You mention taking a spotting scope for casual astronomy rather than a valued scope. How do you feel the PF-65ED compares to the PF-80ED and how do they both compare to the TV-60. Do you use other 1.25" eyepieces rather than the zooms if using the spotters for astronomy, or do you use the Pentax eyepieces?
The TV-60 is the telescope that introduced me to the joys of terrestrial viewing but sometime later I purchased the Pentax PF-65ED-AII to take its place for this task and now I use the TV-60 exclusively for Astronomy. The reason is simple: I can put the little but sturdy PF-65ED in a small backpack and take it on a tour to the mountains without worrying about dust, rain, fog or bumping it against a tree, not so with my TV-60.
For Astronomy from the wild outdoors I prefer the greater light gathering and resolution offered by the bigger Pentax, the PF-80ED-A so that’s what I normally take with me. I am fortunate to own various Pentax XW and XF eyepieces and I found that the XWs are a great match for the PF-80ED-A. They are excellent on the PF-65ED-AII scope too, but they are a little bulky and when I travel light I prefer to take a small Tele Vue 8-24mm zoom eyepiece with it and perhaps my new ES 26mm 62-deg eyepiece for the wide fields. When I travel with the PF-80ED-A I normally use a Pentax XL 8-24mm Zoom as my main eyepiece and also carry a 20mm XW for the low power panoramic views. The zoom is so convenient that I don’t bother to bring the other XWs! When I’m home, for casual Astronomy I prefer to use the TV-60 for quick peeks at the night sky or my regular grab and go refractor, the TV-76. At home I only use the Pentax scopes for terrestrial viewing and birding.
If you mean to ask how these three scopes compare for Astronomy, I would say that the TV-60 provides textbook stellar and planetary images and I can normally take it to 120x with a 3mm TV Radian for planetary or double star observing -I have even taken it to stupid powers with a TV 2-4mm zoom eyepiece without experiencing image breakdown –very dim views of course at these mags because of the diminutive exit pupil! The Pentax spotting scopes provide nice low and medium power views of the night sky (without intrusive vignetting at low power), virtually free from chromatic aberration but stellar images are not textbook perfect, there is the slightest flaring on bright stars and the bright planets. Both Pentax scopes provide similar views, it’s just that the PF-80ED-A has greater reach than the PF-65ED-AII showing dimmer stars and more detail on deep sky objects. IMO the Pentax spotting scopes are not suitable for planetary observing even when using short FL eyepieces, there are simply too many optical elements in the light path that hamper the ability of the nice ED glass to provide clean high magnification views of these objects. But then I don’t travel to wild remote places with pitch dark skies to watch the planets!
I believe the Pentax 65ED and 80ED spotting scopes complement each other instead of competing among them, mostly because of the large size difference. But if I could own only one, it would definitely be the PF-80ED-A, no doubt about it –that’s exactly the scope that I purchased first. If my main activity was hiking on wild trails and climbing mountains, the obvious choice would be PF-65ED-AII!