Re: Detrending Airmass
Posted by karenacollins on Aug 07, 2020; 7:33am
URL: http://astroimagej.170.s1.nabble.com/Detrending-Airmass-tp1437p1438.html
Hi Jessica,
AIJ's detrending works by trying to flatten a light curve by fitting any long term trend to the airmass curve. For an exoplanet transit, the out-of-transit segment, or a full transit model plus out-of-transit of the target star light curve can be used by selecting the corresponding "Fit Mode" in Mult-plot Y-data.
Detrending a comp star light curve does not affect the target star light curve in rel_flux_T1 because the differential photometry is calculated before detrending would be applied to a comp star. Even if the comp star light curve were detrended before the differential computation, it wouldn't necessarily flatten the target star light curve since it's photometry also suffers from differential airmass losses.
This indeed causes a problem for delta Scuti stars since AIJ is not able to model the variations and thus can't de-tangle the larger and smaller pulsations from airmass trend.
I think your best plan is to use a comp star (or stars) with the same color as the target star to avoid the need for detrending.
If you don't have such a comp star on the detector, probably the only other choice using AIJ is to just detrend the full delta Scuti light curve against airmass using the full light curve "Fit Mode" (
) , which
will flatten the over-all light curve based on the full set of light
curve data, but may be skewed by the variable pulse amplitudes.
Karen
URL: http://astroimagej.170.s1.nabble.com/Detrending-Airmass-tp1437p1438.html
Hi Jessica,
AIJ's detrending works by trying to flatten a light curve by fitting any long term trend to the airmass curve. For an exoplanet transit, the out-of-transit segment, or a full transit model plus out-of-transit of the target star light curve can be used by selecting the corresponding "Fit Mode" in Mult-plot Y-data.
Detrending a comp star light curve does not affect the target star light curve in rel_flux_T1 because the differential photometry is calculated before detrending would be applied to a comp star. Even if the comp star light curve were detrended before the differential computation, it wouldn't necessarily flatten the target star light curve since it's photometry also suffers from differential airmass losses.
This indeed causes a problem for delta Scuti stars since AIJ is not able to model the variations and thus can't de-tangle the larger and smaller pulsations from airmass trend.
I think your best plan is to use a comp star (or stars) with the same color as the target star to avoid the need for detrending.
If you don't have such a comp star on the detector, probably the only other choice using AIJ is to just detrend the full delta Scuti light curve against airmass using the full light curve "Fit Mode" (
) , which
will flatten the over-all light curve based on the full set of light
curve data, but may be skewed by the variable pulse amplitudes.Karen
On 8/6/2020 2:07 PM, mtsacobservatory
[via AstroImageJ] wrote:
Hello,
We are new users to AIJ, but want to use it with our students to reduce data for delta Scuti stars. We've done the aperture photometry, but does AIJ automatiaclly make corrections for airmass? I noticed there's a way to detrend the lightcurve for airmass in Multi-plot. I tried a detrend for Comp star 1, and I can see the coefficient. What is reasonable range for this value? Also, does detrending Comp star 1 automatically apply the correction to the target? And do we need to detrend multiple comp stars, or is one enough? Any help would be greatly appreciated.
Jessica Draper
Mt. SAC Observatory
Walnut, CA
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