Forum der Vereinigung der Sternfreunde

Here is an interesting question to start the new year

If a theoretical star with equal flux/Angstrom is measured with a spectrograph with perfectly flat instrument response but a non linear dispersion, the resulting raw profile will be curved instead of a horizontal line. This is because each pixel does not cover an equal number of Angstroms.

When the spectrum is wavelength calibrated and a spectrum produced with equal bin widths using interpolation, the values for each bin need to be corrected for the variation in actual bin widths in the raw spectrum to produce the expected flat spectrum profile. This is not done though in either Visual Spec or ISIS according to my tests. Does anyone know if other programs eg SpcAudace, MIDAS, IRAF etc make this correction?

Note, if an instrument response correction, calculated using the same software, is applied then the effect cancels out but it could give problems if a mix of software is used which behave differently.

Cheers
Robin

In MIDAS you have the choice:

Excerpt form helpfile:

Syntax:
REBIN/LINEAR in out [stepx,stepy] [offx,offy] [stx,sty] [fluxcons]

[..]

fluxcons = YES or NO, only applicable for new larger stepsizes;

if YES the total flux should be conserved, i.e. if n pixels of
value z are rebinned into one, this resulting pixel will have value n*z;

if NO, the resulting pixel will have value z; so with NO you can do
overlapping PLOT and OVERPLOT commands;

defaulted to NO

-- end of excerpt --

Cheers
guenter

_________________
Im längsten Frieden spricht der Mensch nicht so viel Unsinn und Unwahrheit als im kürzesten Kriege. (Jean Paul)

Thanks Günter,

It looks like that command is designed to bin 2D images (x and y coordinates) Is the same command used to resample 1D spectra?

Cheers
Robin

Thinking about it more, for a slit spectrograph the same effect is seen in the flat so if a flat is used the effect cancels out. (If an ideal flat lamp with a level spectrum is used, the flat corrected raw spectrum of my theoretical test star will be flat even though the bin sizes are not all the same width.) In this case it makes sense just to interpolate the data with no correction for width to generate the same number of equal width bins.

Cheers
Robin