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Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 22 > Chapter 5: Imaging > 5.1 Imaging Overview

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.   
Pivot1
Wavelength
c in )
FWHMa
(Δλ in )
Visible - plate scale ~ 0.05078 arcseconds per pixel2
52 52
50064
52 52
25 25
STIS/NUV-MAMA
STIS/FUV-MAMA
25 25
STIS/NUV-MAMA
STIS/FUV-MAMA
25 25
STIS/NUV-MAMA
STIS/FUV-MAMA
25 25
STIS/NUV-MAMA
25 25
STIS/NUV-MAMA
25 25
STIS/NUV-MAMA
25 25
STIS/NUV-MAMA
25 25
STIS/FUV-MAMA
F25NDQ15
F25NDQ2
F25NDQ3
F25NDQ4
ND=10–1
ND=10–2
ND=10–3
ND=10–4
13.4 9.7
13.8 15.1
11.4 15.3
11.8 9.5
STIS/NUV-MAMA
STIS/FUV-MAMA
STIS/CCD6
ND=10–3
25 25
STIS/NUV-MAMA
STIS/FUV-MAMA
STIS/CCD
f
ND=10–5
25 25
STIS/NUV-MAMA
STIS/FUV-MAMA
STIS/CCD
f
1
See Section 14.2.1 for definition of pivot wavelength and FWHM.
2
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.
3
The dimensions are 28 arcseconds on AXIS2=Y and 52 arcseconds on AXIS=X. See Figure 3.2 and Figure 11.1.
4
Values given for the F28X50OIII filter exclude the effects of this filter’s red leak.
5
Information on the F25NDQ aperture can be found on page 342.
6
The neutral density filters can only be used as available-but-unsupported apertures with the CCD detector.

5.1.1 Caveats for STIS Imaging
There are several important points about imaging with STIS which should be kept in mind:
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.
Two of the STIS narrow-band filters (F28X50OIII and F25MGII) have substantial red leaks (see Figure 5.5 and Figure 5.11, respectively).
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 , 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).
The repeller wire in the FUV-MAMA detector (see Section 7.4) leaves a 5-pixel-wide shadow that runs from approximately pixel (0, 543) to (1024, 563) in a slightly curved line. The exact position of the wire varies with the optical element used.
5.1.2 Throughputs and Limiting Magnitudes
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 5.1: STIS CCD Clear and Filtered Imaging Mode Throughputs
Figure 5.2: STIS NUV-MAMA Clear and Filtered Imaging Mode Throughputs
Figure 5.3: STIS FUV-MAMA Clear and Filtered Imaging Mode Throughputs
Limiting Magnitudes
In 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.
[O III]1
Mg II2
2700 continuum3
1
This filter has substantial red leaks see “[O III]: F28X50OIII”.
2
This filter has substantial red leaks see “Mg II: F25MGII”.
3
This filter has substantial red leaks see “2700 Continuum: F25CN270”.

5.1.3 Signal-To-Noise Ratios
In 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 Saturation
Both 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.

Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 22 > Chapter 5: Imaging > 5.1 Imaging Overview

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