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collinsmark
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hutchphd said:I understand 2 and 3 and agree. But for 1 you are saying the sun-earth-L2 line is rotating nce a year but the Webb primary mirror axis is fixed on a star. Is the heat mirror axis rigidly fixed wrt the primary axis? I guess it must be?
Yes, the heat shield is rigidly fixed to the primary mirror. If JWST changes the orientation of the primary mirror, the orientation of the heat shield changes along with it. That means its heat shield will slowly rotate with respect to the sun, while JWST is pointing at a give star. Ignoring JWST's orbit around L2 considerations, this rate is:
[tex] \mathrm{rotation \ rate} = \frac{\left( 360 \mathrm{\frac{deg}{year}}\right) \left( 60 \mathrm{\frac{arcmin}{deg}} \right) \left(60 \mathrm{ \frac{arcsec}{arcmin}} \right) } {\left( 365.4 \mathrm{\frac{days}{year}} \right) \left( 24 \mathrm{\frac{hours}{day}}\right) \left( 60 \mathrm{\frac{minutes}{hour} }\right) } = 2.463054 \left[ \mathrm{\frac{arcsec}{minute}} \right] [/tex]
It goes without saying, that JWST can only point in a limited area of sky for any particular time of the year, because it must keep its heat shield in the general direction of the Sun. Once pointed at a star, JWST can't stay pointed at that star forever, since the solar illumination angle keeps changing at the above rate, and eventually, if JWST doesn't change to a new star, the heat shield's direction won't be far enough away from the Sun's direction.
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Trying to do a little bit better eyballing/ballparking, JWST moved 3.5' dec N, and 10' RA W, in ecliptic coordinates. JWST's midpoint declination was right around -12 deg 55 arcmin declination at the time (this midpoint declination figure might be important for later).
So of JWST's 10' RA motion (ecliptic coordinates),
[tex] \frac{(5 \ \mathrm{hours} ) \left( 60 \mathrm{\frac{minutes}{hour}} \right) \left( 2.463054 \mathrm{\frac{arcsec}{minute}} \right)}{\left(60 \mathrm{\frac{arcsec}{arcmin}} \right) } = 12.31527 \ \mathrm{arcmin}[/tex]
12.3' can be attributed to L2's movement (ballpark).
So yeah, 12.3' and 10' are roughly in the same ballpark. So the "sanity check" checks out. (But there's still more things to consider than that though.)
[Edit: the direction is surprising though. Hypothetically, if JWST was smack at L2, I would expect its apparent motion (with respect to the background stars) to be East in ecliptic coordinates. But here it was moving West. Could the discrepancy be due to its orbit around L2?]
[Another edit: I think it's starting to make sense. JWST must presently be in the southern part of its L2 orbit, yet past the most southern part. So it's heading north and west within it's orbit, as viewed from Earth, in ecliptic coordinates. And this westerly motion in its orbit dominates its eastward apparent motion caused by Earh's/L2's motion around the Sun. In other words, JWST's orbit around the L2 point is the dominant source of JWST's apparent motion. Yes, it has a component of eastward apparent motion caused by Earth's/L2's motion, but JWST's orbit around L2 dominates this. (The surface of the Earth's movement might figure into this a little too. I haven't figured that into things yet...)]
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