Sunday, 29 January 2012

Ha Full Disk - 28th January (Flats and Smart Sharpen)

full disk colour 2 by Mark Townley
full disk colour 2, a photo by Mark Townley on Flickr.

There are days when what you see through the eyepeice and what is seen on the camera preview screen just doesn't equate with what comes out of the software at the end, and saturday was one of those days! The suns limb could be seen to have rolling waves sloshing around it's limb, and I nearly didn't bother to image for this reason, i'm glad I did because this is one of the best image i've got in weeks!

I've been using the unfocussed flat for some time now on my solar images, sometimes with good results, sometimes not so good. Something I now believe is fundamentally crucial to their effectiveness is that the flat needs to be taken exactly mid disk to avoid any gradients or banding in the resultant images. When making a mosaic such as this image which is composed of 6 panes, gradients can wreak havoc in uneven illumination of the final composition. So, if you're using flats with your solar images make sure they're taken dead centre of the disk!

This is my very first image made using the smart sharpen technique instead of wavelets. Recently on Stephen Ramsdens excellent forum (link opposite) professional astrophotographers Chris Schur and Ken Crawford have been advocating the use of an iterative approach with the unsharp mask and smart sharpen functions in Photoshop as opposed to using wavelets in registax. As you are probably aware wavelets are a invaluable tool used by many in numerous forms of astronomical imaging, however they do have a nasty side effect if used over zealouslly; they often produce a halo effect around features such as prominences. It has taken me some considerable time to find a setting I am happy with in terms of results - this seems to be dependant on a number of factors including the focal length the system is working at, pixel size of the sensor, and to a certain extent how the luminosity is spread out through the 'curve' of the image. However the results are very positive - the finer detail on the sun seems much easier to see, giving it a hairy or fluffy like appearance.

Hopefully the high pressure that is over the UK at the moment will hold and I will be able to image again next weekend. The sun is slowly but surely getting higher in the sky now and shouldn't be too much longer when my viewing window extends enough to allow me to image at a variety of wavelengths and a range of image scales!