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The PS1 stack images, described in detail in Waters et al.,=
are the 'optimal' combination of multiple warps on the same skycell. For t=
he 3pi survey there are in general 12 warps per filter, but for the Medium =
Deeps it can be several hundred. Stacks are also astrometrically and photom=
etrically calibrated. These images are part of DR1 and can be accessed and =
downloaded through the PS1 archive image cutout interface.
The following information is taken from Waters et al.= , which should be cited appropriately.
The skycell images generated by the Warp process are added togethe= r to make deeper, higher signal-to-noise images in the Stack stage. The sta= cks also fill in coverage gaps between different exposures, resulting in an= image of the sky with more uniform coverage than a single exposure. The st= acks are on the same PS1 Sky tessellation patterns as the PS1 Warp images. See Waters= et al. for details on the stack combination algorithm.
In the IPP processing, stacks may be made with various options for=
the input images. During nightly science processing, the 8 exposures per f=
ilter for each Medium Deep field are combined into a set of stacks for that=
field. These so-called =E2=80=98nightly stacks=E2=80=99 are used by the tr=
ansient survey projects to detect the faint supernovae, among other transie=
nt events. For the PV3 3=CF=80 analysis, all filt=
er images from the 3=CF=80 survey observation wer=
e stacked together to generate a single set of images with =E2=
=88=BC10 =E2=88=92 20 t=
imes the exposure of the individual survey exposures. The sign=
al, variance, and mask images resulting from these deep stacks are part of =
the DR1 release and are available through the PS1 archive image cutout interface. Stack im=
ages have filenames that include '.unconv' because the are constructed by c=
ombining warp images having observed (variable) PSFs. The PS1 pipeline also=
generated stacked images with seeing convolved to a fixed value before sta=
cking. The convolved image products are not included in the public archive =
because it was found that the unconvolved images are always preferred for s=
cientific data analysis.
As well as the standard masks and weights, stacks come with three other = auxiliary image files:
You might want to check out the page on image format quirks befor= e downloading any data.
The exposure time for a stack is given in the FITS header (keyword EXPTI= ME). It consists of the unweighted sum of the individual warp exposures whi= ch were passed to the stacking process (these are visible as keywords EXP_n= nnn). Note that during the stacking process some of these warps can be excl= uded. This is not reflected in the exposure time, so the actual on-sky time= for each stack is not necessarily the same as the exposure time given in t= he header.
The actual exposure time per pixel is available as the auxiliary 'exp' i= mage for each stack.
At the time of stacking, the pixel values in stacks are rescaled to a ze= ro-point of 25 + 2.5* log10(exposure time), based on the input warp calibra= tion (see keyword HIERARCH FPA.ZP in the FITS header - the individual warp = zero-points are also in the header, ZPT_nnnn, as are the relative scaling f= actors applied to each, SCL_nnnn). However, as the final ubercalibration of= the data has not taken place at this time, this zero-point may be slightly= incorrect (usually at the hundredths of a magnitude level). In contrast, t= he stack fluxes/magnitudes in the PSPS catalog database have undergone the = final calibration process and are more accurate.
So to measure your own magnitudes off a stack image, you need to apply t= he following formula:
MAG=3D-2.5*log10(=E2=88=91data-units)+25+2.5*log10(EXPTIME)
Note that due to the complicated nature of the stacking process, the dat= a-units in a stack are not easily related to actual photons from the target= .
Due to the gaps between the OTAs , and general cosmetic masking, the num= ber of input warps per pixel with valid data on a stack varies. This variat= ion can be quite large in the 3=CF=80 images (it is not so much of an issue= on the Medium Deeps). As a result the true PSF at a given location on a st= ack can vary discontinuously between pixels (if the input warps have differ= ent FWHM). This acts as an increased source of error in the stack PSF photo= metry, as the PSF model fitted to the stack is continuous. Better results m= ay therefore be obtained by using the Forced Mean Warp data (as the true PS= F is expected to be continuous over a warp).
The actual number of warps used in each pixel is available as the auxili= ary 'num' image for each stack.
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Typical coverage (number of warps contributing t= o a pixel) in a g-band 3pi projection cell (4 degrees on a side). Black is = zero, white is 17 (note this image is produced from a summary stack, so the= pixels here are 4" across and do not show the full PS1 resolution). |
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For the processing of the Medium Deep fields, stacks have been gen= erated for the nightly groups and for the full depth using all exposures (d= eep stacks). In addition, a =E2=80=99best seeing=E2=80=99 set of stack have= been produced. We have also generated out-of-season stacks fo= r the Medium Deep fields, in which all image not from a particular observin= g season for a field are combined into a stack. These later stacks are usef= ul as deep templates when studying long-term transient events in the Medium= Deep fields as they are not (or less) contaminated by the flux of the tran= sients from a given season. The Medium Deep fields will be made public in D= R3.