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ACIS-S setup with LETG

Checking your LETG/ACIS-S setup parameters

Updated for new on-axis aimpoint definition Nov 2015

Please note that significant changes to LETG/ACIS setup recommendations were made in late 2015 as a result of increased aimpoint scatter and moving the spectrum to the center of the array.

This page is intended to aid proposers and observers as they select instrumental parameters for LETG/ACIS-S observations, specifically with regard to pointing offsets and subarray configurations. Most of the information presented here is also in the Proposer's Observatory Guide (see especially Table 9.3 and section 9.4.2/Offset Pointing). The additional contributions of this page are specific subarray configurations and links to the Spectrum Visualization Tool.


Default SIM Offset

The default for the SIM Offset is now 0 (formerly -8 mm) in order to maximize effective area at energies below ~1 keV. With this SIM Offset value the spectrum will fall in the middle of the CCD array, around row 512.

Why is this change being made?

Default Z-Offset

Because of the changing thermal environment of the telescope, the aimpoint has drifted over time. For the past several years, observers wishing to make observations at the best focus have had to specify small offsets in Y and/or Z pointing. As of late 2015 this will no longer be necessary as aimpoint position and error will be reviewed every two weeks and automatically incorporated into mission planning software. The default Z-offset is therefore 0.

Choosing Y-Offset

Most LETG/ACIS-S observations have used a Y-offset of roughly 1.5' in order to place the important He-like O lines and the O-K absorption edge on the S3 chip; as seen in the figure below, the Backside-Illuminated (BI) chips, S1 and S3, have much higher QE at low energies than the Front-Illuminated (FI) chips. Over the years, as the aimpoint has drifted and aimpoint errors have increased, the recommended Y-offset has tended to decrease. In late 2015 the aimpoint for Cycle 17 observations was operationally redefined so that Yoffset=0 always corresponds to chipx=200.7, and any aimpoint drift is accounted for during observation planning. The current recommendation for most LETG/ACIS observations is Y-offset=1.0', which will place the O features on S3 while ensuring that 0th order does not get too close to the S2/S3 gap.

The Spectrum Visualization Tool displays where spectral features fall on the ACIS-S detector as a function of Y-offset and source redshift. Positions are only approximate, however, because of aimpoint scatter, which can be up to ±20" (0.33'). One arcminute of Y-offset corresponds to a shift of 3.36 Å. When choosing an offset, observers should keep several margins in mind:

The following Y-offsets are of particular interest.
Figure 1: LETGS 1st order effective area (EA) with ACIS-S and HRC-S; lower panel shows low-EA regions in more detail. The effects of dither and ACIS bad columns are explicitly included. Dotted lines mark ACIS chip boundaries, and HRC plate gaps appear near -53 and +65 Å. ACIS curve is for Y-offset=+1.1' and HRC curve is for Y-offset=0'. The darker ACIS curve is for SIM Offset=0 (placing the spectrum near row 512); the lighter curve is for SIM Offset=-8 mm (row 180). ACIS EAs are taken from fall 2015.     Figure 2: Effective areas at different ACIS rows, normalized to row 180. Contaminant on the UV/Ion Shield is thinner toward the middle of the array, yielding higher X-ray transmission and EA.

ACIS Subarrays and Optional Chips

Very few LETG/ACIS observations use the full ACIS-S array, which has 1024 rows and takes 3.2 seconds to read out. Using a subarray and/or fewer than 6 chips allows a faster readout (shorter frametime) and thus produces less pileup in 0th order and the dispersed spectrum.

From Figure 1 one can see that S0 and S5 are useless for detecting 1st-order photons, although they may be useful for collecting higher order spectra in some cases. The S4 chip is also unlikely to be useful. Because of increasingly stringent ACIS thermal requirements, the S0 and S5 chips should be turned off unless there is a good reason for their use. Note that chips can be marked as "Optional", in which case they will be only be turned on if Mission Planning schedulers determine that thermal constraints will not be violated.

The LETG/ACIS background is weak enough that observers often don't bother with background subtraction. In this case, a subarray as small as 128 rows can be used although this leaves very little margin for error: 10 pixels are needed for the standard |tg_d|<0.0008 deg spectral extraction region, 32 pixels for dithered edges, and 80 pixels for pointing errors of up to ±20", summing to 122 pixels. If background subtraction is to be used, a subarray of at least ~200 pixels is needed to fill the default background regions which extend to 1.06 mm on either side of the spectrum (total edge-to-edge width of 88 pixels and net 75 pixels of background vs the spectral width of 10 pixels). Alternatively, users may reprocess their data by specifying narrower background regions in tg_extract.

To summarize, 3 or 4 chips will suffice for most observers. A 128-row subarray is probably OK but there will be little room for background regions. If you want to use the standard background regions you should use at least 280 rows, although custom background regions can be specified to use whatever area is available. The tables below list subarray parameters for the traditional 1/2, 1/4, and 1/8 subarrays, and for optimized subarrays that maximize the number of rows for a given frametime. SIM Offset=0 and Z-Offset=0 are assumed.

Optimized subarrays (maximizing rows for a given frametime)
CCDs 3 chips 4 chips
Rows    303       268       232       196       160       283       247       211       175       139   
Start 361 378 396 414 432 371 389 407 425 443
FrameTime   1.0 0.9 0.8 0.7 0.6 1.0 0.9 0.8 0.7 0.6

CCDs 5 chips 6 chips
Rows    263       227       190       154       242       206       169       133   
Start 381 399 417 435 391 409 428 446
FrameTime   1.0 0.9 0.8 0.7 1.0 0.9 0.8 0.7

Traditional subarrays (use 1/8 with caution)
CCDs 3 chips 4 chips 5 chips 6 chips
Subarray ½ ¼ ½ ¼ ½ ¼ ½ ¼
FrameTime   1.6 0.9 0.6 1.7 1.0 0.6 1.7 1.0 0.7 1.8 1.1 0.7

Frametime for m active CCDs, using n rows starting with row q, is given by the equation

T(msec) = 41.12*m + 2.85*n + 0.040*m*q - 32.99

and rounding up to the nearest 0.1 sec.

Last modified: 11/09/15

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