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!

Iowa Star Party Results

Back from the Iowa Star Party, and my first field use of my Samyang lens. Mostly the results were good; shooting with the Samyang at f/2 appears to have great potential, but I need practice to make it really great.

The first night of the three-night party was on-and-off cloudy with enough breaks to keep visual observers happy, but it was not sufficient for the deep sky imaging I wanted to do. I collected some frames mainly to practice with the system.

The second night was almost perfect. Thunderstorms had moved through during the day and were gradually drifting off to the east as the night progressed. Frequent lightning flashes illuminated the eastern sky at for a while but diminished so that by the time I stopped (around 3 AM) they were not a factor. I used the first three hours of darkness to image the Heart Nebula / Soul Nebula / Double cluster area:

Heart and Soul Nebulae, Double Cluster

Next up was an hour on the central Orion area (20 minutes per channel). I didn't do a good job with processing this.

Central Orion (M42, Horsehead, Flame)

To illustrate what the lens can do, here's the Orion Nebula from the above image, post-processed for higher dynamic range:

M42 from above image

It's pretty, but it's only 1293 x 1326 pixels, so at 300 DPI it's only good for a 4 x 6 inch print. 

It was getting into the wee hours of the morning, but I couldn't stop--I get so few opportunities to image in Orion. I just had to image the Lambda nebula around Orion's head, something I had imaged eleven year earlier in H-alpha on an unusually warm February evening. This is based on a ridiculously short 30 minutes of total exposure (10 minutes per channel) and really illustrates the power of shooting at f/2: 

Nebulosity at the head of Orion (Betelgeuse is at lower left)

(All three of the above full images are at half-scale. For full scale images, see my AstroBin gallery.)

Alas, the third night was not to be. The cold front came through and the day was windy and brisk. late in the afternoon it started raining; online sources suggested that clearing might happen around midnight -- or later. I decided to call it a star party and get some good sleep before the return drive the next morning.

After an imaging session it's always good to reflect on what was learned, and this trip had some lessons beyond learning that my sleeping bag advertised as being good to 20 F was definitely not.

Rotating the system proved very doable. It was awkward at first, but so is everything. I found it almost impossible take the focusing belt on and off without messing up the focus, but the penalty for that is minor. My NINA advanced sequence design with added pauses seemed to work out fine.

The first night I found that R-G-B-Dither cyclic acquisition was problematic. I was doing this to make drizzle processing possible. Mostly this worked, but occasionally PHD2 went wildly unstable in RA and suffered death-by-overcorrection. I'm not sure if this is a settling time issue or what. The second night I turned dither completely off and guiding was fine. The ASI 2600MM seems not to need dithering, but if I'm to drizzle, I'll need it.

Look again at the above images, and notice the slightly darker circular area at the center of each image: a donut hole. This arose because of bad flats; here is the stretched value of the ratio between older, correct flats and those from Iowa. 

Lovely radial symmetry about the image center, but this should be a nearly uniform field aside from any changes in dust. 

I tried using the older flats in place of the new ones, but that failed. Because my system is currently almost dust free, I tried skipping the flats and using PixInsight to correct the vignetting. That also failed. My next attempt will be to use Photoshop to manually correct this issue. 

My guess for what caused this is improper focus. I deferred shooting flats until I returned from Iowa, and in that time the zero reference point on the EAF was lost. This made it almost impossible to reproduce the focus used for the light frames, resulting in the poor flats. One clear night should make it possible to shoot some new flats at proper focus.

The weather forecast is mildly optimistic for the first week of November with highs in the 60s; will that warmth will bring the clear night I need? 

Canon lens and the ASI 2600MM

In the previous post I suggested a configuration to use in mating DSLR lenses to the ASI 2600MM camera + ZWO 7x36mm EFW. A first test of this has been done, with good results. Here's a simple stack/stretch of some Heart Nebula H alpha data taken using an old Olympus lens with a Canon adapter, mated to the ASI with a thin Canon to M42 adapter + thin spacer rings for 1.4mm

Things to be aware of are 

• The wind was gusting to 30mph the entire time data were being collected.
  
• North is to the right; you can see a substantial drift in RA probably because this is a temporary rig and utterly out of RA balance (PHD was squawking a lot) 
• Focus was far from perfect
• This was 2x2 in-camera binned and roughly calibrated using dark frames from 10C cooler calibration, no flats, no bias. 
• Stretch was just the PixInsight STF
  

Evidence of the drift is seen along the top (west) edge of the image where there is a thin band of partial coverage. As a result of the probable bouncing due to PHD's problems we expect stars to elongate in RA during the ten-minute exposures, and that's just what we see in this composite image of the center and four corners:

 The center shows the poor focus and evidence of the RA drift. (RA drift is in the vertical.) 

Stars at the corners compare well with the center stars. While they show the same soft focus and RA elongation, there's no evidence of aberrations resulting from improper spacing between lens and sensor. 

Sharper focus might well show defects, but this is not bad for a windy night, no balance, and a $40 Olympus 200mm f/4 lens (stopped down to f/5.4).

And one last little observation about the pricing of astro gear. The Canon lens adapter for my SBIG ST-8300 cost me $300. Things were far less expensive for the ASI. The Canon/M42 adapter plus the thin spacing rings came to $75. Over half of that was for the general purpose rings that will see duty in additional ways.