James Webb Space Telescope seen from Earth after Sunshield Deployment

The JWST (James Webb Space Telescope) was observed live using a telescope from the desert in Chile, a few hours after it extended its sunshield while on its way to orbit L2 to take first images.


The JWST (James Webb Space Telescope) was observed live using a telescope from the desert in Chile, a few hours after it extended its sunshield while on its way to orbit L2 to take its first images.

Originally the plan was to measure the change in brightness as both sides of the sunshield were deployed, but unfortunately this step occurred during local daytime.

Q&A: ► Can I observe James Webb? You can observe it with a telescope, there’s a link below get James Webb’s position for your location, but only if you manage to collect enough light. JWST is fairly difficult to detect at above magnitude 15.3 and requires a large aperture or a long exposure time with a camera. It is, now at L2, dimmer than Pluto is on average.

► Why is the top right part of the image darker in the timelapse? The telescope’s pointing was adjusted between exposures. The images before and after the adjustment were aligned and “stretched” (assigning pixel brightness to pure number values to create a viewable image) separately, then overlayed only before uploading.

► What did you take this with? A 0.6m telescope (CDK600, 24" Aperture, 4100mm focal length), V-Band Filter, and full-frame CMOS camera (QHY600M Pro).

► What are the moving dots? Why do they move? The dots are a mix of “hot pixels”, defective pixels found on every camera sensor, and readout noise. These are usually calibrated out in post processing which is why they don’t show up on most telescope images, however as you are seeing the unaltered raw data, both are still present in this timelapse. The hot pixels of course stay in the same physical position on the sensor. They’re only shifted in the video as the individual frames are aligned. The only reason the images actually move at all is due the main telescope accidentally using the incorrect pointing model of a mounted refractor (on the left in the thumbnail).

► Why is it a dot? Can’t we see any details? No, James Webb is too far away to make out details. As the sunshield reflects sunlight back to Earth, it’s practically a point source of light and thus looks similar to background stars.

► How far away was James Webb in this video? At the time of recording, the 2nd of January 2022, James Webb was ~780000km from earth according to the NASA page linked below. That’s (roughly) twice the distance of the moon, but it wasn’t close to L2 yet!

► How fast was James Webb moving? At the time of recording, James Webb had a speed of ~0.6km/s (2160km/h or 1340mph) according to the NASA page linked below. With an exposure time of 25s and almost asynchronous image download from the camera, James Webb moved ~16km (10mi) every frame, or 384km (239mi) per second of video!

► From Petr Mareš: What if Hubble looked at Webb? Would it be able to see a high quality image of the shield?

James Webb Sunshield Diameter: 21m James Webb Distance from Hubble: ~1500000000m (Hubble is actually only ~600000m from Earth!)

arctan(21m/1500000000m)*3600 = 0.0029″ (arcseconds) =~ 3 milliarcseconds

Hubble’s resolution is 0.05″/px, so the entire diameter of James Webb’s sunshield wouldn’t even cover 6% of a single pixel. To get a “high quality” image (let’s say 100 pixels across) James Webb’s sunshield would have to be 37km large

Leon Bewersdorff
Leon Bewersdorff
Computer Science Student at RWTH Aachen University, Engineering/Operations at OurSky