NASA’s James Webb Space Telescope. Credit: NASA Goddard Space Flight Center and Northrup Grumman
After meeting the major milestone of aligning the telescope to NIRCam, the Webb team is starting to extend the telescope alignment to the guider (the Fine Guidance Sensor, or FGS) and the other three science instruments. This six-week-long process is called multi-instrument multi-field (MIMF) alignment.
When a ground-based telescope switches between cameras, sometimes the instrument is physically taken off the telescope, and a new one is installed during the daytime when the telescope is not in use. If the other instrument is already on the telescope, mechanisms are in place to move part of the telescope’s optics (known as a pick-off mirror) into the field of view.
On space telescopes like Webb, all the cameras see the sky at the same time; to switch a target from one camera to another, we repoint the telescope to put the target into the field of view of the other instrument.
After MIMF, Webb’s telescope will provide a good focus and sharp images in all the instruments. In addition, we need to precisely know the relative positions of all the fields of view. Over last weekend, we mapped the positions of the three near-infrared instruments relative to the guider and updated their positions in the software that we use to point the telescope. In another instrument milestone, FGS recently achieved “fine guide” mode for the first time, locking onto a guide star using its highest precision level. We have also been taking “dark” images, to measure the baseline detector response when no light reaches them – an important part of the instrument calibration.
Webb’s guider (FGS) and four science instruments (NIRCam, NIRSpec, NIRISS, and MIRI) share the field of view of the Webb telescope optics, but they actually see different parts of the sky at any given observation. Credit: NASA
Webb’s mid-infrared instrument, MIRI, will be the last instrument that is aligned, as it is still waiting for the cryogenic cooler to chill it to its final operating temperature, just under 7 degrees above absolute zeroAbsolute zero is the theoretical lowest temperature on the thermodynamic temperature scale. At this temperature, all atoms of an object are at rest and the object does not emit or absorb energy. The internationally agreed-upon value for this temperature is −273.15 °C (−459.67 °F; 0.00 K).” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>absolute zero. Interspersed within the initial MIMF observations, the two stages of the cooler will be turned on to bring MIRI to its operating temperature. The final stages of MIMF will align the telescope for MIRI.
You might be wondering: If all of the instruments can see the sky at the same time, can we use them simultaneously? The answer is yes! With parallel science exposures, when we point one instrument at a target, we can read out another instrument at the same time. The parallel observations don’t see the same point in the sky, so they provide what is essentially a random sample of the universe. With a lot of parallel data, scientists can determine the statistical properties of the galaxies that are detected. In addition, for programs that want to map a large area, much of the parallel images will overlap, increasing the efficiency of the valuable Webb dataset.
Written by:
- Jonathan Gardner, Webb deputy senior project scientist, NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It's vision is "To discover and expand knowledge for the benefit of humanity."” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>NASA’s Goddard Space Flight Center
- Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard
Source: SciTechDaily
