I think it is extremely important for us to discuss how we can get the best images possible from our SCT's.
We know that it is best to keep the image near the Back Focus as specified by Celestron.
The engineers at Celestron have performed the "hard" calculations for us.
The optimal Back Focus for the Best Image is simply:
- 5.25" = EdgeHD 800
- 5.75" = EdgeHD 925
- 5.75" = EdgeHD 1100
- 5.75" = EdgeHD 1400
Significant deviation from the above will degrade your image.
In a nutshell the tolerance (offset from the NBF distance) is tighter for good seeing and a large chip (diagonal), as we would expect.
The table below summarizes the maximum tolerances (rounded values) for a 10% spot diameter (encircled energy) increase v.s the diffraction and/or scope optical limits:
APS-C chip Full frame chip
1″ FWHM seeing +/- 20 mm +/- 20 mm*
2″ FWHM seeing +/- 50 mm +/- 20 mm
Those graphs clearly show there is one, and only one, Optimal Back Focus distance - at exactly "0.0 mm Offset from NBF" as shown in the top 2 graphs.
But let's be clear ... the graphs are the Worst Case location on each chip ( APS-C & FF) - in the extreme outer (4) corners only.
FF = 21 mm off-axis from the optical center line ( upper curve )
APS-C = 13.5mm off-axis from the optical center line ( lower curve )
None of those graphs indicate how much better the spot size actually is near the center of the imaging chip.
Cropping can remove much of the outer edge error.
And if the "seeing" is random noise then stacking many images may reduce the bad effects due to the atmosphere.
It would be great to see a 3rd line in those top 2 graphs showing the the error at location 0.0 mm off-axis ( ie the center of the imaging chip ), too.
This 3rd line would certainly be below ( better than ) both blue curves.
Edited by mvas, 17 February 2018 - 11:12 AM.