Spaceship based telescopes

[Anthony]

Originally Posted by David L Pulver

* However, various sources and HIGH TECH's suggestion for Telescopic Vision implies that a Hubble or KH-11 style system is closer to 4,000 to 8000x. That would be 160, 000 to 640,000 lbs.

Diffraction-limited optics have a resolution of (wavelength) * 1.22 / (diameter), meaning you need a minimum lens diameter of (wavelength)*1.22/(resolution). Human 20:20 vision is defined as 1 minute of arc resolution (1/3438 radians) at a wavelength of 400-700 nm, so for 1x you need (700 nm) * (3438) * (1.22) = ~2.9mm optics. Most good telescopes are diffraction-limited, so the Hubble has about 800x human visual resolution.

You can, of course, magnify by more than 800x; it will just be a bit blurry. Also, the diffraction limit is the size of the blurry spot a point-like object becomes; if you know you're looking at point-like objects (which will often be true in astronomy) you can do a bit more processing to find the center of the blurry spot, and maybe even figure out that two blurry spots are overlapping. This will not typically be very useful outside of astronomy.

Originally Posted by David L Pulver

So, which is wrong:
(a) "scale by the square of magnification"

Really, 'scale lens diameter by magnification. This tends to result in an actual scaling factor between 2 and 3, as a larger lens requires more support.

Originally Posted by David L Pulver

(b) the estimate that Hubble / KH-11 is about 4,000 to 8,000x magnification

This one is wrong (well, it might magnify by that much to make the images easier to work with, but it doesn't have that much resolution).

Originally Posted by David L Pulver

(c) the base weight of the 10x system.

Binoculars are not a good model for sensors, as they're limited by the structure of your eye. Also, field of view matters, a telescope with a 1 degree field of view is a lot less weight than a wide angle lens.

Originally Posted by David L Pulver

Also, it looks like if you scale DOWN you get a super-light 1x magnification system. At TL8 what would be a realistic weight for a 1x "human eyeball" camera?

The equivalent of human visual resolution in a 60 degree arc (the human eye only has that resolution in about a three degree arc, but cameras cannot easily emulate that) is about 13 megapixels, so your average cell phone camera is slightly worse than human eyeball.

[Ulzgoroth]

Originally Posted by Anthony

Also, the diffraction limit is the size of the blurry spot a point-like object becomes; if you know you're looking at point-like objects (which will often be true in astronomy) you can do a bit more processing to find the center of the blurry spot, and maybe even figure out that two blurry spots are overlapping. This will not typically be very useful outside of astronomy.

Actually, I think it's also being used in fluorescence microscopy these days, where the point-like objects you are looking to localize are individual fluorescent molecules.

[Anthony]

Originally Posted by Ulzgoroth

Actually, I think it's also being used in fluorescence microscopy these days, where the point-like objects you are looking to localize are individual fluorescent molecules.

True. It's also useful in radar -- aircraft are reasonably likely to be a blurry blip, but you can usefully aim at the center of the blur as long as they're in midair.

A lot of the problem with statistics is that figures like 'magnification' aren't actually meaningful -- you can magnify by as much as you want. What you want is resolution.

This also applies to night vision gear. Typical NVG only has around 100x the raw light-gathering power of a dark-adjusted human eye, but it might well turn the gain up to well over 100x, because if you do, the user can use it at night without waiting half an hour for his eyes to adjust.

[David L Pulver]

Originally Posted by Anthony

True. It's also useful in radar -- aircraft are reasonably likely to be a blurry blip, but you can usefully aim at the center of the blur as long as they're in midair.

A lot of the problem with statistics is that figures like 'magnification' aren't actually meaningful -- you can magnify by as much as you want. What you want is resolution.

The magnification number is just a colorful but inaccurate way of tracking bonuses to the roll; the real number of import is the bonus.

A 800x magnification = +8 (or +16 with narrow focus). If that represents a hubble/KH-11, the question is whether that is sufficient?

The range penalty for 250 miles or so is -32. If you have an analyst with skill 14 and a +16 bonus and perhaps a +2 time spent bonus you end up with effective skill 32.

If the range penalty for 250 miles is -32, this gives a 0 chance of success.

Anthony, this suggests that 800x cannot - in GURPS terms - adequately represents a telescope of hubble sized (KH-11) pointing down. What am I missing?

[Anthony]

Originally Posted by David L Pulver

The range penalty for 250 miles or so is -32. If you have an analyst with skill 14 and a +16 bonus and perhaps a +2 time spent bonus you end up with effective skill 32.

If the range penalty for 250 miles is -32, this gives a 0 chance of success.

Anthony, this suggests that 800x cannot - in GURPS terms - adequately represents a telescope of hubble sized (KH-11) pointing down. What am I missing?

+10 for in plain sight, to start with, and typically target size, spy sat pictures usually look at buildings, vehicles, and crowds, not individual humans. The equivalent situation with naked eye observation is an analyst looking at 35mm pictures taken from a plane at 500 meters -- range penalty -15, skill 14, +2 time bonus, modified skill 1.

[Ulzgoroth]

Originally Posted by Anthony

+10 for in plain sight, to start with, and typically target size, spy sat pictures usually look at buildings, vehicles, and crowds, not individual humans. The equivalent situation with naked eye observation is an analyst looking at 35mm pictures taken from a plane at 500 meters -- range penalty -15, skill 14, +2 time bonus, modified skill 1.

According to Powers: Enhanced Senses, that +10 also is supposed to become +20 for focusing in on small details as opposed to scanning.

[David L Pulver]

Good point about the +10.

Originally Posted by Ulzgoroth

According to Powers: Enhanced Senses, that +10
also is supposed to become +20 for focusing in on small details as opposed to scanning.

Sounds like an odd ret-con to me - are you sure it's intended to apply to sensors? I have to admit I don't understand that. Isn't that what the "double the telescopic vision" bonus was for?

[safisher]

Originally Posted by Ulzgoroth

According to Powers: Enhanced Senses

Enhanced Sensors came long after UT/HT, for one. Secondly, as David said, the important thing in GURPS is not the color text, but the game effect. The suggestions in HT come from trying to figure what it would take in game terms to do what they claim, being generous in terms of how they would likely be used in an adventure game. Applying heavy science to those figures is likely to leave you frustrated.

[David L Pulver]

Originally Posted by Anthony

+10 for in plain sight, to start with, and typically target size, spy sat pictures usually look at buildings, vehicles, and crowds, not individual humans. The equivalent situation with naked eye observation is an analyst looking at 35mm pictures taken from a plane at 500 meters -- range penalty -15, skill 14, +2 time bonus, modified skill 1.

Ah, forgot the +10 in plain sight. Okay, the 800x should just about work for that then.

If I may ask again, could you follow up more on the scaling factor?

I'm still not sure whether you meant to say the "square of the magnification" for weight is a good relationship? The relationship I'd been assuming is:

1x = +0 = 1 x weight
2x = +1 (+2 if zooming in) = 4x weight
4x = +2 (+4 if zooming in) = 16x weight
8x = +3 (+6 if zooming in) = 64x weight
16x = +4 (+8 if zooming in) = 256x weight
512x = +9 (+18 if zooming in) =262,144x weight

(Actually, I might assume 8x or below might actually be roughly linear in weight due to the extra effort needed to miniaturize - plus the weights of scopes given in HT support that - and the 4x for each +1/+2 only kicks in at 16x and up).

If you think a different relationship - SPACESHIPS uses roughly 1-3-10 for every +1/+2 - is better, let me know!

[Fred Brackin]

Originally Posted by David L Pulver

If I may ask again, could you follow up more on the scaling factor?

Considering only the mirror (and big modern telescopes are almost exclusively reflectors rather than refractors) I believe mirror weight might actually go up by the cube and not the square of an increase in diameter.

If you make your mirror wider you need to make the backing material thicker to keep it stiff under gravitational or inertial loads. There are thermal issues too.

The Keck design with its' geometric array of smaller mirrors might be a way around the "one big mirror" limit but it's not obvious that what you save in total weight from thinner mirrors you don't make up in the gear to finely control them.

It's hard to say much about competing mirror designs for large astronomical instruments. Every blessed one of them is a prototype. There are no production models and whatever design gets chosen is in the hands of whoever raised the money and set up the fabrication shop. The choice selected probably represents that person's individual preferences and experience.

Still, as a mirror gets bigger it would have to increase in weight by the square as a minimum and more likely the cube.