Camera response from spectra and calibration

[robin_astro]

If you are seeing a fixed pattern response from the camera I think  this would best be corrected separately using a PRNU (pixel response non uniformity) file generated from a conventional flat which just contains this short spatial variation information.

Cheers

Robin

EDIT as Andrew has just suggested !

Edited October 15, 2019 by robin_astro
typo

[robin_astro]

A while back I wondered if it might be possible to use a conventional flat (without the Star Analyser) specifically to get rid of flat defects produced at or close to the camera sensor ie dust and pixel to pixel variations. It failed with dust (probably as the geometry of the beam in the dispersed spectrum is different). It should work with genuine PRNU pattern defects though if they are an issue I think

Robin

Edited October 15, 2019 by robin_astro

[vlaiv]

1 minute ago, robin_astro said:

A while back I wondered if it might be possible to use a conventional flat (without the Star Analyser) to get rid of flat defects produced at or close to the camera sensor ie dust and pixel to pixel variations. It failed with dust (probably as the geometry of the beam in the dispersed spectrum is different). It should work with genuine PRNU pattern defects though if they are an issue I think

Robin

I was just about to ask that, but yes, good point, with angled beam, flat taken without SA will correct only zero order dust shadows - maybe handy for background subtraction, although background should be uniform so it's just an offset and not pattern.

[andrew s]

For what it's worth I was going to experiment with various "flat" options once my new set up is up and running.

I will report back in due course if I get anything  worth sharing.

Regards Andrew 

[robin_astro]

19 minutes ago, andrew s said:

For what it's worth I was going to experiment with various "flat" options once my new set up is up and running.

I will report back in due course if I get anything  worth sharing.

Regards Andrew 

It would be great if you come up with a solution. This so far intractable problem has been bugging me for 15 years !  It is potentially particularly troublesome when attempting spectrophotometry with the Star Analyser where the target and reference are by necessity in different positions

Robin

[andrew s]

1 minute ago, robin_astro said:

It would be great if you come up with a solution. This so far intractable problem has been bugging me for 15 years !  It is potentially particularly troublesome when attempting spectrophotometry with the Star Analyser where the target and reference are by necessity in different positions

Robin

Yes spectrophotometry is exactly what I what to do! Not sure I can crack it but I will do the sums and experiment.

The only other option  I can think of is to use a photometric slit and rotate the instrument to suit! Something new to build...  ...no stop it use what you have got.

Regards Andrew 

[robin_astro]

Hi Andrew,

A couple of references to how they do it for the grism spectrograph on the HST

https://ui.adsabs.harvard.edu/abs/2005acs..rept...10W/abstract

https://ui.adsabs.harvard.edu/abs/2009PASP..121...59K/abstract

Cheers

Robin

[robin_astro]

Also this paper briefly mentions flat field correction for a slitless spectrograph on a 2.2m telescope at La Silla

https://www.aanda.org/articles/aa/abs/2008/32/aa10157-08/aa10157-08.html

[chiltonstar]

This is a very useful thread which I have been following avidly as part of attempting to reboot my astro spectrometry interest! A question for the experts, in keeping with the title of the thread, are any of the commonly-available cameras able to go further into the UV and IR than the AS1224 I have, or the DSLR I normally use which runs out of steam below 420 nm or above 680 nm?

Chris

[robin_astro]

Hi Chris,

I am guessing the ASI224 is colour camera with an UV/IR block filter?  If you can remove it you would probably get further at both ends but any unfiltered mono camera will go further and will be better for spectroscopy in several other ways (more sensitive, smoother instrument response, less pixellation artifacts). If you are looking to go a long way into the UV then response of the grating, the transmission of the telescope optics and the sensor cover glass come into play but you should be able to get to ~3800-7600A with the Star Analyser and any mono camera. 

Cheers

Robin 

[vlaiv]

ASI224 does not have integrated UV/IR cut filter and one needs to be provided if required for application (cover window is only AR coated). It is also very sensitive in IR part of the spectrum (even more so for color camera because above ~800nm sensitivity of bayer matrix is equal and camera can be effectively treated as mono in that rage). Here is published spectral response for it:

Although graph is cut below 400nm, we can sort of guess it goes down to about ~320nm.

One thing to note when trying to capture "whole" spectrum that sensor is capable to record is overlap of different orders. In order to capture full spectrum you need to capture it in "pieces". If camera is sensitive down to 320nm, second order spectrum will start overlapping at twice that value so already at 640nm.

I'm guessing that DSLR is not astro modded and has UV/IR cut filter in place - that is why it is limited to 420-680nm range. One way to split spectrum would be to use UV/IR cut filter for 400-700nm range, and something like 495nm long pass from baader (yellow) or 570nm long pass (again from baader - orange) to record NIR spectrum. With 495nm long pass, overlap will start somewhere around 980nm, and with 570nm long pass - you don't have to worry about second order since sensor is not sensitive above about 1100nm and there will be no overlap. For UV part of the spectrum - don't use any filters and extract 300-500nm range for example.

[Nigella Bryant]

I'm away at the moment but glad the thread is being useful to other's. It'll take me time to filter through all the advice given here so far but thanks all, keep it up. 

[robin_astro]

2 hours ago, vlaiv said:

If camera is sensitive down to 320nm, second order spectrum will start overlapping at twice that value so already at 640nm.

 

In practise though even if the sensor is sensitive to 3200A, you will not get anywhere near there without taking special measures. (For example the typical AR coatings on telescopes, sensors and the Star Analyser grating response absorb that low and the atmospheric extinction increases in the UV).  If you are interested in these issues then there are some  useful discussions on the ARAS forum about  the UVEX spectrograph which was specifically designed to measure as far as possible into the UV 

http://www.spectro-aras.com/forum/viewforum.php?f=45

The second order with a blazed grating is also much fainter than the first order so unless your star is particularly bright in the UV and you are trying to measure faint IR  features you can go to 7600A using the Star Analyser without significant second order contamination. Here is a typical response curve for my setup (C11, star analyser mono CCD camera.) from the first document on this page

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

 You can see that the  response falls to zero by 3700A 

Cheers

Robin

[chiltonstar]

Many thanks for these very useful replies. The camera (Nikon D750) is un-modded so does have a UV/IR cut off unfortunately, but my AS1224 (colour) doesn't seem to go much lower in wavelength even without one. The reason I need to go down a bit more (apart from the interesting spectral lines below 400 nm!) is because I'm using an objective prism and need the 410 nm H line if possible as one of the three lines I've been using to linearize the spectrum from the prism using the BASS software. The prism is mounted in front of an ED80 (fl 600 mm) which is close to the fl of the original lens in the Hilger 0.5 m spectrograph that the prism came from. That went down to about 320 nm or so, but using film rather than a sensor of course. In the blue part of the spectrum, the objective prism is giving me a resolution of about 3 - 4 times better than I get with the SA100 I have.

Chris

Edited October 17, 2019 by chiltonstar

[Merlin66]

Chris,

Not exactly the same, but I have manged to record down to 3700A with a ATiK314L+ with the slit spectroscope. Without a UV cut it really depends on the optics and camera response.

I'm interested to hear about your objective prism....I have a 20 deg f2 prism which can be mounted in front of my ED80.... early results with a FULL modded DSLR were very promising ( especially in the NIR)

Ken

 

[robin_astro]

55 minutes ago, chiltonstar said:

my AS1224 (colour) doesn't seem to go much lower in wavelength even without one.

Yes a colour camera will really suffer compared with a mono in this region as only 1/4 of the pixels are sensitive.  I am a bit surprised it rolls off quite so early though without UV filter. I  suspect that graph of relative sensitivity may be rather optimistic for the blue channel.   Here is the response curve for my unmodified Canon 350D from my website.  That gives up at ~4100A too

Cheers

Robin

[Thalestris24]

Christian Bull compares a cmos ASI183m and an Atik460 ccc here I have a qhy183m but not sure if there is any difference in response. Both the tested cameras give a reasonable UV response (183 better - I'll check it out one of these days with a UV LED light). There seems to be a rapid rolloff due to the ASI cover glass. Again, not sure if QHY different. He talks about the effects of fringing in other cameras apparently caused by not having anti-reflective cover glass. (Is that the reason for the 'microlense' artifacts reported in the ASI1600mm?)

Louise

[chiltonstar]

7 hours ago, Merlin66 said:

 

I'm interested to hear about your objective prism....I have a 20 deg f2 prism which can be mounted in front of my ED80.... early results with a FULL modded DSLR were very promising ( especially in the NIR)

Ken

 

But resolution in the red and NIR is presumably fairly poor because of the non-linear dispersion of a prism which compresses the spectrum at the red end?

Chris

[Merlin66]

Chris,

Notwithstanding I found the prism easily pick Ha emission, I have some images, I’ll upload later.

 

[chiltonstar]

21 minutes ago, Merlin66 said:

Chris,

Notwithstanding I found the prism easily pick Ha emission, I have some images, I’ll upload later.

 

Yes, eg a prism image (uncorrected) using my prism of Vega. H alpha is nicely shown, although not much further into the red.

Chris

 

Edited October 18, 2019 by chiltonstar

[Merlin66]

I was using a FULL spectrum modded 1000D.

I’ll find the image.....

[chiltonstar]

5 hours ago, Merlin66 said:

I was using a FULL spectrum modded 1000D.

I’ll find the image.....

Interesting,  maybe I should look for a s/h 1000D.

Chris

[robin_astro]

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

[robin_astro]

13 hours ago, Thalestris24 said:

He talks about the effects of fringing in other cameras apparently caused by not having anti-reflective cover glass. (Is that the reason for the 'microlense' artifacts reported in the ASI1600mm?)

Yes the fringing problem is a big problem in high resolution spectrographs (eg the LHIRES with 2400l/mm grating). It is due to internal reflections in the cover glass. It also produces newtons rings in solar h alpha imaging and can arise from subtle changes in sensor design.  eg people who upgraded from an ATIK314 which is free of fringing, to an ATIK 414 immediately ran into problems due to the lack of AR coating.  Not sure if it is connected with microlensing artifacts though which is a different phenomenon. The AR coating has a downside though if you want to work far into the UV.  Here are a couple of curves for different camera windows for example (note in this case this is the protective window on the camera, not the sensor cover glass

http://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2085&start=30#p11740

For optimum UV performance for spectroscopy (but not high resolution) some users are taking ATK414 cameras with no AR coating on the sensor cover glass and replacing the AR coated camera window with uncoated clear glass with good UV transmission

Robin

Edited October 18, 2019 by robin_astro
typo

[Thalestris24]

5 minutes ago, robin_astro said:

For optimum UV performance for spectroscopy (but not high resolution) some users are taking ATK414 cameras with no AR coating on the sensor cover glass and replacing the AR coated camera window with uncoated clear glass with good UV transmission

Robin

I probably won't ever be in that league, lol. I'll have to get the Lowspec built, tested, and up and running first. Even then, it won't be high res so I've probably no need to worry about minutae

Louise