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sharkmelley

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About sharkmelley

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    Tenterden, Kent
  1. sharkmelley

    Determining Camera Tilt

    As you rotate the camera, the sharpest stars are moving from corner to corner. I agree with your diagnosis: the problem is not camera sensor tilt. Mark
  2. Try this: http://www.aicccd.com/archive/aic2008/CCD_stack_calculate_gain_by_stan_moore.pdf Or this: http://casa.colorado.edu/~bally/Current_Course/ASTR_3510/Useful_documents/CCD_Gain.pdf However, there's one complication. If you are using a one-shot-colour camera or DSLR then it is essential to separate the colour channels and do the calculations on each colour channel independently. Mark
  3. sharkmelley

    Determining Camera Tilt

    Here's a different way to explain it, using an (improbable) thought experiment. Suppose someone had somehow bent the one of the corners of your sensor - impossible, but bear with me! This causes the stars appearing in the bottom left of the image to appear distorted and out of focus. Now rotate the camera. Different stars will now occupy the bottom left corner of the image and they will be the ones that are distorted and out of focus because it is the bent corner of the sensor that is causing the problem. Sensor tilt is simply a less severe version of a bent sensor! Mark
  4. sharkmelley

    Help with Aberration

    First check there are no aberrations without the CCDT67 in place. Secondly check the distance of the sensor from the reducer. Thirdly check that neither the reducer nor the sensor is tilted with respect to the main optical axis of the scope. Mark
  5. sharkmelley

    Don't buy the Sony A5000/5100 for astrophotography

    It's nice to know there are multiple solutions. But a proper menu option would be far easier! Take the Sony A7RII for instance - the "star eater" spatial filtering can be worked around by putting the camera in continuous mode. But try it on the A7RIII and you still get the star eater! Go figure. Mark
  6. sharkmelley

    D7500 has high thermal pattern noise

    Having looked at this in more detail, it's certainly true that the D7500 is using less aggressive spatial filtering than the D5300. However this does not explain the observed difference in thermal fixed pattern noise (FPN). The D7500 really does appear to have a higher FPN than the D5300. The practical consequence of this is that you are more likely to need dark frame calibration with the D7500 than with the D5300. Mark
  7. sharkmelley

    a7s star eater with M33?

    Your Sony A7S will definitely have the star eater in bulb-mode exposures because Sony has not released any firmware update that switches it off. However, it's only a problem for very tight stars. A Celestron 8" scope is a long focal length and will produce relatively large stars which are relatively free of star eating behaviour. Mark
  8. sharkmelley

    D7500 has high thermal pattern noise

    Thanks for reviving this thread. I've been playing with a Nikon D5300 recently and noticed that the spatial filtering (a.k.a. hot pixel suppression or HPS)was causing all kinds of problems with star colours - mainly turning them green, just like the latest variant of the Sony star eater algorithm. It's all being discussed over on Cloudy Nights: https://www.cloudynights.com/topic/635441-aa-filter-spatial-filter-and-star-colours/ It's early days and it's not yet fully understood. However, it appears that the Nikon D810A (the one built for astrophotography) doesn't have the same issue because the spatial filtering is much less aggressive. So it's just possible that the D7500 is also using less aggressive spatial filtering than the D5300 and that's why it appears noisier. I'll take a new look at Mike's D7500 darks and see what I can determine. It'll definitely provide another interesting data point for my analysis. Mark
  9. sharkmelley

    Oh God is this the Future?

    In geostationary orbit, the mirror would need to be 500 metres in diameter just to give the brightness of one full moon. Unfortunately, because of the distance of geostationary orbit, it would illuminate an area at least 350km in diameter. NASA produced a technical paper on it in the 80's which included a map of the UK showing what happens for a geostationary mirror - it would illuminate an area including Blackpool to Dover, Aberystwyth to Gt Yarmouth Mark
  10. sharkmelley

    Oh God is this the Future?

    The latest reports say that this mission will be in an orbit 500km high: https://www.bbc.co.uk/news/world-asia-china-45910479 This is not much higher than the ISS and means it will cross the sky in a similar time to the ISS i.e. a few minutes. That means it can illuminate the town of Chengdu for at most a few minutes every 90 minutes. That's assuming they can control the direction of the mirror fast enough. The mission is clearly a proof of concept. Mark
  11. These look like the typical halos caused by a double reflection within a glass filter or glass window. The distance given by the dust donut calculator will be the extra optical path light travels during this double reflection. But halving this distance won't give you the actual glass thickness because there's still a scaling required for the refractive index of the glass. Mark
  12. sharkmelley

    Review of the Sony A7S

    The A7S doesn't have an option to display integrated exposures. You would have to download them to a PC and use something like AstroToaster to do the live integration. However, I've never tried it. The way I made that video was to put the camera in Movie mode, turn the ISO up high and turn the video frame rate right down to 4 frames/sec. Mark
  13. Inaccurate polar alignment? Flexure between main scope and guide scope? Focus tube droop with gravity? Mirror shift (on SCT scopes). Plenty of potential causes. In any case, the original poster earlier confirmed that the star field was indeed drifting from sub to sub (see post #3 in this thread). Mark
  14. sharkmelley

    OSC to suit RC10

    You want an APS-C sized sensor to make the most of the RC10's corrected field of view. If you can afford it, definitely go for the QHY 168C or another camera with the IMX071 sensor e.g. ZWO ASI071. It has low read noise and double the QE (quantum efficiency) of the QHY8, SXVR-25MC and QHY10. Doubling the QE means halving your total imaging time. Unfortunately I don't have any comparison info for the QHY12 i.e. how its Sony ICX613 sensor compares to the Sony IMX071 or even how it compares to the QHY10. QHY doesn't appear to publish QE data. Mark
  15. It's walking noise and walking noise is usually generated by bright pixels "walking" across the image. But these are dark pixels "walking". So what is causing those dark pixels in the first place? Are you using darks and is it possible that the master dark is badly matched to the data? Mark
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