The footprint generally means “what does the survey footprint look like”, and in particular, tends to refer to what does the WFD or high-numbers-of-visits portion of the footprint look like. There can be variations in the filter distribution, but primarily we’ve investigated what part of the sky the WFD (or which part of the sky tends to get 500-1200 visits) portion of the survey looks like.
The relevant footprint runs can all be pulled from the FBS 1.5 release. The sets of runs with relevant information include the “footprint” series, the “bulge” series (because these include heavy coverage through the galactic plane in various ways), and the “filter_dist” series (because these include variations in filter coverage, but also - importantly - they feature a restricted survey area that includes no visits in the North Ecliptic Spur). The specific runs are:
To visualize the difference in these footprints, here are plots of the distribution of total visits for each of the above runs (using the same scale for each simulation):
Limited WFD footprint types …
- all of the filterdist runs have this same map, which covers a declination region similar to the standard survey WFD, with no variation across the galactic plane, and no NES or SCP coverage. The variations on filterdist have different filter distributions.
Standard survey variations
- footprint_standard_goals is the same as baseline_v1.5, basically. This is the old standard survey footprint. The footprint_bluer_footprint survey uses this same map, however it has a bluer filter distribution (more u/g, less z/y).
- footprint_no_ gp_north is similar to the standard survey, but removes the northern extension of the galactic plane.
- footprint_add_mag_clouds adds an extension to the standard footprint at the Mag Clouds
- footprint_gp_smooth covers the (large) galactic plane to the same limits as the standard WFD. The rest of the footprint is similar to standard.
Extended N/S WFD coverage
- footprint_big_sky_dust uses an extended N/S region for the WFD (going about 10 degrees further north and south), with the galactic plane boundaries delinated by dust extinction. There is extended coverage with fewer visits to the north, but no SCP or galactic plane coverage.
- footprint_bigsky uses a similar extended N/S region for the WFD, with the galactic plane boundaries defined by a galactic latitude (=20deg) only. footprint_bigsky_nouiy is similar, but without u, i or y bands. Like in the footprint_bigsky_dust, there is extended coverage with fewer visits to the north, but no SCP or galactic plane coverage.
- bulges_bs and bulges_cadence_bs use the same footprint map, although with different timing for the galactic plane coverage. Note that the WFD is extended N/S beyond the ‘standard’ footprint, so this is like the ‘big sky’ footprint, but with bulge, northern, and SCP coverage.
- bulges_bulge_wfd and bulges_cadence_bulge_wfd, and bulges_i_heavy and bulges_i_heavy_cadence all use this survey footprint map, although with different timings and filter distributions. Note that it is similar to the big sky basis, but with galactic plane and SCP coverage, and these variations also have a band of WFD-level coverage through the galactic bulge.
- footprint_bigwfd is sort of a hybrid between the big sky and the standard survey; it has an extended WFD region, going further north than in the big sky but not as far south. This footprint includes SCP and GP coverage, with a small extension for the NES.
- footprint_newA includes an extended N/S WFD (big sky style), with galactic plane region defined by galactic latitutde (l=20). The bulge direction is covered to slightly fewer visits than the full WFD; the anti-center is covered to normal WFD depths. The SCP and NES (and extended northern coverage) is included, to a more limited depth. The WFD region only achieves about 771 visits per pointing.
- footprint_newB is similar to footprint_newA, but the galactic anti-center is covered to much fewer visits in an attempt to redistribute these into WFD and add to NES. The WFD region here achieves a median of 842 visits per pointing.
In terms of solar system object discoveries, we might expect the footprint to be a significant contributor. This is true particularly for TNOs, as they move very slowly across the sky. NEOs cover large amounts of the sky, thus are less sensitive to footprints. The MBAs are similar to the NEOs, although more constrained toward the ecliptic plane. The Trojan asteroids are also sensitive to footprint, as they are concentrated into limited sky locations due to their resonant nature.
We can again look at the discovery completeness at bright and 50% values, for various populations, across these footprint-related runs: (sorted by completeness for bright TNOs)
It’s clear that the filter_dist* runs, where the NES was not covered, have a severe impact on TNO discovery (-30%). The other populations are not impacted as strongly, although Trojans suffer an almost 20% drop in some of these filter_dist runs (the difference is due to the exact filter distribution — filter_dist 1 and 4 are ‘uniform filter distribution’ and ‘heavy u’ distribution, respectively … the standard survey puts only about 30% of total visits per pointing into u band). None of these filter_dist runs improve solar system object discovery, even though the WFD region will have more visits within its footprint.
Removing the filter_dist runs and replotting to look at more subtle differences in the other runs:
In these runs, the trend for TNOs follows from the big_sky runs (which did not cover the galactic plane region), then the bluer footprint (which is unfavorable due to filter distribution), then through the relatively ‘standard’ survey footprint (footprint_add_mag_clouds, baseline, footprint_no_gp_north, footprint_standard_goals being very similar), picks up at footprint_bigwfd and then increases further with the bulges_* series, which have a ‘big_sky’ WFD footprint with galactic plane coverage. The overall winner here is the newB footprint, which did slightly better than the newA footprint (most likely due to the slightly larger number of visits per pointing), but the general adoption of a big-sky style footprint that includes galactic plane coverage would be ideal*.
*it’s worth noting that difference imaging for TNO discovery may be difficult near the galactic plane - however, transient and variable studies will also rely on this difference imaging, so it may be reasonable to assume it will work to some level (and thus benefit TNO discovery to some level, even if not as far as implied in these trends).
We should also look at the discovery for other populations - their variations are not as strongly dependent on footprint (only 96-102%, compared to the TNO range of ~92%-107%, after discounting the filter_dist runs which produced a -30% completeness for TNOs). The bright PHA and NEO population completeness is better for the ‘big-sky’ style runs (both big_sky* and the bulges*related variants) … presumably due to sky coverage again. However, the fainter PHA and NEO population completeness falls slightly in these big_sky style surveys, likely due to the fewer number of visits per pointing (as these smaller objects reach bright apparent magnitudes for a shorter amount of time, you have to have more visits to ‘catch’ them).
Checking in with characterization, we find more ties with filter distribution across the footprint than the footprint map itself, although the increase in TNO observations in the big_sky style runs is again visible, with a huge increase with newB (which has additional observations through the NES, in comparison to most of the other big_sky footprints).