1791: "Telescopes: Refractor vs Reflector"

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JeromeWest
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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby JeromeWest » Sat Jan 28, 2017 10:14 am UTC

Could this explain the recent surge in popularity of mirrorless cameras? An SLR (whether digital or otherwise) would be of no use for vampire photography, so perhaps we have the vampire hunters to thank for this new development in the photography world?

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orthogon
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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby orthogon » Sat Jan 28, 2017 11:23 am UTC

Pfhorrest wrote:
Sableagle wrote:Hmm. Bullets are pretty much moving particles, but do they refract like waves when entering a move dense medium?

Yes. And wheels rolling diagonally across a boundary between surfaces with different friction refract too. Basically anything moving across a boundary that causes a change in speed.

[Ice] hockey pucks, yes, but why would wheels refract? Unless they're slipping, the different coefficients of friction should have no effect.

ETA: I'm thinking of surface friction at the contact point. If one surface was actually lossy, like long grass or cobbles, then yes, it would refract.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Soupspoon » Sat Jan 28, 2017 1:22 pm UTC

I'm guessing reality is just more messy. There is no contact 'point', and (especially, but not limited to) pneumatic tyres flex with the various forces (axle/hub weight, internal shape-maintaining pressure of air/'solid'-core, the transmission of various torques inwards and outwards across the ground/rim/hub interfaces) and result in a 'footprint' upon the ground (or a changing cluster of smaller subtly-independent footprints, assuming tread and/or ground purturbations) which, when spat through a comprehensive modelling algorithm would indicate differing 'rolling resistances'...

(Or wheels always slip in some manner at the points coming into/leaving contact, relative to the current contact centre-line, because x.cos(θ)≠x for even the smallest non-zero value of θ, and this includes elements of the wheel that are in contact with some elements of ground, but towards the fringes of relevenfy bearing less and less of the wheel-weight, so μsFn is going to be exceeded, or have to be accomodated, according to all those other complicating factors...)

On a consistent surface, it does nothing laterally, on crossing a diagonal divide between significantly different surfaces (which may involve just a different 'grain' of gravel/aggregate, even if held inflexibly static by the tarmacadaming process) where lateral effects occur. Small, but can be felt as a driver (aggrevated by one entire wheel-base width giving differing 'feels' to the road by each of your pair (or more) of driving/braking wheels, even when not being tested to the mechanical limit) and hovering the hands off the steering for the instance could reveal a jink, at least without power-steering divorcing you from all the little clues that you come to know and love.


Or so I think is being suggested.

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drachefly
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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby drachefly » Sat Jan 28, 2017 5:08 pm UTC

Kalium_Puceon wrote:Looks like it's been fixed so there's no reflector in the refractor anymore.


Oh, good –I'd been wondering where the heck this mirror everyone's talking about is.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Copper Bezel » Sat Jan 28, 2017 5:51 pm UTC

I've seen refraction illustrated as two different surfaces meeting, with an axle with a wheel on each end rolling along at a not-perpendicular angle to the interface between the two - like, in textbooks, as a kid. But it was always presented as "this wheel magically moves slower while on this surface than the one that isn't", more like the actual behavior of the light in a medium scenario it was demonstrating and not really presented as anything physical anyway; friction definitely wasn't invoked explicitly. It was just meant as a mental picture for how hitting a change of medium at an angle could change the trajectory.

The real world is indeed messy, but unlikely to be consistently messy in the same way across all scenarios, and I really doubt if it would coincidentally recreate an ideal behavior from another phenomenon. Worth saying as well that the free-rolling wheel or barbell is also going to have a very different behavior than the driven wheels of a car.

Edit: Looking it up, I'm seeing that it's physically demonstrated and friction is invoked. I'm not fully convinced, though.
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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Pfhorrest » Sat Jan 28, 2017 6:36 pm UTC

Going back to the original question: if not for mechanical analogues of diffraction, why do bullets shot into water not continue in the same vector as they traveled in air?
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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby peterdavidcarter » Sat Jan 28, 2017 10:19 pm UTC

Ever since Steve Jobs, and probably before, there has been a common belief that psychedelics help you to understand science and technology. Recent evidence of so called "space vampires" uncovered by researchers who had a few hours previously ingested large quantities of lysergic acid diethylamide and a sizeable quantity of psilocybin seem to confirm this hypothesis.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Soupspoon » Sat Jan 28, 2017 11:50 pm UTC

Pfhorrest wrote:Going back to the original question: if not for mechanical analogues of diffraction, why do bullets shot into water not continue in the same vector as they traveled in air?

They probably do (direction, not velocity), but is this (to someone maybe as accustomed to discharging weapons into water) not a little like "why does my straw bend when sitting in a glass of <foo1>"?


1 Pop, lemonade, soda, etc, whatever your locality prefers.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby morriswalters » Sun Jan 29, 2017 12:33 am UTC

Pfhorrest wrote:Going back to the original question: if not for mechanical analogues of diffraction, why do bullets shot into water not continue in the same vector as they traveled in air?
Shearing at the surface?

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Mikeski » Sun Jan 29, 2017 12:43 am UTC

Bullets fired obliquely into water may not deflect as much as you'd expect, but they do deflect. (If not, you couldn't skip a rock across a pond.)

If fired from a rifled barrel, the bullet will be spinning, and gyroscopic effects will counter some of the expected deflection.

Actually, Mythbusters did a bit ("bulletproof water" in episode 34) about escaping from bullets by diving. A non-spinning black-powder ball missed the submerged ballistic gel target entirely, because it deflected too much. A non-spinning shotgun slug hit; either greater momentum or the cylindrical vs spherical shape controlled the deflection enough. Rifled projectiles hit if they were slow enough (9mm pistol). High-velocity rounds just shattered when they hit the water's surface (.223 rifle, M1, 50cal).

So the James-Bondian movie scene where the soldiers empty their full-auto machine guns into the ocean, then the commanding officer gets ticked and pulls out his sidearm to plink a few shots? And the moviegoers think "really? how is that going to do anything?" Apparently the CO was the only one with a chance of hitting Mr. Bond, and the moviegoers should have been rolling their eyes at the grunts with the AK's...

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby synp » Sun Jan 29, 2017 8:29 am UTC

Expensive? You can get refractor telescopes on Amazon for under $40. Good luck finding a reflector for significantly under $200.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Copper Bezel » Sun Jan 29, 2017 8:38 am UTC

On the one hand, what's really meant is "for an equivalent thing", but it's hard to compare since there isn't, you know, really an equivalent thing, due to the other things. So it is kinda like counting something twice to include cost....
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby pogrmman » Sun Jan 29, 2017 4:29 pm UTC

Copper Bezel wrote:On the one hand, what's really meant is "for an equivalent thing", but it's hard to compare since there isn't, you know, really an equivalent thing, due to the other things. So it is kinda like counting something twice to include cost....


I agree, they aren't exactly equivalent. I own a 10" reflector, and have used a long-focus 10" achromatic refractor from the same site, I have to say that there can be big differences between the two. It may just be differences in baffling and stuff, but the view through that refractor is stunning. (It's not like the reflector is bad -- it's great -- I wouldn't have bought it otherwise).

There is also the difference that while my reflector is something like 45 lbs with the mount and can be carried by me with relative ease, and the fact that that refractor is 10 feet long and is on a multi thousand dollar mount on a huge pier. They aren't super comparable, other than the aperture.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Mike Rosoft » Sun Jan 29, 2017 8:25 pm UTC

IonStorm wrote:Sadly this is old science. See Sandford, Dworkin, and Bernstein (2007) "The Chemical and Physical Properties of Vampires in the Gaseous State" Annals of Improbable Research 13, 20-24. (PDF)


Hmm...

Annals of Improbable Research wrote:It should be noted that the fact that vampires do not cast visible shadows raises a dilemma. If vampires do not absorb or scatter visible light, i.e., they do not interact in any way with incident light, how is it that we can see them in both solid and gaseous state? The only possible solution is that vampires must emit light while in both their solid and gaseous forms. Given their propensity to lurk in shadows, it is likely that vampires fluoresce (or phosphoresce with fairly short triplet lifetimes to allow prompt intersystem crossing) as opposed to behaving like a blackbody.


Looks like vampires do, in fact, sparkle! (Insert Twilight Sparkle joke here.)

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Quizatzhaderac » Mon Jan 30, 2017 3:51 pm UTC

peterdavidcarter wrote:Ever since Steve Jobs, and probably before, there has been a common belief that psychedelics help you to understand science and technology. Recent evidence of so called "space vampires" uncovered by researchers who had a few hours previously ingested large quantities of lysergic acid diethylamide and a sizeable quantity of psilocybin seem to confirm this hypothesis.
Are you aware of how Louis Pasteur discovered stereochemistry?

He literally found it at the bottom of a bottle of wine.
The thing about recursion problems is that they tend to contain other recursion problems.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby orthogon » Tue Jan 31, 2017 6:07 pm UTC

OK, I've now reversed my position on refraction of moving objects crossing a boundary between surfaces at an angle. For the hockey puck, the friction should be directed exactly opposing the movement, so shouldn't have the lateral component required to be able to cause a change of direction. However, one side of the puck will encounter the new surface first, so the net force will be to one side of the centre. This is equivalent to a purely decelerating force through the centre of mass plus a couple (torque) which will make the puck spin.

For a wheeled vehicle, the same thing applies initially, but that the couple will cause the vehicle to rotate such that it's no longer pointing in the direction of motion. Friction on such a vehicle is anisotropic: it's much less parallel to the wheels than perpendicular to them. Hence the deceleration caused by the friction is not parallel to the direction of motion and the vehicle will move in a curve, finally traveling in a different direction to the one it started in, i.e. it will refract.

Third revision: the spinning motion imparted to the puck introduces lateral asymmetry in the friction, so it too will change direction. However, it seems to me that it will deflect the opposite way! Can this be right? ( I'm saying that the vehicle deflects like light: towards the normal when entering the "slower" medium, whilst the puck deflects away from the normal).
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Soupspoon » Tue Jan 31, 2017 7:04 pm UTC

Consider the sport of curling, and the roll of (symmetric) spinning in giving lateral motion. (And with wheels, turning motion outwith the wheel-hub axis must impart gyroscopic precession that may significantly, if not actually noticably, induce lean in the wheel (in the third axis, yet different from either the rolling or twisting ones), changing the footprint to off-centre, to... some effect, no doubt. Needs some advanced modelling, probably.)

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orthogon
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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby orthogon » Tue Jan 31, 2017 8:04 pm UTC

God, I'd forgotten all about curling. Now I remember having gone through a similar thought process when the Winter Olympics was on, without ever fully coming to terms with it.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby Quizatzhaderac » Tue Jan 31, 2017 8:38 pm UTC

orthogon wrote:OK, I've now reversed my position on refraction of moving objects crossing a boundary between surfaces at an angle..
<poe>Wait, so you've changed your position by rotating 180 degrees, by reflecting across the boundary, or by reflecting across the line perpendicular to the boundary?</poe>
Last edited by Quizatzhaderac on Thu Feb 02, 2017 6:58 pm UTC, edited 1 time in total.
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orthogon
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Re: 1791: "Telescopes: Refractor vs Reflector"

Postby orthogon » Tue Jan 31, 2017 10:48 pm UTC

Quizatzhaderac wrote:
orthogon wrote:OK, I've now reversed my position on refraction of moving objects crossing a boundary between surfaces at an angle..
<poe>Wait, so you're changed your position by rotating 180 degrees or by reflecting across, the boundary, or by reflecting across the line perpendicular to the boundary?</poe>

Yes.
xtifr wrote:... and orthogon merely sounds undecided.


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