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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.

Table 19.1: STIS Spectroscopic Capabilities
	Spectral Range (Å)		Spectral Resolution
Grating	Complete	Per Tilt	Scale (Å per pixel)	Resolving Power (/2)	# Prime Tilts ¹	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.

Table 19.2: 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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Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 11/12/97 08:00:28