Optimal focal length for planetary imaging

[Amaranthus]

I was creating a FOV calculator in Excel for my equipment, and this included working out the resolution (arcsec/pixel) for my Neximage 5 planetary cam.

I worked out the following for my C8:
FL = 2032 mm
Chip width = 5.7 mm
Pixel size = 2.2. microns
TFOV = 9.6 arcmin
Resolution = 0.22 arcsec/pixel

If I add my 2.5 PowerMate (which I often do to get a larger planetary image, despite the hassle of holding it in the centre of the FOV), I get:
TFOV = 3.9 arcmin
Resolution = 0.09 arcsec/pixel

Now, the Dawes resolution limit for my C8 (203 mm aperture) is 0.57 arcsec. This tells me that it seems to be pointless ever using the PowerMate for planetary imaging on this scope, because the imager's per-pixel resolution even at f/10 is smaller than my telescope's resolving power. In fact, I may as well leave my f/6.3 FR in, because the resolution per pixel in that case is still 0.35 arcsec/pixel!

Is this correct, or have I overlooked something? :shrug:

[Merlin66]

Rough rule of thumb...
focal ratio around x5 your pixel size....this would give f12 fro your system. Close to the original f10....

[cometcatcher]

Forget the Dawes limit. Everyone on this forum (except me) goes waay over it lol.

[Shiraz]

as Ken says, FNo = 5x pixel size in microns - you are right, you don't need a Barlow with that chip/scope combo. http://www.iceinspace.com.au/forum/showthread.php?t=105057

[John Hothersall]

As has been mentioned Dawes limit does not apply to planetary imaging as Dawes limit applies to human eye acuity of equally bright stars and their separation with no contrast between them.

Planetary imaging is about contrast, the Encke division in Saturn's outer most ring is 0.05" and is easily imaged on good/vgood nights even in 8" scopes. CCD's take many frames a second so can discern even slight contrast differences in amongst the turbulence.

At your FL 0.2/pixel is fine as has been said, you could push further on the best nights which is part of getting to know your scope and seeing.

Regards, John.

[Amaranthus]

Thanks all, very helpful. Confirms some of my thoughts, and fills in the gaps nicely. Ray's primer was useful.

I'll muck about with a few different FL and see what the real-world testing on my setup shows, but it does seem like f/10 is a good default for my setup. I've never had an image at f/25 that I was all that happy with, so I'll give f/16 and f/12.5 a go...

[Clayton]

Listen to John H Barry
He does know what he is talking about, and my own experience backs this up. It is certainly harder to work at greater FL but when conditions permit the rewards are great