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Hi Jonah

Iām a GravitySpy glitch classifier that would like to make some Q-scans of glitches. I believe Omicron does this 24/7 but I would just like to find out how itās done.

I did the GWOSC #5 course (up to q-scans) but it doesnt go into the algorithm. The python code tells us to specify a āQ-rangeā in the Q-scan, 4 - 12 works wonders , but it was confusing that āconstant Qā has a range.

Various sources say Q is frequency divided by its bandwidth. i.e a sampling rate/ window size.

Following your suggestion to someone else on this forum I found the formula for Q in prof Keith Chatterjiās PhD paper.

There are two parameters ( on page 82 equation 3.25 Q(min) and N(Q) equation 3.24c )

Are these the two parameters that go in Q-range in the Q-transform algorithm ?

kindest regards
David

Hi @David . Nice to meet you! Thank you for your work with GravitySpy, and for your question.

Iāll give you some resources, and hope that some of these help. Please let me know if something more would help.

• The equations you point to in Chatterjiās thesis are one method for selecting the lower and upper values of the Q-range. In the `gwpy` method, instead, the user selects the lower and upper range of Q-values.

• Typically, we would select a range of Q-values that captures as much GW signal power as possible in a small number of pixels.

• For a nice picture of what Q means, see Figure 3.1 in the same thesis. The figure shows a sine-Gaussian with a Q of 10, which corresponds roughly to the number of cycles you can see (I count about 7). If we doubled the Q to 20, weād see around 14 cycles instead.

• I put some more notes about how to select a good Q-range in this introductory tutorial. Additional notes can be found by clicking āSee Notesā under the Q-scan plot in the Quickview Web App.

Good luck!

Hi Prof Jonah.
Thanks for your time and help.

May I just check my own understanding:

In fig 3.1 the central frequency is 1 Hz. So tuning Q from 10 to 20 is basically asking the sineGaussian to, do whatever it does, to resolve frequencies down to 1/20 Hz i.e 2 times better?

And also the waveform fits about 2 times more cycles in its packet.

While still constrained to produce a tiling with poorer time resolution.

Anyway I appreciate you replying and I will look at that tutorial.
regards
David

Cool streamit app thankyou.

Oh I see
-5/3 Kolmogorov spectrum
seems very universal and beautiful

Hi @David Yes - thatās right. Adjusting `Q` is a trade-off between better resolution in time and better resolution in frequency.

Typically, high mass signals (around 50 solar masses) are in the LIGO band for a short time, so high time resolution (low Q) is important. On the other hand, lower mass signals (say, a binary neutron star merger) are in the LIGO band for longer, so using a higher Q can be a better choice.

Hi
thank-you and good hunting in 2024!