The James Webb Space Telescope Has Emerged From the Freezer (arstechnica.com)

The James Webb Space Telescope has emerged from a large vacuum chamber that was home to temperatures of just 20 degrees Celsius above absolute zero. Scientists have reviewed the data and given the instrument a clean bill of health. "We now have verified that NASA and its partners have an outstanding telescope and set of science instruments," said Bill Ochs, the Webb telescope project manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We are marching toward launch." Ars Technica reports: The $10 billion telescope underwent tests inside Chamber A at Johnson Space Center, which was built in 1965 to conduct thermal-vacuum testing on the Apollo command and service modules. Beginning in mid-July, after the telescope was cooled down to a temperature range of 20 to 40 Kelvin, engineers tested the alignment of Webb's 18 primary mirror segments to ensure they would act as a single, 6.5-meter telescope. (They did). Later, they assessed the fine guidance system of the telescope by simulating the light of a distant star. The Webb telescope was able to detect the light, and all of the optical systems were able to process it. Then, the telescope was able to track the "star" and its movement, giving scientists confidence that the Webb instrument will work once in space. Webb still has a ways to go before it launches. Now that project scientists know that the optic portion of the instrument can withstand the vacuum of space, and the low temperatures at the Earth-Sun L2 point it will orbit in deep space, they must perform additional testing before a probable launch next year.

Comments

Re:L2 is dark

[Anonymous Coward]

L2 is quite dark because most sunlight is blocked. But in fact, there is still direct sunlight at the exact L2 point.

This is because L2 is only in the Earth’s penumbra, not its umbra. Put another way, viewed from L2 the Earth only eclipses a large central region of the Sun. The outer parts of the Sun’s disc are still directly viewable, forming a hollow disc (donut) of light around the Earth.

In order to receive no direct light from the Sun it would need to substatially closer to the Earth so that it is within the umbra (forming a cone volume shape).

If Earth were a larger planet then the L2 point would be darker with no direct sunlight.
Of course, indirect light via the Earth’s atmosphere or moonlight would still partially illuminate L2.