Detrending

[BPVietje]

Hi All,

I'm new to exoplanet observations, but have analyzed 5 transits to date in AIJ.  I would like to learn more about the detrending process, and what is actually happening when I select airmass or another parameter.  Is there a relevant article or book chapter that describes the process -- maybe "Detrending for Idiots"?

I've read the threads in which the number and appropriate use of detrend parameters were discussed, but I have yet to find information on how the process works, statistically.

Thank you kindly,

Brad Vietje
Newbury, VT
www.nkaf.org

[karenacollins]

Hi Brad,

Detrending is explained mathematically in Section 4.4 and equation 5 of the AIJ paper available here on arXiv:

http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2017arXiv170104817C&link_type=PREPRINT&db_key=PRE

At a high level, first the best linear fit of a "trend" data set (such as airmass) to the light curve is found. That
results in a coefficient that is used to produce a scaled version of the trend dataset that is then subtracted from (or
divided into - if that option is selected) the light curve to remove the trend from the light curve dataset. The region
of the light curve used to find the best fit coefficient is defined by the "fit mode" selected on the "Multi-plot
Y-data" panel and the "left" and "right" markers on the "Multi-plot Main" panel. If the exoplanet transit light curve
fitting mode is selected, the full light curve is used, but the linear fit of the trend data set(s) is found
simultaneously with the best fitting transit model.

Karen

[clkotnik]

I note that trend coefficient is shown in the Multi-plot Y-data panel for the active trend parameter.  The spinner control seems to indicate the coefficient could be changed to an alternate value, but I don't find that possible.  Is there a mode that allows for the user to alter the coefficient?

thanks
Cliff

[karenacollins]

Hi Cliff,
The intended mode is the "user" mode:


However, I see that there is a bug in the current version that is preventing this mode from working properly.

There is a workaround, that is probably easier anyway, since it gives you direct access to multiple parameters. Instead of selecting "user" mode, select the light curve fitting mode ().

On the light curve fitting panel that opens, deselect the Transit fit:


Then in the Detrend Parameters sub-panel, select a detrend parameter of interest (AIRMASS in the example below), select "Lock", and then enter a "Prior Center" value to use for manual detrending coefficient (and change it up or down as needed). You should see your light curve being affected by the detrend parameter when you change the "Prior Center" (coefficient) value.


The "Fit Statistics" boxes will have a red box around them, which normally means the fit failed to converge. That is okay in the this case since there is actually no fitting being done.


Karen

On 3/28/2018 4:46 PM, clkotnik [via AstroImageJ] wrote:

I note that trend coefficient is shown in the Multi-plot Y-data panel for the active trend parameter.  The spinner control seems to indicate the coefficient could be changed to an alternate value, but I don't find that possible.  Is there a mode that allows for the user to alter the coefficient?

thanks
Cliff

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