STIS Instrument Handbook for Cycle 24

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5.1	Imaging Overview

5.1.1 Caveats for STIS Imaging

5.1.2 Throughputs and Limiting Magnitudes

5.1.3 Signal-To-Noise Ratios

5.1.4 Saturation

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

Aperture Name	Filter	Pivot Wavelength1 (λc in å)	FWHM2(Δλ in å)	Field of View (arcsec2)		Detector	Refer to Imaging Reference Material Page#
Visible - plate scale ~ 0.05078 arcseconds per pixel3
50CCD	Clear	5852	4410	52 â 52		STIS/CCD	390
F28X50LP	Optical longpass	7229	2722	28 â 524		STIS/CCD	393
F28X50OIII	[O III]	50065	6	28 â 526		STIS/CCD	396
F28X50OII	[O II]	3737	62	28 â 527		STIS/CCD	399
50CORON	Clear + coronagraphic fingers	5852	4410	52 â 52		STIS/CCD	402
Ultraviolet - plate scale ~0.0246 arcseconds per pixel8
25MAMA	Clear	2250 1374	1202 324	25 â 25		STIS/NUV-MAMA STIS/FUV-MAMA	404 431
F25QTZ	UV near longpass	2365 1595	995 228	25 â 25		STIS/NUV-MAMA STIS/FUV-MAMA	413 441
F25SRF2	UV far longpass	2299 1457	1128 284	25 â 25		STIS/NUV-MAMA STIS/FUV-MAMA	416 445
F25MGII	Mg II	2802	45	25 â 25		STIS/NUV-MAMA	418
F25CN270	Continuum ~2700 å	2709	155	25 â 25		STIS/NUV-MAMA	421
F25CIII	C III]	1989	173	25 â 25		STIS/NUV-MAMA	424
F25CN182	Continuum ~1800 å	1981	514	25 â 25		STIS/NUV-MAMA	427
F25LYA	Lyman-α	1221	72	25 â 25		STIS/FUV-MAMA	449
Neutral-Density-Filtered Imaging9
F25NDQ1 F25NDQ2 F25NDQ3 F25NDQ4	ND=10–1 ND=10–2 ND=10–3 ND=10–4	1150–10,300 å		13.4 â 9.7 13.8 â 15.1 11.4 â 15.3 11.8 â 9.5		STIS/NUV-MAMA STIS/FUV-MAMA STIS/CCD	357, 411 357, 439 357
F25ND3	ND=10–3	1150–10,300 å		25 â 25		STIS/NUV-MAMA STIS/FUV-MAMA STIS/CCD	407 435
F25ND5	ND=10–5	1150–10,300 å		25 â 25		STIS/NUV-MAMA STIS/FUV-MAMA STIS/CCD	409 437

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. c.f. 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 7.2.9.)

•	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).

•	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 “[O III]: F28X50OIII” on page 68.

2

This filter has substantial red leaks see “Mg II: F25MGII” on page 77.

3

This filter has substantial red leaks see “2700 å Continuum: F25CN270” on page 77.

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.