Gain Notes for Revolution 1985

This revolution was devoted entirely to observations of SCO X-1, which is
so strong that it produces two pronounced effects that compromise the
energy calibration of the JEM-X instruments:
1) The sheer flux of the source during on-axis observations means that the grey
	filter is activated automatically, reducing not just the number of
	science photons that are processed, but also removing a significant
	fraction of the calibration photons. So that during the on-axis periods
	of the dithering pattern, it is not possible to calibrate the instrument
	as accurately as with normal observations of fainter sources, or off-axis
	observations of SCO X-1. There simply are not enough calibration photons
	in the data stream to find the position of the Xe instrument lines.
2) The high photon flux suppresses the gain a strong and non-linear way so that
	the Xe line becomes an essential part of the energy calibration, even
	when a good fix on the Fe calibration line is obtained. This non-linear
	effect can only be corrected by use of the Xe line, which may, or may
	not be visible.
3) The strong astronomical source, the effect of grey filtering and the large variation
	in the instrumental gain, combine to wash out the weaker instrumental lines
	from Cu and Mo, which cannot be seen during this revolution.

These effects can be seen directly in the instrumental gain table for the revolution
where deep dips are superimposed on the usual shape of the gain history. Each
dip shows where the instruments have passed over the strong source in the 
dithering pattern, and the gain suppressed according to the strength of the
source in the FOV.

This is one of the later revolutions in the mission where the calibration
sources have become so weak that for the best energy determination and
IC Gain History table is needed.
 

To get optimal results with this revolution it is necessary to use the IC gain history.
See notes in revolutions prior to revolution 1433 to find out how to do this
if you don't already know.

All offline gain/energy corrections (including the IC tables) are based on the
two Fe calibration sources in JEM-X1, for both units.

Latest OSA software (version 10 and higher) automatically cuts out the first
Science windows of data from each revolution to ensure that people do not use data
from the instrument-settling period. This process can be overruled, but only by
experienced users.
	
JEM-X1 rating: Acceptable (Of the well-determined Xe lines, all but one SCW has a
	position within about 4% of ideal. Due to the reasons described above
	this situation cannot be improved upon, and extreme caution should be used
	when using data from the on-axis science windows for energy-sensitive
	applications (spectra, energy-binned lightcurves and images).

JEM-X2 rating: Acceptable. Identical comments as those for JEM-X1.

CAO 10/08/2018