CN FORUMS ARCHIVE Telescope Reviews: reading license plates at five miles

Steve, I have a couple of working rules of thumb that are from a lot of field experience, not a lot of mathematical scenarios. One rule, that seems to work for astronomy and especially for terrestrial, is that often the views start to degrade if the exit pupil is less than 1mm. In astro, there are certainly times when going as low as .5mm works, but they are the exception, not the rule. And in terrestrial, it seems like the atmospheric soup is even more unkind to the really small exit pupils. What that means, is figure the upward limit of your scope to be equal to the objective in mm. For instance, my 70mm Ranger is at it's practical, terrestrial upper end at about 68.57x with a 7mm Nagler. We'll call it 70x. Likewise, a 60mm scope is usually at it's upper end at about 60x, and an 80mm at about 80x. Those are rough upper limits, though they can be pushed, sometimes substantially given the right conditions. However, in real world conditions, looking out through the atmospheric soup, frequently the views are best when the exit pupil is 2-2.5mm. Now, all of a sudden we're talking about a 70mm scope being brighter and showing more detail at 35x.

You've got to remember that the increased magnification also increases any atmospheric distortion, and if the astmospheric distortions are bad enough, when magnified, they can cancel parts of the image. For that reason, many seasoned birders choose a fixed wide angle eyepiece for their spotters of around 30-40x. I used to contend that I could do all the bird scoping I cared to do at about 32-35x. In fact, my most used terrestrial eyepiece in the 70mm Ranger is a Meade 13.8mm SWA that yields 34.78x.

However, recently, I've been experimenting with playing around with the upper limits of practical long range IDs and observations. I find that high magnification is most useful closer in, where you're not having to look through so much atmospheric soup. For instance, looking at birds on the feeders at ridiculous powers like 100x can be fun. But, the longer the distance, and the worse the conditions, the more I have to back off the power. There is a practical limit to that, otherwise we'd be using nothing but our eyes for the really distant birds. I'd put that practical limit somewhere around a 2-3mm exit pupil. So, if you wanted to, you could size your scope based on the idea that it's most useful in the powerband represented by a 1mm(best case)-3mm (worst case) exit pupil. If you couple that with the idea that there's a point of diminishing returns, and that at somewhere between 1/2-3/4 mile the atmosphere adds so much distortion that it negates the effects of very high magnification, then you can see why I think that for practical reasons, a high quality 80mm scope is going to show you just about everything that can be seen.

BVD did this article a long time ago about the merits of CATs for birding. That was one of several factors that led me to eventually get a C5. But, most of his data was from relatively close resolution tests, and he didn't really get into the idea that the atmosphere plays havoc with the magnifications that the C5 is capable of.

However, all that said, there is something to be said for having a scope that not only can push past the 60x barrier that most dedicated spotters are limited to, but can do so without resorting to the image degradation inherrent in even the finest commercial spotting scope zooms. Let me put it this way, as much as I love the sharp, clear bright image in my Leica Televid APO at 30-40, it starts to fall off in quality after 50x and isn't very good at 60x. Whereas the 69x image in the TV Ranger is still pleasing to look at.

Given the current market, if I wanted a scope that could occasionally do high power, didn't have to be waterproof, and could live with a mirror reversed image, I'd look hard at the WO 80 SD or similar. If you don't mind spending a hefty bit more and want a scope that can show everything that there is to see, the TV85 or Sky-90 come to mind as probably being the best there is, but both are much larger than conventional spotters. In fact, there's a trend amongst many knowledgeable birders to go to one of the super high quality 65mm scopes out there simply for portability. IMO, the Swarovski HD is the best of the breed as far as dedicated, compact, spotters, but if I wanted to do that one on a budget, and didn't care about the waterproofing, or the correctly oriented images, I'd look hard at the WO 66 and the new Astro Tech 66.

Mike

--------------------
18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others