|Space Telescope Science Institute|
|STIS Instrument Handbook|
5.1 STIS can be used to obtain images in undispersed light in the optical and ultraviolet (UV). When STIS is used in imaging mode, the appropriate clear or filtered aperture on the slit wheel is rotated into position, and a mirror on the Mode Selection Mechanism is moved into position (see Figure 3.1).Table 5.1 provides a complete summary of the clear and filtered apertures available for imaging with each detector. In Figure 5.5 through Figure 5.6 we show the integrated system throughputs.
Table 5.1: STIS Imaging Capabilities Pivot1
(λc in Å)
Visible - plate scale ~ 0.05078 arcseconds per pixel2 52 × 52 50064 52 × 52 STIS/FUV-MAMA F25NDQ15
13.4 × 9.7
13.8 × 15.1
11.4 × 15.3
11.8 × 9.5
ND=10–3 ND=10–5 The CCD and MAMA plate scales differ by about 1% in the AXIS1 and AXIS2 directions, a factor that must be taken into account when trying to add together rotated images. Also, the FUV-MAMA uses a different mirror in the filtered and unfiltered modes. In the filtered mode, the plate scale is 0.3% larger (more arcsec/pixel). Information on geometric distortions can be found in Section 14.6.The dimensions are 28 arcseconds on AXIS2=Y and 52 arcseconds on AXIS=X. See Figure 3.2 and Figure 11.1.The neutral density filters can only be used as available-but-unsupported apertures with the CCD detector.
5.1.1 Caveats for STIS Imaging
• The filters are housed in the slit wheel, and while they are displaced from the focal plane, they are not far out of focus. This location means that imperfections (e.g., scratches, pinholes, etc.) in the filters cause artifacts in the images. These features do not directly flat-field out because the projection of the focal plane on the detector shifts from image to image due to the nonrepeatability of the Mode Selection Mechanism’s (MSM) placement of the mirror (careful post-processing may be able to account for registration errors).
• STIS CCD imaging slightly undersamples the intrinsic PSF. The use of dithering (see Section 11.3) to fully sample the intrinsic spatial resolution and to cope with flat-field variations and other detector nonuniformities may be useful for many programs.
• Programs requiring high photometric precision at low count levels with the CCD should use GAIN=1; programs at high count levels should use GAIN=4. At GAIN=4 the CCD exhibits a modest read noise pattern that is correlated on scales of tens of pixels. (See Section 7.2.9.)
• At wavelengths longward of ~9000 Å, internal scattering in the STIS CCD produces an extended PSF halo (see Section 7.2.8). Note that the ACS WFC CCDs have a front-side metallization that ameliorates a similar problem in that camera, while the WFC3 CCD does not exhibit this problem.
• The dark current in the MAMA detectors varies with time and temperature, and in the FUV-MAMA it also varies strongly with position, although it is far lower overall than in the NUV-MAMA (see the discussion of Section 7.5.2).
• In Figure 5.1, Figure 5.2, and Figure 5.3, we show the throughputs (where the throughput is defined as the end-to-end effective area divided by the geometric area of a filled, unobstructed, 2.4 meter aperture) of the full set of available filters for the CCD, the NUV-MAMA, and the FUV-MAMA, respectively.Figure 5.2: STIS NUV-MAMA Clear and Filtered Imaging Mode ThroughputsFigure 5.3: STIS FUV-MAMA Clear and Filtered Imaging Mode ThroughputsIn Table 5.2 below, we give the A0 V star V magnitude reached during a one-hour integration which produces a signal-to-noise ratio of 10 integrated over the number of pixels needed to encircle 80% of the PSF flux. The sensitivities adopted here are our best estimate for August 2008. The observations are assumed to take place under average zodiacal background and low earth shine conditions. These examples are for illustrative purposes only and the reader should be aware that for dim objects, the exposure times can be highly dependent on the specific background conditions. For instance, if a 26.9 magnitude A star were observed under high zodiacal light and high earth shine, the exposure time required to reach signal-to-noise of 10 with CCD clear would be twice as long as the one stated in Table 5.2.
Table 5.2: Limiting A Star V Magnitudes [O III]1 Mg II2 2700 Å continuum3
5.1.3 Signal-To-Noise RatiosIn Chapter 14 we present, for each imaging mode, plots of exposure time versus magnitude to achieve a desired signal-to-noise ratio. These plots, which are referenced in the individual imaging mode sections below, are useful for getting an idea of the exposure time you need to accomplish your scientific objectives. More detailed estimates can be made either by using the sensitivities given in Chapter 14 or by using the STIS Imaging Exposure Time Calculator (ETC).5.1.4 SaturationBoth CCD and MAMA imaging observations are subject to saturation at high total accumulated counts per pixel: the CCD due to the depth of the full well and the saturation limit of the gain amplifier for CCDGAIN = 1; and the MAMA due to the 16-bit format of the buffer memory (see Section 7.3.2 and Section 7.5.1). In Chapter 14, saturation levels as functions of source magnitude and exposure time are presented in the S/N plots for each imaging mode.