Uber-Fast Dobs.....Great or not so Great?

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Kawx4d
Post subject: Uber-Fast Dobs.....Great or not so Great?
Posted: Wed Sep 22, 2010 12:01 pm
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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?

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Chopin
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Posted: Wed Sep 22, 2010 3:23 pm
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Of course only you know what your order of preferences and compromises are. But for me, that is if I were in your shoes looking for a dob in that size, it would have to be an Obsession 15" Ultra Compact. At f4.3 you are right at the brink of contollable coma, but even better is how insanely small and lightweight that scope is. I think you could leave it built in your living room and lift it with two hands right out the door for literally grab and go viewing. Just a thought. But then I'm at a point where my 12" Hardin is just large enough that I find it to be a pain in the arse to set up regularly. At 65lbs, the fully built 15UC is 3lbs lighter than just my 12" tube alone. Insane.

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Chopin
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Posted: Wed Sep 22, 2010 4:13 pm
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Although the optics on the Hardin are excellent, you know. Maybe my f5 dob is something you're looking for...say an even trade for your Stowaway??? If that doesn't seem like a fair swap, feel free to throw in a few eyepieces... :mrgreen:

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Kawx4d
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Posted: Wed Sep 22, 2010 4:40 pm
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Chopin wrote:
Of course only you know what your order of preferences and compromises are. But for me, that is if I were in your shoes looking for a dob in that size, it would have to be an Obsession 15" Ultra Compact. At f4.3 you are right at the brink of contollable coma, but even better is how insanely small and lightweight that scope is. I think you could leave it built in your living room and lift it with two hands right out the door for literally grab and go viewing. Just a thought. But then I'm at a point where my 12" Hardin is just large enough that I find it to be a pain in the arse to set up regularly. At 65lbs, the fully built 15UC is 3lbs lighter than just my 12" tube alone. Insane.



Thanks Jason.

Actually I have looked the Obsession UCs up and down several times. I almost pulled the trigger on the 18 a while back, but a couple things made me back away. Weird to say but Dave Kriege pretty much talked me out of it until I repeated for the third time that limited storage and portability were key. It was a very strange email conversation but he was really pushing the traditional 18 over the UC. That was a head-scratcher. All things being equal I'd choose the stability of the traditional in most cases, so maybe that's what he was trying to convey, but I was looking for something that transported better, and whether or not he meant to come across this way, he seemed less confident in his UC product.

The other is that an 18UC owner I used to observe with regularly lives less than half a mile from me, and he's had quite a few issues - mostly minor, but one pretty serious one with the mirror cell. He generally likes the scope, but some of the things he's had to do to it should not be left to the consumer after dropping $7K. I know I'd be a bit peeved if I had to adjust or fix a few of the things he mentioned on a scope of that caliber. His was an earlier model so maybe some of the things got worked out, but I know the mirror-cell design issue has remained the same on all the models, even after he told Dave about it. It has about a half-inch of lateral play on either side, and while transporting it over a rough road the mirror slammed against the stop, and a star-washer chipped the edge of the mirror. Fortunately it was not the usable mirror surface but on the bevel and a little below into the blank. He got lucky. Keep in mind the scope was inside the $500 transport case he purchased separately, so it obviously did not protect from this particular problem.

Now transport is less of an issue for me because I just don't get out, and if I did make the occasional trip, I'd find a way to make it work. I probably will never get to dark skies as much as I once did, at least not in the next 10 years. I just don't see it happening. So I figured if I had a traditional full dob, I'd probably just attach an axle and some pneumatic wheels directly to the rocker box, tilt the entire unit and roll it outside. I'd make the wheel-unit easy to remove when the scope is use. Still I'm seriously looking at backing down to the 12.5 - 16 range with a more reasonable f/ratio, but I could still make the axle work for just rolling out of the house.

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Kawx4d
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Posted: Wed Sep 22, 2010 4:48 pm
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Chopin wrote:
Maybe my f5 dob is something you're looking for...say an even trade for your Stowaway??? If that doesn't seem like a fair swap, feel free to throw in a few eyepieces... :mrgreen:


:hilarious: :hilarious:

Ah, that scope will be buried with me, or actually my wife. She's not into astronomy, but during the 2003 Mars apparition, she spent three hours with me viewing it in the 14.5 & the Stowaway. She loved both scopes but loved the Stowaway so much that she confiscated it and to this day still tells everyone that it's her scope. You should see the looks I get with just the mere mention of perhaps selling it to fund another scope. Honestly though it's a keeper.

I'll tell her your interested in a trade! :mrgreen:

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Chopin
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Posted: Wed Sep 22, 2010 6:30 pm
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He he. Sorry, though. I looked back and realized I veered from your initial post by suggesting the UC. Sucks to hear that about the scope, though. I wonder if that's true of all the UC's or just the 18"???

Anyhoo, I feel f5 is the lowest you can go without corrective coma intervention. So in that regard I concur with your friend. Size is a toughie, though. Climbing above the 14" mark makes f5 scopes a bear to view through. What would you consider the three most important attributes, in order from most to least? Mirror quality? Size? Weight? Mechanical quality? Cost?

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Kawx4d
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Posted: Wed Sep 22, 2010 7:28 pm
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Chopin wrote:
What would you consider the three most important attributes, in order from most to least? Mirror quality? Size? Weight? Mechanical quality? Cost?


Honestly I'm a picky bastard when it comes to equipment. Mirror quality is first, and mechanical quality is a very, very close second.

Cost is important from the standpoint that I don't want to go into debt for a telescope. I don't mind spending more for a quality optic and mechanical structure but I'm not going to be ridiculous about it. So this one could migrate depending on the situation at the time.

I'd say weight, then size. Once I know the size range I want, I don't have to have an instrument at the top end of that range if one of the smaller ones fits better. That also means I'm willing to drop some aperture for a higher quality builder in order to keep cost in line.

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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 :lol: :lol: :lol:


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Chopin
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Posted: Wed Sep 22, 2010 7:59 pm
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I'm like you, optics and mechanics first, although weight is always a close third for me since less weight usually means less vibration. Seems to me, since you seem to share similar desires, I'd recommend sticking to he 12"-14" range and get the best package you can. I'm betting that you'll use it more, too. Big scopes require time to move and cool down. And since time seems to be in low quantity for you these days... Plus f5 is easy enough to work into this aperture range. I know that aperture rules, but why own a Ferrari if you live on a dirt road?

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Kawx4d
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Posted: Wed Sep 22, 2010 8:31 pm
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Aside from it being an f/4.3, I'm beginning to realize that I probably sold the perfect telescope for me when I let the Starmaster go. Although I wasn't going much to dark skies at the time, I really did not figure on this kind of drought, and sold it help fund a larger scope. I was into some serious deep-sky - Hickson Groups, Aarp & Abell Objects. The 14.5 would show them OK but when I'd look at those objects through a 20, I realized it was much more conducive to studying them. Then the bottom fell out of the dark site trips. I would agree that where I find myself now, a 12.5 or 14.5 is probably ideal. These sizes offer some incredible resolution and detail on the moon and planets, and are easy enough to transport should the occasion present itself.

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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 :lol: :lol: :lol:


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Chopin
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Posted: Wed Sep 22, 2010 8:43 pm
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So which manufacturers are you leaning toward?

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Erik
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Based on my own observations, I think that optics down to at least about f/4 (assuming good optical quality) can perform wonderfully on planets. The biggest issue of course is collimation. And obviously, it's much more difficult to figure a fast optic. The depth of focus is a valid point but given the size difference when looking at large aperture scopes, I feel that it's a small price to pay. My 16" f/4.5 is a nice ratio, with the eyepiece height being 72" at the zenith. But if I were looking at a 20", it certainly wouldn't be f/4.5! I'd happily go down to f/4 or faster to maintain the eyepiece height.

Of you could just get a Piece of Schmidt and forget about eyepiece height. :lol:

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Kawx4d
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Posted: Thu Sep 23, 2010 7:19 am
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True, a 20" would be shorter, and if I hit dark skies regularly, I'd go ahead and make that compromise on a scope that size, but I would not go shorter than f/4. My f/4.3 was a great planetary performer as well, but there was some disagreement about whether or not to leave in the Paracorr for L / P use. I found it best to leave it in to reduce the comatic smearing of fine detail. It was subtle, but there, at least to my eyes.


Erik wrote:
Or you could just get a Piece of Schmidt and forget about eyepiece height. lol


Blasphemy! I would buy a department store telescope before I'd ever own another Piece of Schmidt! :lol:

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Kawx4d
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Posted: Thu Sep 23, 2010 8:15 am
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Chopin wrote:
So which manufacturers are you leaning toward?


It will depend on the size and f/ratio I choose I guess. I'm pretty limited if I want to stay f/5 or slower. The 12.5 Portaball is f/5. A friend of mine owns both the 8 & 12.5, so I've used them quite a bit and they are really nice, and will travel anywhere. They pretty much disappear for storage. Dave Juckem is also working on an 18" f/4.5 version which would be OK with me for that size. The downside to those is more sensitivity to balance, but I think Juckem has minimized that with his new models.

Having owned a Starmaster before, I know exactly what I'm getting, so I'd stay with the f/4.3 version of any size there. Same with Webster for the 14.5 f/4.3. Obsession has a 12.5 f/5 and both a 15 & 18 f/4.5, but they would be my last choice of the higher-end builders. In talking with owners out in the observing field it seems they need to do more to their scopes to dial in the mechanicals for balance and the mirror cell than any of the others.

The only higher end dob builder out there that I know, that will custom make for any f/ratio is Teeter. I've never seen or used one of his scopes, so it would be unknown territory for me. If I went that way, I'd have Bob Royce make the mirror and either do a 12.5 f/6 or a 14.5 f/5 and probably choose the continuous-face conical mirror if Teeter could build a cell to accommodate it.

Or, there is the alternative of just not doing anything right now until things change enough that I can again make trips to dark skies. I'd like to have more than an 8" Dall-Kirkham for backyard L/P, so maybe find a 10" or a 12" in a longer focal length for that purpose, and forget about any of the issues associated with future transport. Royce has a really nice 10" f/6 tube Newtonian with a rotating focuser assembly and one of his conical mirrors which will cool very quickly. He'd probably make a 12.5" if I called him. I've got some more thinking to do.

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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 :lol: :lol: :lol:


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Talyn
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Erik wrote:
But if I were looking at a 20", it certainly wouldn't be f/4.5! I'd happily go down to f/4 or faster to maintain the eyepiece height.


I'm getting a 20" sometime next year. I am buying the scope body minus the optics - the owner transferred the optics to a permanently mouted equatorial 20" - and am having a mirror made for it.
The original mirror is an f/3.7, but the guy who will make my mirror would prefer to do an f/4, f/4.5 or an f/5 and I'm happy with this because of collimation and coma issues. I don't want to be collimating it every so often during an observing session.
I also don't mind climbing a ladder, which is just as well as an f/4 would be 6'7" long (plus base), an f/4.5 would be 7'5" (plus the base) and an f/5 would be 8'2" plus the base height. If it stayed as an f/3.7, the height would be 6'1" which, as I am 5'9", would still need a step to reach the eyepiece.
A friend of mine has a 20" f/6 mirror which he's going to make into a scope soon, and that works out at nearly 10 feet high.

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Kawx4d
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Have you decided which f/ratio you will go with, or are you in discussion with the optician? Sounds like it will be a hell of a scope when it's finished!

I'm with you on the f/3.7 and not going there. I just think we're getting into territory that is pretty unnecessary and compromised. Sub f/4 puts too much strain on the entire optical system - tolerances throughout from the mirror cell to the focuser have to be spot-on at all times for maximum performance, and given these scopes are not made like the ones for professional observatories, I think we're just pushing things too far. I want to spend time observing, not dialing in everything and maybe having to do it more than once per evening.

20" f/6? :boggle: Dubbed "The Beast" no doubt! :mrgreen:

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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 :lol: :lol: :lol:


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Talyn
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Kawx4d wrote:
Have you decided which f/ratio you will go with, or are you in discussion with the optician? Sounds like it will be a hell of a scope when it's finished!

I'm with you on the f/3.7 and not going there. I just think we're getting into territory that is pretty unnecessary and compromised. Sub f/4 puts too much strain on the entire optical system - tolerances throughout from the mirror cell to the focuser have to be spot-on at all times for maximum performance, and given these scopes are not made like the ones for professional observatories, I think we're just pushing things too far. I want to spend time observing, not dialing in everything and maybe having to do it more than once per evening.

20" f/6? :boggle: Dubbed "The Beast" no doubt! :mrgreen:


It'll probably be f/4.5. Okay, that means I'll end up with a 7'5" tube but that's no big deal for me - at 5'9" tall I will have to use steps anyway regardless of the f-ratio. It also means doing modifications to the scope (longer truss poles, adjusting the attachment angles, etc), but that's nothing that can't be overcome.
Like you, I want to spend my time observing and not tweaking collimation and all that kind of messing around. Clear nights here are too precious to waste them messing about with fiddly stuff (another reason I don't do imaging :mrgreen: )!

I think that, in the hands of excellent opticians such as Mike Lockwood, etc, fast ratios won't be a problem as regards astigmatism and other errors, but if less experienced people attempt them then they might have problems. We have excellent opticians over here in the UK, but only one of them as far as I know has made a fast mirror (that f/3.7) and he doesn't make mirrors any more.

Yep, when my friend's finished his 20" f/6, I'll be interested to see it, it'll be 10ft tall and quite impressive.

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Dave Mitsky
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A friend of mine (who also has an 18" Obsession, along with a very impressive retinue of other scopes) owns a 22" SDM (Size Does Matter) Dob, the first one to be sold in the United States, with an f/3.6 Kennedy primary.

http://www.sdmtelescopes.com.au/

I've been very impressed with this beautiful telescope. Being able to stand on the ground and make use of an aperture of that size is very addicting. As far as I know, he hasn't had an major problems with maintaining collimation. He has ordered one of the new Tele Vue Paracorrs.

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Talyn
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I've just been browsing that SDM website, those scopes are so beautiful. :soppy:

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Not Sure
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Just too bad you have to be an employee
of the Vatican to afford one.

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Chopin
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Not Sure wrote:
Just too bad you have to be an employee
of the Vatican to afford one.


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Chopin
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Kawx4d wrote:
Chopin wrote:
So which manufacturers are you leaning toward?
The only higher end dob builder out there that I know, that will custom make for any f/ratio is Teeter. I've never seen or used one of his scopes, so it would be unknown territory for me. If I went that way, I'd have Bob Royce make the mirror and either do a 12.5 f/6 or a 14.5 f/5 and probably choose the continuous-face conical mirror if Teeter could build a cell to accommodate it.


I've looked through a 14.5" Teeter, but honestly wasn't in a position to compare against anything. I do remember liking the views, and believe the EP was an older style 13mm Nagler. No clue as to what the mirror was, and honestly it was like 3 years ago. Aren't you glad I'm here to help out. :lol:

As for Royce, I'm fairly confident you can't get a better mirror. If I were to do a custom newt with a new mirror, and funding was in my favor, I'd hire his hands over anyone's.



Kawx4d also wrote:
Or, there is the alternative of just not doing anything right now until things change enough that I can again make trips to dark skies. I'd like to have more than an 8" Dall-Kirkham for backyard L/P, so maybe find a 10" or a 12" in a longer focal length for that purpose, and forget about any of the issues associated with future transport. Royce has a really nice 10" f/6 tube Newtonian with a rotating focuser assembly and one of his conical mirrors which will cool very quickly. He'd probably make a 12.5" if I called him. I've got some more thinking to do.


I don't think I'd be happy dropping a Mewlon 210, just to grow 2". A 10" ?6 Royce newt would be, without question, optically awesome. But I'd venture that you won't notice enough of a quality difference between Royce and Tak. My gut tells me you are looking for aperture, and anything less than 12" will be settling.


It's difficult to be inside another person's head when making theses decisions. Believe me, I've been there myself. Every scope is a compromise. But it would be helpful to know if you plan on doing more of either high mag viewing (planets, lunar, doubles, globs), or do you plan to view mostly faint stuff. Based on the discussion so far, I'm guessing faint stuff.


I also notice you mention Mak-Newt in your sig. Still considering this route? A 10" MN is serious weight commitment. Do you own a mount that can handle one of these? If money were no object, I'd probably go this route, but then it wouldn't be a problem to afford a CGE to throw it on.

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Kawx4d
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Posted: Sat Sep 25, 2010 5:33 am
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Dave Mitsky wrote:
A friend of mine (who also has an 18" Obsession, along with a very impressive retinue of other scopes) owns a 22" SDM (Size Does Matter) Dob, the first one to be sold in the United States, with an f/3.6 Kennedy primary.

I've been very impressed with this beautiful telescope. Being able to stand on the ground and make use of an aperture of that size is very addicting. As far as I know, he hasn't had an major problems with maintaining collimation. He has ordered one of the new Tele Vue Paracorrs.


Thanks for the info Dave. I first came across their website about 3 years ago and they do make some beautiful telescopes.

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Kawx4d
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Talyn wrote:
Clear nights here are too precious to waste them messing about with fiddly stuff (another reason I don't do imaging :mrgreen: )!


Exactly!! :mrgreen:

I'm lucky to have a lot of clear nights, but my time is limited, and I sit in front of a computer all day in the first place. I don't want to be messing with one and imaging software when I get behind a telescope (or in this case with dobs, to the side of one :grin:) I observe to get away from all of that!

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Scopes and eyepieces are only useful if you can see something with them. I'll keep what I have and happily settle for an effective cloud filter...

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Kawx4d
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Chopin wrote:
Aren't you glad I'm here to help out. :lol:


:lol: :lol: You are always a great help!!



Chopin wrote:
As for Royce, I'm fairly confident you can't get a better mirror. If I were to do a custom newt with a new mirror, and funding was in my favor, I'd hire his hands over anyone's.


My friend had a 12.5 f/5 Plettstone with a Royce conical mirror and it was a stunner. I spent quite a bit of time with it and can honestly say it was every bit as good as my 14.5 Zambuto. So yeah, he would be number one choice to make a mirror if I went that direction.

Royce is a really nice guy too. I've emailed with him quite a few times, and he was always willing to take the time to explain things. Heck he even offered to walk me through building a longer planetary Newt once, but I just didn't have the time to put to it. How many people would offer to do that?

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Chopin
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Kawx4d wrote:
Royce is a really nice guy too. I've emailed with him quite a few times, and he was always willing to take the time to explain things. Heck he even offered to walk me through building a longer planetary Newt once, but I just didn't have the time to put to it. How many people would offer to do that?


He has as much passion for the art and science of astronomy as he does for his business.

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Dave Mitsky
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Posted: Sat Sep 25, 2010 8:32 am
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Kawx4d wrote:
Chopin wrote:
So which manufacturers are you leaning toward?


The only higher end dob builder out there that I know, that will custom make for any f/ratio is Teeter. I've never seen or used one of his scopes, so it would be unknown territory for me. If I went that way, I'd have Bob Royce make the mirror and either do a 12.5 f/6 or a 14.5 f/5 and probably choose the continuous-face conical mirror if Teeter could build a cell to accommodate it.



I happen to know Rob Teeter. His scopes are both well-built and attractive.

http://www.teeterstelescopes.com/teeter.html

Dave Mitsky

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Kawx4d
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Dave Mitsky wrote:
I happen to know Rob Teeter. His scopes are both well-built and attractive.


Much appreciated Dave, thank you! I think his scopes are beautiful as well, but had no idea about the mechanics. When I bought my Starmaster my friend had the 16" so I knew exactly what I was getting. I tend to be very cautious when making large purchases, and will stick with what I know, even if it's not exactly perfect for what I want to do at the time. I've read a lot of your posts and know that you tell it like it is, so this makes me feel better about keeping Teeter in the running for a custom focal length. Thanks!

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Kawx4d
Post subject: Re: Uber-Fast Dobs.....Great or not so Great?
Posted: Sat Aug 04, 2012 7:00 pm
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I came across something interesting the other day while searching the archives in the Zambuto Yahoo group, so I thought I'd resurrect this thread. Carl posts a lot to the group and was talking about big, hyper-fast dobs and the important function of exit pupil in those big scopes. He gives a much better explanation than I've ever heard about the "wasted light". Essentially, what size and f/ratio scope you "really" have at your eyeball. As we previously discussed, the extremely fast mirrors are basically to keep the observer closer to the ground, and I might add are really only practically possible because of the Paracorr. Let's face it, no Paracorr and everything in an f/3 mirror from about 10* off-axis is going to look like shit. But folks still talk about how much more of a wide-field you can get as well, and it appears it's not all it's cracked up to be.

Here was Carl's post:
Quote:
Now back to the large and fast. Victor mentions a 32" at F/3. Let's shake that
down- exit pupil equals eyepiece focal length in millimeters divided by focal
ratio. So let's take a 40mm wide field, and divide it by F/3, for a 13.3mm exit
pupil. Well, that's good for my cat. A functional limit of usable exit pupil for
a 40-50 year old is 5mm perhaps, which means anything over 15mm in effective [eyepiece]
focal length is wasting aperture. What will be seen by the eye will be
everything coming off the primary that fits within F/8. If my math is correct,
you're now using a 12" telescope with a very large secondary, which you will see
floating as a black shadow in the field of view. The rest of the aperture
splashes over your iris and lights up your eyeball. -Might even reduce contrast
:)
So, while people buy these to remain on the ground, or at least closer to it, they still stick in these long f/l wide-field eyepieces to try and get a wider field of view for any given aperture. With these really fast big mirrors, if you do that, you aren't getting 30" of performance, at your eyeball, & are not using the full capacity of that huge mirror you just spent several thousand dollars for, and probably 20K or more for the entire scope. I thought his explanation really laid out, and explained the problem nicely.

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Erik
Post subject: Re: Uber-Fast Dobs.....Great or not so Great?
Posted: Sun Aug 05, 2012 9:52 am
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Kerry,

Carl makes good points in his post except for one huge and glaring omission. Who uses a 40mm eyepiece in an f/3 scope? There's absolutely no reason to. A 21mm Ethos can give nearly the maximum possible TFOV in a 2" format. It is very close to the 31mm Nagler, 41mm Panoptic, or a 50mm 2" plossl.

Yes, the Ethos is expensive but if someone is going to buy a 30" telescope, an eyepiece isn't going to be much of a purchase in comparison.

Even at f/4.5, I rarely find a reason to use an eyepiece with less magnification than the 21mm Ethos. For wide field it fits the bill and with enough aperture, the magnification isn't a problem. I've never seen an object in the 28mm UWAN that I couldn't also see in the 21mm Ethos.

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