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Uh oh! My eyes aren't at the top of my head any more!

Why didn't some one tell me this! I mean, I've been following the discussion on the Obsession Users Group of the 12.5 vs the 15 (not to mention the 18, 20, and 25) and one of my reasons for getting the 15 was that I wouldn't need a ladder. Afterall, the advertised eyepiece height is 66-inches - and I'm 68-inches - maybe a bit more.

OK - call it a "senior moment," but that's how I reasoned. So when the scope arrived a few days ago and I set it up I was surprised to find the eyepiece, when pointed near the zenith, was too high for me. I got out my tape and measured - 66-inches exactly - just as advertised. Of course my eyes are at 64-inches!

OK, have you stopped laughing. Picked yourself off the floor? I'm only willing to make a fool out of myself to this extent because perhaps - just perhaps - some other fool out there is following the same irrational rationale.

Now I'm not all bent out of shape over this. I had considered the 12.5, but for me it just wasn't enough of an aperture jump over my 8-inch SCT, So I'll settle for having my feet on the ground most of the time. Besides, much of my observing is done seated - I really like to spend time on target, not jump around a lot - and my observing chair does put my eyes at 68-inches - that I had actually measured in advance ;-)


I posted the preceding on the Obsession Users Group and got some very helpful responses, the most important of which dealt with how seldom you look near the zenith, especially with a Dobsonian. All major telescope designs have blind spots, or sections of the sky that are awkward as the devil to view. With the Dob its stuff directly over head - or near there.

You can point it straight up - that's easy. But tracking something that is straight over head as the Earth turns presents problems that become obvious as soon as you try to move the scope by hand in this position. You have no leverage, so smooth movement is out of the question! And for technical reasons even when powered the scope can't handle being vertical. One veteran observer said he limits his viewing to stuff below about 80 degrees. "Yipes," I thought - you're cutting out a whole 10 degrees of prime sky! Sounds drastic. Then I thought again.

Start drawing concentric circles around yourself - the first one marking the horizon. As you move upward in 10 degree increments the circles get smaller and smaller. By the time you reach 80 degrees there's just a tiny piece of sky left - and stuff in that circle will be out of it, frequently in a matter of minutes, as the Earth turns. So it being the rainy season here, I went to experimenting with the Obsession in the basement - and sky charts on my computer - trying to get a handle on just what eyepiece height meant to me.

First, understand that the tree line in my yard pretty much eliminates objects below 40 degrees - and frankly, I seldom want to look at things much lower than that because the lower you get, the more atmosphere you're looking through and this degrades the image seriously. So 40 is my usual starting point.

I then pulled up my handy observing chair - with its seat that slides up and down- and found that it took me to about 57 degrees - if I want to be really comfortable - and 68 degrees if comfort wasn't a priority, but I wanted some assistance form the seat. Standing flatfooted I could add another 5 degrees, bringing me to 73 degrees. The next seven degrees will require something to stand on that raises me just two or three inches - certainly easy enough to make. And after that I'm getting near the territory where the scope is dysfunctional.

With these ranges in mind I spent about half an hour with "Starry Night" software planning an observing session that would keep me seated. I stopped planning after the first gazillion objects ;-)
Bottom line. This ain't no issue. But I do want a higher observing chair and my choice at the moment is the CatsPerch Pro plans and hardware. It's sold here and described here: - and be sure to check out the "pics" page with that article. The author makes some modifications that solve the problem of having a big front "foot" that with other chairs gets in the way of the Dob. Very interesting.

Posted by Greg Stone at May 23, 2005 03:51 PM Comments? Please email me:

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