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PostPosted: Mon Sep 01, 2008 5:21 am 
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If it weren't for the fact that it didn't require so much money to do so, I'd have already finished (read: started and finished) this project, and saved you all the headache of my endless mental struggles. But now that I actually have a probable budget goal ($2K), and a time frame of about a year and a half, I'm closer than ever. So here goes.

The reason for this is to help [me] understand the effects of various elements in the optical train, and which ones are the most important for an end result. I have this dream of building a premium quality newtonian for viewing planets, and topping it off with a binoviewer. I'll mount it on my Old Meade starfinder pier GEM, so I'm tryng to keep the total weight south of 20 lbs to ensure maximum stability.

In an older thread I discussed my initial plans to use my old Meade 10" ?4.5 mirror to build a planetary specific newtonian, which I should test/refigure to maximize it's usefulness. Since then I've considered two possible alternatives: a Discovery 8" ?7 PDHQ, or an Orion 6" ?8 newt OTA. Refiguring the Meade mirror and buying the Discovery would about equal each other in price. The Orion would save loads of cash, enabling me to put more money into the Binoviewer setup, which is a portion of the scope I will not go without. After comparing BV planetary viewing to single eye viewing several times over the past two years, I can assure you that the differences in detail recognition are remarkable.

I already understand the benefits of a small secondary, a curved spider, and a primary that isn't too big so as to avoid the effects of a larger swath of unstable atmosphere. I'm more interested, for sake of this discussion, to understand wave front error alone.

So I'm looking at the following elements in the optical train:

primary mirror
secondary mirror
binoviewer
eyepieces


Where will the most improvements be gained? Is it better to assure the quality of the primary over all else bacause it is at the front of the light path? Is the secondary more important because it is a condensed congregation of magnified light coming from the primary? Will the BV do more to improve the overall image? Are ALL of the elements best to be near equal (say, around 1/8th wave) to ensure consistent image reproduction at the end of the light path, rather than having a 1/16th wave primary and secondary mated to a 1/4 wave BV?

Too many questions, I know. I'm looking for any opinions, based on real world experience or theoretical knowledge. I've got PLENTY of time here. But that means I can crunch every possible configuration in my head over and over...and over, and over, etc... I only want to do this once. So I want to do it right.

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PostPosted: Mon Sep 01, 2008 6:38 am 
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Well, I'll start the discussion, but since I have no ATM experience, take my suggestions with a grain of salt. I suggest concentrating on your mirror and binoviewer - not because of theoretical considerations of their importance in the optical train, but because of their variability in the real world. I know that tests have shown it takes near-perfect seeing and trained eyes to even see the difference between a 1/4 wave PV mirror and a 1/8 wave PV mirror, and PV measurements are one of the poorest ways to judge mirror quality. Still, I'd get a Discovery f/7 at a minimum, and shop around for an R.F. Royce f/8, or another premium mirror. I see Cary at Optic Wave Labs in the Rancho Cordova (Sacramento) area has an 8" f/7.3 with a claimed .999 Strehl advertised on CN for only $500.

I wouldn't worry about which binoviewer until the end, as I'd do all my judging of planetary images Cyclops style, and try binoviewers to see which degrade the image the least, or improve it the most.

You rarely hear horror stories about poor secondary mirrors.

For eyepieces, you want low element counts to minimize light scattering. A couple of my buddies did comparisons of 5 and 7 mm. eyepieces at a star party a couple of years ago, and we all agreed the TMB Super Monos gave the best views. Nagler T6es came close. TV Radians were mid-pack. Worst were Nagler T1s and Pentax XLs. They were all close in sharpness, but the T1s and XLs had poor contrast from internal scattering. True, the T6es gave much wider fovs, but that's not important for planets. I'd go with T6es for general viewing, and would consider TMBs for strictly planetary viewing. If I didn't wear eyeglasses, I'd go with the TMBs.

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PostPosted: Mon Sep 01, 2008 11:19 am 
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I like the idea getting the 10" f/4.5 refigured. You'll get a guaranteed high-quality surface and the shorter tube length will help keep weight down and balance easier.

One thing to keep in mind is that the binoviewer and the small-size diagonal tend to contradict each other. You may need to go full-size, which makes a superb primary surface that much more critical.


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PostPosted: Mon Sep 01, 2008 11:55 am 
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Thanks for starting the discussion, Darrell. I've seen Cary's mirror. 0.999 looks insane. But if anyone has half a brain, that'll be gone long before I can swing for it, if it isn't already.

Interesting opinion on the 2ndary.

As far as EP's go, I'm leaning heavily toward abbes, such as UO HD orthos. I've read great things about the Supermonos, but are they still available?

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PostPosted: Mon Sep 01, 2008 12:01 pm 
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Chopin wrote:
Interesting opinion on the 2ndary.


I might be changing my mind. :(

I haven't worked out the geometry, but it's possible that a longer focal ratio can allow the secondary to shrink again, even with the bino-viewer.

You might want to look at both possibilities.


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PostPosted: Mon Sep 01, 2008 12:54 pm 
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Interesting, Dave, although I'm not sure I completely understand what you mean. Are you suggesting that the optical corrector lens on the BV, which mathematically increases the focal length, also changes the required 2ndary size for full illumination? If so, that poses an interesting advantage for planetary use.

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PostPosted: Mon Sep 01, 2008 1:43 pm 
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Jason,

If you are designing the scope specifically for planetary viewing with a binoviewer, will you need the BV corrector to reach focus?

As for smaller apertures punching through turbulent skies that spoil views of larger scopes, I routinely set up a 6", 8" and 10" together and, while poor seeing may keep the 10" from performing at maximum, it has never failed to match or better the planetary/lunar views of the 6". Ditto the 8". But on nights when seeing permits, I'd much rather have the resolving power of the 10" over the 8" or 6".

It will be interesting to see what your project develop.


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PostPosted: Mon Sep 01, 2008 4:57 pm 
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Gary, you are thinking the way I am, that the primary can be brought forward to compensate for the BV. However, my concern is that I may not be able to fully illuminate the center of the FOV without the use of an OCS/OCA. Hmmm, possibly that's what Dave was referring to, creating an f/ratio that can sqeeze the light cone down the BV ota while still creating full illumination? Maybe there's a simpler solution that I'm not considering.

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PostPosted: Mon Sep 01, 2008 6:05 pm 
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Are you talking about turning your 10" f/4.5 into something like an f/7, or just refigure it to a finely polished f/4.5? The longer focal ratio would give tremendous views, but the OTA length and size would be a real armful.

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PostPosted: Tue Sep 02, 2008 6:31 am 
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Jason,

You're right of course. f/4.5 creates a special challenge for specific binoviewer setups. Because you're focusing on building a scope that is specialized for planetary work, I keep assuming you'll end up with something much slower than f/4.5, which would mean an entirely different primary. The f/4.5 will require a fairly large secondary, even to illuminate the minimal field needed for planetary work. It may be that your best bet is to trade for a slower primary.

FWIW, I really like a 10"f/6 for all around work. Something slower would be nice for a planetary instrument, but you pay the price of a very long tube and the viewing discomfort that comes with it. My 12"f/7.1 has produced the finest planetary views I've ever enjoyed, but doesn't get a lot of use because the tube is almost 8' long and is therefore a pain to use.

A 10"f/6 with a 2.14" secondary produces an illuminated field that is very close to that of a 10"f/4.5 with a 2.6" secondary. How much will the 26%+ central obstruction degrade perceived contrast over the 2.14%+ obstruction? Not a lot. But enough for you to see a very slight difference and if your goal is to create an instrument that is optimized for planetary work, I assume you want the highest contrast possible.

I'm in the process of rebuilding my 10"f5/6 Newt and was considering compromising with a 2.14" secondary. But, the more I thought about it, the more I realized that I didn't want to compromise. I want to squeeze as much contrast out of it as possible when my target is a planet or the moon and I also want to be able to illuminate a wide enough field to get full benefit from my widest 2" eyepieces. So, I'll switch between two secondaries: a 2.6" for deep-sky and a 1.83" for planetary.

I'd rather the 10" was an f/6, but the f/5.6 is an old ZOC that's such a wonderful piece of glass, I can't bring myself to trade it. Meanwhile, I'll make you a screaming deal on a f/7.1.


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PostPosted: Tue Sep 02, 2008 6:59 am 
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Darrell, can you backward figure a mirror? That question aside, I'd plan to keep it at 4.5. You hit the nail on the head with the tube length. As is, the 8" ?7 PDHQ is as long a tube as I'd want for my current mount, being about 5' long. Even if I could keep the weight down on a 10" ?7 (likely with a phenolic impregnated paper tube), the cantalever effect would be tremendous, and still stress the Star Finder mount. I know there will be compromises in my end product, as far as size, since I'm attempting this project within a relatively strict and low budget. I have no reason to believe that a faster ?/ratio will be a disadvantage for planetary viewing, except for the need to collimate more often, and be spot on.

I'm really leaning toward the PDHQ, since I know the mirrors are of good consistent quality (wiht documentation available), the secondary is already %19 CO, and the OTA comes with a crayford focuser. All I have to do is add flocking, a fan, and a curved spider to finish the scope. For roughly $1200 that is really hard to beat, IME. But if I can find a mirror like Cary's when I'm ready to plunk down the cash, I'd do even better. Then I could just build the OTA from scratch to fit the mirror. I figure the end cost would be about the same.

Gary, you made a great point about the 10" out shining the smaller apertures. Maybe I'm hesitent to push the aperture because "good" nights come so infrequently in my area. I could also be putting too much weight into this fear. Although I can say that my 12" dob rarely shows the planets well, even when appropriately cooled. And when the seeing is poor (2-4/10 P), while my friends' smaller 4" to 7" scopes do show some decrease in sharpness, my 12" absolutley falls on it's face. BTW, I know that the mirror is good, at least by eye, since I've had my dob as high as 563x on one exceptionally still night, where Jupiter had insanely colorful bands, numerous micro storms, and countless festoons. I know the aperture helped here, but without seeing like that I've never been able to replicate that night. Maybe you can counter my fears and pump talk me to consider the 10". You undoubtedly have greater experiences than me with this, so I'm willing to listen and learn. You also mention that you have that you have an ?7.1 you're willing to part with. Is that 10" or smaller? PM me the details if you want. I can't promise that I could swing for it any time soon. -- foget I asked this, for some reason I missed your post until it was in my post editor, where I caught the last sentence. I assume it's the 12", so, yeah, too big. :grin:

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PostPosted: Tue Sep 02, 2008 7:34 am 
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Jason,

No, the f/7.1 is a 12.5" and I really mentioned it as a joke since it's great for demonstrating an extreme case of the problem created by the too-long tube you're trying to avoid.

Also, don't assume that I've any more meaningful experience than you. Yes, I've been in total astro-stuff immersion mode for 4 to 5 years now, but I'm a lowly lawyer by training and, while that background may by great preparation for debating the merits of various scope configurations, that's about all it's good for.

My last post neglected to consider several of your factors, not the least of which is your mount and your location. My current location generally provides better than average seeing - frequent stagnant highs pressure domes - so I regularly get full benefit of the medium aperture Newts. I've no doubt that plays a large role in my perception of the relative performance of the 6" and 8" scopes and it is foolish of me to assume that your experience would be the same.

As for your mount, a 60"+ tube weighing up to 50# with accessories is clearly going to stress the Starfinder (although some would debate that as well). I've tried several scopes on the same mount and the largest I was happy with was the 6"f/8. I haven't had an 8" Newt to try on it, but am satisfied that it would handle it easily.

Given the factors I was neglecting, including budget, I really like the 8"f/7.

Gary


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 Post subject: Relay lens?
PostPosted: Tue Sep 09, 2008 4:55 am 
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So I need an education on how one would introduce a relay lens into the light path of a newt, and what it would do to the light path from a mathematical stand point. It seems that the lens would enable a binoviewer (or camera, etc...) the ability to focus without an optical corrector setup. So, in effect, the relay lens becomes a sort of optical corrector in itself, I think. OK, maybe I don't quite get it. Research on google came up a bit short for me.


Also, I wonder, would this introduce CA, fringing, etc...

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PostPosted: Tue Sep 09, 2008 12:42 pm 
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Jason,

You can have a mirror ground any way you want, turning a concave one to convex, if you've got enough glass and time to spare ;) For planets, the planet killer reflectors tend to be 6" or 8" aperture, and f6 to f/8. You don't need the 10" and larger apertures to view bright planets. The slower focal ratios give you better corrected images, with higher contrast due to smaller secondary mirrors.

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PostPosted: Tue Sep 09, 2008 7:44 pm 
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Size and weight aside, I think the perfect planetary scope would be a flocked 8" f/8, with a nice Crayford focuser, an open mirror cell for fast cooldown, a curved secondary support, and a small secondary mirror. And of course, a well figured primary mirror. 8" is enough aperture that resolution isn't an issue, and f/8 is long enough that a small collimation error will adversely affect high power images.

I've looked through a lot of high-end refractors in the 3-6" range, and they don't come close to a modified Newtonian on the planets, IMO.

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PostPosted: Tue Sep 09, 2008 10:10 pm 
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I like your thinking, Erik. Every spec you stated there is on par with my current plans. If I do decide to risk the ?8 design I can avoid optical correctors or a relay lens and still illuminate the planetary disk with a BV in place. If I go with the ?6, which will be less taxing on my mount, I'll have to consider the corrector/relay lens route.

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PostPosted: Tue Sep 09, 2008 10:43 pm 
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This is my current favorite plan, which just so happens to be an 8" ?8. The green blocks represent estimated weights.

Image


Bryan really runs a good site for newtonian ATMers. Much of the scope will likely be built with his parts. The Discovery primary seems to be the most cost effective, while still offering a high quality figure, and comes with documentaion. I already own a Moonlight focuser for my Hardin, so getting one for this scope is a no-brainer. The final piece is the Denk II BV body. After doing some pretty stout research, it seems you'd have to spend much more to do better, if you can do better. I'm not sure how many people have done a thorough side-by-side of the Denk II against the TV, the Baader or the Zeiss.

At any rate, this is the preliminary stage of design. Comments or suggestions?

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Last edited by Chopin on Mon Nov 24, 2008 6:52 am, edited 3 times in total.

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PostPosted: Wed Sep 10, 2008 5:06 am 
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Jason:
This is an interesting project. A few observations, not in any particular order:
At f/<5 you have a lot of light coming in to the EP so using say 8mm EPs with a 1.3X OC will give some reasonable magnification for our typical viewing conditions. I am working on a similar project, yours will probably be done sooner, but I have a Protostar 1.52 inch quartz diagonal which I hope to utilize with a relay lens assembly. SInce we will both be experimenting with arrangements after the primary is set, you're welcome to try my diagonal in your tube before committing to buying one yourself. My attempt will be to try and increase the distance of the primary to secondary mirrors and use a relay lens assembly. My relay lens will go just inside the tube prior to the focuser and will consist of two high quality (Zeiss or Fujinon) achromats, facing each other separated by one or two mm. This should bring the focal point outside the tube, with an image of correct orientation. Well, that's the plan anyhow. As I mentioned in a previous email, I have a number of the achromats and will give you two of each. Hopefully your experience will let me know if using them in my 12.4 inch f/4 is reasonable. I am reasonably certain they will work well in an f/5 or f/6, but am not positive of what if any chromatic aberation will be imparted.
Also, Normand Fullum and others, will certainly refigure your primary at a reasonable price and since the Protostar is at least 1/10 wave having a primary at that error certainly won't cost much more than 1/8 wave which is about the best you'll be abkle to discern.

George

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PostPosted: Wed Sep 10, 2008 7:51 pm 
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Looks like a good plan, Jason! I'm looking forward to seeing the project as it progresses. :)

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PostPosted: Wed Sep 10, 2008 8:19 pm 
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You might want to be patient on the mirror. There's a thread on CN talking about how long the wait seems to be in getting a mirror from Discovery. Terry Ostahowski prioritizes his own direct orders, and works on the discount Discovery mirrors on his "off" time.


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PostPosted: Wed Sep 10, 2008 8:31 pm 
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They do come up occasionally on AM and elsewhere.

What about Antares? I know they're GSO, but don't they guarantee 1/6th wave and come with some sort of certification?

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PostPosted: Tue Sep 16, 2008 9:34 am 
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I'm designing and redesigning by the minute, my friends. I haven't had the opportunity to sit at the computer to work out my latest train of thought until this morning. Regarding my own initial intention to design this around a BV, I'm considering the possibility of dropping that idea, at least for this moment in time. The more thought I put into the project design, the simpler I want to go. Purity of design will help me achieve a low wave front at the eyepiece, and that is ultimately what I want more than anything. The relay lens is also quite interesting, but since I'd rather not add more air to glass surfaces to the final product, I'm stashing that idea away for a rainy day (thanks for getting me to think outside of the box, though, George). The only areas where I am still grey are the focal length, where I'm teetering between ?6 and ?8, and the secondary spider, which will either be a curved arch or a three vane straight design. The basics I am relatively sure of at this point are: 8" primary, 1.3" secondary, helical focuser, and closed tube design with active cooling.

Here are the current plans:

Image

Image

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Getting closer, maybe...

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Last edited by Chopin on Mon Nov 24, 2008 6:53 am, edited 1 time in total.

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PostPosted: Tue Sep 16, 2008 9:56 pm 
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The theoretical limiting magnitude reported by NEWT (as well as some other calculators) is a bit conservative. This one will get you well below 15 in a dark sky. The best I've done in my 10" is 15.2, but even that should get your 8" to about 14.5 or so.

F6 would give you a shorter tube, but 8" is small enough where even a 72" tube at f8 wouldn't be too unwieldy, unless you're in somewhat cramped conditions. It would also allow you to use a slightly smaller secondary, which would help slightly on planetary contrast.

And it would improve the performance of some of your eyepieces.

I don't really have an opinion on the three-vane or curved spider, although in theory a three-vane curved spider should have less total diffraction than a 4-vane strait one.

My 16" will use (at least initially) a 3-point wire spider. I'm very curious to see how well that works. Then maybe I'll be in a position to have a more informed opinion.


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PostPosted: Wed Sep 17, 2008 12:27 am 
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Dave, thanks for the link. I actually just put it up in the ATM links of interest.

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PostPosted: Sat Sep 20, 2008 3:32 pm 
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Jason,

I was up at Farpoint/Optic Wave Labs on Friday, and Cary has some nice 8" planetary mirrors in stock. He has a 1/10th wave f/7 for $650, as well as some nice lightweight conical mirrors that would make great planetary optics. Just thought I'd let you know since you were looking for a mirror...

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Homebuilt 16" Truss Dob
8" Antares f/5 Newt
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos/Peterson PipeMount


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PostPosted: Sat Sep 20, 2008 3:58 pm 
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Erik, thanks for the heads up. :)

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PostPosted: Sat Sep 20, 2008 8:35 pm 
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Like Eric I favour f/8 or more for planetary observing; however I purchased a 4" refractor for this. Trouble is since I bought the refractor before Christmas we've had nothing but monsoon or fog around here.

I'm not buying any more astro gear for at least a year, in hope that the weather gods will give me a break. Besides, I just bought a new DSLR (Nikon D90). How did I get myself into TWO expensive hobbies???

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PostPosted: Sun Sep 21, 2008 12:49 pm 
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:lol: I'm right there with you John. The only way I was able to get myself on the list for the "hopeful" next run of Supermonos was promising my wife that I wouldn't bug her about that 400 5.6 "L" anymore. :grin:

I wish I could figure a way to grow that kind of "green" in my organic garden. ;)

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PostPosted: Sun Sep 21, 2008 5:05 pm 
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When I discovered astronomy, my good fortune included running into a fellow who advised, "Invest in the highest quality optics you can afford." You're not likely to outgrow the Supermonos and, while others may debate their equal, there are none finer for planetary work. I had a chance to compare them to most of the other well known contenders at WSP this year, including an amazingly prestine and complete set of Claves, and none outperformed the Supermonos.

Be forewarned -- the fellow who gave me the advice failed to anticipate that, when it comes to astro-stuff, my measure of what I can afford would increase even as my post-retirement disposable income fell. Some call it rationalization, but I refuse to assign any rational motive to these discussions;-)

I can think of no more interesting way to design a planetary viewing system than to select the eyepieces first and work toward the ground from there.


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PostPosted: Sun Sep 21, 2008 5:33 pm 
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Good points, Gary. Jason, I may have missed it in one of your posts, but will this scope be driven? As I mentioned earlier, my reluctance towards using narrow ep's is primarily based on the fact that my main scope is a big dob. I may feel differently if I had a scope on a solid EQ mount that tracked well.

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-Erik Wilcox
Homebuilt 16" Truss Dob
8" Antares f/5 Newt
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos/Peterson PipeMount


"Newt Gingrich is what stupid people think smart people sound like."
-The Great Paul Krugman

Evolution is both fact and theory. Creationism is neither. -Anonymous


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