Instrument Science Report STIS 96-024
MAMA Bright Object Limits for
Astronomical Objects
C. Leitherer, E. Kinney, S. Baum, and M. Clampin
September 16, 1996
ABSTRACT
The maximum allowed count rates for the STIS MAMA detectors are converted into magnitudes for a range of stellar energy distributions, using Kurucz model fluxes and observed
flux standards. Tables are provided to assist observers in determining the appropriate
instrument configuration for their targets. We discuss different classes of objects which
can trigger a bright object alert. Both the spectroscopic and the imaging modes are covered. The maximum allowed count rates are reached for relatively faint objects. As a
result, in addition to stars, many non-stellar targets, such as emission nebulae, galaxies,
or globular clusters are around or above the limiting magnitude.
1. Maximum allowed count rates for the MAMAs
The STIS MAMA detectors are subject to damage at high local and global illumination rates. The limits are listed in Tables 1 and 2. The rationale for these limits is described
in STIS ISR 96-028, “STIS Bright Object Protection Observing Policies for the MAMA
Detectors.”
Table 1. Absolute MAMA Count-Rate Limits for Non-Variable or Regularly Variable
Sources
Limit Type
Count-Rate Limit
Applies To
Absolute global
200,000 counts/sec
FUV and NUV, all modes
Absolute global
30,000 counts/sec
FUV and NUV, 1st order spectroscopy
Absolute peak
50 counts/sec/pixa
All NUV observations
Absolute peak
35 counts/sec/pixa
All FUV observations
a. Pixel refers to lowres
1
Table 2. Absolute MAMA Count-Rate Limits for Irregularly Variable Sources
Limit Type
Count-Rate Limit
Applies To
Absolute global
80,000 counts/sec
FUV and NUV, all modes
Absolute global
12,000 counts/sec
FUV and NUV, 1st order spectroscopy
Absolute peak
50 counts/sec/pixa
All NUV observations
Absolute peak
35 counts/sec/pixa
All FUV observations
a. Pixel refers to lowres
The purpose of this Instrument Science Report is to demonstrate how these count rate
limits translate into magnitude and flux limits for actual astronomical targets.
2. Conversion into magnitudes
All STIS targets will be screened by STScI to determine whether they violate any
safety constraint. The checking will be performed by converting the expected flux at the
wavelengths of observation into a count rate. For an initial screening, both by GOs/GTOs
and by STScI staff, it is often convenient to have simple look-up tables where the B/O
constraints are converted into visual magnitudes and are listed for different spectral types.
This has been done, e.g., for the FOS, and we are providing similar tables for STIS in this
Instrument Science Report.
The bright-object constraints were converted from count rates into limiting fluxes and
magnitudes for a variety of spectral energy distributions, or equivalently spectral types,
and for a set of instrument configurations. Input spectra were first selected from flux distributions calculated with Kurucz’s line-blanketed LTE model atmospheres. We chose a grid
of 21 temperature points covering O to M stars, both for dwarfs and supergiants. The grid
is listed in Table 3. Temperature points were selected to match spectral types according to
Schmidt-Kaler’s (1982; in Landolt-Börnstein, New Series, Group VI, Vol. 2b, ed. K.
Schaifers and H. H. Voigt (Berlin: Springer), 1) spectral-type vs. Teff calibration. Two
sequences were calculated for stars of luminosity classes V and I, respectively. The corresponding log g values were obtained from stellar radii and masses. The masses were taken
from evolutionary models by Maeder & Meynet (1988; A&AS, 76, 411). The surface
gravities at the low- and high-Teff ends (types O and M) were slightly increased with
respect to the prediction of the evolutionary models. Otherwise the validity range of
Kurucz’s model atmospheres would have been exceeded. Since the dependence of the
atmosphere fluxes on gravity is minor, this introduces a negligible error.
The results for the bright-object limits are in Figure 1. We have plotted the V magnitude vs temperature of a point source reaching the absolute global count-rate limit of
30,000 counts/sec in Table 1. The long- and short-dashed lines are for dwarfs and super-
2
giants, respectively. Grating G230L was chosen for this specific example. The brightness
limit computed with the model atmospheres is monotonically decreasing with Teff, as
expected from the higher UV flux at higher temperature and fixed V magnitude. The two
different log g sequences are very similar, suggesting that surface gravity plays only a
minor role in the determination of the UV flux in this parameter range.
Also included in Figure 1 are the simulation results for individual stars. Spectra of
well-observed stars were used to calculate the visual brightness limits corresponding to
the limiting count rates in the UV in the same way we did for the model atmospheres. We
used a set of stars from the Bruzual-Persson-Gunn-Stryker atlas. Temperatures were
assigned to spectral types using Schmidt-Kaler’s calibration. On the average, the numbers
for models and observations agree very well for stars earlier than about G. This gives confidence in the reliability of model atmospheres to calculate stellar fluxes. There is,
however, some dispersion around the mean relation: stars of approximately the same spectral type can have bright object magnitudes which differ by a few tenths of a magnitude.
This reflects the intrinsic temperature spread within the same spectral sub-type and/or
spectral classification, reddening, model, etc. errors. In addition, the later the spectral type,
the less the flux is determined by continuum emission. Chromospheric emission is the
dominant source of UV flux in late-type stars. Since chromospheric emission is not
included in Kurucz’s models, the theoretical Vlim values in Figure 1 below 5000 K are
invalid.
Based on the model-atmosphere results and the spread around them, and on the
observed UV flux excess of stars below 5000 K with respect to Kurucz models, we derived
a revised set of MAMA bright-object limits. The new limits were obtained by defining an
envelope to the theoretical and observational limits. This envelope is the solid line in
Figure 1. The same procedure was applied to all other gratings (and the prism) and to all
optical elements in the imaging modes. Limiting magnitudes were calculated for all countrate limits in Table 1. The most stringent limit was adopted for the actual magnitude limit.
Depending on grating and spectral type, any of the limit types in Table 1 can set the magnitude limit.
The resulting limiting magnitudes are in Table 4 and Table 5 for the spectroscopic and
imaging modes, respectively. The limiting magnitudes are more conservative than the values in the tables of the STIS Instrument Handbook, version 1.0, with the exception of a
few entries for late-type stars, which are slightly brighter. The differences are mainly due
to three reasons:
•
The magnitudes in the original tables were in the ST-mag system, whereas the revised
values are in the Vega-mag system (all colors 0). The changes introduced by switching
to the new magnitude system are negligible (less than 0.3 mag).
3
•
The limiting count rates have been revised and the magnitudes have changed correspondingly. Depending on whether the global or local limits apply, either brighter or
fainter magnitudes can result. This effect is of the order of 0.5 mag, or smaller.
•
The new magnitudes were calculated both for a set of input spectra from the BruzualPersson-Gunn-Stryker atlas and from theoretical model atmospheres. The models
were used to smooth out discontinuities in the magnitudes resulting from peculiarities
of individual spectra or observational errors.
These tables will be used by GOs/GTOs and STScI to check if proposed targets are
close to a bright-object violation. They can be used both by proposers during Phase I and
II and by STScI to determine initially if a potential overlight condition exists. Cases which
are not covered in the tables and cannot be easily inter/extrapolated (e.g., pathological
energy distribution), can be evaluated individually using the STIS exposure time
calculator.
In addition to the V magnitudes derived from the model fluxes, we also give V magnitudes corresponding to a 50,000 K black body and an input spectrum with Fν ∝ ν-1. The
tables also give limiting fluxes, at the wavelengths of observation, for point sources and
extended targets. The maximum integrated magnitude of all stars in the field for the imaging mode is listed in Table 5. The stars are assumed to have spectral type O5. In order to
be roughly consistent with our procedure above, we applied a safety margin of a factor of
2.5. This safety factor, together with the factor of 3.3 to 4 more conservative count rate
limit adopted by us initially (see STIS ISR 96-028), leads to a safety margin of about a
factor of 10 for screening. This is similar to what is used for FOS B/O screening. For comparison, the screening limit used for the FOC is a factor of 10000 more conservative than
the actual damage limit.
Note that the tables are based on a worst-case scenario, assuming maximum throughput. The slit throughput has been taken as 100%. Decreasing the slit width can decrease
the bright-object magnitudes by up to 0.75 mag. Neutral density filters will allow observations of correspondingly brighter objects. Zero interstellar extinction was assumed. Even
small extinction values in the visual will translate into large ultraviolet absorptions, which
allows observations of correspondingly visually brighter objects.
3. Observations of point sources
In this section we illustrate the use of Table 4 and Table 5 to evaluate the probability of
violating the bright-object limits for point sources.
Spectroscopy
For the spectroscopic modes, the most likely objects to trigger an alert are hot, unreddened stars. The bright-object limits are quite similar to those for FOS target acquisitions,
4
and we can build on our previous experience with FOS bright-object checking. There are
no stars having spectral types later than mid-G which pose a safety problem for the FUVMAMA. For the NUV-MAMA, any spectral type can cause an alert. When the slit is used
at a given orientation it is extremely unlikely that a random object in the sky will be in the
field of view. Therefore we expect the alert to be almost always due to the target itself.
Exceptions are long-slit spectroscopic observations in crowded fields, such as in LMC/
SMC star clusters, where random objects can fall within the aperture.
If an object is not a single star but a binary having two components of significantly different temperature, Table 4 and Table 5 do not apply. Parsons (1981; ApJ, 247, 560) finds
that roughly 20% of all late-type supergiants have hot companions which are not discernible in the optical spectrum but dominate the ultraviolet light. The bright-object limits for
such objects would be more stringent than the values in Table 4 and Table 5, which are
based on our extrapolations of the optical spectrum into the ultraviolet.
This poses a safety risk for the STIS MAMA detectors if an instrument configuration
is chosen assuming the spectrum is due to the cool star alone. In order to prevent damage
to the MAMA detectors, special rules are in place for spectroscopic MAMA observations
of stars of spectral type F and later and luminosity classes IV–I.
The worst possible case is that the companion is an early-O star. If the O companion
contributes less than 20% to the total light in the V passband, it is considered undetectable
in the optical. Therefore, for screening purposes, one can assume that the cool star has an
O5 V companion which has a V magnitude 1.7 mag (5 times) fainter than the target itself.
By consulting Table 4 one can determine how close to the limit the O5 V star is.
The limiting magnitudes in Table 4 and Table 5 refer to idealized, non-variable stars
whose spectra can be described with single spectral types from the visual to the ultraviolet.
Irregularly variable sources have more stringent limiting magnitudes.1 They can be estimated by scaling the values in Table 4 and Table 5 according to the information in Table 1
and Table 2.
Imaging
The B/O limits in the imaging modes are by far more stringent than for the spectroscopic modes. No spectral class can be excluded a priori from the bright-object screening,
except for certain narrow-band filters and for late-type stars observed with the FUVMAMA. In addition, there is a non-negligible probability of alerts from random stars in
the field of view. In Table 6 we give the average number of stars per square degree as a
function of limiting magnitude. (Note that the MAMA field of view is 4.8x10-5 square
degrees.)
1.
One magnitude more conservative for global limits.
5
The limiting B/O magnitude for a late-B star observed with the 25NUV-MAMA is
about V=19. Table 6 suggests there will be 2-3 stars brighter than 19th mag in a field of 25
by 25 square arcsec having 0˚ galactic latitude. Of course, most random stars would not be
of type B, but rather G or K dwarfs. If a B/O limit of 16 mag for such stars is assumed, we
would still expect to find 0.3 such stars in the MAMA FOV. This is clearly too large to be
insignificant. We conclude that there is a non-negligible chance of B/O alerts due to random stars in the FOV when the MAMA’s are used in the imaging modes.
Random moving targets accidentally in the STIS FOV are not a safety concern. Such
targets are catalogued and uncatalogued asteroids. An empirical formula for the surface
density of asteroids near opposition (K. Noll, private comm.) can be stated as:
–2
N [ deg ] = 2.2
V – 15.8 – 0.01b
2
where V is the V magnitude and b is the ecliptic latitude in degrees. Assuming a solar-type
spectrum and taking the worst case for the instrument configuration (NUV-MAMA, clear,
imaging mode), we find from Table 4 a B/O magnitude of about V = 16. Solving for V=16
and b=0 (worst case) we find N = 1.2 deg-2. Therefore, in the STIS FOV (25 arcsec
square) pointed in the ecliptic at the opposition point there will be 5.8x10-5 objects of
V=15 or brighter. The number decreases away from the ecliptic and away from the opposition point. This is a negligibly small number.
Prism Spectroscopy
We do not give separate discussions of prism spectroscopy. Observations with the
prism have much more stringent bright-object limits than for any grating. The limits are
comparable to those of the filters used with the NUV-MAMAs, and the discussion in the
imaging sections should be used as a guideline for bright object violations.
4. Observations of extended sources
We compare the brightness of the most commonly observed extended targets to the
MAMA bright-object limits and discuss which objects are unlikely to trigger a safety alert.
Moving targets (major planets) are not discussed here. They will be addressed in a separate ISR.
Globular clusters
Spectroscopy
Typical surface brightnesses of globular clusters are V=14 mag/arcsec2 in the center,
declining by 1 to 2 magnitudes at a distance of 10" from the center. V=14 mag/arcsec2
6
translates into a V flux of 9x10-15 erg/s/cm2/Å/arcsec2. Assuming a solar-type spectrum,
this is much fainter than all local spectroscopic bright-object limits of Table 4. Some globular clusters are known to have post-AGB stars. Post-AGB stars have bolometric
luminosities of about 104 Lsun and temperatures anywhere between 3000 and 100000 K.
(This evolutionary phase lasts about 104 yr). If the post-AGB star is in its B phase, like the
well-known example BS-47 Tuc, it has an MV of about -3. At the distance of 47 Tuc (VMV = 14.5), the star will have V=11.5. This is brighter than the bright-object limit for both
the NUV and the FUV MAMAs in the lowest dispersions.
Imaging
The local surface-brightness limit for the MAMAs in the imaging modes is brighter
than the surface brightness of a typical globular cluster (cf. Table 5). However, the global
limits can be violated. Assuming the globular cluster fills the MAMA detector completely
(625 arcsec2), and it has a mean surface brightness of 15 mag/arcsec2 over this area, the
total magnitude is about V = 8. The integrated magnitude for the bright object condition is
7.3 mag fainter than for the local limit (Table 5). If a G2V spectrum is assumed for the
globular cluster, the corresponding bright object magnitude is 16.2-7.3 = 8.9 mag for the
NUV clear MAMA. This is fainter than the expected V magnitude of the hypothetical
globular cluster. We note that the presence of a post-AGB star would trigger an alert as
well—both for the FUV and the NUV MAMA in their imaging modes.
Galaxies
Spectroscopy
An extensive database of IUE observations of normal galaxies (= galaxies with no
strong emission lines or central AGN) exists. There are no galaxies with flux levels
between 1200 A and 3000 A higher than 10-12 erg/s/cm2/Å in the large IUE aperture. The
brightest galaxies (e.g., NGC5253) reach flux levels around 10-13 erg s-1 cm-2 Å-1 in the
IUE aperture. The flux within a typical STIS aperture will be smaller. Only observations
with grating G230L come close to the limit.
Active galaxies may exceed the bright object limits in some cases. For instance, the
bright source 3C273 has a continuum level in excess of 10-13 erg/s/cm2/Å, plus strong
emission lines several times the continuum. This is around and above the limits for G140L
and G230L.
7
Imaging
Galaxies with bright nuclei and clusters can exceed the imaging B/O limits. For
instance, the dwarf galaxy NGC1705 has a measured IUE flux of 3x10-13 and 3x10-14 erg/
s/cm2/Å at 1200 and 3000 Å, respectively. The flux comes from a bright nucleus, which is
unresolved on a WFPC2 image. This object exceeds the B/O limits for all optical elements
in Table 5. More galaxies exceeding the imaging B/O limits can be found. Therefore all
MAMA imaging observations of normal galaxies have the potential for B/O violations.
Galaxies with strong emission lines (AGN, quasars) are even more likely to violate B/
O restrictions than normal galaxies due to their strongly peaked surface brightness towards
their centers and due to their strong emission lines. In fact, one of the few occurrences of
an actual FOS safing due to an overlight condition was caused by a Seyfert1 galaxy.
As a specific example we give again 3C273. The IUE flux in the continuum is a few
times 10-13 erg/s/cm2/Å. This exceeds the MAMA limits for the imaging modes by a factor of several.
HII regions
Spectroscopy
The brightest UV emission lines seen in H II regions are C III] 1907-9 Å and C II]
2325 Å. Orion has the highest (optical) surface brightness of any Galactic H II region, so it
should work as an upper limit. From IUE spectra the central surface brightnesses for C III]
and C II] are about log S = -13.0 erg cm-2 s-1 arcsec-2. The lines are unresolved at IUE resolution. The actual line widths are due to macroscopic turbulence. Turbulence is
supersonic in giant HII regions and subsonic in normal, H II regions. We will use a velocity of 10 km/s as reasonable compromise, resulting in a line width of about 0.1 Å. This
corresponds to about 1 pixel for the G230M grating, and is unresolved for G230L.
Depending on the spatial extension of the H II region within the slits, the line will be
broader and the per pixel count rate lower. Comparison with the limiting surface brightness of the spectroscopic modes (Table 4) shows that this is fainter than the damage limits.
Imaging
There might be a safety concern for the imaging modes. Narrowband filters have limiting surface fluxes per unit surface at least an order of magnitude brighter than the Orion
nebula value. The clear MAMA’s have B/O limits close to the C III] values observed in
Orion, but the values in Table 5 refer of course to a continuum source having constant
brightness over the clear bandpass. If the C III] flux were spread out evenly, it would be
orders of magnitude fainter. On the other hand, an H II region will extend over a significant fraction of the FOV, and the global limit applies. Using the same argumentation as for
8
a PN (see below), it can be shown that the brightest H II regions are an order of magnitude
fainter than the imaging B/O limit if they fill the MAMA FOV.
Planetary nebulae
Spectroscopy
The strongest lines in planetary nebulae are C IV 1550, He II 1640, C III] 1909, C II
2325, and O II 2470 Å. The most complete sample is Magellanic Cloud PNe. The maximum surface brightnesses of the mentioned lines are 10-12 erg/s/cm2/arcsec2. Using
similar arguments as for HII regions, we find that there is no PN bright enough to generate
a B/O alert for the spectroscopic modes.
Imaging
PNe can be above the B/O limit if they are observed in the imaging mode and fill a
substantial portion of the FOV. Consider a PN observed with the clear NUV-MAMA filling the entire FOV. We assume the total emission-line flux from all UV lines is 5x10-12
erg/s/cm2/arcsec2 and this flux is concentrated within 0.1 Å. Spread out over 1000 Å and
over 25 x 25 arcsec2, this gives a total flux of 3x10-13 erg/s/cm2/Å over the FOV. This
needs to be compared with the integrated B/O magnitude, which is 7.3 mag fainter than
the peak B/O magnitude (Table 5). If the line strength is (on average) not a strong function
of wavelength and the mean slope of the emission-line spectrum resembles a mid-A star,
the critical B/O magnitude is V = 18-7.3 = 10.7, or 2x10-13 erg/s/cm2/Å. Consequently the
object reaches the B/O limit.
5. Summary
A summary of possible overlight conditions for various astronomical objects is given
in Table 7.
9
Table 3. Adopted model atmosphere grid (solar abundances).
Dwarfs
Supergiants
Spectral
Type
Teff
log g
Teff
log g
O5
44500
5.0
40300
4.0
B1
25000
3.9
20800
3.0
B3
18700
3.9
16200
2.6
B5
15400
3.9
13600
2.4
B8
11900
4.0
11200
2.2
A1
9230
4.1
9230
2.1
A3
8720
4.2
8770
2.0
A5
8200
4.3
8510
2.0
F0
7200
4.3
7700
1.7
F2
6890
4.3
7350
1.6
F5
6440
4.3
6900
1.4
F8
6200
4.4
6100
1.4
G2
5860
4.4
5200
1.3
G5
5770
4.5
4850
1.1
G8
5570
4.5
4600
1.0
K0
5250
4.5
4420
0.9
K4
4590
4.5
3950
0.5
K7
4060
4.5
3700
0.2
M2
3580
4.6
3450
0.0
M4
3370
4.8
2980
0.0
M6
3050
5.0
2600
0.0
10
Table 4. MAMA spectroscopic bright object limits.
Spectral Type
G140L
G140M
E140M
E140H
G230L
G230M
E230M
E230H
Prism
Local Limit
Surface Brightnessa
5.4x10-11
1.1x10-9
9.5x10-9
2.4x10-8
1.8x10-11
6.4x10-10
1.5x10-9
1.0x10-8
N/A
Local Limit
Point-Source Fluxb
1.3x10-12
3.1x10-11
2.8x10-10
7.3x10-10
3.4x10-13
1.5x10-11
3.6x10-11
2.4x10-10
N/A
O5 Vc
13.8
11.1
10.8
10.0
13.6
10.8
11.4
10.3
17.5
B1 V
13.7
9.9
9.9
9.4
12.8
10.4
11.1
10.0
16.7
B3 V
13.3
9.4
9.5
8.8
12.2
9.7
10.3
9.3
16.3
B5 V
13.2
9.3
9.4
8.6
12.0
9.6
10.0
8.9
16.0
B8 V
13.0
9.2
9.3
8.5
11.4
9.2
9.7
8.7
15.4
A1 V
10.0
6.8
6.6
5.9
10.1
7.5
8.0
6.9
13.9
A3 V
7.9
4.9
4.5
4.0
9.8
7.1
7.6
6.5
13.7
A5 V
7.1
3.9
3.5
3.0
9.6
6.9
7.3
6.2
13.6
F0 V
5.0
1.3
0.4
0.0
9.3
6.3
6.7
5.4
13.6
F2 V
3.9
-0.2
-1.2
-1.5
9.2
6.2
6.6
5.4
13.5
F5 V
2.3
8.8
5.9
6.2
5.1
13.2
F8 V
1.4
8.7
5.8
6.1
5.0
13.0
G2 V
0.3
8.5
5.4
5.7
4.7
12.4
G5 V
-0.1
8.5
5.4
5.6
4.6
12.3
G8 V
8.5
5.2
5.5
4.4
12.3
K0 V
8.1
4.9
5.2
4.1
12.2
K4 V
7.7
4.2
4.4
3.3
11.4
K7 V
7.3
3.6
3.7
2.6
9.4
M2 V
6.7
2.9
3.0
1.9
8.5
M4 V
6.5
2.6
2.6
1.5
8.4
6.1
2.0
2.0
0.8
8.4
M6 V
T~50000
ν
oK d
-1 e
13.7
11.0
10.7
9.9
13.5
10.7
11.3
10.2
17.4
11.4
8.8
7.7
7.0
12.5
8.8
8.8
7.2
14.2
surface brightness in ergs sec-1 cm-2 Å-1 arcsec-2 of the continuum or of an emission line from a diffuse source.
b Peak flux in ergs sec-1 cm-2 Å-1 of an emission line from a point source; for the firstorder slits this is also usually the peak flux from the continuum, but for the echelles the
continuum flux limit is considerably lower, as the global limit and not the local limit dominates.
c Limits are V magnitudes, with the assumption of zero reddening.
d Limits for a black body with a temperature of 50,000 degrees K.
e Limits for a source with a spectrum F proportional to ν-1.
ν
a Peak
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Table 5. MAMA imaging bright object limits.
FUV-MAMA
NUV-MAMA
Spectral
Type
25MAMA
(clear)
F25SRF2
(longpass)
F25QTZ
(longpass)
F25LYA
(Lyα)
25MAMA
(clear)
F25SRF2
longpass
F25QTZ
longpass
F25CIII
(C III)
F25CN182
(182 cont)
F25CN270
(270 cont)
F25MGII
(Mg II)
Surface
Brightnessa
4.7x10-13
7.8x10-13
1.6x10-12
2.4x10-11
1.4x10-13
1.7x10-13
1.6x10-13
1.5x10-11
2.8x10-12
1.1x10-12
5.2x10-12
Point-Source
Fluxb
1.5x10-15
2.5x10-15
5.2x10-15
7.8x10-14
3.4x10-16
4.4x10-16
4.0x10-16
3.6x10-14
6.8x10-15
2.7x10-15
1.3x10-14
O5 Vc
20.9
20.5
19.8
17.1
20.8
20.8
20.8
16.6
18.7
18.6
16.6
B1 V
19.9
19.7
18.3
15.1
20.4
20.5
20.5
15.4
18.2
18.2
16.2
B3 V
19.5
19.0
17.8
14.7
19.9
20.0
19.9
15.1
17.4
17.7
15.7
B5 V
19.3
18.6
17.8
14.4
19.6
19.5
19.5
14.9
16.8
17.3
15.3
B8 V
18.7
17.9
17.6
13.0
18.9
18.8
18.7
14.5
15.5
16.6
14.7
A1 V
17.0
16.1
16.4
8.5
18.1
17.9
17.8
13.6
14.0
15.7
14.0
A3 V
16.5
15.6
16.0
6.2
17.9
17.5
17.5
13.3
13.7
15.5
13.8
A5 V
15.7
14.8
15.3
4.6
17.6
17.2
17.3
12.8
13.2
15.2
13.4
F0 V
13.5
13.0
13.3
-0.1
17.1
16.6
16.7
11.3
11.8
14.6
13.1
F2 V
11.4
12.1
12.4
16.9
16.5
16.5
10.7
11.1
14.3
12.7
F5 V
9.3
10.8
10.9
16.7
16.2
16.2
9.9
10.1
14.1
12.4
F8 V
8.6
10.2
10.9
16.5
16.0
16.0
8.9
9.2
14.1
12.2
G2 V
7.5
8.5
8.1
16.2
15.7
15.8
6.7
8.0
13.7
12.2
G5 V
7.3
8.3
7.7
16.2
15.6
15.4
6.3
7.6
13.6
12.1
G8 V
6.1
7.4
6.6
16.0
15.4
14.9
5.4
7.1
13.4
12.0
K0 V
3.6
5.8
4.2
15.7
15.1
14.7
4.3
6.0
13.2
11.9
K4 V
-3.3
-0.1
-3.4
15.1
14.4
13.7
0.2
2.5
12.4
11.2
K7 V
14.4
13.7
12.1
0.8
11.9
10.7
M2 V
13.7
12.9
11.1
-0.9
11.2
10.1
M4 V
13.4
12.5
10.7
10.7
9.8
M6 V
12.8
11.9
10.0
10.1
9.3
T~50000oK d
20.8
20.4
19.8
16.9
20.7
20.7
20.7
16.6
18.7
18.6
16.5
ν -1 e
19.1
18.0
17.3
14.8
18.7
18.8
18.8
14.4
15.0
16.6
15.0
Integrated
Magnitudef
13.4
12.7
11.6
9.5
13.5
13.3
13.2
9.0
10.9
10.6
8.7
a Peak allowed diffuse-source surface brightness in ergs sec-1 cm-2 Å-1 arcsec-2.
b Peak allowed point-source flux in ergs sec-1 cm-2 Å-1.
c Limits given are V magnitudes, for unreddened stars.
d Limits calculated for a black body at 50,000 degrees K.
e Limits calculated for a source with spectrum F proportional to ν-1.
ν
f Maximum integrated magnitude of all stars in field, assuming all are O stars.
12
Table 6. Log number of stars per square degree brighter than V for different galactic
latitudes (from Allen, Astrophysical Quantities)
V
b
5
10
12
14
16
18
20
mean(0-90˚)
-1.41
+0.91
+1.76
+2.56
+3.29
+3.95
+4.50
0˚
-1.08
+1.25
+2.18
+3.02
+3.78
+4.50
+5.00
90˚
-1.69
+0.55
+1.33
+2.01
+2.54
+3.02
+3.40
Table 7. Summary table showing overlight conditions for various astronomical objects
Object
Spectroscopy
Imaging
single stars
Any star can generate alert for most gratings;
hot stars more likely to produce overlight
conditions
Any star can generate alert for most filters;
hot stars more likely to produce overlight
conditions
multiple stars
Cool, evolved stars with hot companion can
produce alert
Cool, evolved star with hot companion can
produce alert
minor planets
Random object is unlikely to be in FOV
Random object is unlikely to be in FOV
globular clusters
Clusters are too faint to trigger violation
unless they host a bright post-AGB star
Clusters are too faint to trigger violation
unless they host a bright post-AGB star
normal galaxies
The brightest normal galaxies reach safety
limit for G230L
Normal galaxies can exceed the safety limit
for certain filters
active galaxies
Galaxies with strong emission lines exceed
safety limit for G140L and G230L
Galaxies with strong emission lines exceed
the safety limits for some filters
HII regions
HII regions are not bright enough to reach the
safety limits
HII regions are somewhat below the safety
limits
planetary nebulae
Planetary nebulae are not bright enough to
reach the safety limits
Planetary nebulae can reach the safety limits
for some filters
13
Figure 1: Limiting V magnitude versus effective temperature for grating G230L. Stars
brighter than Vlim violate one or more of the brightness limits in Table 1. Short-dashed
line: Kurucz models for supergiants; long-dashed line: Kurucz models for dwarfs; filled
circles: observed fluxes of standard stars. The solid line is an envelope giving a conservative limit to both the observed and the model fluxes. The limiting magnitudes in Table 4
are based on the solid line.
14