HST (Hubble Space Telescope)

We remind the reader that the space telescope had three problems after it was put into operation. The first was image blurring. The second, which was not publicly discussed, concerned the three small telescopes (Star Trackers) whose task was to stabilize the HST’s pointing direction. The third problem concerned the pointing direction itself. After the initial alignment of the HST telescope axis toward a selected space object, the NASA control center obtained a blurred image of a neighboring object. The telescope was effectively squinting.

The Influence of Motion on the Function of the Space Telescope

The velocity of the HST is the vector sum of the velocity of the galaxy, the velocity of the Solar System, the orbital velocity of the Earth, and the orbital velocity of the HST itself. The velocity of our galaxy relative to curved spacetime is approximately 600 km/s. The orbital velocity of the Solar System relative to the center of our galaxy, the Milky Way, is approximately 220 km/s. The orbital velocity of the Earth is approximately 30 km/s. The orbital velocity of the HST around the Earth is 8 km/s. Therefore, the motion of space telescopes relative to curved spacetime is not negligible, especially in the case of telescopes with focal lengths greater than 1 meter.

The figure shows a parabolic mirror moving at a selected velocity v. For better illustration, the telescope velocity is exaggerated. During observation, the HST axis direction is stabilized. The orbital velocity (8 km/s) is negligible compared to the assumed velocity of the HST (600 km/s). In Fig. 1, vectors were used to represent the velocity and direction of light rays, as in the explanation of stellar aberration in the Encyclopedia of Astronomy.

Fig. 1. Influence of telescope motion on the direction of observation and image sharpness.

The blue rays represent rays that, after reflection and subsequent impact, meet at the focal point F of the parabolic mirror when the telescope is stationary. The red rays represent two rays when the telescope moves at the selected velocity, given by the equation v. These rays determine the observation direction for the telescope’s given velocity vector.

After reflection, the two red rays do not meet at the original focal point F, but at point Fx. Moreover, they do not arrive there at the same time, because the paths of the reflected rays are not equal. The telescope camera records the image formed by all red light rays originating from directions that do not coincide with the optical axis. Furthermore, the recorded image is blurred because all red rays striking the parabolic mirror at an incorrect angle do not meet at a single point after reflection.

The corrective lenses installed during the 1993 repair mission eliminated the influence of mirror motion on image sharpness, but not on the direction of observation. This means that the HST still squints.

The validity of this consideration could be verified today, in the modern space age, by a simple experiment:

By measuring the distance between a precisely defined reflecting surface and the source of a laser beam in a device placed in open outer space. If the measured distance changes during the device’s orbital motion around the Earth, this would confirm the correctness of the above consideration.

In 1992, we sent this consideration not only to responsible NASA personnel, but also to the magazine VTM. We include NASA’s response and the VTM editor’s review.

L e t t e r f r o m N A S A

R e v i e w b y t h e V T M e d i t o r

Note: The mirror of the James Webb telescope consists of 18 segments. Interestingly, the telescope mirror segments must be readjusted whenever the direction of observation changes.