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About ollypenrice

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  • Birthday 12/02/53

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    Imaging, Cycling, Thinking, Literature, French culture, Mountains...
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    South east France, Lat 44.19N.

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  1. I'm not sure what point you're making? I don't know the stats for manufacturer returns but there are only two makes on that list of four which I would buy myself. While a Class A chip would be nice, I honestly think that it would make no difference to a final amateur astrophoto. Using AstroArt 5 to stack and calibrate, I can plug in a column (identified by number) for repair and it simply vanishes from the stack. Likewise sigma clip will take out rogue pixels from any dithered set and replace them with the average value of the rest of the set. My chips are noisy but, with the exception of one old camera which has a row of 4 dead columns, I normally have no pixels to clean up at the end. Sure, it would, as I say, be nice to have a perfect chip but once you have the right calibration it simply doesn't matter. (For me the 'right calibration' does not include darks. I use a master bias as a dark, a Defect Map and a hot pixel filter. This is dead easy, the same calibration files working for all exposure lengths.) Frugal's quotation from Atik is on the money, I reckon. Olly
  2. I think it's true that CCD is stagnating in research terms. Sony are not going to keep on making them, they say. Olly
  3. I don't think it matters. A grade A chip makes precious little difference since we use calibration files, dithered guiding and cosmetic repair. I don't know anybody who specified a grade A in buying a camera. A few gamma ray hits and it's a standard grade anyway! I had the cleanest normal grade Kodak 11 meg known to man - for about three weeks. No good keeping it in a lead box, though. I'm sure the cameras are expensive simply because they are in very low volume production. Lots of work is done by hand and a small number of sales need to fund all the R and D. I know the CCD chip is the expensive but but I wonder how much the manufacturers pay for CCD and CMOS? Olly
  4. Hehheh, I opened the thread and thought, 'ere, wot's this?' Ollyu
  5. Lovely image. I asked because the effect on F ratio would be a bit of a killer for DS. Olly
  6. What are you aiming to image? Olly
  7. I posted on the old thread by mistake. Focus using a star located at the intersection of the 1/3 lines. (Lines parallel with the chip sides and a third of the way to the opposite side.) These intersections will give you the best average focus across the chip. It does look like tilt, I must say. I don't think using the camera in a scope tells you much. The slower F ratio gives a far greater depth of field which will mask a tilted chip. Olly
  8. Focus using a star located at the intersection of the 1/3 lines. (Lines parallel with the chip sides and a third of the way to the opposite side.) These intersections will give you the best average focus across the chip. It does look like tilt, I must say. Olly
  9. Very attractive. Your extra chip size has paid off on this one for sure. Olly
  10. It really is a bit complicated, this one. For instance, we actually know nothing about what is happening between guiding commands. We only know the position of the guide star on the chip as each new command is sent. Avalon claim that their belt drive mounts, having no backlash, return very quickly to the target position on receiving a command. They are, therefore 'on target' for longer between guide commands than conventional mounts. I'm prepared to believe this because our Avalon does seem to me to produce better results than the guide trace suggests. It's hard to quantify this, though. And then we must remember that we also know nothing about where the mount is really pointing. We only know where the guide star is on the chip, and the guide star can be moving around due to turbulence. So if we take lots of very short and frequent guide subs we may get a better guide trace. This, alas, ignores the fact that what we have is just a good trace of a star dancing about in the seeing. We would get better real guiding by using longer and less frequent guide subs in which the turbulence has been averaged out. But the rule of thumb (guide error to be no more than half the pixel scale) seems reasonable to me. Olly
  11. There are several. Voila! Olly
  12. I agree with Steve. The theoretical resolution of your system is measured in arcseconds per pixel, meaning, 'How many arcseconds of sky land on one pixel?' The smaller the piece of sky sampled by one pixel, the more detail you will theoretically resolve. (If there is an arcsecond of dark sky between two faint stars then I cannot separate them with our Twin Tak rig because it operates at a coarse 3.5 arcsecs per pixel. Both the faint stars and the gap between them land on the same pixel.) If I use a long FL system working at 0.6 "PP then I can separate the stars. * All of this is theoretical because if the guiding is moving around by an arcsecond during the exposure, the arcsecond gap will be blurred out and will disappear from the final image. In fact, as Steve says, the accepted rule of thumb is that your mount needs to track with about half the pixel scale of the camera-scope in order for the theoretical resolution to appear in the picture. I don't know how this 'accepted figure' is arrived at but I just accept it! (Baaas like a sheep. ) Even if your mount can track without any error whatever you also need the beam of light coming down from the stars to be undistorted by the atmosphere, and it won't be. An imaginary beam from a point source can easily be stirred around so that instead of landing on one pixel, over a few seconds, it lands on several. 0.74"PP is not going to be easy to realize. Also to retain your Quattro F ratio with a long focal length is going to need a BIG scope! (Of themselves focal lengths tell you nothing about exposure times. You can have a fast long FL scope but it won't go on an EQ6! Good image. Working at a long FL it all gets harder and (very easily) very expensive... Olly * In fact stars will never land on one pixel but I'm using the one arcsecond gap as a straightforward way to explain resolution in theory and practice.
  13. 0.68"PP is a big (a very big) ask. I've worked at 0.66 in conjunction with Yves and his 14 inch Dall Kirkham. We were mounted on a Mesu 200 with excellent tracking under autoguiding. Even so, I'm not convinced that we were really acheiving the theoretical resolution. (We had intended to do a lot of binned imaging but the camera wouldn't bin properly.) You can still get a good result even if you can't actually realize 0.68 but, given that the RC can be difficult to collimate and that a coarser sampling rate might still have you at the limit of the guiding and seeing, you might consider the alternative of a slightly shorter FL in an easier optical system. Just a thought. If you got results like John's you'd be a very happy bunny but not everyone does! Olly
  14. I'd certainly do a test without the wire. That would, if nothing else, eliminate or confirm it easily. I don't like the look of the unblackened focuser retaining bolts either. Just in principle I'd blacken them. Use stove or barbecue matt black because those paints use pigments rather than dyes which can be reflective in some wavelengths. If the wire is the culprit it might be worth not running down the vane in a straight line but, counter intuitiviely, in a wide curve to smear out the diffraction effects. Olly
  15. Ah, for NB I think DBE is less important. I never use ot on NB data myself. Olly