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The 2" per pixel is used as a guide only.
The theory goes...
a) Not many sites have seeing that is better than 2".
b) Nyquist theory of sampling means that you should sample every point with at least 2 pixels.
c) That means that the highest sampling rate you should consider is 1"/pixel. Probably more as your seeing will only be 2" on the better nights.
d) Match the focal length of your scope to this sampling rate. In reality most people have a scope and a CCD camera, what you get as a sampling rate is determined by the equipment you own. Of course some cameras allow you to bin the pixels, so your 6.8um become 13.6um.
In your case I would say that the 2.73"/pixel is undersampled (point sources, i.e. stars will end up as squares), and the 0.83 is oversampled. I usually run oversampled (0.87"/pixel) as I find there is more room to use deconvolution software to fix the seeing. Most deconvolution methods require the picture to be oversampled. The trade off is sensitivity and image scale. If I bin 2x2 I get a much better sampling rate in theory, but the image scale goes down. I like the bigger images.
Cheers
Stuart
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