Since I'm actively looking to replace my old Starmaster with another "large(ish)" dob I've been on CN much more than normal as of late. Some interesting discussions have arisen over the pros and cons of uber-fast mirrors and I was curious about people's opinions here, especially since there seems to be a huge sea-change with the higher-end big dob builders to offer mostly sub f/4 instruments. I know this can be very subjective, but with optics being a constant exercise in compromises and trade-offs, at what point is enough, enough?
There was a time I probably would have given a sub f/4 mirror a shot without much worry. There are some fantastic opticians out there with the ability to make high-quality fast parabolas. After years of collimating my f/4.3 I don't have any worry about properly collimating an f/3.7, or even f/3.3, but what does concern me is the structural ability of even the best mirror cells to keep a mirror that fast spot-on during an entire observing session. Even my old 14.5 Starmaster with its excellent whiffle-tree cell could not keep the mirror absolutely still while slewing. I never had to re-collimate during an evening, but I could see slight alignment changes in either the auto-collimator, or with the hologram laser just from moving the scope in altitude. At f/3.7 we are under a 1/2 mm coma free spot on the mirror, and even less with the f/3.3. The mirror also has to have a tiny bit of room to move to keep the optics from pinching, so how does one get a mirror held with virtually no play yet not risk pinching the mirror, and not gluing it down?
I think what we are seeing is the slow acceptance of the many issues that accompany ultra-fast mirrors in order to get a shorter structure so observing can be done without ladders. With truss scopes I don't see that we gain much in the way of portability - the footprint isn't really smaller. The shorter height might make the difference for leaving the telescope fully assembled for "roll-out" purposes for some people, but other than that I don't see any other advantages given that most big truss dobs are not used for imaging.
I have a very close friend with an extensive optics background. Masters in Optics from Rochester, 9 years in the Brown University optics lab, and for 8 years he ran the interferometry lab for Intel , before deciding to go get his Ph.D from Cal-Tech. He's got less than a year left, and although the Ph.D is in a different field, his worked is completely optics based. Having said that I've talked with him at length on the subject of f/ratio and when it comes down to it - coma, astigmatism, controlling zones with steep parabolas, structural tolerances needed for steep parabolas, depth of focus, etc., he really feels that ultimately f/5 should be the cutoff.
Not wanting to possibly put them in any uncomfortable position on a public forum, I am reluctant to put out the names of three top opticians that I have spoken with on the subject, two of which I have met and observed with personally, one quite often. They all make mirrors faster than f/5 due to market demand, but that's really the only reason. Suffice it to say that myself with a few others in my old observing group have discussed the f/ratio question with them, and all of them from both a personal use & manufacturing standpoint prefer f/5 or slower mirrors. This is tough to ignore IMHO.
My own experiences over the last 3 years with not being able to do anything but lunar and planetary observing has also changed my perspective. I have come to appreciate the slower f/ratios much more and especially find the depth of focus far more accommodating. The longer f/l eyepieces are generally more comfortable to use, and as my eyes age, a slower system is a little more forgiving in that respect. Contrast and sharpness seem as good as it gets, and no Paracorr is nice.
I had been looking for an 18 or 20 inch dob to replace the 14.5, but although I've never actually spent any time with these f/3.3 - f/3.7 telescopes, I have to wonder if I could ever be truly happy with the final image in the eyepiece, especially since diameter also affects off-axis coma in conjunction with f/ratio. Given a 10" vs a 20" f/4, the 20" gets further off-axis than the 10 and beyond 10" will continue to get worse. Even with the new Paracorr 2, the scope still must maintain proper collimation for the Paracorr to do its job.
In the end I really don't think I'd ever want anything faster than the f/4.3 I had, and would prefer to remain at f/5 or above if at all possible. I certainly don't want to deal with a 20" f/5 behemoth, but a 14.5" f/5 or a 16" f/4.5 - f/5 is certainly doable, and I think in the end will be far less frustrating overall. Where I once wanted to own a 20, I'm far more inclined these days to reduce the aperture and keep the focal ratio reasonable to gain a lot more advantages than the singular advantage of a short scope with the sub f/4 systems.
Any thoughts out there?
Oh, yeah. The Stifmeister's coming back to Grand Harbor. Deck the halls. Bye-bye, Great Falls. Wipe my a$$ and lick my ba!!s It's Stifler time, baby. Whoo-hoo-hoo. Whoo-hoo-hoo. ~Steve Stifler-American Pie 2