Saturday, August 05, 2006

eyepiece presentation issues

I think I found an error in the The Backyard Astronomer's Guide (TBAG) book with respect to view orientation on p.58...

The spotting scope (a small refractor, right?) on my cat presents an inverted view. I.e. it is upside-down. Or rotated. Up is down and left is right. Or to put it yet another way, south is up. I verified this in the day with a terrestrial target, my Mountain Equipment Co-op Pingo tent.

Drawing of view of my tent logo through eyepiece of finder and newt.

Mom's newt reflector presents the same view through the eyepiece... It's upside-down or rotated. So, when trying to find things by star chart, I simply need to turn the chart upside-down. Or view from the other side of the table.

When I through through the cat telescope, with the standard mirror diagonal or star diagonal of course, I see a laterally-inverted or mirror-reverse view. I'd have to hold a mirror to the charts or look at the backside of them brightly backlit.

Drawing of view of my tent logo through eyepiece of cat with mirror diag; this also assumes I'm looking "down" into the eyepiece from above the 'scope, so to get a "reference" to the real horizon.

OK. The book says, "Inverted images [are] seen in Newtonians." Later, "[But] mirror-reversed views [are] produced by refractors and catadioptric telescopes."

I find this especially ironic when they're at the same time addressing the surprise and frustration people experience regarding this.

The Sep/Oct issue of nightsky magazine, on p.83, offers the following: binos offer a correct, upright view. Interestingly, they also show a refractor with a 45° diagonal here. A compound telescope (they show a cat) or a refractor, both with a 90°, presents a mirrored view. And finally the finder scope or a reflector (or as noted in their text, a "straight-through" refractor) creates an upside-down view.


Perhaps it is not an error. But  the remark that a refractor produces a laterally-inverted or mirror-reversed assumes that a mirror diagonal is being used. That's common. It's standard equipment on a SCT or MCT. Still, I think it should be stated for clarity.

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