Webb Takes a Selfie

NASA recently released some of the very first engineering images from the Webb telescope. We’re not quite at the point where Webb is producing amazingly detailed and colorful images of the cosmos—those will be coming soon!! These are of course calibration images, designed to give Webb’s engineering teams the necessary information to bring the telescope’s segmented mirrors into alignment and help bring Webb’s many instruments online. The “primary mirror selfie” image featured at the top of this post was a delightful surprise to many. We expected to see an image or images showing a single star as viewed by Webb’s 18 individual mirrors and that those images would be used for the overall mirror alignment strategy, but the selfie image was a pleasant surprise to many of us who are not a part of Webb’s engineering teams. I was immediately curious about how this image was made and exactly what instrument provided it. The press release states that there is a specially designed pupil imaging lens (PIL) in one of Webb’s main imaging instruments known as NIRCam. What does that mean exactly? Well, let’s dig in! 

First, what is a PIL anyway? Let’s start with a pupil which is the physical aperture (or opening) which allows light into an optical system. In such a system, a PIL then is a specially designed lens whose sole purpose is to provide a clear image of that aperture allowing you to see where light enters the system. It turns out that the engineers who built Webb’s NIRCam instrument eventually tacked on a requirement for a pupil imaging lens in their design specs so that engineers could periodically check Webb’s mirror alignment optically during ground testing, commissioning and throughout the mission. A little internet sleuthing brought me to a 2005 paper published in the proceedings of the Society of Photo-optical Instrumentation Engineers (SPIE) which goes into great detail about the design and fabrication of this lens. I’ve grabbed two of the figures from the paper and reproduced them below.

The PIL is a very cleverly designed element of NIRCam using a combination of weights and magnets to swing the lens into and out of the light path of the instrument. It also includes a fail-safe spring that is designed to ensure that the lens can never become permanently stuck in the light path which would render the rest of the instrument essentially useless! 

Now let’s take a closer look at the image produced by the PIL. There are a number of interesting elements to this image that become clearer upon closer inspection. Certainly, the most glaring feature is that bright white mirror segment outshining its seventeen friends. This is due to the fact that the telescope was pointed at its alignment star; a very bright star known as HD 84406 near the Big Dipper. In this case, that particular mirror segment and no other is sending light from that star through NIRCam’s optical system, but the PIL is more interested in seeing the mirror, not the star, so that light is defocused into the shape of the mirror itself. There’s enough additional ambient light and reflections to see the other mirrors, and even the support struts of the secondary mirror show through in the vertical and diagonal lines running through the center of the mirror array. There are also a few internal reflections from NIRCam that appear as glints of light near the center and corners of the image. This image gives the engineers a good starting point from which to begin the process of aligning the mirrors.

Speaking of the mirror alignment, the press release that gave us this selfie image also gave us Webb’s first look at HD 84406 as imaged by NIRCam. The observing plan for this first step of the mirror alignment involved imaging a large area of the sky around the target star, roughly equivalent to the size of the full moon. The large field was chosen because engineers did not know the status of the mirror alignment after the rigors of launch and mirror deployments. It was possible that things could have been thrown way out of alignment and each mirror would be sending its own image across a wide field of view. As it turns out, the mirror segments maintained a remarkable alignment given the stresses of launch, and the team found all 18 images of the star clustered near the center of the search region in the initial alignment mosaic image seen below. 

Once the 18 images were found, the team then went to work identifying which mirror provided which image and then aligned the mirrors so that they produced an image aligned in Webb’s familiar hexagon shape.

The team then fine-tuned each mirror and brought all 18 images together into one stacked image as seen in the figure below. These alignments will continue to be refined in the coming months as the observatory cools to its operating temperatures and all of its instruments come online.  

The team that built the PIL met a very challenging set of requirements in building this lens and like everything we’ve seen with Webb so far, it has proven to be a resounding success! This is yet another testament to the dedication, resourcefulness, and creativity of the many engineers and scientists who have worked on Webb over the years. All of that hard work is finally paying off and we cannot wait to see what secrets of the universe Webb will reveal! 

For all the latest news regarding Webb and its commissioning, be sure to follow the official NASA Webb Blog.