Why is the door not rounder?

I understand it's main purpose is to block sunlight from entering the telescope.

Especially the right side (in the provided image) seems to waste material.

But also the two corners on the left side could have a larger radius.

Hubble Space Telescope aperture door

  • 36
    $\begingroup$ If there is any answer besides "so they can have a hinge on it" I don't know what it is. $\endgroup$ Commented Apr 24 at 14:21
  • 5
    $\begingroup$ I literally said that aloud. $\endgroup$ Commented Apr 24 at 16:32
  • 3
    $\begingroup$ @OrganicMarble there are plenty of round terrestrial windows and covers that don't need a long, linear, continuous "piano" hinge. This looks like something one would build out of spare items in one's basement using a screw driver, a circular band saw and a piece of sheet metal brought home years ago they were throwing away at work. Intellectually I know it isn't, but it really does give that impression. And those sharp corners with no rounding, sticking way out just begging to get bumped and bent some time before deployment just look... wrong from a design perspective. $\endgroup$
    – uhoh
    Commented Apr 25 at 6:33
  • 3
    $\begingroup$ @uhoh It doesn't look that way to me. Perhaps because I have seen other space telescopes with similar long straight hinges on their aperture doors. images.app.goo.gl/tdRqbCYTJUmrTFGx7 $\endgroup$ Commented Apr 25 at 11:20
  • 5
    $\begingroup$ A circular shield would perfectly prevent light from any angle from entering while fully closed. Once opened, the hinge point would be the only point blocking stray light from entering the aperture from a source within an angular distance <90 degrees (e.g. sun, moon, earth, etc.). Light would be able to creep around the hinge point via the small gaps that would now exist between the cirular shield and the rim of the aperture. Also, as noted, straight edges are easier for mounting hinges. As for why the other side of the shield is rounded, it had to fit in the Space Shuttle payload bay. $\endgroup$
    – Agbullet
    Commented Apr 25 at 18:16

3 Answers 3


Aperture door structure

When considering the potential weight savings and strength of the material, it is important to note that the aperture door is made of lightweight aluminum honeycomb material (Hubble Space Telescope meteoroid-debris protection analysis p. 25):

Aperture door diagram

The material is lightweight per area, but not very strong for attaching a hinge at the edge. A wider hinge area would increase its durability.

However, the hinge is not actually a long "piano hinge" like could be easily assumed. Instead, it is a pair of hinges, one of which contains a motor (Space Telescope Systems Description Handbook, PDF page 45 onwards), and the door has "heavier core near hinges and latch". It also notes the weight of the honeycomb door as 79 lbs (35.8 kg) and total weight of aperture door assembly as 97.7 lbs (44.3 kg).

Aperture door diagram

Considerations of the sharp corners

The sharp corners were not without problems, as evidenced in e.g. the protective cover used during transportation (Hubble Space Telescope Protective Cover System p. 6):

Since the AD/LS cover must be removed from the Shuttle cargo bay where access is very limited, special design considerations were required. The Aperture Door is not round like the Light Shield but pentagonal with protruding corners on 2 sides. This requires the AD/LS cover must have a way of opening to be larger than the Aperture Door and yet be fairly rigid for the GSE to be able to pick it up in the very confining space of the Shuttle Bay full of HST vehicle.

Interestingly the text calls the shape "pentagonal", but the illustrations show the familiar rectangular shape with two rounded corners.

Use as a light shield

I think the real and largest reason for the large area of the door is to act as a light shield (Hubble Space Telescope: Optical telescope assembly handbook. p. 8):

The Aperture Door (AD) located at the ST forward end is attached to the SSM LS and is positioned to shade and protect the focal plane particularly from scattered earthshine.

Creation of the Hubble Space Telescope:

The “jaunty” angle of this aperture door was selected to allow observing Venus at maximum elongation without sunlight entering the main tube of the telescope.

Space Telescope Systems Description Handbook:

  • The Aperture Door AD located at the ST forward end is attached to the SSM LS and may be positioned relative to the Sun, Moon or Earth to shade and protect the sensitive Sis from electromagnetic radiation and degrading the focal plane image.
  • Assists in providing stray light suppression and contamination control
  • Fully opened, the door permits viewing within 50 deg of +Vl.
  • Fully opened, a ±5 deg roll shades the sun from entering.
  • Stray sunlight in the hinge region will be attenuated.

In fact, it appears that the door is close to the minimum size to meet the angle requirements. The roll requirement of +- 5 degrees displaces the projection at 135 inch (343 cm) distance by +- 12 inch (30.5 cm). The door is 28 inch (71 cm) wider than the aperture, giving +- 14 inch (35.5 cm) margin from the centerline. The bottom corners have to be sharp to ensure shading at shallow angles.

Diagram of shading

Similarly the 50° sun angle, combined with 105.25° door opening angle gives the 119.2 inch (303 cm) length of the door. I'm not quite sure what determines the 105.25° angle, but it's probably related to optics.

  • 6
    $\begingroup$ Supporting the idea of its use as a sun shield, apparently the hinge was placed where it is so that the door would open in a certain direction, basically the "sun facing side" of the telescope. There is apparently an optimal position in relation to the Sun for thermal management and solar power. This was mentioned in this answer to a different question. $\endgroup$ Commented Apr 25 at 12:28
  • 1
    $\begingroup$ Thanks for the links, especially the Systems Handbook. $\endgroup$ Commented Apr 25 at 14:18
  • 2
    $\begingroup$ @OrganicMarble Yeah, it is lovely! $\endgroup$
    – jpa
    Commented Apr 25 at 14:55
  • 1
    $\begingroup$ If you treat the two rounded corners as edges then the door is pentagonal $\endgroup$ Commented Apr 26 at 5:23
  • 1
    $\begingroup$ Excellent answer! It's great when someone really digs in llike this and gets to the bottom of a conundrum. $\endgroup$
    – uhoh
    Commented Apr 27 at 1:18

The flat edge of the door is where it has the hinge and the mechanics that open and close it. The hinge section obviously is most mechanically simple if it's straight, and it's set back from the edge of the aperture to keep it well clear of the light collection area when open.

There's no benefit to be had from cutting it to more closely fit the aperture; the door is fairly light, so any weight savings would be essentially meaningless on a 12-ton telescope, and narrowing the hinge area would possibly weaken the door, requiring reinforcement that would weigh at least as much as the part you cut off. In addition, you want a significant overlap on the door, because that's what keeps sunlight from getting into the sensitive parts of the telescope. A close fit would actually be worse, because it would provide more opportunities for a slight misalignment to wreck the optics.

In short, it's shaped that way because it's convenient and there are no particular drawbacks to shaping it so.

  • 1
    $\begingroup$ Hinges do not have to be straight nor do they need to rest on a straight edge. I agree that it is easier to put a straight edge hinge in production but there are tons of curved doors in the world and the woodworkers and trades that install those probably aren't as good as NASA engineers. So either there is a scientific reason or they just decided that they were going full out lazy on that part. My guess is since it is motor driven the type of motor they used was more efficient on a straight door. It didn't "have to be straight". $\endgroup$
    – blankip
    Commented Apr 25 at 16:15
  • $\begingroup$ Even if you make the door round, the hinges have to be straight or they won't work, and then you need extra material to attach the round door to the straight hinges. No, the door does not technically have to contain a flat edge, but if it doesn't, you have to work around that fact. $\endgroup$ Commented Apr 25 at 16:23
  • $\begingroup$ You are assuming that all hinges are pin hinges as would be what you see on an entry door for a house. However you have hinges (types) like slow close cabinet hinges. With these types you could have any shape of door that you want and the door mechanism will work the same. This literally could be any shape. Looking at the diagram from the answer below on how the hinge was set (and my building background) it looks like the hinge type was set to give the enclosure a more solid "closed" state. $\endgroup$
    – blankip
    Commented Apr 25 at 16:38
  • 1
    $\begingroup$ And honestly from an engineer and building perspective the only reason for the completely flat edge is mentioned in their handbook... basically better to grip it. From a weight aspect it makes no sense to add a few extra pounds in material for an overlapped part. These NASA guys are pretty smart and there are probably 5 reasons they did this given that 2-3 extra pounds is small fraction of a chance to help with the 5 other issues it solves for. $\endgroup$
    – blankip
    Commented Apr 25 at 16:40
  • $\begingroup$ Also, the more complicated you make your hinge, the more chances there are for something to go wrong, and when your something is out in space where it's difficult to get up there to repair it, that's one thing you want to avoid at all costs. The long-term reliability of a simple straight hinge is far more valuable than the small decrease in weight you'd get from cutting off the corners and having to use a more complex mechanism. $\endgroup$ Commented Apr 26 at 18:47

not an answer but a note:

When the HST moves to point in a direction that is near the sun, the shield is the best shape without being overly large to act as a shield or shade.

(when viewing away from the sun, the shape was less important)

enter image description here

When HST performs a maneuver from one target in the sky to another, it cannot allow the tele scope aperture to point within 50 degrees of the sun. For example, if two targets just outside the 50-degree Solar Avoidance Zone are on opposite sides of the zone, HST follows an imaginary circle of 50 degrees around the sun until it reaches the second target

Noting the pitch maneuver along the V2 axis, same axis as the hinge for the aperture door and the Solar Arrays, shows potentially that the door design favors maximum coverage as a sun shield or shade over the tubular light shield that the door is attached to when rotating around that axis.



Not the answer you're looking for? Browse other questions tagged or ask your own question.