Hi Rich: I don't want to leave the impression that a short scope isn't capable of good performance. If the mirror is well-made, the center of the field will be diffraction-limited. It's just that the shorter you go, the smaller that area becomes unless you invoke supplementary optics. Using wide-field eyepieces just lets you look at more field that's outside the "sweet-spot". While a "pleasing" view is certainly provided, it won't be "tack-sharp". Has the definition of a good view changed over the years? Not being able to join the social side of astronomy very often, maybe I'm just out of touch with current paradigms. The dividing-line between "reasonable" and "short" f-ratio is even different depending on who you ask. Vaughn Parsons once told me that he didn't like to go faster than about f/5, 5.5 was better, 6 better still. Some opticians say that f/5 produces an acceptably large diffraction-limited field. Going with f/7, like Jim, pretty much gets the nod of approval from everyone. For casual visual use, this usually isn't even a problem. Comet-hunters are the typical "victims"- what at first glance looks like a probable hit near the edge of the field becomes just another star when moved to the center and focused critically. When we look at planets at high-power, we are only using the very center of the field anyway...an RFT can be an excellent performer in this instance. I've got a 12.5" f/5 mirror that I want to build a scope around one day, as well as a 17.5" f/4.5 that's waiting its turn, also. Would I be happier if they were f/7? Maybe, but they probably wouldn't get used as much simply because I'd break my neck falling off the ladder, slip a disc trying to set them up, or not be able to afford an Expedition to haul them around in. My own physical & financial limitations force me to "shorten-up", against accepted optical wisdom. I like to avoid conflict, but sometimes draw it due to my limited communications skills, darnit. Were I a better writer, I'd have a cubicle next to Joe's, instead of making sawdust for a living! C. --- Richard Tenney <retenney@yahoo.com> wrote:
No, my bad, you did say half the size, I'm the one that got it mixed up with brightness.
And I don't think Dave was too offended; he did add a smiley in there to let you know that. :-)
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