Scrutable

dyqik wrote: ↑

Thu Sep 17, 2020 12:54 pm

basementer wrote: ↑

Wed Sep 16, 2020 6:54 pm

dyqik wrote: ↑

Tue Sep 15, 2020 10:06 pm

Some of my colleagues are not entirely convinced that the claimed detection in the paper is actually a spectral line. And they are among the world's experts on detecting molecules in planetary atmospheres with the telescopes in question, and one is acknowledged in this paper.

I suggest you look at fig. 3 first, then fig. 1 and fig. 2 in the paper.

Additionally, pulling the line out of the background involves fitting a 12th order polynomial to the spectra, which is a potentially difficult thing to get right and not contaminate the data with.

Could you clarify, dyqik, is it that a 12th order polynomial is usually used in this sort of analysis, although it's known to be tricky? Or that this is a novel technique that immediately looks risky?

No, you don't usually do it like that. I'm on vacation, so haven't spoken to colleagues in detail about this.

The main limitation at a small line-to-continuum ratio (hereafter,
l:c ratio) was spectral ‘ripple’, from artefacts such as signal
reflections. We identified three issues (see ‘JCMT data reduction’
in Methods), with the most problematic being high-frequency
ripple drifting within observations in a manner hard to remove
even in Fourier space (Extended Data Fig. 1). We thus followed
an approach standardized over several decades, fitting
amplitude-versus-wavelength polynomials to the ripples (in
140 spectra). The passband was truncated to 100 km s−1 to avoid
using high polynomial orders. (Order is based on the number N
of ‘bumps’ in the ripple pattern; fitting is optimal with order N + 1
and negligibly improved at increased order. A wider band includes
more ‘bumps’, increasing N. For minimum freedom, a linear fit can
be employed immediately around the line candidate, ignoring the
remaining passband—see Table 1 for resulting systematic differences.)
We explored a range of solutions with the spectra flattened
outside a velocity interval within which absorption is allowed. (The
polynomial must be interpolated across an interval, as if fitted to the
complete band it will always remove a line candidate, given freedom
to increase order.) These interpolation intervals ranged from very
narrow, preserving only the line core (predicted by our radiative
transfer models, Fig. 1), up to a Fourier-defined limit above which
negative-sign artefacts can mimic an absorption line. Details are in
Methods, with the reduction script in Supplementary Software 1.
The spectra were also reduced completely independently by a second
team member, via a minimal-processing method that collapses
the data stack down the time axis and fits a one-step, low-order
polynomial; this gave a similar output spectrum but with a lower
signal-to-noise ratio

shpalman wrote: ↑

Wed Sep 16, 2020 8:03 pm

I know the polynomials form a complete set but I struggle to think of any physical reason for using such a high order one here.

Grumble wrote: ↑

Sat Sep 19, 2020 10:04 pm

Is there life on Venus?
It doesn’t seem too likely.
Does it have a penis?
It must be rather spiky.

dyqik wrote: ↑

Wed Oct 28, 2020 1:15 am

There's no phosphine detected on Venus - paper including some of my colleagues among the authors.