söndag 27 november 2016

Removing hot pixels in a stacked image

Sometimes even an aggressive hot pixel filter won't remove all hot pixels. Here's a technique that can remove any residual hot pixels in a final stacked image. I use PixInsight's Morphological Transformation with a starmask to remove these nuisances.
Here's a crop of an image, showing what I'm talking about. The image was taken with a DSLR and consists of a stack of 10 sub frames exposed for 15 minutes each at ISO 800. My camera, a Pentax K20D, is getting old, and I always have lots of hot pixels in my images. Calibration removes most, but frequently a number remain after image integration. The technique which I describe here will dim the remaining pixels.
hot pixels after stacking
I start with making a Luminance copy of the image in its linear state, and apply STF to this grayscale image. Then I use the StarMask tool with a low value for Scale (typically 3 works ok) and a noise threshold of 0.5 (to be experimented with). I decrease large-scale, small-scale and compensation (1, 0, 1) and smoothness (about 6 - 8). Then apply the mask tool to the luminance copy. It may be necessary to tweak the parameters. No stars should be in the "Star-Mask" that is created.
When I'm satisfied, I apply the mask to the original colour image.
For pixel removal I use Morphological Transformation with Morphological Median as operator. Amount to about 0.5, iterations to 4 - 5, and Structuring Element to 9 pixels with a circular pattern.
Apply the tool to the image. If hot pixels of a certain colour remain, I split the RGB channels and use the channel that has the remaining hot pixels to repeat the process. The result is this.
Same crop after hot pixel removal
Further tweaking of the star mask and morphology parameters can improve this result even more, of course.

söndag 20 november 2016

First steps in guiding

Finally I have taken the plunge and invested in a guiding setup. I decided for the SkyWatcher ST80 scope with ZWO ASI120MM camera. The camera is the older USB2 version.
As I don't want to take my laptop out in the field, I intend to use a RaspberryPi as a guiding computer.
The last couple of days and nights, I have been trying to get this to work. My configuration at the moment is this:
ASI120MM connected to RaspberryPi, running Ubuntu Mate as an operating system.
The Pi also holds an INDI server and the lin_guider software. The camera connects to the Pi and receives guiding pulses which it sends on to the mount (SW AZ-EQ6 GT) via the ST4 port.
Installation was quite straightforward, despite warnings that the camera driver may not be stable. Setting the exposure time to 1 sec in Lin_guider seems to work fine though.
Last night, despite partial cloud cover, I was able to test the guiding, and it worked fine.
Lin_guider connected to the camera, and frames started to flow in. Focussing was a bit of a hassle. I had to take my laptop out (despite the dew), and because there is no live view, it took a while to get focus right. In the end I had my setup guiding on Vega (which was grossly overexposed at any gain setting), and later on a nearby much fainter star. This worked fine until the stars disappeared behind my neighbour's trees and clouds rolled in.
I haven't tried imaging yet, and I still have to figure out the best settings for PID gain, but so far so good.