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robin_astro

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About robin_astro

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  1. 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
  2. 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
  3. 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
  4. Also presumably the absorption of a photon imparts some small velocity to an atom to conserve momentum and again in recoil when a photon is emitted? Dont ask me to calculate the dominant mechanisms in a particular scenario though Robin
  5. There are a number of ways photons can transfer momentum eg to electrons, atoms and molecules. For example Directly through scattering By exciting vibration modes in a molecule Through radiationless transitions where electron excitation levels generated by photons are translated into vibration modes within the molecule The atoms/molecules then interact with each other throughout the medium (solid, liquid or gas) raising its temperature
  6. 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
  7. 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
  8. 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
  9. 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
  10. 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 !
  11. 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
  12. 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
  13. 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
  14. 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
  15. The observation that really made me go "yes!" out loud though was, after weeks of planning, watching the ISS streak across in front of Jupiter in 2004.
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