Roll over to display the Hα signal only. North is to the right 1600x1105
Objects: The great spiral close to home
Coordinates of field center: 1h 34m 30s +30° 43'
Notes: Large galaxy with a very extended dynamic range and complex HII regions.
Telescope and camera: ASA 10', f 3.6; SBIG STL 11000.
Exposure: Luminance 52x8'; Hα 94x8'; R, B 12x10''; G 12x8". About 25 hrs total.
Date: 7 nights on September, October 2010.
Software used: acquisition, CCDsoft, The Sky 6, CCDcommander. Calibration; registration and stacking: CCDstack; processing: PixInsight, Photoshop CS.
Often a galaxy needs some special processing to abate the brightness of the bulge without loosing contrast here, as it would happen with a simple non linear stretching. My technique of choice for galaxies is the creation of an artificial luminance model for the central bulge which is subtracted from the luminance frame. In this way more "space" is opened up at the top of the range for the dynamic expansion. This procedure is applied on the 32 bit sum of the luminance frames. Here, for illustration purpose, I will show it on a strongly stretched image.
Roll over to see the luminance model
There are many ways to create the model, my own favorite is based on the use of the DBE processor of PixInsight LE. DBE is normally used to create a background model for the correction of gradients and other illumination artifacts. In brief, DBE places, either automatically or manually, a grid of sampling points on the image. Then a luminance function fitting the readings on the sample points is created. By careful placing the sampling points and with the proper choice of the parameter describing the smoothness of the computed fitting function (Smoothness 0.0025 in this example), DBE can be used to build a very smooth model. In practice it looks like an heavily low-pass filtered version of the luminance frame without any artifact due to the presence of stars.
The model is subtracted from the luminance frame and the result can be blended with the original luminance to regulate the strength of the desired effect. The roll over shows the subtracted data: the relative brightness of the central bulge is less in the subtracted image and the contrast of the spiral arms is much improved.
Roll over to see the luminance frame after subtraction of the bulge model
Copyright © 2011 by Gimmi Ratto. (February 14, 2011)