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 your source will violate the limits in this table, you must first correct the magnitude limit for the aperture throughput for your chosen slit. The maximum magnitude correction achieved without use of a neutral density filter using a supported slit is ~0.75 magnitudes. An exception to this are the values for the local surface brightness limits in row 1 of Table 13.45. They were calculated for a 52X2 slit.
  • The screening limits in the tables assume zero extinction. To determine if your source will violate the limits in this table, correct the magnitude limit for the extinction of your source.
  • The peak flux from an emission line or from the continuum from your source must be less than the flux limit given in row two (for point sources—remember to correct for your aperture throughput) and row one for diffuse sources (remember to correct for the width of your source by scaling by your 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 you must also assure that your source does not violate the local limit, e.g., if it had a bright emission line.
    • If you are observing a source which has high equivalent width line emission (i.e., whose flux is dominated by line emission), you must assure 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 you plan to place multiple bright stars in the long slit, or observe slitless, you must also assure 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; use of a less sensitive central wavelength may have a brighter true limit, allowing you to observe your target. The STIS ETC should always be used to verify the safety of your detailed 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

14.3

11.9

10.13

9.49

13.6

10.3

9.87

8.65

14.7

B1 V

13.5

11.1

9.33

8.59

13.1

9.8

9.37

8.15

14.2

B3 V

12.6

10.2

8.53

7.69

12.5

9.2

8.87

7.55

13.8

B5 V

11.9

9.4

7.83

7.19

12.1

8.7

8.37

7.05

13.4

B8 V

10.7

8.2

6.73

6.19

11.3

7.9

7.67

6.25

12.7

A1 V

8.4

5.3

4.43

3.79

10.4

6.9

6.67

5.35

11.9

A3 V

6.9

4.2

3.13

2.49

10.1

6.7

6.47

5.15

11.7

A5 V

5.1

3.8

1.43

0.89

9.9

6.5

6.37

4.95

11.6

F0 V

2.1

1.1

–1.47

<–1.61

9.4

6.3

5.97

4.65

11.5

F2 V

1.1

0.1

<–1.77

<–1.61

9.1

6.2

5.77

4.45

11.4

F5 V

–1.1

<–2.0

<–1.77

<–1.61

8.7

6.0

5.37

4.25

11.2

F8 V

<–2.0

<–2.0

<–1.77

<–1.61

8.4

5.8

5.17

4.05

11.0

G2 V4

0.5

–1.3

<–1.77

<–1.61

7.8

5.3

4.57

3.55

10.6

G8 V5

2.6

–0.2

<–1.77

<–1.61

7.3

4.8

4.70

3.15

10.1

K2 V6

4.0

1.3

<–1.77

<–1.61

6.5

4.0

3.27

2.35

9.4

KM III7

2.2

–0.9

<–1.77

<–1.61

6.1

3.7

2.87

2.15

9.1

T~50000 K8

14.2

11.8

10.0  

9.29

13.6

10.2

9.77

8.65

14.6

(λ–1)9

9.2

6.6

6.63

4.39

10.4

7.8

6.87

6.25

12.7


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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