Distant limits to optics? (1 Viewer)

Vespobuteo said:

only the sky is the limit.
or atmospheric conditions rather.

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I heard the man in the optics shop reply to a customer who asked how far they could see with binoculars, "in the day only 93 million miles but at night, much, much further". Sorry, could't resist it. I will get my coat!

Hi Colin,
If the man used the binoculars to look 93 million miles during the day, after a few seconds he would not see anything again with his central vision.

Sirius has been seen during the day with unaided eyes.
With binoculars it is quite easy to see during the day.
This is a lot further than our Sun.

I have seen Jupiter, Mars and Venus during daytime without optical aid, except my distance glasses nowadays.
Did not see Sirius.

longboat said:

Verhalen sounded vaguely familiar, so I google-earthed it. Thriving metropolis, that is!

I did spend 11 years in TX (5 in Corpus Christi area, 6 in Abilene). Spent a weekend in Marfa and hiked the Davis mountains, but never did see the Marfa lights. Probably saw a sign to Verhalen on that trip, but that was back in '97 or '98... my memory is not that sharp anymore. B

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Hey, don't be rude . . . why, in its heyday, Verhalen probably had 15 or 20 Residents. When Mom, Dad, and I left Wiville in northeast Arkansas, we diminished the population by 30%! :cat:

Bill

PS Behind me and to the left, you can see where our dirt driveway used to be.

Stretching the limits ... ?

Since I've been called out, as it were, on the Manx Shearwater distance topic, thought I'd best try and provide a little extra info (providing proof not so easy of course).

Anyway, first yes, the ED50 is a cracking little scope ...

Thought my eyesight was quite good, but after recent visit to opticians looks like it's 'normal' (20/20)

Observer location - Pendennis Point, Falmouth. Birds were seen to be flying behind The Manacles off the east side of The Lizard.

I'm sure I measured this as being 8 miles distant on the OS map at the time; however looking on googlemaps https://www.google.co.uk/maps/dir/5...c583915:0x495fdd792e8c51c!2m2!1d-5.05!2d50.05

it only looks to be a little over 10km, ie 6.25 miles or so, which is a lot less (unless I've got my rocks off?) That probably changes things a lot?


That probable error aside (can't check on the ground as I'm away at the moment), both Manx Shearwater and Balearic Shearwater can be relatively numerous on seawatches in season - used to seeing both, and with some overlap when both occur. On this occasion it was the end of the main Manx season and I was hoping for an early Balearic.

The lighting was such that flocks of birds shearing would show a dark grey upperside, followed by a gleaming white underside as they caught the sun (the bright side more noticeable than the dark side of course). Pretty confident that I would have picked out a mid-phase (pale brown) Balearic if one had passed by. A dark bird could have been missed, as could a pale bird, but I was hoping the creamier underparts would stand out. Can't prove one didn't pass by as I was watching, but as I say, don't think an obvious one did. The proof would of course be in picking one out in the future - something to aim for?! And of course it's a case of id'ing birds that aren't rarities requiring a description - picking up on other id features (including even the different shape and length) maybe too much at that range!

Manx Shearwater wingspan (from Collins) - 72 -82cm The width of the wing looks to be c.10% ie 7 - 8cm, but can't see this online to confirm. So how many arcseconds would that be at that range and magnification?


Anyway ...

I'll juggle some numbers.

Just taking the wing span as 80cm and the distance as 6.25 km, that would be 26.4arcseconds to the unaided eye. Even unaided, there is a good chance you would spot s a dark object that size on a white background or vice versa, but you wouldn't be able to see it as a bird. An 8cm wing width would be 2.64". At 27x that would be 71.3" which is still somewhat smaller than your 120" optician diagnosis but you should might make out a cross shape if it presented at the right angle. You wouldn't see the wings clearly, or discern other features. I'm pretty sure you couldn't tell grey from brown at that range either, but I wouldn't rule out seing indistinct light/dark flashes, but the light would have to be very good indeed. Is that enough?

David

typo said:

I'll juggle some numbers.

Just taking the wing span as 80cm and the distance as 6.25 km, that would be 26.4arcseconds to the unaided eye. Even unaided, there is a good chance you would spot s a dark object that size on a white background or vice versa, but you wouldn't be able to see it as a bird. An 8cm wing width would be 2.64". At 27x that would be 71.3" which is still somewhat smaller than your 120" optician diagnosis but you should might make out a cross shape if it presented at the right angle. You wouldn't see the wings clearly, or discern other features. I'm pretty sure you couldn't tell grey from brown at that range either, but I wouldn't rule out seing indistinct light/dark flashes, but the light would have to be very good indeed. Is that enough?

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Enough to disprove? Don't know ...

Not sure if makes the calculation worse, but don't forget it was c.6.25 miles, or 10.1 kilometres.

(Could perhaps point out that the reason I went to the opticians recently was that I thought my eyesight was getting worse - perhaps it was better previously?)

Moving on from the wire discussion earlier - pretty sure that the eye/brain combo can pick up more info from moving objects than merely stationary ones? Or how much is it that the brain is filing in detail that isn't actually there?

Will report back in the spring/summer if I can replicate the observations at all (in France at the moment).

Cheers

A flock of birds would be seen even if a single bird wasn't.