Quote:
Originally Posted by Atmos
The size of the CFZ is determined by the focal radio as it is the steepness of the light curve. The size of the pixels also has to be taken into account as this is the area in which the CFZ occupies.
When you have a larger slower telescope you typically have larger pixels and at higher resolution you are never diffraction limited. Take what Mike and Trish have, a 20” CDK with a 16803. When the seeing is less than perfect the CFZ aid basically made larger via atmospheric blurring.
4-5” F/5 refractors really struggle because their CFZ is the determining factor of perfect focus where as a CDK can largely be determined by the seeing conditions.
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A large CDK scope can be very difficult to focus in poor seeing. So much so that its really a waste of time trying to image with that sort of long focal length scope in poor seeing.
I have had it where it seems in focus you take another focus shot and the seeing pushes it out of focus, you adjust and all you are doing is chasing the seeing and its all over the place and unpredictable.
Temperature compensation is a good tool but its somewhat imperfect because it assumes a straight line response between temperature and focus and I don't know that it is. Its not a bad approximation but I imagine its not as accurate as continuous autofocus or refocusing at some regular interval which matches your scopes focus shift with temperature.
For example I have noticed my Honders will shift focus noticeably after an hour of thermal equalizing and then the differences become much less after its settled.
The carbon fibre scopes so far have been very forgiving of temperature focus shift.
Some scopes have built in adjusters to match the focus shift. Roland Christen has said aluminium tubes focus shift matches reasonably closely the shift in the lens focal point so it tends to cancel out.
Greg.