13.8 MAMA Spectroscopic Bright Object Limits

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As described in Section 7.7, the MAMAs are subject to absolute brightness limits, above which sources cannot be observed or they would potentially damage the detectors. In Table 13.45, we present the complete set of absolute bright object point source spectroscopic screening magnitudes and fluxes for the MAMA spectroscopic modes. These screening magnitudes are presented as a guide. Whether an individual source can be observed is ultimately determined by whether, in the desired configuration, the spectrum of that source is predicted to exceed the global and local observing count rate limits, as described in Chapter 7. The information presented here should be used in conjunction with the material presented in Chapter 7. Remember, sources cannot be observed in configurations where they exceed the absolute bright object limits. A few important points to note are:

  • The screening limits are given either as V magnitude or CGS units as indicated.
  • The screening limits for first order modes and the PRISM in this table have been calculated assuming zero slit losses. For Echelle modes, the 0.2X0.2 aperture was assumed unless otherwise noted. To determine if a source violates the limits in this table, the magnitude limit must be corrected for the aperture throughput for your chosen slit. The maximum magnitude correction achieved without the use of a neutral density filter using a supported slit is ~0.75 magnitudes. An exception to this is the values for the local surface brightness limits in row one of Table 13.45 (values in the table calculated for a 52X2 slit).
  • The screening limits in the tables assume zero extinction. To determine if a source violates the limits in this table, the magnitude limit must be corrected for the extinction of the source.
  • The peak flux from an emission line or from the continuum from a source must be less than the flux limit given in row two (for point sources — it is important to correct for the aperture throughput) and row one for diffuse sources (it is important to correct for the width of the source by scaling by the slit width in arcseconds divided by 2.0).
    • For echelle observations, the global limit of 200,000 counts/s over the detector sets the magnitude limits, but it must also be assured that the source does not violate the local limit, e.g., if it had a bright emission line.
    • If the observations include a source which has high equivalent width line emission (i.e., whose flux is dominated by line emission), it must be assured that the line emission does not exceed the limits. This may be a concern for stars with strong emission lines, such as Wolf-Rayet or T Tauri stars.
  • If the observations include multiple bright stars in the long slit, or the observations are slitless, it be also be assured that the sum from all targets imaged on the detector does not exceed the applicable global limit.
  • The limits in this table are the worst case limits for the scanned gratings; the use of a less sensitive central wavelength may have a brighter true limit, allowing the observations of the bright target to be performed. The STIS ETC should always be used to verify the safety of a target and configuration specifications.


Table 13.45: Approximate MAMA Spectroscopic Bright-Object Limits (V mags. and cgs units).

Spectral Type

G140L

G140M

E140M

E140H

G230L

G230M

E230M

E230H

PRISM

Local limit1
surface brightness

2.6 × 10–11

4.0 × 10–10

3.4 × 10-8

6.7 × 10–8

8.7 × 10–12

2.6 × 10–10

5.0 × 10–9

3.1 × 10-8

7.4 × 10–13

Local limit2
point source flux

5.0 × 10–12

9.4 × 10–11

1.1 × 10–9

2.2 × 10–9

1.4 × 10–12

5.0 × 10–11

1.0 × 10–10

6.9 × 10–10

5.9 × 10–14

O5 V3

14.1

11.9

10.15

9.77

13.6

10.3

9.81

8.57

14.08

B1 V

13.2

11.1

9.30

8.94

13.0

9.7

9.31

8.02

13.46

B3 V

12.3

10.2

8.47

8.07

12.5

9.1

8.75

7.41

12.85

B5 V

11.7

9.5

7.81

7.38

12.0

8.7

8.31

6.95

12.39

B8 V

10.5

8.2

6.68

6.20

11.3

7.9

7.58

6.17

11.60

A1 V

8.1

5.7

4.44

3.79

10.3

6.8

6.62

5.15

10.53

A3 V

6.7

4.3

3.13

2.67

10.1

6.7

6.40

4.94

10.28

A5 V

4.9

3.2

1.46

1.09

9.9

6.5

6.21

4.73

10.04

F0 V

1.8

1.1

–1.47

-2.48

9.3

6.3

5.82

4.32

9.49

F2 V

0.8

0.1

-2.50

-3.57

9.1

6.2

5.63

4.14

9.27

F5 V

–1.4

–2.2

-4.72

-5.60

8.7

6.0

5.28

3.91

8.85

F8 V

-2.6

–3.6

-5.98

-6.72

8.4

5.8

5.05

3.74

8.60

G2 V4

-0.68

–4.29

-4.65

-4.83

7.71

5.26

4.44

3.22

7.97

G8 V5

-0.86

–3.39

-4.61

-5.40

7.24

4.74

3.98

2.73

7.52

K2 V6

-0.39

-2.44

-4.20

-4.81

6.41

3.97

3.12

1.96

6.74

KM III7

2.2

–0.9

<–1.77

<–1.61

6.1

3.7

2.87

2.15

9.1

T~50000 K8

13.89

11.73

10.03

9.63

13.47

10.14

9.75

8.49

14.01

(λ–1)9

10.40

8.14

6.58

6.08

11.19

7.76

7.57

6.15

11.51


1 Peak surface brightness in ergs/s/cm2/Å/arcsec2 of the continuum or of an emission line from a diffuse source. For first-order and PRISM spectra, the calculation was done assuming use of the 52X2 aperture, while for echelle modes, the 6X0.2 aperture was used.
2 Peak flux in ergs/s/cm2/Å of an emission line from a point source.
3 Limits are V magnitudes, assuming zero reddening. Results for first-order gratings assume slitless spectra but neglect geocoronal lines. For echelles, the 0.2X0.2 aperture throughput is assumed.
4 The magnitudes listed for G2 V are for the Solar template in the STIS ETC.
5 The magnitudes listed for G8V are from IUE data for the star Tau Ceti.
6 The magnitudes listed for K2 V are from IUE data for the star Epsilon Eri.
7 The magnitudes listed for KM III are from IUE data for 9 stars of these types.
8 Limits for a black body with a temperature of 50,000 K.
9 Limits for a source with a spectrum Fλ proportional to λ–1.

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