2.4. Photometry

PACS is a dual-band photometer, with a shorter wavelength camera [B] and a longer wavelength camera [R]. The blue camera was equipped with two filters, to allow imaging at 70 microns (blue) or at 100 microns (green), while for the long wavelength camera, a single filter provided observations at 160 microns. In the naming convention of photometric products, the [B] label always refers to the short wavelength camera whether the blue or green filter was used. Which filter for the B camera was used can be checked in the Meta data of the observation, "blue" (value "blue1" is the blue filter, and "blue2" is the green filter) and in the naming convention of the FITS files in the tarball for Level2.5 maps and beyond.

The Level0/0_5/1 contain products of class Frames at various stages of processing and calibration, with Level 1being fully calibration for astrometry and flux. Maps are found at the subsequent levels.

As in the case of the spectroscopy, if you hover with your cursor over these various layers, called HPPXXX (HPP=Herschel PACS Photometry), a banner comes up telling you what class the products are: for example, the level 0.5 HPPAVGR|B are SlicedFrames: this being a type of ListContext, which is a special container (list) of other products (in this case, a list of Frames). There are several products held in the other levels (calibration, auxiliary, satellite, housekeeping, pointing...), which generally have one or more datasets which contain the actual data.

The astronomer will be interested in the following contexts: HPPPMAP[B|R], HPPHPFMAP[B|R], JHPPSMAP[B|R], HPPUNIMAP[B|R]. These hold the final projected maps, which are SimpleImages and may be held in a MapContext, in all cases always for the blue and the red (B|R) camera. The MapContext/SimpleImages are Level 2 and 2.5 products, and at Level 2.5 you also find combined maps of the same class. These are shown in the screenshot below: the other layers contain engineering (HPSENG), satellite housekeeping (HPSGENK and HPPHK), and DecMec (HPPDMC[R|B]) data, which are used in some of the masking pipeline tasks. See also the PDD for definitions of the various HPPXXX slices.

By looking into the level0 HPPAVGB (click on the + next to it) you can see the layers inside it. At Level 0there will always only be one layer (numbered 0), which is a single Frames. A Frames holds various other layers containing your astronomical and associated data. You can see some of these in the screenshot: the Status table, Mask, the BlockTable, and the Signal:

The layers in the level0 and 0.5 product of an ObservationContext (SPG 13)

Figure 2.10. The layers in the level0 and 0.5 product of an ObservationContext (SPG 13)


At Level 1, shown in the figure below, you see the same data structure as in Level 0.5,

The layers in the Level 1 product of an ObservationContext (SPG 13)

Figure 2.11. The layers in the Level 1 product of an ObservationContext (SPG 13)


Finally there is the Level 2, Level 2.5 and sometimes aLevel 3, inside which you will find maps. In Level 2 you find can find the map belonging that that particular obsid (i.e. the scan or cross-scan of the pair): HPPMAPR|B. These maps are high-pass filtered.

The layers in the Level 2 product of an ObservationContext (SPG 13)

Figure 2.12. The layers in the Level 2 product of an ObservationContext (SPG 13)


In Level 2.5 there are several maps which are the combination of the scan—cross-scan pair (the most common observing mode of the PACS photometer, and in which the scan and cross-scan were two separate observations). The difference between the various combined maps provided is the mapping technique used to create them. The HPPHPFMAP[R|B] context contain maps which were all created using High Pass Filtering and PhotProject tasks (they are the mosaic of the Level 2 maps of the scan and cross scan obsids); HPPJSMAP[R|B] maps were created using JScanam (the scan and cross-scan obsids are used together to create directly a Level 2.5); and the HPPUNIMAP[R|B] maps were created by Unimap (the scan and cross-scan obsids are used together to create directly a Level 2.5). See the PDRG for a description of the different mappers.

The layers in the Level 2.5 product of an ObservationContext (SPG 13)

Figure 2.13. The layers in the Level 2.5 product of an ObservationContext (SPG 13)


Some observations also have a Level 3. The maps here are each the combination of all observations taken with the PACS photometer during the Herschel mission of the same object (the same coordinates) from within the same observing programme. Note that since photometry observations always produce a red filter image and either a blue or a green filter image, for observations where not the same blue filter was used, only a red combined Level 3 image will be provided. The structure is similar to that of Level 2.5 except that there is no HPF map.

The layers in the Level 3 product of an ObservationContext (SPG 13)

Figure 2.14. The layers in the Level 3 product of an ObservationContext (SPG 13)


See Chapter 3 for details of the datasets contained in the various maps.

Table 2.9. The contents of the Levels. [R|B] indicates the red or blue camera

Level Product Description
0 History A history of tasks that were run on this object; the most useful histories are held within the Level X HPSFXIT or HPS3DXX layers themselves
0 HPENG Herschel PACS engineering information table
0 HPGENHK Herschel PACS general housekeeping information table
0, 0.5 HPPDMC[R|B] Herschel PACS Photometry DecMec information table; used in the pipeline
0, 0.5, 1 HPPAVG[R|B] Frames or SlicedFrames, holding individual Frames, red and blue
0 HPPHK Herschel PACS Photometry housekeeping information table
2 HPPPMAP[R|B] SimpleImage, holding the final map of the observation projected using PhotProject
2.5 HPPJSMAP[R|B] SimpleImage of the final combined map of the scan and cross-scan pair created by JScanam
2.5 HPPHPFMAP[R|B] SimpleImage of the final combined map of the scan and cross-scan pair created by highpass filtering and photProject
2.5 HPPUNIMAP[R|B] SimpleImage of the final combined map of the scan and cross-scan pair created by UNIMAP
3 HPPUNIMAP[R|B] SimpleImage of the final combined map of all the observation taken on the same object created by UNIMAP
3 HPPJSMAP[R|B] SimpleImage of the final combined map of all the observation taken on the same object created by JScanam


2.4.1. Photometry FITS files on disc

In the screengrab below you can see the organisation on disc of an unpacked tarball of a photometry gotten from the HSA. The layout is similar to that when viewing an observation in HIPE: the top level is the obsid; then you have the various contexts containing the browseProduct, level0..3, quality, etc; within the various "level#" are the product contexts (HPPXXX); and from within those directories are the FITS files of the maps. Each map has a red camera and a blue camera version, distinguished by a B or R in the filename. For the blue camera maps, in addition a filter ("green" or "blue") is in the filename to indicate which of the two blue filters was used. In Table 2.10 we link the levels (2, 2.5, and 3) and product context names as seen in HIPE to the directories and filenames as seen on disc, so you can locate the maps discussed in this manual on disc as well as in HIPE. (All FITS files have a slice index in their name, but this will always be "00" since there is no slicing in the photometry pipeline as there is for spectroscopy.) The standalone browse products (SBPs) are also given in Table 2.10. Not all observations will have a Level 3, since these maps are mosaics from observations of the same programme that cover the same part of the sky. A lack of Level 2.5 means that the observation has only a scan map, not also a cross-scan map.

Screengrabs of parts of a photometry ObservationContext stored on disc, having been downloaded from the HSA as a tarball
Screengrabs of parts of a photometry ObservationContext stored on disc, having been downloaded from the HSA as a tarball

Figure 2.15. Screengrabs of parts of a photometry ObservationContext stored on disc, having been downloaded from the HSA as a tarball


PACS photometry FITS file names include the obsid, the product class name, the filter name ("blue" or "green") for blue camera products, and the pipeline level it comes from. For products of Level 2.5 and 3 the RA and Dec replace the obsid in the filenames.

Table 2.10. The PACS photometry products as found in an unpacked tarball gotten from the HSA. [B|R] and [b|r] indicate a choice between the red and blue camera product/file. The "Directory" is the one in which the FITS file can be found, below the directory that is the obsid (1342xxx).

Originating Level; Product context Directory FITS filename Description
2.5 or 2; HPPJSMAP[B|R] browseProduct/[blue|red]_JScanam_map hpacs_[25|20]HPPJSMAP[B|R]_[filter]_(RA)_[p|m](Dec)_##_v1.0.fits The Jscanam maps taken from Level 2.5 where present, otherwise Level 2
2; HPPMAP[B|R] level2/HPPMAP[B|R] hpacs[obsid]_20hpppmap[b|r]_##_[long number].fits The maps created by PhotProject on highpass filtered frames
2.5; HPPHPFMAP[B|R] level2_5/HPPHPFMAP[B|R] hpacs_25HPPHPFMAP[B|R]_[filter]_(RA)_[p|m](Dec)_##_v1.0_[long number].fits The maps created by mosaicking the scan and cross-scan Level2 highpass filtered maps
2.5; HPPJSMAP[B|R] level2_5/HPPJSMAP[B|R] hpacs_25HPPJSMAP[B|R]_[filter]_(RA)_[p|m](Dec)_##_v1.0_[long number].fits The maps created by combining the scan and cross-scan observations with the JScanam mapper
2.5; HPPUNIMAP[B|R] level2_5/HPPUNIMAP[B|R] hpacs_25HPPPUNIMAP[B|R]_[filter]_(RA)_[p|m](Dec)_##_v1.0_[long number].fits The maps created by combining the scan and cross-scan observations with the Unimap mapper
3; HPPJSMAP[B|R] level3/HPPJSMAP[B|R] hpacs_30HPPJSMAP[B|R]_[filter]_(RA)_[p|m](Dec)_##_v1.0_[long number].fits The maps created by mosaicking the JScanam Level2.5 maps belonging to the same object and the same proposal
3; HPPUNIMAP[B|R] level3/HPPUNIMAP[B|R] hpacs_30HPPUNIMAP[B|R]_[filter]_(RA)_[p|m](Dec)_##_v1.0_[long number].fits The maps created by mosaicking the Unimap Level2.5 maps belonging to the same object and the same proposal