Spitzer Space Telescope

I don't know just how to word this, but does anyone know why the
Spitzer IR Telescope is such a crappy telescope compared to Hubble?
Maybe it has to do with the wavelenght of IR light.

Sure, it's in space, and it's CCD cam is really, really cold. But, I
mean, I could build something less than a meter wide with a 256 x 256
imager in my backyard.

Was it a $$$ thing?

=[ d

In article , "DavidBowman" wrote:
I don't know just how to word this, but does anyone know why the
Spitzer IR Telescope is such a crappy telescope compared to Hubble?
Maybe it has to do with the wavelenght of IR light.

It has a lot to do with that.

Sure, it's in space, and it's CCD cam is really, really cold. But, I
mean, I could build something less than a meter wide with a 256 x 256
imager in my backyard.

But could you build it with a mid-IR detector? Remember, Spitzer is not
using a CCD.

Ground-based Mid-IR astronomical instrumets typically use 256^2 or 320*240
arrays. That's the technology that's currently available and your typical
digital camera that has a huge number of pixels simply can't detect light
at mid-IR wavelengths.

Raytheon is currently the company that supplies most of the mid-IR arrays
for astronomy and they don't have arrays larger than than a few hundred
pixels squared. You can search their website to confirm this. I'm sure
that will change in 3 or 4 years time, but that's where we are at the
moment. The military may well have acess to larger mid-IR arrays right
now, but astronomers only get their hands on those arrays a few years
later.

I'm being lazy and don't know off the top of my head where Spitzer's array
came from, but it almost certainly isn't the latest one available because
the whole instrument had to be designed, necessarily, several years ago.

Also, please remember that in order for these arrays to work you need to
be able to cool them to 15K or less. I'd be more than interested to hear
how you'd do that in your back yard.

Raytheon is currently the company that supplies most of the mid-IR
arrays
for astronomy

Well, I doubtn there's a huge market

and they don't have arrays larger than than a few hundred pixels
squared. You can search their website to confirm this.

Well, I certainly believe you! Do you have any idea why IR photons are
so hard to detect, or why the arrarys are so small? They have more
energy than microwave photons.

=[ d

In article , "DavidBowman" wrote:

Well, I certainly believe you! Do you have any idea why IR photons are
so hard to detect, or why the arrarys are so small? They have more
energy than microwave photons.

Yes, they do, but I'm curious as to why you've brought up microwaves. Is
there a microwave array or detector that has more pixels than your
run-of-the-mill mid-IR array?

In message , Tom Kerr
writes
In article , "DavidBowman"
wrote:
I don't know just how to word this, but does anyone know why the
Spitzer IR Telescope is such a crappy telescope compared to Hubble?
Maybe it has to do with the wavelenght of IR light.

It has a lot to do with that.

Sure, it's in space, and it's CCD cam is really, really cold. But, I
mean, I could build something less than a meter wide with a 256 x 256
imager in my backyard.

But could you build it with a mid-IR detector? Remember, Spitzer is not
using a CCD.

Ground-based Mid-IR astronomical instrumets typically use 256^2 or 320*240
arrays. That's the technology that's currently available and your typical
digital camera that has a huge number of pixels simply can't detect light
at mid-IR wavelengths.

Off topic comment, but that's twice the size of the CCD that has just
worked at Titan :-) I wonder what NASA paid ten years ago.


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From: (Steve Willner)
Subject: Spitzer Space Telescope
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In article ,
"DavidBowman" writes:
I don't know just how to word this, but does anyone know why the
Spitzer IR Telescope is such a crappy telescope compared to Hubble?
Maybe it has to do with the wavelenght of IR light.

If you mean in angular resolution, wavelength is much of the story.
The shortest Spitzer wavelength is roughly ten times the shortest
Hubble wavelength, so equal angular resolution requires a primary
mirror ten times larger than Hubble's. That wasn't in the budget.

More generally, Spitzer has been a heavily cost-constrained mission
throughout. Spitzer cost to launch (plus checkout) was something
like $650M in real year dollars. (Think 2001 or so for a fair
average.) Hubble cost to launch was $2000M in 1985 or so dollars.
Quite a difference. Chandra, by the way, was in the same ballpark as
Hubble. I'm not sure what Compton cost.

The choice of Spitzer primary mirror diameter (85 cm) actually goes
back to the original mission concept, which was a shuttle-attached
payload. (That would have been an amazingly poor idea, but in the
early 1980's the shuttle was the only politically acceptable solution
to any problem.) There were many design iterations after that, but
the pressure was to keep cost and therefore mirror diameter down. In
retrospect, I'm a bit surprised the mirror diameter remained at the
full 85 cm, but the Science Working Group hung tough despite
considerable pressure to accept a smaller mirror. Instrument
capabilities were sacrificed instead, but I think that was the right
tradeoff.

Despite its small arrays, Spitzer's sensitivity still enables it to
map large areas pretty quickly. The extreme example is the GLIMPSE
Legacy Project ( http://www.astro.wisc.edu/sirtf/ ), which has imaged
220 square degrees in only 400 hours of Spitzer time. Hubble could
never compete with that.

Sure, it's in space, and it's CCD cam is really, really cold. But, I
mean, I could build something less than a meter wide with a 256 x 256
imager in my backyard.

As others have mentioned, the detectors are not CCD's, and they have
to be a lot colder than visible-light detectors. The difficulty
building them is partly related to the lower energy of infrared
photons compared to visible photons and partly related to smaller
market demand, hence less money invested in development. The whole
telescope has to run below 5.5 K, and some of the detectors run
colder than that (near 1.3 K, in fact). This was quite a challenge!
The "passive cooling" technique was one of Spitzer's technical
innovations, but testing was a major headache, and nobody knew for
sure it would work until after launch. (It does work; current
lifetime projections are very near pre-flight estimates. I am
amazed.)

Even if larger arrays had been available, it would have been
difficult to fit them into the focal plane without major design
changes. See the diagram at the bottom of
http://ssc.spitzer.caltech.edu/obs/overview.html
to get an idea how crowded the Spitzer focal plane is and the picture
at
http://cfa-www.harvard.edu/irac/
for an as-built image. (That black circle in the middle of all the
instruments is the focal plane area.) Expanding the focal plane
would have required different optics and lots of other design
changes, all tending to make things heavier and more expensive, never
mind the additional costs of the arrays themselves. More money
could have solved a lot of problems, but that was not an option.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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