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Old 10-01-2020, 07:49 AM
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Sa100

While waiting for BW CCD sensor for my Logitech 4000 camera (I will also attempt the long exposure mod then), I was playing with SA100 grism, mounted on C11.

I took spectra of Betelgeuse and Sirius on Wednesday evening... it was quick&dirty job, exposure (couple of seconds) was done with moded Canon450D @ ISO100. To avoid saturation, I drifted the scope along RA, thus obtaining wide spectra like there was a slit in the light path...
Of course, this was possible only because both stars are sufficiently bright.. With fainter objects, I will have to use higher ISO settings and longer exposures with accurate tracking.

It was relatively easy to align the grism in my mechanical solution - grism is rotatable with respect to camera (~+/- 10degrees), and of cource the whole thing can be rotated so the flrame could be aligned perpendicluar/paralel to celestial coordinate grid.


EDIT:
In the current setup, grism distance from sensor is ~100mm (limitated by prism holder diameter and inner diameter of the C11 baffle tube).

There is a bit of overlapping of second order spectrum at IR side (right side, not easily visible on attached images), I will sort it out with 680nm IR filter (I have one left over from Canon 400mm FP lens setup, now I will use it here).
Attached Thumbnails
Click for full-size image (Betelgeuse.jpg)
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Click for full-size image (Sirius.jpg)
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Last edited by bojan; 10-01-2020 at 08:25 AM.
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Old 10-01-2020, 09:31 AM
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Bojan,
Very nice result!
Terminology: the SA100 is just a transmission grating; a Grism is a combination of a prism plus grating.
You first image shows sufficient drift, just needs better focus on the spectrum.
BASS Project will allow you select and sum the spectral columns and present a profile for wavelength calibration...
Looking at the image profiles the dispersion changes between them, 3.66 A/pix for the Betelgeuse and 5.4 A/pix for Sirius. For a grating this is very high, normally 8-10 A/pix target.
(cropped/ changed images for IIS??)

Well done!
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Click for full-size image (Bojan_Betelegeuse_profile.JPG)
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Old 10-01-2020, 10:17 AM
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Ken,
Thank you for your analisys.
The images I attached earlier are full size (from 450D), no cropping.
I did some wl calibration in BASS, it was not easy to identify some features (especially with Betelgeuse)... I (wrongly?) assumed Halpha was marked with red circle...
I also assume the feature marked by Green circle is camera response. Have to learn how to calibrate this.
Will get there :-)
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Old 10-01-2020, 10:40 AM
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Bojan,
Interesting, if both spectra were taken with the same set-up - grating spacing/ camera etc. I'd expect the dispersion to be the same .....

The green dip is the "Canon" dip caused by the overlap of the Bayer filters in the DSLR camera. This can easily be compensated when doing the instrument response processing.
See the example "Astronomical Spectroscopy for Amateurs", p100.

Betelgeuse being a red giant M2/M3 has many Tio bands in the red.
Remember the Saha graph.
(http://www.astronomicalspectroscopy.com/How_to.html)
This shows there is little possibility of Hydrogen in the spectrum.

The red dip looks to me like the pronounced Telluric band at 7600A, a good and easy marker....
EDIT: Looking at the MILES M2 reference...I had another go at re-calibrating (at 4.5 A/pix)
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Last edited by Merlin66; 10-01-2020 at 10:57 AM.
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Old 10-01-2020, 10:53 AM
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Yep, the spectra were taken with the same setup, same evening, one after another.

Difference is in position of the "0" order, I did not take sufficient care of this.. Next time I will use the lap screen for more accurate aim at target.
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Old 10-01-2020, 10:59 AM
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Bojan,
Hmmmm
The position of the zero order doesn't impact on the dispersion...
It's best to position the zero order towards the left of the frame and centre the spectrum across the chip. Focus on the spectrum.
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Old 10-01-2020, 11:14 AM
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Quote:
Originally Posted by Merlin66 View Post
Bojan,
Hmmmm
The position of the zero order doesn't impact on the dispersion...
It's best to position the zero order towards the left of the frame and centre the spectrum across the chip. Focus on the spectrum.

For positioning/ficussing I was using magnifier lens, placed at the centre of the camera screen, this way it is easy to focus on spectrum... but not easy to place "0" order on the edge, as it's not visible with the lens...


BTW, my setup is similar to this one, except I have prism in front of grating.
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Old 10-01-2020, 12:08 PM
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My setup...
It screws on C11 2" thread, grism is centered in the barrel.
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Old 10-01-2020, 12:32 PM
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Ahhh, OK.
Which prism wedge are you using?
The prism obviously impacts on the linearity of the dispersion.
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Old 10-01-2020, 12:54 PM
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Tim Stone used his SA100 to get this result.
Not 100% sure about the line identification.....
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Old 10-01-2020, 01:09 PM
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I tried to recalibrate the spectra (starting with Sirius, using Balmer series as reference).
Came up with 0.432nm/pxl, almost the same as your 2nd result.
Applied the same dispersion to Betelgeuse (corrected for hor shift), both spectra now look consistent (green dip and some other less prominent features).
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Last edited by bojan; 10-01-2020 at 06:54 PM.
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Old 10-01-2020, 01:18 PM
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Quote:
Originally Posted by Merlin66 View Post
Ahhh, OK.
Which prism wedge are you using?
The prism obviously impacts on the linearity of the dispersion.
I ordered the prism from Bxoptic, they had it in stock and it was pretty cheap (I am trying to find the quote...) and the delivery was very quick, I highly recommend them (I used them for other things in the past when I worked for MCE Lasers).
I think it was ~3 or so, almost ideal match for SA100.
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Old 10-01-2020, 01:30 PM
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Wedge prism details as follows:

material :BK7,
Dia :38.1mm
thickness (thick edge ) :4.66mm
wedge angle :4degree +/-1 arcmin
AR coating , 450-650nm

1 pcs, the unit price was 35.00 USD (for one) + US$20 shipping.

Better than Edmund....


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  #14  
Old 10-01-2020, 02:28 PM
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Bojan,
Thanks for that - they have a very impressive set-up.
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