7.7 MAMA Bright Object Limits

STScI has responsibility to ensure that the MAMA detectors are not damaged through over-illumination. Consequently, we have developed procedures and rules to protect the MAMAs. We ask all potential users to share in this responsibility by reading and taking note of the information in this section and designing observing programs that operate in the safe regime for these detectors.

7.7.1 Overview

The MAMA detectors are subject to catastrophic damage at high global and/or local count rates and therefore 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 current observing 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, the STIS shutter will close, and that exposure will be lost.  Details of these procedures may be found in STIS ISRs 1996-28, 1996-31, and 1998-08.


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.

7.7.2 Observational Limits

To ensure both 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 7.8, 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).


Table 7.8: Absolute MAMA Count Rate Limits

Target

Limit Type

Mode

Channel

Screening Limit

Non-variable

Global

All modes other than
1st-order spectroscopy

FUV & NUV

200,000 counts/s

Non-variable

Global

1st-order spectroscopy

FUV & NUV

30,000 counts/s

Non-variable

Local

Imaging

FUV & NUV

100 counts/s/pix

Non-variable

Local

Spectroscopy

FUV & NUV

75 counts/s/pix

Irregularly Variable

Global

All modes other than
1st-order spectroscopy

FUV & NUV

80,000 counts/s1

Irregularly Variable

Global

1st-order spectroscopy

FUV & NUV

12,000 counts/s1

Irregularly Variable

Local

Imaging

FUV & NUV

100 counts/s/pix1

Irregularly Variable

Local

Spectroscopy

FUV & NUV

75 counts/s/pix1

1 Applies to the phase when the target is brightest.

7.7.3 How Do You Determine if You Violate a Bright Object Limit?

As a first step, you can check your source V magnitude and peak flux against the bright object screening magnitudes in Table 13.45 or Table 14.40 for your chosen observing configuration. In many cases, your source properties will be much fainter than these limits.

However, if you are near these limits, then you need to carefully consider whether your source will be observable in that configuration. Note that 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 or (more conveniently) you can use the STIS ETCs to calculate the expected count rates from your source. The ETCs have a host of template stellar spectra available. If you have a UV spectrum of the source, you can also input it directly into the calculators. The calculators will evaluate both the global and per pixel count rates and will warn you if the 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.

7.7.4 Policy and Observers' Responsibilities in Phase I and Phase II


It is the observers' responsibility to ensure that their observations do not exceed the bright object count limits stated in Table 7.8.


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 a 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 a 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.

Following their Phase I approval, proposers of MAMA observations are required to check both their targets and the fields surrounding those targets (as defined below) in detail for excessively bright sources, by the Phase II deadline. The relevant policies and procedures are described here.

STScI has developed bright object tools (BOT) to conduct detailed field checking prior to MAMA program implementation. These tools are based on an 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 and F), 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 (e.g., due to crowding or to the absence of data for one of the filters). In such cases,  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 BOT field report data.

Subsequently, automated GALEX screening was added as a selectable option in the BOT. The AIS (all-sky) sources are screened as unreddened O5 stars and reported as either safe or unsafe. This is a powerful tool, because it is based directly on UV fluxes; e.g., previously unknown hot companions to late-type stars will be revealed. The target should still be checked with the ETC, but if the field passes it can be cleared, subject to verification with the GALEX image or catalogue display that there is complete coverage of the macro-aperture field. Unsafe objects require further investigation.  The GALEX fluxes are upper limits in crowded regions because of the relatively low spatial resolution, and the source may clear with more specific parameter information. Please note that the fluxes and magnitudes given in the current version GALEX catalog do not include any correction for the local count rate non-linearity that affects high count rate sources. This can lead to serious underestimates of the flux for the brightest stars in the GALEX catalog. An estimate of the possible size of this effect is detailed in section 4.4 of Morrissey et al. 2007 (ApJS, 173, 682), "The Calibration and Data Products of GALEX." The GALEX screening done by the BOT now includes this estimated correction. This will sometimes result in the BOT reporting GALEX magnitudes that are brighter than those given in the GALEX catalog itself.

While STScI will check all targets and fields before any MAMA observations are cleared, by policy GOs must provide screened, safe targets for MAMA programs, as well as 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 at first, 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. By doing so, they will more quickly become aware of any problems, such as the need for supplementary data, which might otherwise entail lengthy implementation delays following the Phase II deadline. (One exception is for 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.  The CS will interact with the GO as necessary and requested, both during the Phase II preparations and through program execution, and will perform independent checks on the safety of the observations.

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., POSTARGs, patterns, or mosaics) are specified, the field to be cleared increases commensurately. POSTARG vectors and the enlarged, rotated field circles are conveniently displayed in APT/Aladin. No unsafe star or star with unknown properties may lie within 5 arcseconds of the detector edge at any orientation (or 13.5 arcseconds for very bright sources, see below).  POSTARGs and/or orientation restrictions may be introduced to avoid bright objects in the fields, however.

A GO proposing MAMA observations must send to the designated CS, by the Phase II deadline, the ETC calculations for each discrete target, as well as 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. Although automatic BOP calculations are currently available only for GSC2 and GALEX, many additional data sources are available directly through the APT/Aladin interface, including the STScI Mikulski Archive (MAST), which contains data from IUE as well as from HST. An existing UV spectrogram of the target (or of a similar object) may be imported directly into the ETCIUE spectral data used for BOP calculations must be low resolution, taken through the large IUE aperture. If model spectra are used in the ETC, the original Kurucz models should be used for early-type stars, rather than the more recent Castelli & Kurucz models. For stars later than the Sun, none of the provided models is adequate, since they lack the chromospheric emission lines that dominate the actual FUV flux.  In such cases, actual UV data, either for the target or for a very similar object, must be used. In the worst cases, new HST observations in safe configurations or ground based data may be required to clear the fields for BOP.  In general, any such supporting HST observations must be obtained within 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 automatically clears 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.  See also the special considerations for M dwarfs (below).

It is not expected that all such issues will be resolved by the Phase II deadline, but they should at least be identified and have planned resolutions by then. Another possible resolution is a change to a less sensitive MAMA or to a CCD configuration. Any MAMA targets or fields that cannot be demonstrated to be safe to a reasonable level of certainty in the judgement of the CS will not be observed. It is possible that equivalent alternative targets may be approved upon request in that case; but any observations that trigger the onboard safety mechanisms will not be replaced.

A related issue is MAMA pointing or configuration changes after the targets and fields have been cleared by the STScI BOP review. Any such changes must be approved by the STIS Team on the basis of a specific scientific justification and a new BOP review by the GO, which may be submitted via the CS if absolutely necessary. However, in general such requests should be avoided by ensuring that submitted MAMA specifications are final, to prevent a need for multiple BOP reviews.

GOs planning MAMA observations of unpredictably variable targets, such as cataclysmic variables, are reminded of the special BOP procedures in effect for them, which are detailed in ACS ISR 2006-04.

Special Considerations for M Dwarfs

Because M dwarfs can exhibit strong (and unpredictable) flaring, special procedures have been developed (STIS ISR 2017-02) to check any known or potential such objects within the clearance regions.  For programs targeting known M dwarfs, the CS will provide a spreadsheet based on those procedures for the GO to use to assess the safety of the proposed observations.  Somewhat cruder assessments, based on available photometry, may be used to (conservatively) evaluate any possible additional M dwarfs in the clearance regions around the designated targets, if the information required for the full spreadsheet analysis is not available.

In some cases, the 2MASS JHK may be the only photometry available for an otherwise "unknown" star. It is possible to estimate V and E(BV) 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 (JH)0 = 0.11 for all O stars; and (VJ)0 = 0.67, (VH)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(BV). These data determine the necessary parameters. Note that the ETC also supports direct entry of observed J, H magnitudes along with any specified value for E(BV).  Magnitudes and colors from Gaia, together with the intrinsic values tabulated by E. Mamajek here, may also be of use, particularly for later-type stars (though these are not currently supported in the ETC).

Pointings Close To Objects Violating Safety Limits

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 arcseconds.

Any field object within 5 arcseconds of the edge of an aperture used for a MAMA observation is subject to the same bright object limits as targets that are in the aperture. Targets or field objects falling in an annular region extending from 5 to 13.5 arcseconds from the edge of the aperture are also subject to some restrictions. Any object in this zone producing either a real global count rate in excess of 1.5 x 106 counts/s or a local count rate greater than 500 counts/s/pix is not permitted. See STIS ISR 2000-01 for a discussion of the current screening procedures.

7.7.5 Policy on Observations that Fail Because They Exceed Bright Object Limits

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.

7.7.6 What To Do If Your Source is Too Bright for Your Chosen Configuration?

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:

  • Select a narrower slit that passes only a fraction of the source flux, for spectroscopic observations.
  • Select a higher dispersion grating.
  • For NUV low-resolution and medium-resolution spectroscopy, consider using the CCD G230LB and G230MB modes (see Section 4.1.6).
  • Employ a neutral-density filter.
  • Change configurations to observe a different portion of the spectrum of your target (e.g., change the central wavelength).

It may be possible to avoid bright field objects by specifying ORIENT restrictions to the visit and/or POSTARGs for the exposures.
For further advice, see Section 12.4.

7.7.7 Bright Object Protection for Solar System Observations

Observations of planets with STIS require particularly careful planning due to the very stringent overlight limits of the MAMAs. In principle, Table 13.45 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 with 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 arcsecond) solar system objects the local count rate limit is the more restrictive constraint, while for large objects (>~1–2 arcseconds) 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 13.45 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.

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 (use the Solar Spectrum template in the HST Standard Star menu 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/Å/arcsec2) 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 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.

Jupiter and Saturn

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 STIS ISR 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.

Field Safety Checks for Moving Targets

Multiple stellar fields along their paths must also be cleared for safety of moving target observations. Of course, this cannot be accomplished until scheduling windows are assigned, and it may have to be iterated if the windows change. An automated capability to do that has been added to the APT/BOT.  Your Contact Scientist will advise and assist with this procedure.