More information on Spitzer operations

It might seem, if you're just using Spitzer data (as opposed to planning observations), that you don't need to know much about Spitzer operations. But, it turns out that you do need to know a little bit about how Spitzer works in order to understand why the data look the way they do.

Spitzer is a robot

Spitzer is a robot, designed to operate autonomously for 12-24 hours at a time. We at the SSC prepare schedules for the telescope, and this is done for a whole week at once. We then upload the schedule (in whole or in part) up to the spacecraft. The telescope then goes about taking data all by itself. Every 12 or 24 hours (whatever we've told it to do), it "calls home", downloads the data and gets any new instructions. We use the Deep Space Network (DSN) to communicate with Spitzer; pretty much every other spacecraft not in Earth orbit is also competing for this limited resource. We have to plan out well in advance when we will be able to use certain dishes as part of the DSN. When there is a special event, such as an "orbital insertion" (when a spacecraft goes into orbit around another planet), then that event takes priority over all others, and we have to rearrange our uplink/downlink times accordingly.

This operational constraint is why we can't drop everything and make an observation of whatever you want right this second. This also means that there is NOT someone sitting with a joystick (or joystick-like-thing) at the SSC or at JPL, commanding the spacecraft in real time. All of the observations have to be planned in great detail at least 2 months in advance, usually more in advance than that. This is one way that observing from space is very different than observing from the ground.

Spitzer has many instruments, each with different fields of view

Spitzer has three different science instruments: IRAC, the InfraRed Array Camera, IRS, the InfraRed Spectrograph, and MIPS, the Multiband Imaging Photometer for Spitzer. (Here is the scientist-focused overview page with a high-level summary of each of the instruments.)

Only one of the instruments is on at a time, so that means that you can't do simultaneous observations with, say, MIPS and IRAC. However, when one of them is on, for the most part, all pieces of it are on at the same time, and you can't turn a piece off. This still doesn't mean that you can do simultaneous observations using different pieces of a given instrument, for the following reason.

There is only one moving part on the whole spacecraft (and that is the MIPS scan mirror). We do not have, as might be found on ground-based telescopes, a single field of view with, say, a pick-off mirror that rotates to feed light down different instruments. Instead, for a given pointing of the spacecraft, each camera within each instrument sees a slightly different piece of the sky. This figure has a picture of Spitzer's focal plane, e.g., all those fields of view projected onto the sky.

So, what does this mean in practice? Let's walk through an IRAC observation. IRAC has four cameras, and two fields of view (the two large blue squares in the figure linked above). Each one of IRAC's fields of view has a special kind of optics component called a beamsplitter, which reflects some wavelengths of light and allows other wavelengths to pass through. One IRAC field of view is shared by the 3.6 micron and 5.8 micron cameras (IRAC channels 1 and 3) -- the beamsplitter sends the 3.6 micron photons one way and the 5.8 micron photons another way. The other IRAC field of view is shared by the 4.5 and 8 micron cameras (IRAC channels 2 and 4). If you construct an observation that asks the spacecraft to observe a certain target in just 3.6 microns, then the telescope will slew to center that target in the 3.6 micron field of view. Because you can't turn off the other three IRAC cameras, you will also get data in those other three channels "for free." The 5.8 image will also be centered on the target you asked for, because it sees the same target as the 3.6 micron camera. The 4.5 and 8 micron cameras both see a different field of view, offset from the target you asked for. The edges of the two IRAC fields of view are separated by about 1.5 arcminutes. The direction of the offset is set by the time and location (in the sky) of the observation.