I'm going to start this post with the credits. Usually these are left for the end but the amount of help they've given me earns them top billing.
For using NINA and associated plugins, you could not do better than to watch closely the many tutorials created by Patriot Astrophotography . Chad's presentations are concise and clear; even when he suggests some of the technical portions might be a little on the dull side, they're anything but.
Also well worth viewing are tutorials by the Cuiv, The Lazy Geek . His tutorials are often in demonstration form and tend to be more informal than Patriot's.
You probably already know this great reference for PixInsight, Inside PixInsight by Warren A. Keller. It's been my main resource for starting LRGB processing.
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Although I hated to do it, my first clear night in quite a while was dedicated to playing with NINA's autofocusing (AF).
AF starts at a good (but not necessarily great focus position). From there it racks your focuser out by several "step sizes," takes an image and analyzes it, then racks in a step size and reimages, continuing until it has taken however many steps you specified. Because every geared focuser has some backlash (which has a first guess value of zero distance), that first inward rack will be soaking up that backlash. If your step size is smaller than the backlash no change in focus will result. Through iteration you work you way to producing a nice bowl-shaped hyperbolic curve with the minimum at the point of best focus.
In doing this you will determine two values: step size and backlash. Patriot Astro has a great video showing you how to do this. I was able to determine the values for my FSQ-106-mounted Pegasus FocusCube and was able to get hyperbolic fits with R^2 values uniformly 1.0.
A second convenience that NINA provides is the use of filter offsets; these are basically the distance by which your filters are not parfocal with a reference filter (usually taken as the luminance filter). Using filter offsets allows you to move from filter to filter without having to go through an AF sequence. This saves you time that you can use for collecting light frames.The process of measuring offsets is automated by a plugin called Darks Customs, and this video explains how to use it. So how parfocal are Astrodon LRGB filters? Very!
[Here are the numbers for my FocusCube: step size = 35 and backlash = 450. Filter offsets are luminance = 0 by definition, red = +7 focuser steps, green = -6, blue = -1; The narrowband Baader filter offsets are Ha = -52, O3 = -20, and S2 = -48. For perspective, the FSQ focuser has a travel of about 30mm; this corresponds to about 8000 FocusCube steps. The distance in mm for an offset is therefore equal to
offset in mm = offset in steps * 30mm / 8000 steps = offset in steps * 0.00375
The largest RGB offset is therefore 7 * 0.00375mm or 0.02525mm. That's 1/38th of a millimeter!
Another way to think about the offsets is to compare them to the step size, which can be thought of as the travel needed to change star diameter by 25 to 50 %. An offset of 7 is just 1/5 of the step size, so using the offset in this case may lead to some improvement over assuming parfocal, but it probably won't be much.]
Getting AF ready was one night's chore. The second night was doing a trial image using the Advanced Sequencer. Using filter offsets made for a very simple sequence: Cool the camera, unpark the mount, run AF on the luminance, slew and center on NGC 188, start guiding and shoot 20 frames of each filter in the order of offsets from negative to positive: green, blue, luminance, and red. (This ordering means the focuser only moves in one direction, eliminating the need to deal with backlash.) The triggers for this were AF after a significant change in star width and to dither every third light frame.
Almost everything worked perfectly. But--it was crazy windy and that gave PHD2 fits while trying to recover from dithers. This resulted in a number of frames that had terrible stars, and those triggered AF runs that wasted time. I need to apply a better settling time the next gusty night.
AF worked to perfection. Stars for each filter were near perfect. My target was NGC 188, an open cluster near Polaris. Processing in PI resulted in this image.
I had trouble dealing with the light pollution and the end result is a rather blotchy image. I'm sure flats would have helped a lot.
New moon is coming up, so weather permitting there will be more imaging!
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