# Could Hubble observe the night side of the Moon?

IIUC the Hubble space telescope cannot observe the Sun lit side of the Moon. And Hubble is never pointed closer than 90 degrees towards the Sun. But this still allows for observing the night side of the Moon during phases between full moon and half moon.

• Could Hubble's instruments be damaged if it were pointed to the Sun lit side of the Moon?

• Would Hubble observations of the night side of the Moon be of scientific interest? How would such observations, in terms of resolution and the instruments on Hubble today, compare to observations made by Lunar orbiters, like LRO, and Earth ground based telescopes?

• @Hobbes I didn't realize what it was actually asking, that does make far more sense. I've added in a note to prevent others like me from making the same mistake. Jan 23, 2019 at 17:51

The pixel size of the HST's Wide Field Camera 3 or WFC-3 is 164/2048 = 0.08 arcsec.

The night side of the Moon is illuminated by Earthshine and the brightness depends on the phase angle of the moon (and the weather on Earth (clouds, wind-induced waves on the ocean) and the time (ocean versus land) but we can find some averages. Let's use +15 magnitude per square arcsecond from below with a as a representative number. It gets a bit brighter at small phase angles when the Earth's mostly sunlit side faces the Moon, but then the Hubble would be starting to point dangerously close to the Sun.

A pixel size of 0.08 x 0.08 arcsec is 0.0064 of a square arcsecond, so that's $$-2.5log_{10}(0.0064) \approx +5.5$$ magnitude; so its +20.5 magnitude per pixel.

The Sun is +27 magnitude, 1360 W/m^2, and 2.5E+06 square arcseconds. Assuming HST+WFC-3 are used without a filter, they cover most of the power in the solar spectrum.

So the Sun's -11 mag/arcsec^2 or -5.5 mag/pixel is 5E-04 W/arcsec^2 or 3E-06 W/pixel.

That makes the night side of the Moon (26 magnitudes dimmer) about 1.4E-16 Watts/pixel.

Multiply by 6E+18 (e- per Coulomb) and divide by 2.5 eV (some kind of average energy per photon) and that's about 200 e- per second in the CCD if I throw in a quantum efficiency of 0.8 and a pixel fill-factor of 0.8.

That's fairly dim! A five second exposure gives 1000 e- and the $$\sqrt{N}$$ shot noise is 3%. The cooled CCD's dark current is minuscule here, we're limited by shot noise: 1, 2 and readout noise of roughly ~20 e-.

As the other answers have pointed out, tracking might be a challenge. Kinematically there's no difference between the telescope slewing at 0 arcsec/sec to follow the stars and 15 arcsec/sec to follow the Moon, but there may be problems with the way the star cameras obtain data if the stars are moving, and so it may be necessary to give the HST a "blind kick" of 15 arcsec/sec in order to follow the Moon, and read out images say once per second to off-line stack them correcting for any residual sub-pixel drift.

Abstract

We have used the large database of photometric observations of the bright and dark portions of the face of the Moon from the Earthshine Project at Big Bear Solar Observatory to determine the surface brightness of the earthshine and its variations. Our purpose is to make these observations appropriate for the calibration of lunar flashes according to their magnitude. We have evaluated the daily, seasonal, and annual changes in magnitude for our entire data set and have also calibrated the surface brightness of the entire lunar geography for several lunar phases by means of the observation of lunar eclipses. We find variations between +12 and +17 mV arcsec² with hourly changes upward of the order 0.25 mV arcsec², which are uniquely due to the terrestrial meteorology. This rapid change in the terrestrial flux reaching the Moon is usually neglected when calibrating the magnitude of lunar impact events. We justify this using earthshine observations to determine the brightness for the day, time, and selenographic location of a given event in order to improve the accuracy of its brightness calibration up to 0.25 mag.

Hubble can in fact observe the Moon, and has done so. Here's a picture of the Apollo 17 site (The upper right is from Apollo 17 mission itself). The x shows where the actual site is. You can also see more Hubble pictures of the Moon at this page.

• I can see that the lower right photo was taken from approximately 4 o'clock relative to the red X in the left photo, but I can't match the top right photo to the others. What are the common features to look out for? Jan 22, 2019 at 23:58
• To me it looks like the top right photo was taken standing near the red "x" and pointing towards one of the nearby ridges with a lunar rover on it. Jan 23, 2019 at 13:12
• I've addressed the problem here I don't think daytime images address either brightness, S/N or tracking, since the exposures can be so fast, but it does address the question of damage.
– uhoh
Jan 25, 2019 at 3:52

I believe COS (the sensitive UV instrument) could be damaged if it were pointed at the illuminated moon. (I wasn't able to find any documents online that confirm this though) The instruments don't point in the same direction. So it's possible to orient the telescope so that one is not pointed at the moon while others are.

Lunar observations are difficult because the platform isn't really designed to track moving objects. So it can't sit and stare at a location. I'm sure any hubble images would have some value, but the resolution will be nowhere near that of LRO.

• any thoughts on this HST instrumentation question?
– uhoh
Jan 22, 2019 at 22:50
• "The instruments don't point in the same direction" - do you want to elaborate? I expect the difference to be tiny. They all look though the same telescope, the detectors are placed in different spots of the focal plane. Jan 24, 2019 at 11:10
• Hubble is routinely pointed at the day side of Earth (astronomy.stackexchange.com/questions/26041/…), so pointing at the Moon shouldn't be an issue. Jan 24, 2019 at 15:53