- This topic has 8 replies, 4 voices, and was last updated 1 day, 19 hours ago by
.
-
Posts
-
Good morning everyone
In order to examine some spectra of sunlight I need to know the Quantum Efficiency of my camera with which I have done some shooting. The camera is a 15 year old compact CANON IXUS60. CANON does not provide any information about the QE. I would kindly ask for help to get the values, even if approximate, of QE for the three RGB bands.
Thank you and best regardsAttachments:
I had a hunt online to try and work out what the sensor was in the camera, but to no avail. Hopefully someone else will be able to track down the sensor then it should be easier to find the QE… maybe.
James
In principle you may be able to measure it yourself if you can find another camera with a known QE curve. If you don’t have one to hand perhaps someone could lend you one. Unfortunately all my cameras are in La Palma.
Using the same optical system in front of the detectors and a source of identical brightness, measure the intensity of the spectrum and use one to calibrate the other.
Not saying it will be easy.
Note that even if you know the QE of the camera, this is not enough to flux calibrate the spectrum as there are many other effects between the sun and the camera sensor. The atmosphere and the grating efficiency are the main ones. You need a known light source to flux calibrate spectra. For astronomical spectroscopy this is done by measuring a star with a known spectrum as described in my document here
http://www.threehillsobservatory.co.uk/astro/Relative_flux_calibration_20221222.pdf
but that would not be straightforward with your setup.Can you describe in more detail what you are trying to measure about the solar spectrum ?
Cheers
RobinYou can approximately calibrate the spectrograph in relative flux using a tungsten lamp of known temperature which will have a black body curve but this will not include the effect of the atmosphere. If you then measure the sun at two different elevations you can then also estimate the effect of the atmosphere. You will need to calibrate the spectrum in wavelength first though
Cheers
RobinJames, Paul and Robin thank you for responding to my request. I must say that I am a little challenged by what I intend to start studying. In fact, the sun is not my goal. In addition to my study of the Earth’s Shadow, i.e. the Antitwilight sky, I would like to combine the observation of its spectrum. At least initially, for this I think a low resolution is sufficient. The spectrum I obtain is given by an instrument that is little more than a toy. However, it is necessary to be able to compare the pixel values of one channel (e.g. of a frequency on the Red) with those of another channel.
The fact is that I tried them all a bit: As a sample source, I tried a tungsten bulb. I obtained very uncertain results; the spectral energy distribution is too uncertain. There are QE curves of the nikon D70 and a canon on the web, which nobody owns yet, not even in the second-hand market.
Unfortunately, I do not own a telescope and therefore cannot take spectra of stars. Mind you, with the reconstruction of the city after the earthquake of 6 April 2009, floodlights are shooting light everywhere, so from my site the stars have all but disappeared. Robin, you have given me a lot of work to do, I will study your article thoroughly.
Includo un’immagine dello spettroscopio che uso e un diagramma dei PV di uno spettro costruito usando il soft matlab. The spectrum is bad, but for a broadband study it should be fine.
Again, many thanks to everyone.Attachments:
One thing you could do without having to calibrate is to compare the spectrum of the antitwilight sky with that in different directions. Just divide the two spectra to see the difference (Adjust the exposure so the intensity is similar to reduce problems with non-linearity of the camera and be sure to switch off any automatic colour correction in the camera).
Note though that some reflective gratings respond differently depending on the polarisation which changes depending on the direction in the sky. This is an example of the response of a reflection grating. S and P are for the two directions of polarisation. You can also see why you need to consider the grating response as well as the camera QE if you want an absolute flux calibration
https://www.optometrics.com/assets/M466Sales.pngI am not sure if the transmission grating (grism) used in the direct vision spectrograph you are using is sensitive to polarisation though. This shows typical curves for some transmission gratings but it does not show any effect of polarisation.
https://www.optometrics.com/assets/ARTx.png
The one in your spectrograph will probably be similar to the 600l/mm curveCheers
RobinI am not sure if the transmission grating (grism) used in the direct vision spectrograph you are using is sensitive to polarisation though.
Transmission gratings are not sensitive to polarisation so that is one thing less to worry about 😉
https://www.edmundoptics.co.uk/f/transmission-gratings/13496
Cheers
RobinOk, Robin. Yes, of course, the relationship between spectra has its significance, as does the simple comparison. With soft matlab, it is also straightforward to have them graphically. The problem arises when one wants to establish the relative weight between the pixel values (or relative intensities) of different bands or rather spectral frequencies.
Fortunately, polarisation is not detected. In that case I would abandon the study, I have no desire to tackle more advanced mathematical problems as well.
I spent the afternoon today in front of two spectra, zenith and 30° anti-solar. Tomorrow I will climb a peak of the Gran Sasso (2700 m) to take some spectra above the vapours of the valley floor.
- You must be logged in to reply to this topic.