Hey there!
I'm Dave Hearn and I'm the Director of Kissimmee Park Observatory.
In this Star Hopping "Extra", we're going to continue on our Basic Astrophotography
series, and show you how to perform Basic Image Processing: the procedure to extract
a beautiful color astrophoto from your raw frames you captured through your telescope.
More about all this great stuff right after this - stay tuned.
<Intro Theme>
Hey Hello Hi and welcome to Episode 53 of Star
Hopping!
I'm Dave Hearn, and I'm just enthralled to be your host.
In this series of programs we'll show you the most beautiful sights in the night sky,
and explain exactly how to find them with your binoculars or telescope.
So I'm in that situation again where I just need to wait a little bit for more interesting
deep sky objects to rise.
So while that occurs, I've created another Star Hopping "Extra" episode, to expand
our astrophotography series.
Tonight I'll discuss Basic Image Processing, the process that amateur astrophotographers
go through, in order to extract beautiful astrophotos from the raw images that we collect
with the telescope, during a imaging session.
So let's talk about the basic steps that are used in our imaging workflow to capture
and digitally process an astrophoto.
First we Capture multiple subframe images.
Next we align the subframes on the star pattern within them.
Then we Stack the Subframes.
Next we Process the image stack.
And finally we Finish the image with a general image processing app.
So let's start out with Capturing the subframes.
In the earlier episodes of the astrophotography series, I've discussed different methods
to capture images of the night sky.
In all cases, this imaging process involves capturing a series of digital images of the
night sky.
Each one of these images take several minutes to actually expose, in order to capture a
sufficient image of the deep sky object.
And with all cameras, even CCD cameras, long exposure images generate a lot of electronic
noise.
If you would use only one of the images, known as a sub frame, the image would not be smooth
and would have a lot of noise in it.
So to come back and remove this noise, we combine all of the images that we took, and
aligned them on the stars in the image.
This process is known as "stacking".
When we align all the images and then combine them, the noise is smoothed out and is generalized
across the whole image.
This results in a haze that appears over the image when it is first stacked.
This undesirable artifact can now be removed by digital image processing.
My favorite application that I use for capturing and stacking the subframes is the popular
imaging application "Nebulosity", by Craig Stark.
Nebulosity automates your capture process - it opens and closes the camera shutter for
a preset exposure time, and saves off each sub frame with an ordered image title.
It repeats this process for the number of frames that you want to capture.
Nebulosity also allows you to align all of the subframes on specific stars.
You select a star on the image nearby the deep sky object subject by clicking on it.
Then you move to the next subframe and do the same, progressing through all subframes
in the stack.
Sometimes there is movement in a subframe, which results in star trails or just oblong
stars.
You can just skip or leave out those bad frames, so that they don't degrade the quality of
the image.
Of course you reduce the total integrated exposure time when you do that, so it's
better to have good tracking throughout the series of subframes.
Then Nebulosity automatically aligns all of the images on your selected star and saves
it as a Stack.
If there has been any slight drift during the image series, such as declination drift
due to poor polar alignment, the image will fade out on the edges.
These edges need to be cropped out, as it will affect your ability to process the stack
in the next procedure.
So Nebulosity also has a basic crop function to clip off these drifted areas.
So now we have a cropped and aligned image stack.
It may look good on its own, or possibly only the brightest part of the galaxy or nebula
is showing at this point.
Now we need to Process the image stack to maximize the image.
I use a different application for processing, because Nebulosity isn't powerful enough in
the processing tools area.
What I use is "StarTools".
There are many different processing packages out there with different capabilities.
Since I'm a Mac Guy, I am really only familiar with apps on MacOS.
One other great option on the Mac and Windows, is PixInsight, but I find it expensive and
fairly complicated with a large learning curve.
StarTools is cheaper and is more straightforward once you practice a bit.
Now I can't go into specific processing steps because that would keep us here for
at least a few hours!
But suffice it to say that you'll need to go through these 6 main image processing steps:
The first is Digital Development, where you amplify the image.
This brings out faint details but increases the noise as well.
Then you'll need to perform a process where you increase contrast, and reduce background
sky brightness.
This is called "Stretching" the image's dynamic range.
It may be necessary to repeat the first two steps several times, using different tools
to maximize the image.
This varies with there image processing package you are using.
At this point I usually adjust the image's color, to remove any background tint and get
a black sky with white stars.
This usually results in a true-to-color image of the deep sky object itself.
You may also perform some sharpening to increase contrast in edges within the deep sky object
image.
Lastly you'll need to perform general image noise reduction to remove all traces of noise
which causes graininess in the image.
This step in the processing workflow can be time consuming, and will vary with each astrophoto.
But as you get your imaging capture process streamlined and automated, the image processing
effort becomes more normalized over time.
It's definitely a learning process but it's very rewarding
when you get good results.
So now you have an image that looks pretty good but it doesn't quite "pop".
That's where a general image processing app comes in.
On the Mac I use Apple Aperture, which is getting replaced by the not nearly as good
"Photos" app.
But there are other good ones - Adobe Lightroom is very popular and runs on most platforms.
In any case, a photo finishing app can be used to perform the following improvements:
Final sharpening to bring out the details.
Final color balance and color saturation.
Adjusting the Black Level to get a reasonably black sky.
You have to be careful here because nearly every sky has a little bit of Skyglow in it.
A totally black sky looks a little artificial.
This is an area that you have to practice in order to get a good feel for it.
Then we do Final cropping and centering of the image for composition.
So now we have an astrophoto to share and post on Social Media!
So we discussed the main steps in astrophoto image processing.
They were:
Capture multiple subframes.
Align the subframes on the star pattern.
Stack the Subframes.
Process the Image Stack.
Finish the image.
That's basically the workflow for Digital Image Processing of an Astrophoto.
So that does it for this week's episode.
You can find the show notes on our website at StarHopping.org/SH053, where you can comment
and leave any questions that you may have.
You can also contact me personally on Twitter @StarHoppingMan, where I'd love to help
you with any astronomy or observing questions.
So feel free to reach out with your questions - I'm here to help.
Well thank you very much for joining me here, and I'll see you next time with more Star
Hopping tips
and tricks.
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