It's a dance between several variables. I find it silly when I realize I'm looking at 72mm, 80mm, 90mm, 102mm and fussing over needing a little of all of them only to stop and say to myself... "It's basically one inch difference!" to ground myself and stop thinking I need more small refractors. So I start my selection process by reminding myself that whatever I'm looking at, is really something I already have probably, and that I need to look at what I really need instead of just looking at new paint jobs or fancier display. So I start there... and because of that, I have not replaced my ED80 with another APO because each time I'm about to do it and get a 72mm or 90mm or 102mm APO thinking I need more, I stop and realize... I'm doing all that for 1 inch difference and ground myself once more. I have lots of 80mm achromatic doublets. Lots of them. If I see one for $50 or something, I snatch it up. So I have several short fast 80mm doublets. But again, I've yet to replace my ED80 APO with its FPL53 optics with something "better" because really there's not much that is wildly significantly better and I'm not really interested in spending a bunch (thousands) just to get 1 little inch of aperture that's already still small and have it perfectly corrected, etc. I consider 4" small still for aperture.
After that, it's mounting requirements. I struggle between not caring about weight and physical size because I can just put it on my observatory mount and it's not a big deal. But, what if I want to use it manually on an Alt-Az outside of my observatory or take it with me to a star party or have it out for others to use, etc? So then I start worrying about mounting the thing safely because I cannot stand under-mounted "large" scopes, they're terrifying in the dark. I have tons of little 80mm scopes laying around on small mounts, truly one-handers. I add them each time I see a deal because one cannot have too many and I have kids and people using them often so they're all out in my observatory ready to go at all times on their little mounts. But, when I'm looking at bigger refractors, over 102mm, so 120mm to 150mm where things get big and heavy real fast, the mount is my next big consideration because if I cannot properly mount it, then I have to get a new mount... just for that one scope? That grounds me again. And that's why I have not bought a 150mm F8 ED doublet yet to replace my current 150mm F8 doublet achromat. Because I just don't want to also have to buy a good mount, adding even more cost to the whole thing. And while I could eliminate this problem by just getting a better manual portable mount, I also don't want to buy something big and heavy and expensive that will not see a lot of use. So this keeps me safe from spending all day every day. If I cannot comfortably mount it now, then I really don't need it and likely won't use it a lot visually.
Price obviously matters. Otherwise, I'd have an 200mm triplet in my observatory by CFF. But I don't have a $50k scope in there sadly. Price matters. I like to shop around and look at what options there are for the price. What influences this for me is the overall build and the quality of the focuser. I don't want a cheap scope if I have to replace the focuser unless the price is good enough to allow the replacement of the focuser without going over budget and just having gotten a different scope anyways. So I look at that as part of it. If the price is good but I have to sink more into it for mounting and focuser options, then it's not a good price. I try to set a budget and stick to it. We all go over our budgets. But, I really ground myself with budget because while I have a lot of telescopes laying around, I don't mind if they're laying around when they cost next to nothing, but if it's a pricey telescope I don't want it just laying around because... well, why did I get it then? And again for this reason I haven't replaced my ED80 APO with another small APO because I could spend a lot more and end up with a little more aperture, but hardly enough to care at this point because I have bigger scopes and these are all small apertures frankly. But I of course always want a bigger APO I think. My ED80 does the job. But I'm constantly looking at 102mm and 130mm APOs to replace it or pair with it. Then I see the price. Then I see it's really only an inch or so more than what I already have and the immense cost isn't worth it yet because I have other scopes already far bigger so.... yea, it grounds me, I just don't need it. I want it. But I don't need it.
Lately I've been eyeballing 102mm~130mm APO options, triplets. I want a 150mm ED/APO eventually to replace my 150mm achromatic doublet. The price is reasonable for the new Skywatchers and F8 with ED glass is good enough for me. I don't need a quad design and I'm not sensitive to CA really (hence my backyard full of small fast achromatic doublets). But no matter what, a 6" frac needs a heavy mount. So while it rides on my observatory mount, I also want to use it sometimes manually or go somewhere with it, and I don't have a heavy duty manual alt-az mount and I don't like EQ mounts that are manual for visual use. I will likely eventually get the Skywatcher 150mm F8 because it's price makes sense. But then again, I think to myself... what's wrong with a 150mm F6 reflector for $200? Sigh. This is why I don't have nice refractors. They just cost so much per inch of aperture and are still not perfectly corrected without spending thousands. Kind of silly really. And I love refractors. But dang! So I'm back to looking at 102mm~130mm APO triplets as a real replacement for my ED80 so that it's enough aperture to actually see stuff, good for imaging and still reasonable to mount without needing a crazy mount just to handle it (like an AZ8, TWII, SkyTee2 would be good for me) so that it's still reasonably portable. So I'm constantly looking at 130mm F7 triplets lately and the 120mm/127mm triplet options too. Things that can take reducers for imaging but still be great for visual. And then of course I go right back to thinking... why do that when I can just get a 150mm F8 ED doublet for the same price. But then again, no reducer. So visual and solar system imaging would be its job. And I frankly don't need that since I have much larger aperture mirrors. Sigh. So... I'm still grounded and haven't bought a new scope hah.
For cheap refractors, smaller ones, achromats mostly, I just look at the aperture, focal-ratio and focuser. Almost every time the focuser is garbage and must be replaced. So I consider that in the price. I really like getting used big achromats for cheap. Lots of big cheap aperture is great and I don't mind CA when I know I didn't spend thousands for it.
Funny enough I'm less fussy about optical figures and correction than I am about the rest of the scope. And I probably should be more fussy about this. But then again, maybe this is why I don't have any really nice big refractors and I'm still just looking at them...
At the end of the day, I really don't want a several-thousand-dollar scope of any kind just bopping around the yard or traveling, I don't like worrying about it. I like inexpensive high function stuff that I can replace and not care about it really. So I probably never will really have a "nice" telescope I guess. And I'm not even looking at "nice" scopes, like a Tak or something.
I tend to look at Astrotech/TS ED/APOs for value, versus Skywatcher and ExploreScientific again for value. I don't even look at Taks or Televue, etc. I just can't afford that stuff for the tiny size apertures that they are. So I don't look at the high end scopes at all. I struggle looking at the AT/TS, SW and ES scopes at $2k and less even. They're good, functional, but not "nice" by a lot of people's standards around here. They tend to have good focusers and good enough optics for imaging and are corrected enough to not think about it.
I actually want to get an iStarOptical 200mm F10 objective (R20~R50 range) and have SiebertOptics build a modular OTA for it. Reasonable cost ($5k total maybe?) for what it is. It would never leave my observatory nor come off the mount ever of course. But I would find that reasonable.
So.... all that to just say, clearly after re-reading it, price is my only true factor for the refractor it seems. I'm cheap.
Edited by MalVeauX, 17 August 2019 - 10:38 AM.