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nirvanix
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Reged: 06/07/07

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Re: Plato's challenge new [Re: Asbytec]
      #6305970 - 01/11/14 11:16 AM

Norme in your last photo there is a crater near the rim of Plato at the two o'clock position. That is one that I'm certain I detected last winter and wonder why it's not labeled? I remember that one as being resolved along with 6 others, and crater h was the albedo feature I noted.

A star is a point source of light. If my optic catches enough light from that point source it will form an image my eye can see. Any spot of the lunar surface, like the rim of a craterlet, can act as a point source of light that my optic/eye can detect even though my optic isn't capable of resolving that feature. If I used a photosensitive device and digital image processing, I might be able to discern several point sources of light in a curved line that would indicate the structure, even though my eye couldn't. Those multiple points would have to be separable by my optic of course otherwise they'd appear as one spot.

There is the theory and practice of telescopes on earth being separated by distances so that the total resolution becomes something of the order of the separation distance. I wonder if there are any amateurs engaged in that? Or is it really only practical for radio devices


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Asbytec
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Re: Plato's challenge new [Re: nirvanix]
      #6306050 - 01/11/14 11:52 AM

That looks like "f" as labeled in David's graphic at about a mile in diameter. That's a great catch. In your 10" scope? Do you remember what your seeing conditions were or the moon's distance that night?

So, h was a albedo "speck?" That one would be curious to know how it resolves out - as one elongated object (crater or speck) or two distinct features. I cannot remember which specks I saw, would have to dig way back into the CN archives. I don't remember seeing a double speck, anywhere.

Quote:

Any spot of the lunar surface, like the rim of a craterlet, can act as a point source of light that my optic/eye can detect even though my optic isn't capable of resolving that feature.



Absolutely. I guess that's why we see specks, instead. Normally the smallest craters do not form specks in my scope, they have to be something larger than a point source (being defined as 1/4 Airy disc diameter, or about 0.4" arc. That's pretty small, less than 0.4 miles.) Otherwise Plato should be covered with specks and maybe all of the labeled one in David's graphic, at least.

So, it seems we have crater form to some level of resolution (I'm curious about), bright specks below that (kind of curious about that, too), then we see nothing further down the resolution chain. But, we would if they were bright enough. Most are not. But you know...specks might be a bit easier (and crater from more difficult) on Plato because it is a tad dark.

Anyway, I hope you guys can report sightings, if seeing ever allows good look at Plato I'll join in. Resolution is an interesting topic and Plato is an interesting place to try to understand it and the limits of our equipment and conditions. It gives us a feel for what we can expect to see. Maybe we see more specks per capita on Plato than elsewhere?

It would be very interesting to note the affects of seeing in larger apertures, too. They might be hard pressed to reach the diffraction limit, but they are still supposed to resolve quite nicely since their diffraction limit is much smaller, anyway. So, even if they are not at diffraction limited, they are returning smaller discs (short exposure FWHM) even in less than perfect seeing. That's the theory, does it hold? I'd think so.

Edited by Asbytec (01/11/14 11:56 AM)


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Asbytec
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Re: Plato's challenge new [Re: Asbytec]
      #6306149 - 01/11/14 12:44 PM

Quote:

Those multiple points would have to be separable by my optic of course otherwise they'd appear as one spot.




This whole idea and the mechanics behind our ability to probe the world beyond fascinates me to no end. I like to really push it for everything it's worth in the real world and try to understand it in theory. Often the two are not far apart. It's just a step we take to delving into our hobby, not unlike a surfer waxing his board and reading the waves for the best break.

I had to come back to your idea of the expanding point source. At some point larger than about 1/2 Raleigh, the visible disc expands noticeably to the eye. This may well be the curve of a crater rim as much as it would be a Jovian moon. So, at some intermediate point between point source and full blown expanded object, we see an expansion of the spurious disc. I wonder how many specks we observe on Plato are actually expanded in this way at least somewhat. Are they indeed expanded perpendicular to the sun's angle? Is the floor visible at this point, or is this the range of angular dimension that defines a speck without a visible floor? Not a perfectly round speck like a spurious disc, but an elongated one defining a crater rim as a speck to our eye?

That needs a closer look. If so, then a speck might well be resolution of a crater rim even if the floor is too dim to see. Or it could be the two are not mutually exclusive, once you see the rim you see the floor and a speck is still a speck.

Edited by Asbytec (01/11/14 12:52 PM)


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David Knisely
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Re: Plato's challenge new [Re: Asbytec]
      #6306272 - 01/11/14 02:06 PM

Asbytec wrote:

Quote:

Dawes, or actually Lambda/D as an approximation of Dawes, is about 113.4/Dmm. It changes a little with the magnitude of the point source, but we can assume the moon to be mostly bright (crater rims) and any diffraction affects having relatively stable and modest magnitude.




Dawes limit is *only* applicable as an approximate limit for a pair of equal point sources, not for an object that contains extended detail. A craterlet on the moon is an extended object, and point-source limits cannot apply to them. For that, you have to use a limit that is somewhat larger than Dawes. The British observer Wilkins in 1959 did a detailed set of measurements in various apertures and came up with the approximate relationship that a craterlet half-filled with shadow would be visible on average if it had a diameter (in miles) equal to 9/D, where D is the aperture of the telescope in inches. It is again an approximation, but it is somewhat useful, as in a 150mm aperture (5.9 inch), it predicts a minimum size of about 1.5 miles for t he smallest craterlet visible. I have done some work with high-resolution charts and found this guideline to be fairly accurate. Its predictions are much more realistic than a figure derived from Dawes limit. Smaller linear features such as rilles or tiny mounds can be detected in a given aperture at notably smaller sizes than craterlets due to their simpler diffraction structure, but seeing craterlets as the true pits they are requires a somewhat larger telescope. Clear skies to you.


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nirvanix
Carpal Tunnel


Reged: 06/07/07

Loc: Saskatoon, SK
Re: Plato's challenge new [Re: Asbytec]
      #6306791 - 01/11/14 06:24 PM

You're going deep Norme. Are you angling for a job at JPL

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azure1961p
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Re: Plato's challenge new [Re: nirvanix]
      #6307181 - 01/11/14 10:33 PM

The only caveat in assigning or guesstimating point source contrast on the moon is it would need to be high contrast indeed. A mountain peak at say a quarter mile across seen from the depths of the night side of the terminator could produce such a stellar-like diffraction pattern - though of much less intensity, ie; the .25 mile mountain peak would not be bright enough to illuminate diffraction rings in the said instrument. Of course Im guessing here as I haven't find such a thing but stellar point sources say, in the 1st magnitude certainly are far far higher a surface brightness per unit area producing the pattern. If our telescopic views had planetary bodies with this kind if brilliance wed have a battery of done extremely dark neutral density filters.

Pete


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azure1961p
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Re: Plato's challenge new [Re: azure1961p]
      #6307192 - 01/11/14 10:36 PM

- that all said, if I were looking for detail other than rilles significantly below Rayleigh for example, Id imagine the starry lone peaks emerging from night might be the ticket to some extraudinary examples.

The problem is in measuring off the glint of light creating the sub angular res.

Pete


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Asbytec
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Re: Plato's challenge new [Re: nirvanix]
      #6307283 - 01/11/14 11:23 PM

David, observing E seems to best 9/D by a factor of ~ 0.8. I am looking for a reason why that would be. Yes, Dawes technically applies to two moderately bright point sources with 100% object contrast. As a starting point, I agree Dawes is too optimistic for an extended object and actually find Raleigh a bit more achievable (in the thought process and experience above.) Dawes requires 100% object contrast to retain enough after transferring the object into an image on the focal plane.

Raleigh leaves enough contrast from lunar features to be seen, I suppose, and it might be bested when conditions are favorable. Having observed craters down to about 1 mile in diameter away from Plato strongly suggests Raleigh to be a better resolution criteria, even though it also applies to two moderately bright stars set in the blackness of space.

From the data on craterlet E, it should subtend an angular diameter of 1 to 1.1" arc between perigee and apogee. Raleigh is, of course, 0.92" arc so this one just does not quite resolve down to Raleigh if it is a limiting crater with a diameter of 1.2 miles.

If it is, and it can be seen in a 5.9" scope, then that level of resolution is 7/D and even 8/D if "E" is larger in diameter by +0.2 miles. And this makes sense because the object starting contrast is somewhat reduced, which is the essence of the Plato challenge. If E is between 0.98 to 1.1" arc, then it's diameter can range from 1.1 to 1.3 miles in diameter at mean distance which implies resolution from 6.5/D to 7.5/D.

Again, E was only spotted once in a period of excellent seeing, given the opportunity and as infrequently as I view Plato. But, I think it's telling in terms of what can be done. And again, because there are so many variables involved in resolving a crater, and approximation is very difficult to apply across the board to all observers on average nights. It seems 9/D is a reasonable assumption for average to good conditions covering most nights and most observers who observe on those nights. It accurately describes what I see most of the time.

Being able to go deeper really speaks to how important diffraction limited seeing is to everything we do and when we're lucky enough to have such skies. In those instances, and in real terms of what an aperture /can/ do, I suspect something less toward 7/D is possible as a limiting observation...if nothing smaller can be seen in a 5.9" aperture (on Plato) and if E is about 1.2 +/- 0.1 miles in diameter.

So, that's my challenge to Plato observers, to see what shows up in those best moments when seeing is diffraction limited if it get's there. I got 6 in crater form, Nirv got 7. I doubt I can get more having reached what might be a limiting observation, but I bet larger apertures can until they bump into constantly less than diffraction limited seeing with increasing aperture. From that point, resolution begins to deviate from diffraction limited but larger aperture should still best a smaller one unless seeing is very bad.

A good amount of magnification is required, too, to really inspect that realm of the very tiny. The diffraction discs can be seen at 1mm exit pupil, but I suspect they are too bright causing some physiological eye response. Magnification near 0.5mm exit pupil should dim the moon sufficiently without reducing available contrast on the focal plane. I saw E at 0.47mm exit pupil (320x.) The moon is not so dim where the eye's response looses photopic vision.

I find smaller detail is more easily noticed at smaller exit pupils, of course without any increase in resolution in that realm. They are just bigger and dimmer. The point being, maybe it's not good to use 1mm exit pupil in some cases, especially when the object is very bright. You may not notice a tiny feature or loose it in the glare, maybe missing the crater floor near a bright rim when the image is small and bright.

Nirv, hey, it was your comment that drove me to lay awake in bed crunching numbers and concepts when I should have been sound asleep. Job? Na, this is pure interest and enjoyment. That's what drives such long winded posts full of theory, speculation, experiences, and a mix of all the above. This isn't work, is it?.


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azure1961p
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Re: Plato's challenge new [Re: Asbytec]
      #6307342 - 01/11/14 11:56 PM

BTW- the reason Norme, that you don't see Plato covered in extraordinarily small craterlets as veritable point sources is that very often these small features have very smoothed *walls* often with no discernible rim. As a result transitioning from plain to crater pit is more gradual and as a result contrast on these very small features is softened still. Apollo 12s site for example is covered in these eroded craterlets, smoothed over time.

I think in theory point source craters exist on the moon and in reality too, but the corridor that would present such a view is narrow and conditional.
You need an isolated point of brilliance - again - against the backdrop of the shadowed terminator/night side.

Pete


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Asbytec
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Re: Plato's challenge new [Re: azure1961p]
      #6307358 - 01/12/14 12:16 AM

Quote:

...often with no discernible rim...as a result contrast on these very small features is softened still.

You need an isolated point of brilliance - again - against the backdrop of the shadowed terminator/night side.




Makes sense, Pete, that seeing a speck is a matter of illumination and contrast. As the sun angle rises on even a small rimless crater, it illuminates the far wall. As that source of illumination grows, maybe it will have enough illumination from the rim/wall to be visible as a speck if it's lit surface area reflects enough light against the lower grazing angles of the general lunar surface. That seems to put a lower bound on the diameter one can see as a speck, just speck-ulating.

But, I can't escape the hypothesis that specks form when the crater wall or rim is large enough to reflect enough light (along with any inherent albedo.) And if they are that large relative to aperture, are they large enough to be seen as extended? That will require some careful observation of tiny specks to detect any elongation and with a preconceived notion of what they might look like. Very difficult at best.

But what about specks at higher sun angles? Do we see fewer or more of them? Does the crater need to be much larger at those sun angles? And does Plato's dark floor help bring them out?

We've seen elongated specks over the terminator, sometimes I even focus on and evaluate seeing with them. Thinking about it, though, at those sun angles those features are most likely higher peaks. Yea?


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Sarkikos
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Re: Plato's challenge new [Re: Asbytec]
      #6307789 - 01/12/14 10:12 AM

Not all albedo spots are craterlets. We can be fooled into thinking a spot we see on the lunar surface is a craterlet, when it is not. And many times when a craterlet is at high sun angles, it will be difficult or impossible to see the craterlet as anything but an albedo spot. Depending on aperture, some craterlets can never be seen except as albedo spots.

IME & IMO, the best course is to consult high resolution lunar photos when in doubt about whether an albedo spot is a craterlet or not.

Mike


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Sarkikos
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Re: Plato's challenge new [Re: Asbytec]
      #6307825 - 01/12/14 10:33 AM

Quote:

But what about specks at higher sun angles? Do we see fewer or more of them? Does the crater need to be much larger at those sun angles? And does Plato's dark floor help bring them out?




I see more albedo spots most everywhere on the Moon at higher sun angles. To my eyes, Full Moon reveals more albedo spots at one time than any other phase. The contrast between spot material and surrounding regolith will determine if we see an albedo spot or not. Some albedo spots will be craters that we cannot resolve. Others will be domes. Still others will be unusual contrasts between a local area and the surrounding area, for whatever reason, such as Reiner Gamma.

Quote:

We've seen elongated specks over the terminator, sometimes I even focus on and evaluate seeing with them. Thinking about it, though, at those sun angles those features are most likely higher peaks. Yea?




I've seen these terminator specks many times. To me, they are some of the most enjoyable lunar phenomena to observe. The specks can be most anything that is more highly elevated than the surrounding area. They can be mountain peaks, crater rims, domes, dorsa, even rupes and rimae, I suppose.

Mike


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azure1961p
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Re: Plato's challenge new [Re: Sarkikos]
      #6309018 - 01/12/14 08:18 PM

Full moon is an excellent lab of sorts to study composition independent of light/shadow intrusions. A true full moon I dislike though - even a sliver of some terminator 20 miles side is appreciated Mike. We are fortunate much as I think its taken for granted that we can see the high angle lit moon.

Norme,

The thing - alluded to with smoothed craterlets- is that while Rayleigh makes a nice realistic measure for craterlet limit res - its more difficult in arriving at such a thing due to the variance in crater age/erosion. Interestingly, the aged smoothed craters are almost analogous to Dawes with progressively fainter stars beyond 6V until after around 10V (with a medium aperture) Dawes almost doesn't apply at all. So too with craterlets - fresh, nicely chiseled crater walls on a 6.5 or 7/D craterlet and all as should be. But add some parameters beyond that - bright ejecta blanket or eroded shape - and we've exceeded the ?/D parameters.

This isn't to steer away from arriving at such a measure or limit, but to realize like Dawes, there are qualifiers needed in contemplating what candidate craterlets make the cut. The excellent thing here is that these candidates need not lurk in some mist of unverifiable neverland. We've got astonishing images of the entire moon that go beyond the level of resolution needed to sort things out.

Think - Im going to have a look see myself...

Pete


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Asbytec
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Re: Plato's challenge new [Re: Sarkikos]
      #6309178 - 01/12/14 09:54 PM

Mike, we might see more albedo features at higher sun angles. That seems intuitive and might agree with my own experience. Or at least my own perception. But what would be the mechanism that makes a crater look like a bright spot instead of disappearing into the high sun lit surface?

It's walls are still angled a bit, that doesn't change with sun angle, of course. But, what does change is the reflected radiation from the 'flat' surface. And if that surface is bright, the crater might still be a bit brighter but the difference (contrast) between the crater and the bright surface would diminish markedly. That would suggest we might see less specks per unit area.

It's also likely true that crater features seen at lower sun angles, they may become more bright speck like features instead. This would increase the total number of specks we see as larger craters transition from crater form to speck, but any specks we see at lower sun angles might disappear in the process. So, we may see more specks, but they may not be the same specks.

I am just speck-ulating here as I am not sure I understand the phenomenon of speck formation. Is it an unresolved crater or dome of any other small reflective feature? Or can resolved craters at a range of diameters become large bright specks without a floor at higher sun angles? So, does this increase the number of specks seen, or do we somehow loose these smaller ones at higher sun angles? I don't know, but surely Plato's darker floor would help preserve bright specks seen. (As a Hypothesis...)

So, if a speck can be any feature, how can we call it resolved as a crater without double checking a chart. In other words, when observing Plato and you note some specks, how can you call one of them 'j', for example. It would be interesting to see how many specks can be seen at lower sun angles and at higher ones. And which craters those would be - if we lost the smaller ones and the larger ones became specks. And why do they become specks, does the angle of their walls reflect more light toward us like a giant mirror?

Just thoughts...


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Asbytec
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Re: Plato's challenge new [Re: Asbytec]
      #6309242 - 01/12/14 10:23 PM

Pete, the thing that still keep haunting me is Raleigh for a 6" is 0.92" arc as calculated using 138.4/Dmm. Using the small angle formula, that translates into a crater 1.1 mile in diameter at the moon's mean distance of 238,855 miles. With CO effects, you /might/ get smaller down to 1 mile if seeing is absolutely lab-like at 8/10 Pickering or better and everything else is working in your favor.

As far as I know, I've gotten down to 1 mile (+/-) given errors in measurement. So, let's say 1.1 mile in diameter is doable in a 6" aperture at Raleigh at the moon's mean distance in excellent seeing, cooled, well collimated, and all that. That's already pushing 1.1 mile ~ 6.6/D for the Raleigh limit. On Plato, I know "e" can be seen in a 5.9" aperture when conditions are nearly perfect. If it is 1.2 miles in diameter, that is 5.9 * 1.2 ~ 7.1/D. In measurements from this site, "e" is actually on the small side toward 1 mile or about 1.7Km, or closer to 1.1 miles in diameter.

But, the figure given by 7/D ~ 1.2 miles is only 0.3 miles short of the figure for 9/D ~ 1.5 miles. So, we're in the ball park where seeing could account for any difference easily. So, on Plato, 1.1 miles in diameter is not quite as good as Raleigh, especially if Raleigh for an obstructed scope should show craters down to 1 mile. That seems to rule out Dawes as a limit as does the remaining contrast transferred from such a small feature.

Sorry, in a hurry...back later. But, yes, these are basic assumptions with so much at play...probably all dealing with object and image contrast.


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Sarkikos
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Re: Plato's challenge new [Re: Asbytec]
      #6309310 - 01/12/14 10:53 PM

Quote:

Mike, we might see more albedo features at higher sun angles. That seems intuitive and might agree with my own experience. Or at least my own perception. But what would be the mechanism that makes a crater look like a bright spot instead of disappearing into the high sun lit surface?




I think we must go back to how the crater originated. When the meteorite hit the surface of the Moon, it may have brought up underlying material that has an albedo different from the surrounding regolith. Or volcanic outflow around the crater may have resulted in a difference in albedo. In any case, it is the contrast between the crater region and the surrounding area which produces the appearance of a bright spot.

And since the sunlight is concentrated into a smaller area of incidence at high sun angles, it seems reasonable to me that the contrast between the albedo spot and its environs would be more obvious at that time.

A different mechanism is working along the terminator to make some areas stand out. Then some areas are literally "standing out" to catch the sun rays, while adjacent areas are in shadow. There is the stark contrast between sunlit features and those in darkness, rather than a difference in albedo emphasized by the intense light of high sun.

Mike


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Asbytec
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Re: Plato's challenge new [Re: Sarkikos]
      #6309487 - 01/13/14 02:03 AM

Mike, you're right that craters disturb the dust and have some brighter albedo. At least some do, younger ones, or whatever. Your point is made and it does have an affect. And since it is lighter, it should be brighter. I guess my question is, as the sun rises and the surrounding terrain is more brightly lit by the direct suns rays, if that bright crater falls off in contrast relative to it's brighter surroundings. Maybe to the point it can no longer be "resolved" as a speck in a given aperture. Surely some do, especially smaller ones relative to the resolution of that aperture. This behavior, if it happens, would be interesting to note inside Plato over time...rather than just counting them on a given night.

Possibly some larger craters evolve from crater form at low sun angles into bright spots (larger than pinpoint specks, if specks are pin points) as the sun angle increases probably due to the same albedo material. They are large enough to reflect a lot of light. Maybe being resolvable, they even provide a brightly lit surface area on the retina. If memory serves, A through D - the big 4 - inside Plato, become more like bright spots as the sun rises higher. I am wondering how many of the smaller craterlets can still be seen as specks during Plato's high noon. All of them to some almost resolvable diameter and what diameter would they be? Are there more of the tiny ones, or less of them? And how does that contrast (pun intended) behave with the generally brighter lunar albedo outside Plato?

I dunno, maybe specks do brighten as the shift form being rim lit on less brilliant surface to being fully lit on a brighter surface - effectively neither appearing or disappearing with sun angle. There is just more of them as the larger craters pit's become fully lit, too.

Edited by Asbytec (01/13/14 02:20 AM)


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Asbytec
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Re: Plato's challenge new [Re: Asbytec]
      #6309504 - 01/13/14 02:35 AM

Pete, you know what else I was thinking? I probably do more double star work that small lunar crater spotting. Maybe because doubles are easier. What I mean is, we can split a Raleigh double fairly easily and quite readily almost regardless of the seeing. But to resolve a crater at that same level of resolution requires an exceptional night. When I first got my scope, I was hunting the smallest craters I could possibly see. It took some effort and very steady moments. A Raleigh double is not nearly as taxing, nor is a Dawes split for that matter. Surely that's a difference in object type and contrast involved.

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azure1961p
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Re: Plato's challenge new [Re: Asbytec]
      #6309695 - 01/13/14 08:07 AM

Probably to do with craters appearing smaller than they are since they are dark or black rather and the surrounding contrast bleeds over into this area. Conversely doubles are bright and space is black so they can be resolved closer and easier than a veritable negative of the same spacing. What do you think?

Pete


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Sarkikos
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Re: Plato's challenge new [Re: Asbytec]
      #6309751 - 01/13/14 08:43 AM

Quote:

I probably do more double star work that small lunar crater spotting. Maybe because doubles are easier. What I mean is, we can split a Raleigh double fairly easily and quite readily almost regardless of the seeing. But to resolve a crater at that same level of resolution requires an exceptional night. When I first got my scope, I was hunting the smallest craters I could possibly see. It took some effort and very steady moments. A Raleigh double is not nearly as taxing, nor is a Dawes split for that matter. Surely that's a difference in object type and contrast involved.




I think this is because of the different types of object involved. Stars are pointicular objects, craters are extended.

When splitting doubles, you are dealing with the diffraction patterns of point sources, or with two overlapping diffraction patterns if they are a tight double. When observing craterlets, you are not at the same intense level of examining diffraction patterns as you are when viewing doubles. A craterlet involves a complex blending of many diffraction patterns, more like a continuum, something quite different than trying to split doubles.

I have heard that planet/lunar observation is more demanding than splitting doubles ... though maybe most doubles enthusiasts would never admit it. For top-tier planet/lunar observation, you need excellent seeing and excellent optics. For splitting doubles, you need only decent seeing and decent optics. As I said, I've only heard this. So far, I have much more experience with planet/lunar than with doubles.

Mike


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