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05-18-2013, 04:55 AM   #1
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plastic telescope vs dslr

I have been thinking about getting a telescope for the kids for a while (the cheap plastic under $50 kind). One thing that has made me hesitant is that the quality might just be unusable. Its not worth paying more at this point (they may use it once or twice and loose interest and they may break it).

Reading that the objective lens diameter is as important as the magnification for obvious reasons (ability to gather light), and the fact that most cheap ones are 50mm or 60mm, would a dslr with a lens that had a 72mm objective (actually filter size, I'll have to check the glass size on some) work better due to the better optics? I have an ist-d that I bought specifically for them to use. I have a cheapo 500mm f8 lens with 72mm objective. I have at least a few various vivitar zooms with 72mm filter threads at least, and a few rear mount tel converters to play with, and some front mount telconverters (not sure if any would help with light gathering).

Is the optical formula equivalent? Is a lens with a 72mm objective lens most likely going to have better light gathering that a telescope with a 60mm objective lens?

05-18-2013, 10:41 AM   #2
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I have a 500mm f/8 non-mirrored lens. Its light and its portable, but I'm not sure if I'd say the optics are better than the cheap 70's era scope I have upstairs.

Unless its a really decent lens, the scopes going to have better IQ in a straight up one on one. Once you factor the camera in, things change though.

A lens is designed to go with the camera, a telescope is not. With a cheap scope, you'll most likely have a cheap tripod, which means as soon as you hook your rather heavy camera to it, you're going to be aiming at the ceiling and will then have to do a makeshift counterbalance for it so you can actually use the thing.

Then, once you DO have all the bits together for the scope, you'll be factoring in more IQ loss from the adapter.

An expensive scope > and expensive lens > a cheap lens > a cheap scope, at least in my experience. YMMV of course.

I find the 500mm lens far, FAR more usable than the scope, if only because its far more portable. I can use the thing handheld to take (generally) poor quality shots of birds, while the scope would simply be unwieldy as hell.


EDIT: The lens I have is one of these Quantaray 500mm f8 to f32 Lens Reviews - Miscellaneous Lenses - Pentax Lens Review Database

I think I paid $30 for it used on eBay, they go new for about 100 bucks more than that. I rarely use the thing because its big and annoying and slow as hell. A telescope being used in lieu of a lens is bigger, slower, and also has the 'awkwardness' factor added in.

Honestly, if I were to have $50 or so to blow and wanted something with optics sort of like that, in lieu of a telescope I'd look for a spotting scope that can have its eyepiece swapped out. Spotting scopes generally are more geared for terrestrial use and will be far more portable (and durable!) for the kids if they decide they like it.

If they decide they don't, you'll have a decent (and more importantly, small and lightweight) thing you can try using with your camera instead of a big clunky old low end telescope.

Mind you, this is just personal experience - I'd bought an old 70's era EQ mounted 910mm scope cheap at a yard sale. When I did the math, it worked out as being f/13 or so (yikes!) Despite its age, its tripod and optics are probably better than any cheap $50 Walmart scope would be, so if I had all sorts of issues with mine, I can't imagine your luck being any better trying to adapt it for your camera. I'l have to run upstairs and do some local shots out the window to show you what I mean about quality and such using mine as a makeshift lens - for all practical purposes, its just generally bad. If I had that #50 or so I spent between the scope and the adapters to mount the camera to it, I'd have gone another route entirely and grabbed a spotting scope (and the adapters, of course).

My 2 pennies of course, I could be horribly jaded simply because I became so frustrated with the process I gave up on it.
05-18-2013, 11:59 AM   #3
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Here are shots of my 910mm vs the 500mm Quantaray. The scope has a 60mm objective, FWIW.

I tried taking shots in similar spots just to show the IQ difference (or lack thereof).

All scope shots were tripod (natch), all 500mm were handheld.

The vignetting with the scope is from the weight of the camera causing the scope to bend where the mount and the scope came together. Not much I can do about that short of rebuilding the thing. Bear in mind, a cheap plastic scope will probably have just as bad (if not worse) of a setup than this.

910mm scope



500mm



500mm + 2X TC



scope



500mm



500m+2X TC



Bonus shots at 500mm+2X TC. I would never have caught him with the scope as he landed right across the street. I would have had to fiddle with the scope to allow it to actually catch him inside its depth of field, and by that time he would have been gone.




05-18-2013, 04:14 PM   #4
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Thanks a bunch for the info and comparison shots. If that is what your telescope is getting (probably better than any of the new cheapo plastic ones), perhaps it might be best to try out some of the lenses. The 500mm I have is a t mount tou five star (possibly the same as your quantaray under a different name). I have several rear telconverters but they are not the greatest quality (I think, I haven't used them much). I'll play with them and see what I get. I have a 300mm f5 that I'll check out with a telconverter or two also.

05-19-2013, 11:12 PM   #5
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For fifty bucks, don't waste it on a department store refractor. Even though the optics _may_ be passable, the mounts they come with make them completely unusable.

At that price range, get a Celestron Firstscope (found one for 29 bucks at Fry's a while back) and a better eyepiece in the 25-30mm range. That is actually a useful tool and works great for beginners.
05-20-2013, 03:38 AM   #6
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cheap is as cheap does

The el cheapo refractors are invariably rubbish, especially the mounts. I had such a cheap telescope as a kid- useless for astro stuff. It would take ages to find Mars, it would be a fuzzy blob, then it would be lost in seconds. For Halley's comet, I had a substantially better view looking through the camera viewfinder with a 500mm f/5 ancient Takumar than the telescope(it was a metal tube back then!) You may well be better off with a telescope adaptor(6 x barlow lens) bayoneted on to the back of a lens for a mini telescope. Had the luck of picking one up cheaply a while back Useful ,although it is out on loan at the moment with the 100 f/2.8 macro when I needed the macro lens.maybe should have loaned out the 58 f/1.2 Minolta with the Minolta to Pentax adaptor as the rig was used in low light. For some strange reason, I have not used the telescope attachment with the 300mm f/2.8 on a tripod(cannot use hand held, even with the 300 f/4 or 70-200.) Half decent telescope lenses DO make good camera lenses, although forget about close focus and depth of field. Itching to try the big telescope as a lens, which is the equivalent of a 3,000mm f/10 on 35mm, 5,000 f/10 on Dx format. Can be easily converted in to a f/ 6.3 rig or faster.
05-20-2013, 10:04 AM   #7
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QuoteOriginally posted by fisheye freak Quote
Half decent telescope lenses DO make good camera lenses, although forget about close focus and depth of field. Itching to try the big telescope as a lens, which is the equivalent of a 3,000mm f/10 on 35mm, 5,000 f/10 on Dx format. Can be easily converted in to a f/ 6.3 rig or faster.
Under infinity, I swear my scope has a DoF of about 3 inches at a quarter mile. A quarter turn of the focus knob will throw the bricks I posted above in and then out of focus again, and I'm a block away from them. Since the scope can't be stopped down (I wonder if there is an adapter or TC out there that would do that for me short of a home made rig?) you're basically always going to be shooting wide open, even if wide open happens to be about f/12 or worse.

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