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19.2 Basic Instrument Operations

Target Acquisitions and Peakups

 Once the telescope acquires its guide stars, your target will be within ~1-2 arcseconds of the aperture center. For science observations taken through slits less than three arcseconds in either dimension, and for science observations involving the coronagraphic bars, a target acquisition exposure is taken to center the target in your chosen science aperture and is followed by one or more peakup exposures to refine the target centering of point or point-like sources. Acquisition exposures always use the CCD, one of the filtered or unfiltered apertures for CCD imaging, and a mirror as the optical element in the grating wheel. Peakup exposures use a science slit or coronagraphic aperture, the CCD, and either a mirror or a spectroscopic element in the grating wheel.

Routine Wavecals

Each time the MSM is moved to select a new optical element or to tilt a grating, the resulting spectrum is projected onto the detector with an error (lack of repeatability) of roughly plus or minus 1 to 10 pixels (better for some modes and worse for others). In addition, thermal effects cause the spectrum to drift slowly with time (typical drifts are 0.1 pixels per orbit, extreme cases in the case of forced large temperature swings registered as high as 0.35 pixels per orbit). An internal calibration lamp observation (WAVECAL) is automatically taken following each use of a new grating element or new scan position (grating tilt) and every 40 minutes thereafter, in order to allow calibration of the zero point of the wavelength (dispersion) and spatial (cross dispersion) axes in the spectroscopic science data during post observation data processing. These routine, automatically-occurring, wavecal observations are expected to provide sufficient wavelength zeropoint accuracy for the large majority of GO science.

Data Storage and Transfer

At the conclusion of each exposure, the science data are read out from the detector in use and placed in STIS's internal buffer memory, where they are stored until they can be transferred to the HST data recorder (and thereafter to the ground). This design makes for more efficient use of the instrument, as up to seven CCD or four MAMA full frame images can be stored in the internal buffer at any time. The frames can be transferred out of the internal buffer to the data recorder during subsequent exposures, as long as those exposures are longer than three minutes.

STIS's internal buffer stores the data in a 16-bit per pixel format. This format imposes a maximum of 65,536 data numbers per pixel. For the MAMA detectors this number is equivalent to a limit on the total number of photons per pixel which can be accumulated in a single exposure. The CCD full well (and not the 16-bit buffer format) limits the photons per pixel that can accumulate without saturating in a single exposure, for both GAIN=1 and GAIN=4 observations.

Parallel Operations

STIS's three detectors do not operate in parallel-only one detector can be used at any time. Exposures with different STIS detectors can, however, be freely interleaved in an observing sequence, and there is no extra setup time or overhead in moving from one detector to another. The three detectors, sharing the bulk of their optical paths, also share a common field of view of the sky. STIS can be used in parallel with any of the other three science instruments on HST; however, use of the MAMA detectors in parallel is restricted.


STIS Spectroscopic Capabilities

Spectral Range (Å)

Spectral Resolution

Grating

Complete

Per Tilt

Scale

(Å per pixel)

Resolving

Power (/2)

# Prime

Tilts1

Detector

Slits

(apertures)2, c, d, e

CCD First Order Spectroscopy

G750L

5240-11490

 5030

 4.92

 535-1170

 2

 CCD

52X0.1
52X0.2

52X0.5

52X2

52X0.2F1

G750M

5450-11150

 570

 0.56

 4870-9950

 11

 CCD

G430L

 2900-5700

 2900

 2.73

 530-1040

 1

 CCD

G430M

3025-5615

 286

 0.28

 5330-10270

 10

 CCD

G230LB

1685-3065

 1380

 1.35

 615-1135

 1

 CCD

G230MB

 1635-3190

 155

 0.15

 5550-10335

 11

 CCD

MAMA First Order Spectroscopy

G230L

 1570-3180

 1610

 1.58

 600-1150

 1

 NUV-MAMA

G230M

 1640-3175

 90

 0.09

 9110-17500

 19

 NUV-MAMA

G140L

 1150-1736

 610

 0.60

 935-1440

 1

 FUV-MAMA

G140M

 1145-1740

 55

 0.05

 11500-17400

 12

 FUV-MAMA

MAMA Echelle Spectroscopy

E230M

 1575-3110

 800

 /60,000

 30000

 2

 NUV-MAMA

0.2X0.2, 0.2X0.06

E230H

 1625-3150

 267

 /228,000

 114000

 6

NUV-MAMA

0.1X0.2, 0.1X0.09

E140M

 1150-1735

 620

 /91,700

 45800

 1

 FUV-MAMA

0.2X0.2, 0.2X0.06

E140H

 1150-1700

 210

 /228,000

 114000

 3

 FUV-MAMA

0.2X0.2, 0.2X0.09

MAMA Prism Spectroscopy

PRISM

1150-3100

 1950

 1.2-120

1000-26

 2

 NUV-MAMA

25MAMA (clear)

1 Number of exposures at distinct tilts needed to cover spectral range of grating, with 10% overlap between spectra.

2 Naming convention gives dimensions in arcseconds of slit. For example 52X0.1 indicates the slit is 52 arcsec long in the cross-dispersion direction and 0.1 arcsec wide in dispersion. The F (e.g., in 52X0.2F1) indicates that it is the fiducial on the bar which is specified for coronographic spectroscopy.

c. For the MAMA first order modes, only ~ 25 arcseconds of the long slit projects on the detector.

d. Full aperture clear (50CCD or 25MAMA), longpass filtered (F25QTZ or F25SRF2 in UV), and neutral density filtered slitless spectroscopy is also supported with the first order and echelle gratings. F25MGII is supported with E230H and E230M.

e. A 6 arcsec long slit (6X0.2) is also supported for use with the echelle gratings, but with order overlap.



STIS Imaging Capabilities

Aperture

Name

Filter

Central Wavelength (c in Å)

FWHM

( in Å)

Field of View (arcsec)

Detector
Visible - plate scale ~0.05 arcseconds per pixel

50CCD

 clear

 ---

 ---

 51x51

 STIS/CCD

F28X50LP

 optical longpass

 >5500 Å

 28x51

 STIS/CCD

F28X50OIII

 [OIII]

 5007

 5

 28x51

 STIS/CCD

F28X50OII

 [OII]

 3740

 80

 28x51

 STIS/CCD

50CORON

 clear + coronagraphic fingers

 ---

 ---

 51x51

 CCD

Ultraviolet - plate scale ~0.024 arcseconds per pixel

25MAMA

 clear

 ~1750-3100 Å

~1150-1700 Å

 25x25

 STIS/NUV-MAMA

STIS/FUV-MAMA

F25QTZ

 UV near longpass

 > 1450 Å

 25x25

 STIS/NUV-MAMA

STIS/FUV-MAMA

F25SRF2

 UV far longpass

 > 1280 Å

 25x25

 STIS/NUV-MAMA

STIS/FUV-MAMA

F25MGII

 MgII

 2800

 70

 25x25

 STIS/NUV-MAMA

F25CN270

 continuum near 2700Å

 2700

 350

 25x25

 STIS/NUV-MAMA

F25CIII

 CIII]

 1909

 70

 25x25

 STIS/NUV-MAMA

F25CN182

 continuum near 1800Å

 1820

 350

 25x25

 STIS/NUV-MAMA

F25LYA

 Lyman alpha

 1216

 85

 25x25

 STIS/FUV-MAMA

Neutral Density Filtered Imaging

F25NDQ1

F25NDQ2

F25NDQ3

F25NDQ4

 neutral density filter, ND=10-1

neutral density filter, ND=10-2

neutral density filter, ND=10-3

neutral density filter, ND=10-4

 1150-11000 Å

 12x12

12x12

12x12

12x12

 CCD,

NUV-MAMA,

FUV-MAMA

F25ND5

 neutral density filter, ND=10-5

 1150-11000 Å

 25x25

F25ND6

 neutral density filter, ND=10-6

 1150-11000 Å

 25x25


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