The MAMA detectors are subject to catastrophic damage at high global and local count rates and cannot be used to observe sources that exceed the defined safety limits. Specifically, charge is extracted from the microchannel plate during UV observations, and overillumination can cause a decrease of quantum efficiency in the overexposed region, or even catastrophic failure if excess gas generation from the microchannel plates causes arcing in the sealed tube.
To safeguard the detectors, checks of the global (over the whole detector) and local (per pixel) illumination rates are automatically performed in flight for all MAMA exposures. The global illumination rate is monitored continuously; if the global rate approaches the level where the detector can be damaged, the high voltage on the detector is automatically turned off. This event can result in the loss of all observations scheduled to be taken with that detector for the remainder of the calendar (~1 week). The peak local illumination rate is measured over the MAMA field at the start of each new exposure; if the local rate approaches the damage level, STIS will shutter, and the exposure will be lost.
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Sources that would over-illuminate the MAMA detectors cannot be observed. It is the responsibility of the observer to avoid specifying observations that exceed the limits described below. |
To ensure the safety of the MAMA detectors and the robustness of the observing timeline, we have established observational limits on the incident count rates. Observations that exceed the allowed limits will not be scheduled. The definitive guidelines for bright object limits are given in STIS ISR 2000-01, but the following brief discussion is included here for convenience. The allowed limits are given in Table 7.4, which includes separate limits for nonvariable and irregularly-variable sources. The global limits for irregularly variable sources are a factor 2.5 more conservative than for sources with predictable fluxes. Predictable variables are treated as nonvariable for this purpose. Examples of sources whose variability is predictable are Cepheids or eclipsing binaries. Irregularly variable sources are, for instance, cataclysmic variables or AGN. Here and in general, "pixel" refers to the 1024x1024 format.
| Target |
Limit Type |
Mode |
Channel |
Screening Limit |
|---|---|---|---|---|
| Nonvariable |
Global |
All modes other than 1st-order spectroscopy |
FUV and NUV |
200,000 c/s1 |
| Nonvariable |
Global |
1st-order spectroscopy |
FUV and NUV |
30,000 c/s |
| Nonvariable |
Local |
Imaging |
FUV and NUV |
100 c/s/p2 |
| Nonvariable |
Local |
Spectroscopy |
FUV and NUV |
75 c/s/p |
| Irregularly Variable |
Global |
All modes other than 1st-order spectroscopy |
FUV and NUV |
80,000 c/s3 |
| Irregularly Variable |
Global |
1st-order spectroscopy |
FUV and NUV |
12,000 c/s 3 |
| Irregularly Variable |
Local |
Imaging |
FUV and NUV |
100 c/s/p 3 |
| Irregularly Variable |
Local |
Spectroscopy |
FUV and NUV |
75 c/s/p 3 |
| 1c/s is counts sec-1. 2c/s/p is counts sec-1 pix-1. 3Applies to the phase when the target is brightest. |
As a first step, you can check your source V magnitude and peak flux against the bright-object screening magnitudes in Table 13.44 or Table 14.39 for your chosen observing configuration. In many cases, your source properties will be much fainter than these limits, and you need not worry further.
However, if you are near these limits (within 1 magnitude or a factor of 2.5 of the flux limits), then you need to carefully consider whether your source will be observable in that configuration. Remember the limits in these tables assume zero extinction and for spectroscopic observations do not include slit losses. Thus you will want to correct the limits appropriately for your source's reddening and the aperture throughput.
You can use the information presented in Section 6.2 to calculate your peak and global count rates. Perhaps better, you can use the STIS Exposure Time Calculators available through the STIS World Wide Web site to calculate the expected count rate from your source. They have available to them a host of template stellar spectra. If you have a spectrum of your source (e.g., from IUE, FOS, or GHRS) you can also input it directly to the calculators. The calculators will evaluate the global and per pixel count rates and will warn you if your exposure exceeds the absolute bright-object limits. We recommend that you use the STIS ETCs if you are in any doubt that your exposure may exceed the bright-object MAMA limits.
You should also be aware that when a short (~300 ms) exposure is taken, in order for the local rate monitor to check whether the bright object limit is violated, the actual check does not involve a real measurement of the maximum flux per pixel. Instead, the obtained short-exposure image is binned into "superpixels," each one with a size of 8x8 (imaging) or 4x8 (spectroscopy) regular (low-res) pixels, and the resulting measured flux for each superpixel is transformed into a peak flux per pixel, assuming that a single isolated point source contributes to the flux in that bin. Therefore, you should be extra careful when observing a crowded field or a slightly resolved source in imaging mode, since it is possible for the exposure to be aborted even when no single source violates the local rate limit (e.g., two or more stars fall inside the same bin or a source with a non-point-source radial profile is present in the field). See STIS ISR 96-31 for more details.
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It is the observers' responsibility to ensure that their observations do not exceed the bright-object count limits stated in Table 7.4. |
It is your responsibility to ensure that you have checked your planned observations against the brightness limits prior to proposing for Phase 1. If your proposal is accepted and we, or you, subsequently determine (in Phase 2), that your source violates the absolute limits, then you will either have to change the target, if allowed, or lose the granted observing time. We encourage you to include a justification in your Phase 1 proposal if your target is within 1 magnitude of the bright-object limits for your observing configuration. For MAMA target- of-opportunity proposals, please provide an explanation of how you will ensure that your target can be safely observed in your Phase 1 proposal.
STScI will screen all STIS observations that use the MAMA detectors to ensure that they do not exceed the bright-object limits. In Phase 2, you will be required to provide sufficient information to allow screening to be performed.
Here we describe the required information you must provide.
To allow screening of your target in Phase 2 for spectroscopic MAMA observations you must provide the following for your target (i.e., for all sources which will illuminate the detector during your observations):
If you wish to observe a target that comes within one magnitude (or a factor of 2.5 in flux) of the limits in the spectroscopic bright-object screening table ( Table 13.44) for your configuration, after correction for aperture throughput and reddening, but which you believe will not exceed the absolute limits in Table 7.4 and so should be observable, you must provide auxiliary information to justify your request. Specifically:
The MAMA imaging bright-object screening magnitudes ( Table 14.39) are very stringent, ranging from V = 15 to V = 20.5 for the different imaging apertures, and apply to all sources imaged onto the MAMA detector (i.e., not just the intended target of interest). Table 14.39 can be used to determine if the target of interest is above the bright-object limit. Since the start of Cycle 9, STScI has been using the second-generation Guide-Star Catalog (GSC II) to perform imaging screening for objects in the field of view other than the target itself. The GSC II contains measurements from photometrically calibrated photographic plates with color information for magnitudes down to at least V = 22 mag. This information will be used to support bright-object checking for fixed and for moving targets (major planets). STScI will make a best effort to perform the imaging screening using GSC II. However, observers should be prepared for the possibility that under exceptional circumstances GSC II may be insufficient. For instance, fields close to the Galactic plane may be too crowded to obtain reliable photometry. If for any reason the screening cannot be done with GSC II, the observer is responsible for providing the required photometry. In the case of moving targets, STScI will identify "safe" fields, and the observations will be scheduled accordingly. Observers will then be updated on the status of their observations. We anticipate that bright-object considerations will not have a significant effect on the scheduling of such observations.
Pointings close to objects violating safety limits must be screened since (i) the possibility of HST pointing errors exists, and (ii) the light of a bright point source may pose a safety threat even if observed at a distance of several arcsec.
The typical HST pointing accuracy is about 0.5 arcsec, but cases have been observed when HST was pointing 1 - 2 arcsecs off its expected position. This results from some guide stars having less accurate coordinates or because they are not single. STScI will perform a screening of not only the targets in the field of view themselves (spectroscopy and imaging modes), but also of targets within 5 arcsec of the boundaries of the used apertures (full field of view of the MAMA detector for imaging and slitless spectroscopy, field covered by slit for spectroscopy). If objects are found that would exceed the Bright Object Protection limit for the particular instrument configuration, the observations will not be executed.
Targets or field objects falling in an annular region extending from 5 to 13.5 arcsec from the edge of the aperture used in a MAMA observation also have additional restrictions. Any object in this zone producing either a real global count rate in excess of 1.5x106 counts s-1 or a local count rate greater than 500 counts sec-1 pixel-1 is not permitted. See ISR 2000-01 for a discussion of the new screening procedures.
If your source passes screening, but causes the automatic flight checking to shutter your exposures or shut down the detector voltage causing the loss of your observing time, then that lost time will not be returned to you; it is the observer's responsibility to ensure that observations do not exceed the bright-object limits.
If your source is too bright for one configuration, it may be observable in another configuration (e.g., in a higher-dispersion configuration). The options open to you if your source count rate is too high in a given configuration include:
G230LB and G230MB modes (see Section 4.1.7).
For further advice, see Section 12.4.
Observations of planets with STIS require particularly careful planning due to the very stringent overlight limits of the MAMAs. In principle Table 13.44 and Table 14.39 can be used to determine if a particular observation of a solar-system target exceeds the safety limit. In practice the simplest and most straightforward method of checking the bright object limits for a particular observation is to use the STIS Exposure Time Calculator. With a user-supplied input spectrum, or assumptions about the spectral energy distribution of the target, the ETC will determine whether a specified observation violates any bright object limits.
Generally speaking, for small (<~0.5-1 arcsec) solar-system objects the local count rate limit is the more restrictive constraint, while for large objects (>~1-2 arcsec) the global limit is much more restrictive.
As a first approximation, small solar system targets can be regarded as point sources with a solar (G2 V) spectrum, and if the V magnitude is known, Table 13.44 and Table 14.39 can be used to estimate whether an observation with a particular STIS grating or filter is near the bright-object limits. V magnitudes for the most common solar-system targets (all planets and satellites, and the principal minor planets) can be found in the Astronomical Almanac. This approximation should provide a conservative estimate, particularly for the local limit, because it is equivalent to assuming that all the flux from the target falls on a single pixel, which is an overestimate, and because the albedos of solar-system objects are almost always < 1 (meaning that the flux of the object will be less than that of the assumed solar spectrum at UV wavelengths where the bright-object limits apply). A very conservative estimate of the global count rate can be obtained by estimating the peak (local) count rate assuming all the flux falls on one pixel, and then multiplying by the number of pixels subtended by the target. If these simple estimates produce numbers near the bright-object limits, more sophisticated estimates may be required to provide assurance that the object is not too bright to observe in a particular configuration.
For large solar-system targets, checking of the bright-object limits is most conveniently done by converting the integrated V magnitude (Vo, which can be found in the Astronomical Almanac) to V magnitude/arcsec2 as follows:
V / arcsec2 = Vo - 2.5 log(1/area)
where area is the area of the target in arcsec2. This V / arcsec2 and the diameter of the target in arcsec can then be input into the ETC (choose the Kurucz model G2 V spectrum for the spectral energy distribution) to test whether the bright- object limits can be satisfied.
Alternatively, an observed spectrum obtained with a known slit size can be used as input to the ETC. Most calibration techniques produce units of flux (e.g., ergs sec-1 cm-2 Å-1), even for extended targets. Such a calibration implicitly assumes a flux per solid angle (i.e., the angle subtended by the observing slit or object, whichever is smaller), and it is more appropriate to convert to units of surface brightness (ergs sec-1 cm-2 Å-1 arcsec-2) by dividing the calibrated flux by the appropriate area (slit size or object size, whichever is smaller). If such a spectrum is available, it can be immediately examined and compared with the local limit in units of surface brightness given in Table 13.44 and Table 14.39, or passed to the ETC as a user-supplied spectrum. It can also be easily converted to counts sec-1 pix-1 by using the diffuse-source sensitivities for the appropriate grating or filter provided in this Handbook. Note that the sensitivities in this Handbook assume a specific slit width, so they need to be scaled by the desired slit width. The ETC provides another check of the local limit: if the peak count rate per pixel exceeds the local limit of 75 (for spectroscopic observations) or 100 (for imaging observations) counts sec-1 pix-1, such an observation would not be allowed. The global limit can be checked by summing the count rate per pixel over wavelength, and multiplying by the desired slit length (in arcsec) divided by the pixel size (0.0247 arcsec) to produce total counts per second for the observation. If this number is larger than the appropriate global limit, the observation should not be performed because it will cause the instrument to enter safe mode. For such cases, a smaller slit size or higher-resolution grating could then be considered.
To further aid the observer, we provide results of test simulations with Jupiter and Saturn; the input spectra are observed surface fluxes of Jupiter and Saturn from 1200 Å to 7000 Å. Jupiter and Saturn have corresponding visual magnitudes of V = 4.9 and 6.5 mag arcsec-2, respectively. No variation of the surface brightness over the disk is taken into account. We assume that both planets have diameters >25 arcsec, i.e., exceeding the field size of the MAMA detectors. This assumption is appropriate for Jupiter and conservative for Saturn.
We adopt slit lengths of 6 and 25 arcsec for echelle and first-order grating observations, respectively. Slit widths are 1 arcsec in both cases. The slits are centered on the planets so that the planets overfill the apertures. The results for the local limits are in Table 7.5. The table gives the observed V magnitudes arcsec-2 of the two planets and the limiting magnitudes for all echelle and first-order gratings. If the planet were as bright as the limiting magnitude, it would reach the limiting local count rates (75 counts sec-1 pix-1 for the FUV and NUV). No safety margin was added to the magnitudes listed in this and the subsequent tables. Table 7.5 suggests that only Jupiter observed with G230L would come close to the local brightness limit.
Next we discuss the global limits. The global limits for echelle spectroscopy are in Table 7.6. As for point sources, they are determined by the screening limit count rate of 200,000 counts sec-1. Both Jupiter and Saturn are fainter than the limit for the FUV echelles and brighter for the NUV echelles. Recall that a slit length of 6 arcsec was adopted, with the planets filling the aperture. This assumption makes the brightness limits 1.9 mag (2.5 x log 6) more stringent than for a point source having the same spectrum.
| |
Observed |
E140M |
E140H |
E230M |
E230H |
|---|---|---|---|---|---|
| Jupiter |
4.9 |
-0.6 |
-1.2 |
10.1 |
8.9 |
| Saturn |
6.5 |
-1.8 |
-2.7 |
9.9 |
8.7 |
The results for first-order spectroscopy are in Table 7.7. Because we assumed that both Jupiter and Saturn completely fill the 25 arcsec long slit, the limiting magnitude is determined by the global screening limit value of 200,000 counts/sec. This situation differs from that of point-source observations where the limiting magnitude is usually determined by the count rate per pixel or by the total number of counts along the dispersion direction. For extended objects, the total number of counts accumulated over the whole detector sets the limit. Note that the limits for the two planets are always set by the global, and not by the local rates. As was the case for echelle spectroscopy, both Jupiter and Saturn are fainter than the limit for the FUV and brighter for the NUV gratings.
| |
Observed |
G140L |
G140M |
G230L |
G230M |
|---|---|---|---|---|---|
| Jupiter |
4.9 |
1.4 |
0.0 |
12.9 |
10.1 |
| Saturn |
6.5 |
1.3 |
-1.0 |
12.8 |
9.9 |
The examples demonstrate that careful choices of slit sizes and neutral-density filters are required to prevent instrument damage.
As is the case with spectroscopy, the global limit is much more restrictive than the local limit, and careful simulations are required to determine which filters can be safely used for imaging.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |