For solar physicists, a complete picture of the sun can never be made by merely taking images.  In taking a basic optical image, an unbelievable wealth of information slams into the detector, just waiting to be picked apart by a curious physicist.  Spectroscopy (the process of breaking apart and studying the individual wavelengths of the sun's energy) is one of physicists' greatest tools in studying the sun.  But one of the nagging difficulties of solar spectroscopy lies in obtaining co-temporal images of a full-disk sun.
    To illustrate, a normal slit spectrograph will only take pictures of the sun in "slices."  Say, for example, that an instrument's imaging slit is oriented along the y-axis.  With this arrangement, a single image will provide a picture of the sun in "y" and "lambda" coordinates.  In other words, we could image how the sun's intensity varies with vertical position and wavelength.  The drawback here is that this image only corresponds to one spot on the x-axis.  In order to get a full-disk image, the instrument must be swept laterally across the sun's surface, taking images at different x-locations until information about the entire disk is obtained.  This is a problem.  Why?  Because this process of sweeping across the sun takes time.  And in the time it takes to image the sun from one side to other, some of the features of the sun that were imaged first will have changed in the time it took to get to the opposing side of the sun.  Thus, the images are not co-temporal.  Ideally, we'd like a way to control the time element, such as making the sweep time approach zero.  This isn't very practical.  So instead, another solution is to eliminate the need for "sweeping" altogether.  This can be done by using a multi-order slitless spectrograph.   Using this technique, physicists can get a picture of how the sun's intensity varies as a function of x, y, wavelength, and time--or I(x, y, lambda, t) .
    But this is not to say that interpretation of this data is easy.  With a slit-spectrograph, the only thing that has to be dealt with is a series of parallel emission lines.  But with the slitless spectrograph, a series of whole "suns" is obtained.  At the center of the instrument detector, there will be a zeroeth diffraction order sun image.  With this image, there will be no dispersion, and all wavelengths will combine in intensity in a straightforward fashion.  But away from the center, you will find other diffraction orders (ie -2, -1, +1, +2, etc.).  With, say, the n = +1 order, there will be one complete sun to show up on the CCD camera for EACH emission line.  As you might guess, this can lead to some very complicated overlapping. Any adjacent lines (or nearly adjacent lines, for that matter) may have their data combining on many of the pixels.  Here is an illustration of the technique used by the Skylab mission:

Figure 1:  A skematic of the instrument used inthe Skylab mission.  Such an arrangement produced images such as that seen below.

Figure 2:  +1 order slitless spectrogram from the Skylab mission.

    In Figure 2, the bright solar image to the right of center is He II emission.  There are several "suns" to the left of this, each representing different wavelengths.  Some of them are quite difficult to pick out, especially the Si XI sun, which has its left limb JUST left of the He II disc.  It is quite clear how the interpretation of all this data might be rather difficult.  It's not always going to be obvious which photons belong to each wavelength!
    As the slitless spectrograph has been used on solar missions before, such as Skylab, it will be used again on the MOSES mission (Multi-Order Solar EUV Spectrograph).  However, there are a couple of key differences between MOSES and those missions that have come before it.  On Skylab's slitless spectrograph, the passband (range of detected frequencies) was quite large.  As a result, there were dozens of "suns" on the each image. For that reason, the instrument was sometimes described as the "overlappograph".  Further, only the +1 order was imaged--see Figure 1.  Actually, the passband is so large such that there is some +2 order data is mixed up in it.  With MOSES, the -1, 0, and +1 orders will be imaged.  Further, the passband will be narrow (about 21 Angstroms, centered at 304 Angsroms). The only two significant emission lines in this region will be Si XI and He II (303.3 and 303.8 Angstroms, respectively).  The main line of study for the MOSES mission will be the He II 304 Angstrom line.  Here is a conceptual diagram of the MOSES instrument:

Figure 3:  A skematic of the MOSES mission data collection.

    The main focus of study for the MOSES mission will be the He II data.  The proposed mechanism for the He II emission line is "collisional excitation."  If this is true, solar physicists will expect to see a Gaussian profile for all of the He II emission (or at least a Gaussian core).  And they would also expect to see "upflows" in the brightest of surface elements.  Also, the relationship between HeII intensity and doppler shift is not yet well understood.  Hopefully, physicists will gain some insight into this question.   And while HeII intensity and line width are already understood to be inversely related, it is hoped that this correlation can be nailed down with greater accuracy.

This page was originally composed by Mark Reiser, an REU student who worked for the MOSES group at Montana State University during Summer, 2001.

The MOSES team includes physics graduate students Melissa Cirtain and Lewis Fox; engineering students Will Roesch, Dustin, Lindsay, ; and principal investigator Charles Kankelborg.