size or brightness?

siebren klein Sep 18, 2001

Hello Paul,

1) I always think logarithmically about intensities. Eyes do and films do.
Only CCDs don't. Certainly the eye can not distinguish 5% at the lowest
level of intensity, but we are talking about a display, not the sky. There
it remains approximately true. Any grayscale you see completely has about 16
levels you can just distinguish. Never more than 20. Talking about analog
displays has no meaning if the eye digitizes into 16 steps. The only reason
a 256-level intensity scale is meaningful is that our colour sense is good
in detemining the ratio of green/red and translating that into a colour. And
we want that colour to be good even at a 20 % intensity level; below that we
accept that "in the dark all cats are gray".

2) It is even more impossible to have a pixel on your screen that
approximates the brightness of a mag 0 star. In Planetariums stars are
projected high-efficiency lightsources and therefore they can be much
brighter than a CRT or LCD display.
Star brightness could be approximated (not realized) if you had a beamer and
projected the
image on a 30cm x 40 cm screen. The limited resolution would make them much
too bright for practical use.
But any pixel can be darkened completely in CRT tubes. You just switch the
e-beam off.

3) We are discussing GUIDE and its possible displays. For pointlike
representations, the above 15-level intensity scale is invalid, you must be
happy to discern more than 5 brightness levels.

4) Larger but dimmer representations of weak stars would be nearer to what
you
see in the sky. If star brightness is at the 6th mag level, you must be very
sharpsighted to split a 10' double without optical help. The eye contains
wonderful (soft) hardware to help us see in the dimmest lighting conditions,
but at the expense of resolution.

Those to who real stars look like points are the happy few. Eyes have higher
order deviations from the ideal shape which are not corrected by spectacles.
It is possible to find out those deviations and correct for them, and in
(very expensive) experiments it has been shown that eyes with 100 %
sharpness can be improved by as much as 100 %. See Sky and Telescope,
October 2001, page 34.

To me the brighter stars have 5 to 7 points. Not that I would appreciate
GUIDE to represent all stars that way!

Siebren Klein
s.s.klein@...
http://www.geocities.com/siebren2001/index.html

----- Original Message -----
From: "Paul Schlyter" <pausch@...>
To: <guide-user@yahoogroups.com>
Sent: Saturday, September 15, 2001 8:22 AM
Subject: [guide-user] size or brightness?


------------------------------------------------------------------------:
1)therefore, we are able only to distinguish about 20 levels of intensity
: > more or less simultaneously.
:
: Again, that depends on the intensity of each level. You here seem to
: assume that the levels increase linearly in intensity. But they
: could just as well increase logarithmically, such that each level is
: 10% brighter than the previous level - in that case we'd be able to
: distinguish all levels from one another.
----------------------------------------------------------:
2)Divided evenly among 10 magnitudes, this would be about 0.5 mag steps.
: > Then, do you really want to be 6th mag stars in GUIDE output to be as
: > difficult to see as in the real world?
:
: Definitely not. But i think it would be quite hard to make a point on
: the video screen really shine with an apparent magnitude of 6. It's the
: same with planetariums - the artificial stars on the planetarium dome
: shine brighter than the corresponding stars in the real sky.
---------------------------------------------------
3) To be visible at all, the lowest brightness of that 20 level scale would
: > have to be rather large, at least several mm across, and that is exactly
: > the problem we meet when representing Sirius in the diameter scale. Not
: > very attractive, I think!
:
: I though we were discussing representing stellar brightnesses with points
: of varying brighness but the same size, not points of varying sizes.
----------------------------------------------------
4)Now, if the faintest dots on the screen would have to be several mm
: large to be seen at all, shouldn't the same be valid for the real stars
: in the sky? I.e. wouldn't a star of magnitude 6 have to be, say, at
: least 1/10 degree or so in apparent size to be visible to our eyes?
:
:
: I was considering 1-pixel approximations.... after all, real stars are,
: to our eyes, point sources, not small squares or circles.
:
: ----------------------------------------------------------------
: Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
: Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
: e-mail: pausch at saaf dot se or paul.schlyter at ausys dot se
: WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch