Mode = ALIGN/APER Config = COS

This Mode is used to move the COS aperture mechanism via stored commanding for the purpose of on-orbit alignment of the aperture to the light path. ALIGN/APER exposures do not acquire science data and must be interleaved with science mode exposures to obtain data, with the aperture mechanism at a variety of positions for the ground alignment process. The values of the Optional Parameters specify the position of the aperture plate in motor steps in two dimensions, relative to a nominal zero position.

The mechanism positions at nominal zero will be set via real-time command. This allows a preplanned scan of the aperture mechanism to be specified without knowledge of where in the mechanism range the scan will be executed. Since the absolute positions are set in real time, a check that the specified offsets relative to nominal zero do not violate the mechanism range limits cannot be performed by the ground system. The proposer is responsible for ensuring that the aperture mechanism remains within its allowed limits at all times. If an attempt is made to command the mechanism beyond allowed limits, the flight software will reject the request and the mechanism will not be moved. Note that for relative moves in a non-preferred direction, a certain number of additional steps must be reserved for "overshoot" commanding.

Separate nominal zero positions are defined for each aperture. The first ALIGN/APER exposure in a visit should normally be preceded by a science mode exposure which selects the aperture; if this is not done, the aperture mechanism will start at the PSA position. If a science mode exposure is used to specify a different aperture from previous science exposures in the visit, the aperture mechanism will be placed at the nominal zero position for the new aperture and any offsets from previous ALIGN/APER exposures will be lost.

Target Name

The special value NONE must be specified.

Aperture or FOV

Do not enter a value.

Spectral Element

Do not enter a value.

Wavelength

Do not enter a value.

Optional Parameters

Note that no Optional Parameters may be used with ALIGN/APER mode when the REQ UPLINK Special Requirement is used.

XAPER (Restricted)

= 0 (default); -560 to +560 (steps)

Specifies the relative position from a nominal zero position of the COS aperture mechanism X (cross-dispersion axis) position, in motor steps. The absolute position of the nominal zero is set via real-time command.

YAPER (Restricted)

= 0 (default); -200 to +200 (steps)

Specifies the relative position from a nominal zero position of the COS aperture mechanism Y (dispersion axis) position, in motor steps. The absolute position of the nominal zero is set via real-time command.

Note: If a particular position for the aperture mechanism is needed for a series of ALIGN/APER exposures, the appropriate parameters (XAPER and/or YAPER) must be specified with the same value on every ALIGN/APER exposure in the series. If either parameter is specified on an ALIGN/APER exposure and then omitted on the subsequent exposure, the mechanism position in the corresponding dimension will be moved back to nominal zero.

Number of Iterations

The Number_Of_Iterations must be 1.

Time Per Exposure

Proposers may specify a positive value of Time_Per_Exposure to provide for additional time after the move has completed to wait prior to the start of the subsequent COS exposure. This time may be desirable if, for instance, the mechanism move could adversely affect the thermal environment (due to running the motors) and a cool-down time is required before the science exposure. If no wait is needed, enter 0 seconds.

Special Requirements

An ALIGN/APER (or ALIGN/OSM) Mode exposure with the REQuires UPLINK special requirement is used as a placeholder for real-time commanding to uplink new nominal zero positions. The mechanisms will be moved to their home positions at the end of this exposure, so no mechanism moves need be commanded in real-time to begin using the newly-uplinked nominal position(s).

Mode = ALIGN/OSM Config = COS

This Mode is used to move the COS OSM1 and OSM2 mechanisms via stored commanding for purposes of optimizing the distance between the gratings and the secondary mirror, and positioning the gratings with respect to the detectors. ALIGN/OSM exposures do not acquire science data and must be interleaved with science mode exposures to obtain data, with the OSMs at a variety of positions for the ground alignment process. The values of the Optional Parameters specify the positions of the OSM1 focus and the OSM1 and OSM2 rotational mechanisms in motor steps, relative to a nominal zero position.

The mechanism positions at nominal zero will be set via real-time command. This allows a preplanned scan of a mechanism’s position to be specified without knowledge of where in the mechanism range the scan will be executed. Although the OSM1 and OSM2 rotational mechanisms have unlimited rotation, the OSM1 focus mechanism has range limits. Since the absolute positions are set in real time, the ground system cannot check that the specified offset relative to nominal zero does not violate the mechanism range limits.

The proposer is responsible for ensuring that the OSM1 focus mechanism remains within its allowed limits at all times. If an attempt is made to command the focus mechanism beyond allowed limits, the flight software will reject the request and the mechanism will not be moved. Note that for relative moves in a non-preferred direction, a certain number of additional steps must be reserved for "overshoot" commanding.

Separate nominal zero positions for the OSM1 focus and rotational mechanisms are defined for each FUV grating and central wavelength, as well as for the NCM1 mirror which is used with the NUV channel. Similarly, separate nominal zero positions for the OSM2 rotational mechanism are defined for each NUV grating and central wavelength, as well as for the two mirror positions. The first ALIGN/OSM exposure in a visit should normally be preceded by a science mode exposure which selects the spectral element and wavelength. If this is not done, the mechanisms will begin from their default positions: the G130M grating at 1222 angstroms for OSM1, and MIRRORA for OSM2. If a science mode exposure is used to specify a different spectral element or central wavelength from previous science mode exposures in the visit, the proposer should first remove the existing mechanism offsets by inserting an ALIGN/OSM exposure with zero offsets (focus and rotation) before the grating/central wavelength change. If this procedure is not followed, there may be timing problems in the resulting command sequence. The science mode exposure with the different spectral element or central wavelength will then be executed with the mechanism placed at the nominal position for that spectral element/ central wavelength. Note that it is not possible to cycle through gratings while retaining offsets to the OSM1 focus position.

Target Name

The special value NONE must be specified.

Aperture or FOV

Do not enter a value.

Spectral Element

Do not enter a value.

Wavelength

Do not enter a value.

Optional Parameters

Note that no Optional Parameters may be used with ALIGN/APER mode when the REQ UPLINK Special Requirement is used.

FOCUS (Restricted)

= 0 (default); -2200 to +2000 (steps)

Specifies the relative position from a nominal zero position of the COS OSM1 "z" focus position relative to the HST secondary mirror, in motor steps. The absolute position of the nominal zero is set via real-time command.

OSM1ROT (Restricted)

= 0 (default); -600 to +600 (steps)

Specifies the relative position from a nominal zero position of the COS OSM1 rotational mechanism, in motor steps. The absolute position of the nominal zero is set via real-time command.

OSM2ROT (Restricted)

= 0 (default); -700 to +700 (steps) (TBR)

Specifies the relative position from a nominal zero position of the COS OSM2 rotational mechanism, in motor steps. The absolute position of the nominal zero is set via real-time command.

Note: If a particular position for any of these mechanisms is needed for a series of ALIGN/OSM exposures, the appropriate parameter (FOCUS, OSM1ROT, or OSM2ROT) must be specified with the same value on every ALIGN/OSM exposure in the series. If any parameter is specified on an ALIGN/OSM exposure and then omitted on the subsequent exposure, the corresponding  mechanism position will be moved back to nominal zero.

Number of Iterations

The Number_Of_Iterations must be 1.

Time Per Exposure

Proposers may specify a positive value of Time_Per_Exposure to provide for additional time after the move has completed to wait prior to the start of the subsequent COS exposure. This time may be desirable if, for instance, the mechanism move could adversely affect the thermal environment (due to running the motors) and a cool-down time is required before the science exposure. If no wait is needed, enter 0 seconds.

Special Requirements

An ALIGN/OSM (or ALIGN/APER) Mode exposure with the REQuires UPLINK special requirement is used as a placeholder for real-time commanding to uplink new nominal zero positions. The mechanisms will be moved to their home positions at the end of this exposure, so no mechanism moves need be commanded in real-time to begin using the newly-uplinked nominal position(s).

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