Shopping for scopes online (1 Viewer)

Sorry for delay in responding. I very much appreciate the tips. I'm afraid I'm still not exactly sure what I should be looking for in a star test. I mean to try one with my current scope as soon as we get some sun! (It is November, though, so I might have to wait a while). I admit I'm a little worried that I'll find that my current scope is real junk (although that would justify getting another one!)

Not sure I could describe a star test to the guy at Eagle Optics (although you'd think they'd test the scopes as a matter of course; they don't though). I suspect I'd just ask him to do the following (suggested by Scampo):

a) Directly inspect all lens surfaces for any oddities or unevenness to the coatings or tiny bubbles, scratches or marks of any kind. Look also at the inner lens surfaces as best you can. Try this in varying lights and at different angles of view.

b) Look through the scope at minimum magnification at a blank wall or empty sky (it doesn't matter at all about focus for this test). Can you see any tiny bubbles, grease, smears, hairs or dirt anywhere in the field of view? Send it back!

c) Zoom to full magnification and focus in an out - again, any dirt, grease, smears, particles or bubbles? Send it back!

d) Any roughness in zooming and focusing? Bear in mind the focus will be stiffer when new, but eases with use. By the way, I find myself only ever using the "fine" focus control - you soon come to get used to it, and it really is a delightful focusing mechanism. Keep an eye on this if you feel roughness - but it will almost certainly soon wear off. If it doesn't improve, get in touch with the dealer.

Incidentally I talked to my local optics store. They don't tend to carry any top scopes - they order them in for customers. They promised they'd check out any scope thoroughly before actually selling it to me. However, their price, before 15% tax, was over US$500 more than the cheapest one on the Internet. By going to the US (which I'm doing anyway) to pick up an Internet scope I can save most of the tax and pay just a low shipping cost. So I'm probably looking at paying almost US$700 more if I buy it here. Ouch!

Paul,

The tests you list are good for some things but they won't show the most likely optcal defects. Focus on a "star" at 60x(45X). As you focus you should see a bull's eye pattern of diffraction rings on either side of best focus. In your scope (or any other birding scope) there should be strong rings on one side of focus and weak or even no rings on the other side. This indicates spherical aberration which is unavoidable in this type of scope. The most common defects are: 1) astigmatism, which causes the rings to appear oval instead of circular, with the axis of the oval switching by 90 degrees from one side of focus to the other. 2) miscollimation, which causes the central bright spot (the airy disc) in the diffraction pattern to be off-center and 3) pinched optics, which causes the shape of the diffraction pattern to be distorted, sometimes with flattened or pointed areas. My friend's scope that I wrote about had all 3 of these defects in combination. Severity matters. Small defects (a very slight oval, a tiny bit of airy disc de-centering or a minor flattening along the edge of the diffraction pattern) probably won't do much damage, but nearly zero defects is possible and desirable. In more than very small amounts these defects cause the image to lose sharpness and contrast, especially at high magnifications, compared to a defect free scope.

Henry

Hi Henry

Thanks very much for explicitly describing the star test so clearly. Will be very interested to see how my current scope fares! I'm hoping that someone at Eagle Optics would be willing to do such a test before shipping a scope. As mentioned, the savings would be huge compared to buying locally. By the way, are the defects usually in the eyepiece or the body of the scope? Can one tell without switching eyepieces/bodies?