Taking night images

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This section is mainly to add notes and information that is useful during planning an observing session and then working on it.

A. Deep Sky

  1. Choice of camera
  2. Using SGP
  3. Using PHD
  4. Exposure time versus number to stack

           Choice of camera

  • Use Trius H694 or Trius Pro 35 according to the field of view required. With this set up and the large filter wheel in place the telescope will need rebalancing, if previously used for lunar work. This involves sliding the tube along the dovetail. Currently this is a two person job.

     

           Using SGP

  • Load the file and make sure it has loaded the correct equipment file.
  • Be sure to connect all equipment before starting.
  • Move to a star nearby and centre it. Then sync with Carte du Ciel.
  • When it now slews it should get the Plate Solve software to work OK.
  • SGP gives up if the scope is already at the coordinates of the object it is trying to slew to, so always start a small distance away. This is bizarre but how it is.
  • It is a good idea to close the plate solve window if it is doing more than one adjustment. Seems to work better that way.

          Using PHD

  • Occasionally PHD seems to wander too much. Then a re-calibrate is needed. Remove the old calibration data then it will automatically recalibrate when used. Use an area near the galactic equator and it then tends to work really well.
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          Exposure time versus number to stack

  • This is difficult to gauge. Generally, it is better to bin the luminance at 1×1 and the RGB 2×2. This makes for shorter RGB exposures. Also, since the RGB are just supplying colour information (at least that’s the way photoshop seems to work) the number of exposures can be more limited. If there is noise in the dark areas of the sky it does not matter if the luminance image is free of noise in those areas.

    With galaxies and nebulae it seems that 300-600s is needed with as many frames as can be fitted in the observation time. Although there may be parts of the image that are bright the fainter outliers need this longer exposure. In cases like the Orion nebula, it may be that two separate exposures are chosen with the shorter on used for the brighter areas. Then use layer masking in photoshop. It seems logical, to me at any rate, that if the exposure is short and there is not much of a signal in the pixels, then no amount of stacking will do much. Yes, there will be low noise, but the final image will be difficult to pull out.

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A. The Moon (and Planets eventually)

  •  The camera will be one of the ZWO cameras. The ASI 120 to take small areas, for example of individual craters, or the ASI 174 to make a mosaic of the whole moon with many fewer frames to align. Use a red filter in the camera. If helps a lot with a wobbly atmosphere.

    The capture program used is SharpCap which takes a movie in either avi or ser format. (Does not seem to matter which). The number of frames will depend on the seeing. If the moon is low in the sky or the seeing is poor then about 1000 plus frames are best to take as only about 5% will turn out to be useful. If using frames to mosaic, set the exposure with a frame that will have the brightest areas and set the exposure so the histogram reaches almost 100%. Then use this for any other frames in the mosaic. Otherwise balancing the images in the mosaic is difficult.