Herschel Space telescope

[Dishmaster]

Welcome. Your avatar is intriguing, what is it?

 

This is a model of the Herschel Space Telescope that was launched in 2009 and is still operating. I'm working in that project.

Edited July 31, 2011 by Dishmaster

[michel123456]

This is a model of the Herschel Space Telescope that was launched in 2009 and is still operating. I'm working in that project.

 

Fascinating. It looked to me a mix between an archaic egyptian sculpture and some Edison's lost invention.

 

The way the Herschel Space Telescope is designed makes me think it cannot be turned to look at any target at any time, since its protection from the sun is fixed & perpendicular to the mirror. Is that correct?

[Dishmaster]

Fascinating. It looked to me a mix between an archaic egyptian sculpture and some Edison's lost invention.

 

The way the Herschel Space Telescope is designed makes me think it cannot be turned to look at any target at any time, since its protection from the sun is fixed & perpendicular to the mirror. Is that correct?

Yes, you are pretty smart. The sun shield is indeed fixed as you describe it. It also serves as a solar power cell. The telescope is 1.5 million km away from Earth from where the Sun, the Earth and the Moon are basically always at a mutual direction. The telescope is operated such that its main mirror is always looking more than 60° and less than 120° away from the Sun. During the full revolution around the Sun during one year, all sky positions are observed at least for a certain duration.

[michel123456]

Yes, you are pretty smart. The sun shield (...)

 

Thank you. But I am also quite ignorant. I wondered why a sun shield was required since the Lagrangian 2 is opposite to the sun. Google (and Wikipedia) saved me by explaining that the Lagrangian 2 was out of Earth's umbra.. There must be some kind of traffic jam at L2...

[Dishmaster]

Thank you. But I am also quite ignorant. I wondered why a sun shield was required since the Lagrangian 2 is opposite to the sun. Google (and Wikipedia) saved me by explaining that the Lagrangian 2 was out of Earth's umbra.. There must be some kind of traffic jam at L2...

 

Luckily, no. Although it is becoming quite popular these days to park satellites there, it cleans up itself, because it is not an attractive Lagrangian point. It is more like a saddle. Frequent small manoeuvres are required to keep the telescope on its orbit around L2. Without them, it will eventually drift away.

[Airbrush]

Welcome aboard Dishmaster. How frequent are the maneuvers? How long will the fuel last to keep doing that?

 

Any advice to posters here on how to get a job in space programs? How did you find your job? I'd rather sweep up around the telescopes for minimum wage than continue to be full time bookkeeper for a restaurant. I don't have much science education, but I've been bookkeeping and accounting for 20 years, so could that number crunching come in handy anywhere you know?

[Dishmaster]

There are slots reserved for this during every other week. But the velocity corrections are usually of the order of a few cm/s. A lot of fuel was saved during the launch, because the telescope was injected into its trajectory quite perfectly without much additional corrections needed. It will easily last until the foreseen lifetime of the telescope which is currently expected for early 2013. The remaining fuel will then be used to actively move it away from its current orbit. Some people are even speculating on letting it crash onto the moon.

 

My personal background is science, i.e. Physics, or more precise, astrophysics. I am employed by a research institute that contributes to the operation of the spacecraft. Other opportunities are certain degrees in engineering. I am afraid, it does not go below that expertise.

Edited August 3, 2011 by Dishmaster

[michel123456]

Could you also explain why it is so important to keep freezing the telescope, I used to think it was pretty cold there anyway.

 

(...) Other opportunities are certain degrees in engineering. I am afraid, it does not go below that expertise.

 

Tragic error.

If you are using only very intelligent people you will lose any element of common sense.

[Ophiolite]

If you are using only very intelligent people you will lose any element of common sense.

Empirical data demonstrate that engineers are not intelligent.

[Dishmaster]

Could you also explain why it is so important to keep freezing the telescope, I used to think it was pretty cold there anyway.

Not cold enough. Even with the sun shield, the main mirror has a temperature of nearly 90 K. The Sun irradiates the satellite and heats it up. This is much to warm for any instrument that tries to detect far-infrared radiation of wavelengths between 60 and 500 µm. The telescope is desinged to observe objects that are as cold as a few kelvin. In order to be sensitive enough to detect these objects, the detectors must be much colder. We are cooling them with superliquid Helium. Some detectors are cooled down to just below 3K, others are below 0.3 K. Otherwise, they would radiate themselves strongly and affect the actual scientific signal. Already the 90 K "warm" mirror is quite problematic, because with such a temperature, it is by far the brightest signal for most the objects we observe. Some exceptions are planets like Mars and Uranus. We already have to apply a few tricks to remove the signal from the mirrors.

 

The amount of Helium restricts the lifetime of this telescope. As soon the Helium is fully evaporated, the instruments are blinded by their own heat radiation.

[michel123456]

What if the telescope was totally in Earth's umbra?

[Airbrush]

Empirical data demonstrate that engineers are not intelligent.

 

 

So we can relax. Since engineers are generally not intelligent, they have enough common sense. I love this Alice in Wonderland logic. Hahahaha.

 

Not cold enough. Even with the sun shield, the main mirror has a temperature of nearly 90 K. The Sun irradiates the satellite and heats it up. This is much to warm for any instrument that tries to detect far-infrared radiation of wavelengths between 60 and 500 µm. The telescope is desinged to observe objects that are as cold as a few kelvin. In order to be sensitive enough to detect these objects, the detectors must be much colder. We are cooling them with superliquid Helium. Some detectors are cooled down to just below 3K, others are below 0.3 K. Otherwise, they would radiate themselves strongly and affect the actual scientific signal. Already the 90 K "warm" mirror is quite problematic, because with such a temperature, it is by far the brightest signal for most the objects we observe. Some exceptions are planets like Mars and Uranus. We already have to apply a few tricks to remove the signal from the mirrors.

 

The amount of Helium restricts the lifetime of this telescope. As soon the Helium is fully evaporated, the instruments are blinded by their own heat radiation.

 

Very interesting, thanks for that. So this means that the helium will fully evaporate sometime in 2013 and then the telescope goes dead? Could it ever be serviced by a mission to deliver helium to it?

[Dishmaster]

First of all, the telescope is never in the Earth's umbra. At a distance of 1.5 million km, the apparent diameter of the Earth is always smaller than the apparent diameter of the Sun. In addition, also Earth and Moon are strong infrared radiators that heavily affect the satellite.

 

No, a servicing mission would probably be too expensive and complicated. In any case, it would have to be robotic. However, many astronomers would be pleased, if it were possible.

Edited August 4, 2011 by Dishmaster

[BJC]

What are the technical problems (overview) of alignment that allows images from the various cameras to be synchronized? With the Hubble i had listened to a brief talk on the "dither and drizzle" problems encountered when they obtained their million second exposure for the Hubble Deep Sky survey.

 

Is there problems matching the infrared frequencies? To other telescope images?

 

Sorry for the rather general question - i do not know enough about the operations of telescopes to ask specific questions.

[Dishmaster]

What are the technical problems (overview) of alignment that allows images from the various cameras to be synchronized? With the Hubble i had listened to a brief talk on the "dither and drizzle" problems encountered when they obtained their million second exposure for the Hubble Deep Sky survey.

 

Is there problems matching the infrared frequencies? To other telescope images?

 

Sorry for the rather general question - i do not know enough about the operations of telescopes to ask specific questions.

Are you asking about the alignment of the three instruments on-board Herschel? Well, it is technically impossible to align all three instruments perfectly on the optical axis. In fact, they are all looking in slightly different directions, separated by a few arcminutes on the sky. These offsets are cancelled out by software. Every target has its coordinates. As a first step, we point the boresight (optical axis of the main mirror) of the telescope there. Then, there are offsets applied, depending on which instrument/camera is used for the observation. This is done via a rotation matrix, the so called SIAM (spacecraft instrument alignment matrix). Of course, this SIAM must first be determined, which we did by systematically observing known pointing targets (e.g. IR bright stars) and measured ther offsets from the expected position.

 

A different technical issue is the decoupling of the telescope and its instruments. The data of both are first gathered separately and only merged during the offline data analysis. The merging relies on a perfect synchronisation of telescope (pointing) and detector (signal) data, so that finally the signals appear at the correct position of the final map. You must know that we do not take images with a single shot, but by scanning the telescope and contunuously gathering data. The final map - hence not image - is constructed offline. If the synchronisation for some reason (signal delays, on-board computer errors, non-constant scanning speed, etc.) is faulty, this immediatly affects both the quality of the final map and the precision of the coordinate grid.

 

Another problem is aligning the IR images with e.g. optical images. Celestial objects have a wide variety of brightness distributions across the wavelength range. So, an optically bright star could be very faint at far-IR wavelengths and vice versa. If you know the exact position (coordinates), then an a posteriori re-alignment is easy. But sometimes you detect several targets simultaneously, and then the non-zero pointing errors could lead to a confusion about which object seen in the visual range corresponds to which IR detection. This is also connected to an issue that we call the "background confusion noise". This is produced by the zillions of distant galaxies and quasars that emit at far-IR wavelenghts, and they are everywhere across the sky. Usually, they are very faint and barely detected. But if you have an instrument like Herschel that is highly sensitive, you easily could get hundreds of sources in one image. The challange then is to decide which IR source actually belongs to the optical counterpart.

 

Then again, offsets between the optical and the IR image could also be real. For instance, if a normal star is surrounded by a cold cloud of dust and gas, the centroids of both could be at different locations. The star is usually seen at visual wavelengths, while the cold dust only shows up in the far-IR.

Edited August 9, 2011 by Dishmaster

[BJC]

Are you asking about the alignment of the three instruments on-board Herschel? Well, it is technically impossible to align all three instruments perfectly on the optical axis. In fact, they are all looking in slightly different directions, separated by a few arcminutes on the sky. These offsets are cancelled out by software. Every target has its coordinates. As a first step, we point the boresight (optical axis of the main mirror) of the telescope there. Then, there are offsets applied, depending on which instrument/camera is used for the observation. This is done via a rotation matrix, the so called SIAM (spacecraft instrument alignment matrix). Of course, this SIAM must first be determined, which we did by systematically observing known pointing targets (e.g. IR bright stars) and measured ther offsets from the expected position.

Yes, the alignment of three instruments - but more the alignments of a single instrument for a time exposure on a specific target.

 

A very good answer, Thank You. Explains why there is such a long "processing time" :)

 

I suppose the raw images are not particularly useful for cosmological interpretations - which means that data processing is not only necessary but can affect theories. The process seems similar to the problems we encountered with processing 3-D geophysical seismic off-shore surveys - although i suspect we relied more on data redundancy.

 

 

Do you deliberately vary the alignment to allow the processing to identify and eliminate local (solar and galaxy) rotational and movement variations?

[Dishmaster]

I suppose the raw images are not particularly useful for cosmological interpretations - which means that data processing is not only necessary but can affect theories. The process seems similar to the problems we encountered with processing 3-D geophysical seismic off-shore surveys - although i suspect we relied more on data redundancy.

 

Do you deliberately vary the alignment to allow the processing to identify and eliminate local (solar and galaxy) rotational and movement variations?

I am not fully sure I understand your questions.

 

The raw images are completely useless, because the bright mirror outshines everything else. Special data processing techniques are needed to suppress these contributions. There is a lot of data redundancy involved in the measurements. For the photometric scan map observations, the detector is passed across the sky in a number of parallel scans. Ideally, each individual detector pixel covers the target at least once. The scanning speed determines for how long one detector pixel "sees" the target. This redundancy is actually needed to look e.g. for glitches in the signals and to reduce the statistical signal noise.

 

As with all data handling, the results somewhat depend on the processing. But we make sure that all influences are documented and the scientist can estimate the amount of uncertainty involved. However, the signal calibration (conversion between detector signal and physical photon flux density) is highly accurate. Of course, if you observe very weak objects, the accuracy of the measured flux can be pretty low, which certainly can affect the interpretation. This is why I always say that a single image of an astronomical observation doesn't tell you much. You need to put it into perspective with other results.

 

However, there is a large number of cosmologically relevant results published already. See:

http://herschel.esac...lications.shtml

 

Local influences to the precision of the telescope pointing (revolution around the sun, proper motion of the sun in the galaxy, relative motion of the Milky Way to other galaxies) are well known and are being taken care off automatically when uplinking the observering programme to the telescope. There is no need to do this empirically afterwards. The actual positioning is done mainly by a star tracker camera. This is a small optical telescope that looks into the opposite direction. The observed stellar pattern is compared with a list of well known pointing stars. This tells the main telescope, where it is looking at. In addition, there are gyroscopes installed that also can monitor the attitude of the telescope. But they turned out to be less reliable than previously expected. Moving the telescope itself is done via reaction wheels. There are spinning wheels built inside, one for each telesope axis. By changing the spinning speed, the telescope receives additional angular momentum and moves across the sky. The thrusters are basically only needed to correct the orbit around L2.

 

More information is available here:

http://herschel.esac...test_news.shtml

[michel123456]

(...) Moving the telescope itself is done via reaction wheels. There are spinning wheels built inside, one for each telesope axis. By changing the spinning speed, the telescope receives additional angular momentum and moves across the sky.

 

And how do you stop motion? There are no brakes.

[BJC]

I am not fully sure I understand your questions.

Thank You - you are replying to my ill-formed questions with well-formed answers.

 

However, there is a large number of cosmologically relevant results published already. See:

http://herschel.esac...lications.shtml

I like the page Herschel First Results Symposium

 

The Hershel site contains a lot of data - perhaps too much detail for me. Difficult to find answers to questions such as: "Is there earlier limits on initial star formation?" ; "Cosmic Infra-red Background"; etc.

 

Again Thank You for your explanations.

 

BTW - How do they eliminate the effect of the mirror?

[Dishmaster]

And how do you stop motion? There are no brakes.

You just brake the wheels. If you can let them spin faster, you can also slow them down. This has an effect as if you removed the angular momentum.

 

How do they eliminate the effect of the mirror?

There are several techniques.

Chopping/Nodding

This is an old technique coming from radio astronomy. In principle, you take two images at slightly different positions on the sky, and then subtract them during the data processing. This should in principle cancel out the back ground from the mirror, since it is roughly identical in both images. The only thing that remains is one positive and one negative image of the target in the resulting picture. We do this with a flipping mirror inside the camera. This is called "chopping". The wording is due to historical reasons, when he mirror was like a rotating fan that cuts through the beam whenever one wing of the fan obstructs the main light path and redirects it. Unfortunately, this is not entirely enough, because chopping slightly redirects the light path also on the mirror. It is not fully homogeneous, so there is still structure from the emitting mirror left. This is removed by additionally moving the telescope and then do the chopped imaging again. The result then is a mirror-free background with two positive and two negative images of the target. But we have abandoned this technique, because it turned out to be rather inefficient.

 

See also:

http://herschel.esac.esa.int/Docs/PACS/html/ch05s03.html

 

Highpass filtering

The dominant observing mode with he Herschel Telescope is on-the-fly scan mapping (as described before). The data is treated like a continuous stream of signals that contains signal modulations of different frequencies. The telescope background is static, hence has a very low modulation frequency. The real data coming from the targets is modulated depending on their extent across the sky. The data are filtered during the data processing with a passband that eliminates the bright but static background. It turns out to be vey effective, but also has a few problems that need to be considered when working with the data.

 

See also:

http://herschel.esac.esa.int/Docs/PACS/html/ch05.html#sec-surveyscan

 

Let me know, if you want to know more.

[BJC]

Chopping/Nodding

This is an old technique coming from radio astronomy. In principle, you take two images at slightly different positions on the sky, and then subtract them during the data processing...

See also:

http://herschel.esac...ml/ch05s03.html

This was what i was referring to when i asked about "Do you deliberately vary the alignment".

 

The reference to the "PACS Observer's Manual" is very helpful - answers many of my questions.

 

 

Highpass filtering

The data are filtered during the data processing with a passband that eliminates the bright but static background.

Similar to eliminating power line (60 hertz) interference in seismic processing???

[Dishmaster]

Similar to eliminating power line (60 hertz) interference in seismic processing???

Possibly!

[michel123456]

You just brake the wheels. If you can let them spin faster, you can also slow them down. This has an effect as if you removed the angular momentum.

(...)

 

But that will make the telescope oscillate around a point. Without friction, how does it come to halt?

[Dishmaster]

But that will make the telescope oscillate around a point. Without friction, how does it come to halt?

Why would it oscillate? You spin up the wheels to apply an angular momentum to the spacecraft so that it slews to the desired target. In order to brake it, you reduce the spinning speed of the wheel which again removes angular momentum until it comes to a full stop.

 

http://en.wikipedia.org/wiki/Reaction_wheel