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andrew s

Simple transmission spectrograph

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Posted (edited)

New very low resolution spectrograph with rotator and filter wheel.

The filter wheel has SA200 and 100 gratings along with clear and photometric V filter plus shutter.

The collimator is a 40mm projection eyepiece and the collimator a 4/3 25mm f0.9 camera lens.

The rotator allow you to position the spectra avoiding field stars.  The ASI 1600MM will allow fast downloads as I plan to do high cadance (20s) exposures of Red Dwarf flare stars.

Regards Andrew 20190825_103022.thumb.jpg.77b3a4e0b7f20cef8edbcb54bf513cb1.jpg

Edited by andrew s
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Could you please describe layout of this?

Since this is "relay" based system, have you thought of adding a half slit to it? By half slit I mean following:

image.png.4c829429df936b14e710c68ca94a011e.png

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Posted (edited)

No slit, it is a slitless design. This way you get a spectrum of all the stars in the field so you can do differential spectroscopy.  The down side is more difficulty in doing sky subtraction and the resolution depends on the seeing.

With a slit you need a guide system to place the target on the slit and keep it there. Also with a siit it has to be wide to be photometrically accurate.

Using the grating in a parallel beam avoids the aberrations of using it in a converging beam.

Regards Andrew 

PS sorry the layout is collimation eyepiece,  transmission grating and then focusing  camera lens . The spacing is such the the 40mm eyepiece images the telescope entrance pupil (mirror) approximately on the entrance pupil of the camera lens for maximum throughput. 

Edited by andrew s
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21 minutes ago, andrew s said:

No slit, it is a slitless design. This way you get a spectrum of all the stars in the field so you can do differential spectroscopy.  The down side is more difficulty in doing sky subtraction and the resolution depends on the seeing.

With a slit you need a guide system to place the target on the slit and keep it there. Also with a siit it has to be wide to be photometrically accurate.

Using the grating in a parallel beam avoids the aberrations of using it in a converging beam.

Regards Andrew 

PS sorry the layout is collimation eyepiece,  transmission grating and then focusing  camera lens . The spacing is such the the 40mm eyepiece images the telescope entrance pupil (mirror) approximately on the entrance pupil of the camera lens for maximum throughput. 

I gathered general layout, I just wondered how you managed to put everything together (adapters and connections) and what sort of lens did you use?

As far as I can tell from the image, lens is narrow enough to fit inside T2 extension or something? You also happen to have two rotators in this setup one manual and one electronic?

EP is probably this one (or similar) with T2 thread?

https://www.teleskop-express.de/shop/product_info.php/info/p1428_TS-Optics-40-mm-1-25--SuperView-eyepiece-with-T2-connection-for-cameras.html

 

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@vlaiv Yes it is a SuperView eyepiece which has T2 thread. The 1 1/4 inch fitting goes into a T2 threaded helical focuser then various T2 spacers onto the rotator. 

The camera lens is a zhongyi speedmaster which has a 43mm filter fitting so I used an adapter to T2 from Baader intended for Tak telescopes. The other end is an ASI 4/3 camera adapter.  I also use a TS T2 rotator between the grating and the camera lens to align the spectrum to the pixel rows.

I will do a diagram later but current,y baby sitting our granddaughter of 3.5yrs, a hand full.

Regards Andrew 

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Quick grab of the famous double 61 Cyngi  a pair of K dwarf stars (K5V and K7V) also known as the Flying star. Stack of 10x6s  exposures.

Still need to refine the alignment with the pixel rows. SA200 giving ~ 7.4A per pixel.

Regards Andrew

61Cygni.png.2a71556c70c54541e8d7a8fc28af6b58.png

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I meant to ask in regards to setup, how do you find filter thread connection on lens - is it strong enough to carry both lens and the camera without any issues?

I was always under impression (maybe due to cheap/stock canon lens) that this filter thread and top of the lens is not most firm to be used for attachment.

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2 minutes ago, vlaiv said:

I meant to ask in regards to setup, how do you find filter thread connection on lens - is it strong enough to carry both lens and the camera without any issues?

I was always under impression (maybe due to cheap/stock canon lens) that this filter thread and top of the lens is not most firm to be used for attachment.

Good question.  On this camera lens yes, as it is metal and with a reasonably long thread but I have tried others where it would not. I have glued the lens to the ASI 4/3 adapter as it was not regid enough. One other point the back focus of 4/3 lenses is short and is not comparable with cameras with back focus much longer than that of the ASI1600. 

I thought I might need to brace it all but I don't now think I will need to.

One impressive thing is that "Source Extractor" can pick out the stars from the spectra so The Sky X can plate solve directly through the grating although  normally I would use the clear filter.

Regards Andrew 

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2 hours ago, robin_astro said:

Hi Andrew,

A very nice evolution of the junk box spectrograph.

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

Here's  to catching some flares !

Robin

Thanks Robin, yes an almost direct copy  but with a few extra bits and a faster camera lens to better match the small pixels of the ASI. The 40mm projection eyepiece is ideal for matching the various aperture stops. I have to admit shamelessly stealing your basic design from the link you posted! Expanding the image to read the writing on the lens.

I did manage to grab some spectra the other night and I am getting about 7.4 A per pix.

The clear filter in the filter wheel allows easy plate solving and the rotator allows aligning the spectra to avoids overlapping other stars so automation is easy so much so I can get packaged software to do it rather than use Python.

Regards Andrew 

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I think this is very nice setup, and I would really love idea of "half slit" integrated with it, or "quarter slit" at least - that way one can still have 3/4 of the field usable for multiple star spectra at once, plate solving and such and use slit to get better spectrum, or at least easier to calibrate and remove background glow.

Out if interest, 40mm + 25mm reduces things about 40/25 = x1.6 am I right about that? I was thinking about similar setup for EEVA - 32mm EP + something like 10-12mm cheap Chinese "megapixel" lens for CCTV (actually a bit larger than standard 1/3 ones - I've found 1/1.8 one for reasonable price of $25 on aliexpress).

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Posted (edited)
1 hour ago, vlaiv said:

I think this is very nice setup, and I would really love idea of "half slit" integrated with it, or "quarter slit" at least - that way one can still have 3/4 of the field usable for multiple star spectra at once, plate solving and such and use slit to get better spectrum, or at least easier to calibrate and remove background glow.

Out if interest, 40mm + 25mm reduces things about 40/25 = x1.6 am I right about that? I was thinking about similar setup for EEVA - 32mm EP + something like 10-12mm cheap Chinese "megapixel" lens for CCTV (actually a bit larger than standard 1/3 ones - I've found 1/1.8 one for reasonable price of $25 on aliexpress).

You could do the split thing but as I pointed out you would need a guide module.  Doable but adds cost and complexity.  Interestingly The Sky X can plate solve directly throught the SA200 as the Source Extractor selects the stars and not the spectra. Robin's Aply200 is a good example of what can be done with a low res slit system.

On the lenes you need to consider not only the focal length but also the apertures to avoid vignetting and limiting the field. You need fast camera lenes to obtain enough aperture.  Your calculation is correct. 

My design was for a specific program. All spectrographs really need to be designed for the telescope (scales with the diameter of the scope)  targets and intended resolution/measurement.

Regards Andrew

PS This shows the current field 33' by 25'

 Field.png.09b6905eb5175a8eac66656aef132589.png

Edited by andrew s
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Interestingly Claudio Balcon in Italy built a similar imaging/spectroscopy setup using an SA100 where he can introduce the grating and an adjustable slit.  It included an off  axis guider.  Like the ALPY200, he has used it in slit mode to spectroscopically confirm and classify a couple of supernovae. 

Cheers

Robin

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Re Plate solving...I found that Astrometry.net handles SA200 images and does a good plate solve - ignores the spectra.....

 

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