Top axis options

[hcferguson]

I suggest the following labels:

• Redshift -> Redshifted
• Channel -> Pixel

Also add a new option:

• Restframe -- this would multiply wavelength by 1/(1+z) ....I think this actually will be at least as common as applying a redshift in the forward direction.

[ibusko]

What I see in the top axis when loading any of the test spectra is just the speed of light. I suppose that happens because the spectrum has no associated rest frame velocity or anything that can act as a zero point? In that case, shouldn't the axis labels be hidden? Otherwise they just add to the visual clutter.

[pllim]

Also see #13

[nmearl]

@ibusko The default rest wavelength is 0.0. We can set the default of the axis to be either redshift or channel if velocity isn't defaultly useful.

[ibusko]

What about this: setting the axis default to channel would ensure that at least the quantity is varying along the axis. And channel numbers is a property that every digitized spectrum has by definition. In case the spectrum has an associated reference point, then the axis would display a velocity-like quantity.

How is the redshift / rest frame / reference wavelength property associated with any given spectrum ? Is it input by the user somehow? Does it come with the spectrum already, as in a FITS header keyword or VO SSAP property ?

[hcferguson]

+1 for this suggestion. However, I would call it "pixel" or "index". Channel often means something different (e.g. when there are two arms in a spectrograph).

[nmearl]

@ibusko There is a pop-up dialog where the user can input the reference wavelength or redshift (it's that first gear icon in the plot window).

I cannot speak to whether or not this information can be withdrawn from the source files directly.

@hcferguson Right. Pixel always made sense to me. I had it that way with the original specview, but it was suggested to use channel somewhere down the line.

[nmearl]

See the latest commit 5bcdbdecc3f2bf533c81cb94d50098c434804719.