Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Why the crossmarks on Apollo era photos?
Date: Mon, 19 Feb 2001 16:00:17 GMT

In article <3A90B4C7.877D4ECF@ix.netcom.com>,
Robert Casey  <wa2ise@ix.netcom.com> wrote:
>What purpose did the crossmarks on NASA photots from
>Apollo moon missions serve?  They were also present on
>Ranger moon probe photos IIRC.

Photographic film can stretch or distort, especially after long storage,
so if you want to be able to do accurate measurement from film, you put a
grid in the camera.  That way, each and every exposure has built-in
calibration marks.

Similar techniques were used for old-style electronic image tubes, where
the image was formed on a continuous surface scanned by an electron beam,
so that distortion caused by component drift etc. in the scanning
circuitry could be calibrated out.  Which is why you'll find such
calibration marks in images from Ranger, Surveyor, the Viking orbiters,
and Voyager.  (They are sometimes less conspicuous than the Apollo
crosses, and they are sometimes taken out during image processing, but
they're always there in the data as sent.)

The reason why you don't see them in modern spacecraft images is changes
in imaging technology:  modern solid-state image sensors have a physical
structure in the sensor surface itself for each pixel of the final image,
and the sensor surface is as stable -- if not more stable -- than any
calibration grid would be.

>That awful Fox show mentioned that some crossmarks
>were bled over by very bright objects in the photos.
>Implying that someone forgot to draw the crossmarks over
>the bright objects.....

As usual, the people who can somehow keep something like this secret for
30+ years apparently couldn't be bothered to hire competent technicians
to actually do the work.

As others have noted, there is some scattering of light within the camera
and within the film, and very bright objects can wash out such thin marks.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)