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In this case I have zoomed in to the line of interest, an H alpha profile of a z = 1.7 galaxy. Now, rather than defining the center of the aperture in spatial coordinates, you must define the bounds of the wavelength region of interest where you want to fit the profile. Since Since wavelengths run vertically in FIREHOSE frames, this means you need to set the bottom and top of the bounding region. . You do this by hovering the mouse over the desired bounds and pressing capital "B" and "T". The display will show black lines indicating where it interpreted your top and bottom bounds to be drawn.
Once you have set the top and bottom bounds, the software will sum up the flux from pixels in that wavelength region and present the normalized pixel profile in a new window, where you can interactively fit a parametric model to the pixel values:
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There is a small help menu indicating the range of commands one may use to control the fit. Common options are to delete/undelete bad points (right mouse button) force a point to be added in a desired location (left mouse button), increase the fit order ('u') or decrease the fit order ('d'). You can also select a particular region for fitting using the combination "s-s" on the left and right bounds of the region of interest. When you are satisfied with the fit, press 'q' or click "done" and the software will return you to the image GUI. For reasons I don't understand, the colormap gets messed up when you do this, but do not worry, this is normal.
Press "q" again in the image GUI to move on to extraction. The software will then create a 2D profile image based on the fit you just completed. It will present you with another view of the 2D crude-sky-subtraction, this time showing the profile peak position, and the +/- 1 * FWHM boundary.
If your object is close to the edge of the slit (as in the example shown), one of the bounds sometimes lies outside of the order: do not worry. This aperture is only used to run a quick boxcar prior to optimal extraction, for determining in what sequence the individual orders will be extracted. The optimal extraction will not use any "aperture" per se, it only uses the profile weighting function that you have specified, and does not extend outside of the slit. Generally, if the center trace lands on the middle of your object, you are good to go.
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