Re: [ATM] Betr.: 48" Dob at OMI -- My potentially 'dumb' questions...

[Alexis Cousein <al@sgi.com>]

RC Stargazer (Dave T.) wrote:
> Hi, Folks.
> 
> Sorry to barge into the conversation, but the theory here is kind of 
> fascinating to me, and I can see how this thread usually relates to basic 
> scope design.  So, I guess I'm brave enough that I'll ask my potentially 
> 'dumb' questions.
> 
> I think I 'get it' that the angular brightness of an extended object will 
> remain the same, by reason of it's definition, and that the major hang-up of 
> this thread seems to be that we're all easily confusing angular brightness 
> (using the definition) with the apparent brightness (of how well we see it).

Not really. With the distances involved and the scope dimensions we're talking
about, a real point source and a star can't be differentiated: they're both
mapped on the focal plane to a diffraction pattern *much* larger than the
source's size would make it.

But that diffraction does mean that at exit pupils smaller than 1-2mm,
stars (and point sources) *do* also behave like extended objects:
the eye starts to perceive them as small discs surrounded by diffraction
rings rather than points of light hitting the retina on one spot.

> 2)  And, if a star really is angularly an extremely small extended object, 
> wouldn't it really follow the same rules as an extended object, and not 
> increase in angular brightness?

Once your eye can see the extent, yes. The total integrated brightness over
the entire object will stay the same, but instead of being a point of light
the image will become a small diffracion pattern, with smaller surface
brightness as the diffraction pattern really becomes noticeable and larger.

The same happens in pretty bad seeing at lower magnifications: very dim
stars can be "blurred" into a speckle pattern and can become too dim to
see well, even in good transparency.

With my scope, I can only see the central star of M57 when the transparency
*and* the seeing are good enough.



-- 
Alexis Cousein                                  al@sgi.com
Senior Systems Engineer/Solutions Architect     SGI/Silicon Graphics
--
<If I have seen further, it is by standing on reference manuals>

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