Depth of field in sensor imaging plane /tilt tolerance - Forums

Topic starter

Jun 4, 2023 · Edited: Jun 4, 2023

Hi everyone,

Does anyone of you have a link (or even better the knowledge) regarding the following question:

RASA11 has an aperture of 1:2.2, focal length is 620mm… using a full frame sensor like the 61MPx of the QHY600M - how much tolerance can I expect regarding tilt to make sure, that all four corners are still with the imaging side depth of field?

My images most recently taken are perfectly in focus in the center but show radially distorted stars in the corner, which is usually a sign that the sensor is slightly too close to the last lens…right? It is not dramatic and does not disturb at normal viewing distance, but I would like to get it "perfect"

I am using 3mm thick 50x50mm filters from Baader om Germany, just for information ( L,Ha, OIII, SII, R, G, B)

Any suggestions would be highly appreciated!

Thanks and CS

Georg

andrea tasselli

Frequent contributor

Jun 4, 2023

depth of focus = +/-4* lambda*FN^2, assuming diffraction limited optics and imaging purposes, with good seeing <2" (visual would be half as much).

Using central wavelength in green @ 550nm this would yield => +/-10.6 microns

Georg N. Nyman

Topic starter

Jun 4, 2023

andrea tasselli:
depth of focus = +/-4* lambda*FN^2, assuming diffraction limited optics and imaging purposes, with good seeing <2" (visual would be half as much).

Using central wavelength in green @ 550nm this would yield => +/-10.6 microns

Thanks Andrea, that is very narrow… wow! I do need to do some corrections…

CS
Georg

Kevin Morefield

IOTD Staff

Jun 4, 2023

Georg N. Nyman:
andrea tasselli:
depth of focus = +/-4* lambda*FN^2, assuming diffraction limited optics and imaging purposes, with good seeing <2" (visual would be half as much).

Using central wavelength in green @ 550nm this would yield => +/-10.6 microns

Thanks Andrea, that is very narrow… wow! I do need to do some corrections…

CS
Georg

I believe this is also your critical focus zone so any remaining tilt after corrections would further shrink your CFZ. Deciding on a focus program and method would be critical with such a small margin for error. I would think that a full field focus routine would be safer than a single star. If you did use a single star, it would be good to know what the curvature of the image field was so that you could pick focus star's within the midpoint of the curvature. But full field focus should take care of that.

Kevin

Helpful Insightful Respectful