1. Phil Appleton based on material from a study by Roland Vavrek (ESAC)
Range Spectroscopy
A review of chop-nod and Unchopped
results
Phil Appleton based on material from a study by Roland Vavrek (ESAC)

2. Apparent “Fringing” but in reality “aliasing”
(beating) between large grating steps and 16 “spectral” pixels
High Frequency
“Noise” due to
aliasing

3. The problem can be solved
if the response between adjacent
spectral pixels can be normalized
(a kind of second-order rsrf).
Fit a smooth function (spline or
something like it) to each spectral
pixel along time dimension (==
scan up and down) and use this to
adjust the response from one
spectral pixel to the next. This removes
the periodic jumps. It is a kind of low-
pass filter applied to each spectral
pixel. The 16 curves are then
normalized to the mean of the
means.

4. NEPTUNE B2A Filter (“Green” 2nd order)
Before correction After correction

5. NEPTUNE B2A Filter (“Green” 2nd order)
Zoomed in shows even better behavior in corrected spectrum
Before After
Spectrum totally dominated
by the aliasing effect
Huge increase in S/N
ratio

6. RANGE CHOP NOD SPECTRUM
Zoom even further
shows that some residuals still remain
but significant improvement

7. This correction is now incorporated into
the standard chop-nod pipeline

8. CHOPPED (RED) VERSUS UNCHOPPED (BLACK) FOR SAME SPECTRAL RANGE AND WITH CORRECTION APPLIED
Note excellent baselines for Unchopped but 20-30% variations in continuum
Unchopped
mode is not
a good one
if you want
to reproduce
the continuum
accurately.
Other issues
remain (see
talk tomorrow
on large-range
scans.

9. Conclusions
Corrections to “second-order” rsrf reduces fringing effects significantly
Unchopped mode seems to produce very stable baselines but although line fluxes are reliable, the continuum is very unrelieable. However, as we shall see tomorrow, the chop-nod mode also has
large-scale rsrf-type ripples that lead to problems.
-- One solution will be discussed tomorrow—using
the bright telescope emission as a spectral reference