Finally, A Few Nights Under the Stars

August 19

The router I talked about in my last post worked well, and I should be able to monitor my imaging remotely from now on. Other things happened this night that made it less than successful.

Imaging started with NINA doing a slew, center and rotate, or trying to. I use NINA's manual rotator, which has always worked well for me in the past. This night it did not. My first rotation resulted in NINA requesting an even greater rotation in the same direction, and so did the next. I didn't want to sort it out, the image wasn't going to amount to much given the omnipresent overhead smoke, so I redid the sequencer script to just slew and center and pause so I could do a rotation by eyeball. Not at all optimal but it started the collection of light frames.

After a while I noticed that the images were coming in fuzzy-looking. It wasn't a focus issue; the stars were sharp but diffuse. It was a very dewy night so that was my first guess. I put everything on pause and looked at the objective. It was dry. I could have examined all the other optical surfaces but instead I looked at the camera settings. The camera was at the correct temperature (-5 C) but the dew heater on the sensor window was off. I set it back on and decided to wait and see if it would clear; I could monitor this on my tablet indoors. The images slowly improved, but I was running out of time. I decided to call it a night and started breaking down the setup.

The next afternoon I found that there was a setting in the manual rotator that allowed me to reverse the sense of corrections. Having it set the wrong way would have produced exactly the symptoms I saw the night before, so I was confident that things would work next time. 

Both issues were probably a consequence of my having to restore NINA after a full lobotomy of my laptop earlier this year; the clean reinstall had put everything back to default settings.

August 20

So, two problems, both probably fixed, and a clear night. The moon was at first quarter and there was an too much overhead smoke, but I wanted to see if things were fixed so out I went to the club's Eagle lake site. Everything worked and I decided to do a quick and dirty two-pane mosaic of the Veil Nebula. I had planned for a minimum of two hours, one for each pane, and started collecting light frames. The northern pane finished with 60 x 60 s frames, then the sequencer did its thing and went to the southern pane. 45 frames into that the clouds swept in and the night was over; the second pane got short-changed. 

An hour per pane was not really enough to for a decent image, but it did give me data to practice mosaic-making. Here is the result:

Veil Nebula 2-Pane Mosaic (1/2 scale, click to enlarge)

Comments:

• This needs more time on target!
• Shooting 60 s subs really helps preserve star color and allows essentially total eradication of satellite tracks, Yay!
• I would like to thin out the stars better during the next reprocess.
• I don't know if the variations in background brightness are representative of reality. In particular, that lighter area to the right of the Western Veil: is it real or is it an artifact of how I flattened the field? It compares very well to this image in which it shows structure. Since the veil is often imaged using LP filters, generally colorless features might be taken as light pollution and filtered out.
• The image was calibrated using dark frames, flats, and flat darks. Lights were dithered, but not sufficiently for drizzling. I had about a half dozen runs at processing this before I found a good way to merge the panes. The workflow I found that worked best is given below.
• Can you find the two seams? I can't despite knowing right where they are!

August 22

The next night was cloudy, but the one after that was clear. The moon was now two days past first quarter, and there was still too much smoke aloft; the sky looked very bright. My first target (Sh2-114, the "Flying Dragon Nebula") was such a lost cause that I stopped collecting data early hoping to have enough time for a quick and dirty image of the Cocoon. I got 45 x 60 s of light frames: here's how that turned out:

Cocoon Nebula (1/2 scale, click to enlarge)

This would be gorgeous from a dark sky site with no moon and no smoke! There's a lot of entrained gas/dust around the dark lane that extends to the left, and it's only hinted at in my image.

One other glitch did arise this third night of four. The mount stopped unexpectedly well before it should have and signaled it had reached its safe limit. The next day I set it all up and made sure the mount had proper limits and was also correctly configured for meridian flips. I also made sure NINA was ready for flips, too. I'm going to attribute the glitch to the usual guilty party: operator error.

Verifying the change will need to wait for the next new moon, though. I'd also like to see if my setup can do a proper meridian flip.

Mosaic Workflow

This was all performed within PixInsight 1.9.4 build 1695, and I've provided most of my non-default settings for each process or script. PixInsight settings are meant to be explored, and you should find those that work best for your imaged objects and esthetics.

If you don't have the same scripts and processes installed as I use here, you will need to obtain them or find suitable substitutes. I also assume you have at least a rudimentary knowledge of PixelMath; there are two points at which PixelMath will come into play.

Calibration

WeightedBatchPreProcessing (WBPP) is used to do the calibration, and for these images I used the default settings. When it finished I had two master light frames, Pane1 and Pane2. I chose to have WBPP do autocropping even though it can insufficiently crop out edges with weak signal. By the way: I've now stopped using bias frames as part of calibration; they're mainly useful for scaling dark frames with mismatched temperature or exposure time, and I never have those.

Linear Image Cleanup

Run BlurXterminator to take care of any aberrations. 

Use the ImageSolver script to solve the images and give them the solutions.

The Veil field is packed full of stars to the extent that I considered using DynamicBackgroundExtraction to be iffy, so I used AutomaticBackgroundExtractor with degree set to 1 and the subtraction as the correction. Why degree 1? This eliminates the mean and gets rid of the overabundance of green often seen in One-shot color masters along with the field-uniform light pollution. During relatively brief imaging sessions, the light pollution may also have a significant spatially linear component, so that's removed, too. 

Preparing and Merging Panes

I use star alignment to create registered panes ready for merging. There are other ways to do this, but it worked best for me for this particular data. This mainly entails creating an image full of synthetic stars that is larger than the eventual mosaic will be and then registering the Panes to it.

I make a rough guess at the RA and DEC of the eventual mosaic's center. With only two panes it's simple to just average the RA and DEC of the panes. I also guess the eventual size of the mosaic in pixels, and add some "elbow room" to make sure the panes will fit into it with black margins. The center coordinates and estimated dimensions are used by CatalogStarGenerator; it produces a star map image named CatalogStars. If this image appears black to you, apply autostretch.

As noted above, the automatic crop feature of  WBPP sometimes leaves some data-poor edges uncropped. Use DynamicCrop to trim these off. Note that this destroys the previous solutions so we will need to re-solve as needed. 

The pane backgrounds must be matched, so use DNALinearFit. This can be obtained at David Ault's website, and more specifically here.

Use StarAlignment to register the panes (the correct settings are reference image = CatalogStars, Registration mode = Two-Dimensional Surface Splines, Radial basis function =  DDM Thin Plate Spline.) This results in two registered panes.

Next is more cropping! Use PixelMath with one line max(pane1,pane2) where pane1 and pane2 are the identifiers of the two registered panes. Remember to tell PixelMath that you want to create a new image, not overwrite one! This will produce a quick and dirty mosaic image. Use DynamicCrop to crop away all the black portions of the quick mosaic, then apply the crop to both registered panes. Save these newly cropped images.

Merge the newly cropped panes using GradientMergeMosaic with type of combination = overlay, Shrink radius = 1, Feather radius = 10, and Blackpoint = 0. If you see pinched stars, you can rerun it with an increased feather radius or you can black out offending stars. At this point you will have a merged mosaic image we will call MyMosaic.

Traditional Single-Pane Post Processing

At this point processing follows your usual workflow for a single linear image.

Do serious color calibration by using ImageSolver followed by SpectroPhotometricColorCalibration.

Do serious background extraction by running SpectroPhotometricFluxCalibration followed by MultiscaleGradientCorrection. I use MGC as it usually gives nice results.

Now you have a choice to continue post processing on the mosaic as it is, or to split it into starless and star images. I went with the latter by applying StarXterminator.

At this point I did see some mild star pinch effects in the starless image, but decided that they weren't worth correcting in this preliminary processing. If I had wanted to correct them I would have just gone back to the application of  GradientMergeMosaic and increased the setting for feather radius or blacked out some stars.

Because I was seeing some residual green in the star image, I applied  SubtractiveChromaticNoiseReduction.

The star image I basically left unchanged.

Stretching

Starless image: I used MultiscaleAdaptiveStretch with target background = 1.25 and scale separation = 256.

Stars image: the usual STF-HT method wildly overstretched the stars, so I used StarStretch with the stretch amount = 4 and color boost = 2. The stretch amount is where you can get very fiddly. It's worth exploring the entire stretch range of 4 to 6.

Combine Starless and Star imagesCombine the nonlinear starless and star images with a PixelMath line ~((~$T)*(~stars)) where stars is the identifier of the stars image.

Final Tweaks

At this point you're free to adjust saturation, apply curves, get fiddly with dynamic range; it's up to you.

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Next time: imaging at the September new moon. Will it be clear? Will it be smoke-free? Will I be local or camped at Lac qui Parle state park?