Response Curve and Vertical Spectrum Location

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Andy Wilson

Hi Tony,

When creating the response curve in BASS do not tick “Free Draw” or “Linearise”. You should find that neither is needed unless you have a particular problem you need to overcome. The “Free Draw” means you can click anywhere, which is usually a bad thing as it means you can click outside the response curve. Leaving it unticked forces each point you click to sit on the curve. Of course the proof of whether it works is always to see if your standard star spectrum is adjusted to be a good match to the Miles or other reference spectrum.

Where possible, I try to make all the spectra I take fall on roughly the same vertical position on the CCD chip. This is a tip I picked up from Olivier Thizy at one of the workshops. It is not essential, but he recommended it as a technique to get the maximum accuracy from spectra. While, flat fields, rotation, tilt, wavelength calibration should all work to allow you to place the spectrum anywhere on the chip, by placing the spectrum in the same physical location on the chip it means these corrections need to do ‘less work’. That is always a good thing as it means the corrections tweak the result rather than have to be relied on for large corrections. In reality my spectra are never exactly on top of the same chip position, but I try to get within a few pixels.

It might be an idea to send the workflow to John for comment before publishing. With my tutorial I posted it to the BASS Yahoo Group, and I got useful feedback from the user community enabling me to improve the tutorial. My preference would be not to have the “Lamp”, “Standard” and “Target” as 3 separate top level streams. Instead using the top level sections of the tutorial as the top level streams. That way it enforces the same geometric corrections on each image or master image.