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Jupiter is magnitude −2.94 to −1.66 and Sirius is −1.46 and Venus stays between -3.8 and -4.8, while Canopus is only −0.74. (more negative is brighter)

Question: Other than their namesake, why do Canopus Star Trackers track Canopus? Why not Sirius, or a bright planet? Planets have ephemerides and spacecraft have computers, so what were all the attractive aspects of choosing Canopus above all other possibilities?


Related: How did the Canopus Star Tracker work? Are there any still out there in service today? (they are still in use today!)

From Hackaday's MARINER 4: OUR FIRST UP-CLOSE LOOK AT MARS which links to Mariner Mars 1971 attitude control subsystem TM 33-681:

Canopus star tracker from Hackaday's "MARINER 4: OUR FIRST UP-CLOSE LOOK AT MARS"(https://hackaday.com/2021/06/03/mariner-4-our-first-up-close-look-at-mars/

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    $\begingroup$ Shirley you can't be Sirius. $\endgroup$
    – DrSheldon
    Jun 4 at 3:20
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    $\begingroup$ Wikipedia seems to have a justification. There is a citation. But i didn't visit it to confirm. $\endgroup$
    – AJN
    Jun 4 at 7:46
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    $\begingroup$ Heh, given the slightly awkward phrasing of the title's second question (the back-referencing "so" doesn't really agree with phrasing of the first sentence) were you trying to get in a punning play on Joker's "Why so serious?" line? Also, "Sirius or some other bright planet"? Putting aside that it would be the primary if it were orbiting nearby, Sirius may be close in stellar distance terms, but it's not that close. ;) $\endgroup$
    – Jacob C.
    Jun 4 at 19:00
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    $\begingroup$ @JacobC. I've updated the body of the question, yes "other" was supposed to be something like "other bright object such as a planet" but got shortened imperfectly. Thanks! I'm too attached to the title now though, which is hardly a tennis racket :-) (R.I.P. Heath Ledger) $\endgroup$
    – uhoh
    Jun 4 at 19:17
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    $\begingroup$ @ DrSheldon; I am serious, and don't call me Shirley. $\endgroup$
    – JohnHunt
    Jun 4 at 19:36
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As someone who has designed a Canopus tracker, it's really very simple. The Sun is generally the easiest thing for a spacecraft to orient to, regardless of orbit or location in the Solar System. So, once you've found the Sun, orient your spacecraft accordingly. You usually have solar panels that you want oriented toward the Sun, radiators oriented away. Maybe other such constraints, too. But you may also want to know your roll angle around the spacecraft-Sun axis.

For a spacecraft near the ecliptic (nearly all of them), the handiest thing to use is a bright star at high ecliptic latitude. If it's at low ecliptic latitude, like Sirius, its angle to the axis varies a lot, but an object at the ecliptic pole is always ~90 degrees from the Sun. This makes it reasonably easy to protect the sensor from being blinded by the Sun, and it simplifies the search: the spacecraft can scan a narrow strip on the sky to find the object. Canopus is the ideal object1.

Note that the issue isn't damage to the sensor: it's not hard to design a sensor that won't be damaged by direct Sun exposure unless it's on the Parker Solar Probe. High sensitivity isn't required. But, of course, if it's swamped by sunlight it won't detect the star.


1As noted in comments, Canopus is at -76° ecliptic latitude while Sirius is at -39° putting them at 14° vs 51° away from the poles of (perpendicular to) the ecliptic.

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    $\begingroup$ What is the actual ecliptic latitude of Canopus? $\endgroup$ Jun 4 at 21:22
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    $\begingroup$ OK, it seems to be -76° for Canopus (and -39° for Sirius) $\endgroup$ Jun 4 at 21:26
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    $\begingroup$ @PeterMortensen And when you think in terms of distance from the pole, it's 14° versus 51°. $\endgroup$
    – John Doty
    Jun 4 at 21:45
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    $\begingroup$ I've added that back in to your answer because I think it's really the smallness of that 14° half-angle cone that a Sun sensor equipped spacecraft needs to view that answers the question Why not so Sirius? $\endgroup$
    – uhoh
    Jun 5 at 0:46
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This answer assumes that the spacecraft is in an orbit which is mostly aligned to the ecliptic plane.

A target, whose orbit is also close to the ecliptic plane, can occasionally go in front of the Sun or behind it. In both cases, the star tracker camera should not point towards the Sun, even by accident.

It may damage the sensor.

Planets orbit the Sun and have complicated apparent movements. Compared to that, stars can be assumed to be inertially fixed. Computation becomes much easier. No ephemeris needs to be stored.

Wikipedia

Canopus's brightness and location well off the ecliptic make it useful for space navigation. Many spacecraft carry a special camera known as a "Canopus Star Tracker" plus a Sun sensor for attitude determination.

Canopus seems to have a significant declination (latitude, not declination; see the other answer) (approximately 52°) which makes it well separated (angular) from the Sun. So it is safer for the tracker camera.

It is the second brightest star in the night sky. Sirius is the brightest.


As pointed out by asdfex, Canopus is farther away from the solar system compared to Sirius. 310 light years versus 8.6 light years. This means that the apparent angular position change (parallax) for different points on the spacecraft orbit will be smaller for Canopus.

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    $\begingroup$ Sirius declination seems to be 16° approx from Wikipedia. It has to be compared to the apparent angular size of the sun for the respective spacecraft orbit, and, the difference then further compared to the sensor FOV to check if adequate safe angular gap would have been present. Moreover, in the Wikipedia entry, the sun and canopus sensors are used to derive the attitude information. For that purpose it is better that the angular distance between the two objects be as close to 90 deg as possible. The attitude estimate will otherwise be more sensitive to measurement errors. $\endgroup$
    – AJN
    Jun 4 at 8:32
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    $\begingroup$ You might want to add the significant parallax as another argument against Sirius. 360 arcseconds vs. 10. $\endgroup$
    – asdfex
    Jun 4 at 8:34
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    $\begingroup$ @asdfex i didn't check the distance of Sirius and canopus from the solar system. But, now that you mention it, it may be significant. Canopus seems to be 310 lightyear away while Sirius is only 8 lightyear away. $\endgroup$
    – AJN
    Jun 4 at 8:36
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    $\begingroup$ The ecliptic latitude of Canopus seems to be -76° (and -39° for Sirius) $\endgroup$ Jun 4 at 21:29
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    $\begingroup$ It is the ecliptic latitude that matters, not the declination. $\endgroup$ Jun 4 at 21:30

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