Re: Silly question....


"John D. Tanner" <j.d.tanner@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:433163dc$0$22970$cc9e4d1f@xxxxxxxxxxxxxxxxxxxxxx
> <snip>
>
>> You are making the assumption though, that the optics can resolve to
>> the pixel level. This is not actually the case. the resolution of the
>> planetary camera, was selected to just about meet the critical sampling
>> criteria of two pixels across the highest frequency component
>> resolvable by the optics.
>
> Not quite sure what you mean by that I'm afraid Roger. I've just checked
> the WFPC2 handbook
> (http://www.stsci.edu/instruments/wfpc2/Wfpc2_hand6/ch2_instrument3.html#440477)
> and the quoted pixel scale is ~100 milli-arcseconds (Wide Field) and ~46
> milli-arcseconds (Planetary). Email and Newsgroups are such a pain to
> get over what you are trying to say :-)
>
> I feel that the value of 79pix/m for the Moon is still reasonable
> considering the assumptions made and that NASA back me up at the
> Astropix website that I quoted :-)
>
> All the best,
> John
> http://physics.open.ac.uk/~jdtanner
The point is a question of what you mean by 'resolve'. Just because you
have a pixel of a particular effective angular 'size', does not mean you
can resolve something at this level. Now there are a number of different
definitions of 'resolve', which apply in different situations. One is the
definition of 'fully resolving', where if you have parallel white lines on
a black surface, you have at least 90% modulation of the resulting image.
This corresponds to the size of the Airy disk of the optics, and
corresponds to 'resolution', that is significantly coarser than the camera
pixel size in this case. However the one that the camera is designed for,
is the limiting situation, where you have two white lines/peaks, with a
dark area between, and can _just_ detect that the signal dips for this
area. The spacing of the pixels on the camera, just about corresponds to
that needed for this (which suggsts this was considered in the design).
However this does not mean you can inherently 'see' an object the size of
a pixel. If (for instance), you have a small dark object, that is standing
on an area of lighter soil, the size of a pixel, in an area of darker
soil, the effect of the optical blurring, can make the lighter area
disappear, since the average albedo is exactly the same as the
surroundings.

Best Wishes


.

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