Untracked Seeing

T. Jarrett, IPAC
(980422)
updated 981125

Go directly to the seeing plots for 980503n.

Separating small compact galaxies from stars relies upon accurate measurement of the time-varying PSF -- that is to say, the "seeing" conditions. If the seeing is not tracked (due to a rapid change in the atmospheric conditions, e.g.) then false sources (re: stars) creep into the extended source database and reliability is compromised.

We have written a simple diagnostic to measure or at least indicate when the seeing is not properly tracked. A description of the algor is given below. Results for two RTB nights, 971205n and 980125n, are given at the end. The two nights provide a nice contrast: 971205n had much better seeing conditions than 980125n, with the latter containing several scans subject to significant untracked seeing.

The following description can be found in the "seetracker" SIS.

SEEMAN attempts to measure the "seeing" (PSF profile as measured on the coadds) on spatial scales (or equivalently, time scales) of about once per coadd. GALWORKS performs the same calculation except on slightly different time scales (it is set by the stellar number density). "Tracked" seeing means that the seeing has not changed significantly between measurements (thus, times scales of a few minutes). "Untracked" seeing refers to a significant change in the real seeing between measurements -- this is a condition that is bad news to both PROPHOT and GALWORKS.

Algor:
Since we measure the "sh" (quantitative seeing diag) for each star, we effectively measure the seeing on very high frequency time scales. Individual measurements will give clues as to significant changes in the seeing. We can exploit this feature to derive a measure of "tracked" or "untracked" seeing.

A running window is passed along the scan direction. The window defines the region in which seeing will be measured as a group. The window has a size of 1000 arc sec (with steps every 500 arc sec). We require at least 7 stars per window. We compute the lower quartile value of the "sh" distribution. We then compare this value with the actual measurement made by SEEMAN and/or GALWORKS. If the seeing is stable, then the measurements should track each other within the measurement uncertainty, with the galworks lower quartile value slightly lower in general than the median value calculated by seeman. The difference is computed. The difference is then normalized by the measurement uncertainty (thus resulting in a N-sigma "score"). The measurement uncertainty is taken from the "global" seeing "ridge" measurements performed by GALWORKS (information located in "seeridge_JHK" files). We write to file the maximum "score" of the "tracked" (or "untracked") seeing. Score values greater than +1 or +2 indicate "untracked" seeing and the scan should be flagged accordingly.

981205n & 980125n Results

The following plots were generated from tables that can be found in the QA ("quality") scan & nightly reports.

971205n Seeing Diags
(Click on graph for bigger and better image)
Notes: blue dots = J, green = H, red = K 980125n Seeing Diags
(Click on graph for bigger and better image)
Notes: blue dots = J, green = H, red = K
yellow dashed line denotes scan 045

Scans 045, 046 & 061 for 980125n have been examined in detail (looking at the extended source results from GALWORKS). The seeing is dreadful for these three scans (and they have company). In addition, according to the untracked seeing diagnostic, see above, H band is clearly untracked for scan 045 and J&H for scan 046 & 061. This can be more easily seen by plotting the seeing ("sh" values) internal to the scan (on roughly a "coadd" spatial/time scale).

For comparison, the following gif shows a case in which the seeing is tracked properly. Galaxies appears as points well above the seeing track (double stars will do the same).

Seeing for Scan 036, 971205n

The untracked seeing results in a a number of spurious sources (including bright stars) being detected and classifed as galaxies. Scans with an untracked seeing "score" greater than 1.0 are clearly undesirable and should be flagged accordingly. Fortunately, only 1 to 2% of the scans in 971205n and 6-7% for 980125n would fail this criterion. See below.

Seeing Track Score Distribution

What about the transition between tracked and clearly untracked seeing? From theoretical grounds, we believe that a score greater than 0.5 will result in a reliability hit that jeopardizes the level-1 spec. We can get a better handle on the "grey" region in which the untracked seeing becomes significant by looking carefully at the results coming from the RTB data. A report on this empirical approach is forthcoming.

Addendum

971110n Results

The following plots were generated from tables that can be found in the QA ("quality") scan & nightly reports.

971110n Seeing Diags
(Click on graph for bigger and better image)
Notes: blue dots = J, green = H, red = K

Comments:

TChester:

i'm going to do a little simulation to see if i can reproduce your basic histogram, and i'll quantify the theoretical argument to see more clearly the effect on star-galaxy separation.

the readers of your memo should note that the sigma which is used for normalization purposes is sigma(pop), and thus 0.5 sigma(pop) is actually more like a minimum of 1.8 sigma(mean). to derive that number i used a density of 330 stars per sq. deg at K, which would be 13 stars in the window. since j and h have 2x as many stars, the 0.5 sigma(pop) would be 2.5 sigma(mean).

Untracked seeing from the night of 980305n

NOTE: the untracked seeing scores are based on the "seeridge" ridgeline determination. The seeridge values are computed by GALWORKS and they are driven by the source density, and NOT by the coadd area (of which SEEMAN ridgelines are restricted to). The seeridge algorithm wants to use a minimum of 20 to 30 stars for any given "sh" ensemble measurement (in contrast the SEEMAN minimum ensemble is 10 stars) in order to minimize the affects of noise and non-stellar contamination. The upshot is that the "seeridge" solution can have time scales as small as about one-quarter of a coadd (in high stellar number density fields) and as big as two or three coadds in low stellar density fields. SEEMAN generally has a solution for every coadd. Hence, the solutions will differ and the untracked seeing will differ (sometimes SEEMAN will do better with fast-changing seeing and low stellar density, and sometimes seeridge will do better with fast-changing seeing and moderate to high stellar density). The examples given below demonstrate this difference. Also see Generalized Point Spread Funtion: 2MASS Seeing for additional examples (showing the seeridge to be superior).

980503n Untracked Seeing Scores

scan	Jsh	Jdsh	Jdiff	Jscore	Hsh	Hdsh	Hdiff	Hscore	Ksh	Kdsh	Kdiff	Kscore	J-H	J-K	H-K
001	1.163	0.040	-0.007	-0.180	1.124	0.038	-0.008	-0.210	1.121	0.048	-0.016	-0.326	0.045	0.042	0.012
002	1.139	0.036	-0.007	-0.200	1.102	0.034	-0.011	-0.321	1.100	0.042	-0.011	-0.262	0.050	0.049	0.009
003	1.150	0.034	-0.018	-0.534	1.111	0.034	-0.005	-0.151	1.097	0.047	-0.015	-0.313	0.042	0.078	0.038
004	1.110	0.036	0.022	0.611	1.074	0.036	-0.002	-0.056	1.071	0.050	0.020	0.400	0.044	0.052	-0.025
005	1.097	0.038	-0.004	-0.109	1.074	0.040	-0.008	-0.205	1.058	0.050	-0.007	-0.137	0.033	0.044	0.024
006	1.064	0.037	-0.009	-0.243	1.036	0.039	-0.007	-0.185	1.037	0.040	-0.016	-0.412	0.035	0.034	-0.011
007	1.106	0.038	0.004	0.111	1.074	0.041	0.000	0.008	1.072	0.051	-0.002	-0.039	0.053	0.063	0.028
008	1.119	0.039	0.008	0.202	1.085	0.039	0.004	0.099	1.088	0.051	-0.002	-0.045	0.055	0.053	-0.024
009	1.129	0.037	-0.001	-0.035	1.094	0.041	0.001	0.022	1.095	0.054	0.002	0.033	0.055	0.061	-0.023
010	1.138	0.039	0.008	0.208	1.101	0.044	0.003	0.072	1.102	0.051	0.006	0.126	0.050	0.053	0.026
011	1.144	0.040	-0.005	-0.117	1.108	0.041	0.012	0.285	1.108	0.055	-0.002	-0.036	0.050	0.062	-0.030
012	1.199	0.049	-0.003	-0.059	1.158	0.052	0.015	0.290	1.150	0.062	0.016	0.257	0.070	0.087	0.038
013	1.230	0.052	0.024	0.454	1.183	0.049	0.006	0.132	1.168	0.059	-0.006	-0.108	0.078	0.107	0.043
014	1.225	0.050	0.019	0.384	1.177	0.050	0.034	0.677	1.163	0.060	0.003	0.050	0.079	0.114	0.056
015	1.219	0.073	0.091	1.240	1.166	0.085	0.123	1.442	1.179	0.064	0.046	0.713	0.059	0.064	-0.069
016	1.204	0.062	-0.005	-0.074	1.167	0.058	-0.004	-0.069	1.161	0.079	-0.017	-0.213	0.054	0.083	0.053
017	1.274	0.060	0.004	0.071	1.219	0.049	0.001	0.028	1.185	0.053	-0.010	-0.195	0.061	0.100	0.048
018	1.254	0.067	-0.025	-0.372	1.204	0.056	0.000	0.000	1.171	0.074	-0.001	-0.019	0.064	0.098	0.043
019	1.152	0.039	0.002	0.054	1.106	0.045	-0.003	-0.067	1.098	0.050	-0.010	-0.191	0.051	0.062	0.020
020	1.175	0.046	-0.001	-0.012	1.131	0.048	0.001	0.011	1.109	0.051	0.011	0.216	0.057	0.074	0.043
021	1.208	0.051	0.031	0.613	1.157	0.054	0.029	0.540	1.137	0.059	0.029	0.498	0.081	0.112	0.060
022	1.223	0.053	0.009	0.170	1.168	0.054	0.040	0.742	1.154	0.060	0.008	0.135	0.093	0.096	-0.045
023	1.254	0.053	0.014	0.271	1.205	0.056	0.025	0.440	1.178	0.063	0.011	0.177	0.082	0.126	0.054
024	1.287	0.059	0.025	0.417	1.235	0.061	0.074	1.220	1.209	0.065	0.157	2.413	0.085	0.111	0.057
025	1.262	0.055	0.045	0.827	1.206	0.054	0.083	1.531	1.181	0.067	0.044	0.650	0.089	0.121	0.051
026	1.263	0.053	0.040	0.745	1.213	0.057	0.030	0.520	1.186	0.062	0.016	0.254	0.079	0.122	0.061
027	1.227	0.044	-0.001	-0.023	1.177	0.043	-0.003	-0.070	1.159	0.055	-0.018	-0.327	0.051	0.072	0.028
028	1.345	0.060	0.018	0.307	1.279	0.068	-0.012	-0.174	1.253	0.063	0.046	0.724	0.082	0.129	0.101
029	1.322	0.049	-0.005	-0.102	1.257	0.048	-0.001	-0.021	1.228	0.049	-0.014	-0.286	0.075	0.120	0.060
030	1.362	0.052	-0.017	-0.327	1.299	0.053	-0.011	-0.215	1.268	0.061	-0.010	-0.167	0.077	0.108	0.046
031	1.297	0.061	0.038	0.617	1.237	0.058	0.001	0.017	1.213	0.071	0.015	0.213	0.073	0.110	0.039
032	1.371	0.064	-0.008	-0.128	1.311	0.060	-0.012	-0.197	1.262	0.068	-0.003	-0.037	0.073	0.136	0.067
033	1.228	0.055	0.111	2.019	1.179	0.060	0.074	1.234	1.153	0.066	0.116	1.762	0.074	0.117	0.064
034	1.264	0.058	0.227	3.921	1.215	0.053	0.087	1.647	1.189	0.064	0.130	2.026	0.078	0.151	0.107
035	1.267	0.057	0.173	3.028	1.212	0.057	0.093	1.639	1.187	0.067	0.071	1.067	0.099	0.102	0.045
036	1.308	0.060	0.046	0.775	1.242	0.056	0.033	0.582	1.211	0.064	0.036	0.568	0.097	0.135	0.066
037	1.309	0.060	0.044	0.737	1.245	0.061	0.057	0.933	1.213	0.063	0.044	0.696	0.106	0.125	0.065
038	1.277	0.064	0.181	2.824	1.222	0.062	0.086	1.392	1.200	0.068	0.184	2.709	0.091	0.106	0.054
039	1.275	0.052	0.033	0.641	1.229	0.053	-0.020	-0.381	1.219	0.087	-0.004	-0.046	0.069	0.073	0.026
040	1.346	0.063	0.003	0.052	1.286	0.058	-0.006	-0.096	1.248	0.061	-0.028	-0.458	0.075	0.119	0.045
041	1.327	0.084	0.090	1.066	1.270	0.079	0.011	0.143	1.240	0.096	0.044	0.458	0.078	0.124	0.056
042	1.311	0.074	0.033	0.444	1.267	0.072	0.040	0.556	1.233	0.086	0.064	0.744	0.069	0.114	0.049
043	1.297	0.057	-0.011	-0.185	1.243	0.054	-0.017	-0.313	1.199	0.072	-0.011	-0.153	0.055	0.117	0.062
044	1.276	0.049	-0.003	-0.061	1.217	0.047	-0.003	-0.064	1.201	0.082	-0.008	-0.093	0.071	0.083	0.039
045	1.226	0.062	0.147	2.376	1.169	0.057	0.182	3.195	1.136	0.068	0.172	2.531	0.118	0.148	0.070
046	1.267	0.063	0.060	0.952	1.203	0.061	0.049	0.798	1.172	0.067	0.048	0.713	0.098	0.151	0.069
047	1.237	0.050	0.052	1.032	1.189	0.058	0.055	0.941	1.152	0.068	0.057	0.841	0.073	0.110	0.061
048	1.245	0.052	0.023	0.449	1.187	0.057	0.040	0.702	1.157	0.058	0.039	0.664	0.088	0.156	0.093
049	1.092	0.043	0.000	0.007	1.068	0.050	-0.013	-0.256	1.049	0.055	0.000	0.004	0.036	0.055	0.032
050	1.106	0.047	-0.008	-0.175	1.066	0.049	-0.018	-0.363	1.048	0.060	-0.006	-0.107	0.057	0.070	0.040
051	1.132	0.049	-0.001	-0.020	1.099	0.051	-0.019	-0.375	1.078	0.051	-0.007	-0.147	0.046	0.067	0.031
052	1.104	0.037	-0.001	-0.028	1.060	0.060	0.002	0.040	1.049	0.056	-0.002	-0.039	0.050	0.063	0.024
053	1.098	0.047	-0.001	-0.021	1.069	0.052	0.007	0.135	1.030	0.068	-0.005	-0.077	0.039	0.086	0.059
054	1.126	0.039	-0.002	-0.057	1.095	0.043	-0.011	-0.256	1.072	0.073	-0.009	-0.127	0.051	0.072	0.040
055	1.139	0.044	0.038	0.860	1.092	0.048	0.104	2.166	1.070	0.060	0.018	0.300	0.068	0.094	0.054
056	1.131	0.048	0.079	1.635	1.082	0.047	0.132	2.809	1.065	0.053	0.049	0.921	0.083	0.107	0.038
057	1.129	0.051	0.025	0.498	1.085	0.048	0.023	0.483	1.067	0.061	0.006	0.097	0.062	0.077	0.035
058	1.143	0.046	0.029	0.631	1.091	0.049	0.038	0.772	1.070	0.053	0.016	0.300	0.071	0.090	0.048
059	1.153	0.052	0.335	6.450	1.125	0.056	0.110	1.956	1.120	0.066	0.068	1.028	-0.079	0.073	0.042
060	1.217	0.058	0.164	2.826	1.164	0.053	0.144	2.716	1.136	0.058	0.011	0.185	0.086	0.106	0.061
061	1.190	0.059	0.028	0.476	1.138	0.059	0.013	0.220	1.097	0.061	0.009	0.148	0.073	0.095	0.058
062	1.085	0.058	0.025	0.431	1.047	0.043	0.016	0.372	1.023	0.051	-0.009	-0.186	0.044	0.069	0.028
063	1.133	0.053	0.000	0.001	1.088	0.043	0.007	0.163	1.036	0.056	-0.005	-0.091	0.056	0.105	0.059
064	1.216	0.081	0.044	0.541	1.152	0.081	0.077	0.948	1.144	0.073	0.023	0.319	0.089	0.107	0.018
065	1.163	0.059	0.052	0.881	1.118	0.047	0.036	0.766	1.093	0.060	0.009	0.150	0.048	0.074	0.031
066	1.221	0.054	-0.005	-0.087	1.167	0.038	0.000	0.000	1.106	0.068	-0.003	-0.047	0.067	0.132	0.066
067	1.239	0.066	0.130	1.967	1.188	0.059	0.181	3.064	1.167	0.068	0.159	2.334	0.089	0.112	0.083
068	1.192	0.045	0.149	3.301	1.142	0.041	0.134	3.276	1.117	0.060	0.043	0.725	0.072	0.090	0.050
069	1.229	0.048	0.105	2.192	1.173	0.057	0.046	0.801	1.145	0.063	0.055	0.867	0.084	0.127	0.043
070	1.212	0.056	0.081	1.445	1.164	0.053	0.041	0.779	1.136	0.058	0.007	0.123	0.071	0.101	0.064
071	1.221	0.052	0.108	2.079	1.169	0.051	0.081	1.591	1.143	0.066	0.034	0.517	0.088	0.119	0.056
072	1.219	0.059	0.058	0.975	1.161	0.062	0.059	0.948	1.146	0.073	0.071	0.978	0.092	0.091	0.055
073	1.295	0.063	0.118	1.872	1.238	0.061	0.075	1.235	1.202	0.063	0.121	1.914	0.091	0.122	0.063
074	1.259	0.059	0.008	0.141	1.201	0.063	0.011	0.175	1.157	0.063	-0.015	-0.238	0.073	0.123	0.069
075	1.293	0.049	0.004	0.076	1.234	0.055	-0.003	-0.056	1.182	0.071	-0.009	-0.126	0.072	0.130	0.073
076	1.307	0.060	0.023	0.382	1.250	0.063	0.007	0.111	1.187	0.073	-0.008	-0.106	0.078	0.135	0.093
077	1.207	0.054	0.038	0.704	1.154	0.065	-0.001	-0.015	1.133	0.071	-0.007	-0.093	0.068	0.097	0.053
078	1.381	0.056	-0.008	-0.151	1.324	0.060	-0.011	-0.183	1.256	0.061	-0.012	-0.203	0.087	0.137	0.130
079	1.253	0.056	-0.016	-0.279	1.192	0.063	-0.015	-0.234	1.156	0.067	-0.022	-0.332	0.085	0.113	0.060
080	1.283	0.060	0.020	0.326	1.229	0.063	0.025	0.397	1.180	0.066	0.030	0.454	0.103	0.162	0.118
081	1.185	0.054	0.041	0.766	1.138	0.056	0.006	0.104	1.098	0.064	0.006	0.094	0.093	0.128	0.079
082	1.225	0.057	0.005	0.081	1.173	0.060	0.007	0.117	1.131	0.060	0.008	0.130	0.093	0.175	0.105
083	1.195	0.055	0.018	0.331	1.140	0.058	0.155	2.679	1.108	0.063	0.053	0.840	0.301	0.156	-0.190
084	1.163	0.054	0.035	0.651	1.114	0.057	0.026	0.458	1.080	0.061	0.043	0.697	0.101	0.141	0.080
085	1.169	0.052	0.022	0.425	1.120	0.055	0.036	0.663	1.086	0.060	0.038	0.632	0.100	0.179	0.091
086	1.202	0.057	0.037	0.652	1.151	0.059	0.027	0.458	1.111	0.064	0.016	0.257	0.088	0.133	0.108
087	1.183	0.045	-0.004	-0.089	1.142	0.059	-0.020	-0.335	1.102	0.063	0.002	0.029	0.068	0.103	0.063
088	1.141	0.046	0.000	0.000	1.104	0.046	0.013	0.283	1.064	0.066	0.006	0.091	0.051	0.097	0.055
089	1.147	0.042	-0.005	-0.119	1.101	0.042	-0.006	-0.149	1.072	0.048	-0.008	-0.176	0.053	0.089	0.045
090	1.156	0.038	0.034	0.895	1.118	0.056	0.002	0.036	1.074	0.043	-0.001	-0.023	0.043	0.086	0.052
091	1.153	0.047	0.028	0.603	1.114	0.043	-0.012	-0.274	1.068	0.062	0.002	0.036	0.052	0.099	0.049
092	1.124	0.038	-0.001	-0.029	1.075	0.040	-0.011	-0.265	1.045	0.037	-0.006	-0.174	0.058	0.100	0.066

See the Untracked Seeing SIS for details and field code.