Phase 2 Proposal Instructions for Cycle 10

10.8 STIS Internal Calibration Targets

---

The following internal calibration targets are available for the STIS. These should not be included in your proposal if the routine calibrations are sufficient for your program. See the STIS Instrument Handbook for details.

10.8.1 Target_Name = WAVE

An observer-specified wavelength calibration exposure for a selected spectral element, central wavelength, and aperture. Permitted in the STIS/CCD, STIS/FUVŠMAMA, and STIS/NUVŠMAMA configurations, only in ACCUM Mode. Few optional parameters may be specified (see below 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 TIMEŠTAG 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). If the exposure time for a WAVE exposure is specified as DEF, the same default exposure time as the automatic wave calibrations will be used. Otherwise, the specified exposure time will be used. To avoid overuse of the line lamps, the exposure time should not exceed 5 minutes.

An ACCUM Mode wavelength calibration will be automatically performed for external STIS/CCD, STIS/FUVŠMAMA, and STIS/NUVŠMAMA spectrographic science exposures which use ACCUM or TIMEŠTAG Mode at a single central wavelength with any of the spectral elements in Table 10.2. 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 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. Setting WAVECAL=NO will disable all automatic wavecals for that entire visit.

For a detailed description of wavelength calibration exposures, see the STIS Instrument Handbook.

10.8.2 Aperture

Any legal aperture.

10.8.3 Spectral Element

Any legal value except MIRROR.

10.8.4 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.

10.8.5 Target_Name = CCDFLAT

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

Observations utilizing 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 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).

A CCDFLAT exposure is permitted only with the STIS/CCD configuration in ACCUM mode. No optional parameters except BINAXIS1 and BINAXIS2 are permitted. 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 as a function of Spectral Element and Wavelength is given in Table10.3: Time_Factor for Spectral Elements and Wavelenghts..

Width (in arcsec) can be obtained from Table10.4: Supported STIS Apertures and BINAXIS1 and BINAXIS2 are defined on page 180. 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) which uses the same spectral element and central wavelength as the CCDFLAT exposure.

The STIS Instrument Handbook describes the use of CCDFLAT and recommends the following procedure in RPS2:

• Specify Target_Name=CCDFLAT to indicate that the exposure is a fringe flat. GAIN=4 is automatically used, and the exposure utilizes 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 Table10.3: Time_Factor for Spectral Elements and Wavelenghts..
• Specify Aperture, using one of those listed below for CCDFLAT. For a point source, you will want to use the appropriate small echelle slit; for an extended source, you will want to use the same slit for the flat as for the science.
• Specify Time_Per_Exposure as DEFAULT. The default exposure time (in seconds) is the Time_Factor in Table10.3: Time_Factor for Spectral Elements and Wavelenghts. and assures a signal to noise of 100 per pixel over the wavelength range of 6000  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 moved into the occultation period by RPS2 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 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 10.3: Time_Factor for Spectral Elements and Wavelenghts.

Spectral Element	Central Wavelength	Time_Factor (sec)
G750L	7751	25
G750M	6768	90
	7283	80
	7795	80
	8311	85
	8561	85
	8825	70
	9286	60
	9336	50
	9806	85
	9851	90

 

10.8.6 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 listed under target name CCDFLAT are intended for use on point-source targets, and the remaining slits are intended for use on diffuse targets. To choose the best slit, see the STIS Instrument Handbook.

10.8.7 Spectral Element

Must be G750L or G750M. A central wavelength must be specified for CCDFLAT. Use only the central wavelengths in Table10.3: Time_Factor for Spectral Elements and Wavelenghts. above; do not use Table 10.2.