Iowa Star Party Images: Repaired!

To recap: I took one night's data at the October Iowa Star Party.  As was my usual (rather lazy) 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 "donut 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 an even larger donut hole than 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 only real solution is to collect future data and flats at a slower speed so that focus is less of 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 it?

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 would create a round mask that I could use to stretch the hole area a bit, or even to create a synthetic secondary flat to use in recalibrating the already calibrated light frames. Nice thought, but 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 any other flatting problem, but it seemed to work well enough for the Iowa data. The nice thing is that it used two standard PixInsight processes.

Because the data for two of the targets was low in the eastern sky over a rather conspicuous light dome, it suffered from a considerable light pollution gradient. I used PI's ABE to make a first pass at getting rid of that. This was a fairly typical application of ABE, using subtraction and a function degree of 4:

ABE settings

This essentially revealed the hole and whatever else the bad flat didn't correct. GradientCorrection was then applied with some 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 masters and processed normally. Without the hole I could process a little more aggressively to bring out Barnard's Loop.

My image based on Iowa data

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

Below my image is the picture of Barnard's loop on its Wikipedia page for comparison. I've rescaled and cropped the image to match mine. Wilson's acquisition data is RGBHa, which probably accounts for the very red nebulosity.

Am I totally happy with my image? Not entirely. Theres still some weak signal suppression at the the very center of the image, and I wish the Running Man was bluer. Given that this is based on only half an hour of total exposure (ten minutes per color channel) I am pleased with is how it reveals a lot of the blue reflection nebulae in western Orion. It's 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 huge 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 long-sought wide field image of the Polaris-area IFN after all.

But now it's time for winter hibernation and reprocessing of 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 over a year!

Next time: the Lambda Orionis Nebula (Sh2-264) after de-holing!