Testing of MDS image quality - Durham (Jim Geach)

I have been looking at fields MD07 and MD08, checking out image quality, and trying to highlight potential issues that will affect extragalactic science.

General image quality

There are some significant flat-fielding issues, especially near the edges of the field. Zooming in, there are some strange re-sampling effects in the stack - these are going to seriously mess up faint-source science:

I’m quite worried about the ‘chunky’ effect seen in (e.g.) the abrupt level changes, and the mottling effect. I’m guessing that all of these are a combination of convolution, dealing with NaN-ed regions in the input frames, and how the sky is dealt with. I’m trying to use the deep CFHTLS Groth Strip and EN1 (INT) images to figure out how much of that mottling is noise, and how much is contributed by faint galaxies - I hope to come up with something quantitative. We should be worried about that that.

More worrying: here is an example of a comparison of g and r-band stacks of skycell 030. Something seems to have gone quite badly wrong - especially with g. I suspect this is a masking issue - looks like something from a bright star has infected the image?

Finally, just looking at a few of the z-band warped images, there might still be an issue with correlated readnoise. Not yet clear to me how much of an issue this is with the other filters, until we look at all of the data. Will do that in the next few days.

Object detection the .cmf catalogue seems only to contain fairly bright (and mainly point-like sources). I ran Sextractor on the frame with a very simple 3-sigma / 5-pixel threshold (quite low, I admit - but I wanted to push it to see what came up). I got the following (red circles are Sextracted and green boxes are from the supplied cmf catalogue). Note - should the cmf catalogue have extended / faint sources in it? There might be some de-blending going on, because some of the big bright things have been split into two.

Depth/photometry I have overlaid objects with their r (Megacam) magnitudes (in yellow boxes) on two zoom-ins of skycell.056 here and here. This gives you a rough impression of what the faint blips correspond to in a deeper image - getting the CFHTLS image to compare to now... will update this later with a new figure.

You can see that (roughly) a 5-sigma detection is at about r=24 mag. Here is a plot showing the MDS instrumental mag (I’ve bootstrapped the z.p. from the CFHTLS deep catalogue) compared to the Megacam magnitude (note that the CFHTLS catalogue should be going down to r=28 mag). You can see where the scatter blows up at about r = 24 mags (I’ve shown two versions of the plot). This was only rough, I need to do something a bit more rigourous to work out the true depth. Also, this is for just one cell.