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The following sections describe the sequence of steps required for reducing FIRE data from scratch.
Data Preparation and Launching FIREHOSE
Initial Setup
Locating the Order Boundaries
Generating Flat Fields
Generating the FIRE Structure
Arc Solutions and Object Extraction
Telluric Correction / Flux Calibration
Combining Orders and Exposures to 1D
Special Topics
Accessing 2D sky subtracted/Wavelength calibrated images
Using "Blank Sky" fields for Sky Subtraction on Extended Objects"
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Data Preparation and Launching FIREHOSE
Assuming you have properly installed FIREHOSE and its dependent packages, you are ready to start.
First, you should prepare your data by copying all raw files into a directory named "Raw" (if you prefer you can change this later, but firehose uses this as a default). Create a second directory named "redux" in the same parent directory as Raw and "cd" into the redux directory. From there, start an idl session and type "firehose" to bring up the GUI.
unix> mkdir Raw
unix> cp <path>/fire*.fits Raw
unix> mkdir redux
unix> cd redux
unix> idl
IDL> firehose
Initial Setup
You are now presented with the firehose GUI. It is organized into a series of tabs which guide you through the linear process of reduction steps.
By default, you are started in the "setup" tab, which requires only two simple operations. First, you can choose the directory where firehose looks for the raw data, if you did not use the default value suggested above. Firehose will never overwrite files in this space, so your raw data will remain safe.
Next you can enter your Magellan-formatted observing catalog into the second box. This is used by the software to match exposures of the same object when organizing for the reduction process. It is more efficient than using header information since it groups objects by where the telescope was pointing and matching that to your object list, thereby avoiding operator errors at the telescope. It also checks against the FIRE list of known telluric standards to determine which exposures are science targets versus calibrators. If you choose not to enter the catalog, firehose will attempt to organize based on header information. All settings can be over-ridden manually later in the process.
Locating the Order Boundaries
The first reduction step consists of indexing the echelle order boundaries. This is done using a flat field exposure. Using the browser, select a single internal quartz lamp spectrum from your run and then click "Trace Orders." The software will run for 15-20 seconds and then present an atv window showing the flat field image with order boundaries marked in either black or another color, depending on your display settings.
Browse around the image for a bit to make sure that the fit to the edges makes sense everywhere. About 1 out of 10 times, the fit is bad in the upper left corner of the array. If you find this condition, try another exposure - this usually works. Or, if you select multiple exposures the software will average them together and fit on the combined image.
You will know that the software worked correctly if: the slit boundaries are located correctly on the image, and a file is produced in the subdirectory redux/Flat with the name "Orders_0123.fits" where 0123 is the frame number of whatever file you used to trace the slits.
Generating Flat Fields
The next step is to generate the pixel flat field and slit illumination images. The pixel flat corrects gain variations on the detector, and the illumination function corrects for the fact that the internal quartz lamp has a slight gradient in intensity across the slit (~5%).
Firehose needs 4 pieces of information to perform this operation: (1) The flat field file itself (internal quartz), (2) A sky flat for illumination correction, (3) The file indicating slit positions and order numbers generated in the trace step, and (4) a data frame which helps firehose calculate the "tilt" of the slit on the detector, to facilitate its fitting out the wavelength dependence of the lamp and/or twilight sky.
Clicking in the "Flat" tab will allow you to enter these exposures in for the combine process:
In the "Flat Field Files" area, click the browse button to select the files you'd like to combine for the flat. Typically one should not combine together flats that were taken after moves of the slit wheel. Instead, you should generate a new composite flat for each time you moved the slit, and then the software will apply the appropriate one to your data. You may select multiple files in the dialog box; these will be combined using an avsigclip algorithm when generating the composite.
In the Illum flat files area, enter in the names of the sky flat files you'd like to use for illumination correction.
For the Slit Tilt file, enter in the name of a long science exposure (to use OH lines to fit the slit tilt) or an Arc lamp file. We've found that the OH lines work slightly better if you have a suitably long frame (>300 sec).
Finally, enter in the name of the order mask file (i.e. Orders_*.fits) generated in the previous step.
Click "Make flat field" and the software will run. This process takes some time, roughly 10 minutes per flat field generated. At the end of the process, the illumination flat and pixel flat will be displayed for inspection.
You will know that the software worked correctly if: The pixel flat looks like a flat image centered around 1.00 with fairly little structure, and the illumination image shows a smooth gradient with amplitude of 1.0 +/- ~0.05. Also, two files should be created in the redux/Flat directory. One named Pixflat_0123to0223.fits where the numbers again represent the range of files used. And another named Illumflat_0.60_0122.fits where the 0.60 represents the slit used, and the 0122 is the frame number of the sky flat.
Generating the FIRE Structure
Firehose stores all information about which flat, arc, and telluric files are paired with which science frames in an IDL structure for portability. The next step of the reduction process is to generate this structure, and then edit it to ensure it reflects the pairings you want (e.g. eliminating saturated tellurics, etc.). This procedure is done by a largely automated, and fairly complicated procedure. It performs the following procedures without user intervention:
- Read in the headers for all objects, and record telescope RA, DEC
- Check telescope pointing against the input Magellan object catalog, and assign each science target a unique object ID and common object name
- Cross-check pointing against a list of known A0V telluric calibrators; flag standards accordingly and record B and V mags if available.
- Assign ThAr arc frames to science and telluric exposures by checking for matches in telescope pointing, and asserting that lamps are on and mirrors in.
- Match telluric frames to science targets by searching for the closest match to each science target in UT obs, airmass, and sky angle
- Match flat field frames to each object or standard
These operations are all performed when the "Generate Structure" button is selected. You may select the "verbose" or "Loud" (= very verbose) buttons to produce more output, but these are mostly only useful for debugging purposes.
Once you have created the structure, it is advisable to inspect and (if appropriate) edit it by hand to ensure there are no errors. To do this, click "Edit Structure."
Object Extraction
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Telluric Correction / Flux Calibration
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Combining Orders and Exposures to 1D
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