20220109, 15:05  #78 
If I May
"Chris Halsall"
Sep 2002
Barbados
2^{2}·2,719 Posts 

20220109, 15:32  #79  
Feb 2017
Nowhere
2×43×71 Posts 
Quote:


20220109, 16:13  #80 
6809 > 6502
"""""""""""""""""""
Aug 2003
101ร103 Posts
10791_{10} Posts 
It would still be subpixel size for any detector working in the visible and would be too faint for the Mark I eyeball.

20220109, 16:41  #81  
Feb 2017
Nowhere
13732_{8} Posts 
Quote:
Oh, no! This means the James Webb Space Telescope will increase Global Warming! ;) Last fiddled with by Dr Sardonicus on 20220109 at 16:41 Reason: fignix posty 

20220109, 18:17  #82 
"TF79LL86GIMPS96gpu17"
Mar 2017
US midwest
7,103 Posts 
Let's try to run some numbers on that.
When I looked recently at https://jwst.nasa.gov/content/webbLa...ereIsWebb.html JWST was ~686170 miles from earth (~1104300 km). sunshield length ~21.2 m https://jwst.nasa.gov/content/observ...sunshield.html max width 14.16 m, about 2/3 length. Since it is roughly a slender hexagon, not rectangular, the largest diagonal is close to the length. angle subtended by length = 21.2 / 1104300 / 1000 = 1.92e8 radians = 0.192 microradians = 192 nanoradians = 0.0396 arc sec. That's well below the ~0.4 arc sec diffraction limit of a 12" optical telescope. Diffraction limit at 550nm/300mm = 1.83 microradians = 1830 nanoradians. https://telescopeoptics.net/telescope_resolution.htm At a 2m focal length of an amateur's 8" CST, the theoretical image size of the JWST is at most (when oriented sunshade length perpendicular to viewing direction) 384 nanometers at 1x (pinhole camera). CCD image sensor pixels are typically at 26 microns (20006000 nanometers) pitch. Suppose the telescope is used with a 20mm eyepiece for a 100x magnification, and therefore has a field of view of 1/4 degree, 15 arc minutes, 900 arc seconds, ~4500 microradians. https://telescopicwatch.com/fieldof...omycalculate/ A 4144wide CCD would be resolving ~1.09 microradians ~1086 nanoradians. JWST might land entirely within one pixel or straddle up to 4 partially, in the best case of sunshield perpendicular to the line of sight. Zooming up to 300x, the diffraction limit of a 300mm/12" aperture scope per CCD pixel won't change that much; a pixel at 300x, 1/12 degree field of view is imaging ~362 nanoradians of sky, still wider than the JWST length. (Unless I've fubared the math along the way in a fatal fashion.) It gets somewhat more difficult as JWST moves further from earth and nearer L2, ~1,500,000 km away. Using 30GHz = radar interferometric imaging from multiple locations around the earth (6378 * 2 km diameter) wavelength = 3e8 m/sec /(3e10 waves/sec) = 0.01 m wavelenth. Diffraction limit = 0.01 m / 6378000 /2 m ~0.8 nanoradians, much better. I have no idea how much radar transmitter power it would take to image JWST or what effect on the craft it would have. Counting the JWST sun shade as 21.2x14.16 x ~0.6 m2 area and assuming 99% reflectivity, 100% capture by earth of a reflected beam, of (93/94)^2 intensity due to JWST's greater distance from the sun, & compared to earth's pi/4 * 6378000^2 m^2 apparent area, reflected power ratio ~ 21.2 * 14.16 * 0.6 * 0.99 * 1.0 * (93/94)^2 / ( pi/4 * 6378000^2 ) = 174.5 / 31,949,120,782,657.8 = 5.5E12 of normal solar power directed at earth. Now if you could focus that roughly 1KW/m^2 of sunshield area remaining after atmospheric filtering (~180KW) into a small spot, well, a small rodent or small bird would disappear in a puff of smoke or at least get singed as they passed through the focal spot due to the earth's rotation and their own motion (a well known issue with focusing solar power generation), or you could melt most metals in it if you prevented the roughly sonic wind from chasing the spot from cooling the crucible too much. It's a rather costly and infinitesimal start on a Dyson Sphere. Last fiddled with by kriesel on 20220109 at 18:24 
20220109, 19:20  #83  
Bamboozled!
"๐บ๐๐ท๐ท๐ญ"
May 2003
Down not across
2·3·1,931 Posts 
Quote:
Given that Gaia, which is much smaller and less reflective than JWST, is visible from L2 with my kit, I look forward to imaging JWST when the observatory is working again. Gaia is ~mag 2022 which takes 30180 minutes for a decent SNR; JWST should be 1618, or 40 times brighter. Ten magnitudes way too faint for nakedeye detection (a factor of 10,000) in general but a specular reflection from the entire sun shield is not "in general". 

20220109, 20:41  #84 
"Vincent"
Apr 2010
Over the rainbow
3^{2}×11×29 Posts 
Lego has a JWST st apparently or someone claim.. anyway
https://twitter.com/LEGO_JWST/status...87469361479681 
20220124, 19:41  #85 
"Vincent"
Apr 2010
Over the rainbow
3^{2}·11·29 Posts 
JWST arrived home
https://blogs.nasa.gov/webb/2022/01/...arrivesatl2/ 
20220124, 22:29  #86  
Feb 2017
Nowhere
2×43×71 Posts 
Quote:


20220125, 16:23  #87  
"Marv"
May 2009
near the Tannhรคuser Gate
3^{2}·89 Posts 
Quote:
Cold, indeed. Its final operating temp on the cold side will be about 40 degrees Kelvin !!! It's around 62 K now. Since heat is light, the colder it is the less interfering light there is. 

20220126, 01:18  #88 
"TF79LL86GIMPS96gpu17"
Mar 2017
US midwest
7,103 Posts 
Radiated heat is photons; convected heat is enthalpy of moving matter; conducted heat is phonons and electrons and maybe more. Space near L2 is not quite a perfect vacuum, even without the discharge of stationkeeping thrusters of JWST. The energy conveyed by solar wind to L2 orbit is probably usually very low density, but not zero.
http://www.dept.aoe.vt.edu/~cdhall/c...nvironment.pdf 
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