Planetary Astro F-Ratio Vs Pixel Size

[Tom30887 (Tom)]

Question for the forum, should the following two image trains yield similar results? I’ve just moved from A to B in the hope that a shorter focal length and faster f ratio should be more forgiving under poor seeing.

Both trying to follow the rule that F-ratio should be 5X the pixel size for standard seeing.

A. C11 XLT with a Baader 1.3X Barlow and ASI664 (2.4um pixel) giving 3600mm focal length at F13

B. C11 XLT at native focal length with ASI676 (2um pixel) making 2800mm focal at F10

Is my logic flawed or am I on the right track?

[Saturnine (Jeff)]

Hi Tom. If the seeing is poor planetary imaging is hardly worth the effort regardless of camera and focal length. The C11 is quite a capable planetary scope and either configuration will give good results when the seeing allows but if you do image in poor seeing, the native focal length would be less compromised by the conditions regardless of which camera you use. The set ups you mention are so close in resolution etc that the differences are hardly worth thinking about. Also, I believe that the ASI664 has 2.9um pixel size, doesn't really make much difference for this exercise though. Try both configurations out and compare the results.

[rmuhlack (Richard)]

As Jeff has mentioned, the resolution of both setups is quite similar (A=0.164 arcsec/px, B=0.147 arcsec/px).

If you wanted a setup that was more resilient to sub-optimal seeing then i'd consider configurations which give a more coarse resolution. eg setup A without the barlow (0.214 arcsec/px), and/or a camera with larger pixels eg ASI220 with setup A (with barlow) would give 0.227 arcsec/px, and with setup b (without barlow) would give 0.295 arcsec/px.