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Comment by lamontcg | Hacker News Reader

What is the exposure difference between the Hubble and JWST images?

I couldn't find an exact exposure time for the Hubble image, the press release by the ESA has this quote though: "This deep field, taken by Webb’s Near-Infrared Camera (NIRCam), is a composite made from images at different wavelengths, totaling 12.5 hours – achieving depths at infrared wavelengths beyond the Hubble Space Telescope’s deepest fields, which took weeks." [1] There is also another comment further down this thread stating Hubble was 140 hours. [2]

[1] https://www.esa.int/ESA_Multimedia/Images/2022/07/Webb_s_fir... [2] https://news.ycombinator.com/item?id=32063214

Those exposure times (weeks / 140 hours) are for these images [0,1], Hubble's deep fields. Hubble's photo of this galaxy cluster, the one our root comment shows superimposed over JWST's, took 5 Hubble orbits [2]. I think that's around 2-3 hours of exposure time.

[0] https://en.wikipedia.org/wiki/Hubble_Deep_Field

[1] https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field

[2] https://archive.stsci.edu/prepds/relics/ ("For each cluster, the team observed to 5-orbit depth with ACS and WFC3/IR")

(If you want to verify [2] is talking about the same photo, you can retrieve it from the "SMACS J0723.3-7327" row, from the "Color Images" column/field).

LEO should be >= 80 mins so 5 hubble orbits should be >= 400 mins / 6.6 hrs?

EDIT: nasa.gov says 95 mins, so ~8 hrs.

Yes, but Earth obstructs the field of view for about half that time. The way HST refers to an "orbit" for scheduling, only part of the elapsed time is usable observational time, for a single target.

https://hst-docs.stsci.edu/hsp/the-hubble-space-telescope-pr...

- "HST GO observing time is counted in terms of orbits. Each 96-minute orbit contains a certain amount of useful time when the target can be observed, called the orbital visibility period..."

Ah right, because it is basically in LEO so the Earth is enormous.

> This deep field, taken by Webb’s Near-Infrared Camera (NIRCam), is a composite made from images at different wavelengths, totaling 12.5 hours – achieving depths at infrared wavelengths beyond the Hubble Space Telescope’s deepest fields, which took weeks.

12.5 hours total exposure for the JWST image, "weeks" for the HST image

Is the actual comparison HST image being used here from the hubble deep field?

EDIT: Doesn't look like it is -- it is from a more recent 2019-published study of SMACS J0723.3-732 as part of the Reionization Lensing Cluster Survey (RELICS).

Some background info in this paper:

https://arxiv.org/pdf/2207.05007.pdf

Although that paper does mention that this image is the deepest image of the Universe to date, and that the Fine Guidance Sensor image may be the second deepest, both exceeding the Hubble Deep Field image.

I think it was around 20 days for the Hubble and around 12 hours for the JWT

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