Residual Images / Ghosting
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Bright Stars/Objects, Blooming and Residual "Ghost" Images
**Please note: Some of the information in this section is very important as it refers to observing practices that you may carry out that could inadvertently affect the observations of subsequent users of the telescopes. You should read this section carefully**
The Faulkes Telescopes (FTs) are research class telescopes with tremendous light gathering power. It is sometimes easy to forget that many of the common objects that we know of in the sky are far too bright for the telescopes to image. For example, the Moon has an apparent magnitude of about -12 and many of the stars that are visible to the naked eye are around magnitude 6 or brighter. All of these objects are too bright for a FT camera to easily image successfully and they should be avoided.
The purpose of this web page is to show you what happens when these bright objects are imaged, explain the science behind the resulting images and help you avoid these problems.
The image below (M16 - The Eagle Nebula) shows something known as "blooming".
The camera on a Faulkes Telescope contains a charge coupled device (CCD) chip. The CCD is the part of the camera that receives the light collected by the telescope and converts this into something that electronic systems can turn into an image. The CCD in an FT camera contains around 4 million pixels. Each pixel receives light in the form of photons and converts this to charge. Each pixel is effectively a "charge well". If the pixel receives too many photons (i.e. bright light) then the charge well becomes full. At this point, charge can overflow into neighbouring pixel charge wells. This causes those pixels to appear brighter than they should be and in extreme cases can cause the familiar streaks that are often seen coming from stars such as the image of M16. The reason the streaks appear vertically is due to the structure of the CCD allowing charge to overflow more easily in one direction than the other.
Another, and more irritating phenomenon, are residual images or "ghost" images. The two images below show an image taken of a 1st magnitude star and then an image taken afterwards of a distant galaxy. The vertical streaks on the second image are caused by a residual charge or "ghost" image left by the star in the first image.
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In the case described here the FT team investigated and found that the residual image lasted for about 4 subsequent images before it apparently disappeared. Each time the residual image got fainter until it was not detectable. Whilst this is clearly something you would want to avoid, it is also worth remembering that it may not only be your images that it may spoil. It could also appear on the images of a subsequent user.
The problem occurs when a bright object is observed and the pixels become saturated with charge. If the saturation is very great then when the CCD is "read out" not all charge is removed from the pixels. Over time the charge is removed in the read out of subsequent exposures.
**You should avoid imaging bright stars - users who cause this problem to occur and affect other users images will be contacted by the Faulkes Telescope Operations Centre to discuss their observing procedures**
