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.
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 7.8, Absolute MAMA Count Rate Limits
, which includes separate limits for non-variable 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 non-variable 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 1024 ×
1024 format (low-res pixels).
You can use the information presented in Section 6.2
to calculate your peak and global count rates. More conveniently, you can use the STIS ETC
to calculate the expected count rate from your source. They have available to them a host of template stellar spectra. If you have a UV spectrum of your source 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.
You should also be aware that the local rate monitor does not perform a measurement of the actual flux per pixel. Instead, the check image is binned into ‘superpixels’, each one with a size of 8 ×
8 (imaging) or 4 ×
8 (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 1996-31
for more details.
It is your responsibility to ensure that you have checked your planned observations against the brightness limits prior to proposing for Phase I. If your proposal is accepted and we, or you, subsequently determine (in Phase II), that your source violates the absolute limits, then you will either have to: a) choose a different configuration, if possible, b) change the target, if allowed, or c) lose the granted observing time. We encourage you to include a justification in your Phase I 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 or strategy of how you will ensure that your target can be safely observed in your Phase I proposal. The observing strategy might include additional observations, obtained over a time-scale appropriate to the particular type of object, with either HST or ground-based telescopes. Proposers should be aware that this type of observation requires extra resources. STScI reserves the right to limit the number of visits requiring quiescence verification observations within 20 days or less of an HST observation to no more than 12 such visits per Cycle.
STScI has developed bright object tools (BOT
) to conduct detailed field checking prior to MAMA program implementation. These tools are based on automated analysis of the fields by means of data from the second Guide Star Catalogue (GSC2) and displays of the Digital Sky Survey (DSS). GSC2 provides two magnitudes (photographic J
), hence one color, for most fields down to about 22nd mag, which, combined with conservative spectral type vs. color relationships, supports determinations of safety or otherwise for individual objects. In the best cases, these procedures allow expeditious safety clearing, but in some cases the GSC2 is inadequate because of crowding or absence of one of the filters, for instance. Then supplementary information must be provided by the proposers to support the bright object protection (BOP) process. The target should always be checked directly in the ETC
with the more detailed information generally available for it, rather than relying on its field report data.
STScI will check all targets and fields before any MAMA observations are cleared. However, by policy GOs must provide screened, safe targets for MAMA programs, and supplementary data as needed to verify target and field safety. The APT/BOT
, including an Aladin
interface, makes the BOP procedures accessible for GO use. Extensive help files and training movies are available. While the procedures may appear complex on first exposure, their convenience and straightforward application rapidly become apparent. All MAMA proposers must conduct BOP reviews of their targets and fields in conjunction with their Phase II preparations. Thus, they will become aware of any problems earlier, such as the need for supplementary data, which may otherwise entail lengthy implementation delays following the Phase II deadline. (An exception is moving target fields, which must be cleared after the scheduling windows have been established.) To assist with these procedures, a Contact Scientist (CS) who is a MAMA/BOP specialist will be assigned to each MAMA program, to interact with the GO as necessary and requested during the Phase II preparations, and through program execution.
Briefly, for a single default MAMA pointing with unconstrained orientation, a circular field including a buffer around the rotated aperture must be cleared. The APT/BOT
automatically reports on all GSC2 stars or GALEX sources within that field. If any displacements from the default pointing (e.g., POS TARG
s, patterns, or mosaics) are specified, the field to be cleared increases commensurately. POS TARG
vectors and the enlarged, rotated field circles are conveniently displayed in APT/Aladin
. No unsafe or unknown star may lie within 5 arcseconds of the detector edge at any orientation (or 13.5 arcseconds for very bright sources, see below). Conversely, POS TARG
s and orientation restrictions may be introduced to avoid bright objects in the fields.
A MAMA GO must send to the designated CS, by the Phase II deadline, the ETC
calculations for each discrete target, and reports on any unsafe or unknown stars from APT/BOT
for each field, either showing that the observations are in fact safe, or documenting any unresolved issues. In the latter case, including inadequacy of BOT
/GSC2/GALEX to clear the observations, other photometric or spectroscopic data sources must be sought by the GO to clear the fields. Many of these are available directly through the APT/Aladin
interface (although automatic BOP calculations are currently available only with GSC2 and GALEX), including the STScI Multi-mission Archive (MAST), which contains the IUE as well as HST data. An existing UV spectrogram of the target or similar class member may be imported directly into the ETC
; IUE data must be low resolution, large aperture for BOP. If model spectra are used, the original Kurucz models for early-type stars in the ETC
should be used rather than the more recent Castelli & Kurucz. None of the provided models is adequate for stars later than the Sun, since they lack chromospheric emission lines that dominate the actual FUV flux for these stars; actual UV data must be used for them. In worst cases, new HST observations in safe configurations or ground based data may be required to clear the fields for BOP; in general, the former must be covered by the existing Phase I time allocation.
If a given star has only a V
magnitude, it must be treated as an unreddened O5 star. (The older Kurucz O5 model with higher Teff
in the ETC
should be used for BOP purposes.) If one color is available, it may be processed as a reddened O5 (which will always have a greater UV flux than an unreddened star of the same color). If two colors are available, then the actual spectral type and reddening can be estimated separately. The APT/BOT now clears automatically stars with only a single GSC2 magnitude, if they are safe when assumed to be unreddened O5 stars. Any other “unknowns” must be cleared explicitly.
In some cases, the 2MASS JHK may be the only photometry available for an otherwise “unknown” star. It is possible to estimate V
from those data on the assumption of a reddened O5 star, and thus determine its countrates in the ETC. F. Martins & B. Plez, A&A, 457, 637 (2006), derive (J−H)0
0.11 for all O stars; and (V−J)0
0.79 for early O types. (The K
band should be avoided for BOP because of various instrumental and astrophysical complications.) M.S. Bessell & J.M. Brett, PASP, 100, 1134 (1988), Appendix B, give relationships between the NIR reddenings and E(B−V)
. These data determine the necessary parameters. Note that the ETC also supports direct entry of observed J
magnitudes along with any specified value for E(B−V)
It may be possible to avoid bright field objects by specifying ORIENT
restrictions to the visit and/or POS TARG
s for the exposures.
Observations of planets with STIS require particularly careful planning due to the very stringent overlight limits of the MAMAs. In principle Table 13.45, Approximate MAMA Spectroscopic Bright-Object Limits (V mags. and cgs units).
and Table 14.40
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 ETC
. 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.
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 13.45, Approximate MAMA Spectroscopic Bright-Object Limits (V mags. and cgs units).
and Table 14.40
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.
– 2.5 log(1/area
is the area of the target in arcsec2
. This V
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/s/cm2
/Å), 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/s/cm2
) 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 13.45, Approximate MAMA Spectroscopic Bright-Object Limits (V mags. and cgs units).
and Table 14.40
, or passed to the ETC
as a user-supplied spectrum. It can also be easily converted to counts/s/pix 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/s/pix, 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 arcseconds) divided by the pixel size (0.0247 arcsecond) 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.
Detailed calculations and observational experience show, that for Jupiter and Saturn, all FUV-MAMA
imaging and spectroscopic modes are safe. Most NUV spectroscopic modes can also be used, but in many cases it will be necessary to use a very small or neutral density aperture to avoid excessive count rates. Of course the STIS CCD G230LB and G230MB gratings can be used without any bright object limitations, but for a red planetary object these CCD spectra will suffer substantial contamination from scattered light at shorter wavelengths.
Note that the global rate limit of 30,000 counts/s for first order spectroscopy was imposed because of the STIS Bright Scene Detection Monitor (see ISR STIS 96-028
), which samples the output of every 32nd row. It could be triggered if a bright point source spectrum fell directly on one of the monitored rows. For extended sources observed with a long slit, the larger global limit of 200,000 counts/s is the relevant one for both echelle and first order spectroscopic observations.
Jupiter and Saturn are much too bright to be observed with most STIS NUV-MAMA
imaging modes. However, the UVIS channel of the Wide Field Camera 3 (WFC3) has a number of UV filters that provide a better alternative for most NUV imaging science, as they have a larger field of view and no safety related bright object limits.