wachipilotes
Well-known member
With which magnification ?
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Compact spotter under $400 (1 Viewer)
- Thread starter sixtus
- Start date
PYRTLE
Old Berkshire Boy
Not sure what you mean by realistic magnification.......its x20 to x40, so at it's maximum you're going to get a darker, narrower image than at x20. A " comfortable" image would be anything from x20 to x25 for me.
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gr8fuldoug
Camera Land / Supporting Vendor
Most spotters hold up to about 85% of maximum power then start to give up some of the quality
Well I don't see many of the high end brands trying to go as high as 40 so my assumption is it wasn't realistically useful. For example I peered through some 20-60 x 60mm mid level scopes and the 60x was not just uncomfortable, it was a waste of time. That is what I was referring to.
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PYRTLE
Old Berkshire Boy
Some of the " high end brands" perform brilliantly at x40 and above but it's all down to........you get what you pay for.
A $300 50mm scope just isn't going to perform like a $3500 scope.
Regards, P
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henry link
Well-known member
If the scope is "perfect" and there are no atmospheric limitations you can easily calculate the magnification required to make its line pair resolution just large enough to become barely visible to an observer with a given eyesight acuity.
Divide 115 by the scope's aperture in millimeters. Then divide the observer's acuity by the result. For a 50mm scope and an observer with 20/20 line pair acuity (120") that would be about 52x. I find in the real world the smallest line pairs are much easier for the eye to resolve when they are a bit larger than barely visible, so I would say 60x would be a bit better for that observer to easily discern the smallest details.
Also in the real world many people have better than 20/20 acuity and most telescopes are not "perfect" and the air is seldom perfectly steady, but I would start with that calculation and modify it to match reality. Personally I wouldn't consider any scope, no matter how inexpensive, with arc second resolution much worse than 140/D (where D is the aperture in mm) and I wouldn't consider a really expensive scope with resolution worse than about 120/D. Unfortunately, to add to the variables there is considerable sample variation among scopes specimens, so one unit may be worse than 140/D and another unit of the same model may be 120/D. To paraphrase PYRTLE......you should get what you pay for (but don't count on it.)
Henry
Divide 115 by the scope's aperture in millimeters. Then divide the observer's acuity by the result. For a 50mm scope and an observer with 20/20 line pair acuity (120") that would be about 52x. I find in the real world the smallest line pairs are much easier for the eye to resolve when they are a bit larger than barely visible, so I would say 60x would be a bit better for that observer to easily discern the smallest details.
Also in the real world many people have better than 20/20 acuity and most telescopes are not "perfect" and the air is seldom perfectly steady, but I would start with that calculation and modify it to match reality. Personally I wouldn't consider any scope, no matter how inexpensive, with arc second resolution much worse than 140/D (where D is the aperture in mm) and I wouldn't consider a really expensive scope with resolution worse than about 120/D. Unfortunately, to add to the variables there is considerable sample variation among scopes specimens, so one unit may be worse than 140/D and another unit of the same model may be 120/D. To paraphrase PYRTLE......you should get what you pay for (but don't count on it.)
Henry
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Cheers, so possibly no for the $350 scope I am thinking. My eyesight isn't perfect either.
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Thanks Henry.
Another question I have always wanted to ask was say in equatorial tropics with temps 90F +, humidity, or heat mirage etc would the case that a very good spotting scope still helps with this?
Or is it a case these conditions negate a lot of the advantage in a good scope and tend to equalize the options?
henry link
Well-known member
Really bad air turbulence totally obliterates detail for any scope, but in milder cases of turbulence better optics do help. That's because external turbulence and optical aberrations have similar and additive bad effects on resolution.
For instance, if unsteady air that would decrease the resolution of a perfect scope from 115/D to 140/D is combined with a scope with aberrations that already reduce its resolution under perfect conditions to 140/D then the final image will have effective resolution of only about 165/D. I've seen this effect a number of times when a mediocre scope and a really good one are set up next to each other under less than ideal conditions. Neither is resolving detail as well as it would under perfect conditions, but the better scope still outresolves the lesser one.
Henry
For instance, if unsteady air that would decrease the resolution of a perfect scope from 115/D to 140/D is combined with a scope with aberrations that already reduce its resolution under perfect conditions to 140/D then the final image will have effective resolution of only about 165/D. I've seen this effect a number of times when a mediocre scope and a really good one are set up next to each other under less than ideal conditions. Neither is resolving detail as well as it would under perfect conditions, but the better scope still outresolves the lesser one.
Henry
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Stephen Mark
Well-known member
My Kowa 601 is really good with the 30mm fixed eyepiece. I imagine thats where you would be using it most.
Steve
Steve
gr8fuldoug
Camera Land / Supporting Vendor
Weposted a Kowa sale here yesterday on some demos at great pricing, https://www.birdforum.net/showthread.php?t=369927
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