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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3957
Loc: "Metrowest" Boston
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Quote:
Oh and as for BIG DOBs, I've honestly never looked through a Dob larger than about 12" that threw up anything but slightly soft, mushy, unsatisfying-compared-to-a-regractor stellar views. Probably a combination of miscollimation and lack of adequate cooling, but that's my experience.
That could be it, and in many cases I'm sure it would be. In some cases though, you may experience a certain, ahem, misalignment of expectations. A thought experiment is in order.
If you hold the magnification constant as you increase the aperture, the Airy disks get smaller and smaller, which theoretically would give you more pinpoint views in perfect seeing than in a smaller scope. However, while seeing conditions may permit the light for a star image to flutter around (geometrically-speaking, not physically) inside the big, fat apparent Airy disks of a small refractor, so that the Airy disk might at most appear to move slightly, the same might not be said of the slim Airy disks in the larger scope. (Please remember that we're holding magnification constant here and talking about what the eye sees, not what happens at prime focus.) The easiest way to carry out an experiment like this is to look through a big scope with these "mushy" star images and have a buddy progressively mask down the scope while you're looking through it. If the star images actually shrink, then the cause of the degradation is something else (like you said), but if the Airy disks simply expand to engulf the mushy star images as the scope gets masked down, then a different conclusion is in order.
Bottom line, I guess, is that if you're only used to smaller scopes, you might be giving the the smaller scopes an unfair "pass" because at any given magnification they are actually under-performing compared to the larger scope. Put differently, it's not really a good thing that one scope is less affected by seeing conditions than another. OTOH, it might be exactly what you want for an eyepiece comparison. Dunno.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt and equatorial platform
- 6 eyepieces and a Barlow
- 120mm f/8.3 home-built grab-n-go Newt
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2928
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In practice the quality of an eyepiece is easy to determine based on how clearly it shows dim stars at the magnitude limit of the telescope. That's basically what happened last year and it was a good test. Find the dimmest possible star with eyepiece X - if Y finds a dimmer one or pulls in X more easily, it is almost certainly a better eyepiece in an objective sense. For some eyepiece pairs it will be no contest - for others a lot of X/Y back and forth will be needed. In any case it's a completely objective criterion and only needs a good high contrast scope and dark skies to test.
The complication is individual observer's eyes, so I would think you would make up a magnitude chart of a densely packed field down to 14th magnitude or so and let each observer find his own test objects, e.g. in the region of Orion's sword and belt. You also have there B33 as an arbiter of contrast.
-drl
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jrbarnett
Eyepiece Hooligan
Reged: 02/28/06
Posts: 4349
Loc: Petaluma, CA
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Mike:
I completely understand the benefits of aperture on resolving power...in theory, and have experienced amazingly sharp, refractor-esque views with reflecting telescopes up to about 12". I've also owned and frequently view through scopes larger than 12". In practice, for me the bigger scopes have never delivered on their theoretical potential.
I am virtually certain that the reason for my experiences is a combination of imperfect collimation and incomplete cooling. The former ought to be fixable easily. The latter is harder to fix if the mirror is massive and the temperature drops very quickly throughout the night. Currently we're having 84-degree days and 42-degree nights. By the time a 60# piece of glass reaches equilibrium (if it ever reached equilibrium under these circumstances), we'd likely be watching the sunrise.
With 4 or 5 observers each viewing three targets through 10 different eyepieces for 3 to 5 minutes per eyepiece, we're already faced with the near impossible. In fact, I'm considering limiting the number of observers to three, and scaling back the number of targets to two, perhaps using Sirius as a third "tie-breaker" target. Alternately were we to have two TEC 140s and two 5-slot turrets, we could parallel process observations, but it's still a very time consuming exercise to be conducted in a single night.
I also find the correlation between published and semi-pro double star observers and smaller apertures (12" and under) to be "interesting". Were more aperture better for the task in practice rather than in theory, I would expect to see a larger number of semi-pros running big mirrored rigs than seem to do so. 
In any case, it should be fun, moderately informative, mildly provocative and a shade irreverent, just like last year. We're not out to build the best mouse trap; we're out to build an entertaining one instead. If the mouse gets away, so be it. 
Regards,
Jim
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"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
- Sir Issac Newton
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3957
Loc: "Metrowest" Boston
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Quote:
I also find the correlation between published and semi-pro double star observers and smaller apertures (12" and under) to be "interesting". Were more aperture better for the task in practice rather than in theory, I would expect to see a larger number of semi-pros running big mirrored rigs than seem to do so.
I wasn't extolling the virtues of larger instruments, only speaking to your use of the term "refractor like". Because large refractors are rare as hen's teeth, peoples' idea of "refractor like" is shaped not only by the lack of obstruction and scatter, but also by modest aperture, generally 6" or less. Consequently, in addition to what people go on about regarding contrast and such, "refractor like" is also connected to less flattering things, too. I would not want a large scope that was too "refractor like", even if it were a large refractor, if you follow what I'm trying to say.
If you think it's collimation or cooling, few things are easier to confirm with a simple star test. There is little ambiguity in that. OTOH, if something else is going on, there are ways to get at that, too. It's just an aside, not really relevant to your plans. I mean, I didn't intend to make a case for you to do anything about it. I was just commenting on something you said.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt and equatorial platform
- 6 eyepieces and a Barlow
- 120mm f/8.3 home-built grab-n-go Newt
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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PJ Anway
Pooh-Bah
Reged: 06/04/03
Posts: 1141
Loc: Michigan's U.P.
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Ah yes, the "Second Annual World Cup Discussion of Resolution"
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PJ
_________________
Lookum Observatory
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square_peg
Postmaster
Reged: 03/26/04
Posts: 29524
Loc: Maple Valley, WA
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I think matching 4mm eyepieces against 5mm eyepieces is inappropriate but I will never the less enjoy reading about your World Cup just as I enjoyed last years. 
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Tom (Pegster)
DSH-8 (GSO Dob)
15x70 Oberwerks
SVP/ED80
WO 66 Petzval
Sears Discoverer EQ 60/900
8x42 Regals
History is Philosophy teaching by examples.
Thucydides
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SteveC
Carpal Tunnel
Reged: 06/15/06
Posts: 1775
Loc: The Garden State
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Quote:
I think matching 4mm eyepieces against 5mm eyepieces is inappropriate but I will never the less enjoy reading about your World Cup just as I enjoyed last years.
I agree. Doing 2 f/ls per year means that we'll done with the EP World Cup in about 15 years, give or take. What will we do then?
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SteveC
TEC 140
Intes Micro 715 deluxe
TEC 110 (on order)
SolarMax 40
Losmandy G-11 w/Gemini, DM-6 w/Sky Commander on SV wood tripod, Vixen Skypod
TEC EP Turret, TMB Supermonos, ZAO II, Naglers, 32mm Konig, 24mm Panoptics, 14mm Meade UWA
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jrbarnett
Eyepiece Hooligan
Reged: 02/28/06
Posts: 4349
Loc: Petaluma, CA
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Awwww, Pegster, what's an extra 45x among friends?
Seriously, though, I am thinking we can easily settle on 10 5mm eyepieces for this given all of the creative suggestions.
Of course we could also run a turret full of 4mm eyepieces for a total of 15 in 3 groups of 5. If we omit 4mm, we lose the ZAO-II. I would really like to have the ZAO-II square off against the 4mm TMB Supermono and perhaps the 3-6mm Nagler Zoom set to 4mm. I have a 4mm Meade Research Grade Orthoscopic and 4mm University Abbe Orthoscopic (or 4mm TMB Planetary) that could round out the 4mm set.
We're still noodling the format a bit. Thanks for the input.
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"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
- Sir Issac Newton
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2928
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If that 4mm Meade wins we're going to have to call those eyepieces the Boise State of astronomy!
-drl
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Alvin Huey
Carpal Tunnel
Reged: 10/18/05
Posts: 1831
Loc: NorCal
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Quote:
Quote:
How about some aperture? Perhaps Dan's 20" Obsession.
The problem with the SCTs and relectors, that would seriously trump a TEC140, is that the usual f/ls will result in magnifications approaching 500x. That would be fine if viewing conditions support that. How often is a 5mm ortho/supermono used with reflectors and SCTs? I don't believe I've ever attempted using a 5mm Supermono in my 7" Mak.  Perhaps once out of curiosity.
I use my 5mm Supermono or 5mm Tak LE on my 22" often. It may not be the sharpest view, but it does a pretty good job in busting apart very small galaxy groups or pairs.
I have to admit that the seeing isn't usually sufficient to use the 5mm on planets. But my 22" is fairly fast at a focal length of 2300mm, which is shorter than your 7" f/15 Mak or any SCT larger than 8". So it isn't so bad. 
And I don't have any wide-field eyepieces, just the 20mm XW as a finder eyepiece. Wide-field eyepieces are "light hogs" as my buddy Jimi calls them.
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Clear Skies,
Alvin #26
22" f/4.0 reflector and 30" f/4.3 StarMaster
FaintFuzzies | TAC | TAC-Sac
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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Edz says >>>In fact, a faster f# (given two scopes of equal aperture) will not change the angular size of the Airy disk since angular size is dependant only on aperture, not f#. F# will change the magnification, which is related to my discussion. Faster f#, since it would lower the magnification, in fact would make the apparent size of the object in the eyepiece smaller with the result being that it would be harder (not easier) to discern eyepiece performance wrt resolution.<<<
Perhaps we're talking about the same thing in a different way. The way I'm looking at it is the OTA produces a real image (like what would fall on an image sensor at prime focus) and the EP magnifies that real image for visual viewing. For testing EPs, you might view a very fine resolution or MTF target placed at the focal plane of the EP. (simple EPs with no integral Barlow/Smyth lens). The finer and crisper that target, the better with which to evaluate the EP's performance. The OTA in his case is simply used to generate the real image to with which to test the EP, and the crisper and finer that real image, the better with which to evaluate the EP.
This is a bit simplistic because the OTA being used will introduce aberrations in the real image (e.g. field curvature) , aberrations that will add/subtract from aberrations of the EP. However, you still benefit from having the sharpest detail in that real image... the sharpest detail as measured by size, since it is a real image that's being examined.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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I suggest you read some reference papers on angular resolution and apparent size to understand why, as you suggested, "for a given focal length EP, a faster f/# will produce a smaller input to the EP making it easier to discern the EP performance wrt resolution", is not appropriate, nor would it make it easier to discern resolution.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Rob E
Pierrot
Reged: 05/20/09
Posts: 1132
Loc: Eastern Virginia
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Two Words
"Baader Planetarium"
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Rob E.
Some of us are actually paid to be funny.
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Zhumell 16"
Orion 120ST
Orion Starmax 127
Baader Planetarium Hyperions
Zhumell SWA EP's
There's two ways to do anything...right and twice
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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Edz sez >>> "for a given focal length EP, a faster f/# will produce a smaller input to the EP making it easier to discern the EP performance wrt resolution<<<
Angular size of the Airy disk is inversely proportional to aperture. Linear size is directly proportional to focal length. Focal length/aperture= f/#
A 50mm f/12 will produce a lot larger Airy disk than a 50mm f/6. If I'm testing EPs, I want a more detailed real image to use the EP on.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
Edited by Luigi (11/04/09 02:58 PM)
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Lake Sky
member
Reged: 03/12/08
Posts: 50
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I say this year toss out all eyepieces no longer available new .
Let's make it market current and market interesting by comparing new native 5 mm eyepieces AND a few barlowed eyepieces such as Ethos 10/ 2X Powermate as suggested . In my experience, I prefer a barlowed Ethos to native Nagler of similar focal lenghth on double stars and it would be intersting if others find the same result in a " shootout ".
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jrbarnett
Eyepiece Hooligan
Reged: 02/28/06
Posts: 4349
Loc: Petaluma, CA
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You must be a mind reader...
On further review, I agree that the magnification difference of ~20% between 4mm and 5mm units is just too large to ignore. Last year's World cup spanned a ~13% difference without ill effect (i.e., the four quarter finalists out of the original twenty featured both 7mm and 8mm eyepieces). Accordingly I would like no greater a magnification delta than the delta last year.
I would also really like to have a ZAO-II and TMB Supermono in the mix. Rightly or wrongly, I view these as the "platinum standard" against which all other eyepieces are measured when it comes to resolution, contrast and sharpness, despite their used-only availability.
This means that we'll exclude 5mm eyepieces - this year. I'd like to re-focus on eyepieces in the 3.5mm to 4mm range. I am looking for 10 of them. The magnification difference between a 3.5mm and 4mm in the TEC is about 12.5%; that is, less than the insignificant delta last year.
To keep to 10, and to make the report more practically relevant, I think limiting participation to currently available eyepieces (and the ZAO-II and TMB Supermono, of course) makes loads of sense.
Here are some candidates:
3.5mm Nagler Type 6
3mm to 6mm Nagler Zoom
4mm Radian
3.5mm Pentax XW
3.5mm Baader Hyperion
3.5mm Vixen LVW
4mm ZAO-II
4mm University Abbe Orthoscopic
4mm TMB Supermono
4mm TMB Planetary
3.6mm Takahashi LE
Other suggestions are welcome,
Jim
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"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
- Sir Issac Newton
Edited by jrbarnett (11/04/09 02:14 PM)
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3957
Loc: "Metrowest" Boston
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What no 3.7mm Orion Epic ED? 
A couple of others:
4mm Vixen NLV
3.8mm Ultrascopic
I would totally understand not wanting to get into the business of comparing Barlowed alternatives as well, since that gets the Barlows themselves into the mix, but just for grins, you might want to consider a "brand homogeneous" Barlow combination or two. I think the one that would be of most interest would be the 8mm TV Plossl in a 1.25" 2x TV Barlow. Maybe consider just that one.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt and equatorial platform
- 6 eyepieces and a Barlow
- 120mm f/8.3 home-built grab-n-go Newt
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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jrbarnett
Eyepiece Hooligan
Reged: 02/28/06
Posts: 4349
Loc: Petaluma, CA
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Having one Barlowed entrant sounds interesting. "Conventional wisdom" suggests that the Barlowed unit should under-perform the others, but will it? After all, several of the listed eyepieces actually include an integrated "Barlow" in the glass stack.
I also like the idea of using a Televue 2x Barlow and 8mm Televue Plossl. This "keeps it all in the family" in terms of coatings and specifications, and the 8mm Televue Plossl is readily available and ubiquitous.
Thanks Mike,
Jim
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"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
- Sir Issac Newton
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Quote:
I agree that the magnification difference of ~20% between 4mm and 5mm units is just too large to ignore.
25%, not 20. the 4mm has 25% higher magnification and will produce a 25% larger image, giving a 25% advantage.
Quote:
The magnification difference between a 3.5mm and 4mm in the TEC is about 12.5%;
14+% , greatest variance, denomiator is lowest power. 3.5mm eyepiece will produce 14% larger image than 4mm eyepieces.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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PJ Anway
Pooh-Bah
Reged: 06/04/03
Posts: 1141
Loc: Michigan's U.P.
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Quote:
Other suggestions are welcome
4mm CZJ
3.8mm Pentax XP
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PJ
_________________
Lookum Observatory
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