The STIS aperture wheel has 65 positions, several of which correspond to multiple slits or masks. Only a small fraction of the full complement of spectroscopic apertures are currently supported. Table A.1 provides a complete list of STIS apertures for spectroscopy, together with comments regarding their characteristics or purposes. Schematics of three complex apertures are given in Figure A.1, Figure A.2, and Figure A.3.

***Table A.1: STIS Spectroscopic Apertures. Supported apertures are in boldface. For most apertures, the name gives the length by the width in arcseconds.***
Apertures	Comments
*Apertures that can be used with the First-Order Gratings and the NUV-PRISM*
52X0.05 52X0.1 52X0.2 52X0.5 52X2	Long slits; the names give the length in the spatial direction by the width in the dispersion direction in arcseconds.
52X0.05E1 52X0.1E1 52X0.2E1 52X0.5E1 52X2E1	Long slits centered at 19.7 arcsec in +Y position. (Places compact target at row ~900 on the CCD to reduce CTE losses.). Supported with all CCD gratings. Use with the MAMAs is not allowed.
52X0.2E2 52X0.5E2 52X2E2	These CTE aperture positions are also centered near row 900 on the STIS CCD detector. However, the positions are offset from the physical aperture centers to give better alignment with fringe flats done using the `52X0.1` aperture. The E2 positions are supported for the `G750L` and `G750M` gratings only.
52X0.05D1 52X0.1D1 52X0.2D1 52X0.5D1 52X2D1	These aperture positions put a target near the bottom edge of the `FUV-MAMA` and are intended for observation of very faint targets where it is necessary to minimize the contribution of the `FUV-MAMA` dark current. They are all supported with the `G140L` and `G140M` gratings. `52X0.05D1` and `52X0.1D1` are also supported for CCD peakups.
`52X0.05F1 52X0.05F2 52X0.1F1 52X0.1F2` 52X0.2F1 `52X0.2F2 52X0.5F1 52X0.5F2 52X2F1 52X2F2`	Fiducial bars on the long slits which can be used for coronagraphic spectroscopy. The `52X0.2F1` is supported for all first-order gratings and is available-but-unsupported for the `PRISM`. The other fiducial bars are available-but-unsupported with all gratings and the `PRISM`. The F1 and F2 bars are 0.5" and 0.86" long, respectively (see Figure 13.1).
`52X0.1B0.5 52X0.1B1.0 52X0.1B3.0`	A long slit with 3 fiducial bars where the bar length in arcseconds is given by the third dimension. See Figure A.1.
`31X0.05NDA 31X0.05NDB 31X0.05NDC`	Neutral-density-filtered long slits of the given dimensions in arcsec, which may be used for bright objects or flat-field calibration. The dex ND factors are 0.4, 0.8, and 1.2, respectively.
`0.5x0.5 2X2 6X6`	Square apertures of the given dimensions in arcsec.
0.1X0.03 0.1X0.06 0.1X0.09 0.1X0.2 0.2X0.06 0.2X0.09 0.2X0.2 0.2X0.5 0.3X0.06 0.3X0.09 0.3X0.2 1X0.06 1X0.2 6X0.06 6X0.2 6X0.5 0.2X0.05ND 0.3X0.05ND	As of Cycle 12, the `0.2X0.2` aperture is supported for use with all first-order gratings; it is available-but-unsupported with the PRISM. While not supported for first-order observations, the other echelle apertures can be used in first order as available-but-unsupported apertures. This includes the echelle neutral-density slits `0.2X0.05ND` and `0.3X0.05ND` with dex ND factors of 2.0 and 3.0, respectively.
25MAMA 50CCD	Supported full-field clear apertures.
F25QTZ F25SRF2	`F25QTZ` and `F25SRF2` are supported for all MAMA gratings and `PRISM,` but are unavailable with the CCD`.`
25MAMAD1 F25SRF2D1 F25QTZD1	These aperture positions put a target near the bottom edge of the `FUV-MAMA` and are intended for observation of very faint targets where it is necessary to minimize the contribution of the `FUV-MAMA` dark current. They are all supported with the `G140L` and `G140M` gratings.
F25ND3 F25ND5 F25NDQ1 F25NDQ2 F25NDQ3 F25NDQ4	Supported neutral-density filters for MAMA detectors. Can be used as available-but-unsupported apertures with the CCD gratings. The numbers give the dex ND factors.
F25MGII `F25CN270 F25CIII F25CN182`	`F25MGII` is supported and these other filters are available -but-unsupported with the `NUV-MAMA` gratings and `PRISM` only.
`F25LYA`	The `F25LYA` filter is available-but-unsupported with the `G140L`, `G140M`, `E140H`, and `E140M` gratings only.
`F28X50LP F28X50OII F28X50OIII`	These CCD imaging filters are available-but-unsupported with the CCD gratings, but are unavailable with the MAMA detectors.
`36X0.05P45 36X0.05N45 36X0.6P45 36X0.6N45`	Long slits of the given dimensions in arcsec which are inclined at ±45° to facilitate observations of moving targets at off-nominal rolls. See Figure A.1.
*Apertures that can be used with the Echelle Gratings*
0.1X0.03 0.1X0.06 0.1X0.09 0.1X0.2	The latter two are supported with `E230H` only. The `0.1x0.03` is supported with all echelle gratings.
0.2X0.06 0.2X0.09 0.2X0.2 `0.2X0.5`	`0.2X0.06` and `0.2X0.2` are supported with `E230M` and `E140M`, while `0.2X0.09` and `0.2X0.2` are supported with `E230H` and `E140H`.
0.2X0.06FP 0.2X0.2FP	Masks with five apertures, suffixed A through E, for reduction of fixed-pattern noise; see Figure A.3.
0.2X0.05ND 0.3X0.05ND	Neutral-density slits. Dex ND factors are 2.0 and 3.0, respectively.
`0.3X0.06 0.3X0.09 0.3X0.2 0.5X0.5 1X0.06 1X0.2`	Miscellaneous available-but-unsupported echelle apertures.
2X2 6X0.06 6X0.2 6X0.5 6X6	The `6X0.2` long slit is supported with the echelles.
52X0.05 `52X0.1 52X0.2 52x0.5 52X2`	Long slits for use with the echelles. `52X0.05` is supported, and the others are available-but-unsupported with the echelle gratings.
52X0.05F1 52X0.05F2 52X0.1F1 52X0.1F2 52X0.2F1 52X0.2F2 52X0.5F1 52X0.5F2 52X2F1 52X2F2	The fiducial bar slits are available-but-unsupported with the echelle gratings.
52X0.1B0.5 52x0.1B1.0 52x0.1B3.0	A long slit with 3 fiducial bars. The bar length in arcseconds is given by the third dimension. See Figure A.1.
`31X0.05NDA 31X0.05NDB 31X0.05NDC`	Neutral-density-filtered long slits, which may be used for bright objects. The dex ND factors are 0.4, 0.8, and 1.2, respectively.
25MAMA	Full-field clear aperture.
F25QTZ F25SRF2	Supported long-pass filters, with all echelles.
F25ND3 F25ND5 F25NDQ1 F25NDQ2 F25NDQ3 F25NDQ4	Supported neutral-density filters for MAMA detectors.The numbers give the dex ND factors.
F25MGII `F25CN270 F25CIII F25CN182`	Narrow-band filters. `F25MGII` is supported for `E230H` and `E230M`. Others are available with the `NUV-MAMA` only.
F25LYA	Available-but-unsupported with the `FUV-MAMA` only.
`36X0.05P45 36X0.05N45 36X0.6P45 36X0.6N45`	Long slits which are inclined at ±45° to facilitate observations of moving targets at off-nominal rolls. See Figure A.2.

The 45° slits (Figure A.2) are nominally inclined at plus or minus that angle with respect to the dispersion and normal long slit, in order to increase the scheduling flexibility for moving-target observations with specified position angles. Otherwise, the HST roll angle constraints severely limit the windows for such observations. These slits have different widths in their central and outer segments, and there are two pairs with different dimensions.

The fixed-pattern slits (Figure A.3) comprise two masks with five apertures each, which are spaced to place the spectrum at different detector locations designed to optimize the reduction of fixed-pattern noise. The target is moved from one aperture to another, and the slit wheel is repositioned, with the intention that the spectrum shifts along the dispersion direction only. See Section 12.5.2 for further details.

It is not feasible to provide detailed transmission properties of all of the unsupported apertures at this time. They can in many cases be estimated from the data provided for similar supported apertures in Chapter 13, and from the imaging data in Table 14.38.

Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 14

A.2 Full Aperture Complement