Focus Analysis for Gratings, etc.

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.

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.

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

• The sky X and Y minima appear at -0.87 mm and -1.06 mm, respectively; average is -0.97 mm.
• The FWHM focus plots, FWHM in table above, show that values of ( FWHM/MIN(FWHM) )^2 agree pretty well with a quadratic function of the commanded focus value. The dashed line gives the fit to the five data points. The solid line is the result of performing 100 fits to the data points with each point "resampled" by adding a gaussian random value of its error bar to its value.
• The value of 1/ACF can also be used in place of the FWHM and the same focus plot created, see 1/ACF in the table above.
• The value at the X minimum, 0.938, implies that the FWHM at minimum would be SQRT(0.938)*38.5 ~ 37.3 microns.
• The value at the Y minimum, 0.87, implies that the FWHM at minimum would be SQRT(0.87)*45.1 ~ 42.0 microns.
• A MARX simulation of an on-axis source has a measured FWHM of 36.9 microns.
• This performance is very close to that expected!
• The FWHM values, in MICRONS, for the data on the plot are given here (click the FWHM value for a plot):

  Location =	-1.50 mm	-1.25 mm	-1.00 mm	-0.80 mm	-0.50 mm
  Sky X FWHM =	72.5 um	50.7 um	40.0 um	38.5 um	52.1 um
  Sky Y FWHM = labeled X,Y should be switched	61.7 um	46.5 um	45.1 um	46.5 um	73.5 um

• For reference, similar analysis of the earlier sim motion test, done with aimpoint ~ 1.5 arc minute off axis, yielded a minimum FWHM of 49 microns at -0.543 mm commanded focus.

Introduction

8/16/99

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 dd@space.mit.edu.