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#1 Aquila1185

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Posted 24 June 2013 - 10:18 PM

So I'm outside, and I'm somehow able to find Mercury in the horizon haze with my 3" telescope. It's weak and faint, but it showed up just barely, just for a few minutes, before it was finally occluded by those low clouds.

I'm reading that Mercury is about 10 arcseconds wide. When the phase reduces below a certain amount (say 15% or so), is the crescent's percentage going to become "unreadable" due to the Dawes Limit of my telescope? The sliver of light would then be approximately 1.5 arcseconds across, the (theoretical) limit of my resolution. Am I thinking correctly, or does the extended nature of the *rest* of the body change the dynamics a bit? Thanks everyone.

#2 Rick Woods

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Posted 25 June 2013 - 12:45 AM

I'm thinking you should be able to make it out, if you use moderately high power.

#3 brianb11213

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Posted 25 June 2013 - 03:06 AM

I'm reading that Mercury is about 10 arcseconds wide. When the phase reduces below a certain amount (say 15% or so), is the crescent's percentage going to become "unreadable" due to the Dawes Limit of my telescope? The sliver of light would then be approximately 1.5 arcseconds across, the (theoretical) limit of my resolution. Am I thinking correctly, or does the extended nature of the *rest* of the body change the dynamics a bit? Thanks everyone.

You will be able to see the crescent all right, however thin it gets, providing you can get a transparent enough atmosphere and the sky is dark enough.

Diffraction effectively blurs the image in a very similar way to a minor focusing error. The Airy disc is the diffraction modification of a perfect (dimensionless) point of light; if the Airy disk were allowed to trail across the frame of a long exposure frame, the result would be a bright line of the same width as the diameter of the Airy disk, surrounded by dark spaces and fainter lines in the same proportions as the diffraction pattern around a star. The same diffraction pattern would be given instantaneously by a bright but very narrow line source of light. The argument is easily extended to more complex shapes, including those of actual planets.

The above of course depends on perfect optics & perfect seeing. Whilst very nearly perfect optics are not too uncommon (especially in smaller refractors) the seeing in the vcinity of the horizon, where Mercury lurks, is always going to have a serious impact. You're doing pretty well to see the crescent phas of Mercury at all, let alone estimate its apprent width with any accuracy.

"Unreadable"? No ... but when the crescent is very thin, diffraction & seeing effects will tend to lead to an overestimate of the phase. However terminator darkening will reduce the apparent phase ... these two effects tend to counterbalance each other but there are so many variables (sky brightness, transparency, accuracy of focus, seeing as well as aperture & optical quality) that you should never expect the tabulated phase to be anything other than an indication of what you might see.

#4 E_Look

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Posted 26 June 2013 - 12:06 AM

I am jealous! I've only seen Mercury really once, and I think it was too full for me to discern any phase. It'd be some sight to see a crescent phase! Congrats on seeing any of it at all.

#5 Aquila1185

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Posted 27 June 2013 - 08:13 PM

Thanks!

I've seen the crescent many times. The key is to catch it as high as possible, so the horizon haze doesn't mess it up. I've found that it's possible as low as 70x, but I usually push the scope to my theoretical maximum by Barlowing that to 140x (10mm + 2x). Mercury is bright enough per unit to take that, so it works out. At my "finder magnification" of 28x, I can't do it.

IMO, it's not much of a sight, though. In the future, I'm going to get a much larger telescope (10-16 inches) and then I'll be able to magnify more and perhaps pick it out more cleanly. It's very tiny and usually embedded in horrible seeing, so it takes a lot of patience (at least with my scope) to see.

My question was really centered on whether or not it was possible to discern the correct phase after it had reduced below the width of the telescope's Dawes Limit. Sorry if I was being unclear. To this day (and now that the apparition is pretty much over) I've never been able to say I saw the published phase. It was just too small and too difficult to pick out anything exact. Talking to E_Look directly, I do count myself very lucky to be able to catch it. I know we're all part of a group, among humans, that is uniquely able to do such things, and I feel very privileged to be able to read what is, in fact, in plain sight to everyone if they would just look up.

#6 Rick Woods

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Posted 28 June 2013 - 01:56 AM

IMO, it's not much of a sight, though. In the future, I'm going to get a much larger telescope (10-16 inches) and then I'll be able to magnify more and perhaps pick it out more cleanly. It's very tiny and usually embedded in horrible seeing, so it takes a lot of patience (at least with my scope) to see.


I found the trick to be catching it in the pre-dawn sky, and following it up as the sun comes up. Then, the seeing gets steadily better instead of worse, and you have a chance of making out some albedo features. If you give it the patience, it can reward you.

#7 blb

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Posted 28 June 2013 - 05:09 PM

IMO, it's not much of a sight, though. In the future, I'm going to get a much larger telescope (10-16 inches) and then I'll be able to magnify more and perhaps pick it out more cleanly. It's very tiny and usually embedded in horrible seeing, so it takes a lot of patience (at least with my scope) to see.


I found the trick to be catching it in the pre-dawn sky, and following it up as the sun comes up. Then, the seeing gets steadily better instead of worse, and you have a chance of making out some albedo features. If you give it the patience, it can reward you.

How true! This is the only way to get a good view of either Mercury or Venus. In the evening as it get's dark the seeing only get's worse as the planets get closser to the horizon and set. As to your question Aquila1185, Dawes Limit only applyes to point sources of the same mgnitude, for observing double stars. Lines of contrast, much smaller than dawes can be seen, such as a thin crescent of a planet. Look here for a good explanation of dawes limit.

#8 azure1961p

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Posted 28 June 2013 - 06:36 PM

It can actually have a size smaller than Dawes and still be a recognizable crescent through an 80mm. Diffraction transitions from resolved to unresolved, there is no off-switch beyond the Dawes that would preclude an observation at 1.4" for example. The crescent would soften as it got smaller but also appear a little thicker than it actually is as previously mentioned it can have the appearance of slight defocus. A larger aperture with greater resolution would show the cusps terminating to a finer degree.

Pete

#9 blb

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Posted 29 June 2013 - 08:11 AM

Yes Pete, when we move beyond looking at stellar points of light, we often see many things smaller than dawes would lead us to believe. Here is a quote from the above web page that shows this to be true.

"This “Dawes’ limit” (which he determined empirically simply by testing the resolving ability of many observers on white star pairs of equal magnitude 6 brightness) only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini’s Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5” telescope – which has a Dawes’ limit of 1.8 arc seconds!"

Once we go beyond point sources of light, it is all about contrast. Just think about the fact that Cassini’s Division was discovered with what was about a 60mm telescope, Wow. Where was Dawes in that?






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