![Smile :)](./images/smilies/icon_e_smile.gif)
In theory there are so many adjustments that you could make...
* Tilt of the primary mirror in 2 directions
* position of the primary mirror in 2 directions across the tube
* Tilt of the secondary mirror in 2 directions
* position of the secondary mirror in 3 directions (2 accross the tube, 1 up/down away from the primary)
* twist of the secondary mirror about the axis of the primary
* position of the focuser in 2 directions (along the length of the tube and around the tube)
* tilt of the focuser in 2 directions
Phew... that's 14 separate adjustments if I've counted right.
Fortunately you don't really need to worry about all of these... Some we almost never change (position of the primary mirror relative to the center of the tube for instance). Some don't need adjustment because they can be covered by one of the others - for instance moving the secondary position up/down/across the tube allows you to correct for the primary position and/or the focuser position. Some we tend to adjust infrequently - the position of the secondary left/right/up/down is typical for this - get it right once and don't mess with it again (I may write another post later on how to get this all set up correctly).
The ones we adjust frequently are the tilt of the primary and secondary mirrors, and that's what I'm intending to discuss in more detail here.
So... let's think what we mean by getting good collimation when adjusting the tilt of the primary/secondary mirros. Good collimation means
1) There is zero coma at the center of the image frame. All Newtonians show coma off-axis - we want light that is coming into the telescope along the optical axis of the primary mirror to be hitting the camera sensor at or near to the middle of the sensor. That way the effects of coma will be evenly spread around the edge of the frame with the sharpest stars in the middle
2) No tilt of the camera sensor with respect to the focal plane of the telescope. If there was a tilt then one side of the sensor would be out-of-focus (too close in) and the other side would be out-of-focus (too far out). Another way to look at this is that the light that comes in along the optical axis of the telescope needs to hit the sensor perpendicular to its surface.
That's it... nothing else (as far as I can work out). Note that there is no mention of right angles, 45 degrees or anything similar in these descriptions. In theory it's possible to satisfy these conditions with the secondary mirror at 30 degrees or any other angle to the axis of the telescope (the focuser would have to be at a pretty odd angle, but it could be made to work). In fact there's no need for a secondary at all - you can have a prime focus reflecting telescope where the camera is inside the tube at the prime focus of the primary.
So...
How can we use SharpCap to arrange good collimation according to the two conditions above? The key is to realise that both conditions are satisfied if
A ray of light leaving the primary mirror from its center spot along the optical axis of the mirror reflects off of the secondary and then strikes the imaging sensor at right angles to the sensor surface at (or close to) the center of the imaging sensor
Actually, this hypothetical ray of light is interesting, because to leave the primary from the center spot along the optical axis, it must have *hit* the primary at the center spot along the optical axis, which means that it must double back on itself - any light on this path must start at the center of the camera sensor (or very close to), hit the secondary, hit the center of the primary along the optical axis and then follow the reverse path back to the sensor. Realising this gives us the ability to collimate
![Smile :)](./images/smilies/icon_e_smile.gif)
Contd in part 2
![Smile :)](./images/smilies/icon_e_smile.gif)