Imaging Overview

STIS can be used to obtain images in undispersed light in the optical and ultraviolet. 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 5.1 through Figure 5.4 we show the integrated system throughputs.   

Table 5.1: STIS Imaging Capabilities

Aperture Name	Filter	Central Wavelength (c in Å)	FWHM ( in Å)	Field of View (arcsec)		Detector	ref. page
Visible - plate scale per pixel
50CCD	Clear	5850	4410	52 x 52		STIS/CCD	376
F28X50LP	Optical longpass	7230	2720	28 x 521		STIS/CCD	379
F28X50OIII	[O III]	5007	5	28 x 52 1		STIS/CCD	382
F28X50OII	[O II]	3740	80	28 x 52 1		STIS/CCD	385
50CORON	Clear + coronographic fingers	5850	4410	52 x 52		STIS/CCD	388
Ultraviolet - plate scale ~0.0246 arcseconds per pixel2
25MAMA	Clear	2220 1370	1200 320	25 x 25		STIS/NUV-MAMA STIS/FUV-MAMA	390 417
F25QTZ	UV near longpass	2320 1590	1010 220	25 x 25		STIS/NUV-MAMA STIS/FUV-MAMA	395 422
F25SRF2	UV far longpass	2270 1480	1110 280	25 x 25		STIS/NUV-MAMA STIS/FUV-MAMA	402 429
F25MGII	Mg II	2800	70	25 x 25		STIS/NUV-MAMA	404
F25CN270	Continuum near 2700 Å	2700	350	25 x 25		STIS/NUV-MAMA	407
F25CIII	C III]	1909	70	25 x 25		STIS/NUV-MAMA	410
F25CN182	Continuum near 1800 Å	1820	350	25 x 25		STIS/NUV-MAMA	413
F25LYA	Lyman-	1216	85	25 x 25		STIS/FUV-MAMA	432
Neutral-Density-Filtered Imaging
F25NDQ1 F25NDQ2 F25NDQ3 F25NDQ4	ND=10-1 ND=10-2 ND=10-3 ND=10-4	1150-10,300 Å		12 x 12 12 x 12 12 x 12 12 x 12		STIS/CCD STIS/NUV-MAMA STIS/FUV-MAMA	76
F25ND3	Neutral-density filter, ND=10-3	1150-10,300 Å		25 x 25		STIS/NUV-MAMA STIS/FUV-MAMA	393 420
F25ND5	ND=10-5	1150-10,300 Å		25 x 25		STIS/NUV-MAMA STIS/FUV-MAMA	395 422

1 The dimensions are 28 arcsec on AXIS2=Y and 52 arcsec on AXIS1=X. See Figure 3.2 and Figure 11.1 on page 213. 2 The 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). Further information on geometric distortions can be found in Chapter 14

Why Image With STIS?

Despite its limited complement of filters, STIS brings several valuable imaging capabilities to HST:

• The STIS CCD detector, although it covers a much smaller field of view (51 x 51 arcsec) than ACS and WFPC2, has higher throughput over a much wider range of the spectrum (2000-11,000 Å) than does WFPC2. The STIS CCD also has a low read noise and dark current; thus STIS CCD observations with the clear aperture have significantly higher sensitivity to faint sources than WFPC2 ( Figure 5.6). The CCD long-pass imaging filter, F28X50LP, has similar sensitivity longward of 5500 Å.The STIS CCD also has a low read noise and dark current; thus STIS CCD observations with the clear or longpass apertures are extremely sensitive to faint sources (see Figure 5.6). The STIS CCD clear imaging mode is especially useful when no color information is needed, for example, for finding faint variable sources, or imaging the faintest possible sources in a given integration time.
• The wings of the point-spread function in the STIS CCD imaging modes are suppressed by an internal Lyot stop. This feature provides a significant advantage for detecting faint sources near much brighter sources.
• The STIS CCD detector can also be used with the coronographic aperture. This aperture contains two occulting wedges and an occulting bar and is especially useful for detecting faint material surrounding a bright source. See Coronographic Imaging-50CORON.
• The STIS FUV-MAMA detector enables true solar-blind imaging with high throughput from 1150 to 1700 Å with a considerably higher throughput than WFPC2 and with a lower dark rate and a better PSF sampling than ACS. The NUV-MAMA is also relatively insensitive to red light and has also high relative throughput. Both provide near-critical sampling of the PSF (0.024 arcsecond per pixel). See Unfiltered (Clear) MAMA Imaging-25MAMA.
• The STIS MAMA detectors enable very high time resolution ( ~125 microseconds) imaging in the ultraviolet, by means of TIME-TAG mode.

Caveats For STIS Imaging

There are several important points about imaging with STIS which should be kept in mind:

• 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 placement of the mirror (careful post-processing may be able to account for registration errors).
• Accurate across-the-detector photometric calibration of the imaging modes requires mapping of the two-dimensional illumination pattern of the sky; for the MAMAs this is a time-intensive process, which will limit the photometric accuracy obtained (see Summary of Accuracies).
• The focus varies across the field of view for imaging modes, with the optical performance degrading by ~30% at the edges of the field of view for MAMA imaging and less so for the CCD (see "Spatial Dependence of the STIS PSF" on page 436).
• STIS CCD imaging slightly undersamples the intrinsic PSF. The use of dithering (See "Patterns and Dithering" on page 229) 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 narrowband filters (F28X50OIII and F25MGII) have substantial red leaks (see Figure 5.7 and Figure 5.14, respectively).
• Relative to WFPC2, a STIS CCD image will have a slightly larger proportion of the pixels affected by cosmic rays and "hot" dark current.
• 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 Analog-to-Digital Conversion: Selecting the CCDGAIN.)
• At wavelengths longward of ~900 nm, internal scattering in the STIS CCD produces an extended PSF halo (see Optical Performance).
• The dark current in the MAMA detectors varies with time, 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 MAMA Darks).
• The repeller wire in the FUV-MAMA detector (see The MAMA Detectors) leaves a 5-pixel-wide shadow that runs from pixel 0,543 to 1024,563 in a slightly curved line.

These caveats are not intended to discourage observers from using STIS for imaging; indeed, for many imaging projects, particularly those not requiring a large field of view or range of filters provided by WFPC2 and ACS, STIS may be the best choice.

STIS versus ACS Imaging

ACS will be installed in a future Servicing Mission. ACS is designed primarily as an imager, whereas the main scientific capabilities of STIS are spectroscopic. Therefore observers will find that many imaging programs are more efficiently done with ACS than with STIS. This applies to the optical wavelength range in particular, where STIS offers only a limited choice of filters. Together with its larger field of view, this will make ACS the instrument of choice for imaging at optical wavelengths in most cases.

The choice between STIS and ACS for ultraviolet imaging largely depends on the details of the science requirements. ACS offers a larger variety of filters. On the other hand, STIS has better sampling of the PSF, thus providing higher spatial resolution, and it has a much lower dark current. Observers must critically examine how the different field of view of STIS and ACS affect their science. ACS with its larger field of view will be the preferred instrument for survey-type studies. For high-resolution imaging of individual objects whose size is small in comparison with the detector, bright object considerations may become important. Remember that it is the observer's responsibility to provide UV flux information of all field objects that happen to fall into the field of view. The larger the field size, the higher the likelihood of including such field objects.

The ACS Instrument Handbook for Cycle 11 provides additional comparisons between the capabilities of STIS and ACS. Ultimately, observers should use the exposure time calculators of STIS and ACS to decide which of the two instruments is better suited for their science.

Throughputs and Limiting Magnitudes

In Figure 5.1 below, we show the throughput (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 three STIS clear imaging modes, with the CCD, the NUV-MAMA, and the FUV-MAMA. Superposed on this plot, we show the broadband WFPC2 throughputs. In Figure 5.2, Figure 5.3, and Figure 5.4, we show the throughputs of the full set of available filters for the CCD, the NUV-MAMA, and the FUV-MAMA, respectively.

We are currently revising the throughput for the CCD imaging modes at wavelengths longer than 6000 Å. Our current estimates suggest that the throughput of the 50CCD imaging mode should be decreased by roughly 10% near 6000 Å and by as much as 35% near 10,000 Å. The ratio of the F28X50LP throughput to the 50CCD throughput will be increased by about 20%. The exact wavelength dependence of these changes is still under review and the adopted changes will be reported in the HST Spectroscopy STScI Analysis Newsletter (STAN) and on the STScI website.

Figure 5.1: STIS's Clear Imaging Throughputs Versus WFPC2

Figure 5.2: STIS's CCD Clear and Filtered Imaging Mode Throughputs

Figure 5.3: STIS's NUV-MAMA Clear and Filtered Imaging Mode Throughputs

Figure 5.4: STIS's 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 observations are assumed to take place in shadow, with average zodiacal background. These examples are for illustrative purposes only. Since sky background becomes important for the source count rates in Table 5.2, the exposure times will be much longer outside SHADOW. For instance, if a 27.7 mag A star were observed in Bright Earth, the exposure time to reach signal-to-noise=10 with CCD clear would be about 4 times longer.

Table 5.2: Limiting A Star V Magnitudes

Detector	Filter	Magnitude	Filter	Magnitude
CCD	Clear	27.7	[O II]	21.9
CCD	Longpass	26.7	[O III]*	21.2
NUV-MAMA	Clear	24.1
NUV-MAMA	Longpass quartz	24.1	Longpass SrF2	24.1
NUV-MAMA	C III]	19.9	1800 Å continuum	22.0
NUV-MAMA	Mg II*	20.1	2700 Å continuum*	22.0
FUV-MAMA	Clear	21.5	Lyman-	16.8
FUV-MAMA	Longpass quartz	22.3	Longpass SrF2	22.9

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. A web based version of this tool can be reached from the STIS web page (http://www.stsci.edu/instruments/stis/), under Tools.

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 CCD Saturation and MAMA Saturation-Overflowing the 16 Bit Buffer). In Chapter 14, saturation levels as functions of source magnitude and exposure time are presented in the S/N plots for each imaging mode.