Telescope tradeoff: aperture vs portabilityAugust 14, 2010
In a comment on the last post, Jon Lindberg brought up some good points about the aperture/portability tradeoff with telescopes. It’s fertile ground for discussion, because there is always a tradeoff.
The big up-front ground rule for this discussion is that when it comes to portability, your mileage may vary. Some people consider 8″ or even 10″ scopes to be “grab-n-go”. For me, scopes break roughly into two categories: those that require one hand and one trip–my definition of grab-n-go–and those that require some more setup, either two hands or multiple trips or other fiddling. And at any given time, there is only one scope in each category that I’m using heavily.
Right now my big gun is a 10″ Orion dob. It weighs about 55 lbs assembled, which is about half in the tube and half in the base. I can move it while it’s assembled, but usually not without having a few twinges in my back the next day. So I usually carry the base to where it’s going to be set up, then put the tube on, then set up some kind of chair next to the eyepiece. Including a trip for my eyepiece case and some charts, it’s usually about four trips. But that’s okay, because I only tend to set it up when I’ve got some serious observing to do, or when I want to impress houseguests.
My old “big gun” was my first telescope, a 6″ Orion dob. It weighs about 33 lbs assembled, and I always carry it out in one piece. But it lives out in the garage with the 10″ and also requires a chair, so I’m still making two or three trips to get it set up. If I’m going to go to that much effort, I might as well get out the 10″ and get the benefit of nearly three times the light-gathering ability and almost twice the angular resolution. So I’ve barely used the 6″ at all since I got the 10″.
The 6″ is also facing competition from the other end, from my 5″ Skywatcher reflector on a homemade Dob mount. That one weighs just under 20 pounds and is short enough that I can use it sitting on the ground, so it’s more grab-n-go-able and still delivers most of the performance of the 6″ scope.
One of the lessons of all of this might be that I have too many telescopes. The more broadly applicable point is that the goodness or badness of a telescope for any particular application depends on what else you’ve got in the stable. When I only had one telescope, it was of necessity both my big gun and my grab-n-go scope. But my enthusiasm for hauling out a 30 lb scope on short notice declined markedly when I had something under 10 lbs to use for quick peeks.
But let’s get on to the meat of Jon’s question, which I am going to interpret as, at what point as you go down in aperture do you start noticing the compromises?
Again, the answer will be different for different observers. Some people think that anything smaller than 8″ is a waste of time. Obviously I disagree. I think that the vast majority of observers would say that a 3.5″-4″ telescope is probably at a threshold between noticeable compromise and being to see most familiar targets–moon, planets, Messier objects, the occasional comet–with rewarding vibrancy and detail. I base that in part on the massive commercial success of 90mm Maksutovs and 4″ refractors, especially apochromats. Also, some of the best deep-sky observers in the world like Stephen O’Meara and Sue French use 4″ refractors as their primary scopes.
That’s not to say that smaller telescopes aren’t popular as well. Refractors in the range of 60-80mm have always sold well and probably always will, especially short focal length, widefield scopes like Orion’s ShortTube 80 (pictured above). And you can have a lot fun pushing these little scopes to their limits, as Jay Reynolds Freeman did when he completed the Herchel 400 with a 55mm scope. But achievements like that get noticed because nobody expects to be doing serious deep-sky work with a tiny telescope. Sub-3″ scopes are almost always intended to be either introductory-level instruments or purpose-built grab-n-go and travel scopes.
So what’s the real word? My little SV50 is well into the realm of trading away performance for portability. So far it has shown every Messier object I’ve tried for, but all but the biggest and brightest have been faint fuzzies at the eyepiece, without a great deal of detail. And a scope that small absolutely requires dark skies to do any meaningful deep-sky work. Here in town it just doesn’t have the horsepower to cut through the light pollution. But that’s okay, because I didn’t get it to use here in town. I got it mainly for airline travel, and if I’m flying, it’s usually to someplace darker than the LA area, so it fills its very specialized niche admirably.
One thing that the little scope excels at is putting a truckload of stars in my eyes. Bigger scopes with longer focal lengths have smaller fields of view, that’s just an inescapable fact of optics. I’ve noticed that when I’m using bigger scopes I’m usually hunting for particular targets. With the SV50 I have a lot of fun just panning around the sky. It is the only scope that I have used that delivers the same super-wide field of binoculars, but with the advantages of being solidly and comfortably mounted (image crouching behind a pair of mounted binoculars when they’re pointed at a target more than 45 degrees above the horizon) and having variable magnification.
For a little more than double the weight and volume, the C90 is still very portable and delivers a LOT more light and a LOT more detail. But for me it has two distinct disadvantages compared to the SV50. First, it’s just big and heavy enough to require a bigger tripod, so the whole kit-and-kaboodle won’t fit into a tiny bag that I can stuff into the bottom third of my backpack. So if I’m traveling with it, it becomes one of the focuses of my packing, instead of something I just shove in the bag and forget about until I reach my destination. Also, the folded light path gives the C90 a very long focal length for its size–900 mm–which makes reaching high powers a breeze. That makes it easy to power up on planets and specific deep-sky targets, but it also means that the scope has a fairly narrow field of view. So my mindset when I’m using it is more along the lines of, “what individual small thing am I going to look at next”, and not, “let me pan around the sky and see what I stumble across”. If you want the latter experience in a more capable scope than the SV50 that still only weighs about 5 lbs and is eminently airline portable, consider a Short Tube 80.
If you’ve got a little more space and don’t mind a little more weight, a 4″ Mak or a 5″-6″ Schmidt-Cassegrain will put a lot of aperture into a decently small space. Something like a Celestron C5 is about the size of a big coffee can but gives you enough light grasp and resolution to go after just about anything you want, especially if you have dark skies. The caveat I’ll add from my own bitter experience is that at this size of scope you have to put as much or more thought into the mount. When I got my first Mak, a 4″, I put it on a cheapo camera tripod from Wal-Mart. That was a disaster–the mount was so shaky that using the scope was an exercise in almost terminal frustration. Moving down to a 90mm scope didn’t really help, and my little scopes didn’t get much use until I got a decent tripod. And by “decent” I mean “costing as much or more than the telescope itself”.
I brought up the Short Tube 80 and all of the catadioptric scopes (Maksutov-Cassegrains and Schmidt-Cassegrains) first because they’re probably the most airline-portable of the bigger scopes. If portability is important but you don’t plan on flying with the scope, at least not regularly, the Orion StarBlast 4.5 and Edmund Astroscan both put some serious aperture into a one-hand telescope. Both are bulkier than a 5″-6″ SCT, but in both cases the bulk includes a base so you don’t have to worry about buying a separate mount and tripod (although you may want something, even a picnic table, to get them up off the ground). The StarBlast has better optics and a better focuser, but the Astroscan is almost indestrucible. As with any optics purchase, read around to find out the good and bad about them both before you make any decisions. The links to telescope reviews on the sidebar are good places to start.
Most telescopes are made in China and Taiwan these days, and the same models that are sold by Orion and Celestron in the US are usually available from SkyWatcher or Konus in the rest of the world. Happily, just about all of the scopes I’ve discussed can be had for $200-300 or even less if you’re willing to shop used (for example, at the Cloudy Nights Classifieds, where I’ve bought and sold just about all of my astro gear). If you have any questions, feel free to ask in the comments. I’m always happy to talk about telescopes.
UPDATE March 11, 2013: Here’s Doug Rennie’s StarBlast 6 hanging out amongst the flora–see comments for explanation!