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Everything posted by dph1nm

  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. According to another forum It has large inherent periodic error, so you really need to guide. NIgelM
  9. 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
  10. 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
  11. 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
  12. 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
  13. 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
  14. The Skywatcher Aplanatic corrector is very good in my f4 Quattro (for which they are designed). Not sure how well they work at f5? NigelM
  15. Try http://www.astro-baby.com/EQ6 rebuild guide/EQ6 Development and Software Releases.htm NigelM
  16. I think vlaiv's picture answered that! I cannot see the difference between the two noise fields, so I personally would be quite happy with a sky 5x the square of the read noise (when measured in electrons of course). After all, what matters for amateur imaging is what the final image looks like - if you were doing precision photometry you might make a different choice. But it is all a matter of personal taste I suppose - I prefer my exposures to be as short as possible. NigelM
  17. Yes, but my advice is not to do the maths on how long it would take or you will just get depressed! Enjoy what you have ... NIgelM
  18. Seems very over cautious to me! 8.7e- RN is equivalent to 75e- signal. A sky 5x this signal (not noise) - i.e. 375e- will only see the overall noise increase by 10% or so due to the RN contribution and I am pretty certain most people could not spot such an effect NigelM
  19. Although you get 4x the signal (per binned pixel) the noise also goes up by the sqrt of this. So your S/N per binned pixel improves by 2x not 4x. I suspect this is where some of the confusion comes from. Of course, the total number photons you collect from an given object has not changed - only exposure time can alter that. NigelM
  20. Cleans the sensor of dust - and very effective it is too. NigelM
  21. I think the EQ5 has a 10min worm period, so it should be easy to tell if this is your issue as the stars will move back and forth in RA on this timescale. You will have periodic error, it is inevitable. It can be pretty large - e.g. several tens of arcseconds. NIgelM
  22. Beware of DSLR histograms! I did some tests with my Canon 1000D and discovered that it only shows 8-bits of data and the right hand end of the display corresponds to 1024. But saturation in RAW on this camera is 4096. So setting flats to 1/3 would result in a very underexposed flat. In fact, even if you hit the right hand end you are only using 1/4 of the available well-depth! NigelM
  23. Probably the secondary dewing up. I have had that problem (and those exact symptoms) with both Newtonians I have owned. Solution - a hairdryer, but I do have mains power available. NIgelM
  24. Yes - obviously if there was no background noise at all then you would do best with an infinite aperture (i.e. collecting all possible photons). If background noise is the dominant source of noise then it comes out at about 0.67*FWHM (for a Gaussian PSF) http://wise2.ipac.caltech.edu/staff/fmasci/GaussApRadius.pdf NIgelM
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