Odd even column effect

The odd-even column effect is the difference in the flux level between odd and even columns in the Hawaii array. The cause of this effect is unknown. It was found that reducing the read speed of the array can reduce the severity of the odd-even column effect. When the read speed is reduced the minimum DIT is increased from 1.77 to 3.55s,which has a significant impact on overheads. The variation in the severity of the effect, and in the minimum possible DIT are summarized below. For service mode observations the allowed minimum DIT is always 3.55s.

Date Severity Minimum DIT
Jul 2012 After a detector intervention, the OE was reduced. The read speed factor was set to '6' on 2012-07-23. 1.7
Jun 2012 OE effect was noted on twilight flats of 2012-06-08. After several tests, the read speed factor was increased on 2012-06-24. 2.95
Feb 2012 OE effect was noted on twilight flats of 2012-02-25. To remove it, the detector was baked for a few hours in the morniing of 28th. The twilight flats taken in the evening of 2012-02-28 did no longer show the OE. 1.7
Nov 2011 After short warm-up of the detector the OE effectis gone. Speed factor changed to 4. Also the effect in the darks has disappeared. 1.7
Oct 2011 Even with increased speed factor, the OE continues. Speed factor changed to 11. There seems to be an additional effect in the darks on the two right quadrants. 3.25
Sep 2011 OE effect rises again reaching about 3%. 2.5
Aug 2011 OE effect back to normal, speed was reduced to old value. 1.777
Apr 2011 OE effect increases in strength. Speed factor was changed to cope with the feature. Minimum DIT was thus changed. 2.96
Feb 2011 ISAAC HAWAII detector starts showing several unstabilities among them a variable O/E effect on quadrant 1 and 3. 1.77
Sep 2008 Again,after an intervention, the detector temperature was set to value belowthe nominal temperature, resulting in the OE effect to appear again.The detector was heated up to the nominal value and the OE effectdisappeared. 1.77
Apr 2008 Shortre-appearance of the OE effect due to lower detector temperature afterthe intervention. The detector was heated up to the nominal value andthe OE effect disappeared. 1.77
Jan 27 2004 - present Disappeared, read speed increased 1.77
October 25 2003 - Jan 27 2004 Disappeared for read speed 12 (still present for read speed 6, so read speedkept at 12) 3.55
August 6 2003 - October 25 2003 Re-appeared. Read speed decreased on August 8. 3.55
April 1 2003 - August 5 2003 Disappeared after intervention, read speed increased 1.77
November 21 2002 - March 31 2003 Re-appeared, read speed decreased 3.55
March 14th 2002 - November 21 2002 Disappeared after instrument intervention. Read speed increased. 1.77
December 25th 2001 - March 14th 2002 Effect reappeared 3.55
October 21st - December 24th 2001 Disappeared after instrument intervention 3.55
August 24th - October 20th 2001 Effect reappeared 3.55
July 24th - August 23rd 2001 Read speed reduced. Effect <1% for flux range 0-20000 ADU 3.55
March 31st - July 24th 2001 Noticeable for low flux levels, became much stronger around May 11 1.77
Before March 30th 2001 Noticeable for flux levels > 10,000 ADU 1.77

The dramatic change that occurred during May 2001 is visible in oddeven5.ps and oddeven1.ps.The first plot shows how bias frames are affected. The lower axis is the number of days since April 1st. The second plot plots the amplitude of the effect as a function of flux for the 11th and 12th of May. The amplitude also depends on the quadrant, so the effect is plotted for each quadrant individually.

Since the effect is a function of flux, it cannot be removed by normal sky subtraction techniques, particularly when the background is varying greatly as can be the case when observing with the H filter or at the beginning of the night. In imaging, the effect can be removed very well by Fourier filtering techniques and the photometry, at the level of 1/100th of a magnitude, is unaffected. See the data reduction guide for more details.