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Photographic Techniques at Almaden Observatory

While Photoshop is a good general purpose image processing tool,  I discovered several years ago that astronomical images are better processed with specialized tools.  I recently switched to a newer tool PixInsight. Pixinsight offers a number of tools that were specifically selected for astronomic processing.  I give a sample of a typical processing workflow here.

Depending on the target there are several processing strategies I can use.

High Dynamic Range Composition

For objects composed of a wide range of brightness such as M42 or globular clusters it is impossible to capture the entire object in a single length of exposures.  Taking M42 as an example, the Trapezium is properly exposed in a 60 second image, but the nebulous outer regions require exposures of 20 minutes or more.

Image sensors and computer monitors are linear devices.  By that I mean that a pixel value 2 times another value means that the first is twice as bright as the second.  Human eyes are logarithmic devices.  I don't know the exact base, but assuming it is 10 then the a value 2 times a second would be 10 times differnent.  If you take a picture where the stars and bright nebula are just below saturating a sensor (say assigning a value of 50,000 to their intensity) then the dimmer parts of the nebula could have counts that are below 1000 (and maybe closer to 100).  If you are limited to only 256 different intensities (which is what a normal computer monitor can display with your browser) then either the dim parts will be black or the bright parts will burn out.  This is the source of public confusion over the Apollo photographs.

Astronomical image processing software (such as PixInsight) has ways of "stretching" the image so that both the dim and bright portions can be represented at the same time on a linear computer monitor; however, the technique is limited by the linearity of the sensor.  If you expose the image long enough to capture the dim portions then the bright portions will saturate (losing information).  If you adjust your exposure until the bright portions are not saturated then the dim portions will be lost in the sensor's noise.

The solution is to use a PI Tool called High Dynamic Range Composition.  This combines several images taken with varying exposures so every portion of the image is properly exposed.  HDR can do this since it is an entirely mathematical representation of the image that is not limited by either the sensors or the display.  To view the image in the end you will have to stretch the image as before, but now you can work with a representation that covers several orders of magnitude in brightness. As a result you can stretch to display both the very bright and very dim portions of the objects.

Narrowband Astronomy

Many of the leading amateur astrophotographers image in color.  The camera sensor captures just intensity.  To get color they place Red, Green, and Blue filters before the sensor1.  The 3 individual images are then combined with images from a clear filter to form a color image.

I cannot take high quality LRGB images since my local spectra is dominated by junk light from my environs.  To avoid light pollution (and the effects of the moon) I am using primarily narrowband imaging. In narrowband imaging I only look at a much more restricted portion of the spectra.  These images can either be monochrome (i.e. just intensity) or two or three monochrome images captured using different filters can be combined to form a false color image.  The Hubble image of M16 is one of the most famous of all astrophotographs. The Hubble palette uses images from three narrow line filters, S II, H alpha ,and O III, to be the red, green, and blue parts of the image.

For more information see my page on Narrowband Imaging.

Using Photometric Filters to avoid Light Pollution

Narrowband imaging works great on object that glow in specific wavelength such as nebula.  Galaxies and Star Clusters are composed of stars.  Stars emit light over a broad set of wavelengths.  Normally these objects would be photographed with LRGB, but these filters also capture significant background light in my location.  To work around this I am developing a technique using the filters used in Photometry. These are narrower than the standard color filters.  Tests so far indicate they will produce interesting results.

For more information see my page on Using Photometry Filters to Avoid Light Pollution.