Aspect Histogram File (ahst3.fits)
The aspect histogram file consists of a single FITS format file for each observation that records the histogram of aspect pointing and roll offsets during the observation. The aspect histogram is used in catalog processing to compute the response matrix averaged over the observation.
FITS HDUs (CXC Data Model blocks):
- ACIS: asphist0, asphist1, asphist2, asphist3, asphist4, asphist5, asphist6, asphist7, asphist8, asphist9
- HRC: ASPHIST
![[NOTE]](../../imgs/note.png)
Aspect Histogram
![[Thumbnail image: HRC-I aspect histogram]](imgs/hrc_6541_field_ahst3.png)
[Version: full-size]
Aspect Histogram
The aspect histogram of an HRC-I observation, which is the binned 3D histogram of the aspect solution of duration versus pointing offset and roll offset.
The aspect histogram FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_ahst3.fits
where 〈i〉 is the instrument designation, 〈s〉 is the data source, 〈obs〉 is the observation identification, 〈obi〉 is the observation interval identification, 〈v〉 is the data product version number, and 〈c〉 is the cycle. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.
![[CAUTION]](../../imgs/caution.png)
CIAO users should be sure to specify a block extension of the ahst3.fits file ("ahst3.fits[asphist{#}]") in the parameters of tools/scripts which accept aspect histogram files (not asol1.fits), e.g. the 'obsfile' parameter of mkinstmap and the 'asphistfile' parameter of mkexpmap.