Difference in field of view between JWST and Euclid

I was watching this video about the Euclid telescope. At the 8 minute mark he compares the field of vide of Euclid with that of the JWST, showing that the Euclid field of view is much larger.

While I understand that the telescopes are built for different purposes and Euclid needs to survey the entire sky, I would like to understand why Euclid have such a larger field of view compared to JWST in the following sense: what are the characteristics of each telescope that determine this big difference? (the two telescopes looks quite similar to me in their design, with Webb being much larger).

If I may add: what prevents us from building a telescope with the same resolution of JWST that can see as far in the past as JWST but with the field of view of Euclid?

• Euclid's VID camera has a 6 x 6 array of 4k x 4k CCDs. Each pixel subtends 0.1 arcsecs. So they cover about 40 arcmin in each direction of their focal plane. JWST NIRCAM long wavelength channel is 2 2040 x 2040 CCDs. Each pixel subtends 0.063 arcsec. So each CCD covers 2.14 minutes. The mirrors of both telescopes provide usable focal planes that are bigger than the areas covered by their CCDS. But no one set of detectors totally fills that area. Commented Jun 13 at 16:38
• @eshaya Thanks, it's a bit clearer. For Euclid: "Each pixel subtends 0.1 arcsecs", for JSWST "Each pixel subtends 0.063 arcsec". I guess my question is: what are the characteristics of the telescopes and detectors that determine this? I guess it's basic question about optics which I is a topic I know very little about. Commented Jun 13 at 20:06
• To calculate arc seconds per pixel , divide the pixel size, in microns, by the focal length, in mm, then multiply by 206.265 Commented Jun 14 at 14:45
• @eshaya Ok, so it is the focal lenght the key quantity? Basically due to the focal length of Euclid, 0.1 arcsecond is 12 microns on the detector. If I increase the focal length, then 0.1 arcseconds will be larger than 12 microns on the detector, and therefore my FOV smaller? So I would need larger focal length and larger detector to have the resolution of JWST and FOV of Euclid. In addition to a huge mirror, as Pela said in his answer, to collect enough light. Am I right? Thanks again Commented Jun 16 at 17:47
• @Erontada Yes, a larger focal length would decrease the angle subtended by each pixel. And if the mirrors are ground and polished well enough, it would result in better resolution. But, then to retain the large field of view, the CCD would need to have more pixels. Commented Jun 18 at 4:20

The exact field of view (FOV) of a telescope is often not a unique number, but depends on the instrument in use. For JWST, the FOV is a typically few arcmin (1/60 of a degree). Here you can see the possible FOVs for all JWST's four instruments, NIRSpec, NIRCam, MIRI, and NIRISS/FGS:

Fields of view of JWST's instruments, measured in arcsec, i.e. 1/3600 of a degree. Credit: JWST User Documentation.

As you say, compared to JWST (indeed, to most other telescopes), Euclid has an enormous FOV. Its two instruments, VIS and NISP, have nearly the same FOVs of 0.75×0.75 square degrees, or 45×45 arcmin2.

A few weeks ago, the first scientific studies from Euclid were released. One of my favorites is this image of the galaxy cluster Abell 2390 and its surroundings. The zoomed-in part shows roughly the FOV that JWST would see:

Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi; CC BY-SA 3.0 IGO or ESA Standard Licence.

Why are they different?

The reason for the wildly different FOVs is that the two telescopes have very different science goals. JWST's main purpose was, originally, to look for the very first galaxies. As these galaxies are expected to be small, but numerous, are large FOV is not needed. What is needed is, in addition to being sensitive to infrared because the light is so redshifted, to catch as many photons as possible, since these galaxies are so extremely faint, being so far away. That is, a huge mirror. The galaxies are faint, but numerous, so one can be certain to have many of them in the FOV, no matter which direction we look.

Another goal, which was added later, is to probe (the atmospheres of) exoplanets, which also doesn't entail a large FOV.

On the other hand, Euclid is built to observe a huge number of galaxies throughout most of the history of the Universe — but not its earliest epochs — in order to probe how the Universe has expanded through time. Another goal is to observe a statistical sample of the most massive galaxies in the Universe. Since galaxies are rarer, the more massive they are, one needs to observe a large area on the sky to get them. In contrast, JWST is unlikely to catch even a single one in one pointing.

Why not both?

So why not just built a Webbclid Telescope that peers as deep as JWST over the area of Euclid?

In astronomy, even bright objects are usually faint enough that you want as many photons as possible. You get this by 1) exposing for a long time, and 2) using as big a mirror as possible. The larger FOV you spread these photons over, the fainter your image will be, and hence a large FOV means that you need 1) to expose for longer, or 2) have a larger mirror.

JWST's FOV is $$\sim20^2$$ times smaller than Euclids, and its mirror is 6.5 m in diameter. For the same exposure time, Webbclid would therefore need a mirror with a diameter of $$\sim 20\times6.5\,\mathrm{m} = 130\,\mathrm{m}$$.

That is

what prevents us from building a telescope with the same resolution of JWST that can see as far in the past as JWST but with the field of view of Euclid.

As an analog, when you use a projector to project your movie on the wall, you can get a larger screen by moving the projector farther from the wall. But this comes at the expense of getting a fainter image.

• Good analogy (or is the inverse of a problem not an analogy?) in the last paragraph :) Commented Jun 13 at 8:54
• Thank you, just to be sure I understood, you say "a large FOV means that you need 1) to expose for longer". So in principle, JWST could increase the field of view of its instrument a little bit by exposing for longer? In other words, the FOV is not fixed or "hardcoded" in how the telescope is built, but it's a combination of the optical characteristics of the tlescope + exposure time? Commented Jun 13 at 14:22
• And maybe my question wasn't clear, what are the physical characteristics of the two telescopes that determine such a big difference? Euclid can have a larger filed of view with a much smaller mirror because it has a lower resolution? Maybe I'll add this to my question Commented Jun 13 at 14:25
• @Erontado In principle yes, but when you increase the FOV, you also _de_crease the resolution. To keep the same resolution, you could, in principle, build a slightly larger detector (CCD), say, 2100² pix instead of 2040² pix. This goes beyond my knowledge, and a more engineer-like scientist or observational astronomer will answer this better, but I think there are some technical reasons that detectors don't come in any pixel size. As for your last question, I'm also not sure I'm the right one to answer; probably eshaya knows more about the optical paths and instrument setups that enable this.
– pela
Commented Jun 13 at 21:31

Here’s why there is a difference in the field of view between the JWST and Euclid telescopes:

Both Euclid and JWST have mirrors that create usable focal planes larger than the areas covered by their respective CCD arrays. However, no single set of detectors (CCDs) completely fills the focal plane area. This means that while the telescopes could theoretically image larger sections of the sky, the actual field of view is limited by the coverage of the detectors.

Focal length is the critical parameter of a telescope in determining the size of its field of view (FOV) and the angle subtended by each mm in the FOV. But, instruments on a telescope may have focal reducers inside them to alter their internal effective focal lengths. A larger focal length reduces the angle subtended by each pixel of a given size (usually several $${\mu}m$$). This means that for a given pixel size, a telescope with a longer focal length will have a higher resolution, being able to resolve finer details, if the mirrors are ground well enough and the atmosphere supports it. However, this also means that each pixel covers a smaller portion of the sky, thus reducing the field of view unless the number of pixels is increased.

If the mirrors are ground and polished to a high standard, the resolution improves with focal length. However, to maintain a large field of view with this increased resolution, the CCD arrays would need to have more pixels. At the largest CCD sizes this requires advancements in detector technology and increases in the data storage and processing capabilities.

JWST and Euclid have different fields of view, each suited to their respective scientific goals.

Euclid’s VIS Camera: 6x6 array of 4k x 4k CCDs, each $$12 {\mu}m$$ pixel covers 0.1 arcseconds, resulting in a 40 arcminute field of view.

JWST’s NIRCam longwave camera: Two 2040x2040 pixel CCDs, each 18 $${\mu}m$$ pixel covers 0.063 arcseconds, resulting in a 2.14 arcminute field of view per CCD after the light passes an f/9 triple lens inside the instrument which reduces the effective focal length of the camera.

The design choices for each telescope reflect different priorities. Euclid is optimized for wide-field surveys, capturing large portions of the sky in each image. JWST, on the other hand, is designed for high-resolution imaging, focusing on detailed observations of smaller areas.

Mechanically, the difference between JWST and Euclid is that JWST has a focal length of 131.4 m, while Euclid has a focal length of 24.5 m. Telescope focal length doesn't directly control field of view, but it does strongly influence it: a Euclid instrument would need 5.3 times as much on-instrument magnification as a JWST instrument to get the same field of view. Such intense magnification would greatly degrade image quality and brightness.

• Thank you. So it is the focal lenght the key quantity? Basically due to the focal length of Euclid, 0.1 arcsecond is 12 microns on the detector. If I increase the focal length, then 0.1 arcseconds will be larger than 12 microns on the detector, and therefore my FOV smaller? So I would need larger focal length and larger detector to have the resolution of JWST and FOV of Euclid. In addition to a huge mirror, as Pela said in his answer, to collect enough light. Am I right? Thanks again Commented Jun 16 at 17:48