EnA8BkqQ What is an acceptable tolerance for camera sensor tilt using a long focal length (1500-2500) versus a shorter focal length (200-600)
Tilt causes the corners of a sensor to no longer be in focus even though the center of the sensor is in focus. When there is tilt, the corners can be intrafocus, or extrafocus. And in general, you want the amount of defocus to be within the critical focus zone (CFZ) of the OTA. There are CFZ calculators that you can find on the web.
If all else is equal, you will find that the CFZ of faster lenses are more critical (smaller) than slower lenses.
This article has some drawings, and also mentions the CFZ of his f/4 OTA as being 30 microns.
https://aiastro.wordpress.com/2019/01/24/sensor-tilt-adjustment/
Assuming the sensor corner is 22mm away from the optical axis (i.e., full frame camera), then in his case, the angle of the tilt would need to be adjusted to within .03/22 radians or less than 1.5 milliradians. For rough guesstimates, I usually use 1 milliradian as the desired accuracy -- that is basically tilt causing a target that is 1m away being 1mm off centered -- easy to remember :-).
Another way to look at this is that if the tilt adjustment screws are 40 mm from the center (the screws for the ASI2600 and ASI6200), the screws will need to be adjusted to about .055 of a mm. A fine pitch M3 screw has a pitch of 0.35mm, so the 0.055mm would represent about 1/6 of a turn of an M3 adjustment screw. (A standard pitch M3 screw has a pitch of 0.5mm)
That should give you a rough back-of-the-envelope idea of the type of tolerances required.
This is why you read recommendations of applying very small fractional turns to the tilt screws when adjusting.
Notice that an APS-C frame sensor can withstand a larger tilt angle and still have the corners within the CFZ of the OTA. Thus less sensitive to tilts than full frame cameras.
Chen
