Improving from Hubble and Spitzer Space Telescopes to JWST

Improving from Hubble and Spitzer Space Telescopes to JWST

Hubble Space Telescope (HST) has given us much information that is in use in HST’s successor, the James Webb Space Telescope (JWST). Like the now retired Spitzer space telescope, JWST operates mostly in infrared. Here we review improvements from HST to JWST and compare images from HST and Spitzer with those from JWST.

Dr. MMM of AstroPicionary has taught astronomy to 10,000+ students over 15+ years at a USA Carnegie Level R1 University. Dr. MMM continues to teach astronomy and physics. She authors textbooks in astronomy that are in use worldwide. Here, Dr. MMM discusses JWST images compared to HST images.

Lessons To Learn for JWST

Hubble Space Telescope (a.k.a., Hubble) has given us much science that we can use in the design of its successor, JWST. Hubble primarily observes in the visual (V) band of the electromagnetic (EM) spectrum, with some capabilities of observing in infrared (IR) and ultraviolet (UV). The YouTube channel AstroPictionary reviews astronomy vocabulary words like the one below on the EM spectrum.

AstroPictionary YouTube Channel on Astronomy vocabulary words

JWST primarily observes in infrared. Let’s look at lessons to learn from HST for JWST. These lessons to learn include wavelength, size, orbit, lookback distance,

Lesson To Learn – Wavelength

One lesson to learn for James Webb design is that the further we look back in distance and time (a.k.a., spacetime), the more we need to observe in infrared. The further we look back in spacetime, Hubble flow causes recession of space; thus, far away objects from Earth are brighter in infrared than visible. Therefore, JWST needs to search primarily in IR! That is exactly what the James Webb telescope is doing — observing in IR.

Image courtesy of NASA, ESA/JPL-Caltech/B. Mobasher (STScI/ESA) under Public Domain

The above, top right image shows Hubble’s image of a distant galaxy (HUDF-JD2) in the Hubble ultra deep field. The bottom right image shows the same field of view (FOV), but in infrared. A new galaxy appears in the Spitzer image (bottom right) that is not in the V band image of Hubble (upper right). A NIR image from Hubble (bottom left) also finds the new galaxy and combining Hubble and Spitzer images generates the upper left image. The reason why this galaxy appears in infrared and not in V band images is due to molecular and atomic hydrogen gas. Hydrogen gas absorbs V band light. Thus, with more hydrogen gas between a celestial object and the telescope, the less a V band image shows.

This lesson to learn is that JWST needs to primarily observe in IR to see very distant objects. JWST operates primarily in IR, with some capabilities in V band, namely in red and orange.

HST Lesson Learn – Size

As telescopes are cylindrical, size refers to diameter of a telescope. Size relates to two powers of telescopes – light gathering power and resolving power. We look at each of these in a comparison between HST and JWST.

Light-Gathering Power

The above Hubble ultra deep field and Spitzer images show a new discovery of a faint, distant galaxy. This discovery is due to size of the primary mirror, which is large for space telescopes at the time of deployment. The HST primary mirror is about 2.4 m in diameter. The Spitzer space telescope primary mirror is only 0.85 m in comparison. As the object is bright in IR and non-existent in V band, a primary mirror of at least 0.85 m diameter is a need for discovering previously unknown objects in IR. This lesson to learn on size is that for JWST to discover a higher number of previously unknown faint and distant objects, a larger primary mirror is a need.

The sketch below compares the diameter of the HST primary mirror with the JWST primary mirror. As the sketch shows, both primary mirror diameters are large in comparison with the size of a human.

Sketch courtesy of Bobarino under CC BY-SA 3.0

Light gathering power of a telescope relates to the diameter of a telescope. The larger the diameter, the more light a telescope can gather. As the JWST mirror is (6.5 m / 2.4 m) = 2.7 times larger than the HST mirror, the JWST telescope gathers 2.7 times more light each second of observing. More light is needed to discovery previously unknown distant and faint objects.

Resolving Power

Ability of a telescope to discern between two objects that look nearby to each other in space is resolving power. The figure below from textbook Astronomy Essentials shows a telescope in space around Earth observing two distinct celestial objects.

Figure from Astronomy Essentials, Volume 1, textbook by Michele M. Montgomery, PhD, ISBN 978-1-77330-947-7, with image
courtesy of Two Micron All Sky Survey under the Public Domain, Image courtesy of NASA/Apollo 17 crew; taken by either Harrison Schmitt or Ron Evans under the Public Domain, and Image courtesy of Clkr-Free-Vector-Images under Pixabay License, combined and modified by author and used with permission

For objects to be distinct from each other, or resolved, angle θ needs to greater than a minimum value. For a NIR wavelength of 1 micron (or 1×10-6 m), the 2.4 m mirror of HST has a minimum angular resolution of θ = 0.1 arc seconds.

As shown in the sketch above, the further a celestial object is, the smaller the angle θ. In addition, the more difficult for a telescope to resolve celestial objects at a particular wavelength. This lesson to learn on size is that a larger diameter telescope is a need to improve minimum angular resolution.

With a 6.5 m diameter primary mirror, JWST has a minimum angular resolution better than 0.1 arc seconds at a 1 micron NIR wavelength. In comparison with HST, the primary mirror of JWST has 2.7 times better resolving power than HST at the same NIR wavelength. Bottom line is that JWST has equal to or better resolution than HST, and JWST obtains equal to or clearer images than HST.

The image below compares lookback distances of various different telescopes. Before Hubble, ground based telescopes could see to a redshift parameter of z ~ 0.1. HST gets our lookback distances to a redshift parameter of z ~ 10, which is about 96% to a time of the Big Bang. JWST should be able to see infant-aged stars and galaxies.

Graphic courtesy of ESA under CC BY-SA 4.0

Did JWST Improve Over HST and Spitzer?

The image below compares similar FOVs of JWST (left) and HST (right). The comparison successfully shows the crisper image of JWST compared to HST due to improved resolving power. Also successfully shown in the comparison is the ability for JWST to find more objects that are fainter in the HST image due to improved light gathering power. What took HST more than ten days to obtain in the right image, JWST obtains in about 1/2 day.

JWST image (left) courtesy of NASA, ESA, CSA, and STScI under Public Domain and HST image (right) courtesy of NASA, ESA, CSA, and STScI under Public Domain

The image below compares a Spitzer space telescope image (left) with a James Webb space telescope image (right). Both are NIR images: The Spitzer image is at 8 microns, and the JWST image is at 7.7 microns. Both are looking into the Large Magellanic Cloud. As the images show, JWST provides are crisper image.

Image courtesy of NASA/ESA/CSA/STScI under Public Domain

Comparing Locations of HST and JWST in Space

JWST is primarily an infrared telescope like the now retired Spitzer space telescope. Infrared telescopes need to operate at cold temperatures. Space is a good place to locate infrared telescopes.

A significant heat source that negatively affects infrared telescopes in space around Earth is the Sun. Like Spitzer, JWST has a sun shield to protect the telescope and instruments from solar heating. In addition, the JWST sun shield blocks light from the Moon and Earth, keeping the telescope and instruments colder.

The sketch below shows the orbit of HST around Earth. As we see in the image, light from Earth is difficult to shield due to close proximity of HST with Earth. With the shield pointing in the direction of the Sun, not always will light from Earth be shielded.

Sketch of HST orbit courtesy of NASA under Public Domain

Not in the above image is the Moon in orbit around the Earth. Earth’s Moon is another source of heat to HST that can’t always be shielded.

To eliminate heating from Earth and Moon, JWST orbits around Earth’s Lagrange point #2 (i.e., L2). JWST does not orbit Earth. While revolving around L2, JWST also orbits around the Sun along with Earth. The animation below shows this orbit of JWST near L2 and in its simultaneous orbit with Earth around the Sun.

Bottom Line

The bottom line is that JWST is a good improvement over HST and Spitzer. In the early days of JWST, images are clearer, more objects are seen. Both angular resolution and light gather power are equal to or better with JWST than with HST or Spitzer.

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About the author

Michele M. Montgomery earned a B.S. Degree in Nuclear/Mechanical Engineering from the Pennsylvania State University, an M.S. Degree in Physics from The University of Alabama with a concentration in Solar Physics, and a Ph.D Degree in Physics from Florida Institute of Technology with a concentration in close binary star systems. She joined the faculty at The University of Central Florida Physics Department in 2004 where she regularly taught astronomy, astrophysics, and cosmology. In 2006, she noticed that a large, urban college nearby to UCF did not teach astronomy at one of their largest campuses. She began teaching astronomy at this East Campus of Valencia College, a college that has more than 60,000 students; she still teaches four courses of astronomy each fall, spring, and summer semesters. The astronomy program atValencia College East has grown significantly with several more faculty added who teach astronomy.

By 2019, Dr. Montgomery has taught astronomy to more than 10,000 college and university students, both online and face-to-face. Many of her students have gone on to take her astrobiology, astrophysics, and space physics courses. 

By 2016, Dr. Montgomery had co-authored several astronomy texts and quiz/exam banks. Her work appears in several domestic and international astronomy text books (e.g., Horizons by Cengage, Universe by Cengage, Foundations of Astronomy by Cengage) that are used both at the higher education as well as at the high school levels. Starting in Fall 2019, Dr. Montgomery switched gears to authoring digital textbooks and research full time, while still teaching 12 courses of astronomy and up to eight conceptual, algebra, and/or calculus-based physics courses each year. Her research interests are numerical simulations using Smoothed Particle Hydrodynamics of close binary star systems. She also regularly is granted telescope time on the NASA's Kepler space telescope for observing eclipsing binary star systems. She has also observed using Gemini South, Keck, and Kitt Peak ground-based telescopes. Her major teaching areas are Astronomy, Astrobiology, Astrophysics, Cosmology, Space Weather/Space Physics.