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Instrument Science Report ACS 00-05 MEASURED THROUGHPUT AND BANDPASS OF THE RAMP FILTERS R. C. Bohlin and Z. Tsvetanov 2000 June

ABSTRACT ACS has 12 narrow band ramp filters with 2% bandpasses of / and three ramps with medium bandpasses of 9%. Both the narrow and medium bandpass sets cover the range 3700-10700е with a nominal 40x70 arcsec field of view. The transmission of the bandpasses have been measured as a function of wavelength and are fit with analytic functions. These characterizations are used to generate a uniform grid of 11 transmission curves over the wavelength range of each of the 15 ramp filters. This grid of transmission curves has been formatted and delivered to the STScI Synphot data base for use in the ACS Exposure Time Calculator and for instrumental absolute flux calibrations. Synphot computes the transmission curve at any wavelength in the range of a ramp filter by interpolation between the shapes of the two curves nearest in wavelength from the grid of 11.

1. Measurements and Analytic Fits The final transmissions of the ramp filters T() are empirical analytic fits to the data as a function of wavelength in Angstroms with the form: T() = BP + Ped + Z, where BP is the primary component of the transmission, which sits on a broader pedestal level (Ped), all of which has a minimum out-of-band zero level transmission of Z=1e-6. The BP and Ped are of the form: BP = Tavg F() where
Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.


Instrument Science Report ACS 00-05

F()=1/{[(-W Ped = 7

Ped = 7 The peak transmission is characterized for each filter by a quadratic fit: Tavg() = b0+ b1 Wcen+ b2 Wcen^2, where the wavelength is in Angstroms. The fractional FWHM is fit by: FWHM() = c0+ c1 Wcen+ c2 Wcen^2. Wpc is the pedestal central wavelength in е that is used in the filter name, eg. for FR656N, Wpc=6560. Wcen is the central wavelength of a passband in the range of the minimum to the maximum wavelength in Table 1. The 15 ACS ramp filters are listed in Table 1, along with the b and c coefficients and ancillary data.
Table 1. ACS Ramp Filters
Filter Name FR388N FR423N FR462N FR505N FR551N FR601N FR656N FR716N FR782N FR853N FR931N FR1016N FR459M FR647M FR914M Ramp Band OII OII OII OIII OIII OIII H H H IR IR IR Broad Broad Broad Minimum (е) 3710 4049 4420 4824 5266 5748 6274 6848 7474 8158 8905 9719 3810 5366 7574 Maximum (е) 4049 4420 4824 5266 5748 6274 6848 7474 8158 8905 9719 10609 5366 7574 10709 Segment Location middle inner outer middle inner outer middle inner outer inner outer outer middle inner middle Filter Wheel Number 36 36 36 50 50 50 38 38 38 48 48 49 49 49 48 b0 3.5965 1.2132 14.606 1.1781 5.4225 6.3219 -1.5151 -1.1078 -2.8885 6.3224 -13.680 -4.1362 -2.7452 -0.0227 1.056 b1 -1.877E-3 -3.647E-4 -6.143E-3 -1.756E-4 -1.735E-3 -1.900E-3 6.860E-4 4.744E-4 8.326E-4 -1.289E-3 3.103E-3 1.005E-3 1.458E-3 2.271E-4 -6.92E-5 b2 2.794E-7 5.136E-8 6.778E-7 1.717E-8 1.607E-7 1.616E-7 -5.121E-8 -2.880E-8 -4.559E-8 7.569E-8 -1.658E-7 -5.081E-8 -1.500E-7 -1.559E-8 4.213E-9 ACS # 25M 25I 25O 26M 26I 26O 27M 27I 27O 28I 28Ob 28Oc 24Ma 24I 24Mc c0 -0.0400 -0.0658 -0.1250 -0.2545 -0.0930 -0.0638 0.1285 -0.2437 -0.4023 -0.0324 -0.4723 0.1584 -0.2946 -0.0641 0.001 c1 2.486E-5 3.437E-5 5.735E-5 1.046E-4 3.714E-5 2.624E-5 -3.385E-5 7.193E-5 1.045E-4 1.165E-5 1.036E-4 -2.741E-5 1.423E-4 3.720E-5 1.000e-5 c2 -2.243E-9 -3.411E-9 -5.693E-9 -9.922E-9 -3.113E-9 -2.072E-9 2.626E-9 -4.931E-9 -6.471E-9 -6.80E-10 -5.448E-9 1.360E-9 -1.351E-8 -2.362E-9 1.000e-9

8 cen)/(FWHM Wcen/2)] + 1}. 10-4 e-{[-Wpc]4/(2[0.047Wpc]4)} for the narrow band filters and 10-4 e-{[-Wpc]6)/(2[0.197Wpc]6)} for the medium band filters.

2. Results From 11 profiles spaced equally by 10% of the wavelength coverage of each ramp, Synphot finds the closest pair in wavelength from this set of 11

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Instrument Science Report ACS 00-05

and accurately interpolates the transmission curve for any wavelength within the bandpass of each ramp. Figure 1 demonstrates that the interpolation of the shapes of the adjacent bandpasses produces errors that are usually less than 0.1%. The 11 profiles for each ramp filter are computed by the IDL routine RAMPROF.pro and formatted as FITS binary tables for delivery to Synphot by RAMPFITS.pro. Figures 2-3 illustrate the smooth, continuous change of the bandpass width with wavelength for three ramp filter throughput profiles on each of the 15 ramp filters. 3. Acknowledgements Thanks to W. Sparks, G. Meurer, and M. Clampin for careful reading, comments, and corrections to drafts of this ISR.

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Figure 1: Heavy solid line is the bandpass of the FR656N filter at 6303е, while the light dashed and dotted lines are the adjacent bandpasses at 6274 and 6331е, respectively, that are used in the interpolation scheme. The light solid line is the error expressed as the percent difference between the interpolated shape at 6303е and the computed shape at 6303е. The error in the interpolation procedure is usually less than 0.1% but does exceed 1% in the short wavelength wing, where the transmission is <0.001. The analytic approximations to the transmission curves are non-physically asymmetric near the ends of their wavelength coverage; but the error in these computed transmissions is less than ~0.001.

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Figure 2: Sample transmission profiles for each of the 12 ACS narrow band ramp filters. The three samples are at the minimum, middle, and maximum of the wavelength coverage and are shown with the linestyle alternating between solid and dotted from one ramp to the next. The longest wavelength passband of a filter nearly coincides with the shortest passband of the next ramp. The peak transmission of the pedestal is just below 10-3.

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Figure 3: As in Figure 1, except for the medium band ramp filters.

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