Apollo 11 Lunar Surface Journal

Apollo 11 Scan Converter Spots

Copyright © 2004 by John Sarkissian, Bill Wood, Colin MacKellar, Ken Glover, and Eric Jones
All rights reserved.
Last revised 4 October 2011.


Note: The following discussion refers to full-size Apollo video. Video sources viewed on conventional TV sets are usually seen with cropped borders and the 'Parkes spot' discussed below will not be visible. The same sources viewed on computer monitors will usually be seen at full size and the 'Parkes spot' will be visible.


During the Apollo 11 EVA, NASA had a choice of using received television signals from three stations: Goldstone (GDS) in California; Parkes (PKS) in central New South Wales, Australia; and Honeysuckle Creek (HSK) near Canberra, Australia. During the first few minutes of the EVA, NASA used each of the three signals at various times and, in this note, we discuss artifacts unique to each of the signals which allow determination as to which of the three signals was in use at any particular time. Much of this information is presented in John Sarkissian's On Eagle's Wings: The Parkes Observatory's Support of the Apollo 11 Mission.

As Sarkissian notes, a number of peculiar image artifacts were seen in the video signal provided to the global television audience. One set of these artifacts originated in the TV camera and were either transient, after-images of bright objects in the field-of-view or were secondary images of bright objects produced by extra reflections in the camera optics. An example of the latter is a prominent bright streak which, in Figure 1, appears to the right of the US flag. It is a secondary image of the bright, sunlit, north (plus-Y) strut of the LM. Whenever either of the astronauts walked through the portion of the picture containing the bright streak, it appears to be visible through them. Additionally, when either of the astronauts obscures part of the north strut of the LM, the corresponding portion of the bright streak fades. These camera-related artefacts produced the ghostly effects remembered by the public.

labeled artifacts in an Apollo 11 TV image

Figure 1 - Labeled artifacts in the Parkes signal. On the left side of the image, Buzz Aldrin is deploying the solar wind collector, partially blocking the LM's sunlit north strut with his suit and backpack. The corresponding portion of the secondary strut image on the righthand side of the image has disappeared as a result. The Parkes spot is just above the horizon near the righthand edge.

A second set of artifacts had a terrestrial origin. These were small 'spots' - usually white - seen in the images scan-converted from the Goldstone and Parkes signals and were products of the optical conversion systems. The scan converters were located at Goldstone, at Honeysuckle and, in the case of the Parkes signal, in Sydney at the Overseas Telecommunications Commission's Paddington gateway - known as 'Sydney Video'

What was the origin of the spots? Bill Wood, a USB (Unified S-Band) Lead Engineer at Goldstone during Apollo, suggests that the spots were defects in the targets of the Vidicon camera tubes. Wood writes, "Back in the late 1950s I was responsible for the maintenance of various closed-circuit vidicon TV cameras used to monitor rocket engine tests at the USAF Rocket Engine Test Laboratory and, later, at the Jet Propulsion Laboratory. I would often see white spots on vidicon camera video outputs, especially if the camera was located close enough to the rocket engine being tested to be subject to vibration. Small particles could be dislodged from either the electron gun filament or the target with only moderate vibration. I even found white spots on some new vidicons that came in shock-mounted shipping cartons from the manufacturer. I believe the RCA Slow Scan converters used during Apollo were shipped by air to the several MSFN stations with the vidicon tubes already installed, which would make them more susceptible to vibration in transit. It does not surprise me to see small spots in the Apollo Slow-Scan converter outputs, caused by vibration encountered during shipping and/or installation of the scan converters."

When the signals at Goldstone and Parkes were scan-converted, the spots were always present in the scan-converted signals. The Goldstone images had a white spot located about one quarter of the image height up from the bottom, except when an inversion switch in the electronics was 'on' to compensate for the TV being upside down on the MESA. When the inversion switch was 'on', the Goldstone spot was about one quarter of the image height down from the top. (During one Goldstone sequence, the image is shown in 'negative' mode and the spot is black). The Parkes pictures had a white spot located about one third of the image height down from the top near the righthand edge, except when the inversion switch was 'on', which put the spot one third of the image height up from the bottom near the lefthand edge. Finally, no such spots have been identified in the scan-converted Honeysuckle Creek signals. The fact that images from each of the stations has a distinctive 'mark' has proved to be fortuitous, because it allowed Sarkissian to easily identify which of the three signals NASA used at any time during the moonwalk.

Figure 2 shows the Goldstone spot with the camera upside down on the MESA and the inversion switch 'on'.

Goldstone spot in an Apollo 11 TV image

Figure 2 - Goldstone spot one quarter of the frame height down from the top.

Figure 3 shows the Parkes spot - just above the horizon on the righthand side. At this time, the camera was on the tripod looking back at the LM.

Parkes spot in an Apollo 11 TV image

Figure 3 - Parkes spot near the righthand edge.

The following table gives details of NASA's switching between tracking stations during the first 9 minutes of the EVA. Thereafter, the Parkes signal was used exclusively. The table has beeen adapted from a similar table in On Eagle's Wings. Note that the times given in the righthand column differ from the corresponding times in the original version. In the early 1990s, the "Net 2 and Alpha" audio tape (recorded at Honeysuckle), which was the source of these times, was played back at Tidbinbilla and transferred to compact cassette - and then eventually played into a computer and converted to an audio file. Somewhere in this copying process, the tape was slowed down by a factor of 11 percent. Time markers common to both the EVA video and the "Net 2 and Alpha tape" (especially the "TV on" announcement and the switch to Parkes) were used to determine the original speed of the tape. Happily, once a single speed correction was applied, all of the time markers in both the video (times of switching between station feeds) and the audio tape fitted to within a second or two.

Comparative timings from the EVA video and the Net 2 communications loop

Video Transmission Time (mm:s) Net 2 and Honeysuckle internal comms Dialogue Time (mm:ss)
TV on (upside-down)
Picture is from GDS
Time is 02:54:00GMT
00:00 Goldstone TV on line
Honeysuckle Video on line
00:00
GDS picture is turned upright 00:31 We are in reverse 00:31
Picture is switched to HSK 01:42 All stations, we have just switched video to Honeysuckle 01:45
Armstrong steps onto the Moon
The time is 02:56:20GMT
02:20
Picture is switched to GDS
(GDS picture is negative)
04:45 All stations, Houston TV. We have just switched to Goldstone video. 04:57
Picture is switched to HSK 05:39 Honeysuckle, Houston TV. We have just switched back to you again. 05:46
Picture back to GDS 06:50 All stations, Houston TV. Switching to Goldstone 06:50
(Honeysuckle internal comms:)
Can you confirm we are receiving Parkes data now?
07:57
Roger, Parkes main into demod 4 PM 08:28
Houston TV, Sydney Video. Please be advised I have a very good picture from Parkes, shall I give it to you?
Roger, beautiful picture, thank you
08:36
Picture from PKS on main beam
Time is 03:02:53GMT
Momentary switch back to GDS and then back to PKS
08:53 All stations, we are switching to Parkes at this time 08:53
Honeysuckle, Network
Right, you might pass on to the Parkes people that their labour was not in vain, they’ve given us the best TV yet.
Roger, thank you very much, they’ll appreciate that, they’re monitoring.
10:30
Timings revised by Colin Mackellar, May 2004. (After Sarkissian, 2001.)
Source: audio tape recorded at Honeysuckle of the tracking station network (Net 2) and the Honeysuckle comms loop.

Colin Mackellar has provided a complete transcript of this portion of the Honeysuckle Creek recording along with the corresponding audio clip. The playback speed has been increased by 11 percent as discussed above.

 


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