Testing CTE Correction on ACS Data

[acviana]

The ACS data will require an additional step that the WFPC2 data does not, CTE correction.

• http://www.stsci.edu/hst/acs/software/CTE/

Pull a file from ACS from our archive area on /astro/mtpipeline/ and see if you can get the software to run the CTE correction over each.

[ktfhale]

I'm examining the ACS CTE correction options. The page linked references a module called PixCteCorr, that looks like it can do what we want. However, there's also a whole software package for ACS called CALACS, the documentation page of which is more recent. Both CALACS and PixCteCorr exist within the acstools module. The astrotools documentation lists them separately, but it appears that CALACS does CTE correction as an early step, in addition to other steps like cosmic ray rejection and drizzling. CALACS seems to be its own pipeline. Are there issues with our solar system data that prevents us from using it?

For the moment, I'll look int just using the function PixCteCorr.

EDIT: I see that CALACS is the standard pipeline for reducing ACS data. Since we haven't grabbed anything after the _flt.fits files for our own archive, I assume there's a reason we are foregoing it.

[ktfhale]

PixCteCorr only supports ACS WFC images. The random saturn images I got off of our ACS archive used the SBC detector instead, so PixCteCorr is not an option for some subset of our ACS data, unless we want to hack out the if-statement that checks to make sure the detector is WFC.

[ktfhale]

I went and talked to Max. A summary of what I heard from him:

We don't know what effects CTE correction has on images of planets, or other significantly extended sources. CTE correction was developed for point sources, like stars. When it comes to objects like comets and asteroids, which are somewhat starlike, the effect of CTE correction should be beneficial. But we should examine planetary images before and after CTE correction to see if there's actually an improvement, or if it's actually destructive.

CALACS is the first part of the ACS reduction pipeline, and seemingly deals with the hardware-based issues. Before it included CTE correction, its final output were _flt.fits files, which we have in our archive. After a new version was released that included CTE correction, its final outputs were _flc.fits files, which we don't have.

I'm going to request some _flc.fits from MAST, and compare them to the _flt.fits files we have in our own archive.

[deustua]

Hi kevin, With respect to ACS it is correct that the pipeline runs CTE correction only for the WFC. However, while not tested with point sources, i think there is a cte effect for extended sources, though perhaps more subtle. it is also true that faint sources can 'disappear' in the absence of cte correction. so testing is agood idea. Check with Marco Chiaberge and find out if anyone has considered CTE correction with the SBC.

susana

[deustua]

also, MAST does apply automatically the cte correction for acs images. it didn't used to, but does now.

[ktfhale]

It appears that the majority of our ACS data is not taken using WFC, the Wide Field Channel. The majority seems to be HRC - the High Resolution Channel, which makes sense for things like planetary observations. Because i/o takes a long time, I don't have an exact breakup of the files yet, but WFC files so far seem to be on the order of hundreds, whereas there are thousands and maybe tens of thousands of SMB HRC files, respectively. I'll know more exactly tomorrow.

[ktfhale]

Our ACS _flt.fits files have the following distribution over the three detectors:

WFC: 595 files SMB: 8542 files HRC: 9159 files

[acviana]

From Marco Chiaberge:

We published an ISR back in 2009 that includes a photometric correction formula for the HRC. You can find it here: http://www.stsci.edu/hst/acs/documents/isrs/isr0901.pdf

Note that the correction for ACS/WFC in that ISR is NOT the most updated version, but it is still ok for pre-SM4 data. For post SM4 data, the best photometric correction formula is published in the following ISR: http://www.stsci.edu/hst/acs/documents/isrs/isr1205.pdf

[ktfhale]

For the reasons noted in Ticket#100 for WFC3 data (limited applicability to our data, long computation times), we will probably not implement CTE correction into the pipeline. This ticket can be closed.