Pre-processing:
Stellarium/Cartes du Ciel (CdC).
Stellarium is a free open source planetarium for the PC.
It has a database of over 210 million stars as well as illustrations of constellations and images of nebulae.
It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.
It's also possible to point to a target and the telescope will slew to that target, however personally I use SGP (see below) to perform this.
I also use CdC, but I find its not so intuitive and I just use it to confirm where the telescope is slewing to on a particular target.
AstroPlanner:
AstroPlanner is a commercial software application which is useful for observation planning and logging,
As well as details about the chosen target, it shows the best time of year and time of night to image it.
It also shows the position of the moon.
Both of the above programs can be run on my main PC in the house.
When the actual imaging is taking place and to enable me to be tucked up in bed or in a nice warm house, I use two software packages listed below, one commercial (SGP) and one free (PHD2).
These programs need to be run on the NUC in the observatory.
Sequence Generator Pro (SGP):
SGP is a commercially available software and one of the most capable and complete image capture suites available on the market. It controls in one place all the actions one needs to perform in order to take 'stunning' images.
It’s written by two very clever programmers Ken & Jared, both living in Austin Texas and used to control the camera, filters, focuser, mount (telescope), plate solving, guiding and a lot more.
Plate solving is a technique to initially image a portion of the sky (called plates in the old film days) then by means of SGP work out exactly where the telescope is pointing.
Using this and the known position of the target, adjust the telescope to point exactly at the target.
One of the options which is well worth the money is to be able to select a target and then define the area one wishes to image. One can even select multiple images to cover a large target. SGP will then generate a sequence which will automate the initial slewing to the target, then by means of plate solving get you accurately onto the target, night after night.
In astrophotography there is no such thing as auto-focus, so SGP combined with the focuser (stepper motor controlling the telescope’s focus knob) takes a series of images moving the focuser a small amount each time. By analysing the width of the all the stars (HFR) in each image, correct focus is judged when the width is at a minimum.
As focus changes with temperature during the night, SGP will rerun its focus routine on a nominated change in temperature.
SGP uses the sequence (or sequences) to automatically (i.e. unattended) take a series of images of the chosen target, perhaps multiple targets in one night with different filters.
Push Here Dummy (PHD2):
The telescope mount is constructed so that if correctly set up, it’s motors will track stars as they apparently move across the sky.
However equipment made for the amateur, however well-made, will never be able to track stars perfectly over a long period. We are talking about very very tight tolerances here.
Astrophotographers talk about angle of measurements in arc-seconds.
Using my equipment and on a good still night, PHD2 can often achieve guiding to within ± 1 arc-sec.
What PHD2 does is use a second camera in a guide-scope. This locks onto a nearby star, then if the star drifts slightly over time PHD2 will make minute corrections to the mount’s motors.
It's main screen shows various data about the guide star the software has locked onto.
The view of the star is through the guide-scope.
Post Processing:
PixInsight:
PixInsight is described as a modular, open-
In normal speak, its a framework for a series of scripts which will perform various post processing functions.
Despite taking 20 minute long exposures the results are almost totally black (see first image below).
Only after all post processing is completed, the detail finally shows through (second image).
(And of course, one has to repeat this for each RGB or NB filter).
The first image above, contains additional information introduced by the CCD camera (called noise) which is unwanted.
Also not all the points of light in the first image above are stars, some are what is known as 'hot pixels'. These are individual pixels which look much brighter than they should and are normal in long exposures.
Pixinsight uses algorithms to remove noise thereby cleaning up the image, and also removing other blemishes caused by stray cosmic rays entering the camera, satellite & aircraft trails.
To assist in increasing the signal (stars) and reducing noise, numerous images are taken (the more the better).
However each image may not be perfectly aligned with its predessors, and Pixinsight will also align the images into one final image. But the image still looks totally black and further post processing is needed.
Pixinsight is capable of this, but I am still learning the product and so far I have completed the remainder of the processing in Photoshop.
Photoshop:
The first image below, shows how the histogram is hard up aganst the left-hand side indicating that all the pixels in the images are dark.
But the information is there, it just has to be teased out by moving (& stretching) the histogram towards the right and away from the left-hand edge As shown in the second image below after three stretches.
It's better to perform several small stretches than one large one, as one has to be careful about increasing noise in the final image.
The second image shows there is now a gap to the left-hand side of the histogram, and would normally be removed by moving the black point to the right.
But as the histogram base is wider, the detail is now starting to show.
(The second smaller peak is caused by the varying black border, which is a left-over from the alignment process, and is removed by cropping. It also shows a small rotation of the aligned images, which probably means my polar-alignment is slightly out).
Different people will have different techniques for completing the post-processing process, and to be honest if I edited the same image twice I would complete it in different ways.
Its just a matter of trial & error and see what looks good.
Some time ago I purchased the Nik Collection from Google (now free), and it contains Silver Efex Pro.
This is a Photoshop plug-in which is especially good at enhancing black & white images.
I use it to finally tweek the mono image, and I find it works very well.
Finally one need to combine the edited results from all the wide or narrow band filters into one full colour image. Two Photoshop actions are very useful and very modestly priced:
- Annie's Astro Actions by Anna Morris.
- Astronomy Tools by Noel Carboni.