# How close to itself can the JWST see? Inside our own system, or must it look beyond like it is far sighted? Essentially, can JWST see Mars or others?

How close to us can we see with JWST, would we be able to look at our neighbor planets and/or their moons (when in the right place logically) or would things that close be out of focus? I tried looking it up, but it seems everyone is (understandably) more into how far away it can see.

• I think there’s a very similar question somewhere, where the answer is calculated that everything farther away than 1.5 km is “infinitely far away”. There has already been images of Jupiter.
– pela
Sep 7, 2022 at 20:08
• @pela $$\frac{1}{f} = \frac{1}{d_1} + \frac{1}{d_2}$$ and a focal length of 0.13 km suggest otherwise; you'd have to move the focal plane by a dozen meters to do that! I think your number applies to 60 mm f/15 refractor in the back yard. If you could move the focal plane back by 10 cm for example you'd be focused at 172 kilometers. So the challenge here is to find out what the range of motion is for JWST's "focus knob". Related in Space SE Has Hubble ever focused on something close enough that it had to move away from being focused at infinity?
– uhoh
Sep 7, 2022 at 20:50
• – uhoh
Sep 7, 2022 at 20:52
• @uhoh Ah, thanks! Good thing I'm not an observer 😅
– pela
Sep 7, 2022 at 21:00
• On a related note, objects near the JWST are likely to be bright and hot. Sep 7, 2022 at 21:13

Mars was a (possibly de facto) requirement. The JW attitude control system was given slew rates compatible with observations that far. Some limit had to be stated, and faster slew would cost more, either outright, or in impacts to other spacecraft elements (systems engineering), or in programmatic impacts (e. g., test requirements or lifetime or image jitter or whatever).

Slew rates are important because targets move, and JW moves. For extragalactic work etc., exposures measured in many seconds do not produce relevant motions (>PSF). For Solar-System work, motions are not just relevant but 3D- targets produce not just streaks but arcs if close enough (therefore fast enough).

In flight, part of the delay of several months (before first science light) was as-built, as-flown testing and demonstration. Ops has now reported that the attitude and slewing functions, of science-grade (not merely transit from one observing field to the next) have come in better than spec. This means JW can see closer than Mars.

• A temporary, reversible -1 for complete lack of supporting sources. There's no way for readers to verify if "slewing rate" is really the limiting factor beyond "take my word for it". Good Stack Exchange answers need to support their facts with verifiable sources. Thanks!
– uhoh
Sep 10, 2022 at 14:21
• And what would readers do with that? Finish their Cycle 3 proposals (more likely Cycle 4 at this point)? A qualified, working astronomer would have already known the answer, because they already read the necessary documentation on submissions, because they need not waste their time submitting notions instead of viable work. i. e., the act of asking belies not already knowing…even knowing who to speak to. Verify…why? If an internet commenter were qualified, they would not be an internet commenter . What work will be left undone? Sep 15, 2022 at 12:12
• Welcome to Stack Exchange! This site works a little differently than others you may have used. It's not for everyone; SE may not be your cup of tea.
– uhoh
Sep 15, 2022 at 12:36