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AstroImageJ Model Fit question

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ldj
ldj's picture
AstroImageJ Model Fit question

I have a couple of questions I hope someone (Dennis?) can answer.

1. In the Data Set 2 Fit screen, what time standard is the fitted Tc given in? Is it based on the "Default X-data" setting from the Multi-plot Main screen? When I change that value from, for example, JD_UTC to BJD_TBD the result changes, however I just wanted to make sure that this is a direct link and not an accident.

2. Is there any way to get error estimates out of the model for the principal values computed from the model? (eg. transit duration, planet size, Tc)

-- Dave

ldj
ldj's picture
I think I can answer my own

I think I can answer my own questions:

1. With some certainty, the answer is yes.

2. The answer is no - the AstroImageJ paper, section 4 says "... and does not provide estimates of the parameter uncertainties." 

rzellem
rzellem's picture
If you'd like, you can use

If you'd like, you can use our data reduction pipeline EXOTIC that does estimate the uncertainties on the model fit parameters. EXOTIC takes raw FITS files and fully-reduces them, then models them, and then produces an output file that can easily be uploaded to the AAVSO Exoplanet Database.

You can download EXOTIC here: github.com/rzellem/EXOTIC

dennis-conti
dennis-conti's picture
AIJ Model fit question

HI  Dave,

Yes, you correctly answered your own question. However, just to add to the uncertainty in planet size, because planet size is estimated from transit depth, which =(Rplanet/Rstar)^2, without considering the uncertainty in Rstar, it makes it difficult to estimate uncertainty in planet size.

Dennis

WBY
WBY's picture
AIJ Comp Star Detrending for model Fit

When I select a set of detrend parameters for the target star that gives the best fit (lowest BIC) for the transit fit, I have been simply detrending the comps with airmass applied to the whole dataset (excluding any trim regions). However, it seems to me that I should be detrending the comps in a corresponding manner to the Target. For example, suppost I am detrending the model fit with Airmass, Sky/Pixel TI and Y(FITS) T1, then perhaps I should be detrending the comps similarly. Using C2 as an example, I should detrend using Airmass, Sky/Pixel C2, Y(FITS) C2.  

Is their an expert model fitter out there who knows the best practice for this? I have tried both ways and it doesn't seem to make much difference in the fit but that may just specific to the data sets i have been working with. 

Brad Walter, WBY

dennis-conti
dennis-conti's picture
AIJ Comp Star detrending

Hi Brad,

Yes, it would seem logical that applying a set of detrend parameters to the target in order to minimize the affects of systematics should also apply to the comp stars. However, I have seen in practice that the pros primarily use just Airmass. Keep in mind that when deterend parameters are applied to the comp stars, they do not affect the model fit results of the target. The purpose of applying the detrend parameters to the comps is primarily to judge whether their resulting RMS is indicative of them being a "good" comp - namely that the assumption that they are a non-varying star is valid.

Dennis

WBY
WBY's picture
AIJ Comp Star Detrending

Thanks Dennis,

Actually it makes sense, thinking more carefully, that detrending the comps doen't affect the model very much since youar are using a  ratio of target flux to total of the comp fluxes. Detrending is primarily for graphical represetation. The only way I see that comp detrending makes a difference in model fit is if the detrending has significantly different effects on different stars within an image and that difference varies over time. That could happen due to inhomogeneous sky conditions or simply that each ot fhe stars being measured drifts across different pixels. If conditions are good inhomogeneities are small withiin a single image.

Brad  

 

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