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Fine Focus in Nebulosity

We’ve covered focusing here a few times before, but I thought it would be worthwhile hitting it one more time with one more video. Previously, I’ve talked about fine-focusing in Nebulosity with a Bahtinov Mask and there is also a movie showing and older version of the tool up in the Tutorials section. So, the question is:

Q: How do I get critical focus in Nebulosity?
If you’ve not read the section on fine-focusing in Nebulosity with a Bahtinov Mask you may want to head on over there for a longer treatment, but the upshot is that I firmly believe you don’t need an auto-focus system to easily reach critical focus. Auto-focus is great if you’re running unattended (be it a remote observatory or having the camera change filters, etc. while you’re sleeping). But, you can hit crisp focus without it and without tearing your hair out. The Bahtinov mask is one way, but without this you can get quick, clear, numeric and graphical feedback on your focus.

I run routinely at f/4 and I don’t even have a motor on my focuser these days. I’ve done this on an f/4 Newt (where the motor really did help) and I currently do this on my Borg 101 f/4 completely manually. It only takes a minute and it’s not something I fret about.

How? Rough focus is obtained with the Frame / Focus command. Click on it and you’ll loop through images. Don’t obsess here and just get the stars to be fairly small. Then, click on Abort, Fine Focus, and then some star in the field. The video below will show the Fine Focus in action. Personally, I pay attention to the HFR (Half Flux Radius) and make small adjustments while watching the graph (allowing for the scope to settle between adjustments). Keep in mind, with a 1s exposure, you’ll always have a bit of variation from frame to frame. As you go towards focus, the HFR will get smaller (graph goes down). Once you go past it, the graph will go up. You can then back off, knowing the sharpest focus you obtained, and using that value as your target. Despite being a fast touch-typist, it took me longer to write this paragraph than it often takes to focus.


PixInsight Tutorials

PixInsight has recently been released for the Mac. While it’s not very Mac-ian in its look and feel (OK, it’s about as non-Mac as you can get), it’s an incredibly powerful program with a lot of high-end routines. It’s also got a built in programming language to let developers build their own tools, leveraging off of what it has built in. Who knows ... perhaps some day the UI will be more refined (keep in mind that OS X is really BSD Unix with a great UI riding on top). Until then, I’ve decided to put up some video tutorials to help others get over the initial hump in using PixInsight. As new tutorials get done, they’ll go up here with this blog entry serving as the master.

1) The Screen Transfer Function: 12/14/08 (click here).
2) Preview windows: 12/14/08 (click here)
3) Removing background gradients (vignetting) with the Automatic Background Extraction tool: 12/14/08 (click here).

PixInsight: Screen Transfer Functions

Your screen has only 8 bits of intensity resolution and your astro-images probably have 16. That’s 256 shades of intensity vs. 65536. Before actually stretching the data, it’s nice to see what’s inside there. PixInsight’s version of the “B” and “W” sliders in programs like Nebulosity is the Screen Transfer Function. It’s a lot more powerful that simple black and white points and it’s at least as nice as having a temporary adjustment layer in Photoshop. Here’s an introduction to them.


PixInsight: Preview Windows

The intensive processing done by PixInsight can take up a lot of CPU time and it’s really nice to be able to quickly see how things will look on a representative portion of the image. PixInsight lets you define any number of Preview images -- portions of your main image -- that you can use for this. This video shows how to make them.

PixInsight: Automatic Background Extraction

We all end up with gradients in our images. Sure, flat-fielding (if you do it well) will take out the optical issues like vignetting, but skyglow gradients will still make it such that you’ll often really want an artificial flat. PixInsight has both an automatic tool and a manual tool for doing this. Here, I go over using the automatic one.