I believe jp is asking small an exit pupil is reasonable for viewing the planets.
With my 10 inch, I usually top at about 400x on the best nights, that's a 0.6mm exit pupil. With a 4 inch, I would push it a little more, about 200x or a 0.5mm exit pupil.
My eyes agree with Jon and at least one other CN contributor. For Jupiter's bright low contrast detail I normally top out at 0.6mm exit pupil through my MCT and about 0.5mm in my Newt. (Truthfully, I may be as low as 0.5mm in my MCT, but there is some leeway in the operating focal length and resulting magnification). Much higher than that and, for me, bright low contrast detail tends to fade. The limb of the planet is still sharp, just the low contrast detail fades as the planet's image becomes larger and a bit dimmer (per unit of area, normally per square arc second).
On Saturn, the same story. Saturn has bright and a lot of low and very low contrast detail in it's cloud tops. I do not notice much degradation that might be visible at lower magnification, so I top out around 0.5mm exit pupil, too. The rings are much brighter with a bit higher contrast detail (and some low contrast detail). I can push the rings a bit higher, say around 600x in my 8" Newt or about 0.3mm. Mars is bright and has some high contrast detail. In my MCT, I have hit Mars as high as 400x or about 0.4mm with good results.
Typically, in my experience, the brighter and more high contrast detail, you can push it a little higher as much as seeing might "allow" or until the image becomes too dim for our eyes to see well. This seems to be generally true with everything, including the moon, small bright planetary nebulae, and even galaxies. I have heard some folks are amazed at Jupiter's bright low contrast detail at low as 0.3mm exit pupils. But, for me, such ludicrous magnification does not do me any favors.
And it sounds like the .45 exit pupil (444x) is trying to push it too much and I would be better off with the .65 exit pupil (308x) on the 8" SCT.
You could do both. It depends on the planet and the bright high and low contrast detail visible. And seeing, of course. But the former might be good for the outer gas giants, Mars, and Saturn's rings. The latter is better for Jupiter and Saturn, generally. Be sure your scope is well collimated and thermally stable.
Yes, if you wear glasses some generous eye relief is needed. However, you may find the small exit pupils help correct astigmatism. We're using so much less of our astigmatic iris, so you may not need glasses if astigmatism is the problem. You can consider a Barlow lens if you can find one that delivers the desired exit pupils with your existing eyepieces. They generally extend eye relief while adding magnification to an eyepiece.
Isn't exit pupil defined as the focal ratio of the scope divided by the focal length of the eyepiece?
I am not sure the exit pupil is defined by that, but it can be used to calculate the exit pupil. Using the focal ratio, the math contains the aperture and focal length and magnification with the eyepiece focal length. Really, in my view and since the exit pupil is an image of the entrance pupil, magnification is the ratio of the entrance pupil (aperture) to the exit pupil. Or equally, the exit pupil is the ratio of aperture to magnification. I prefer this as a definition, but either work to calculate the exit pupil.
Edited by Asbytec, 28 July 2021 - 08:07 PM.