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Refractor vs. Reflector ?

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#101 Daniel Mounsey

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Posted 11 March 2005 - 11:41 AM

Hi Daniel,

Hear hear now :smash: ! This is THE most exciting post I've seen all year :applause:

I'm sure many CN'ers would agree! Bravo!


You crack me up Ron. :lol: :lol: :roflmao: :roflmao:

#102 sixela

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Posted 11 March 2005 - 11:42 AM

OK. Take for instance the Maksutov Newtonian. It requires a greater cooldown time because of the Meniscus. However, they produce stable images almost from the word 'go',

When they're small (and they usually are). When they're large, many of them switch to an open tube design (Klevtsov-Cassegrain)...

I've observed tube currents in (large) SCTs, and there's nothing that would lead me to believe that a Maksutov-Cassegrain would behave any differently, though I'm prepared to accept any evidence to the contrary (and preferrably, a falsifiable hypothesis as to the cause).

#103 sixela

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Posted 11 March 2005 - 11:46 AM

By the way, not familiar with a Lymax cooler, please explain.


A Lymax cooler is a device that injects ambient (but filtered) air into an SCT or Maksutov, a kind of reverse vacuumer. You insert it through the focuser end into the baffle.

The fact it exists (and is very popular) is a vivid reminder that large SCTs do *not* cool down fast and are hindered by tube currents.

I seem to have seen recent threads from people with SCTs who posted out of focus star images that were typical for tube currents hugging the tube edge...

#104 Daniel Mounsey

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Posted 11 March 2005 - 12:01 PM

Rob,
I'll let all you guys know when Grissoms curves are available. Ed said his shop is almost up and running. As far as mirror cell design, my 10" and 12.5" both use a nine point flotation. In these medium apertures, the visual results of varios designs look more crucial on the bench, rather than in the actual eyepiece. I'm not saying mirror cell design is not important by any means. They are important, but the majority seen in most high end scopes are very well constructed and designed in my opinion, so I wouldn't worry too much. Pons and Grissom both use old Novak cells and their images are the finest I've ever seen on planets. I'm also not surprised about the curve spider you used giving off a halo. That's why I think so many observers have been discouraged from the curve.

#105 Daniel Mounsey

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Posted 11 March 2005 - 12:06 PM


I wish I could gather all you guys over for a star party at Wilson. I guarantee the odds would be 10-0 in favor of the curve no matter what physics appear to say.

The physics favour those vanes as well, though ;).


Thanks. Now I understand. My goodness, you take no prisoners.

#106 Daniel Mounsey

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Posted 11 March 2005 - 12:34 PM

I'd deffinitely have to say no on the optical window. I would take me forever to explain why. I've had numerous discussions with Pons regarding this matter. The Grissom curve is still way better.

#107 Daniel Mounsey

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Posted 11 March 2005 - 12:37 PM

b.) Creates a more stable image as it closes the tube off.


Not necessarily true. Sadly for CAT owners, they too have to deal with tube currents, and CAT scopes aren't exactly known for eliminating thermal issues and fast cool-down, and those with a Lymax cooler aren't observing with the Lymax cooler in place ;).

The optical window, BTW, isn't neutral and generates its own set of tube currents, given that it's an excellent radiator of energy in to the sky (as CAT owners with dew problems will tell you).

Unless you start building a setup where you temperature control everything (and cool the mirror with a Peltier, heat the optical window, with sensors galore,...).


I totally agree with you Alexis.

#108 Daniel Mounsey

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Posted 11 March 2005 - 01:17 PM

The chimney effect is virtually eliminated in Pons and Grissoms scopes because of their fan set up. All the eddies are completely flushed out on a constant bases, so there isn't any time for build-ups. The view of planets clearly varify this. The fans are always left on. The system as a whole provides completely reliable images from the get go when set up in this manner. Grissom's got a 4" refractor he roles out of his garage. All he does with it is check the seeing before he goes through the work of rolling out the big scope. He turns on the fans and there you have it. I'll tell you what, the guys I view with are the most arrogant swines when it comes to optical perfection. If there's a differnce, they'll pick it up quick, just like that! You wouldn't believe what they're like. They'll settle for nothing less than excellent optical quality. Like I said, I'd love to share these views with people so they could see for themselves how great these scopes really are. My Starmaster has been set up very much similar except for a couple of differences I had to make because it's a truss. Detail on Jupiter looks like braided threads in all these scopes. It's a powerful and exciting experience to witness. You couldn't ask for any better. There's nothing I'd love more then to see someone bring what ever scope design they believed was good and just watch them get floored. There was a friend who brought up a 10" F-20 TEC Mak. He realized it just wasn't the same animal. He sold it a week later. It's not because the TEC was bad, it was because it's all relative. How good is good? What does one consider good? The power lies in the views! No matter how much you debate the numbers, the view which is seen in the eyepiece is the deciding factor! :)

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#109 Guest_**DONOTDELETE**_*

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Posted 11 March 2005 - 01:23 PM

Nothing would please me more than if the Newtonian were given its due reverence, manufactured properly, with the same design thoughtfulness that we see in the Teleport, and AP refractors for example. The Newtonian is my favourite telescope design. I have seen what it can do on many occasions. It is sometimes quite amusing to see the jaws drop when owners of 6" APOs look at Saturn through a cooled 14.5" Teleport. You are quite right about making the effort to reduce thermals and to minimise contrast loss by the use of curves. All these things help improve the performance, although I still maintain that for me, the optics are paramount. Taking the primary and flat from a Chinese 8" f/5 Newtonian and placing them in a wider tube, with curved vanes, a fan at the rear and any other tweak you might want to add, will no doubt improve the performance. The image will still be so-so. Even if Newtonian owners are inspired to make changes to their tubes, there is still one overwhelming issue which will maintain the belief that refractors are the 'cool' telescope for planets. The 'jewelry factor'. There is no getting away from it, refractors look cool.....and.... fortunately for refractor owners, they produce great images.
Having said all that, I actually think the Teleport with the carbon fibre struts, the Spandex accordian shroud, stainless steel mini counterweights, and stainless corners and plaque looks cool. I also think that the Astrosystems Telekit truss tube Dobs, in a light wood with that cute wooden finder also looks cool. But for most astronomers, Newtonians are not 'instrumenty' enough, and do not have the appearence of a precision engineered beautiful tool.
Amateur astronomers who are proud of their telescopes can be placed into two catagories, a.) Don't give a *bleep* what it looks like or what size it is, as long as the image is emotionally involving (here is the key), b.) I want a refractor or an SCT or a Russian Mak or a Mewlon because they look so sexy, and this aspect is the emotional involvement.
The importance of emotional involvement with any product we purchase or want to own should never be underestimated. Whether we like to admit it or not, we are all captives of emotional involvment with regards to decision making. I do not really want to bring aspects of the Sociology of Conspicuous Consumerism into this debate, but it does lie at the heart of this issue.
Most of the time in modern life, content takes a back seat to presentation. It is the main reason, I would suggest, that Newtonians have such poor press when it comes to ultimate imaging tools. If it were not the case, there would have been a movement within amateur astronomy many years ago, to elevate the humble Newtonian to a position where it is generally accepted as the ultimate imager, because of the aperture for money aspect.
The initial spark of intrigue and excitement that this thread has aroused, is because of the statements surrounding the refractor-beating images of the properly designed Newtonians. This is the 'orgasm' (for want of a better word) of any visual astronomical experience. The sheer aesthetic of a great contrasty image.
I see it and hear it all the time within my own group of observing friends. "7 inches is the ultimate aperture for the planets in the UK". "My Mak Newt or my APO gives the best images of all the telescopes on this observing field". (Justification for owning a cool looking APO or an 'instrumenty' Mak Newt, or a statement of fact)? I would suggest the former. I have often found that such people are either blind to reality or self deluding, although to be fair, quite often it is simple inexperience, yes, even amongst those who can afford all the high end gear. People see what they want to see at the eyepiece of another's telescope, in order to feel content with their own. Its that emotional involvement thing again.
Sure the APO and the Mak Newts are great instruments for stable contrast images....but....they are 6" and 7" and 8" apertures. So what? Take the power up to 300X or 400X and you have reached empty magnification.
Now....take a 10" f/7 or 12" f/6 mirror from Zerodur, or AstroSital or Quartz, have CZ figure it, get a small Protostar quartz flat. Now make a wide tube that looks like the tube from a Tak 10" Mewlon, with the nice sexy curve to the end of the tube. Make a matching dewshield from the same material and same product finishing, so that it looks like a giant refractor. A dewshield on a Newtonian? Yes, a dewshield on a Newtonian. Try it, it works well. Make sure the paintwork and product finishing is beautiful and immaculate, give it great baffling, pay attention to detail, in other words, design its looks not just its function. Chrome plate the fixtures and fittings, or at least brushed aluminimun or stainless. Now introduce your curved vanes and fans. Turn the secondary holder into a work of art, something that you would actually want to look at. The same for the rear cell. Place a low profile Starlight or Moonlight Crayford on the side, an FS60C as a finder, place the OTA in running-rail tube rings.
Now you have a Newtonian that you want to sleep with...Now you have a Newtonian that appeals to the refractor lovers...Now you have Newtonians appearing on AP 1200s... Now everyone can suddenly see how good a Newtonian can be. Of course we all knew it all along didn't we?? Refractor? For the planets? Why, when I can use my 12" crowdpuller? Sir Isaac's ghost at Woolsthorpe would be saying - "Well I could have told you all that if anyone had listened in the first place".
Here endeth the rant.

#110 Ron B[ee]

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Posted 11 March 2005 - 01:26 PM

Hi Ron,
When I compared the 10" with the straight spider to the 12.5" using Grissoms curve and noticed even less halo in the 12.5", that's saying a lot. More so than if I were comparing it to a 12.5" with a straight vane. Since the 10" was producing a dimmer image, it gave the 10" all the advantage it needed, and that is a dimmer image to help cut down the halo at nearly the same magnification, but it was still more aesthetic in the 12.5". As I metioned before, if we study the way straight spiders look, they bleed off the limbs and then they start to get thinner, so there's still light glow all the way around the limbs in a straight vane spider. Another valid issue here is the study of Saturn's "A" ring. If we observe the Keeler division, which is now wrongly known as Encke's division, the straight vane in many cases bleeds right at that ansa, making it harder to distinguish the contrast of this crucial area. All I know, is what I'm seeing. I couldn't be more happy than I am with Grissoms curves, they're awsome! The airy disc looks nicer, cleaner and less disturbed.


Daniel, you're beginning to aesthetically entice me into trying one myself :question:: say the Protostar curve spider perhaps. Since I think I may keep my 8" Discovery Dob, but being mechanically inept :(, I may start to turn into an "awkward ATMer" ;-) after all and do it myself ;)!

Thanks for the info Daniel.

Ron B[ee]

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Posted 11 March 2005 - 03:44 PM

When they're small (and they usually are). When they're large, many of them switch to an open tube design (Klevtsov-Cassegrain)...


I don't think that size was foremost in my mind when mentioning closing a tube off, although having observed with a couple of 8" and one 10" Mak Newt, there is certainly a more refractor-like appearence of the image during cool down, than is typical of a Newtonian during cooldown. My point is, that for most observers that want to choose an instrument for stable high contrast planetary images, they initially look for a refractor or a Mak. It is more immediate, and certain aspects of my rant above apply here.
Not sure I agree with the Klevsov point. I don't know anyone who prefers a Klevsov to a large Mak, although it may also have something to do with the quality of the Klevsovs available in the UK.
I too do not think that an SCT and MAK Cass would be fundamentally different in the way they cool. However, again, the point is that for the vast majority of people it is the closed tube aspect that is appealing. Amateurs over the decades have become accustomed to being told, and seeing that refractors, and more recently Mak Newts, produce stable images 'straight out of the box', and good planetary images. From this, the assumption is that closed tubes = stable images.

#112 sixela

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Posted 11 March 2005 - 05:24 PM

Well, there's usually no point in a Klevtsov for apertures for which a full-aperture meniscus lens is still manageable (and economically feasible). I'm just saying that Maksutov-Cassegrains tend to be less prone to thermal issues because they tend to be smaller, and that their sub-aperture meniscus lens larger brothers don't have a closed tube, preventing a more direct comparison with larger apertures.

Not sure about those Mak-Newts - I've rarely observed through them. But SCTs are plagued by cooldown issues just as much as Newtonians, and when they're large, they can be just as frustrating as Newtonians with thick mirrors - and a closed tube means you cannot fix the issues by forcing air through the tube *while* you are observing.

#113 Daniel Mounsey

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Posted 11 March 2005 - 05:34 PM

I've gotta admit. You absolutely knocked me out with your excellent post, I couldn't agree more. Refractors truly are beautiful precision instruments. My favorite are Takahashi and AP's. Perhaps I should post a few picks for the group to see. I think a Newtonain could be made to look absolutely beautiful, in fact I'm working on that right now, but sitting behind refractors does have a mystique appeal which will never leave us.

#114 Daniel Mounsey

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Posted 11 March 2005 - 06:20 PM

When I was testing the 10" TEC Mak, it wasn't a bad scope. It certainly looks beautiful. The cooling system at the time basically stirred the currents inside to keep them at bay, but there was still something just a bit odd with the images it produced. At the time we had some 6" and 8" apos set up and I've gotta admit, the apos were better. Even the owner of the TEC agreed. He sold his Mak and now has and 8" TEC apo. I'll get some pictures of it in this forum for you guys to see. When comparing the 8" F-9 TEC apo to the TMB F-9 apo, I do recall some differences. I have not literally had the two side by side just yet because my friend just got the TEC not too long ago, but there is no question that the color correction in the TMB is a cut above. That is by no means at all to say the TMB was better just because of that. The truth is, is that I didn't see any color on Saturn in the TEC. I only saw it on Vega or very bright stars and it wasn't that much. The coatings on the TEC are incredible. I couldn't even see my own reflection. Images of Saturn in it were absolutely beautiful to say the least. I think Maks are still a great design, but when it really comes down to it, the Newts take no prisoners on planets. I remember when Grisson told me about his blue tube 6" AP Starfire many years back and Alberto, a friend of his had a 12.5" F-6 set up with the curve, fans, etc. Grissom said he didn't even want to look through his AP anymore after that. This is one of the problems. It's like once you see these beautiful images, that's all you remember. Your looking through all these other scopes, but you're still thinking about that one image you can't forget. This is a picture of the 10" F-16 Zeiss apo looking up the tube.

There are still some other issues I did not cover which deserve VERY serious consideration.

1.) Seeing conditions and proper observing locations.
2.) F-ratios
3.) Proper cooling for truss designs
4.) Some discussion on exit pupil for planets
5.) Some discussion on magnification.
6.) Light pollution.

I intend to discuss them all in this forum and help dispell some myths once and for all.

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#115 Rusty

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Posted 11 March 2005 - 11:16 PM

For more info about curved spiders, cooling fans etc., stay tuned to the REFLECTOR forum please. This is the REFRACTOR forum. This has been an interesting thread, but let's stay on topic please. Personally, I think this whole thing belongs in the Equipment forum since it has had little to do with refractors.


This is the Equipment Forum! (I'll have what you're having - make mine a double! :jump:

#116 luca

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Posted 12 March 2005 - 12:32 AM

This is the Equipment Forum! (I'll have what you're having - make mine a double!



I think that this started in another forum (refractors?), and was moved here at one point... :shrug:

Luca

#117 werewolf6977

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Posted 12 March 2005 - 04:36 AM

Thank the Good Lord!! The SRF's were having hissies!!

#118 Daniel Mounsey

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Posted 12 March 2005 - 10:11 AM

Another issue regarding cooling is the location you set your scope. For instance if you have a dob and you are setting it on cement, this is one of the worst things. I usually set a rug down below. Cement will produce eddies, which eventually make their way up the mirror box or tube. Dobs are simply vulnerable to this. It's just something to be aware of. The refractor is practically immune to it. I would say the only legitimate way ground currents would effect a refractor are currents from the ground rising in front of the optical path, but at least the objective is higher off the ground then a dob mirror. Grass is one of the best sources you can observe from, and a tripod on this kind of surface is usually more stable than a dob is in this manner.

I have friend who has a balcony who never get's good images. Remember too that the location you observe from should also be thermally stable for the best results. Houses, roofs, cars, water heaters, etc. are all potential threats. A wide open space like a baseball field with grass of course is a better location. What's even better then that is ocean breeze. Laminar flow on the west coast has helped produce some stunning images. Lot's of moisture and dew are good signs at lower altitudes. If the weather is dry, it's usually a bad sign if you live near an ocean because the moisture is most likely being zapped ou Santa Ana. This brings in unwanted, dry desert air from the east if you're living on the west coast, which leads me to the next problem, deserts. Many people believe that deserts are great observing locations because they're dark, but deserts are the worst locations for observing and darkness doesn't do anything to help planets anyway, especially with larger aperture. The images are saturated enough as it is. Saturn is the only planet in my opinion which can sometimes benefit from dark skies.

If you're near an ocean, the key is a constant, steady flow from its direction. It doesn't matter that it's a 5-10 mph. Check the jet streams too. Heavy areas are usually marked in grey. The clear sky clock is also a good source. I'd say it's been pretty accurate about 80% of the time we've used it. But their cloud cover has been very inaccurate for us.

I believe that once people get more experienced with this stuff, they'll begin to realize the benefits of it. At Wilson a couple of points of an arc second can be gained because of the transparency.

#119 celestial_search

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Posted 12 March 2005 - 12:59 PM

Actually this thread started out by comparing refractors and reflectors for planetary images among other things. It is an excellent thread and contains much useful information, another benefit of Cloudy Nights! Many of the finer points apply to all types of scopes. This post cannot be "quick-scanned." Keep the info flowing! I wish I could contribute, but I am following it and learning a lot from it.
Thanks again Daniel and others who are sharing. And, by the way, ignore that little gremlin behind the refractor curtain. :grin:

#120 Chriske

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Posted 13 March 2005 - 07:13 AM

So, some of you want to make or buy curved spiders..? This is a possibility to make them yourself... :p
I hope someone uses it.

#121 celestial_search

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Posted 13 March 2005 - 11:59 AM

Thanks for the link. I admire hand-crafted stuff, but I don't have the inclination to make one (at least now). Hats off to those who do!

#122 Daniel Mounsey

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Posted 13 March 2005 - 02:34 PM

Last night we just made a run to Mt. Wilson. This was a very fun and interesting night. We would carefully examine and compare the images of two scopes. A fantastic 5" F-15 D&G achromatic refractor vs. a 12.5" F-5 Portaball. Vic uses the latest Zeiss Badder V bino with an AP Maxbrite diagonal and prefers TMB Monos, Edmund RKE's and also uses Zeiss Abbe Orthos. Vic is very much a purist. The scope sits on a Losmandy G-11 and is surprisingly solid with the long tube which is actually not that heavy. Vibrations stop in three seconds. Vernon uses the 12.5" Portaball reflector with Zambuto optics. He has a nice Protostar secondary and uses a Televue bino with Tak LE's and Edmund RKE's. The 15mm RKE in particular are his favorite with that scope on Saturn when seeing holds. I personally love the Edmund RKE's as many of you already know.

Seeing was rated at about 8 and we started viewing at about 8pm. Saturn was just about right over head. Vic would use magnifications on both Saturn and jupiter between 180x-250x with the D&G, while Vernon worked with about 300x-400x on this particular night. When I tested the D&G I was utterly impressed with the image quality to say the least. It's too bad so many people under rate good achromats. There's much to be said for a very good achromat! Saturn was sharp and just so darn pretty to look at. Color correction was fabulous for an achromat and at F-15, it should be. We also have another 5" F-15 D&G and for some reason isn't as well color corrected. Vic's is a newer model and I don't know if they've made any changes to the glass these days.

When I went to look through the Portaball, It was just so shocking to see how big and beautiful the image was. It was absolutely sharp. The detail and color was incredible! Both left and right ansas revealed hints of Keelers division in an out. It was literally like a Voyager photo with color and band structure on the globe. The Crepe ring just jumps out at you. The rings just looked like vinyl with the transparency this good. Clearly the Portaball won out on Saturn. When I went back to the refractor, it looked dim in comparison and the detail was quite limited. It was like a small pretty picture with nice contrast. When Vic went up in magnification, the image still looked sharp, but the limited aperture was making the image dark.

Refractors are amazing sometimes. Their images can stay sharp at really high magnifications, but the magnifications are dispreportionate to the light being collected. It was like that's all we were gonna get out of it but it still wasn't over just yet. The D&G wasn't going down without a fight.

Jupiter was slowly rising and sadly, seeing had also dropped to about a 7. This was going to place issues on the Portaball for reasons I'll say later. In the Portaball, Vernon had to drop down in magnification to sharpen things up a bit and he'd also have to add a #13 ND filter, but not only that, the exit pupil in the eyepiece at these lower magnifications was rising and would make the images suffer as well. More on that later.

The Portaball's image of Jupiter still looked saturated. When I went over to the D&G, I liked the image better. It was just more pretty to look at. It wasn't that there was more detail. It was just the fact that the image was more pleasing to view BUT here's the mistake many observers make. They usually interpret this sharp looking image as one that is providing more detail an this is not what's really happening if both optics, etc. are good quality. It's a total mind game. When I walked back to the Portaball, the detail was all there, in fact maybe just a bit more, but once again, it just didn't look as pretty.

With limited aperture, you have a choice you can make. You can look and say, hey I like this pretty picture, but the detail you will see is going to be limited. So this night the winner was The Portball on Saturn and the D&G on Jupiter. Refractors sometimes have there place, but there will be nights when the seeing is good and you'll wonder why you even bought a refractor. The key to success in this hobby is to have as many different kinds of scopes you can afford for every occasion. I know that's hard for many.

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#123 Daniel Mounsey

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Posted 13 March 2005 - 02:35 PM

Post deleted by Daniel Mounsey

#124 Daniel Mounsey

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Posted 13 March 2005 - 03:07 PM

This is how a good night can sometimes look from certain areas near Mt. Wilson. Notice the marine layer in the lower basins. On some nights you may not see too much marine layer but you'll have what's called an inversion layer. This inversion layer keeps smog down and allows laminar air flow from the ocean to glide along the top of it. The result in some cases is sub arc second seeing. On some nights, I've seen the guys use in access of a 1000x to split sub arc second doubles but even if your scope can resolve this finer detail, it doesn't mean your eyes will.

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#125 celestial_search

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Posted 13 March 2005 - 03:11 PM

Now that's a refractor! Again, thanks for the interesting info Daniel. I liked your reference to the "mind" thing. The psychology of observing is often negelected in reports and is a factor. Expectations can influence what you "see." Gracias.


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