In summary, we find that observations of the moon could potentially harm the FUSE detectors, so a "safety snap" (SAFTSNP) observation (as described in section 2.2 of the Phase 1 Proposal Instructions ) would probably be required before attempting any scientific observations. Tracking the motion of the moon may not be technically feasible, and certainly requires further development of FUSE's moving target capabilities. Observations at inertially-fixed positions in which the moon is allowed to drift across the apertures is another possible mode of observation, but doing so requires special planning by FUSE ground system personnel. For all these reasons, FUSE lunar observations are unlikely to be executed any earlier than the end of Cycle 1.

Potential Detector Damage

First and foremost, proposers must demonstrate that lunar observations do not violate the FUSE bright object flux limits, which are:

1.0E-10 ergs/cm^2/sec/Angstrom (for continuum) 

3.3E-12 ergs/cm^2/sec (for an emission line integrated over 0.033 A, which
				is the minimum FUSE resolution element)

The above limits cannot be violated anywhere in the FUSE bandpass. Further discussion of the FUSE flux limits can be found in section 4.5.2 of the FUSE Observer's Guide (FOG).

The FOG also contains instructions on how to estimate count rates (section 4.2) and points you to various tools for estimating FUSE's response to your target (e.g., Exposure Time Calculator, spectral simulators, etc).

Given the potentially damaging effects of observing the moon with FUSE, we have already made an initial assessment of the risks. We used absolute intensities of the moon measured by the Hopkins Ultraviolet Telescope (HUT; Henry et al., Ap.J.Lett.,v.454,L69-L72,1995) to scale the solar data from the SUMER experiment on SOHO (Curdt et al.,A&A,v.126,281-296,1997), thereby producing an approximate lunar spectrum at FUSE resolution (Figure 1; click on the figure for an expanded version; click here to download the corresponding 152KB postscript file).

Figure 1. Estimated lunar flux in a 30" x 30" aperture at FUSE spectral resolution. The dashed horizontal line is the nominal FUSE flux limit.

We then ran this simulated spectrum through the FUSE Simulator tool to estimate the count rates for a lunar observation by FUSE (Figure 2; click on the figure for an expanded version; click here to download the corresponding 345KB postscript file).

Figure 2. Estimated total lunar count rate in a 30" x 30" aperture, as output from the FUSE simulator tool using the data displayed in figure 1 as the input spectrum.

The strongest emission in the lunar spectrum is the CIII line at 977.03 A, which has a line-integrated specific intensity of ~2E-03 ergs/cm^2/sec/sr. The Lyman-beta line (1025.722 A) and OVI doublet (1031.924 and 1037.614) are comparably strong and fall in a region where FUSE is intrinsically more sensitive (by a factor of ~3). All of these lines are potentially damaging to FUSE.

It is not clear whether the solar CIII line is spectrally resolved by SUMER. The observed FWHM in the SUMER spectrum we downloaded from the web is 0.2 A, but that spectrum was taken through a 1 arcsec wide slit and may have been slightly blurred by the spatial extent of the emission. In our simulation, we assumed that the width observed by SUMER is the actual linewidth (i.e., the specific intensity is ~1E-02 ergs/cm^2/sec/sr/A). The resolution of SUMER is supposed to be comparable to that of FUSE, but our reading of the SUMER documentation indicates that the FUSE resolving power may be 4 to 6 times higher. Thus, if the solar CIII line is unresolved in the SUMER spectrum, then the intensity at the center of the CIII line could be 4 to 6 times larger than the value assumed here.

We also must recognize that these emission lines vary with the solar cycle, roughly by a factor of two. The HUT observations were made on 3/17/95, which is closer to solar min than solar max. So taking into account solar cycle variability and uncertainty in the intrinsic linewidth, we may be underestimating the lunar flux by a factor of ~10.

Since the moon is an extended object, the flux collected scales linearly with the area of the aperture. Unfortunately, all four of the FUSE apertures will be illuminated simultaneously (there are no shutters for individual apertures), which means that we must consider the flux collected in the 30" x 30" aperture. Given the above assumptions, the peak flux in the CIII line is ~2E-10 ergs/cm^2/sec/A, which is a factor of two above the limit quoted above (the nominal limit is indicated by the dashed horizontal line in figure 1). However, this flux is spread over many pixels relative to the case of a point source having the same flux, so the situation is not quite as bad as it first appears. On the other hand, we also said that the flux could very well be 10 times larger than our estimate, so caution is clearly advised.

Bottom Line : A very short duration (e.g., of order seconds) exposure of the moon probably would not cause significant damage to the instrument. (Note, however, that it could take many minutes for the moon to drift across the FUSE apertures when the spacecraft is trying to maintain an inertially-fixed pointing.) Consistent with FUSE policy regarding all bright targets, observations of the moon could possibly be attempted late in Cycle 1, or in future cycles, if such observations are given scientific approval by the TAC. The additional fact that the moon is an extremely fast-moving target further complicates the situation, as described below.

Rapid Motion

The moon is a very fast-moving object with a huge parallax. The apparent motion of the moon can be as high as ~4"/sec as seen by FUSE, of which only ~0.6"/sec is due to its geocentric motion. Although FUSE can slew at rates of up to 240 arcsec/sec, the design pointing requirement is that FUSE maintain a pointing stability of < 5 arcsec while tracking targets moving slower than 0.2"/sec. Thus, observations that track the motion of the moon are well beyond the current design requirements.

Observations of the moon could possibly be scheduled as fixed target pointings in which the moon passes across the FUSE field of view at a specified time. However, the planning of such an observation would require intensive manual effort as a special project.

The FUSE project is currently focussing on the support of "routine" operations, with the hope that more complex observations (including those of moving targets) will be enabled as soon as possible after simple observations become routine.

Since the PI team (including the PI himself, Warren Moos) has a strong interest in solar system astronomy, we anticipate that limited support of relatively simple moving target observations will be available during the latter part of the first year of operations (i.e., Cycle 1). Further discussion of FUSE's support of moving targets can be found in section 4.5.4 of the FOG.

In light of the above discussion, we cannot guarantee that observations of the moon will be possible during Cycle 1. However, if a proposal specifying the moon as one of its targets is accepted by the scientific peer review, we would work toward developing the necessary ground-system capabilities for executing lunar observations, assuming that we could demonstrate that the flux levels do not damage the FUSE detectors.

Solar Elongation Angle Constraints

During Cycle 1, FUSE can only observe targets having solar elongation angles that lie in the following range:

75 deg <= solar elongation angle <= 165 deg

This means, for example, that FUSE could not observe within +/- 1 day of the time of full moon.

Minimum Total Exposure Time

Proposals for Cycle 1 must request a minimum on-target exposure time of 10,000 sec. So if your program requires less than 10,000 sec, you must either identify other objectives that push your requested time up, or you must "piggyback" on a related scientific program. The FUSE project has imposed this 10 ksec total exposure time constraint in order to limit the total number of programs that have to be processed.

In addition, any individual observations within a program that require less than 2000 sec will still be charged 2000 sec because of the overhead associated with short exposures.

Please read section C.1.1 of Appendix C of the FUSE NRA for further discussion of the latter two points and for a general overview of the FUSE GI program.

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