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This answer to Why does the Gaia space telescope have two main mirrors says:

According to the GAIA FAQs which does an excellent job: http://www.cosmos.esa.int/web/gaia/faqs:

Why is there an angle of 106.5 degrees between Gaia's 2 telescopes?

The choice of the so-called basic angle of GAIA was a non-trivial one. On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky. On the other hand, it should not be a harmonic ratio of a 360-degree circle (e.g., 60 deg, 90 deg, or 120 deg). Taking these considerations into account, acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc. Accommodation aspects identified during industrial studies subsequently favoured 106.5 deg as the value finally adopted for Gaia.

I don't understand any of that.

Questions:

  1. "On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky." Well any large angle is a large angle, why should it be 90?
  2. "should not be a harmonic ratio of a 360-degree circle" I can imagine for instrumental reasons, but why are "acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc." and what does "etc." mean here? Is there a pattern I'm not seeing?

Any angle with an uncertainty of +/- 0.1 deg can be reached with some rational number with a denominator of 1800 or less, that's as far as I've gotten.

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    $\begingroup$ The down vote is for the question being too challenging? Too mathematical? Not every question can be "How many stars are there?" $\endgroup$ – uhoh Apr 4 at 21:41
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    $\begingroup$ My answer on April 1st would have been: "The bond angle of liquid water is 106.1 ± 1.8°, so using an angle inside the errorbars of this for GAIA sounded cool." That's obviously not the answer. I also do not see any pattern (yet) in the series of possible angle choice. Could it have something to do with the position of the maxima of spherical harmonics? Something like "we want want to compress the data of our measured view as spherical harmonics with highest possible resolution"? $\endgroup$ – B--rian Apr 6 at 14:39
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    $\begingroup$ @B--rian speaking of the H2O bond angle: How to find the angle between Mickey Mouse's ears as specified in an official trademark? $\endgroup$ – uhoh Apr 6 at 15:39
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    $\begingroup$ From what I can tell, it has basically nothing to do with details of the instrumentation. It's more to do with the mathematics of trying to convert an integer number of scan observations into (preliminary) coordinates along a great circle on the celestial sphere (the term of art seems to be "great circle reductions"). They had exactly the same issue with the Hipparcos satellite even though the satellite and the detectors were very different (the chosen angle in that case was 58 degrees, so there's evidently some dependence on details of the spacecraft, scan mode, etc.). $\endgroup$ – Peter Erwin Apr 10 at 0:24
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    $\begingroup$ I'll see if I can come up with some kind of hand-waving summary, even though it won't be a proper answer because I don't understand all the details. $\endgroup$ – Peter Erwin Apr 10 at 2:27
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I did not find anything conclusive on the web, and I am yet waiting to receive feedback from ESA's contact page. In a German speaking IT forum there is something a bit less vague than the FAQ cited in the question:

The reason for the 106.5 degrees is not easy to see. It has to do with the fact that we are (also) doing an astrometric solution on a great circle and for this all angles 360x(n/m) [n, m = small integers] have to be avoided because otherwise one would have a congruence state that would hinder the solution. Values other than 106.5 degrees would also be possible. 90 degrees would be ideal for parallax determination, but unfortunately 90=360x(1/4).

This said, why not simply choosing 91° as $\gcd(360,91)=1$ (with $\gcd$ being the greatest common divisor)?

Maybe it is really only a numerology of the engineers, just like the Mars parachute Morse message. An argument for that would be that Gaia also has also exactly 106 CCD sensors. Or it is simply a logistical issue on how to mount things on the spacecraft. I am really curious what the official ESA statement will be.

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    $\begingroup$ Okay so 106.5 is 360 times (71/240), so perhaps 71 is considered a "not small" integer. How about the others? This is a question about rational numbers and the largeness of integers, so an answer will be based on rational numbers and the largeness of integers as well. But you also want to be comfortably far away from a small integer rational number, the smaller it is, the farther you want to be away from it. 90 and 120 degrees are 1/4 and 1/3 and systematic errors those low harmonics may be strong. Not by coincidence our magic angle is almost equally far away from both of them. $\endgroup$ – uhoh Apr 7 at 18:26
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    $\begingroup$ @uhoh I guess you are asking about the other possible values in your question, right? $\endgroup$ – B--rian Apr 7 at 18:32
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    $\begingroup$ "but why are 'acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc.'" $\endgroup$ – uhoh Apr 7 at 18:39
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    $\begingroup$ The 0.1 deg error bars irritate me - it could be mechanical limitations which limit us to 0.1 deg steps. I am so curious what ESA will answer, if they answer. $\endgroup$ – B--rian Apr 7 at 18:52
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    $\begingroup$ The focal plane detector is huge, of order 0.5 meters wide a focal length of 35 m the samples both star fields (of the two primaries) over a rotation of about .8 degrees, so not sure what the 0.1 degrees is from. Unrelated but just asked in Space SE: What causes GAIA rotational axis to precess the way it does? How exactly is this accomplished? $\endgroup$ – uhoh Apr 7 at 21:25
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1/ 90 degrees gives the most precise angle for triangulations on a 2D Cartesian plane. You can try it out by programming 3D mesh and by glancing sideways travelling on a bike or on a satellite: glancing at 90'C, you have the highest measurement of the movement of X relative to Y compared to looking at 45'. If you have binoculars on the bike helmet (i.e. a narrow field of vision) you will find that 90 degrees views gives the best relative difference.

2/ The second statement is wrong: On the other hand, it should not be a harmonic ratio of a 360-degree circle (e.g., 60 deg, 90 deg, or 120 deg). Taking these considerations into account, acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc. . The suggested values don't impart notable harmonic ratios in the same way that phi rotations give the best coverage of light for a plant:

  • The CCD time is set to 4.4 seconds and can be changed to 4.47, rendering angular resonance NEARLY TRIVIAL.
  • The rotation time is 1 degrees every minute, one rotation every 6 hours (4009 photographs at 4.4 seconds apiece), which can be changed to i.e. 6:20.
  • GAIA has already rotated sideways by ~1 degrees every time the cameras have scanned round once, and the aperture of the CCD is 1.7 x 0.6 degrees, The resonance shifts mostly depend on those figures.
  • The sweep has to also keep the solar panels towards the sun and the sun away from the CCDs and to provide a very constant thermal environment. some very decisive constraints compared to rotation resonance.

Here's a video of the complexity of the supposed "360 degrees".

https://www.gaia.ac.uk/science/parallax/scan

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    $\begingroup$ I'm not so confident that all this "ESA is wrong" is really true. In Stack Exchange it's important to support assertions in answers using authoritative sources or at least going through the argument and the math in some details. Comparing GAIA to a plant isn't helpful, and writing "NEARLY TRIVIAL" and "IS A WRONG STATEMENT" in ALLCAPS is a little dramatic especially for an answer so thin on substance. This currently reads like an opinion and a little bit like a rant, can you elaborate on these points? Thanks! $\endgroup$ – uhoh Apr 8 at 12:02
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    $\begingroup$ This isn't correct. There certainly are small ranges of angles which are possible and have seen a plot showing the expected errors vs basic angle that shows the peaks and troughs and that the troughs occupy narrow ranges of angles. $\endgroup$ – ProfRob Apr 8 at 12:05
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    $\begingroup$ I don't think the CCD exposure time has anything to do with it. The important factors are the time interval between when a star appears on the detector and the length of time it is on the detector. $\endgroup$ – ProfRob Apr 8 at 12:08
  • $\begingroup$ The satellite rotates at most by 0.06 degrees for every image capture (4.4 seconds). Do you mean that it's important for the 106.5 angle or for the exposure time? What kind of resonance were you referring to? vibrations in the steel? I'm interpreting the suggested resonance issue as timing the camera wrong relative to position so as to miss some views. $\endgroup$ – DeltaEnfieldWaid Apr 8 at 13:06
  • $\begingroup$ @uhoh Sorry i had edited the ALLCAPS to bold and forgot to press enter. I program 3D models of platonic solids a lot using quaternions and Euclidian space, and digital signal processing of sine waves with specific resonances, so I am interpreting angles and degrees from that training: I didn't compare GAIA to a plant i compared the 106'5 degrees "harmonic ratio" to the harmonic ratio of Phi, a mathematical constant used by plants. My point is: harmonic resonance is a function of time. it's tweakable using time differences. $\endgroup$ – DeltaEnfieldWaid Apr 8 at 13:24

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