Chandra X-Ray Observatory (CXC)
Proposal Planning Toolkit: Star Checker Help

This is a help page specific to using Star Checker online.

Description

The celestial pointing and aspect solution of Chandra relies on the Aspect Camera Assembly (ACA), which is an optical telescope, CCD detector, and associated electronics onboard the spacecraft. The aspect solution depends on acquiring 2-8 guide stars (see POG Section 5.8). The 'quality' of a guide star field can depend on the spacecraft roll angle and on the exact offsets/pointing of the selected instruments. As a result, some targets may have suitable star fields that are only available at certain times of the year (since the roll angle changes with time). Small pointing and/or offset adjustments can improve the quality of the star field for the ACA.

Star Checker uses the current ACA detector performance to provide an assessment of the quality of the star fields for user-supplied target and offset configuration. Star Checker will notify the user via email with the range of roll angles (and corresponding dates) that achieve acceptable star field quality for the user-supplied target and offset configuration.

It is possible that Star Checker may find no viable path for acquiring guide stars, such star fields are 'difficult' but may still be possible. The user who receives such an outcome should contact the CXC Helpdesk to determine the viability of those star fields for their target.

Inputs

Input Coordinates

The user may enter the input coordinates or resolve a Target Name using the Resolve Name button.

Coordinate System

Select the coordinate system for specifying the column direction. The Star Checker supports the following choices:

Coordinates

Input a set of coordinates in the format of the selected coordinate system or resolve a target name which will set the RA/Dec coordinates in J2000. The supported coordinate ranges are listed below.

Target Name

Enter a valid target name and click the Resolve Name button. This will query the catalog[s] specified in the Name Resolver field.

If one, and only one entry is found, the coordinates will be displayed in J2000. If no entry is found or more than one entry is retrieved, the user will be asked to modify the Target Name and try again.

Name Resolver

Name of the service or services that will be used to resolve the target name when the Resolve Name button is pressed. The services will be chosen in the specified order. For example, "NED/SIMBAD" will first search for the target name in NED and then, if the name was not found, in SIMBAD.

Science Instrument

Specifies which detector will be on the optical axis during the observation. The choices are: ACIS-I, ACIS-S, HRC-I, or HRC-S.

Y Detector Offset

The detector offset in the y direction, in arcminutes. If no value is provided, an offset of 0 is assumed.


Z Detector Offset

The detector offset in the z direction, in arcminutes. If no value is provided, an offset of 0 is assumed.


SIM Translation Offset

Offset from nominal translation position, in millimeters. This is a motion of the SIM and thus the aimpoint away from the default position on the detector along the z-axis (which is along the axis from ACIS-S to ACIS-I). If no value is provided, an offset of 0 is assumed.


SubArray

Choices are: None, Standard 1/2, Standard 1/4, Standard 1/8, and Custom. The default is 'None'.

A subarray is a reduced region of all of the active (turned on) CCDs that will be read. A reduced region may also help to reduce the effects of pulse pile-up.

PLEASE ENSURE THAT YOUR TARGET FALLS WITHIN THE CHOSEN SUBARRAY (e.g., use the Observation Visualizer (ObsVis) available in the latest CIAO downloadable package).

Note that the minimum Frame Time depends on the size and location of the subarray. Read the ACIS chapter of the Proposer's Observatory Guide (POG) for more information.

The standard subarrays can be selected to avoid pileup approximately by the factor of the subarray size. The standard subarrays for ACIS-I use the values as listed in the table. For ACIS-S, the starting rows differ because of the different position of the aimpoint relative to the readout node. The ACIS chapter of the POG Guide contains a schematic of the locations of the default Subarrays.

Standard Size ACIS-I
Start Row 		Number of Rows ACIS-S
Start Row
1/2 513 512 257
1/4 769 256 385
1/8 897 128 449

For Custom Subarray, a Start Row and Number of Rows is required.

Subarray: Start Row

The starting row that will be read for processing custom subarrays.

Valid range is 1 - 924.

Subarray: Number of Rows

The number of rows that will be read for processing custom subarrays. A minimum of 128 rows is required if multiple ACIS chips are selected. A minimum of 100 rows is required if a single ACIS chip is selected.

Valid range is 100 - 1024.

Results

Star Checker will notify the user via email with the range of roll angles (and corresponding dates) that achieve acceptable star field quality for the user-supplied target and offset configuration.

It is possible that Star Checker may find no viable path for acquiring guide stars, such star fields are 'difficult' but may still be possible. The user who receives such an outcome should contact the CXC Helpdesk to determine the viability of those star fields for their target.

DS10.13 Monday, December 09, 2024
The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email: cxchelp@cfa.harvard.edu Smithsonian Institution, Copyright © 1998-. All rights reserved.