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Setting a specified transition wavelength in coadd.abut() #3

@nkerman

Description

@nkerman

Idea:

Users would benefit from being able to choose the point at which two abutted spectra transition from the lower-λ (A) to longer-λ spectrum (B)

Problem:

At present, the transition wavelength is always decided by the function, find_transition_wavelength, which chooses the center of the overlap.

This is less than ideal, as often you would get a better abutted spectrum by using A for the vast majority of the overlap.

Potential Fix:

This does not have to be automatic within the wrapper (that would be great, but there's a lot of variables and data-dependent decisions). Instead, the user should be able to do this piecewise, i.e.

imports...
initialize SegmentList, make WL array, coadd (or use convenience function to do these steps)
abut1 = abut(blue, middle, transition_wl=1400)
abut2 = abut(abut1, red, transition_wl=2000)

Where the abut function checks for a user-specified transition wavelength, before using find_transition_wavelength, i.e.

def abut(product_short, product_long,    transition_wvln = None):
    # check for user trans_wvln:
    if transition_wvln:
        # check it's in the right region:
        if (user_trans_wvln>product_long.min) & user_trans_wvln<product_short.max):
            transition_wvln =  user_trans_wvln
        else: # user specified a wvln outside of the overlap
            print(‘invalid wvln specified’)
            exit
    else: # user didn't try to specify a transition wvln
        transition_wvln = get_transition_wvln()

@jotaylor (who helped draft this idea) @stscirij

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