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!

Recent Imaging

I've been imaging over the past few days. The horsehead image from the last post did get its green and blue parts taken:

Flame and Horsehead nebulae

The Moon was Gibbous and nearby; some of its light snuck in and caused the blue-green flare at lower right. This has since been solved by extending the dew shield with some craft foam.

I also had a couple of clear nights during which to image the Rosette nebula. I collectd H-alpha the first night, but it was partly cloudy the second night and I didn't get very many OIII frames.  Consider this a preliminary composite image:

Rosette Nebular (NGC 2244)

The Moon was even brighter and closer to the target than for the previous image, but the dew shield extender worked fine. Next chance I get I will collect more OIII data, and for the first time try to get some SII as well.  I'll get a Hubble Palette image yet!

AT72ED First Light!

Tonight is the first imaging night for the AT72ED (at least for me with this telescope). My target is the area around the horsehead and flame nebula in Orion. A preliminary look at the images is encouraging. I think I may need to adjust the distance between the flattener and the CCD a little. It was H-alpha tonight of course.

The evening has had waves of altocumulus and altostratus move through, so it was fortunate I didn't set up the autoguider. It would have lost its lock several times. Speaking of lost, I wonder how many light frames will have to be rejected because of the clouds.

If tomorrow or Thursday night is clear I'll try a broadband blue filter (perhaps in combination with an Orion astrophotography filter) to pick up the reflection nebulae in the field of view.

Crazy weather for this time of year; the temperature was in the mid 20s with a very light breeze.

Added note: a couple of images taken using the AT72ED are included in the next blog entry.

Serendipity and a cranky cable

Tonight I thought I'd go back to the Horsehead and redo the H alpha exposures. But when I asked the GoTo to take me there, it instead put me right on the Flame Nebula. I decided to go with the what the mount apparently preferred.

I'm current imaging with my MK-67, an f/12 Mak-Cass. Its focal length is 1800 mm. The ST-8300M's pixel size of 5.4 microns  is about 1/3 the size it should be (about 16 microns for the average poor seeing I get here). I'm therefore binning 3x3 on the camera, which give me about 1.65 arcseconds per pixel. Looking at some of the subs, this seems to work very well.

As I write this I've got 22 subs in for the Flame. Tomorrow night I'll go back (unless the mount has other plans) and shoot some other bands if it holds clear.The odd lines I mentioned last time are gone. I asked on the MAS forum about the noise I was getting the other night. It looks like the best suggestions were about bad or loose cables. Something was acting up tonight when PHD went crazy. The oddest thing was that each exposure caused the PHD window to jump back and forth by about the window frame's width (8 pixels or so). It also couldn't calibrate correctly. Multiple restarts didn't fix this, nor did unplugging the USB cable at the laptop.  However unplugging and reseating the cable at the autoguider did the trick. I begin to suspect that the autoguider's socket is the problem.

The Horsehead image came out okay, but needed more exposures. I think the flame will look much better. I aim to get 30x5min exposures for it.I'll post the Ha images in the next couple of days.