Imaging Techniques
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Overview  Polar Align  Focal Length  Focussing  Find & Take  Processing  Color Imaging

Before I began imaging I read two books which proved very helpful in getting me started.   In the following webpages I have summarized the equipment and techniques that I currently use to take my images.  Hopefully this information will prove useful to you.  The imaging techniques and equipment you need to use vary slightly depending on the type of imaging you want to do.  The two main types of imaging are Deep Sky and Planetary, Lunar and Solar imaging

Deep Sky Imaging

  • You need to choose a focal length  appropriate for the object you wish to image.
  • You need to be able to achieve good focus on the object
  • You need to be able to find and centre the object on the CCD chip.
  • Good quality deep sky images require long exposures (e.g. up to several hours).  This is because objects such as galaxies and nebulae are very dim and a long exposure is needed to collect enough light to form a good image.  It is therefore important to have a good polar alignment so that the guide corrections needed during the exposure are minimized.  Fortunately with CCDs you do not need to take a single image lasting several hours.  It is better to take say 24 images of 10 minutes each and add them together afterwards using image processing software.  This helps minimize the affect of any misalignment of the polar axis and RA drive tracking errors.  The other advantage, is that combined with a technique called dithering it allows you to use statistical rejection techniques as you combine the images which can remove hot/cold pixels and satellite trails.   The need to take long duration exposures, rules out the use of webcams for imaging all but the brightest of deep sky objects as they do not have the capability to take images of more than a few seconds at best.

  • Taking a color image with a monochrome CCD camera requires separate images to be taken through red, green and blue filters.  These images can be combined using image processing software to create a true color image.  Alternatively a single color image can be taken if you use a color CCD camera such as the Starlight Xpress MX5C, a color webcam  or Digital SLR.

  • During a long exposure, noise (known as dark current) builds up on the CCD chip.  This noise would severely affect the final image.  Fortunately it can be subtracted from the image using dark frames.  Even with this technique it is still good to minimize the dark current generated on the chip.  The dark current generated is proportional to the temperature of the chip.  The lower the temperature of the chip the lower the dark current.  So for the best quality deep sky images it is best to use a CCD camera with the ability to cool the the CCD chip to around 30 degrees Celsius below ambient air temperature.  CCD cameras from Starlight Xpress and SBIG have this ability, whereas webcams and Digital SLRs do not.

  • Taking Flat Field images is recommended to achieve the highest image quality.
  • Once the images have been captured, the final step is image processing

Planetary, Lunar and Solar Imaging

  • You need to choose a focal length  appropriate for the object you wish to image.
  • You need to be able to achieve good focus on the object
  • You need to be able to find and centre the object on the CCD chip.
  • Good quality planetary images require short exposures (e.g. a few tenths of seconds or less).  This is because the planets, moon and sun are very bright compared to deep sky objects such as nebulae and galaxies. The short exposure has the added benefit of 'freezing' atmospheric turbulence allowing finer detail to be resolved than in deep sky images.  Important: When taking an image of the Sun, the use of a solar filter is essential.  Looking at the Sun through a telescope without a solar filter, even for a second, will cause permanent blindness.  Using CCD imaging equipment to image the Sun without a solar filter will damage the equipment beyond repair.

  • As the exposure duration is very short, a good polar alignment is not critical as the the object will not drift much in a few tenths of a second.  However, a good polar alignment will help keep the object on the CCD chip as you take multiple exposures.

  • The key to good high resolution planetary, lunar and solar images is to take as many images as possible as quickly as possible.  If you can take a large number of images you can sort through them afterwards and keep the ones that were taken at the best moments of seeing and so have the most detail.  These 'best' images can be added together using image processing software to produce an image with much less noise than a single image.  This permits more detail to be enhanced using image processing techniques such as Unsharp Mask.

  • The need to take a large number of images quickly means that webcams are ideally suited to planetary, lunar and solar imaging.  Webcams can take tens of images per second.  It is not uncommon for an imager to take 3,000 images of an object using this technique and then sort through them later looking for a few hundred 'best' images that can combined.

  • Taking a  color image with a monochrome CCD camera requires separate images to be taken through red, green and blue filters.  These images can be combined using image processing software to create a true color image.  Alternatively a single color image can be taken if you use a color CCD camera such as the Starlight Xpress MX5C, a color webcam  or Digital SLR.

  • Even during a short exposure, noise (known as dark current) builds up on the CCD chip.  This can affect the quality of the image.   Fortunately it can be subtracted from the image using dark frames

  • Taking Flat Field images is recommended to achieve the highest image quality.
  • Once the images have been captured, the final step is image processing