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About dph1nm

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    Brown Dwarf

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  1. Err, well it took me a while to see it, and I am not sure I can help much. With my DSLR I usually point the telescope relatively near the zenith at dusk and wait until 0.5s exposures just saturate the back of the camera histogram (in blue of course, not green or red). Note that on my 1000D 'saturation' in the histogram is actually when the jpeg saturates - there is another two stops (4x) to play with in RAW. I usually aim for about 40 before I get bored (or it gets too dark). Professionally it is easy - you just look up a suitable 'blank' field and then ask the night assistant when to start observing or sneak a look at the logs to see when the previous observers did theirs)!! I have also used night flats professionally - they waste too much time amateur-wise given how little clear sky I get round here. If you targets are small and you dither (or you take lots of different fields) then they can work extremely well. Obviously they do not work well if you are imaging a large nebula! NigelM
  2. Remember that if you are dominated by read noise, which this shot probably is, then lower ISO on a 450D is bad news - it just makes the signal-to-noise worse. The only real answer is to take multiple shots and stack them. If you do use a tracker then I would take a separate static shot for the foreground stuff (with the same total exposure as your tracked shots) and combine the two in software. NigelM
  3. What does the entropy weighted stacking do? It seems this is designed to combine data taken with different exposure times or on different nights. NIgelM
  4. When I started CCD imaging many,many years ago this is exactly what was done. An average value was calculated from the bias frames (or indeed the overscan region) and subtracted from the lights and flats. We never subtracted bias frames to avoid adding noise. It worked perfectly well. NigelM
  5. Both CMOS & CCD manufacturers tend to add a constant value to the image before it is read out, in order to avoid negative values. This needs to be subtracted (both from flats and lights) before flats will work correctly. Easiest way to find out what this value is is to take zero-length exposure (i.e. with no light getting in), known as the bias. For DSLR cameras the best approximation to 'zero-length' is to use the shortest shutter speeds available with the camera in the dark. You could use darks and flat-darks instead, as darks also have the bias signal added, so subtracting a dark gets rid of bias as well. NIgelM
  6. I happily re-use flats, even taken months previously. The only issue is if there are significant dust spots, but my Canon does sensor cleaning whenever you switch it on and off, and there are very rarely any visible dust spots. If there are they can be fixed in post-processing. NigelM
  7. As I read it, the main benefit of harmonic drives is zero backlash. They still have periodic error, and as with most mounts it would seem that the more you pay the better it is. NigelM
  8. This is because stars are essentially single pixels at this resolution - so DSS tends to think they are hot pixels! It is possible to get caught by this on shots at much longer focal length as well, if the seeing is good. I have had the central pixels of stars removed at 1200mm focal length! For this reason I am very wary of the cosmetic cleaning, but if you turn it off all together I find that some real hot pixels get through. NigelM
  9. According to another forum It has large inherent periodic error, so you really need to guide. NIgelM
  10. Note that Skywatcher do two coma correctors at very different prices. The expensive one is the Aplanatic, which is optimised for the Quattro f/4 scopes. I can vouch for the fact that this one works. NIgelM
  11. Because this encourages people to think that it is their camera which is the cause of all the noise, and that by spending lots of money they will do better. In fact the signal from the source really does fluctuate and there is nothing you can do about this. NIgelM
  12. A hairdryer (I am in a mains powered observatory)! Which usually works, although you do have to wait a few mins for the focus to settle back down. NigelM
  13. Sorry vlaiv, but this is misleading. Software binning does not change the S/N per sq arcsec on the sky, so the two halves of your images should look identical. It looks like you threw away 3/4 of the data when downsampling the unbinned shot. NigelM
  14. DSS needs 8 stars in common (at least) to stack frames. If you can't get it to detect 8 due to trailing etc then try selecting superpixel mode for the debayering. In crude terms this reduces the resolution by 2x so makes it easier for DSS to detect stars. NigelM
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