LASCO C1 Offset Bias versus Time

LASCO C1 Offset Bias Versus Time
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The Offset Bias

Images taken by the LASCO C1 camera have an offset bias or dark current which increases with time. The raw signal (S_r - in DN/binned pixel*) is related to the "true" signal (S - in DN/pixel/sec) by:

	S = (S_r/b-f)/t_x

where b is the binning (pixel/binned pixel), f is the offset bias (DN/pixel) and t_x is the exposure (sec).

* NOTE - pixels may be "binned" or grouped together. These are specified by the "LEBX" and "LEBY" parameters in the header and the binning is given by b==LEBX * LEBY.

Calculation of the Offset Bias

Calculations of the LASCO C1 offset bias have been done using dark images for various days during the mission. The value of the offset bias for a particular image was taken to be the value assumed by the most number of pixels in the image. In other words, a histogram of the signal values in the image was created, and the value corresponding to the maximum of the histogram function was taken to be the offset bias for that image. This was done in order to eliminate the effect of cosmic rays. In addition, the smallest exposure times possible were used in order to minimise the effect of stray light.

A functional approximation to the offset bias as a function of time was assumed to be given by:

	f = a + b*( 1 - exp(-t/t0) )

where f is the offset bias, a, b, and t0 are constants, and t is the time in days from julian day 50395. That is:

     t = mjd - 50395

A least squares fit of the offset bias to the above equation yields the following values for the constants:

     a  = 351.958	DN/pixel	
     b  = 30.7349	DN/pixel	
     t0 = 468.308	days

The graph below shows the individual data points as diamonds with the solid line being the fit using the above equation:

Fig. 1 - Offset Bias vs. Time

The above method has been implemented in the IDL program named fpc1_bias.pro This program has the following advantages to recommend it:

It is a rather accurate representation of the offset bias as a function of time.
It uses the mjd time accurate to the millisecond so that there are no discontinuities across day boundaries.
The exponential method will not diverge or become too unrealistic with the passage of time.

The files from '/net/corona/cplex2/lz_data/misc/c1/dark/' were used in this analysis. Those images with offset bias calculated to be above 450 were discarded. Only those images using port C and with exposure times less than 9 seconds were used.

PR Nov 21, 1997
PR Oct 23, 1998
PR Nov 01, 1999 - fixed units