Re: Fw: ATM Slightly off topic- Moon viewings

[Bill McHale <wmchal1@umbc.edu>]

On Thu, 9 Jul 1998, Charles Hovatter wrote:

> Not to worry; you did no harm.   It's just the difference between
> night and day.   It is not really as bright as it seems, but will
> quickly destroy any visual purple that you eye has made. - This is
> what allows our eyes to become dark adapted.   Not as harmful as it
> sounds as normal daylight does the same thing.   No matter how large
> a scope you use visually, you cannot increase the surface brightness
> past the naked eye view.   You do collect more light, but you are
> spreading it out over a larger area.   The view of the moon with any
> telescope is actually dimmer (per unit of area) than the naked eye
> view, because of losses in the optical system.   This applies to all
> "extended" objects I.E., objects of some size rather than just a
> point source, like a star.

This does not sound right to me.  Granted considering the moon's
relatively low reflectivity it probably is not a cause for concern, but I
should think that it is possible to make the object brighter in a
telescope, depending on magnification.

Consider that a dark adapted eye is 7, possibly 8 mm across at the pupil,
and a 6 inch telescope is 150 mm.  Now that means that the scope (ignoring
for the moment light lost by the central obrstruction, and absorbed by the
optical elements) is gathering more than 350 times as much light as the
eye normally would.  Even allowing for a moderate sized secondary, and
absorptions by the eyepiece, I figure the is still a minimum of 70% of the
light reaching the eye...   So as long as the scope is being used at say
150x I would think that the image of any object should be brighter than
normal.

I think this also rings true when you consider that Binoculars and
Telescopes can be used to observe Nebula and other objects that are fairly
large in the sky, but normally somewhat dim.

> With the moon, you can only make it bigger with any scope; Never any
> brighter surface.   No worse than going to the beach without
> sunglases.   Uncomfortable, perhaps, but then that is why they make
> sunglasses.   BTW - They make them for eyepieces too.   A 30%
> transmission neutral filter may make viewing the moon, a much more
> comfortable pastime.   On a big scope, I would say, stop it down, but
> with a 6" that will not leave you with much.   I would go with a
> filter.
> 

Personally I would never stop it down if I could help it... After all that
also cuts down on the detail you can see.


Bill

***************************************************************************
The main problem with my job is that they expect me to actually work.
---------------------------------------------------------------------------
Home page - http://www.gl.umbc.edu/~wmchal1
***************************************************************************