Pseudo-HDR Deep-sky Photography



There are many great amateur photos of the Great Nebula in Orion to be found everywhere in astronomy forums. The below isn't one of the greatest but about what I can accomplish this time of the year (Early February) when urban light pollution and unsteady air conspire against my astrophotographic ambitions:


Figure 1: 20 images of 10s at ISO6400 stacked in Registax and contrast enhanced in PhotoImpact.


Because of the lousy seeing usually experienced in the winter time under city skies, exposure times have to be kept reasonably short and ISO correspondingly high. But it is fun to experiment with deep-sky astrophotography from the city anyway, and here then was the first clear skies for weeks and thus an opportunity to test my newly acquired Pentax K-5 for its astrophoto- and high ISO qualities.


The optics used for all images shown here was my old 4" f/10 Vixen Polaris R-100L Newtonian reflector on a Meade LXD75 equatorial mount.


So, I got my M43 photo - but as with many photographs of this nebula the inner core around the Trapezium is overexposed as the price one pays to get more of the fainter outer nebula. Stacking this type of images first of all improves the signal-to-noise ratio but it also does improve upon the limited dynamic range of normal digital images.


But not enough for my taste for a very high dynamic range like M43.


Next, therefore, I produced a view of the central quadruple star, the Trapezium and surroundings at a lower exposure value:


Figure 2: Contrast enhanced stack (tighter crop) of 10 images of 4 seconds at ISO 1600


As always, the stacked images themselves are a bit dull in appearance - but they aren't! They contain much more subtle light and colour that can be enhanced in a few, fairly simple steps that I have described here.


One day, I shall try to make a genuine HDR-image based upon series of images of this type. But for now, I just blended these two images to get this:


Figure 3: Blend of the two stacks above using layers and "Luminosity Only" - plus a bit more of fiddling with curves and levels.


What I have done is that I made a sandwich of two layers in PhotoImpact and blended the two images in "Luminosity Only" mode - a bit similar to the way some astrophotographers produce images using LRGB-technique or "luminance layering"


I think the reulsts of this first attempt are encouraging and that the technique could be worthwhile pursuing as soon as weather- and observing conditions improve.






Copyright 2012 - Steen G. Bruun