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Exp osure Time and S/N calculated by the ETC for the G130M/1055 and G130M/1096 Settings are not correct The focus of the G130M/1055 and G130M/1096 settings were recently adjusted to increase their resolution. As a consequence, the sp ectral height and size of the resolution element (resel) have changed. Because of these two changes, exp osure times may b e underestimated by as much as a factor of 4, and S/N may b e overestimated by as much as a factor of 2. The ETC assumes a resel of 30 pixels and an extraction height of 47 pixels, but the true resels are smaller and dep end on wavelength; the true extraction heights are larger. The ETC will b e up dated for phase I I of Cycle 21, in the meantime the correction factors given in the Known Issues page of the ETC should b e used. Conversely, users can refer to the formulas b elow to determine their own correction based on more detailed information. ETC Corrections for resel size and extraction height The correction factors were calculated in the following manner by utilizing the standard exp osure time and S/N calculations as detailed in the ETC Users guide. The ETC assumes that all flux from the source is captured within the default extraction height multiplied by the sp ectral resel width to determine the total count rate p er resel. The corrections can b e determined by ratioing the equation for exp osure time or S/N with the correct resel size and extraction height and with what the ETC calculates. If one defines the ratio of source count rate p er pixel to dark count rate p er pixel as: = C B
dar k

(1)

The correction for exp osure time is given by: t= 30 + E H t R + 47
ET C

(2)

where EH refers to the extraction height (listed in Table 1) for a given central wavelength, R is the resel size in pixels (which can vary with wavelength and is listed in Table 2) and tE T C is the ETC calculated exp osure time. As an example, consider a faint source where 0 at =950° in the G130M/1096 central wavelength. The extraction height for this A setting is 59 pixels and the resel is roughly exp ected to b e 12 pixels. The full correction factor would b e 3.1вtE T C . If 59 as would b e the case for a very bright source, then the full correction factor would b e 2.5вtE T C . Similarly, one can calculate a new S/N: S/N = R +1 30 + E H 47
1 2

SN R

ET C

.

(3)

Tables 3 and 4 give corrections to the ETC derived S/N and exp osure time for given values of as a function of wavelength.

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G130M/1055 G130M/1096

63 59

Table 1: Extraction Heights for the 1055 and 1096 central wavelengths Caveats Since these central wavelengths have not b een fully characterized, the quoted resel sizes may change at a later date. The resel sizes quoted here are estimated from raytrace models of the settings at the current focii that include the mid-frequency wavelength errors which broaden the COS FUV line spread function. If the wavelength of interest is near a bright geocoronal line such as Lyman , the sky will need to b e taken into account. In those cases, one must return to the original S/N and exp osure time equations and determine the correction for the sp ecific situation while taking the sky contribution into account. Similarly, the equations ab ove are most appropriate for p oint sources. Observers doing calculations for extended ob jects should contact the STScI help desk for further information. Setting Segment B 1055 A Wavelength 900 950 1040 1050 1120 1190 940 1000 1050 1080 1100 1170 1230 Resel Size 10.6 8.3 19.0 21.5 32.1 42.6 11.9 8.4 8.6 11.9 15.3 23.9 31.6

B 1096 A

Table 2: Predicted Resel Sizes

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Setting

Extraction Height (pixels)

Segment

B 1055 63 A

B 1096 59 A

° (A) 900 950 1040 1050 1120 1190 940 1000 1050 1080 1100 1170 1230

Resel (pixels) 10.6 8.3 19.0 21.5 32.1 42.6 11.9 8.4 8.6 11.9 15.3 23.9 31.6

=0 3.79 4.85 2.12 1.87 1.25 0.94 3.16 4.47 4.36 3.16 2.46 1.57 1.19

=1 3.77 4.83 2.11 1.86 1.25 0.94 3.14 4.45 4.34 3.15 2.45 1.57 1.19

=10 3.62 4.64 2.03 1.79 1.20 0.90 3.04 4.31 4.21 3.05 2.37 1.52 1.15

=100 3.13 4.01 1.75 1.55 1.04 0.78 2.72 3.85 3.76 2.73 2.12 1.35 1.03

= 2.82 3.62 1.58 1.40 0.93 0.70 2.51 3.56 3.48 2.52 1.96 1.25 0.95

Table 3: Exp osure Time correction factors

Setting

Extraction Height (pixels)

Segment

B 1055 63 A

B 1096 59 A

° (A) 900 950 1040 1050 1120 1190 940 1000 1050 1080 1100 1170 1230

Resel (pixels) 10.6 8.3 19.0 21.5 32.1 42.6 11.9 8.4 8.6 11.9 15.3 23.9 31.6

=0 0.51 0.45 0.69 0.73 0.89 1.03 0.56 0.47 0.48 0.56 0.64 0.80 0.92

=1 0.52 0.46 0.69 0.73 0.90 1.03 0.56 0.47 0.48 0.56 0.64 0.80 0.92

=10 0.53 0.46 0.70 0.75 0.91 1.05 0.57 0.48 0.49 0.57 0.65 0.81 0.93

=100 0.57 0.50 0.76 0.80 0.98 1.13 0.61 0.51 0.52 0.61 0.69 0.86 0.99

= 0.59 0.53 0.80 0.85 1.03 1.19 0.63 0.53 0.54 0.63 0.71 0.89 1.03

Table 4: S/N correction factors

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