The Jet With a 17-Ton Telescope That NASA Uses as a Flying Observatory | Hacker News Reader

The Jet With a 17-Ton Telescope That NASA Uses as a Flying Observatory

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114 pointsnealabq46 comments

Very cool, I worked with one of the engineers who did the stress & airflow analysis for cutting that big hole in the plane, it was really interesting to hear his stories! And yes, as goodcanadian stated elsewhere in this thread, there are airflow management modifications in front of and behind the door to make this all possible.

I wonder if the plane flies during the day or night? I would assume that for a given mission its one or the other, which makes me also wonder how fast or slow it flies. Even though it is looking in the infrared - wouldn't the daylight still affect it?

We only fly at night. Well, technically speaking, we can observe with the sun up, but the guider cameras are optical which makes it nearly impossible to track any targets when the sky is bright. Moreover, we have to stay well away from the sun as the telescope would focus the sunlight with enough intensity to destroy itself. So, yeah, we only fly at night.

Airspeed is roughly mach 0.85.

Is there some way that civilians admirers can ride SOFIA and get to participate in some of those activities set aside for grade schoolers? I would certainly be as excited as any 6th grader to have that chance!

Grade schoolers don't fly on SOFIA. The Airborne Astronomy Ambassadors program is for grade school teachers and other educators. I can't think of any obvious way for the general public to fly on SOFIA. If you are a professional astronomer, apply for time, and you may be invited to fly. If you are an educator, apply to the Airborne Astronomy Ambassador program.

Nice to see another NASA-related astronomer here. I work on Fermi Gamma-Ray space telescope project, although I'm deep into the data processing and data management side of things.

I also have been helping out with a ton of LSST camera related things.

Next time you guys are in CHCH please do some outreach stuff. It would be great to be able to come and talk with you about the project.

Outreach isn't really my area, but I presume it will be done. There is no southern deployment this year due mostly to the heavy maintenance, but a deployment is expected next year (July? 2015).

EDIT: To make my comment sound less off hand, Education and Public Outreach is an important part of the project, just not the area I work in, so I do not know for sure what form Outreach will take, but I would be very surprised if there wasn't something.

Do you have more info about the airplane performance? I'm working on a flight simulator and this plane is one of the 747's we're going to add. I'd love to get more details about the differences compared to a regular 747SP.

During one of my many Lyft rides I learned the driver was also one of several managers on this project. We had an interesting chat about it, and it left me wishing that the people involved in this sort of work were paid substantially more.

Agreed, unfortunately meaningful projects such as this are not a high priority for the US electorate (and thus the US Congress) [1]. My hope is that growing interest by college students in STEM subjects [2] will ultimately lead to an increased understanding of the importance of these projects in the long term.

[1] http://en.wikipedia.org/wiki/Budget_of_NASA

[2] https://www.stemconnector.org/sites/default/files/store/STEM...

One could always ask for support saying a given experiment could prove the, say, biblical flood or somethimg equally silly. If someone believes that all animals on Earth descend from the living cargo of a wooden ship, it should be possible to convince them of anything.

Hacking a right-wing nutty congresscritter is an intriguing concept.

And, I guess, Bill Maher is a right-wing nutty? He believes Mars and Moon missions would be a great waste of money.

The Bible isn't holding us back....it's our lack of imagination and lazy jokes and the conflation of capital and fiat currency.

... or both you and the GP are partially right and _both_ are holding us back. :/

If we took religion away from history, would we have science?

> He believes Mars and Moon missions would be a great waste of money.

If they fail to inspire people to continue exploring, yes. I would prefer Moon missions because they'd be cheaper and, as far as inspiration goes, they'd achieve the same results. How many brilliant people have chosen to dedicate their talents to building web technologies because space is no longer cool?

Also, being able to build self sustaining off world colonies may prove handy if we end up unable to stop climate change...

I think if something is inspirational, it must be realizable; thus, unless we're talking about mining the moon and requiring human's for labor, we're going to miss the point.

Climate change on Earth is a constant, and if time is x, human existince is a speed bump.

NASA is so prestigious that they basically can get away with paying everyone very little. They aren't stuck with bottom of the barrel people even though they pay them bottom of the barrel wages.

I'm not saying it's right or wrong, but I don't think more funding would really change much (other than allow them to fund more projects, but I'd argue that there are some serious diminishing returns at this point)

oh interesting. I'm also a lyft driver/scientist. I'm doing a series of in-car interviews of scientists, I'd love to try to talk to him, would you be able to contact me (details in profile)?

Scientists in Sedans getting Sauced?

How long are the exposures? One would think that due to vibration etc. the exposures need to be extremely short, but long exposures are beneficial in astrophotography.

EDIT: Wikipedia answers the vibration issues: http://en.wikipedia.org/wiki/Stratospheric_Observatory_for_I...

It's normal to take many short exposures and stack them up to get a long one. The length is dictated by the amount of noise in your detector. From a look at the Wikipedia article, it looks like they need a lot more than just short exposures on Sofia.

Yes, you can stack short exposures, but long exposures are better in astrophotography (as a single 10 second exposure has a higher signal-to-noise ratio than 10x 1s exposures). If you take it to the extreme, it's very difficult to differentiate between the signal and the noise if your exposures are really short, which makes astrophotography in cities without a tracking quite mount difficult.

SOFIA, like all major observatories, uses tracking. A guider camera tracks on the target itself if it is bright enough, or it does offset tracking on another star in the field. You don't need to have any visible signal from your target at all in order to stack the images. Note also, that not all instruments and observing modes are imaging. Often, spectroscopy is done instead.

Maybe, I shouldn't reply to myself, but I feel like I have oversimplified. First, not all observatories use guiding cameras, but most do. An obvious exception are radio telescopes which do passive tracking, but will check their pointing on a bright source periodically.

Second, the sky is very bright and variable in the infrared, so the dominant source of noise is from the sky. For this reason, you take an image on source followed shortly by an image off source to use to subtract the background sky from the on-source image. As the infrared sky can vary quite quickly, you want to sample the background quite often. If there is a "blank" patch of sky near enough to your target, this is accomplished by "chopping" the secondary mirror back and forth between the source and the off position. This occurs at frequencies on the order of 1Hz, so exposures are less than a second. If you have to move further to find "blank" sky, it is accomplished by "nodding" the telescope, i.e. moving the primary to the off position. This takes a bit longer and may only be done every ~30s.

Because there is active guiding, there is no issue co-adding an arbitrarily large number of these images, and observations can actually be several hours long. And, because the background sky varies fairly rapidly, stacking short exposures (with the background subtracted) will actually give better noise performance than taking long exposures.

That's what I meant by "The length is dictated by the amount of noise in your detector." More noise, and you want longer exposures. Less noise, you'll want short exposures. It's easy to compute in advance, if you've measured the noise in your detector.

Can you explain the cause of the difference between one long exposure and stacking short exposures? I'd naively expect one 10s exposure to produce equivalent data to 10x1s exposures.

What I suspect nawitus is referring to involves the read noise of a CCD. There is always some low level noise involved in reading out a CCD. If this noise dominates, which may be the case for amateur astrophotography, it can explain the noise difference between a single exposure and stacking an equivalent number of shorter exposures.

If for the sake of illustration, the read noise is 10 counts and the signal for a 1 second observation is 10 counts, you will have a signal to noise of 1. Stacking 10 of these will give you a signal to noise of ~3 (square root of 10). Conversely, for the 10 second observation, you will have 100 counts for a signal to noise of 10.

However, there are other sources of noise, and instruments at major observatories are generally designed such that the read noise is not the dominant source of error either through more sensitive detectors or gathering more light (a bigger telescope or larger pixels) or some other means. If even a short exposure constitutes 1000000 counts, the difference between one 10s exposure and ten 1s exposures will be negligible.

Exactly the sort of thing I was looking for. Thanks for the explanation, it makes perfect sense. I hadn't realized that there would be sources of noise that hit you for each read.

Vibration isn't the reason why exposures tend to be short, and at the longer wavelengths SOFIA operates at it's not the dominant source of the image quality. The real reason is that at the infrared wavelengths seen on SOFIA the sky that we're looking through is very bright, and can quickly saturate detectors.

If you're interested in the nitty-gritty (with pictures!) there's a PDF that describes one of the methods: http://www.sofia.usra.edu/Science/instruments/FORCAST_observ...

Edit: More nitty-gritty here: https://www.sofia.usra.edu/Science/ObserversHandbook/FORCAST...

There have been a couple of good answers with regard to exposure time. With regard to vibration, the telescope is floated on an oil bearing and gyro stabilized. It is well isolated from aircraft vibrations.

I imagine some interesting ideas of cabin noise management could be derived from research around this...

My favorite 747! It's the 747SP. I love how short and fat it looks.

Here is one of the runs they did last year: http://flightaware.com/live/flight/NASA747/history/20130717/...

The arches are when they are doing their observations.

Edit: For some reason the map is centering over the Atlantic when I open the link, the flight path is actually from New Zealand if you can't see it to start with.

I wonder how it'll compare to eventually to be launched James Webb Telescope.

At wavelengths which they both observe (~1–28.5 microns; SOFIA goes out to longer wavelengths than JWST), JWST will be far superior in terms of both sensitivity and resolution. For one, the mirror on SOFIA is less than half the diameter of that on JWST ([0],[1], so the resolution is less than half as good and the collecting area is down by more than a factor of four). Even though SOFIA flies above a lot of the infrared-absorbing atmosphere, it still suffers from some absorption. Additionally, the aircraft and the remaining atmosphere emit in the infrared, so the background levels for SOFIA are much higher.

That being said, SOFIA operates at far-infrared wavelengths as well, which JWST will not support. There are satellite missions being propose which would work in the far-infrared; none have been fully approved yet (to my knowledge), but any of them would surpass SOFIA in most metrics (except perhaps spectral resolution).

[0] http://www.sofia.usra.edu/Science/telescope/index.html

[1] http://jwst.nasa.gov/faq_scientists.html#whatis

Indeed, there are a couple of key things to note:

1) JWST has not been launched, yet.

2) SOFIA's wavelength coverage is different that JWST's or anything else that is in the pipeline.

So, really, they are complimentary observatories.

> 2) SOFIA's wavelength coverage is different that JWST's or anything else that is in the pipeline.

Depending on what you mean by "pipeline", that isn't quite true. Work is being done on several far-inrared missions, such as SPICA[0]. I'd consider SPICA to be in "in the pipeline" and it will cover much (if not all) of the far-infrared that SOFIA does. So I wouldn't call JWST and SOFIA complementary; rather, SOFIA is a precursor observatory and a potential technology development platform (for things like SPICA; not so much for JWST, since the JWST instruments are already being built).

[0] https://en.wikipedia.org/wiki/SPICA_telescope

Funding for SPICA is still far from certain, and if it happens, it will likely be toward the end of the SOFIA mission. I would agree, however, that it is appropriate to call SOFIA a precursor to SPICA.

I wonder how much more expensive Herschel would have been, if ESA had gone with an closed-cycle cooler instead of using a limited liquid Helium supply directly.

I'm not sure closed-cycle coolers can get as cold as evaporation of liquid helium? Herschel was cooled to ~1.7 K [0] while MIRI will only be cooled to 7K [1]. Also, with regard to reliability, evaporation of liquid helium is a pretty foolproof design, while closed-cycle systems can potentially break and end a mission early.

[0] https://en.wikipedia.org/wiki/Herschel_Space_Observatory#Ins...

[1] http://jwst.nasa.gov/miri.html

Are the individual instruments cooled to that temperature though? Because if they are not, then the difference doesn't matter.

Had a look at the SPIRE manual [0][1] and it seems that Herschel does include an Helium cooler to cool SPIRE and PACS to 300mK.

On the other hand a closed-cycle system can also extend the mission, since you won't run out of cooling at a fixed point in time.

[0] http://herschel.esac.esa.int/Docs/SPIRE/html/spire_handbook.... [1] http://herschel.esac.esa.int/Docs/Herschel/html/ch02.html

There's a fantastic BBC Horizon documentary called, "Seeing Stars" which includes, among other things, a SOFIA mission: http://www.dailymotion.com/video/x1kacli_bbc-horizon-2011-se...

Opening that door must do abysmal things to the plane's drag coefficient.

As I understand it, opening the door has negligible effect on the performance of the aircraft. The shape of the fuselage is modified to send airflow past the aperture.

  The telescope on board is 10 times as sensitive 
  and has triple the resolution of NASA’s Kuiper 
  Airborne Observatory, originally launched in 
  1975 on a converted C-141 military cargo plane, 
  and decommissioned in 1995. That telescope was 
  the first to spot the rings around Uranus.

"...the rings around Uranus."

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