5.1 Imaging Overview

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 5.13 we show the integrated system throughputs.  


Table 5.1: STIS Imaging Capabilities

Aperture NameFilterPivot Wavelength1c in Å)FWHM2  (Δλ in Å)Field of View (arcsec2)Detector

Visible - plate scale ~ 0.05078 arcseconds per pixel 3

50CCD

Clear

5852

4410

52 × 52

STIS/CCD

F28X50LP

Optical longpass

7229

2722

28 × 524

STIS/CCD

F28X50OIII

[O III]

50065

    6

28 × 526

STIS/CCD

F28X50OII

[O II]

3737

   62

28 × 527

STIS/CCD

50CORON

Clear +
coronagraphic fingers

5852

4410

52 × 52

STIS/CCD

Ultraviolet - plate scale ~0.0246 arcseconds per pixel 8

25MAMA
(NUV, FUV)

Clear

2250
1374

1202
  324

25 × 25

STIS/NUV-MAMA
STIS/FUV-MAMA

F25QTZ
(NUV, FUV

UV near longpass

2365
1595

  995
  228

25 × 25

STIS/NUV-MAMA
STIS/FUV-MAMA

F25SRF2
(NUV, FUV

UV far longpass

2299
1457

1128
  284

25 × 25

STIS/NUV-MAMA
STIS/FUV-MAMA

F25MGII

Mg II

2802

    45

25 × 25

STIS/NUV-MAMA

F25CN270

Continuum ~2700 Å

2709

  155

25 × 25

STIS/NUV-MAMA

F25CIII

C III]

1989

  173

25 × 25

STIS/NUV-MAMA

F25CN182

Continuum ~1800 Å

1981

  514

25 × 25

STIS/NUV-MAMA

F25LYA

Lyman-α

1221

    72

25 × 25

STIS/FUV-MAMA

Neutral-Density-Filtered Imaging 9

F25NDQ1
F25NDQ2
F25NDQ3
F25NDQ4
(Aperture,
NUV, FUV

ND=10–1
ND=10–2
ND=10–3
ND=10–4

1150–10,300 Å

13.4 × 9.7  
13.8 × 15.1
11.4 × 15.3
11.8 × 9.5  

STIS/ALL

F25ND3
(NUV, FUV

ND=10–3

1150–10,300 Å

25 × 25

STIS/ALL

F25ND5
(NUV, FUV

ND=10–5

1150–10,300 Å

25 × 25

STIS/ALL

1 See Section 14.2.1 for definition of pivot wavelength.
2 See Section 14.2.1 for definition of FWHM.
3 The CCD plate scales differ by ~1% in the AXIS1 and AXIS2 directions. See Section 14.6.
4 AXIS2=Y is 28 arcsec; AXIS1=X is 52 arcsec. See Figure 3.2 and Figure 11.1.
5 Values given for the F28X50OIII filter exclude the effects of this filter's red leak.
6 AXIS2=Y is 28 arcsec; AXIS1=X is 52 arcsec. See Figure 3.2 and Figure 11.1.
7 AXIS2=Y is 28 arcsec; AXIS1=X is 52 arcsec. See Figure 3.2 and Figure 11.1.
8 The MAMA plate scales differ by ~1% in AXIS1 and AXIS2 directions. FUV-MAMA uses different mirrors in filtered vs. unfiltered modes; the filtered mode plate scale has 0.3% more arcsec/pixel. Cf. Section 14.6.
9 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:

  • 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).
  • The quality of the low-order flat fields for the MAMA imaging modes limits the photometric accuracy obtained over the full field of view (see Section 16.1).
  • The focus varies across the field of view for imaging modes, with the optical performance degrading by ~40% at the edges of the field of view for MAMA and by ~30% for the CCD (see Section 14.7).
  • 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).
  • The STIS CCD will have far more "hot" pixels and a much higher dark current than the newer CCDs.
  • 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 7.2.10.)
  • 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).
  • 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.
  • The Charge Transfer Efficiency (CTE) of the STIS CCD is decreasing with time. The effects of the CTE decline are most serious for the lower rows of the detector and for faint sources with low background levels. For further details see Section 7.3.7.

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.


Table 5.2: Limiting A Star V Magnitudes

Detector

Filter

Magnitude

Filter

Magnitude

CCD

Clear

26.7

[O II]

21.3

CCD

Longpass

25.8

[O III]1

20.5

NUV-MAMA

Clear

23.9


 

 

NUV-MAMA

Longpass quartz

24.1

Longpass SrF2

24.1

NUV-MAMA

C III]

19.4

1800 Å continuum

21.5

NUV-MAMA

Mg II2

20.5

2700 Å continuum3

22.2

FUV-MAMA

Clear

20.5

Lyman-α

16.0

FUV-MAMA

Longpass quartz

21.8

Longpass SrF2

21.6

1 This filter has substantial red leaks see Section 5.2.4.
2 This filter has substantial red leaks see Mg II: F25MGII.
3 This filter has substantial red leaks see 2700 Å 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. STIS ISR 2015-06: STIS CCD Saturation Effects provides an in-depth analysis of the CCD saturation, in which it is found that the center of the detector has a full-well depth of 128,000 e but drops off towards the sides. The top of the detector has a substantially higher full-well depth at ~160,000 e, which can cause serial transfer artifacts.