Sunday, 10 July 2011

Ha Full Disk - 9th July: Using Flat Fields...

ha full disk b&w by Mark Townley
ha full disk b&w, a photo by Mark Townley on Flickr.
There has been some discussion of late on the Cloudy Nights Solar forum as to the use of flat fields in solar imaging. My DMK31 chip is generally kept spotless in terms of dust and other contaminants, so have no use for a flat field there. However various lenses I use are susceptible to the diffraction effect known as 'Newtons Rings', this manifests itself as a series of light and dark bands across the image caused by interference patterns generated as a result of working in monochromatic light. In addition there is also a fainter (but still there) banded pattern that appears to be etalon related. I knew that a flat field should remove both of these banding artefacts.

The purpose of a flat field is to generate an image that can be 'subtracted' from your final image that removes all artefacts caused by variations in pixel to pixel sensitivity, plus any distortions in the optical path.

Generating a flat field was relatively simple: as I was imaging using a barlow it is possible to image the very centre of the solar disk. Defocussing removes all detail that is present on the solar disk, but all the banding artefacts remain easily visible. An AVI is taken of this which can then be fed into Avistack2 during stacking.

This image is the result: There is no banding visible and the frame as a whole is very evenly illuminated. Bear in mind, this is a 6 pane mosaic also. Fine detail is also greatly enhanced. I'm really pleased with this, and next time the sun cares to return I will also take a dark frame and include this in the processing workflow.

Hope you like it!