1.4 Data from the MAST Archive
All flt.fits or flc.fits images downloaded from the MAST archive contain many extensions with useful data. The formatting of a single WFC3/UVIS image is shown here as an example.
--> from astropy.io import fits
--> fits.info('icb701osq_flc.fits')
| Filename: icb701osq_flc.fits | ||||||
|---|---|---|---|---|---|---|
| No. | Name | Ver | Type | Cards | Dimensions | Format |
0 | Primary | 1 | PrimaryHDU | 305 | () | |
| 1 | SCI | 1 | ImageHDU | 224 | (4096, 2051) | float32 |
| 2 | ERR | 1 | ImageHDU | 51 | (4096, 2051) | float32 |
| 3 | DQ | 1 | ImageHDU | 43 | (4096, 2051) | int16 |
| 4 | SCI | 2 | ImageHDU | 220 | (4096, 2051) | float32 |
| 5 | ERR | 2 | ImageHDU | 51 | (4096, 2051) | float32 |
| 6 | DQ | 2 | ImageHDU | 43 | (4096, 2051) | int16 |
| 7 | HDRLET | 1 | NonstandardExtHDU | 18 | (8640,) | |
| 8 | HDRLET | 2 | NonstandardExtHDU | 26 | (112320,) | |
| 9 | HDRLET | 3 | NonstandardExtHDU | 26 | (112320,) | |
| 10 | HDRLET | 4 | NonstandardExtHDU | 26 | (112320,) | |
| 11 | HDRLET | 5 | NonstandardExtHDU | 26 | (112320,) | |
| 12 | WCSCORR | 1 | BinTableHDU | 59 | 14R x 24C | [40A, I, A, 24A, 24A, 24A, 24A, D, D, D, D, D, D, D, D, 24A, 24A, D, D, D, D, J, 40A, 128A] |
| 13 | WCSDVARR | 1 | ImageHDU | 15 | (64, 32) | float32 |
| 14 | WCSDVARR | 2 | ImageHDU | 15 | (64, 32) | float32 |
| 15 | D2IMARR | 1 | ImageHDU | 15 | (64, 32) | float32 |
| 16 | D2IMARR | 2 | ImageHDU | 15 | (64, 32) | float32 |
| 17 | WCSDVARR | 3 | ImageHDU | 15 | (64, 32) | float32 |
| 18 | WCSDVARR | 4 | ImageHDU | 15 | (64, 32) | float32 |
| 19 | D2IMARR | 3 | ImageHDU | 15 | (64, 32) | float32 |
| 20 | D2IMARR | 4 | ImageHDU | 15 | (64, 32) | float32 |
| 21 | HDRLET | 6 | NonstandardExtHDU | 26 | (112320,) | |
| 22 | HDRLET | 7 | NonstandardExtHDU | 26 | (112320,) | |
| 23 | HDRLET | 8 | NonstandardExtHDU | 26 | (112320,) | |
For ACS/WFC and WFC3/UVIS, the first seven extensions ([0] thru [6]) will always remain in this order:
- The primary header (group [0]).
- The chip 2 image, error, and data quality headers ([1], [2], and [3], respectively).
- The chip 1 image, error, and data quality headers ([4], [5], and [6], respectively).
Extensions [7] through [23] contain the distortion correction information beyond that captured in the world coordinate system (WCS) and the headerlets noted in Section 1.3.3. The number and order of these extensions are subject to change depending on the amount and placement of the headerlets. While the extensions of other images might look slightly different, they will be made up of these four types:
- The D2IMARR extension in [15], [16], [19], and [20] are a two-dimensional vector containing corrections for physical distortions in the detectors from pixel grid irregularities due to the manufacturing process.
- The WCSDVARR extension in [13], [14], [17], and [18] describe distortion corrections not modeled by the polynomial fit which are mostly due to hard-to-model local distortions introduced by filters. For ACS/WFC and WFC3/UVIS these are four 64 × 32 images.
- The WCSCORR extension is a binary table that records the history of changes to the WCS in the header by Tweakreg, where each line records a single update to a single chip.
- Each HDRLET extension contains independent WCS solutions available for the data.
Other instruments, such as WFPC2 and WFC3/IR, may have different numbers of extensions for the data, error, and DQ arrays depending on the number of detectors for that instrument.
Please see Chapter 4 for more details. Most importantly, the image data in the flt.fits or flc.fits from MAST is not modified irregardless of any changes to the formatting of the extensions, although the improvements to the WCS can cause differences in the drizzle products.