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APO ED

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APO ED

Postby Guest on Mon May 16, 2011 8:57 am

I got some conflicting reviews on the internet, can you please help on making a choice about scopes?

How do you compare the 80 ED APO telescopes to the 80 ED Spotting Scopes?

Considering the use is for terrestrial view only. So the telescopes will require the "amici prism" or another 45o erecting prism.

I am worried about that prism. In one hand I see people saying that the amici prism is perfect and work 100% even at 100x(what is already more-than-enough for terrestrial)... and its only problems are for high magnifications like 200x... and so the telescopes + prism would give better erect view when compared to the scopes. In other hand I read on televue site that they do not recommend the use of erecting prism for their scopes.

So, ignoring the most obvious differences, like: Waterproof, portability, durability, eyepiece versatility, weight and etc... ignoring all that and considering visual quality alone...

How do you compare a 80ED APO telescope with erecting prism vs a spotting scope... for the same price(terrestrial view only).
(Say a televue 85 vs prominar 88... or a Skywatcher 80ED vs Brunton/Bushnell 80ED)
Guest
 

Re: APO ED

Postby Jne_K on Tue May 17, 2011 9:17 am

Hi

Good questions, thank-you.

The vast majority of spotting scopes are, in fact, refractor telescopes, the difference being the spotting scopes are specifically setup for day use, whereas the label "telescope" usually denotes astronomy use. However, there is a lot of crossover, especially with scopes like the Televue 76 and Televue 85, which can do double duty, quite nicely.

ED, HD, Fluorite, APO are terms that denote better color correction than standard achromat objectives, either in a spotting scope or a telescope. If you do not see one of these special glass designations, then you have a standard glass achromat. Better color correction leads to better resolution and contrast, but, in a spotting scope, you really need to be above 40x to see much difference, visually, over a standard glass version. For digiscoping, though, ED and so on is an advantage at any magnification, same as in camera lenses.

Now, then, there is MUCH, MUCH more to the story than simply sticking and ED element in an objective and claiming better color correction and contrast. A cheap scope with ED is still a cheap scope as far as optical performance and is easily put to shame by a high end scope like a Swarovski in a standard glass version. In other words, don't read too much into labels and claims. You still have to put a scope to the test to know what you are getting. Then, too, the eyepiece, whether in a spotting scope or a telescope, can make or break a scope as to performance. It is the eyepiece that is often the limiting factor, not the objective, and, make no mistake, it is very, very expensive to make a high grade eyepiece. Well worth it, though, if you want to bring out the best in either a spotting scope or a telescope.

There are no hard and fast rules as to labeling, but, in general, a scope that is labeled APO (apochromatic) will be fully color corrected, whereas a scope labeled ED, HD and so on may or may not be fully color corrected. In the high end stuff, though, like a Kowa 880 Prominar or a Televue, APO performance is taken for granted, whether they use the APO label or not. On the low and mid-priced stuff, that ED model may or may not achieve a high level of color correction.

We haven't even discussed other optical issues and there is still mechanical construction to consider, too. Again, don't read too much into that ED label. Price, not labels, is a better guide as to what to expect.

As to 200x by day, that is a myth. NO telescope made, regardless of size or quality, is going to let you use 200x by day, or even 100x on most days, due to limitations in the atmosphere. Read my article, What is a spotting scope? for more on this. There is a reason that most conventional spotting scopes only go to 60x.

If you use a standard 90 degree star diagonal on a Televue, you absolutely will get better performance than if you use one of these fine scopes with a 45 degree image erecting prism. Image erecting prisms on small, high end refractor telescopes like a Televue do rob you of performance, period. That's why I use my small APOs with the star diagonal by day. With a star diagonal, you still get upright, erect images, but they are reversed, right to left. With practice, this is not a big deal for day use. If the 90 degree thing bothers you, there is always the Televue 60 degree diagonal.

Lastly, there are weight issues. Small refractor telescopes like a TV-76 or TV-85 were never designed to be throw them over you shoulder and head out into the field kind of scopes. They're fine for setting up at a fixed location, but much too heavy to be carried all day out in the field. For that kind of day work, you will be much better off with a conventional spotting scope.

Hope this all helps. I know it can be overwhelming. Guess the first question to ask is exactly how you plan to use a scope and go from there.
Thanks for posting with us
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