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Nigella Bryant

Camera response from spectra and calibration

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41 minutes ago, robin_astro said:

I know  the 350D is pretty old technology now but Christian Buil made some measurements of the solar spectrum in the UV with a modded 350D. It got down to the Ca H,K lines and still had a little sensitivity down to ~3700A

http://astrosurf.com/buil/lhires_ir/obs.htm

About a third of the way down the page. 

You can see his response curves here

http://www.astrosurf.com/buil/350d/350d.htm

Robin

But presumably the lower sensitivity of the 350D and the higher noise levels were less significant for solar measurements than they would be for much fainter stars....?

Chris

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3 hours ago, chiltonstar said:

But presumably the lower sensitivity of the 350D and the higher noise levels were less significant for solar measurements than they would be for much fainter stars....?

Yep there would be plenty of signal (It was actually the daylight sky.)  I think it was  just a demonstration by Christian Buil that there was still some sensitivity down there. The actual curves show how much (or little! ) there actually is (from none below 4000A as standard to "some" with the filter removed)

Robin

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For me the one of the most telling measurement that Christian Buil makes is in fig 5 on this page where he compares the effective QE of a colour DSLR with a good mono CCD camera. 

http://www.astrosurf.com/buil/50d/test.htm

With or without UV/IR blocking filters, all colour cameras come with a huge sensitivity penalty when used for  spectroscopy

Edited by robin_astro
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3 minutes ago, robin_astro said:

With or without UV/IR blocking filters, all colour cameras come with a huge sensitivity penalty when used for  spectroscopy

the gap has narrowed where mono CMOS is concerned though and Christian now reports he is using only CMOS for spectroscopy

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10 hours ago, robin_astro said:

the gap has narrowed where mono CMOS is concerned though and Christian now reports he is using only CMOS for spectroscopy

So a camera like the ZWO ASI 120MM-S USB 3.0 Mono Camera would be one possibility to get an extended low wavelength range>

Chris

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Chris,

Don’t know about the ASI 120, but I can record well down in the UV with the ASI 174mm when using the spectroheliograph.

The CaK lines make an easy target......

 

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Just now, Merlin66 said:

Chris,

Don’t know about the ASI 120, but I can record well down in the UV with the ASI 174mm when using the spectroheliograph.

The CaK lines make an easy target......

 

This is the response curve (QE) from the FLO site:-

Chris

zwo_asi120mm_qe_curve.jpg

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Probably 50% at 4000 A and going down to zero at 3500 A.

would probably give you some results.....

(I’d need to check the ASI 174 for comparison)

 

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On 19/10/2019 at 07:57, Merlin66 said:

Probably 50% at 4000 A and going down to zero at 3500 A.

would probably give you some results.....

(I’d need to check the ASI 174 for comparison)

 

I notice that the Mini version of the ASI 120mm doesn't appear to have a UV-IR cut filter, and therefore has a slightly better response in the UV.

Chris

zwo_asi120mm_mini_qe.jpg

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I have not checked the response of my old  ASI120 which I use as a guider. It does not have an UV/IR block (though the window on either model may by AR coated which would ultimately restrict coverage in the UV ) Any modern unfiltered mono camera will reach the Ca HK lines without difficulty though, even with an AR coated window/cover glass.

Note the  120 is a pretty small sensor though so check you have enough width for the range you want to look at

Cheers

Robin

Edited by robin_astro
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8 hours ago, robin_astro said:

 

Note the  120 is a pretty small sensor though so check you have enough width for the range you want to look at

Cheers

Robin

Good point: my spectra with the objective prism are 8mm long, and with the SA100 about 11mm, and of course they will be longer with a wider spectral response from the chip.

Chris

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Chris,

I’d be very interested to see how you mounted and aligned your objective prism.

I modified a PVC pipe adaptor to hold the prism and fit the scope.

 

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On 22/10/2019 at 02:55, Merlin66 said:

Chris,

I’d be very interested to see how you mounted and aligned your objective prism.

I modified a PVC pipe adaptor to hold the prism and fit the scope.

 

Crude & functional Ken! See attached.

I just mounted the prism on a square of ply, with the prism held by two strips of 24mm Al L-section, locking it in position with a pair of bolts. The aperture of the hole is 45mm, about as large as I could get with the 60mm x 60mm prism. There are rubber spacers on the back so that it fits snugly into the end of the ED80 and stays in place.

With the base of the prism towards the ground, I point the scope (ED80) with the prism in place at a distant Na streetlamp and move the scope up and down until the spectrum is centred, then adjust an RDF mounted on a small photographic ball & socket head until the red spot is on the target (in use, the RDF normally points in the opposite direction to way it does in the image). It stays aligned then as I rotate the end dew cap.

Chris

objectiveprism.png

Edited by chiltonstar

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Chris,

Neat and functional, I would say. It obviously does the job. Well done!

I was lucky to find a 100mm diameter FS5 20 deg prism and mounted it into a PVC joiner, see images. It's a bit difficult to see as the prism doesn't show up too well.

This is then mounted on the ED80. I also, like you, use an off-set finder.

Ken

 

100_3580.JPG

100_3577.JPG

100_3578.JPG

100_3579.JPG

Edited by Merlin66
correction

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