HETG Orbital Activation & Checkout

back to HETG on Orbit

Under Revision... 8/3+/99




What are the goals of the data taken during insertion/retraction? Currently they are:

Notes & To-do

OAC Timeline for HETG

See the CXC In-flight Calibration page and the Phase 2 timeline Version 5.0 for specifics of "Phase 2 OAC and Calibration".

The first HETG observations are summarized here:

(ks)(ks)No.No.on???refTarget or ActivityOBSIDnumberInstrument(J2000)(J2000)Comment
--------HETG Activation and Checkout------
2.8---y---slew and acquisition------
1---yy-ASH-1.1CAPELLA52290021ACIS-S/NONE05:16:42.30+45:59:52.4To verify target acquisition. Changed: now use 6 S-chips full-frame.
5---yy-ASH-1.2CAPELLA1098280048ACIS-S/NONE05:16:42.30+45:59:52.4Start normal 6-chip full-frame readout of ACIS-S on Capella.
--------------5 ks exposure includes Launch Lock release and HETG insert. done by realtime comm.
--------------Grating insertion (or retraction) takes <3min.
--------CAPELLA: plate focus------
15-----yASH-2.1CAPELLA1099280049ACIS-S/HETG05:16:42.30+45:59:52.4At nominal focus. Edit DOT to remove HETG insertion command.
15-----yASH-2.1CAPELLA1235280185ACIS-S/HETG05:16:42.30+45:59:52.4At nominal focus.
15-----yASH-2.1CAPELLA1100280050ACIS-S/HETG05:16:42.30+45:59:52.4Focus -0.2 mm from nominal
15-----yASH-2.1CAPELLA1236280186ACIS-S/HETG05:16:42.30+45:59:52.4Focus -0.2 mm from nominal
15-----yASH-2.1CAPELLA1101280051ACIS-S/HETG05:16:42.30+45:59:52.4Focus 0.2 mm from nominal.
15-----yASH-2.1CAPELLA1237280187ACIS-S/HETG05:16:42.30+45:59:52.4Focus 0.2 mm from nominal.
-----y--request data dump------
--------HETG Calibration Targets------
1.6-------slew and acquisition------
10------ASH-3.1CRAB PULSAR168590036ACIS-S/HETG05:34:32.00+22:00:52.1TE mode
5------ASH-3.2CRAB PULSAR-CC170590038ACIS-S/HETG05:34:32.00+22:00:52.1CC mode.
2.6-------slew and acquisition------
3------ASH-5.2"CYG X-2, HETG-CC"110490000ACIS-S/HETG21:44:41.20+38:19:17.0CC mode
--------------Analyse Capella focus data during above observations.
--------------Need to be at best focus for following observations. Do by near-realtime OR or DOT edit.
15------ASH-5.1CYG X-21102480052ACIS-S/HETG21:44:41.20+38:19:18.1TE mode, w/window
15------ASH-5.1CYG X-21238480188ACIS-S/HETG21:44:41.20+38:19:18.1TE mode, w/window
2.3-------slew and acquisition------
0.5-------------HETG retraction. Done by realtime command.
--------------Edit DOT to remove HETG retraction command.
0.5-------CAPELLA1199280149ACIS-S/NONE05:16:42.30+45:59:52.4Defocus by +1.0 mm. Verify grating retraction and defocus.
--------------Note: 30 ks observation of MKN 421 deferred from here to normal operations
--------------Note: 40 ks observation of 3C 273 deferred from here to normal operations

HETG OAC Observation Simulations

4/3/99 Update


Based on Herman Marshall's Capella studies (see his colorful In Flight Calibration Observations web page) the HETGS focus test will consist of three 30 ks exposures of Capella taken at 0.0, +0.2 mm, and -0.2 mm with respect to the then-nominal ACIS-S focus position. Simulations and analyses of these specific observations have been carried out to produce flight-realistic results. Three sets of results are created coresponding to the condition that the "then-nominal" focus is actually at a value of 0.0, +0.3 mm, or -0.3 mm with respect to the true focus. The plots in the defocus analysis section , below, show that these cases are clearly identified by the in-hand data.

Simulated data set that would be measured if the then-nominal focus coincides with the real focus location. Simulated data set that would be measured if, unfortunately, the then-nominal focus actually coincides with +0.3 mm from the real focus.

Capella Simulated Observations

The observations are simulated using MARX 2.20 and include dither. The procedure flight_sim.pro sets up the MARX parameter file, etc. and spawns the simulation. Parameters of the observation are read from the simple "database" file: obsdb.rdb. The observations are then analyzed by the general purpose procedure obs_anal.pro. "Auto-aspect reconstruction" is performed on the simulations by obs_anal.pro using the motion of the zero-order image. This technique produces overall E/dE values in good agreement with expectations and may actually be the analysis method for these first-light images. Due to "apodization" of the time series in the aspect determination algorithm, these 30 ks exposures produce only 20 ks of valid aspect corrected events - this efficiency can likely be improved.

Observation summaries of the simulations including links to line-lists are given here for the 3 simulated sets of three exposures ('3.001 - '3.009 in obsdb.rdb):

Capella Spectral Model

MARX input spectra representing Capella and HR1099 (a plausible alternative line source) were created by Dave H (nH effects are not included currently.) The desired ("real") model flux is based on ROSAT measurements. The spectra given are not properly normalized; the MARX flux value given yields the desired inband flux, properly normalizing the spectrum. Plots, including flux calculation, were made by plot_marx_spect.pro.

SourceSource flux
erg/cm^2 s
0.1-2.5 keV
Desired model flux
0.32-12.4 keV
MARX spectrum
data / plot
File flux in:
erg/cm^2 s
photons/cm^2 s
Corrected MARX flux
photons/cm^2 s
0.32-12.4 keV
Capella1.5E-10 erg/... 1.4E-10 erg/... data / plot 6.756E-11 erg/...
0.05337 phot/...
0.1105 phot/...
HR10991.5E-10 erg/... 2.0E-10 erg/... data / plot 5.207E-09 erg/...
2.888 phot/...
0.1109 phot/...

Summary properties of Capella models and BCS data are presented in the table below; this comparison suggests that the simulated fluxes are accurate to within a factor of 1.5 .

QuantityEnergyUnits Dave's ModelBCS Data [1] Brickhouse [2]
total flux 0.32-12.4 keV E-10 erg/cm2s : 1.40 - 0.66
total flux 0.32-12.4 keV photon/cm2s : 0.110 - 0.054
O VIII flux 0.653 keV 10-3 photon/cm2s : 3.42 5.06 +/-1.39 5.18
Fe XVII flux 0.826 keV 10-3 photon/cm2s : 8.29 6.65 +/-1.34 4.70
Fe XX flux 0.966 keV 10-3 photon/cm2s : 0.40 2.00 +/-0.79 -
Ne X flux
3 lines
1.022 keV 10-3 photon/cm2s : 3.63 - 0.95
Mg XII flux
1.472 keV 10-3 photon/cm2s : 0.37 - 0.19
[1] Vedder and Canizares, ApJ 270:666-670, 1983 July 15
[2]  XSPEC model vmekal+vmekal with parameters from Nancy Brickhouse 

Defocus Analysis

Goals and Results

The goals of the defocus analysis are:

The Capella observations and routine analysis ( simulations, above) produce detected line lists of FWHM values for Gaussian fits to the spectral lines. There is a wealth of data here specified by: the line energy, HEG or MEG, plus or minus 1st order, and the defocus condition (0.0, +0.2, -0.2). The zero-order FWHM vs defocus is also measured. The results here are shown for these specific measured widths:

The plots below where produced by oac_anal.pro and carry out a simple quadratic fit of FWHM^2 vs defocus, see Functional Form below. The FWHM values in a defocus set were normalized by the minimum measured FWHM in order to keep the plots on the same scale; other presentation methods will be investigated. The three plots correspond to the case where the actual nominal focus is 0.0, +0.3 mm and -0.3 mm with respect to the real HRMA best focus. The data allow these cases to be distinquished.

Plots covering a wide range of defocus are presented below to show the expected global variation of width vs focus. Previous MARX 2.11+ and 2.04 simulations are included to show effects of variation in the mirror model. Note that it is unlikely that a full range set like these will actually be obtained in flight!

Functional Form for FWHM vs Defocus

The width of a feature, FWHM, as a function of defocus, dX, can be approximated as:

      FWHM   =   SQRT{ (w_const)^2  +  ( beta(dX-Xo) )^2 }
           w_const = FWHM at best focus
              beta = FWHM per defocus distance
                Xo = axial location of best focus
Note that "beta" will differ primarilly due to different mirror shells sets (e.g., zero-order - 1,3,4,6; MEG = 1,3; HEG = 4,6). "w_const" can vary for diffracted orders if the line has a natural width. Thus FWHM^2 is a quadratic function of dX.
      FWHM^2 =  w_const^2  + beta^2 (dX-Xo)^2

             =  (w_const^2 + beta^2 Xo^2)    1
                            - 2 beta^2 Xo   dX  
                                 + beta^2   dX^2
The measured FWHM^2 vs dX values can be fit to a quadratic and the measured best focus location, Xo, calculated from the fit parameters:
         FWHM^2  =  a + b dX + c dX^2
and so
         Xo = -b/(2c)

Error estimate and counts needed

The measurement of the FWHM has a fractional error of order 1./SQRT(number-of-counts-in-line-order); the error on FWHM^2 is twice this. Looking at the MARX simulation plots, the zero-order or MEG curves have a slope (at dX ~ 0.4mm) of order :

  dFWHM^2 / FWHM^2   |
  -----------------  |                          ~  200%/mm   
     defocus(mm)     | at dX ~ 0.4 mm,
                         where (FWHM/FWHM(min))^2 ~ 2
So, if we want a 1-sigma error of 0.025 mm (1 mil), we need to measure FWHM^2 to 5% and FWHM to 2.5%. This requires about 1600 counts in the line-order.

OAC Contacts

This web page is: http://space.mit.edu/HETG/flight/oac.html.

Please send any comments and updates to Dan Dewey at dd@space.mit.edu.