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robin_astro

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  1. Hi Louise Small aperture APO's work really well with the Star Analyser. See Jim Ferreira's work for example http://www.lafterhall.com/spectroscopy.html I ran your setup using an SA100 and minicam5 through the calculator. With 5arcsec seeing, distances 20-46mm give all ok. For best resolution aim for the top end. For the faintest objects at lower resolution you might want to reduce the spacing. Larger distances (up to 60mm when it gets difficult to fit the spectrum in the frame) will still work ok though Cheers Robin
  2. I see the minicam5s is cooled and can do long exposures too. That makes it ideal for spectroscopy with the Star Analyser and will allow you to go fainter. Cheers Robin
  3. Hi Louise, Using the Star Analyser at too large a distance is a bit like using too high a magnification eyepiece. You get to a point where you dont see any more detail, things just get fainter. The calculator is just warning you that you have reached that limit. Colour cameras are not ideal for spectroscopy because in some regions of the spectrum (red and blue) only 1 in 4 of the pixels are sensitive due to the Bayer pattern. Also the red green and blue filter responses dont match perfectly so you get humps and bumps in the overlapping regions in the spectrum which should not be there. As a beginner you will find the mono minicam5 easier to get to grips with using the Star Analyser 100 (It was designed exactly for this kind of camera) Focusing is critical to getting good resolution but is tricky to learn and a fast updating webcam type camera is ideal for this. Being monochrome is an extra bonus too. (Software like RSpec can even give you a live view of the spectrum profile) With cameras like the minicam5 it is usual to just screw the grating onto the end of the 1.25 inch nosepiece (with spacers if needed to get the distance optimum) and put it directly into an eyepiece holder. The same is usually done with DSLR, using a T2 converter and a 1.25 inch nosepiece I know it sounds a bit counter intuitive but when used behind a telescope, the SA100 used at twice the distance works better than the SA200. This is because the angle the beam is deflected is less which gives less aberration and less focus shift along the spectrum. This is why I chose a 100l/mm grating for the original Star Analyser 100. The SA200 was developed later for situations where the larger distance cannot be achieved. (Note in the case where the grating is positioned in front of a camera lens, the aberrations are less so the spectrum can be spread out more to gain resolution. Here SA200 can have some advantages over the SA100. See here for list of pluses and minuses in that case http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm ) Cheers Robin
  4. Hi Peter, Which camera are you using now? Is this with the colour camera or the monochrome Lodestar? Your spectrum is ok but the line identification is not right. See here on this RSpec forum thread for more information on the problem of identifying lines in cool stars like Betelgeuse https://groups.yahoo.com/neo/groups/RSpec_Real_Time_Spectroscopy/conversations/messages/6188 Robin
  5. Hi Matt, Ah OK, from the original noisy spectrum, I thought it was going to turn out much hotter (A perhaps). This excellent spectrum looks closer to the catalogued type but it gets tricky at Star Analyser resolution in the late F/G/early K region. You have to be careful going just by the broad shape of the continuum as it can be distorted interstellar extinction. Precise classification is done by looking at the lines at 10A or more resolution traditionally , higher than the Star Analyser can usually give. I am not an expert on that though. Cheers Robin
  6. Hi Ken, This is not entirely true. While it is true you can subtract the sky background, it still adds noise in the spectrum and particularly in slitless systems like the Star Analyser, ultimately dictates how faint you can go. The limiting magnitude with the Star Analyser with my reasonably dark skies is ~mag 14. By using a slit spectrograph (which keeps most of the sky background out) I can get to ~mag 17 at the same resolution eg http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm Cheers Robin
  7. Hi Louise, If you can get the grating far enough away from the sensor then the SA100 works better than the SA200 in any case because the aberrations and shift in focus along the spectrum is less. (I developed the SA200 specifically for situations where you cannot get the grating far enough away from the sensor, like in a filter wheel for example.) 100mm distance with a Star Analyser 100 and DSLR should be fine. (What warnings did you get on the calculator?) You can see more about the discussion of SA100 v SA200 on my website here. http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm Cheers Robin
  8. Australian Rob Kaufman who took some of the comet spectra has some other impressive examples using this technique over on the "ice in space" forum, like these spectra of Mira and Eta Carinae (How I wish we had something like that in our northern skies !) http://www.iceinspace.com.au/forum/showthread.php?t=153450 Cheers Robin
  9. Hi Louise, Using the Star Analyser as an objective grating in front of a DSLR lens is an interesting arrangement as it generally gives a sharper spectrum compared with mounting behind a telescope. It is good for some extended objects too like comets for example as they appear star like so the spectrum can be recorded. It is only for brighter objects though as the effective aperture is very small ~25mm. It helps if you can track so you can take longer exposures. You can see a description of the technique on my website http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm and a discussion on the relative adavantages of the SA100/SA200 using this arrangement here http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm and there are some comet spectra taken using this type of arrangement included in a talk I did for the BAA comet section a while back here (eg slides 6-8 Not my observations, credit to the original observers is on the slides) http://www.britastro.org/projectalcock/RL spectroscopy_BAA_comet_section_may13.pdf Cheers Robin
  10. Hi Peter, Good to see you are managing to make some progress. (No Methane in Sirius though. This is just one of the bumps from the colour camera response) Your Denebola posted on "Cloudy Nights"/BAA forum is even better Robin
  11. Just measuring the redshift of a galaxy is not sufficient to determine the rate of expansion. Measuring the redshift of distant galaxies spectroscopically is not the problem. It is knowing the distance. In order to determine this you need for example to measure the apparent brightness of objects with known luminosity (ie "Standard Candles" like type 1a supernovae for example) Robin
  12. Very nice ! I had a look in Skiff's database to see what classification these two stars had been given. http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk The brighter one (HD 94363) is variously classified around late G / early K which agrees with your spectrum but there is only one classification documented for the fainter companion (BD-01 2458) as G0v which is clearly incorrect from your spectrum or the visual appearance so it could be interesting to try to get a less noisy spectrum. Cheers Robin
  13. Hi Peter, I think you need to work on your focusing to bring out the Balmer lines more clearly. Compare with this spectrum using the SA200 recently posted on cloudy nights for example http://www.cloudynights.com/topic/569031-all-the-baby-stepsin-a-sprint/ If you have a focal reducer, use it with your 10 " SCT to reduce the size of the star image. Focus on the zero order first then move the focuser in slightly until the H beta line is sharpest (The zero order will then be out of focus) Cheers Robin
  14. Hi Peter, Do you mean how do I make my spectrum match the shape of the spectrum of the same spectral type in the VSpec library ? See my reply on the BAA forum and also the recent post in this forum https://stargazerslounge.com/topic/287199-ccd-sensitivity/ Do these answer your question ? Cheers Robin
  15. Yes many things affect the shape of the spectrum (the main ones are the response of the grating and the camera sensor but the telescope and spectrograph optics also have an effect) The absorption of the atmosphere (which is greater in the blue and depends on how high in the sky your target is) also affects the spectrum. All these are usually corrected for by measuring a "standard" star (so you know what the spectrum should look like) at roughly the same location in the sky as your target (so the affect ofthe atmosphere is the same). If you then divide the standard star spectrum you measure by what it should look like, the result is the "instrument response" You can then apply this to the spectrum of your target to correct it to produce the true spectrum. You can see the steps using the Star Analyser for example in a couple of presentations I have given at BAA workshops. (Using Visual Spec and ISIS software here but other software like Rspec or BASS can be used. http://www.threehillsobservatory.co.uk/astro/spectroscopy_10.htm slides 31-48 http://www.shelyak.com/dossier.php?id_dossier=36 Low resolution slitless spectroscopy - observing a fast transient of a T Tauri star, Robin Leadbeater (3MB) Cheers Robin