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Night with binos 2011-08-21


parvee
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Had a great fun watching the skies last night. Very good seeing, I cannot remember last time I could see entire Ursa Minor with a naked eye. All neighbours went to bed earlier which helped a bit :hello2:

I'm quite limited to the field of view, only N, NE, E accessible from my back garden. I mainly use Cassiopea to find my way around the sky this time of year.

Mizar-Alcor for starters, just to get my eyes adapted. No drama here, probably best known optical double.

M31; although this is good friend by now, an object that really got me into astronomy, I still find it most enjoyable. Located within seconds (Mirach, hop, hop, hop :icon_scratch:). Still just a misty cloud, but with somewhat distinguishable shape. Loved it as usual.

Double Cluster; got my eyes adapted by now. Very easy to spot, unmistakeable formation.

Then I looked high up, holding my 15x70s by hand. Still learning constellations, so it's taken a trip back home to have a look at stellarium and made out the Cygnus. No problems here, with Deneb almost in zenith.

M39; first time I've seen this one. Wow. This one is made for binos. I though it will be trickier to spot, but again, looked at stellarium and memorized the star pattern. Quite large object, could distinguish at least 20 stars.

By that time Triangulum was reasonably high up, so I raised the bar and decided to go for Triangulum Galaxy. No luck. 5.7 Mag it says, but could not spot it. No luck with NGC752, same Mag :(. But I was able locate the nearby 56And double quite easily.

M34; another open cluster of the night, visible, but less impresive than the M39 and Double Cluster. Got my tick in the box ;)

Jupiter was up by that time, so had a first proper view of it. Excellent, this was another first-timer :) Could spot 3 moons (Europa, Ganymede and Callisto; Io was in transit). Tiny, yet sharp points, all distinguishable in Jupiter's glare. Jupiter itself very bright, its light is specific, you can tell it's reflected. Had a bit of a problem getting it sharp, could not get it pin-point. :/ (any hints?)

Spent the rest of the night jumping through all of above and Moon and learning constellations.

I will be getting myself a proper telescope in the future, but I thought I'll learn the basics first. I tell you: grab yourself a pair of decent binoculars first. It is a fantastic introduction to the night sky.

Cheers, parvee.

Edited by parvee
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Great report. Bins are a great way to learn the skies. M33 is very large, and has low surface brightness. This makes it hard to spot. You need really dark skies to see it well. I had my best views of it with my 15x70s and my 80mm APM at 22x magnification, from a dark site in France this year. You could go for the other Messier in Cygnus: M29. A smaller than M39, but easily found.

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Great report. I spent a year using an old pair of 10x50s, then a better pair of Bressers. Helped me learn my way around and spotted a few targets too.

Even though I've got a telescope I still use them today, and they're great for quick views and widefields :)

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Great report. I spent a year using an old pair of 10x50s, then a better pair of Bressers. Helped me learn my way around and spotted a few targets too.

Even though I've got a telescope I still use them today, and they're great for quick views and widefields :)

Absolutely, as I discuss here, 52 of the first 108 Messier objects I found were found with bins, 48 of them with the 15x70s I bought years after I got my 8" telescope.

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How long had Jupiter been up? I was looking at him on Sunday evening and had a similar problem getting sharp focus in my 15*70s. I put it down to him being a bit low in the sky. Saw the moons well enough though.

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so I raised the bar and decided to go for Triangulum Galaxy. No luck. 5.7 Mag it says, but could not spot it.
As Michael said, it is a large, low surface-brightness object. It nearly fills around half the field of your 15x70; you will probably only detect it as a slight brightening of the sky as you pass over it with averted vision. It is one of very few objects that is actually better in a 7x50 binocular, because the FoV is sufficiently large to enable some darkness to be seen around it and for it to be sufficiently condensed that its light is not smeared out over too much of the sky. Ditto for M101. Edited by tetenterre
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I would prefer my 10x50 wide field (7 deg FOV) bins to 7x50 bins, as they have a 5mm exit pupil, which means a darker background, and more light of the object entering the pupil of your eye. Having said that, M33 is about 70'x40', so roughly 2x1.5 times the moon. That is framed quite nicely in the 4.4 deg and 3.76 deg FOV of the 15x70 or 80mm APM with 22mm Nagler.

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I would prefer my 10x50 wide field (7 deg FOV) bins to 7x50 bins,

Oh, so do I -- I don't even own a 7x50 -- but there are a few objects that are actually easier to see in a 7x50: M33, M101, NGC7000 are the obvious ones.

as they have a 5mm exit pupil, which means a darker background, and more light of the object entering the pupil of your eye.

Not quite sure what you're getting at here. If you increase magnification (and keep everything else the same), all extended objects are darker, so not only the sky gets darker, but also (in this case) M33. Even if your eye pupil cannot accept the 7mm exit pupil of a 7x50 (mine cannot), the light from M33 is focused to a smaller area of the retina so that, even if your pupil is only 6mm, the light intensity is greater at the retina; this, I believe, is what makes these very low surface-brightness objects easier to see at 7x50, even if your pupil does marginally vignette the exit pupil.

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Oh, so do I -- I don't even own a 7x50 -- but there are a few objects that are actually easier to see in a 7x50: M33, M101, NGC7000 are the obvious ones.

Not quite sure what you're getting at here. If you increase magnification (and keep everything else the same), all extended objects are darker, so not only the sky gets darker, but also (in this case) M33. Even if your eye pupil cannot accept the 7mm exit pupil of a 7x50 (mine cannot), the light from M33 is focused to a smaller area of the retina so that, even if your pupil is only 6mm, the light intensity is greater at the retina; this, I believe, is what makes these very low surface-brightness objects easier to see at 7x50, even if your pupil does marginally vignette the exit pupil.

Suppose your pupil dilates to 5mm maximum. In a 7x50, half the light gets wasted, though this may be compensated by having a lower magnification, i.e the surface brightness on the retina stays the same (half the light, over half the surface area of retina). However, the brain (which does the actual seeing) is less inclined to ignore a larger area of slightly increased contrast than a smaller (up to a point) because evolution has lead to a certain innate "understanding" of statistics: more photons means more significance (or bigger is closer by, and therefore more of a threat/opportunity). I find the view at slightly smaller exit pupil distinctly better than at 7mm.

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Suppose your pupil dilates to 5mm maximum.

I often see this 5mm figure quoted, but I wonder where it comes from. It seems to be one of these urban myths perpetuated on the internet that by <insert age of choice, (usually 40 +/- 10 years)> your eyes are restricted to 5mm. It is quite simply wrong. I am 61, my eyes open to 6.3mm and 6.5mm respectively; given the age poll last week, most forum members are at least 20 year younger than I am!

In a 7x50, half the light gets wasted,

Only based on the 5mm myth. Even in my smaller eye, less than 20% of the light is wasted; for younger people, it is likely to be even less than this.

I find the view at slightly smaller exit pupil distinctly better than at 7mm.

As do I. My favoured hand-helds are 10x42 (4.2mm exit pupil); many of the cheap 10x50s (eg the Lidl/Bressers) are internally vignetted and have an exit pupil of 4mm-ish. I usually use my big binocs with an exit pupil of 2.7 mm (the other eyepieces offer 5mm).

However, all theory aside, in my experience (i.e. empirical evidence) most people find those three objects I mentioned easier at 7x50 than at 10x50. You are clearly an exception, and it may be that there is something odd about my "sample" but...

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I often see this 5mm figure quoted, but I wonder where it comes from. It seems to be one of these urban myths perpetuated on the internet that by <insert age of choice, (usually 40 +/- 10 years)> your eyes are restricted to 5mm. It is quite simply wrong. I am 61, my eyes open to 6.3mm and 6.5mm respectively; given the age poll last week, most forum members are at least 20 year younger than I am!

Only based on the 5mm myth. Even in my smaller eye, less than 20% of the light is wasted; for younger people, it is likely to be even less than this.

As do I. My favoured hand-helds are 10x42 (4.2mm exit pupil); many of the cheap 10x50s (eg the Lidl/Bressers) are internally vignetted and have an exit pupil of 4mm-ish. I usually use my big binocs with an exit pupil of 2.7 mm (the other eyepieces offer 5mm).

However, all theory aside, in my experience (i.e. empirical evidence) most people find those three objects I mentioned easier at 7x50 than at 10x50. You are clearly an exception, and it may be that there is something odd about my "sample" but...

I said assume 5mm mainly because the arithmetic is easier. At 6 mm dilated pupil an 8.4 exit pupil the ratios are the same. Regardless of the numbers, if the dilated pupil is smaller than the exit pupil, the surface brightness remains the same, but the total light is smaller.

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To the OP: Apologies for the thread-drift, but we hope you find it informative! :)

At 6 mm dilated pupil an 8.4 exit pupil the ratios are the same.

Sorry, you've lost me completely now -- what astronomical instrument does this 8.4mm exit pupil come from?

Anyway, did a bit of research and found this (recent, and gives SD & range plus sample size, so it's a bit meaningful).

pupil_size.png

Basic lessons (for me, at least):

* My eyes fit within 1SD of the mean for my age cohort.

* The majority of SGL users are statistically likely to have DAPD in excess of 6.5mm

* A minuscule minority of SGL users are statistically likely to have a DAPD of 5mm or less.

Edited by tetenterre
To correct inconsistent formatting
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Fair enough, so for many 7 mm is fine. What I am getting at is that for optimum contrast your exit pupil must match your dilated pupil. If your dilated pupil is 6 and your exit pupil is 7, as in your case (to one significant number), the reduction of light is (6/7)^2, which equal to the change in solid angle, and so the surface brightness does not change, with respect to a perfectly matched 6 mm exit pupil. The number of photos has gone down, however. An 8.4mm exit pupil can be obtained by using a 42 mm EP in an F/5 scope. I just wanted a ratio of 1.4 between exit pupil and dilated pupil. This yields (roughly) a factor of 2, for ease of computation.

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An 8.4mm exit pupil can be obtained by using a 42 mm EP in an F/5 scope. I just wanted a ratio of 1.4 between exit pupil and dilated pupil. This yields (roughly) a factor of 2, for ease of computation.

Ah, I think I see where we are getting tangled. You appear to be addressing theoretical situations (such as using a 42mm e/p in an f/5 scope), whereas I am addressing actual practical situations, such as the experience of observing large low surface-brightness objects with a 7x50 binocular.

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Ah, I think I see where we are getting tangled. You appear to be addressing theoretical situations (such as using a 42mm e/p in an f/5 scope), whereas I am addressing actual practical situations, such as the experience of observing large low surface-brightness objects with a 7x50 binocular.

I have seen a 42 EP in an F/5 scope :), however, the main issue is the effect of an oversize exit pupil, no further gains in surface brightness, but loss of magnification.

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