Target_Name = WAVE

An observer-specified wavelength calibration exposure for a selected spectral element, central wavelength, and aperture. Permitted in the STIS/CCD, STIS/FUV–MAMA, and STIS/NUV–MAMA configurations, only in ACCUM Mode. Few optional parameters may be specified (see Optional Parameter). The Number_of_Iterations must be 1. Note: For the STIS/CCD configuration, a gain of 4 e/DN will be used. Observer-specified wavecals may be used in addition to or in place of automatic wavecals.

Exposures with the WAVE internal target must immediately precede or follow an external science exposure (i.e., an ACCUM or TIME–TAG Mode exposure with an external pointing) which uses the same spectral element and central wavelength as the WAVE exposure. These exposures will use the same calibration lamp configuration as the automatic wave calibrations discussed below (based on the spectral element, central wavelength, and aperture specified). The exposure time for a WAVE exposure must be set to DEF, which will use the same default exposure time as the automatic wave calibrations.

An ACCUM Mode wavelength calibration will be automatically performed for external STIS/CCD, STIS/FUV–MAMA, and STIS/NUV–MAMA spectrographic science exposures which use ACCUM or TIME–TAG Mode at a single central wavelength with any of the spectral elements in Table 8.2 Supported Central Wavelengths for STIS Gratings. A wavelength calibration exposure will be added prior to the first such science exposure and after each subsequent science exposure at the same central wavelength if more than 40 minutes of orbital visibility time have elapsed since the previous wavelength calibration (see the STIS Instrument Handbook for an explanation). The calibration lamp configuration, exposure time, and calibration aperture (if different from the science aperture) will be based on the grating, central wavelength, and aperture of the science exposure. Specifying the Optional Parameter WAVECAL=NO in any exposure line will disable all automatic wavecals for that entire visit. PIs are advised to use an Exposure Group Container (Type: Sequence) with their wavelength calibrations and science observation so they schedule close together. For a  detailed description of wavelength calibration  exposures, see the  STIS Instrument Handbook.

Aperture

Only certain aperture-grating combinations can be used for MAMA TARGET=WAVE observations because the line lamps can be too bright for the MAMA detectors when used with wide slits. The apertures that may be selected depend on the grating being used, and are given in the following table.

Table 8.5: Legal aperture-grating combinations when TARGET=WAVE                                       

Spectral Element

Any legal value except MIRROR.

Optional Parameter

In general, no Optional Parameters are allowed for a WAVE exposure. However,

CR-SPLIT=NO is allowed. No other value of CR-SPLIT is legal.

Target_Name = CCDFLAT

A calibration exposure using the internal STIS CCD flat field lamps.

Observations using the G750L and G750M elements at wavelengths longer than about 7000 Angstroms are affected by fringing in the CCD. Due to shifting of the spectrum relative to the detector caused by the Mode Select Mechanism uncertainty (see Section 11.2 of the STIS Instrument Handbook), the reference flats taken by the Institute and maintained in the Calibration Data Base will often not rectify the fringes as well as a flat field taken contemporaneously with the science data at the same Mode Select Mechanism setting (i.e., without moving the grating wheel between the science and flat field observations). See Section 11.2 of the STIS Instrument Handbook for the most recent suggestions and recommendations for implementing a "fringe flat".

A CCDFLAT exposure is permitted only with the STIS/CCD configuration in ACCUM mode. No optional parameters except BINAXIS1 and BINAXIS2 are permitted and should match what is used in the corresponding science exposure.. The number of iterations must be 2. A CCDFLAT exposure is taken to calibrate fringing. The user may specify Time_Per_Exposure as DEF, in which case the default exposure time is used, or as a duration less than the default exposure time. The default exposure time is: MAX {0.1 sec, [Time_Factor × 0.1/(Width × BINAXIS1 × BINAXIS2)]} where:

• Time_Factor is a function of Spectral Element and Wavelength and is given in Table 8.6: Time_Factor for Spectral Elements and Wavelengths, 

• Width (in arcsec) can be obtained from Table 8.7: Supported STIS Apertures,

• and BINAXIS1 and BINAXIS2 are defined in CCD Modes Optional Parameters. The default is set to BINAXIS1=1 and BINAXIS2=1, but you may specify a value of 1, 2, or 4 for either quantity.

Note: A gain of 4 e/DN will be used with Target = CCDFLAT.

Exposures with the CCDFLAT internal target must immediately precede or follow an external science exposure (i.e., an ACCUM exposure with an external pointing), or the wavecal associated with that external science exposure, which uses the same spectral element and central wavelength as the CCDFLAT exposure.

Section 11.2 of the STIS Instrument Handbook describes the use of CCDFLAT. It recommends the following procedure:

• Specify Target_Name=CCDFLAT to indicate that the exposure is a fringe flat. GAIN=4 is automatically used, and the exposure uses two bulbs in the tungsten lamp.
• Specify Number_of_Iterations=2, to allow cosmic ray rejection and to assure adequate signal to noise. (Other STIS pure parallel exposures may not have Number_of_Iterations>1, but CCDFLAT is an exception.)
• Specify Config, Opmode, Sp_Element, and Wavelength. Config must be STIS/CCD. Opmode must be ACCUM. Sp_Element and Wavelength must be from Table 8.6.
• Specify Aperture, using one of those listed below for CCDFLAT. For an extended source, you generally should use the same slit for the flat as for the science exposure. For a point source, you should use the appropriate small echelle slit (0.3X0.09, 0.2X0.06). However, for a point source observed with the E1 and E2 series of apertures (except for the 52X0.05E1 aperture), you should use the 52X0.1 slit for CCDFLAT exposures. For example, if you were using the 52X0.2E1 slit for your science exposure, you would use the 52X0.1 (for a point source) or the 52X0.2 slit (for an extended source) for the CCDFLAT exposures. (See Section 11.2 of the STIS Instrument Handbook for further details and discussion)

• Specify Time_Per_Exposure as DEFAULT. The default exposure time (in seconds) is given by the formula in the third paragraph above which assures a signal to noise of 100 per pixel over the wavelength range of 6000 Å to 1 micron. You may also specify your own value for Time_Per_Exposure, but it must then be shorter than the default.

Note that CCD flats are often moved into the occultation period whenever they occur in the first or last exposure in an orbit. Fringe flats only work if they are taken with no movement of the Mode Select Mechanism. Be sure to place the fringe flat exposure immediately before or after the science exposure or associated wavecal to which it pertains. In some instances (such as a long series of exposures), you may wish to bracket your science observing with fringe flats in order to take into account any possible thermal shifts.

Table 8.6: Time_Factor for Spectral Elements and Wavelengths.

Aperture

52X0.05, 52X0.1, 52X0.2, 52X0.2F1, 52X0.5, 52X2, 0.3X0.09, or 0.2X0.06.

The 0.3X0.09 and 0.2X0.06 slits should be used for point-source targets. The 52X0.1 slit should be used for point sources observed in the E1, E2 apertures. The remaining slits are intended for use on extended targets. To choose the best slit, see Section 11.2 of the STIS Instrument Handbook.

Spectral Element

It  must  be  G750L or G750M. A central  wavelength  must  be  specified  for CCDFLAT. Use only the central wavelengths in Table 8.6: Time_Factor for Spectral Elements and Wavelengths.

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