Iowa Star Party Images: Repaired!

To recap: I took one night's data at the October Iowa Star Party.  As was my usual practice I deferred shooting flat frames until my return home. When I processed the data I got something of a shock. the stacked calibrated frames had very obvious dark "holes" in the middle. Here's what I mean:

Master Green after ABE

This was the result of not matching the exact focus used while gathering light frames. The remedy would be to obtain correct focus and retake the flats, so the next clear night when the temperature was close to what it had been in Iowa I set up in my back yard, got focused, shot flats, and hoped for the best. Something was off, though; the new result after recalibrating the light frames was a larger dark ring instead of a what you see above.

I've read that flatting at f/2 is a fiddly thing, and based on this I have to agree.

The real solution is to collect future data at f/2.8 or slower so that the focus is less an issue. And to take flats in the field at the time of data acquisition. But that's too late for the Iowa data, which I really don't want to throw away. So what can I do to salvage that data?

How about creating a sort of secondary multiplicative layer of my own? One way to do this might be by using using the GAME script for PixInsight, which basically allows you to make a round mask--I could use the inverse of the mask to stretch the hole area a little bit, or even to create a synthetic secondary flat to use in recalibrating the already calibrated light frames. I tried both methods and didn't get satisfactory results. 

More labor-intensive would be to create applications in a high level programming language that could directly adjust pixel values in the master frames. It's been maybe 12 years since I did any serious coding and had none of the needed compilers installed on my computer, so no -- that would be too involved. Much simpler would be to use PixInsight's PixelMath to do the same thing. I used Microsoft Excel to model the hole as a 6th-order polynomial and used a simple PixelMath script to transform the master images. This actually worked to some extent, but not well enough. I could have fiddled with that and eventually found the right polynomial, scale size, and amplitude to make a good correction, but I really didn't have the patience for that. There had to be a better way.

And in this case, there was. I wouldn't recommend this as a cure for every flatting problem, but it really helped here.

Because the data for two of the targets was low in the eastern sky over a bright light source there I used PI's ABE to make a first pass at getting rid of the light pollution. This was a fairly typical application of ABE, using a Function degree of 4:

ABE settings

This essentially exposed the hole and whatever else the bad flat didn't correct. GradientCorrection was then applied with some very non-default settings designed to work better on the hole's small-scale structure.  

GradientCorrection settings

Here is the result. The hole is almost completely gone, as are the edge issues left behind by ABE.

I applied this to all three channel masterss and processed normally. Without the hole I could process a little more aggressively to bring out Barnard's Loop.

Samyang 135 mm at f/2 (1/4 scale)

Wikipedia image (an RGBHa image by Hunter Wilson),
cropped and scaled to match my image above.

Below it is a picture of Barnard's loop on its Wikipedia page for comparison. I've rescaled and cropped the image to match mine. I don't know what Wilson's acquisition data is, but mine is from a mere half hour of total exposure (that's ten minutes per channel). 

Am I totally happy with this image? No. there are issues with the upper left and lower right corners, for starters. What I am pleased with is how it reveals a lot of the reflection nebulae in western Orion. A shame it didn't quite extend far enough to catch the Witch Head. It would be wonderful to devote hours to this area, but that's not going to happen.

Here's the other de-holed image from Iowa:

L to R, Soul Nebula, Heart Nebula,
and the Double Cluster (1/4 scale)

As with the Orion image, the hole is essentially eradicated. Too many stars, though. 

Reprocessing the third image, the Lambda Nebula, will be problematic as the hole is very entangled with the nebula. I may need  to use my PixelMath script method for that.

Next spring I'll try to get dithering working more reliably so that I can drizzle process and get better stars. With some luck I may get my wide-field image of the Polaris-area IFN after all.

But now it's time for winter hibernation and reprocessing old data. With what I'm learning from the Masters of PixInsight folks, I may even tackle that big old Veil Nebula mosaic data I've been sitting on for a year!