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ASCA: Frequently Asked Questions about Data Analysis


Imaging Analysis

Q1 I use ftools to extract an image, but I can't get the equatorial coordinates to show up in my display program, only the pixel coordinates.
A1 Try extracting the image in sky coordinates instead of detector coordinates with: 'set image sky'. The old practice of recording the approximate detector to sky conversion factors in detector images has been discontinued, due to the inherent inaccuracies involved.

Timing Analysis

Q1 I have been looking at some of the most recent ASCA observations. There appear to be gaps in the data lasting for a few orbits at a time. Could you please tell me what these are likely to be caused by? Also, what exactly is the orbital period of ASCA?
A1 Such gaps are most likely due to DSN downlink dropouts. Some of the ASCA data are telemetered down to the Kagoshima station in Japan, while others are telemetered to NASA DSN stations. There are occasional loss of downlinks to the DSN stations, due to last-minute changes to DSN usage, or due to equipment problems. See the list of data dropouts for mmore.

The satellite orbital period was 96.1 min at the beginning of the mission in 1993 February and 94.0 min in mid July, 2000. The period for your observation can be found by examining the FITS orbital element files ("frf.orbit.nnn" where "nnn" is the version number).

Spectral Analysis

Q1 What are the PI channel - energy relationships?
A1 For GIS with 1024 channels, 5.9 keV = channel 500; data with fewer channels are always linearly rebinned.

For SIS, 1 channel = 3.65 eV but Bright and Fast mode data are rebinned

	channel 0-1023:		x1	0-1023
	channel 1024-2047	x1/2	1024-1535
	channel 2048-4095	x1/4	1536-2047
these are further binned into a linear scale when making spectra by default.

Appended below is a precalculated table for convenience: in addition to rounding errors, there are calibration uncertainties in the gain, so please use this as a rough guide. This table should be good enough for making hard vs soft images or light curves, or quickly identifying spectral features seen in your data.

 E (keV)  GIS              SIS
         (1024) Faint  Bright  1024   512
------------------------------------------
  0.5            136    136     34     17
  1.0      85    273    273     68     34
  1.5     127    410    410    103     51
  2.0     169    547    547    137     68
  2.5     212    684    684    171     86
  3.0     254    821    821    205    103
  3.5     297    958    958    240    120
  4.0     339   1095   1059    274    137
  4.5     381   1232   1127    308    154
  5.0     424   1369   1196    342    171
  5.5     466   1506   1264    377    188
  6.0     508   1643   1333    411    205
  6.5     551   1780   1401    445    223
  7.0     593   1917   1470    479    240
  7.5     636   2054   1537    514    257
  8.0     678   2191   1571    548    274
  8.5     720   2328   1605    582    291
  9.0     763   2465   1639    616    308
  9.5     805   2602   1674    651    325
 10.0     847   2739   1708    685    342

Q2 When extracting spectra in sky coordinates, I get a warning about "inconsistent image OPTIC" such as
Input event files have inconsistent image OPTIC
   139.79341       138.30974      difference =   1.4836730
   123.17679       123.15044      difference =  2.63520000E-02
Should I worry about this?
A2 This is not a problem, just a manifestation of the initial attitude wobble.

The OPTIC keywords record the optical axis position of the detector; those associated with the sky coordinates are the average of optical axis position (fixed in detector coordinates) for your data. This can be different, particularly if you include the data from the first 20 min or so of the observation when the attitude is still settling down. If you do not exclude this period from your spectral extraction, your vignetting correction can have a larger-than-usual error.

Q3 I used 'addascaspec' to combine two ASCA spectra, and now I'm getting a much lower chi-squared than when I fitted the two simultaneously in xspec. What's going on?
A3 This is a feature of 'mathpha' in dealing with the Poissonian regime; by default it uses a rather conservative method to estimate errors (i.e., it tends to overestimate the size of the error bars in the combined background spectrum). There is a full description in the "Calculation and Propagation of Errors" section of 'fhelp mathpha'.

Q4 I have a high latitude source observed in 2-CCD mode that fills both chips. I would like to use the Blank Sky observations for background but those were taken in 4-CCD mode. What kind of errors would this cause?
A4 You will probably have to use the blank sky data, with the following caveats in mind:

It is true that the internal (non X-ray) background is clocking mode dependent. However, below about 5-6 keV, the cosmic X-ray background (CXRB) dominates over the non X-ray background (see this plot), of CXRB+internal background as data and a model of the internal background). Below about 5 keV, the dominant source of background subtraction errors will be the point-to-point fluctuations of the CXRB. The uncertainties in the internal background are usually about 10% of the total internal background, or about 10-4 counts/sec/keV level.

Q5 After I extracted spectra from the ASCA GIS-3 data of Seq. no. 51022000, I found the spectral channel number is only 128 rather than 1024 so that it doesn't match the GIS response (with 1024 channels). What is the reason, and how can I use the GIS-3 spectrum?
A5 This was due to on-board software problem that affected data taken between 1994 February 10 and 1994 April 8. All you need to analyze your GIS-3 spectrum is to download a vresion of the response rebinned to 128 channels ( gis3v4_0_128ch.rmf).

You can read the details of what happened, and how the data have been processed here but please note the the fix this page speaks of has already been applied to your data.


This file was last modified on Tuesday, 23-Apr-2002 11:53:08 EDT
Curator: Michael Arida (SP Sys); arida@milkyway.gsfc.nasa.gov
HEASARC Guest Observer Facility
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This file was last modified on Tuesday, 23-Apr-2002 11:53:08 EDT

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