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Gadget or Chip?

There are two broad categories of focus methods, indirect and direct. The indirect methods use a separate device to focus the camera whereas the direct methods use the CCD itself.

Indirect methods (Gadgets)

The Cutting Edge?
Pity the poor film astroimagers! They do not have direct and immediate access to the imaging surface. As a result, they have developed less direct but quite effective methods such as the use of a knife-edge focus device to focus their film cameras. It should be possible to build one of these for your CCD camera, but probably is more work than it is worth since using one of these is no more accurate and a lot more effort than the more direct methods.
Focussing Eyepiece
Remember eyepieces? Some imagers use an eyepiece designed to be parfocal with the CCD. The idea here is that you focus visually with the eyepiece, then remove the eyepiece and insert the camera which is now (theoretically) focused.
The Test
I decided to see how accurate the eyepeice method was. I used a 12.5 mm Ultima eyepiece with a locking collar for the test. First I placed the camera in the drawtube tightly against the scope and focused the camera at that point using my normal (diffraction) technique. I then removed the camera and inserted the focusing eyepiece. I slid the eyepiece in and out until it was focused and then locked the eyepiece collar in that position. Next, I randomly twirled the focus knob out of focus and refocused with the eyepiece. I then removed the eyepiece and replaced the camera and checked focus using diffraction. I repeated the eyepiece refocus and camera check focus sequence six times. The results? Four out of six times I was out of focus. So much for the eyepiece method. In fact, here is my motto.

Next: Direct Methods (Chips)


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