I have been trying to find a formula for determining the physical size ( not field of view ) of the image circle (at infinity focus) for any telescope of a given aperture and focal length.
It seems that this should be a pretty basic thing so consequently it must be that I am missing something pretty basic.
Short answer: There are many image circles. One for each quality you want to consider like illumination, spot size, coma, etc.
TL;DR answer follows...
I agree with Bob. Bob may find that amusing. But I love Bob. We actually agree on like 99% of things. Then we get into it on the little bit left because we are both old and like to yell at each other because we are deaf and we can't hear each other.
On really high end scopes where they are on the far end of the quality curve, they actually measure many different scope attributes and each has an image circle. So, it's actually "image circles" that you need to worry about. Each of these things can be calculated, and then compared to an ideal. Then you have to figure how each influences each other. Ha. Pretty complicated stuff.
- Spot size
- Surface flatness
- Surface curve accuracy
- Field flatness
You have to define the size of the aberration you are willing to put up with at a certain distance from the center of the sensor - this is the image circle for that aberration.
And all of this stuff gets worse as you move away from the center of the field, and the image circle gets bigger.
Then you also have to consider stuff like mechanical vignetting, not associated with the optics at all, but stuff like drawtube size at a given distance from the sensor.
Bigger sensors require larger corrected image circles and larger everything else associated with it.
So, really, the important thing is that you need to think about all of these things and the size of the sensor you want to cover.
Then you balance that with your checkbook.
Then you decide how much you can put up with each parameter as you make the system faster, plus how much that costs in money in terms of materials, design, and actually manufacturing a system that can hold those tolerances when you carry it in the back of your truck bounding down the dusty roads to your dark sky observing site.
In the end, you have to compromise somewhere. I put up with some aberrations in fast systems because I can't afford the expensive pieces of glass (meaning everything mentioned) that corrects them better at the apertures I want to shoot at.
But you would be surprised at what you can get away with if you don't flopping pixel peep.
And if you are one of those people who just have to have the best because you can afford it, there usually is a pretty good correlation between quality and cost - as there is for most things.
Edited by Jerry Lodriguss, 19 November 2018 - 05:19 AM.