Drizzle integration

[Ikonnikov]

10 hours ago, jimjam11 said:

I reran the experiment with some natively undersampled data. This was captured with an EF200/ASI1600 yielding 4.04"/px. The results are somewhat different:

 

Top left is 4.04"/px in theory undersampled. In reality the stars dont look anything like as bad as the 3.48"/px binned data above but they are slightly blocky. FWHM is 5.802"

Top right is the data resampled prior to integration yielding 2.02"/px. FWHM is 6.145"

Bottom right is drizzled. FWHM is 6.190"

Bottom left is integrated as per the top left, but the integrated stack was then resampled. FWHM is 5.578". This is the lazy option!

 

Some observations:

Resampling a bunch of subs and then integrating them is time/space consuming compared to drizzle integration (in the land of PI at least). PI is very fast at drizzle integration despite it being a two step process. 

 

Whilst my data suggests drizzle has some benefit over resampling prior to stacking (acknowledging I might/probably have done something wrong) I am struggling to see the benefit of drizzle at all unless you have a very strange equipement config or image somewhere exotic (like mauna kea) which can yield very small star FWHM. Based on these results it is really hard to get undersampled data which properly benefits from drizzle because you need the combination of large aperture, short focal length and large pixels; something like a rasa paired with an Atik 11000? I guess the HST is one such example :).

 

 

 

I also had a go at a drizzle experiment recently, and came to a similar conclusion:

Capturing subs on my E130D with a ASI290MM (giving 1.39 arcsec/pix image scale bin 1x1) with medium SGPro/PHD2 dither between each sub, using PI for (pre)processing.

Capture 1: 64x subs bin 1x1 15sec exposure, registered with auto settings and integrated with default linear clipping rejection settings. Average FWHM about 2.8 arcsec by PI FWHM eccentricity script (Moffat 2.5), similar values by dynamic PSF of a selection of stars. 

Capture 2: (back to back with capture 1, no obvious change in sky conditions) 80x subs bin 2x2 5 sec exposure registered with bicubic spline setting (to avoid horrible dark ringing resulting from auto/lanczos interpolation) and create drizzle files, integrated with linear fit clipping rejection or additionally drizzle integrated with scale set to 2.

In short the drizzled 2x2 data never really got close to the original 1x1 image in terms of star FWHM . I kept reducing the dropshrink setting which did lower the FWHM slightly but 0.5 was the lowest drop shrink value I could use without major artifacts (caused by lack of coverage) and this still gave stars with ~30% higher FWHMs than the integration of 1x1 subs. Also the FWHM in the drizzled image (drop shrink 0.5) is barely lower than that of the undrizzled 2x2 integration and using the default drop shrink of 0.9 it was actually higher! I appreciate that at 2x2 binning here the FWHM measurement accuracy might be compromised due to undersampling but I can't say my results fill me with confidence as to drizzle being worth the bother either! Maybe a different selection of pixel interpolation in image registration might help?

 

Paul

Integrated masters attached

 

 

 

drizzle_integration_BicubicSpline_2x2_DropShrink_0p5.fit integration_bicubic_spline_2x2.fit integration1x1.fit