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It is quite easy to focus a ccd camera with the help of diffraction patterns caused by an objective mask.
Such masks are either realised by aperture plates or gratings. I use the old grid of a barbecue and
put it in front of the telescope. The procedure of focusing is explained here, see also pictures:
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A bright star (about 2-3 mag) is centred on the ccd chip. This star should be near to the object
to be exposed after focusing.
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The grid is attached to the scope carefully.
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The parameter of the ccd control software are entered to allow a quick and continuous read-out of
the image. Exposure time should be below 1 second, binning is 3x3. If necessary the image size is reduced
by the software.
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At the beginning the ccd is not in focus normally. The image of the star is a bright disk.
If you use a SCT a ring appears instead of a disk because of the secondary mirror.
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For some type of telescopes, e.g. refractors, it is not important from which side the focus
will be reached - not so for SCTs: best procedure is an approach by moving the mirrow upwards,
corresponding with a counter-clockwise rotation of the focus knob. If you have a SCT, you should
check first if the ccd is on the correct side of the focus. Rotate the knob counter-clockwise and
observe the image of the star. If it increases, you have to go to the other side of the focus.
In this case, rotate the knob clockwise until the image of the star increases.
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Approach to the correct focus should be done carefully by rotating the focus knob counter-clockwise.
Stop, if the start is nearly point-like.
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Before doing the last step, check your control parameters. Set binning to 1x1 or 2x2 and adopt the
size of the read-out image. Exposure time and contrast should be optimised to show the
diffraction streaks on both sides of the star.
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Move the focus knob clockwise very carefully and observe the image on the PC screen. The streaks will
move closer together and will form a sharp line on both sides of the star at the end. Now, you have
reached the focus.
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If you have a SCTs rotate the knob for another 1/30 of one revolution (counter-clockwise), to take
into account that this type of telescope does not produce a flat field.
Focus has to be controlled during the night, especially for SCTs where the focus can shift if the
scope is moved. If necessary, focusing has to be repeated.
Fast telescopes may show different focuses for different colours. Therefore, the focusing procedure
may be applied for each filter when taking LRGB image.
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