Impact of New Southern Point Source Photometric Sensitivity Scoring Factors

I. Introduction

The empirical dependence of photometric sensitivity on atmospheric seeing and sky background levels was derived for northern data. These dependences were converted into scan quality scoring factors that have been applied to northern survey data since the start of operations, and to southern survey data since an empirical analysis of the achieved sensitivity from the CTIO facility had not yet been carried out.

John Gizis has recently evaluated new photometric sensitivity dependencies for both southern data, as well as northern data taken with the new H-band array in his memo 2MASS Calibrator Repeats. This new analysis shows that the northern seeing and background factors overestimate the achieved southern photometric sensitivity by a small amount.

The photometric sensitivity parameter (PSP) is related to mean scan seeing shape and background via the relation:

As the seeing shapes or frame backgrounds increase, the PSP value increases, and the photometric sensitivity decreases. The table below gives the current northern and new southern scoring factors as a function of PSP range. Note that the H and K_s factors were the same for the northern data, but are different in the south (the new northern H-band array yields a different relationship than the northern K_s factor now).

	North	South
Scoring Factor	PSP (H and Ks)	PSP (H)	PSP Ks)
1.0	< 10.85	< 9.6	< 10.6
0.8	10.85-11.11	9.6-9.8	10.6-10.9
0.5	11.11-11.35	9.8-10.3	10.9-11.1
0.3	11.35-12.40	10.3-11.7	11.1-11.7
0.1	>12.40	>11.7	>11.7

II. Scans in the Working Database

A best-case estimate of the impact of the new sensitivity parameters can be made by examining southern scans in the working database. There are currently 13,690 southern scans in the working DB scan information table. The table below shows the cumulative number of scans and the percentage of all scans having sensitivity scoring factors less than the value specified in the first column, using both the old northern PSP-score relationship and the newly derived southern relationship. This is a useful count because it shows explicitly the number of scans that might have to be reobserved at various scoring thresholds. Note that the lowest scoring factor is 0.3 since only scans with net quality >2 have been entered into the working DB.

	Old		New
Score Factor	Number (< Score)	% of Total	Number (< Score)	% of Total
1.0	13690	100.0	13690	100.0
0.8	34	0.2	483	3.5
0.5	17	0.1	355	2.6
0.3	5	0.04	137	1.0

The new scan scoring plan proposes to set to quality zero scans with average seeing shapes >1.25. The table below shows the same information as that above, but considering now only scans with mean seeing shapes < 1.25. This doesn't affect many southern scans because of the generally good seeing achieved at CTIO.

	Old		New
Score Factor	Number (< Score)	% of Total	Number (< Score)	% of Total
1.0	13670	100.0	13670	100.0
0.8	31	0.2	472	3.5
0.5	15	0.1	344	2.5
0.3	3	0.02	130	1.0

This best-case analysis indicates that if all scans with net quality scores < 0.5 are scheduled for rescan, using the revised southern PSP-seeing-background relationships will tag at least ~1% of southern scans for reobservation. Of course, this is a lower limit because any reduction of the sensitivity score factor below 1.0 reduces the margin for the scan to pass due to other potential quality downgrades such as photometricity, untracked seeing or airglow.

III. Scans in the Production Archive

A worst-case estimate of how the sensitivity factors will affect southern scan scoring can be derived by comparing the old and new scoring factors of all survey and calibration scans in the production archives (PARC). This includes all processed data, photometric and otherwise. There are seeing and background information available for 48,417 scans in the PARC.

The table below shows the cumulative number and percentage of all survey and calibration scans having sensitivity scoring factors less than the value the the first column.

	Old		New
Score Factor	Number (< Score)	% of Total	Number (< Score)	% of Total
1.0	48417	100.0	48417	100.0
0.8	724	1.5	4213	8.7
0.5	491	1.0	3192	6.6
0.3	330	0.7	1564	3.2
0.1	83	0.2	207	0.4

Finally, the table below shows the scoring statistics only for scans having an average seeing shape < 1.25.

	Old		New
Score Factor	Number (< Score)	% of Total	Number (< Score)	% of Total
1.0	48087	100.0	48087	100.0
0.8	540	1.1	3940	8.2
0.5	325	0.7	2931	6.1
0.3	189	0.4	1353	2.8
0.1	13	0.03	90	0.2

The percentages of sensitvity scores < 1.0 are most likely overestimated in this analysis because calibration scans are included. Cal scans are regularly taken at higher airmasses to monitor atmospheric extinction, and this can result in higher backgrounds and worse seeing. But this worst case model suggests that at most 2.8% of southern scans will have sensitivity score factors less than 0.5. Most likely, the final number having factors < 0.5 will be between the best and worst-case estimates, or perhaps 2%.