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Clicking on images of historic satellites led me to the airvectors.net page Foundations Of Automated Space Technology which includes this image and passage:

A number of Samos satellites were also flown in 1960 and 1961. Most were film-readout satellites, though a few were Zenit-style film-and-camera-return spacecraft. Details were never revealed, and the program remains somewhat mysterious; in fact, the Zenit-style film-and-camera-return spacecraft appear to have been partly developed, under Bernard Schriever's direction, as a means of giving the Air Force an option of flying an astronaut if the service had so desired, in much the same way that Zenit operated as a "cover" for Vostok. If so, that aspect of the program was conducted with extreme discretion.

It appears Samos was very unsuccessful, though it was played up in the press as America's official space reconnaissance program to distract attention from Corona. Some authors have suggested that it the main reason Samos remains classified after all this time is that nobody involved wanted to remember it, much less say anything about it.

The first Midas missile early warning satellite was launched on 26 February 1960 but failed to achieve orbit, and although Midas II was successfully launched on 24 May 1960, its telemetry system failed two days after launch. Midas III was finally successfully launched in 12 July 1961. A total of 12 Midas satellites were launched into 1966, but it appears that this program wasn't very successful either, with infrared sensors that picked up glints of sunlight off clouds as missile launches. Details are unclear, since specifics of the Midas program were also never revealed. In any case, the military used the experience to consider better solutions.

enter image description here

MIDAS IR Sensor (~CLF1066 / CCSA)

Question: Why does this "Midas missile early warning satellite" have a tilting cone with a complex black-and-white checkerboard pattern? While this question covers checkerboard patterns on launch vehicles, presumably these are too small to have been seen from Earth in the 1960's.

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    $\begingroup$ "The spin table would rotate 360 degrees at 2 rpm about the vertical axis of the satellite in a nose-down attitude.", The tilting cone rotates to increase the observable area of the IR scanner. Later Midas Version spun with higher rpm. But I'm puzzled with the checkerboard pattern, especially because the sensors were covered during launch so they would've never be seen after payload integration. $\endgroup$
    – GittingGud
    Commented Jul 22, 2019 at 8:12
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    $\begingroup$ I wonder if that could be a ground test article for the sensor and they wanted to do something like tune a rotation control loop and optically observe the rotation $\endgroup$
    – Erin Anne
    Commented Jan 27 at 3:14
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    $\begingroup$ "The answer is always thermal control" at least for the checkerboard. See para 2.3.1.1 ntrs.nasa.gov/api/citations/19730018163/downloads/… $\endgroup$ Commented Jan 27 at 3:30
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    $\begingroup$ @OrganicMarble it's not the 50:50 black/white that catches my eye, nor even the checkerboardedness (I would have gone with more of a holstein fractal than a Belted Galloway look, but that's just me), it's the variable row height and the alternating duty cycle (one row is mostly black, the next is mostly white) that has me scratching my head. $\endgroup$
    – uhoh
    Commented Jan 27 at 6:16
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    $\begingroup$ @OrganicMarble even the "gloriously striped" Mercury capsule retropack at the NASM shows a constant, uniform duty cycle even thought the period is continuously variable. (btw your NTRS link would probably make for an excellent supplemental answer there!) $\endgroup$
    – uhoh
    Commented Jan 27 at 6:21

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This is a two component question:

  • Why the tilted cone?
  • Why the complex checkerboard pattern?

The Angled Cone

Answer: The tilted cone is the rotating sensor telescope

From https://space.skyrocket.de/doc_sdat/midas-6.htm The MIDAS Series 3 was a development model for the MIDAS early warning system. It used a rotating 8” aperture Bouwers telescope with a 25* field of view. The telescope rotated on a turntable at 6 rpm. It had lead sulfide IR detectors in a 8 x 23 array. Note: the source does not make it clear if the “field of view” (FOV) is the optical FOV of the telescope or the scanning angle.

A Bouwers concentric telescope is a type of catadioptric telescope. “Catadioptric” means it combines reflective and refractive optical elements. This design maximizes FOV (in contrast to astronomic telescopes which usually maximize magnification and resolution.) The MIDAS Series 3 telescope’s 25* field of view (FOV) is much greater than the 7.5* field of view common in binoculars.

enter image description here

Two other features of the Bouwers (which will be mention again below) are that its center of gravity and prime focus are near the center of the telescope.

In addition to the inherently wide FOV, the telescope was mounted on a turntable with a 25* angulation to the axis of rotation, This effectively doubles the optical FOV as it completed a scan every 10 seconds.

From https://en.wikipedia.org/wiki/File:MIDAS_infrared_sensor.PNG and http://www.astronautix.com/graphics/r/rts1.jpg

enter image description here

Midas 2 Credit: Manufacturer Image

enter image description here https://en.wikipedia.org/wiki/File:MIDAS_infrared_sensor.PNG

The satellite orbited “nose down” as shown in the illustration below of the MIDAS 3 predecessor, the WS-117L Subsystem G of Weapons System 117 (WS-117L) with a W-17 payload From https://www.drewexmachina.com/2023/07/19/the-promise-of-midas-the-first-experimental-early-warning-satellites

enter image description here

The presence of a turntable begs the question of how electronic connections were made between the non-rotating satellite bus and the rotating telescope. The following is speculation without backup evidence.

The presence of two optical elements at opposite ends of the Bouwers telescope places the center of mass near the geometric center of the telescope (unlike a Newtonian or a traditional refractor telescope). This is an advantage since, in a satellite, the turntable-mounted telescope must rotate around its center of mass. The location of the prime focus behind the corrector plate makes it easy to redirect the image beam along the rotation axis using a convex mirror:

enter image description here

With this design, the sensor array and related electronics can be mounted on the satellite bus instead of the rotating turntable. https://space.skyrocket.de/doc_sdat/midas-1.htm mentions the Aerojet payload built for the first two demonstration flights (MIDAS 1 and 2) had the sensors mounted “in a fork beneath the spin table” which is consistent with the above speculation about the optical design.

The Checkerboard Pattern

Answer: Possibly for thermal management

As for why there is a complex checkerboard pattern on the rotating cone portion of the satellite, I have inadequate references so I will speculate:

During the 1960’s, before the development of Multi-layer Insulation (MLI), checker-board patterns were used for passive thermal control of spacecraft. Paints and surface treatments were sometimes not available with the required combination of absorbance and emittance .

enter image description here https://ntrs.nasa.gov/api/citations/19730018163/downloads/19730018163.pdf

Alternating areas of different surface treatments would allow properties not available in a single treatment. A checkerboard would allow creation of a coating half way between two alternatives.

The 50/50 surface area of a traditional checkerboard could be varied to, say, 56/44 by changing the relative width of the rectangles.

enter image description here

It is a challenging problem to map a equal-area checkerboard onto a cone, similar to creating a equal-area map of Earth onto a flat plane.

https://upload.wikimedia.org/wikipedia/commons/d/da/USGS_map_Albers_conic_tall.gif

Equal area mapping necessitates distortion, which is apparent in the MIDAS 3 checkerboard pattern.

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  • $\begingroup$ Incredible answer, thank you! $\endgroup$
    – uhoh
    Commented Feb 1 at 1:04
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    $\begingroup$ Since there are two main sections you may want to add headings. Here is an example, I didn't make the edit myself because it may not be what you want. $\endgroup$ Commented Feb 1 at 14:29
  • $\begingroup$ @StevePemberton ... great suggestion. please go ahead with the edit $\endgroup$
    – Woody
    Commented Feb 1 at 16:15

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