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
Depending on the target there are several processing strategies I can use.
For most objects a series of exposures as long as the sky
background will permit will be combined into a single grayscale
image. The typical exposure times that my site supports vary with
the filters. The broadband filters such as Red, Green, and Blue
filters can only be exposed for 10 minutes in a moonless sky before the
sky background starts becoming a problem. A Clear filter is limited to
My current standard exposure for the Narrowband filters is 15
minutes. These can be taken even with a full moon at long as the
moon is > 30 degress (40 for O III) from the target.
Objects that have a wide range of brightnesses benefit from High
Dynamic Range discussed below.
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
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
The solution is to use a PI Tool called High
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.
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.
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.