WFC Build1 Noise Problems

AUTHORS : Marco Sirianni, Gabriel Perdue

DETECTOR : WFC Flight build #1

RESULTS :

During the Thermal Vacuum campaign the WFC flight build #1 was on board of ACS. The presence of severe electronic noise, prevented any useful calibration measurements. The noise pattern showed amplitude and time variations. In the following table we list few sample frames (bias and dark) to show the problem. The frames were taken between 19 February and 13 March 1999. For each bias frame we show the image of the two chips, the read noise associated to each amplifier, and a plot for each CCD. Each plot consists of two columns, one for each amplifier: plot1 for ampl. A+B and plot2 for ampl. C+D. The first row shows the average of 2000 lines of the chip, the second row shows the average of columns in the region where the bias level is approximatively constant and no bias jumps occur. Finally, the third row shows the histogram of a reduced region of the chip where only the small scale noise is present.

Due to the time dependence of the noise pattern, the subtraction of the bias frame slightly reduced the overall noise. This image shows the dark #3486 after the subtraction of the bias frame #3485. A strong pattern noise is still present and after the correction for the bias level offset between the two frames the dark rate measured was 2.8 +/- 27.9 e-/pix/hr. In some cases the bias level in the physical and virtual overscan regions resulted higher than the dark level in the image area, yielding an unrealistic negative dark rate.

Table 1 : WFC Bias and Dark Sample Frames

Exposure Type	Cal. Log Number	CCD AMP	READ NOISE DN rms	Images	Plots
Bias	3466	A-B C-D	24-47 8.7-19	IMAGE 1 IMAGE 2	PLOT 1 PLOT 2
Bias	3479	A-B C-D	24-43 8.5-17	IMAGE 1 IMAGE 2	PLOT 1 PLOT 2
Bias	3485	A-B C-D	18-30 8-14.5	IMAGE 1 IMAGE 2	PLOT 1 PLOT 2
Bias	3520	A-B C-D	31-14 8.5-13.7	IMAGE 1 IMAGE 2	PLOT 1 PLOT 2
Dark	3470	A-B C-D	- -	IMAGE 1 IMAGE 2	- -
Dark	3486	A-B C-D	- -	IMAGE 1 IMAGE 2	- -
Dark	3522	A-B C-D	- -	IMAGE 1 IMAGE 2	- -