|
| J-band | |||
| 90161 - North | 90312 - South | Blink Comparison | |
| Release | * | * | * |
| "A" (best sens.) | * | * | * |
| "A" (worst sens.) | * | * | |
| "AAA" | * | * |
| H-band | ||
| 90161 - North | Blink | |
| Release | * | * |
| "A" (best sens.) | * |
| Ks-band | ||
| 90161 - North | Blink | |
| Release | * | * |
| "A" (best sens.) | * |
Observations:
- Completeness fraction is a well defined function of magnitude. Changing observing conditions have only a modest effect. This may be a bit illusory as the best PSP values (best seeing in particular) tend to dominate in the database. Ultimately the variation in completeness with PSP will be used to translate all-sky PSP maps into all-sky completeness maps (see future work below).
- The "catalog" as defined by a source having an "A" in any one
band is complete to a depth approximately 0.5 mag brighter than the
entire release dataset. Completeness in this case is defined by
asking how many times a "fiducial" source which received an "A" also
received an "A" in its other apparitions.
- For the J-band, the entire release is 99% complete to J-mag=16.2. The "A" selection is 99% complete to J=15.7. These numbers are to be compared to the J-band Level 1 spec of 99% in the 1/2 magnitude bin brightward of J=15.8.
J-band H-band Ks-band Level 1 15.8 15.1 14.3 "A" level 15.7 14.8 14.5 Release 16.1 15.5 15.1 - The definition of completeness (in terms of the ph_qual
flag) is muddled. Completeness is well defined -- i.e. the number
of times a source got an "A". What is not well defined is the
bulk completeness of the integrated catalog (i.e. what is the
average completeness in each magnitude bin for all sources which received
an "A").
- If the sample was observed under excellent sensitivity conditions, then the sample turns out to be relatively incomplete, because faint stars that would otherwise get "B"'s get "A's". All of their other apparitions are necessarily "B" which makes them appear incomplete.
- At the other extreme, sources which receive an "A" when conditions are poor necessarily receive an "A" in all of their other apparitions. This is the difference between the "ja" and "jab" plots. They show the same points, but the "fiducial" list of stars is different because one was chosen in poor conditions, the other in good conditions.
- "AAA" sources are very difficult to characterize in terms of completeness vs. mag (see plots labeled "jaaa"). This fuzzyness results from hybridization of the results from various bands. Some sources have their completeness determined by their J-band others by their K-band. The result is a poorly characterized turnover in completeness. Those interested in working with a sample with well-characterized completeness are strongly advised to stay away from the "AAA" sample. Of course this was exactly our motivation in going from a single encompassing ph_qual flag for each source to a band-specific ph_qual flag - an excellent choice in retrospect.
Future work:
- Compare southern and northern fields with similar source density.
- Divide according to PSP ranges to examine how completeness maps to this variable.
Addendum
Tom Chester pointed out that the fainter sources will suffer from flux overestimation as their detection is biased by positive noise fluctuations. Roc Cutri provided Ks-band photometry from a deep image stack of the P161 calibration field compared with the average photometry from the cal scan detections. It shows that
- Down to Ks=15.5 the photometry is largely unbiased.
- The faintest detected sources are actually Ks=16.5 (and one real one at K=17!)
- When compensating for this flux bias, the plots above will look the same down to Ks=15.5 and then the remainder will be expanded into the Ks=15.5-16.5 range.
- Completeness falls off less sharply with magnitude than the plots indicate.
- The conclusions for the 90% completeness levels are unaltered.
- Flux overestimation does not affect conclusions at the magnitude level of the Leve1 specifications and does not affect the completeness curves for "A" sources.
Roc's comments on the plot: The attached plot shows the difference between the aperture mags measured from a "deep" stack of ~1300 northgoing scans and your mean mags for the P161D field. The deep mags are from simple aperture photometry using IRAF. Mags are for 4" radius apertures curve-of-growth corrected to "infinite" aperture. The c.o.g. correction was -0.067 mags, larger than the typical correction from the pipeline.. The corrected-aperture mags were then normalized to the mean cal-scan mags using the trimmed average offset for stars having 8 < Ks < 14, where there is no systematic bias. The points scattered downward off the general curve are due to confusion - I let IRAF recenter the apertures, so for some fainter stars, the apertures were pulled off the nominal star positions towards brighter objects. They can be disregarded. The effects of flux-overestimation in the single scans is clear and dramatic. The result is that your completeness curves will be stretched out somewhat. It is also rather amazing that there are some real detections and measurements of Ks>16 mag objects! Also notice the rising offset for the two stars brighter than Ks=8. Must be some saturated samples that snuck into the stacks.