less aberration and less focus shift along the spectrum.  This is why I chose a 100l/mm grating for the original Star Analyser 100. The SA200 was developed later for situations where the larger distance cannot be achieved.

(Note in the case where the grating is positioned in front of a camera lens, the aberrations are less so the spectrum can be spread out more to gain resolution. Here SA200 can have some advantages over the SA100. See here for list of pluses and minuses in that case

http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm  )

Cheers

Robin

Edited March 30, 2017 by robin_astro

[robin_astro]

I see the minicam5s is cooled and can do long exposures too. That makes it ideal for spectroscopy with the Star Analyser and will allow you to go fainter.  

Cheers

Robin

[robin_astro]

Hi Louise

Small aperture APO's work really well with the Star Analyser. See Jim Ferreira's work for example

http://www.lafterhall.com/spectroscopy.html

I ran your setup using an SA100 and minicam5 through the calculator. With 5arcsec seeing, distances 20-46mm give all ok.  For best resolution aim for the top end. For the faintest objects at lower resolution you might want to reduce the spacing. Larger distances (up to 60mm when it gets difficult to fit the spectrum in the frame) will still work ok though

Cheers

Robin

[Thalestris24]

Thanks Robin - I need to do some more reading

Louise

[Thalestris24]

1 hour ago, robin_astro said:

Hi Louise

Small aperture APO's work really well with the Star Analyser. See Jim Ferreira's work for example

http://www.lafterhall.com/spectroscopy.html

I ran your setup using an SA100 and minicam5 through the calculator. With 5arcsec seeing, distances 20-46mm give all ok.  For best resolution aim for the top end. For the faintest objects at lower resolution you might want to reduce the spacing. Larger distances (up to 60mm when it gets difficult to fit the spectrum in the frame) will still work ok though

Cheers

Robin

The distance from front of nosepiece to the sensor looks to be about 42mm. Not sure if I need to add a few mm to allow for where the grating would be. However, 42mm gives a spectral range of 11429A but that's probably ok? Range increases if I reduce the sensor distance. I don't have exact dimensions to hand and have just measure using a digital caliper

Louise

[robin_astro]

Perfect !  That will be a very nice setup. Good resolution and plenty of room to fit the spectrum in the frame. Can't wait to see first light

Good Luck

Robin

[Thalestris24]

9 minutes ago, robin_astro said:

Perfect !  That will be a very nice setup. Good resolution and plenty of room to fit the spectrum in the frame. Can't wait to see first light

Good Luck

Robin

Just ordered a SA100

[Thalestris24]

21 hours ago, Thalestris24 said:

Just ordered a SA100

Received today  - but who knows when I might get a chance to try it... Continuing permacloud here  

[Pig]

Louise, I have bought the SA100 in the not so distant past but the weather has not played ball for a number of weeks now thus it hasn't had much use recently. Whilst I was fact finding I am sure I read on the RSpec pages the SA100 delivers better results when directly mounted onto a DSLR rather than though a scope.

Edited April 4, 2017 by Pig

[Thalestris24]

2 hours ago, Pig said:

Louise, I have bought the SA100 in the not so distant past but the weather has not played ball for a number of weeks now thus it hasn't had much use recently. Whilst I was fact finding I am sure I read on the RSpec pages the SA100 delivers better results when directly mounted onto a DSLR rather than though a scope.

Hi

See my other thread on the SA100! I think I was getting quite good spectra with the 80mm APO and Minicam5s

Louise

 

[Pig]

11 minutes ago, Thalestris24 said:

Hi

See my other thread on the SA100! I think I was getting quite good spectra with the 80mm APO and Minicam5s

Louise

 

I will go & have a read.... I think its a really wonderful subject and have enjoyed using the SA100 with my scope so far, its just a pity the cloud cover has put the brakes on proceedings for around 4 weeks  

[robin_astro]

Yes you can get a bit sharper spectra with the grating mounted on a camera lens but it is much less sensitive compared with mounting it behind a telescope as you effectively only have a ~25mm diameter aperture (Even an 80mm aperture telescope will collect ~10x more light), so it only works for brighter targets. 

Cheers

Robin