Maybe decide the biggest aperture you can afford and handle… and then buy the next size down.
That is to protect us from these factors which can nudge us into buying too big a scope:
# We always overestimate our capacity to handle a big scope. At the end of your normal day, lift one or two bags holding a weight that equals the scope and carry them from storage to your viewing place. Sit outside for an hour in the cold and tired. Now carry the two bags back to storage. Repeat a few more nights and then decide.
# We tend to overspend on the initial purchase without considering the long term subtle costs like “bought a 10”… so I bought a paracorr… bought premium EPs… bought an 80mm refractor as the 10” was too cumbersome…made a wheel trolley for the 10”… bought a 14” (instead of a 12” after an 8”)… bought a slightly bigger next car..."
# We overweigh the vivid evidence, like seeing blue circles on a forum showing how a 10” diameter is bigger than an 8”.
# We’re easily swayed by authority figures who don’t know our own life busy-ness away from astronomy. Forum regulars may often recommend one aperture size too big for us, because they’ve arranged life better to fit around that bigger scope and they’re doing the nice thing of recommending to you the same source of their own personal happiness.
# We’re misled by anchoring our judgement to all the other sizes of that scope category. So an 8” dob seems small when dobs go from 4” to 32”. But compared to all scope types, 8” of aperture or roughly a 6” refractor is huge.
# Finally the regret notion of “you’ll always be wondering what the views in the next bigger size would look like.” How do we really think this? Since if you’re upgrading from a smaller scope, any aperture bigger than what you currently have is wonderful. If you’re downsizing from one scope, you've tested the loss of light grasp with an aperture mask. If you’re adding to a collection, you already have a bigger scope.
The biggest factor in deciding on which scope to buy is how it will fit in your overall life. That is something none of us can answer.
For example, I’ve considered only having a 6” f8 for the next 10 years; it can be 9kgs for easy carrying down stairs, it can be designed to cool as fast as a doublet refractor, it's the equivalent aperture to a decent-sized 115mm refractor, very aesthetic views are easy to produce, it rarely needs collimating and keeps you in touch with astronomy when the rest of life may get a little hectic.
Perhaps get the next size down from the biggest you could realistically manage.
Edited by Max T, 06 February 2016 - 05:16 AM.