RE: ATM A few questions

["Chalfan, Richard D" <Richard.Chalfan@PSS.boeing.com>]

I would like to expand upon a comment by JoAnn.

        "Also with the straight vane the spikes are outside of the planet
instead of 
> spread around the whole image where they can reduce contrast."
> 
        Actually an extended object, like a planet, can be thought of as
containing a huge number of discrete point sources all packed together very
tightly.  Each of those "point sources" acts just like the image of a star
would.  If you have a spider with arms, the diffraction pattern from each
and every "point source" within the extended object will have spikes.

        Those spikes are generally dimmer than the adjacent "point source"
central maxima, so you do not see separate spikes within the image.  But the
light energy in those spikes is present, and it adds a background of "noise"
that is spread around within the extended object and lowers the contrast of
the image.  At the edge of the planet, there are no more "adjacent point
sources" off the edge, so you can see the spike off the edge of the planet.
With a circular spider, the diffracted energy is moved out from the central
maxima to the rings.  This does the same thing, spreading a "noise"
background around adjacent "point sources" thus lowering contrast. 

        It would be nice to do some experiments to see if curved or straight
spiders are better for planetary observing.  Looking at the mechanisms
involved I doubt that you could see a difference in the image if the total
area obstructed by the veins were kept constant.  The diffracted light is
there with either curved or straight arm spiders.  However, when you look at
the whole optical system, including the eye, there may be an advantage to
one or the other method.

        Any experiment would have to hold LOTS of difficult to control
factors constant.  It would be nice to look at, say, Saturn with the
identical optical system varying only the spiders (which have been carefully
crafted to present the same total light obstruction).  In practice that
would be difficult to get scientifically valid data.  Maybe with "quick
change spiders", a number of observers in a "double blind" set-up, at a
thermally stable site, with superb seeing....  well, not in my back yard.  

        Further, I agree with others who have commented that there are other
more important image degradations for most of us most of the time.  But it
is worth thinking about for an optimized planetary Newtonian.

        Richard Chalfan
        Kent, WA