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Nigella Bryant

Camera response from spectra and calibration

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Hi all my spectroscopy gurus. My zwo ASI224mc definetly has a response dip on spectrum data. Can I make a camera response data capture and use this to calibrate the spectrums in future so this can be extracted from the spectrum of an object? 

Attached are two spectrums with what I assume is the camera response for clarity. I my be wrong. 

Hope the above makes sense. 

IMG_20191014_131416.jpg

IMG_20191014_133130.jpg

Edited by Nigella Bryant

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Since you are using OSC camera, you will have two major "dips" in camera response - R-G transition and G-B transition. The one you marked is R-G transition. Here is "generic" response curve for that sensor:

image.png.51a389c52c2883ae73771eb899b1940a.png

So first dip B-G is at about 480nm, and second is at about 580nm

In your spectrum, you can also see first dip but it is not as pronounced:

image.png.21ca89966a44990f64eb8100e0e3fd27.png

In any case, way you create instrument response (both sensor and optics if coatings don't have uniform frequency response, and often they don't) is to take your spectrum and divide it with reference spectrum of that star. This means that you should record the spectrum of a known star - one that you have reference spectrum of.

You will also need to "smooth" result or "smooth" reference spectrum if it is more detailed than your spectrum, otherwise features of reference spectrum not recorded in your spectrum will be part of instrument response (but they are not).

Here is video tutorial for RSpec that describes that process (RSpec is used to make the video - but you can figure out important steps and reproduce those in software of your choice):

https://www.rspec-astro.com/videos/InstrumentResponse/InstrumentResponse.mp4

 

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2 hours ago, vlaiv said:

Since you are using OSC camera, you will have two major "dips" in camera response - R-G transition and G-B transition. The one you marked is R-G transition. Here is "generic" response curve for that sensor:

image.png.51a389c52c2883ae73771eb899b1940a.png

So first dip B-G is at about 480nm, and second is at about 580nm

In your spectrum, you can also see first dip but it is not as pronounced:

image.png.21ca89966a44990f64eb8100e0e3fd27.png

In any case, way you create instrument response (both sensor and optics if coatings don't have uniform frequency response, and often they don't) is to take your spectrum and divide it with reference spectrum of that star. This means that you should record the spectrum of a known star - one that you have reference spectrum of.

You will also need to "smooth" result or "smooth" reference spectrum if it is more detailed than your spectrum, otherwise features of reference spectrum not recorded in your spectrum will be part of instrument response (but they are not).

Here is video tutorial for RSpec that describes that process (RSpec is used to make the video - but you can figure out important steps and reproduce those in software of your choice):

https://www.rspec-astro.com/videos/InstrumentResponse/InstrumentResponse.mp4

 

Thanks vlaiv, much appreciated. So I guess if I use my mono asi120mm that will give less of a camera response and less to calibrate. 

Edited by Nigella Bryant
Comment revised.

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Did I mention there's an article on spectroscopy in the October issue of Astronomy Now, not into it myself but just read through it and interesting subject, worth a read.

Dave

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1 minute ago, Davey-T said:

Did I mention there's an article on spectroscopy in the October issue of Astronomy Now, not into it myself but just read through it and interesting subject, worth a read.

Dave

Thanks Dave, unfortunately I have sky at night not astronomy now, lol. I'm very taken with the subject but a huge learning curve. 

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12 minutes ago, Nigella Bryant said:

Thanks vlaiv, 

So the dip I've marked in black is not a camera response but something else. Is it part of earth's atmosphere as it doesn't match the reference star spectrum. 

I did not say that - I've mentioned that it is in fact camera response (transition from green to red) and that there is another one like that, but less pronounced - transition from blue to green.

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1 minute ago, vlaiv said:

I did not say that - I've mentioned that it is in fact camera response (transition from green to red) and that there is another one like that, but less pronounced - transition from blue to green.

I realised that and changed my response, lol. 

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27 minutes ago, Nigella Bryant said:

Thanks vlaiv, much appreciated. So I guess if I use my mono asi120mm that will give less of a camera response and less to calibrate. 

Yes, mono is better option. You will still need to do camera response calibration but it will be smoother without sudden dips.

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33 minutes ago, vlaiv said:

Yes, mono is better option. You will still need to do camera response calibration but it will be smoother without sudden dips.

Thanks vlaiv, much appreciated. 

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Nigella,

I'd recommend Andrew's Tutorial ( see p 36)

https://britastro.org/sites/default/files/BAA_BASS_Tutorial_0.pdf

I usually crop the profile from say 3800 to 7000A, to make things easier.

(The Astronomical spectroscopy website has recently been updated - worth checking out....   http://www.astronomicalspectroscopy.com/   )

 

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1 hour ago, Merlin66 said:

Nigella,

I'd recommend Andrew's Tutorial ( see p 36)

https://britastro.org/sites/default/files/BAA_BASS_Tutorial_0.pdf

I usually crop the profile from say 3800 to 7000A, to make things easier.

(The Astronomical spectroscopy website has recently been updated - worth checking out....   http://www.astronomicalspectroscopy.com/   )

 

Thanks Ted.

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26 minutes ago, Merlin66 said:

Ted????

Ken, sorry, no idea why I wrote Ted, but at age 60 I'll put it down to senile decay, lol.

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19 minutes ago, Nigella Bryant said:

Ken, sorry, no idea why I wrote Ted, but at age 60 I'll put it down to senile decay, lol.

Do you not sometimes get that thing where your brain thinks a word, yet your fingers type a different word of the same length and sharing some of the same letters (and not because you have one hand in the wrong place on the keyboard :) ?

I even do it occasionally when I realise I've mistyped, delete the incorrect word and retype the wrong one a second time...

James

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17 minutes ago, JamesF said:

Do you not sometimes get that thing where your brain thinks a word, yet your fingers type a different word of the same length and sharing some of the same letters (and not because you have one hand in the wrong place on the keyboard :) ?

I even do it occasionally when I realise I've mistyped, delete the incorrect word and retype the wrong one a second time...

James

Sometimes James, at least I'm not alone, lol. I frequently read a word and I've read it as a completely different "CART" (word). Gotten me in trouble a few times and once embarrassingly so in a parent's meeting,  lol. 

Edited by Nigella Bryant

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I think it might be worthwhile  taking a step back here.  There are several sources for the difference between the shape of the spectrum as measured and how it should look.  The camera response is one of them and mono cameras certainly have a smoother response than colour cameras. There are other effects though including the response of the grating, the telescope optics and the earth's atmosphere so knowing the camera response is not enough. Fortunately a simple way to  account for all these effects is by measuring a star that we know what the spectrum should look like and dividing our spectrum by the published spectrum. (We normally choose a hot eg A star as these have a simpler spectrum. If using the Star Analyser, this is  typically the same one which is used to initially focus and wavelength calibrate the spectrum). This gives us the response of the whole system including the camera.  We can then use this to correct the spectrum of any star. The process is the fundamentally the same whatever software you use. There are examples of doing this for both the Star Analyser and ALPY in my tutorials here

http://www.threehillsobservatory.co.uk/astro/spectroscopy_10.htm

and documents here

http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm

Cheers

Robin

Edited by robin_astro
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23 minutes ago, robin_astro said:

There are examples of doing this for both the Star Analyser and ALPY in my tutorials here

http://www.threehillsobservatory.co.uk/astro/spectroscopy_10.htm

and documents here

http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm

Cheers

Robin

And here is an example specifically for a colour camera using a Star Analyser

http://www.threehillsobservatory.co.uk/astro/spectroscopy_11a.htm

Cheers

Robin

 

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Note it is very important to accurately remove the sky background before attempting to measure the instrument response otherwise it will not work on other spectra.

Other tips:-

Turn the image gain up to make sure you select the whole height of the spectrum when binning and that the sky background does not contain any of the measured spectrum or any other background stars or spectra.

Smooth the published spectrum first to approximately match the resolution of the measured spectrum (ie make the Balmer lines look similarly wide so they divide out more accurately.)

Remove any remaining artifacts from dividing the spectrum lines but not other features that are actually in the instrument response

Do not over smooth the result. In particular, make sure it fits well at the blue end. If it does not your spectrum will rapidly shoot off either high or low.

When you have the instrument response,use it to correct the spectrum you produced it from. The result should of course closely match the published spectrum. If it does not, investigate why it is wrong

Edited by robin_astro
typo
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While we are on this topic, hope no one minds me asking another question related to spectrum calibration.

How do I do flat calibration? Same as with any imaging - just place flat panel on scope with the same gear arrangement (focus included) and I take subs? I divide spectrum image after dark calibration as normal?

I'm asking because there is issue of vignetting - spectrum can "overlap" parts of "zero order" of vignetting and flats in this case will maybe produce wrong results? Or maybe it does not matter at all, and we flat calibrate to eliminate pixel to pixel QE variations and dust on sensor / any filters after SA?

I assume that it might not matter because any vignetting will be incorporated in system response, but that means that recording needs to be done with zero order image at exact same spot each time (for both reference star and one that we are measuring)? It also means that spectrum orientation should remain the same with regards to sensor (but not necessarily to star field - as we can rotate whole assembly to get spectrum clear of background artifacts) - that is probably better as we want spectrum to be horizontal anyway, I guess.

Anyone tried background removal procedure with "clear exposure"? I understand that it can be tricky as SA needs to be removed from optical train which can lead to slight focus change, and one needs to do linear fit on zero order stars to match intensity.

I guess one can monitor FWHM / HFR to get roughly the same defocus of zero order image.

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Flat correction is useful for slit spectrographs and I used to flat correct Star Analyser spectra though no longer recommend it. The problem is spectroscopic flats are a mixture of position and wavelength dependent effects. With a slit spectrograph the position of a particular wavelength in the flat is fixed and defined  by the position of the slit so a conventional flat taken through the spectrograph can be used but in a slitless system any particular wavelength ends up at any location across the flat image. 

Professional slitless systems also have this problem and the solution for them is to build up a 3D flat ie a separate "flat" for each location in the image. This is obviously impractical so the advice I generally give now is:-

Keep the sensor as clean as possible to minimise dust donuts and place your reference star and target at the same location in the field. The instrument response will then take care of any vignetting type issues

If you want to explore the possible errors due to not taking a flat or you are forced to measure spectra at different positions in the field then I can suggest taking spectra at different locations and seeing how much the spectrum changes  for your particular setup

Cheers

Robin 

Edited by robin_astro
typos, clarification
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15 minutes ago, robin_astro said:

Flat correction is useful for slit spectrographs and I used to flat correct Star Analyser spectra though no longer recommend it. The problem is spectroscopic flats are a mixture of position and wavelength dependent effects. With a slit spectrograph the position of a particular wavelength in the flat is fixed and defined  by the position of the slit so a conventional flat taken through the spectrograph can be used but in a slitless system any particular wavelength ends up at any location across the flat image. 

Professional slitless systems also have this problem and the solution for them is to build up a 3D flat ie a separate "flat" for each location in the image. This is obviously impractical so the advice I generally give now is:-

Keep the sensor as clean as possible to minimise dust donuts and place your reference star and target at the same location in the field. The instrument response will then take care of any vignetting type issues

If you want to explore the possible errors due to not taking a flat or you are forced to measure spectra at different positions in the field then I can suggest taking spectra at different locations and seeing how much the spectrum changes  for your particular setup

Cheers

Robin 

I'm sort of worried by pixel to pixel differences of my sensor (ASI1600), as I've noticed particular pattern - checkerboard pattern is present for some reason. QE is not varied greatly (1-2%) and with regular imaging flat calibration works well, but like you said - here it can be a bit of a problem. I'll try different approaches to see how much difference I get in spectrum.

Here is "zoomed in" section of my regular flat for this sensor (note it is in fact mono sensor and what you are seeing is not bayer pattern):

image.png.df76f34c782d0b5836f6ef562b2dff7c.png

Pixel to pixel variation is real and not noise - I used x256 flat and flat dark subs to create it, and each sub has SNR of about 50 or so, stack has SNR of about 850-900. Variations are, on other hand, few percent (about order of magnitude larger than any residual noise).

Here is "profile" of the flat (flat has been scaled to relative QE):

image.png.b7eb4ed5a282701495ed41c74b090018.png

I think there are at least couple of "wavelengths" present here - one very fine on pixel level, and one maybe 4-5 pixels in wavelength. In any way, I think it will result in strange pattern in spectrum even if I over sample (and I will by factor of at least x2-x3 in order to get good dispersion and minimize seeing effects, I can bin data later to improve SNR back a bit).

I will certainly take flats, and I'll make sure that I place zero order star at roughly the same position for each measurement (SGP does have crosshair in preview so I can use that). After that I can try both with and without flat calibration to see which one works better.

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You can make a PRNU (pixel response non uniformity)  "flat" as described by C Buil which can then be use. Don't have the reference to hand as access difficult  on the road.

When looking at IRAF examples they tended to apply different types of flat for different reasons.  Low frequency for vignetting, higher for dust doughnuts,  PRNU etc. Not saying you should do the especially for Star Analyser for the reasons Robin gave.

Regards Andrew 

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