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OSCIR Data Format |
The raw FITS files from OSCIR are 6-dimensional simple FITS files.
Dim Name Size
1 X dimension of array 128
2 Y dimension of array 128
3 Chop position Number of chop positions (1 or 2)
4 Savesets Number of savesets per nod position
5 Nod position Number of nod positions (1 or 2)
6 Nod sets Number of nod sets
The file format is much different from a visible or near-infrared FITS file, which usually contains a single 2-D image representing a single readout of the detector. Briefly, the OSCIR detector must be read out at several tens or hundreds of Hz in synchronization with the chopping secondary, so frames are co-added into two hardware buffers (one buffer for each chop position). The buffers are periodically (every 2 sec for most data) written to disk, forming a "Saveset". After several savesets are recorded, the data collection stops while the telescope nods, then resumes. A cycle during which the telescope nods to a new position and then back is called a "Nodset".
This pattern is summarized in the following example where the number of savesets per nod position is 3 and the number of nod sets is 4. With two frames per saveset this results in a total of 48 frames; the IRAF task IMHEAD will report the file dimensions as [128,128,2,3,2,4].
Frame ChopPos Source Saveset NodPos Nodset IRAFimage
1 A * 1 1 1 [*,*,1,1,1,1]
2 B [*,*,2,1,1,1]
3 A * 2 [*,*,1,2,1,1]
4 B [*,*,2,2,1,1]
5 A * 3 [*,*,1,3,1,1]
6 B [*,*,2,3,1,1]
---Nod---
7 A 1 2 [*,*,1,1,2,1]
8 B * [*,*,2,1,2,1]
9 A 2 [*,*,1,2,2,1]
10 B * [*,*,2,2,2,1]
11 A 3 [*,*,1,3,2,1]
12 B * [*,*,2,3,2,1]
---Nod---
13 A * 1 1 2 [*,*,1,1,1,2]
14 B [*,*,2,1,1,2]
15 A * 2 [*,*,1,2,1,2]
16 B [*,*,2,2,1,2]
17 A * 3 [*,*,1,3,1,2]
18 B [*,*,2,3,1,2]
---Nod---
19 A 1 2 [*,*,1,1,2,2]
20 B * [*,*,2,1,2,2]
21 A 2 [*,*,1,2,2,2]
22 B * [*,*,2,2,2,2]
23 A 3 [*,*,1,3,2,2]
24 B * [*,*,2,3,2,2]
---Nod---
etc 3
---Nod---
36 A * 1 1 4 [*,*,1,1,1,4]
37 B [*,*,2,1,1,4]
39 A * 2 [*,*,1,2,1,4]
40 B [*,*,2,2,1,4]
41 A * 3 [*,*,1,3,1,4]
42 B [*,*,2,3,1,4]
---Nod---
43 A 1 2 [*,*,1,1,2,4]
44 B * [*,*,2,1,2,4]
45 A 2 [*,*,1,2,2,4]
46 B * [*,*,2,2,2,4]
47 A 3 [*,*,1,3,2,4]
48 B * [*,*,2,3,2,4]
The "IRAFimage" column gives the IRAF notation for accessing the specific frame, e.g. if the example given above is contained in a FITS file "image", the first frame would be accessed as "image[*,*,1,1,1,1]" and the last frame as "image[*,*,2,3,2,4]"
The "Source" column indicates whether the source is in Chop Beam A or B. This changes when the telescope nods. For the first nod position the object is in beam A and the sky is in beam B. For the second nod position the object is in beam B and the sky is in beam A. Thus, for the first nod position the difference beam_A - beam_B will be a sky subtracted image. For the second nod position the difference beam_B - beam_A will be a sky substracted image.
The usual method of reducing the images is to difference the chop pairs (compute [beam_A - beam_B] or [beam_B - beam_A] for each saveset, depending on the nod position), then average these differences to form a final image. The task OREDUCE* does this.
Sky flat fields are usually taken without chopping and nodding. For an image containing 8 savesets the generic IRAF task IMHEAD would report the dimensions [128,128,1,8,1,1], since there is only one chop position, one nod position, and one nod set. The task OFLAT* can be used to derive the flat fields. The task OREDUCE* can be used to apply the flats after deriving the average of the chop and nod differences.
* The Gemini IRAF package contains tasks for processing OSCIR data: OREDUCE and OFLAT
In original form; Inger Jorgensen
Last update February 6, 2001; Phil Puxley