FAQ - Frequently Asked Questions

By using the button "contact" you can send questions to the Hubble Europeans Space Agency Information Centre. A selection of answers will be published on the page below.

16. Q: Given the latest advances in Hubble instrumentation, image processing etc. what would the absolute theoretical limit for the smallest object visible on the Moon be?

A: Since this question really asks for the "full treatment", let's look at it in detail.
The so-called Rayleigh criterion gives the maximum (diffraction limited) resolution, R, and is approximated for a telescope as
R = Lambda/Diameter,
where R is the resolution [Radians] and Lambda is the wavelength [m]or with "normal" units as
R = 0.21 * L/D
in the units [arc-seconds or "] = [microns]/[meter]

So for Hubble this is:
R = 0.21 * 0.500/2.4 = 0.043" (for optical wavelengths, 500 nm)
or
R = 0.21 * 0.3/2.4 = 0.026" (for Ultraviolet light, 300 nm)

The CCD detectors in Hubble's instruments should, in theory, have been adapted to this number and follow the Nyquist-Shannon sampling theorem i.e. have 2 pixels per resolution element (i.e. 0.0125" pixels), but for good scientific reasons, like getting a larger Field of View, this is not the case. The pixels in the different instruments are typically 0.05"/pixel and the best sampling is 0.025"/pixel for the ACS/HRC instrument.

With so-called dithering and drizzling one can improve sampling and minimise resolution loss of undersampled images by shifting the telescope back and forth between exposures with a fraction of a pixel.

When these methods are applied, the final effective resolution of telescope and detector can at best be calculated as:
R= sqrt (telescope optical resolution^2 + pixel size^2)
i.e. for Hubble with ACS/HRC, and in the UV (300nm):
R= sqrt (0.026^2 + 0.025^2) = 0.036"

Then, in an extreme case, such as the Moon where there are lots of light (a high Signal/Noise ratio), one can do image processing (image restoration) and retrieve roughly a factor of 2 better resolution at the expense of some artifacts, i.e. for Hubble 0.018" (best best case in Ultraviolet).

On the Moon, we can transfer this to the following linear extension with the help of the Small-angle approximation:
linear extension = distance * R/206264 in units [m] = [m] * ["]/[1/"]

On the Moon, with its minimum distance, this would give a linear extension of:
363,000,000 * R /206264 = 32 metres

So 32 metres should be the minimum size of an object visible as more than a dot.

15. Q: Does ESA have any robotic proposal that could be useful for servicing Hubble and other space equipment?

A: No, and none is planned.

14. Q: Do you think it was a good idea that NASA decided not to carry out Servicing Mission 4?

A: The Columbia Accident Investigation Report stipulates that a "safe haven" is required for the crew of the Space Shuttle in case the Shuttle cannot return to the ground. The International Space Station provides this capability; Hubble does not. Rescue could only be performed by launching a second Shuttle within a few days. This is very difficult from an operational stand-point.

Whether it was a good or a bad idea is irrelevant. It was the only option available in ageement with the guidelines of the Columbia Accident Report and within the available budget. For the science of astronomy it is a setback, especially since there will not be a replacement for Hubble any time soon.

Currently there are attempts to 'stretch' Hubble's life as far as possible, and we will only know how successful these attempts are when Hubble fails one day. Whether that will be 5 years earlier than expected (2010) or only one or two remains to be seen.

13. Q: Wouldn't a robotic mission to service Hubble be even more expensive than sending astronauts?

A: No. Depending on what one includes in the calculation (actual costs, or including the fraction of initial investment, etc.) a single Space Shuttle launch costs up to 1000 million US Dollars. Even developing a robotic mission from scratch would cost less than that. In addition, some limited robotic capability is required in any case in order to bring Hubble down in a controlled manner: Several of the heavy elements of the telescope, like the main mirror, will survive re-entry, and they might cause damage on the ground. So it is important to de-orbit the telescope in such a way that it lands in the Pacific Ocean (or lift it into a much higher orbit).

12. Q: Do you think Hubble is still necessary?

A: Hubble currently provides the best resolution in the optical and Ultraviolet (UV) wavelength range. This is extremely important for the identification of interesting objects. Adaptive optics techniques can be used with ground-based telescopes to obtain a similar resolution to that of Hubble, but only for small fields-of-view and in the infrared wavelength range. Also adaptive optics currently is limited to certain areas of the sky where the necessary 'guide stars' are available.

As mentioned, Hubble provides access to the ultraviolet wavelength range. The UV light is blocked by the Earth's atmosphere, so observations in the UV can only be performed from space. They are very important in order to understand the physical processes that take place in astronomical objects.

11. Q: I heard that Hubble had problems in its first years. What really happened?

A: Yes, it is true. During its first three years Hubble suffered from what is known as spherical aberration. Spherical aberration is an optical defect and simply put, although Hubble's main mirror had a perfect shape, it was two microns too flat. The problem was caused by a faulty measuring device used during the polishing.
During the First Servicing Mission the problem was solved by installing an extra optical device in Hubble called COSTAR.
The spherical aberration in Hubble's main mirror is still there, but today it does not affect the observations - partly due to COSTAR and partly due to built-in optical corrections in the newer instruments.

10. Q: How much has the Hubble Telescope cost overall so far?

A: As you may know, Hubble is a collaboration between ESA and NASA. ESA has 15% of the observing time, and Europe's financial contribution to Hubble is 593 million Euros as of 1999 terms (including the development of FOC and the Solar Arrays, the participation in the operations and in the relevant Servicing Missions). The US expenditure is currently an estimated 4.5 to 6 billion US$.

9. Q: What do you think is Hubble's best photo so far?

A: Oh - that is almost a naughty question ;-) Hubble has produced so many fantastic pictures. And to tell you the truth: I change my taste once in a while. These days it is the Keyhole Nebula which is my favourite.

8. Q: Why is Hubble able to see so much better than telescopes on earth?

A: Because it is above the Earth's atmosphere. The atmosphere disturbs the starlight (a bit like looking through water) and blurs the images. So Hubble's images are much sharper than those from other telescopes.

7. Q: What has the Hubble Space Telescope found out about the beginning of the universe?

A: It is a bit difficult to state in just a few sentences. Hubble has measured the age and size of the Universe better than before (by refining value for the Hubble constant - or the expansion rate). It has also seen details in the first galaxies that are not visible from the ground. Today we know that galaxies were formed earlier than previously thought and most scientists also believe that they evolve by colliding and merging together.

6. Q: What are the Hubble deep space fields?

A: These are two very long (> 100 hours) exposures showing details of the remote Universe never seen before. I can recommend reading the new 10 year brochure that is also available on the web (under "Further information").

5. Q: What is going to be so much better about the NGST?

A: The Next Generation Space Telescope will be much larger than Hubble - it will be able to collect ten times more light and thus see much fainter objects. A special feature is that it will mainly be sensitive to Infrared (heat-) radiation. This means that it can look further out into the Universe than with any other telescope and see the first galaxies as they form. Check also the pages on NGST.

4. Q: As the Big Bang could be similar to a conventional blast, and therefore the Universe developed around it, how is possible to state that there is not a center of the universe? And then how is possible to take a picture of the most ancient radiation and see it as it were around the photographers (Hubble and then us)?

A: The similarity of the expansion of the Universe to a conventional blast is a typical misconception in the popularizations of the cosmological model, possibly due to the unfortunate choice of the name 'Big Bang'. As a matter of fact the expansion of the Universe is totally different from a conventional blast, which happens 'within' a given space. The expansion of the Universe 'is' the expansion of the space-time itself, together with its energy-matter content. Admittedly it is not easy to imagine the correct scenario, because it is so different from our experience of everyday life. An additional difficulty related to your second question is that this is due to the fact that the velocity of light, although very high, is still finite (~ 300.000 km/sec). Therefore, if we look at objects which are at some distance from us, we see them as they were sometime ago, the elapsed time being exactly the time needed by the light to reach us. For example, the image of the surface of the Sun that we can observe is always 8 minutes old, because it takes about 8 minutes for the sunlight to reach the Earth. If you look at the stars of the Orion constellation, which are roughly at a distance of 900-1000 light years, you see them as they were 900-1000 years ago. If some of them would have exploded today as a supernova, you (or your future relatives!) would know about this event only in a millennium.

This explains (we hope) why we can see pictures of the very old Universe: we just have to have powerful enough telescopes to look very far away. We see 'Universe' around us up the limit that cosmologists call the 'horizon'. As time passes by, we get a larger picture of the Universe, as the horizon gets bigger and bigger. This scenario must be true for any 'observer' in the Universe, and he or she will think to be at the 'centre' of the expansion. In this sense, there is no centre. Your questions are very profound because they go back to the very nature of space-time and of the cosmological models. It is not easy to answer properly in a few sentences. We would suggest you take the time to read some serious scientific book on Cosmology and meditate on the difference between the 'world' that we model around us on the basis of our daily experience and the one that describes the entire Universe. The difference in scale matters a lot!

3. Q: I was wondering a few things. First how far can the Hubble Space Telescope actually see?

A: Actually the telescope itself has no limits - but the Universe itself does. Hubble is a medium-sized (2.4 meter) telescope with very sharp optics and very good instruments. This enables the telescope to see very faint objects despite its relatively modest size. According to the theory of Big Bang, the absolute observational limit to telescopes (as we know them today) is a 'sphere' of opaqueness surrounding us positioned approximately 13-14 billion light years away. It is called the 'surface of last scattering', and is also known as the source of the 'microwave background radiation'. Up to 300.000 years after Big Bang, the Universe was totally opaque to light. This means that we know that we (when we look out in the Universe and thus back in time) will never see past, or through, this barrier.

Today galaxies that have been seen with Hubble are at a distance of approximately 12-13 billion light years. In the coming years more distant galaxies will undoubtedly be detected, but the limit for our observations will not progress a lot for two reasons. Firstly, the galaxies have to have time to form stars after Big Bang (this takes roughly one billion years). Secondly, the young galaxies will be enshrouded in large amounts of dust that will - at least to some degree - obscure our view of the early Universe.

2. Q: In the Hubble FAQ you mention that the telescope will never be aimed at the Earth. What about the Moon? Would it still be damaging for Hubble's instruments? If feasible, could these observations take a picture of the landing site of the Apollo missions?

A: Indeed there has been a few cases where Hubble has been aimed at the Moon - see here. It has to be done with greatest care (since the Moon is very bright and can damage instruments etc.), and is normally avoided. Even with Hubble's incredible sharpness (resolution) only objects the size of a football field can be seen (~100 metres)... so no Apollo spacecrafts will likely be visible. With novel image processing techniques this may be improved in the future, but we will have to wait and see.

1. Q: What if the hubble telescope watched the Earth, what resolutions would the images possible have? I would guess approx. 10 - 20 cm?!?

A: Hubble's so-called angular resolution - or sharpness - is measured as the smallest angle on the sky that it can be 'resolved' (i.e. see sharply). This is 1/10 of an arc-second (one degree is 3600 arc-seconds). If Hubble looked at the Earth - from its orbit of approx. 600 km - this would in theory correspond to 0.3 meter or 30 cm (almost the same as you found). Quite impressive! But the 'blurring' of the atmosphere downwards would make the actual resolution worse. Unfortunately, Hubble will never be turned to Earth since a) the brightness of the Earth could be damaging for the telescope and the instruments, and b) there is not any particularly interesting astronomical research to be done there (the type of science that deals with the Earth is called geophysics).