5.5 Ramp Filters

5.5.1 Introduction

Five slots on ACS filter wheel #2 each contain a ramp filter unit that is made up of an inner, an outer, and a middle segment. For the sake of simplicity, these three segments will be referred to as individual inner (IRAMP), outer (ORAMP), and middle (MRAMP) ramp "filters" although strictly speaking, they form part of the same physical ramp filter unit.

All fifteen ramp filters can be used with the WFC over specified regions of the WFC1 and WFC2 chips. IRAMP and ORAMP filters can only be placed on the WFC1 or WFC2 chips, respectively, over regions which define the WFC1-IRAMP and WFC2-ORAMP apertures. The MRAMP filter is, by default, placed on the WFC1 chip over the WFC1-MRAMP aperture, but it can, in principle, also be placed on the WFC2 chip over the WFC2-MRAMP aperture. The different ramp apertures and their reference points on the WFC chip are shown in Table 7.6 and Figure 7.4 of the ACS Instrument Handbook. In practice, the observer specifies a ramp filter and a central wavelength, and the filter wheel is appropriately rotated in order to place the central wavelength at the reference point of the relevant aperture.

With the HRC, only the five middle ramp filters were available and they cover the region over the HRC chip defined by the HRC aperture (Table 7.7ACS Instrument Handbook).

The fifteen ramp filters provided a total wavelength coverage of 3700 Å10,700 Å. Twelve of the filters have a narrow bandpass (Δλ/λ) of 2% and three have a medium bandpass of 9% (Table 5.2, ACS Instrument Handbook).

5.5.2 Existing Ground Calibrations

For each of the 15 ramp filters and different specified central wavelengths, the transmission was measured as a function of wavelength over the bandpass. For each ramp, analytical fits to these measurements generated eleven transmission profiles spaced by 10% of the wavelength coverage of the ramp (ACS ISR 2000-05). Using these delivered profiles, pysynphot interpolates the transmission curve for any wavelength within the bandpass of each ramp filter. The interpolation errors are below 1% which is better than the measurement accuracy.

WFC and HRC flat fields have been derived from pre-flight data by illuminating the ramps with continuum and monochromatic light using the Refractive Aberrated Simulator/Hubble Opto-Mechanical Simulator (RAS/HOMS) (ACS ISR 2002-01). Each of the five WFC ramp filter units associated with the five wheel #2 slots is made up of three segments (inner, outer, middle) which cover three different adjacent wavelength ranges. A composite WFC flat was made for each of the five ramp filter units by combining three pieces of full field broad-band surrogate LP-flats (flats that incorporate both low frequency and pixel-to-pixel variations) nearest in central wavelength. For instance, for the ramp filter unit made up of FR505N (middle), FR551N (inner), and FR601N (outer) segments, surrogate broad-band LP-flats of F475W, F555W, and F606W were used.

HRC ramp flats were made from surrogate HRC flats appropriate for the middle segment. The broad-band flats reduce the rms structure in monochromatic P-flats from 1% to 0.2%, except at the shortest measured wavelength of 3880 Å.

After flat-fielding some test ramp images with the relevant composite L-flats, correcting for anamorphic geometric distortion, rotating, and median filtering to remove residual noise, the response for FR505N is found to be uniform to below 1% over a 30 × 80 arcsec field of view for the WFC and 11.5 × 24 arcsec FOV for the HRC. Out of the five middle ramp filters, the FR388N shows the least uniform response (< 10%) over such regions (ACS ISR 2002-01).

Values for the dispersion of the middle ramp filters FR388N, FR459M, FR505N, FR656N, and FR914M are, respectively, 0.192, 0.0399, 0.146, 0.120, 0.0219 mm/Å at the ramp filters' wavelength.

Due to the extra divergence of the beam from the filter to the detector, the dispersion at the WFC and HRC detector is estimated to be a factor of 1.2 and 1.3 lower respectively.

5.5.3 In-Flight Calibrations

An attempt has been made to check the wavelength calibration of the ramp filters by crossing them with the grism (ACS ISR 2007-11). In these tests, the wavelength setting of HRC ramps agreed well with the calibration measurements. In contrast, some of the WFC ramp filters showed notable offsets from their expected centers when tested in this fashion.

There is reason to believe, however, that the offsets seen in the WFC ramps may be caused by a "filter wedge" produced by using the grism and ramp filters in combination. One of the filters that showed the largest offset with respect to its width was FR782N. This filter was used to observe red-shifted Hα sources, and showed that the HST fluxes agreed with those measured from the ground to 20%, which is within their continuum subtraction error (Ovezier and Heckman, personal communication). For now, observers are advised to use the standard settings of the ramps for their observations when using the WFC.