Detrending Airmass

[mtsacobservatory]

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

[karenacollins]

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

---

If you reply to this email, your message will be added to the discussion below:

http://astroimagej.1065399.n5.nabble.com/Detrending-Airmass-tp1437.html

To start a new topic under AstroImageJ, email [hidden email]
To unsubscribe from AstroImageJ, click here.
NAML

[mtsacobservatory]

Hi Karen,

I tried detrending the target delta Scuti star with airmass using two different fit modes (the full curve transit dip that you suggested, and also the full curve flat line), but both skewed the lightcurve.  I also tried running an airmass detrend on two of the comp stars.  Even if that detrending doesn't automatically apply to the rel_flux_T1, it should at least give us an idea if airmass was a significant problem, right?  Both comp stars look fairly flat, as they should, and the airmass coefficients I got were -0.0012 and -0.0045.  Is there a reasonable range that people use to know if this value is acceptable?  

Thanks again for your help,

Jessica Draper
Mt. SAC Observatory
Walnut, CA

[karenacollins]

Hi Jessica,

Airmass coefficients of -0.0012 and -0.0045 seem reasonably small, but there's no real upper or lower bound on reasonable values that I could quote.

From your description, it sounds like your comp stars are fairly flat without detrending. That is helpful and possibly gives you information on the amount of airmass trend that might be contributed from those light curves. However, if they have significantly different effective temperatures (Teff) from the target star, the differential target star light curve could still have some airmass trend. I think the only way to disentangle the pulsations from airmass trend is to look up the Teff of your target star and then try to find some comparison stars in your field with similar brightness and Teff. The TIC (TESS Input Catalog) on MAST lists a reasonable Teff for almost all stars. There's an interactive mode that lets you click on stars in an image to see the Teff of stars in nearby your target, or you can click on the catalog entry and it will highlight the corresponding star.

If you are not familiar with the TESS catalog on MAST, go to:
https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html

-Then in the pull down menu at the top-left, select "MAST Catalogs", and then just below a second pull down menu will appear. Make sure it says "TESS Input v8". Then enter your target's ra and dec and a radius of a circular region to search in the "and enter target" box, like this:

-Then click search (r=5m means show the catalog for all stars within 5 arcmin of the target coordinates).

You will the see a display similar to the one below with the catalog values on the left and the image display on the right. Look for stars in your field with a similar magnitude and Teff.


I can 't support the above tool, but wanted to give you an idea about it, in case it's helpful.

Karen

On 8/12/2020 10:56 AM, mtsacobservatory [via AstroImageJ] wrote:

Hi Karen,

I tried detrending the target delta Scuti star with airmass using two different fit modes (the full curve transit dip that you suggested, and also the full curve flat line), but both skewed the lightcurve.  I also tried running an airmass detrend on two of the comp stars.  Even if that detrending doesn't automatically apply to the rel_flux_T1, it should at least give us an idea if airmass was a significant problem, right?  Both comp stars look fairly flat, as they should, and the airmass coefficients I got were -0.0012 and -0.0045.  Is there a reasonable range that people use to know if this value is acceptable?  

Thanks again for your help,

Jessica Draper
Mt. SAC Observatory
Walnut, CA

---

If you reply to this email, your message will be added to the discussion below:

http://astroimagej.1065399.n5.nabble.com/Detrending-Airmass-tp1437p1439.html

To start a new topic under AstroImageJ, email [hidden email]
To unsubscribe from AstroImageJ, click here.
NAML