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Subject: SPACE Digest V13 #356

SPACE Digest                                     Volume 13 : Issue 356

Today's Topics:
              Re: Space Profits
          Re: Solar Eclipse (viewer design)
                   re: faq
              Re: Railguns, EM launchers

Administrivia:

    Submissions to the SPACE Digest/sci.space should be mailed to
  [e-mail].  Other mail, esp. [un]subscription requests,
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----------------------------------------------------------------------

Date: 3 Apr 91 01:05:07 GMT
From: mips!spool.mu.edu!sol.ctr.columbia.edu!emory!wa4mei!ke4zv![e-mail]  (Gary Coffman)
Subject: Re: Space Profits

In article <[e-mail]> [e-mail] (Paul Blase) writes:
>
>The PC does demonstrate a very important point, however, which
>is that the best example of a technology does NOT always become the
>standard, rather it is the item that gets to the most people first.
>Another example is video tape.  In actuality, Betamax gives a superior
>picture to VHS, however the companies that supported VHS managed to get
>more pre-recorded tapes and better players on the market faster.  Something
>to watch out for.

Actually Sony cut their own throats on Betamax. They refused to license it
to other companies. Panasonic licensed VHS to anyone who would build one.
This created the vital critical mass of machines that convinced the software
producers to bring pre-recorded tapes to market. Also the original Betamax
recording time was short of that required to hold most feature movies so
you needed to change tapes. VHS could record an entire movie on one tape.
Probably the major use of VCRs is time shifting and the Betamax lacked
the features necessary for this service, programable timers and channel
changers, and long enough recording time. Therefore VHS was the right
technology built for the right reasons for the right market at the right
price while Betamax was a closely held proprietary technology with slightly
superior graphics (video quality), high price, and low flexibility. This
really does start to sound like a PC versus Mac contest.

Sony has finally folded their tent and bought a license from Panasonic
to produce VHS machines. Their new slogan is "never say never again".

Gary

------------------------------

Date: Wed, 3 Apr 91 12:42:00 EST
From: John Roberts <[e-mail]>
Disclaimer: Opinions expressed are those of the sender
    and do not reflect NIST policy or agreement.
Subject: Re: Solar Eclipse (viewer design)


>From: [e-mail] (Jon Edelson)
>Subject: Re: Solar Eclipse
>Date: 3 Apr 91 02:25:46 GMT
>Organization: Princeton University

>Maybe John could post his projector design.

>-Jon Edelson ([e-mail])

OK, here's the old message...

.........................................................................

Date: Fri, 12 Jan 90 01:52:20 EST
>From: John Roberts <[e-mail]>
Subject: Plans for a solar eclipse viewer (long)

>From: mephisto!eedsp!chara![e-mail]  (Donald J. Barry)
>Subject: Electronic Journal of the ASA, Vol. I, No. VI

>                        THE ELECTRONIC JOURNAL OF
>                THE ASTRONOMICAL SOCIETY OF THE ATLANTIC
>                    Volume 1, Number 6 - January 1990
>...
>                     EXPLAINING SOLAR AND LUNAR ECLIPSES
>                               by Brent Studer
>...
>        An important note about eclipse observing:  Observing a solar
>    eclipse can be dangerous to your eyes - NEVER look directly at the
>    Sun, particularly through unfiltered telescopes or binoculars.  One
>    alternate method for observing a solar eclipse is to project the image
>    of the Sun onto a piece of white cardboard, either through a telescope
>    or through a small hole cut into another piece of cardboard, but it is
>    highly suggested that even this viewing method should be done with
>    caution and experience.  Lunar eclipses, by comparison, are quite safe
>    to observe directly, either with the unaided eye or through optical
>    instruments.

>                         Copyright (c) 1990 - ASA

*****************************************************************************

This is the "camera obscura" technique, which is the principle behind the
earliest (lensless) photographic cameras. I have developed and used a
refined approach, which I hereby offer freely for general use (unless
by some chance it's covered by a previous patent):

Select a good-sized cardboard box with the top flaps intact, which is not
too heavy to hold comfortably, and a cardboard tube. (The wider the tube is,
the easier it will be to aim the device.) The box will be upside-down in use,
so what was the bottom will be referred to as the top. Near the top of one
end, centered horizontally, cut a round hole the same size as the outside
diameter of the tube. Insert the tube a short distance, and attach it (tape,
glue, support struts, etc.) so that it is rigidly attached to the box and
is perpendicular to the face of the box. Over the other (far) end of the
tube, fasten a piece of opaque cardboard, with a very small round hole of
a carefully-selected diameter centered in the end of the tube. (You can try
various sizes - a sixteenth of an inch might be good as a first approximation.)
This hole is called the aperture of the viewer.

Turn the box over, and attach a piece of white paper to the inside of the side
opposite the tube, to serve as a viewing screen. Now close up the flaps of
the box and tape them shut, then cut a hole through the flaps near the
tube end, large enough to stick your head through with room to spare.

Here is a side view of the assembly:

     +------------------------------+
     ||                             |
     ||white                        +-----------------------------+
     ||paper                                                      |aperture
     ||(screen)                     +-----------------------------+
     ||                             |
     |                              |
     |                              |
     |                     hole     |
     +-------------------  for   ---+
                           head

Use:
The first step is to align the tube with the sun. (Do *not* look through
the tube at the sun. You'll injure your eye.) Either prop the box up,
or get an assistant to help you. Point the tube approximately at the sun,
then move the assembly around until the shadow of the tube exactly covers
the base. The tube is now pointed at the sun. Next, stick your head in
the hole and look at the screen. What you should see is an image of the
sun, far superior to what can be produced using the "two pieces of cardboard"
trick. Make sure you have sufficient ventilation to breathe. With a little
practice, you can keep the tube aligned with the sun, and watch an eclipse for
several minutes.

Theory:
In a "camera obscura", the rays of light from different parts of the sun pass
through the tiny aperture (at the far end of the tube), and since they come
in at different angles, they hit the screen at different spots. In this way
an image is formed on the screen. Since the light from the highest part of
the sun is projected onto the lowest part of the image and vice-versa, the
image will be inverted (upside-down). The purpose of the box is to make the
area around the screen dark, so you can see a much dimmer image than would
be possible in broad daylight. The purpose of the tube is to increase the
length of the path from the aperture to the screen to produce a larger image,
without adding much to the weight of the box. (If you want, you can use
just the box and the aperture without the tube, but good long boxes are
hard to find and difficult to hold for long periods.)

If you do not see an image, there are three possibilities:
 - The aperture is too small, so the image is too dim (try a larger aperture).
 - The tube is not pointed directly at the sun (realign the tube).
 - You stuck your head in too far, and the image is projected on the back of
   your head (pull your head a little further out, or use a higher box).

Design principles:
 - The diameter of the aperture determines the smallest "pixel size", and
   therefore the resolution of the image, as well as the brightness. A small
   aperture gives a very detailed, but dim image. A large aperture gives a
   bright but fuzzy image. A very large aperture gives a bright fuzzy blob,
   and you could possibly hurt your eyes looking at it in the dim light (it's
   no brighter than the outside sunlight, but your eyes are dark-adapted).

 - A round aperture gives the best image, since it has the highest "area
   to maximum dimension" ratio. Any other shape blurs the image more than
   necessary in the direction of elongation of the aperture.

 - The length of the path from the aperture to the screen affects image size,
   resolution, and brightness. A short path gives a small, bright, fuzzy
   image (which is what you would get using two pieces of cardboard - just
   enough to see the crescent shape of the sun). A long path gives a bigger,
   more detailed, but dimmer image. With a longer path length, you can use
   a larger aperture to get a bigger image with the same brightness and
   resolution, which makes it easier to see details. I've been able to
   see brightly-lit clouds near the sun. I suppose in principle the device
   could could be improved to show sunspots. Some solar observatories do this.

 - If you use a fairly wide tube, alignment is easier, since it does not have
   to be exact. A longer tube gives a bigger image, but is harder to align.

 - It is possible to get a bigger/brighter image by replacing the aperture
   with an arrangement of lenses. I've never tried this. It is also possible
   when trying this to get a very small, bright image that will hurt your eyes
   and burn a hole through the screen. If you try this, be careful!

 - A further refinement in the design is possible, which will make alignment
   easier. Make a tiny hole in the side of the box (just large enough to
   cast a spot of light that you can see) on the side of the box where the
   tube is attached, somewhere to the side of the tube. Get an assistant to
   line the tube up with the sun, stick your head in the box, and observe where
   the new spot of light hits the side of the box where the screen is attached.
   Mark this spot in a clearly visible manner (you may have to temporarily
   open the flaps of the box to do this). Now you can align the device
   yourself, by sticking your head in the box and moving the assembly around
   until the new spot of light strikes the mark you made. If you make the
   sighting hole too large, the spot of light it produces (which is itself
   an image of the sun) will wash out the larger image you're trying to
   view from the tube. If you make the sighting hole too small, its
   image will be too dim, making it hard to align the tube.
   Here is a view from the tube end of the box:

                 +---------------------------+
                 |    o       ___            |
                 | sighting  /   \ tube      |
                 | hole      \___/           |
                 |                           |
                 |                           |
                 |                           |
                 |           . . .  your     |
                 |          .     . head     |
                 |          .     . (in box) |
                 +---------------------------+

 - Another possible refinement is to install some sort of viewer in the
   back of the box, pointed at the screen inside, with covers to shield
   your eyes from the daylight. If this is done, the large hole in the
   bottom of the box can be eliminated.

I like this design, because it can be easily, quickly, and cheaply built
from readily-available materials, and it actually works very well. You
can use it to view solar eclipses, or any time to look at brightly-lit
distant objects, and you can throw it away when you get tired of it.
Just make sure to tell your neighbors what you're doing (or better yet
write "Solar Eclipse Viewer" on the side), so they won't have you hauled
off to the asylum. :-) During the last partial eclipse, I had about twenty
people looking through my viewer. Most of them were favorably impressed.

Disclaimer: Never look at the sun. Also, never perform any solar observation
unless you know what you're doing. Make sure you're in a comfortable position,
and that you have plenty of air. It's probably safest to sit down while using
this viewer. Don't blame me if you get a crick in your neck or hurt your eyes
or suffocate or trip over the dog. Also don't blame me if you paint the inside
of the box (except the screen) black and the paint rubs off on your head or
you breathe the fumes and get sick. Also don't do anything dangerous. I built
the viewer as described, and it worked very nicely, with no harmful effects.

(my part) Copyright (C) 1990 by John Roberts. Unlimited distribution
permitted provided my name is included.    January 12, 1990

    John Roberts
    [e-mail]

------------------------------

Date: Wed,  3 Apr 91 12:00 CDT
From: [e-mail]
Subject: re: faq
Original_To:  BITNET%"spacelist"
Original_Cc:  TPM4017

I have been trying to access the NASA-Ames archive-server (archive_server@
ames.arc.nasa.gov, ames!archive-server) and have been unable to do so.  I am
sent an error message stating that no such node exists.  I do not have
access to internet and must operate non-interactively via Bitnet.  Is
the archive-server accessible thru bitnet?  If anyone can help me access
this server please email me or post it to the list if you consider it
of general importance.

Your considered opinions are greatly appreciated,
Tim McCollum (tpm4017@panam)
--------------------------------------------------------------------------
 /////   /////  |     |         My goal is simple:  complete understanding
|     | |     | |     |          of the universe. Why it is as it is and
|____/  |-----| |     |          why it exists at all.
|       |     | |     |
|       |     |  \___/                   - Stephen Hawkings

------------------------------

Date: 3 Apr 91 17:43:48 GMT
From: snorkelwacker.mit.edu!usc!rpi!news-server.csri.toronto.edu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Railguns, EM launchers

In article <[e-mail]> [e-mail] (Tom McWilliams) writes:
>No one has given a good explanation of how to beat the frictional problems
>of a PURE gun-style launcher.

Brute force.  Just give it enough extra velocity to punch on through, and
enough ablator to keep the payload temperature under control.  It does help
if you put the muzzle end at the highest possible altitude.

>Use a short rail, with moderate acceleration to put a fully-loaded, already-
>been-tested-and-budget-approved chemical rocket moving up at around, say,
>300mph, then fire the rockets.

Unfortunately, those chemical rockets aren't designed for a horizontal
takeoff.  You could get useful gains by catapulting them at such speeds
*vertically*, and in fact there was a recent Japanese proposal to do
just that.
--
"The stories one hears about putting up | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 are all true."  -D. Harrison|  [e-mail]  utzoo!henry

------------------------------

End of SPACE Digest V13 #356
*******************