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Depending on what telescope and what camera you are using generaly most CCD cameras will focus when you place them into your telescope.
However if you find you cannot obtain focus the you either need more or less travel in the focusing range of the scope.The most common being lack of out focus travel by this I mean the telescope fosucer needs to come out more, I have found that the Meade DSI cameras on my C6 SCT will not focus if I place the camera straight into the back of the scope.

I need to use other bits of equipment and these are A: Diaganol B:focal reducer C: a barlow lens with the glass lens removed using only the tube as an extention tube.Either one of these enables my DSI to obtain focus.

I would think that this principle would apply to other CCD cameras, The DSLR however sometimes needs more in focus travel and this is a problem many people suffer from, low profile adapters usualy help, I have not personaly used a DSLR so I really cannot comment too much on this subject.

Make sure you do this in daylight to test your focus, as there is nothing worse that messing around in the dark wasting valuable imaging time, do it during daylight on a distant tree or hills or whatever you can obtain focus with and remember this make a note if you have a couple of telescopes i.e refractor, SCT,Newtonian etc this will soon become second nature.

Once you have obtined focus during daylight when you come to focusing at night also make sure you use a focusing mask this will enable you to obtain near perfect focus, remember focusing shifts during the evening, due to movement of the scope and fast slewing not to mention seeing, seeing effects focus too, just because you focussed on a nice bright star early evening might not be the same when some thin cloud rolls over creating all sorts of effects. So it's worth while checking focus from time to time during the evening.

Use a bright star with the mask don't try to use a dim star you will not be able to see the focusing marker very well on dim stars, this I have found to be a problem when using my Williams Z66 the stars are very small and difficult to see the focusing marker properly.

Incorrect focus will show up dust bunnies and dirt in the optical train more. Focus is very critical with the planets especialy with bad seeing, you cannot use a focus mask on a planet it just doesn't work, so if the moon is about first focus on the moon then go onto jupiter, saturn etc if the moon is not present then you may need to slew to a really bright star and focus using the mask. 
Another thing to remeber is make sure your camera is seated flush when you place it into the scope or diaganol, if the camera is slightly offset when you tighten up the screws then your stars will look like miniture comets!! usually obvious straght away but not always!.

Is yours floppy!! Mirror flop with larger SCT's is another problem this happens when the temperature drops and the mirror which is quite heavy decideds to slip down, a good remedy for this is to always focus back, in other words focus in past the focus point then focus back into focus this way you let gravity do the flopping for you under control, this then should not drop during the session flop happens more or less when you are imaging near the zenith and all the weight is being pulled by gravity.


See link  explaining the most common and accurate focusing mask





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