So you started with the AT60ED, then bought the 80mm f5 Achro that you adapted for a 2 inch focuser. What does the 72mm you bought later give you that is not already covered by these two? No challenge in my question - I'm really just trying to learn about these smaller refractors and what to expect from them.
The AT60ED gave me the absolute max true field of view for a well corrected refractor, and the smallest overall package. It also is the most forgiving of a light mount. For taking in as much as possible of Barnard's Loop all at once, this is the one for me. Ditto for the Lambda Orionis nebula (Angelfish.) It can also do some light planetary, and partially resolve the brighter globulars at high power. It is the most airline portable of what I have, although I have not used it that way...so far. I still would like to see the Large and Small Magellanic clouds through it.
The 80 f/5 (Meade Adventure Scope which is an ST80 clone with different colors/badging) was a pleasant surprise since it has decent optics. My ST80 has some aberrations, but makes an excellent finder on the 20" and sees some service at moderate powers as well. So when I realized that this particular sample of Meade 80 f/5 had a good figure, I decided to take full advantage of it by adding a 2" GSO two speed focuser--which could be had for $86 delivered at the time. I added rings, and moved the internal tube baffle back to maximize the fully illuminated field.
In practice the 80 f/5 achro is a mix visually. This sample is capable of some planetary, but of course that is still its weakness. The chromatic aberration is such that the AT60ED is preferable for high power viewing, lunar and planetary. Some of this is aesthetic, but there is loss of fine detail to CA which results in the 80 achro performing more like a 60ED at high power. (Filters don't fix the loss of fine detail from CA in my experience.)
Of course, I didn't buy the achro for planetary or high power. I bought it for rich field; any other capability is just bonus. While I can use a 41 Pan for ~6.6 deg true field of view, the result is an 8.2mm pupil. The result is that my eye's pupil will effectively stop the scope down to some extent. So a better pairing is the 31 Nagler for 6.0 degrees and a 6.2mm pupil. There is still a lot of field curvature inherent to the scope's short focal length. It does quite well with filters on large nebulae complexes. The extra aperture helps in this regard, so for all but the most gargantuan nebulae it has an advantage.
The 80 f/5 with 2" focuser, rings, and its fixed dew shield is considerably bulkier than the AT60ED or even the AT72EDII. It is still a lot smaller than the ubiquitous ED80 f/7.5. The ED80's are great optically, but they have fixed dew shields, and the tube ID is actually for a 100mm objective. Therefore they have not been optimized for portability/travel. And of course the focal length of the ED80 is considerably longer.
I am finding the AT72EDII is somewhat of a Goldilocks scope between the AT60ED and the 80 f/5 achro in 2". It isn't quite as small as the 60, but it fits in the same size Apache 3800 case with 2" diagonal and RDF. It provides more resolution than the 60 and is better at planetary than the 80 f/5 achro. At f/6 it is a better match for the 41 Pan than the 80 f/5, providing 6.1 deg true field and 6.8mm pupil.
The net result is that the 72EDII has been getting a lot more use than I thought it would. When I needed something to put in the car quickly and find a spot along the road up the mountain for the Jupiter-Saturn conjunction, it provided the best balance. When I was tracing the Eridanus super bubble during a DSO session beside the 20", it became the tool of choice over the other refractors I could have brought. I wanted somewhat more light than the 60 was bringing in. The 80 might have done even better for this, but the 72 was more conveniently packaged. Seeing in the backyard is usually pretty bad this time, with vigorous twinkle to the stars, so a small scope has an edge most of the time since there won't be much to see. The 72 is more convenient than the ED80 for quick looks at planets in the backyard, so the 72 is on a light mount near the door. If the seeing is better then bigger scopes than the 72 or 80 will come out...unfortunately that hasn't been worthwhile in recent months.
Edited by Redbetter, 25 June 2021 - 12:02 AM.