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

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

#21

Post by aunty_helen »

goodcanadian wrote: post #8
netcraft wrote: post #5

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.

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.

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

#22

Post by rbanffy »

goodcanadian wrote: post #14
nawitus wrote: post #7

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...

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...

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

#23

Post by goodcanadian »

aunty_helen wrote: post #21
goodcanadian wrote: post #8

Earlier quoted context omitted.

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.

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.

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

#25

Post by nawitus »

greglindahl wrote: post #10
nawitus wrote: post #7

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.

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

#26

Post by goodcanadian »

rosser wrote: post #24

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.

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

#27

Post by goodcanadian »

nawitus wrote: post #25
greglindahl wrote: post #10

Earlier quoted context omitted.

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.

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

#28

Post by goodcanadian »

goodcanadian wrote: post #27
nawitus wrote: post #25

Earlier quoted context omitted.

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.

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

#29

Post by greglindahl »

nawitus wrote: post #25
greglindahl wrote: post #10

Earlier quoted context omitted.

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.

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.

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

#30

Post by Serow225 »

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.
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