Focus Analysis for Gratings, etc.

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ACIS-I Focus Test: "-1.036" +/- 0.020 mm

8/25/99, 4pm

OBSIDs 1276, '77, '78, '79, and '80 were taken at focus positions of -0.502, -0.752, -1.002, -1.252, and -1.502 mm. The best overall focus is determined as the average of the locations of the best 1-D projected focuses ( projected to Sky X and Sky Y). Fits are made using both the Gaussian effective FWHM and "1/ACF". The result is -1.036 +/- 0.020 mm.

OBSID 1278 may be the best focussed image seen to date?! It has 1-D FWHM's of 37-38 microns and an encircled fraction of 63.8% (.ps , .gif) in one arcsecond (relative to 10 arcseconds = 1.00) for energies between 0.73 and 1.13 keV and nominal grade set, 02346.

Sky X projection: FWHM & 1/ACF plots: .ps or .gif or .txt
Sky Y projection: FWHM & 1/ACF plots: .ps or .gif or .txt

Grating Dispersion Focus (ACIS-S): -1.00 mm OK !

8/22/99, 10pm

The focus data sets were analyzed for the FWHM in the dispersion direction by rotating the sky X,Y coordinates by 220 degrees to put the jet at the position angle it has in the TDET coordinates. The result is a best dispersion focus at -0.96 mm -- consistant with the adopted -1.00 value and giving a FWHM ~ 40.0 um.

Dispersion projection: FWHM & 1/ACF, inner points .ps or .gif or .txt

Complete ACIS-S Focus Test: -1.00 mm OK

8/22/99, 10pm

Five more focus measurements were taken: obsids 472, 473, 474, 475, and 476 at focus values of 0.0, -0.9, -1.1, -1.2, and -2.0 mm using 1/4 frame readout of S3. The measured SkyX and SkyY projected FWHM agree with the previous data points and give a best overall focus of -0.97 mm -- consistant with the adopted -1.00 value.

<-- The "-2.00 mm" defocus image shows
a clear donut hole!
Sky X projection: FWHM & 1/ACF, inner points .ps or .gif or .txt All points, .ps or .gif
Sky Y projection: FWHM & 1/ACF, inner points .ps or .gif or .txt All points, .ps or .gif

Preliminary Focus Test: Intensity contours

8/18/99, 9pm

The ellipticity of the intensity contour changes as the telescope moves through focus ( .ps and figures below), this is another way of seeing the slight astigmatism suggested by the FWHM focus results. Note that the Sky X, Y axes here are at ~ 45 degrees in the Chandra Y, Z system - SAOSAC raytraces suggest some astigmatism along the diagonal at -0.4 mm and +0.4 mm defocus.

Preliminary Focus Test: FWHM analysis

Focus analysis of Obsid's 1185 to 1189 (62553 to 62549) is summarized below. Note that the reported FWHM's are along the Sky X and Y axes; grating resolution depends on the detector X axis projection.

Standard grade set (0,2,3,4,6) and energies in the 0.5 to 2.0 keV range were used.

Sky X projection: FWHM, .ps or .gif, 1/ACF, .ps or .gif,
Sky Y projection: FWHM, .ps or .gif, 1/ACF, .ps or .gif,



Grating focus concentrates on the 1-D projection of the HRMA PSF along the dispersion axis. The FWHM of this projection is measured as 2.35 times the best-fit Gaussian sigma.

Please send any comments to Dan Dewey at

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