Executive summary:
- In regard to narrow lines, early O1 data resembles early ER8 data: a pervasive 16-Hz comb persists throughout the CW search band (below 2000 Hz); there is a notable 1-Hz comb below 100 Hz (0.5-Hz offset); and other sporadic combs persist.
- On the other hand, there are distinct improvements: noise floor is cleaner nearly everywhere and substantially lower in the 10-70 Hz band; non-linear upconversion around quad violin modes and harmonics is much reduced; some combs and isolated lines have disappeared; and the OMC alignment dither frequencies have moved out of the CW search band (hurray!).
- Oh the third hand, new artifacts have appeared or strengthened: a sporadic 8-Hz comb suspected before is confirmed; a 1-Hz comb (no offset) has emerged below 70 Hz; a broad bulge appears in the 1240-1270 Hz band.
Details:
Using 104.5 hours of FScan-generated, Hann-windowed, 30-minute SFTs, I have gone through the first 2000 Hz of the DARM displacement spectrum (CW search band) to identify lines that could contaminate CW searches.
This study is very similar to prior studies of
ER7 data and
ER8 data, but since this is my first O1 report, I will repeat below some earlier findings.
Some sample displacement amplitude spectra are attached directly below, but more extensive sets of spectra are attached in zipped files. One set is for O1 sub-band spectra with labels (see code below), and one set is an overlay of early ER8 spectra (50 hours) and the early O1 spectra. As usual, the spectra look worse than they really are because single-bin lines (0.5 mHz wide) appear disproportionately wide in the graphics
A flat-file line list is attached with the same alphabetic coding as in the figures.
Findings:
- A 16-Hz comb pervades the entire 0-2000 Hz band (and well beyond, based on daily FScans)
- A typically much weaker and sporadic 8-Hz comb (odd harmonics) is also pervasive - previously suspected, now confirmed (all harmonics are labeled in figures, even when not visible)
- A 1-Hz comb with a 0.5-Hz offset is visible from 15.5 Hz to 78.5 Hz (slightly wider span than before)
- A new 1-Hz with zero offset is visible from 20.0 Hz to 68.0 Hz
- A 99.9989-Hz comb is visible to its 8th harmonic (was previously visible to its 13th harmonic)
- The 60-Hz power mains comb is visible to its 5th harmonic (was previously visible to its 9th harmonic)
- There is a sporadic comb-on-comb with 0.088425-Hz fine spacing that appears with limited spans in three places near harmonics of 77, 154 and 231 Hz (ambiguity in precise fundamental frequency)
- There is a 31.4149-Hz comb visible to its 2nd harmonic
- The OMC alignment dithers have been moved to above 2000 Hz (thanks!)
- Upconversion around the quad violin modes and their harmonics is much reduced, although the strengths of the higher harmonics themselves remain high. To be more specific, the fundamental and higher harmonics of the upconversion itself (integer harmonics) due to the fundamental violin harmonics are highly suppressed, while the higher harmonics of the violin modes themselves (not integer harmonics) remain high.
- A number of previously seen combs are no longer apparent: 59.3155 Hz, 59.9392 Hz, 59.9954 Hz and 75.3 Hz
- A variety of single lines have disappeared, and new ones have appeared (see attached line list)
- Compared to early ER8 data, the noise floor is slightly lower in most of the band and significantly lower in the 10-70 Hz band, but there is a significant new bulge in the 1240-1270 Hz band
Line label codes in figures:
b - Bounce mode (quad suspension)
r - Roll mode (quad suspension)
Q - Quad violin mode and harmonics
B - Beam splitter violin mode and harmonics
C - Calibration lines
M - Power mains (60 HZ)
s - 16-Hz comb
e - 8-Hz comb (odd harmonics)
O - 1-Hz comb (0.5-Hz offset)
o - 1-Hz comb (zero offset)
H - 99.9989-Hz comb
J - 31.4149-Hz comb
K - 0.088425-Hz comb
x - single line
Figure 1 - 0-2000 Hz (Early O1 data with line labels)
Figure 2 - 20-100 Hz sub-band (shows complexity of combs below ~70 Hz
Figure 3 - 1300-1400 Hz sub-band (shows how clean the noise floor is away from 8-Hz, 16-Hz lines at high frequencies
Figure 4 - 0-2000 Hz (Early ER8 and O1 data comparison, no labels)
Attachments:
* Zip file with miscellaneous sub-band spectra for early O1 data (with line labels)
* Zip file with sub-band spectra comparing early ER8 and early O1 data
* Flat-file list of lines marked on figures