STIS has a slit wheel that contains the apertures and slits. It also includes the grating wheel (called the Mode Selector Mechanism - MSM) that contains the gratings, the echelles and prisms for spectroscopy and mirrors for imaging. To achieve the full wavelength coverage, there are specific MSM positions (grating tilts) at specific central wavelengths that can be specified.

5.4.2 MAMA detectors

The MAMA detectors are photon counting detectors sensible to UV radiation. They can be used to take data in an accumulate mode or as a time series. They offer high spatial resolution and two-dimensional imaging over a large field of view.

Incident photons strike the photocathode creating photo-electrons that pass into the curved Micro-Channel Plates (MCPs). They are multiplied to a pulse that is recorded by an anode array behind the photocathode and detected by the detector electronics. In front of the photocathode two different materials have been deposited that make each detector more sensible to near and far-UV radiation.

The peak photocathode response of the FUV-MAMA occurs at Ly while for the NUV is almost flat between 1800Å and 2600Å. Other properties of the detectors are listed in the table below.

It is very important to remember the observational limit brightness when designing STIS observations with the MAMAs as brighter objects might damage the detectors. If the source exceeds these limits, it is necessary to specify an alternative configuration of the instrument.

MAMA detectors are capable of recording data in highres mode producing 2048 x 2048 images with a 10-30% increase in resolution (but smaller S/N per pixel). This mode also suffers from a lack of flat field reproducibility making it difficult to be of use for high S/N observations.

5.4.3 Spectroscopic Modes

A detailed description of the 15 spectroscopic modes can be found in Chapter 4 of the Instrument Handbook (Version 2.0 1998), in particular the summary in Table 4.1. The expected limiting magnitude, after integrating an orbit, per spectral resolution element and a S/N = 10 for an A-type star is V ~ 21 using the G750L grating, V ~ 14.9 for the G230M and V ~ 11.4 for the E140H echelle (the complete list is in table 4.2).

5.4.4 Imaging Modes

Selecting STIS instead of FOC or WFPC2 to obtain images has several advantages the detector has a higher throughput over a wider spectral range, a lower read noise and dark current (resulting in higher sensitivity images), true solar blind UV observations with the MAMAs are possible as well as high time resolution UV images

The imaging capabilities with the CCD were designed for target acquisitions. Images can be obtained without a filter, with a longpass, an [OIII]5007 and an [OII]3740 filter. With the MAMAs, the filters available include two longpass, two continuum (2700, 1820), a MgII 2800, a CIII] 1909 and a Ly filter. More details can be found in the instrument handbook.

5.4.5 What constitutes a STIS observation

All STIS data products are FITS files and should remain this way. They do not need to be read into GEIS or other formats. As any FITS file, the header stores the global properties of all the exposures in the files. A series of image extensions contain the data and a specific header to describe it.

Each STIS ACCUM data file will contain the following extensions:

• SCI: stores the science values
• ERR: contains statistical errors
• DQ: contains data quality flags

As explained in the section above, STIS (and NICMOS) FITS files will contain multiple science exposures when an associated set is taken. Associations are recognized because they have a zero as the last character in the rootname. The following table summarizes the different naming suffixes. Note that the final calibrated spectra are the _x1d, _x2d, _sx1 and _sx2 ones.

The following table (which as the previous one was extracted from the latest edition of the HST Data Handbook) details the different extensions that can be found for different observation types. Example of STIS products for different observation types:

To determine the different files in an association table the STSDAS task tread can be used. It will not "read" the FITS file, just display the information. The task infostis produces an output similar to iminfo for STIS data.