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Hi Troy
Its similar to terrestrial photography, a 200mm tele lens will give a larger image scale than a 28mm wide-angle lens, albeit at the expense of a narrower field of view.
So, the longer the focal length of your telescope, the bigger the image scale which will produce a more magnified view of the Bahtinov diffraction spikes.
If you plug say, a 20mm focal length eyepiece into your ED80 and then the same eyepiece into your C8, forgetting the differing fields of view, the image scale (magnification) will be bigger in the C8 due to its longer focal length.
On the issue of where to place the Bahtinov Mask, I usually place it on the naked optical tube ( i.e. minus the dew shield), although I did once do a quick test, placing it on the dew shield and it appeared to work/focus just as well. My dew shield is a homemade foam tube that easily slides off and my Bahtinov Mask sits more snugly on the naked OTA hence I focus with the dew shield removed.
If the ED80 has a rigid dew shield that will support your Bahtinov Mask, then I’d fit the mask to the dew shield to avoid fiddling around with moving/re-fitting the dew shield.
Cheers
Dennis
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