Casscam Image Dissector Star Tracker

This is an experimental post. It's the first to expand the scope to cover the history of computing at UT's McDonald Observatory and Center for Space Research, as discussed in the About. And it's related to an excellent new post on Ken Sherriff's Blog. Ken's blog is a direct inspiration, much like the TCHC Blog. Ken's post explores a historical star tracker. Star trackers are a fascinating topic and played a role in the history of computing at UT. This post will focus just on the image dissector star tracker used in the McDonald Observatory 82-inch telescope Cassegrain Camera. Later posts will discuss the CCD star trackers used by the Center for Space Research for the NASA ICESat and ICESat-2 missions, and the associated computational modeling and data analysis. It's a long and complex story, covering different eras of computing at UT, and best explored gradually over time. There are also three good books for background reading on these topics [1], [2], and [3]. 

Around 1990, the 82-inch telescope at McDonald Observatory was used to make glass plate photographs of asteroids for the Texas Minor Planet Project (TMPP) and the Hubble Space Telescope Astrometry Team. It was the Indian Summer of traditional analog glass plate imaging, digital CCD imagers had not yet taken over this niche. The heart of the TMPP system was the suitcase-sized Casscam and its integrated star tracker. The Casscam was among the last and most advanced plate cameras. It's possible that the star tracking control loop was entirely analog electronics. Based on operational experience and the environment at McDonald, it's also possible that at least the encoders and logic were digital. It was directly descended from the first instruments used on the 82-inch. Below is an old photo of a direct ancestor, probably from the thirties. [4] The knife-edge focus frame and glass lens were still used in 1990, as discussed below.

An 82-inch telescope instrument and direct ancestor of the Casscam. The knife-edge focus frame lies to the left and has the basic outer dimensions of a glass photographic plate. The cone mounted on top is a heavy glass lens. Peering through the lens, celestial objects appeared much as in a long-exposure full-color astrophotograph.

Even the largest asteroids were small and faint, requiring a relatively long exposure to build up an adequate spot in the photographic emulsion on the glass plate. While building up an asteroid image spot, the Casscam had to track the asteroid’s apparent motion to hold the spot still on the glass plate. This apparent motion was principally from the Earth’s own motion and parallax effects. Against the background of effectively fixed stars, an asteroid moved appreciably when viewed from the Earth, especially when imaged with the magnification of the 82-inch telescope. The Casscam had to nullify this apparent motion during asteroid tracking. The photographic plate holder was rotated to align the asteroid's motion along the Casscam's primary axis. Then, during asteroid tracking, the Casscam moved the plate at the same speed as the asteroid's apparent motion, nullifying it. This was open-loop tracking, without feedback or active error correction.

Star tracking was also needed, separately from asteroid tracking. The stars in the image near the asteroid were also faint, and it was essential to build up adequate spots in the photographic emulsion for them as well, as they were the means to computationally tie the asteroid to the celestial reference frame. The computational modeling and data analysis aspects of this are subjects for later, dedicated posts. In this post, the focus is purely on the Casscam’s capability to track the stars and hold their image spots still on the glass plate. It did this using a combined image intensifier and image dissector tube star tracker locked onto a guide star. The image intensifier was a close relative of a photomultiplier tube. Incoming photons initiated a cascade of electrons down a cylindrical tube, roughly twelve inches long and a few inches in diameter. The circular end of the tube was a glass phosphorescent screen with a cross-hair etched on it. In normal operation the green fuzzy ball of a star image was kept centered in the cross-hair. Below is an example with a much lower magnification and wider field of view. Imagine this zoomed in on the central bright star, with a cross-hair on it.

Stars in an image intensifier. This one has a much lower magnification and wider field of view than the one on the Casscam.

Image dissector tubes seem to have been named to suggest dissecting or taking apart an image, in other words sampling an image. Image sampler may be a more suggestive name to modern eyes. An image is formed using electrons, and that image is then sampled, all within the tube. In the picture below, the lens on the right forms an electron image on the photo-electric plate while the aperture and valve samples the electron image.

An image dissector tube in its early role as a television camera. [9]

The Casscam's combined image intensifier and dissector was sampling the electron image just at the center of the field of view, around the cross-hair. Once a star was placed in the cross-hair, the sampling would output an error signal whenever the star began to drift away. The control loop would then move the plate holder to zero the error signal and correct the drift. The control loop was running at about 1 Hz, and produced a loud clicking noise every second. This soon became a familar sound in the darkness of the 82-inch dome, a steady click click click while the star tracker control loop was active.

In note [5] below there's mention of a 64x64 image dissector at McDonald in the seventies, so clearly something like sampling of a pixel grid was possible and a viable technology until solid-state CCD imagers became available in the eighties and nineties. The transition to CCD star trackers will be explored in a future post about the Center for Space Research and the NASA ICESat star trackers.

Since this post includes a photo showing the knife-edge focus frame, its use can also be described. At the beginning of an observing run, early steps included preparing the Casscam and focusing the telescope. The Casscam was a heavy instrument, about the size of a suitcase. The McDonald operations staff would mount it onto the back of the 82-inch using a lift and heavy bolts. The combined system then needed to be focused by moving the secondary mirror, which was roughly fifty feet overhead. The secondary mirror was moved by an electric motor controlled from a control paddle on the observing platform. Focus was achieved using a knife-edge technique within the focal plane. By placing a metal frame with a straight knife-edge into the Casscam’s plate holder, the observer could adjust the secondary mirror until the light from a star was cut off instantaneously rather than gradually. On occasions when time permitted, the knife-edge focus frame could be replaced by the massive glass eyepiece also shown in the photo. Needless to say, star-gazing through the 82-inch telescope was something very special. 

Notes and photos

[1] MacKenzie, Donald. Inventing Accuracy: A Historical Sociology of Nuclear Missile Guidance. Cambridge, MA: MIT Press, 1990.

[2] Spinardi, Graham. From Polaris to Trident: The Development of US Fleet Ballistic Missile Technology. Cambridge: Cambridge University Press, 1994.

[3] Grewal, Mohinder S., Angus P. Andrews, and Chris G. Bartone. Global Navigation Satellite Systems, Inertial Navigation, and Integration. 4th ed. Hoboken, NJ: John Wiley & Sons, 2020.

[4] Evans, David S., and J. Derral Mulholland. Big and Bright: A History of the McDonald Observatory. Austin: University of Texas Press, 1986.

Direct ancestor of the star tracker discussed in Ken's post. [1] 

Another direct ancestor.

[5] Though the image dissector is capable of scanning a two-dimensional image, it is difficult to do so before the phosphor of the last intensifier has decayed substantially. McDonald observatory did indeed build an area photometer which scanned a 64x64 two-dimensional array (P. M. Rybski, G. W. Van Citters & G. F. Benedict, IAU Coll. 40 Astronomical Applications of Image Detectors with Linear Response, 1976). Bull Astr Soc India, 406-423 December, 1985. This could very well have been related to the Casscam star tracker. Fritz Benedict was a member of the Hubble Space Telescope Astrometry Team into the nineties.

[6] Image dissector tubes have found widespread use in astronomy, beginning with the pioneering work of L. Robinson & J. Wampler in the early seventies. Though occasionally used as imaging devices for either recording extended fields or guiding in automatic/remote-manual mode, the more popular usage has been in intensified scanning spectrometers. Such a system was first developed at Lick Observatory (Robinson & Wampler, Publ. Astr. Soc. Pacific 84, 16 1972), who subsequently duplicated it at the Anglo-Australian Observatory. Kitt Peak National Observatory, European Southern Observatory and Ohio State University have subsequently built similar instruments, some of which are still maximally used. The introduction of more sensitive detectors like the image photon counting systems and charge-coupled devices, and resultant shift in the emphasis of observing programs to fainter limits, have rendered the image dissectors less popular in recent years. However, the image dissector remains the most useful detector at intermediate light levels where avenues remain open for astronomical research. Ibid.

[7] The Intensified Image Dissector Scanner has been in routine use at Kitt Peak National Observatory for two years ... In this instrument, the output phosphor of a three-stage image intensifier is used as a temporary storage medium for incoming photon events. An image dissector tube is used to rapidly scan this output phosphor. Instrumentation in astronomy III, Proceedings of the Society of Photo-optical Instrumentation Engineers, v172, p86, 1979.

[8] An image dissector, also called a dissector tube, is a video camera tube in which photocathode emissions create an electron image which is then swept up, down and across an anode to produce an electrical signal representing the visual image. It employs magnetic fields to keep the electron image in focus, and later models used an electron multiplier to pick up the electrons ... they continued to be used for imaging in early weather satellites and the Lunar lander, and for star tracking in the Space Shuttle and the International Space Station. Wikipedia

[9] https://www.earlytelevision.org/baird_and_farnsworth.html