This is a report describing variations and/or contaminations of the NACO Lamp Flats that have been obtained since June 2004.

The above figure trends lamp flat standard deviation since April 2005. The large increase at the beginning of June was due to problems with detector overheating that started at that time. Obvious change in lampflats occurred between June,04 and June,05 2005 and has continued since then. It stabilized toward mid August and has remained constant since then.

General timeline of variations:

• June 2004 - June 2005 - image of a defocussed dust particle visible in frames obtained with camera S27. Small effect.
• June 4-10, 2005 - significant change to NACO lampflats most likely connected with period of detector overheating - dimming observed at edges of the FOV. Until July 08, 2005 the average flux and flux std were quite unstable.
• August 05, 2005 - dimming visible in corners of the lampflats remains but statistical values of lampflats generally stabilized.

Before June 2005 the typical NACO lamp flat looked as in the example below. The lampflats in all filters taken with camera S27 were contaminated by the defocussed image of a dust particle. This effect appeared for the first time on June, 30/July, 1, 2004. The position, and size of the feature were changing with time. Note that the contamination practically cannot be distinguished from the larger gradient seen in the cross-section of the frame. It is below 1% level. The lampflats taken with cameras S13 and S54 were generally free from this type of contamination.

NACO lampflat from July 01, 2004; Filter Ks, Camera S27

The below examples show difference master calibration lampflats from July,01 2004 with corresponding frames from June,29 2004. Click on each image to get enlarged version.

Filter Ks, Camera S27	Filter H, Camera S27	Filter J, Camera S28

Since June 05, 2005 we've started seeing changed lampflats with dimming at the edges of the field of view. Below are examples of the master calibration frames from that day taken in filter H, cameras S27 and S13. They are difference images where the reference (from 2004-11-05) was subtracted. The corresponding lampflats for other filters - J and Ks, looked similar.

Filter H, Camera S27	Filter H, Camera S13

Since beginning of June 2005 we have seen the same kind of pattern in the lampflats in all filters. There are consistent trend observed:

• in S13 the dimming is visible in two upper (left and right) and lower right corners,
• in S27 we have dimming in just the bottom right corner,
• in S54 the dimming affects the top right corner and below.

See below examples from Aug.17, 2005 (before break in operation) and Oct.10, Nov.05, 2005 (after the break). Again the differences with corresponding reference frames are shown. Please note that there are no significant variations during that period of time, although the standard variations of the flux of the NACO lampflats decreased slightly toward October and November 2005.


Difference image of lampflat from Aug.17, 2005; Filter H, Camera S27

Difference image of lampflat from Aug.17, 2005; Filter H, Camera S13

Difference image of lampflat from Oct.19, 2005; Filter H, Camera S54

Difference image of lampflat from Nov.05, 2005; Filter H, Camera S27

Difference image of lampflat from Nov.05, 2005; Filter J, Camera S13

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