Astrophotography
Intro
I've always been interested in looking at the night sky, and also the technical aspects of photography. I've built a good understanding of what makes digital photography work, and how to choose the correct settings for a given environment and shot. However, up until recently I haven't made much of an effort to learn much about astronomy, what we can see in the sky with our own eyes and what is possible to observe with the help of some optics and modern cameras.
I bought myself a telescope a few years ago, and have enjoyed using it to explore the night sky, and the basic model I have is OK for looking at the larger and brighter objects, such as the moon, Jupiter, and Saturn. It is also astonishing how it enables one to see many more stars in the sky, even if they are only visible as tiny point lights. Somehow, simply applying some optical magnification also brings into focus a lot more than can be seen with the naked eye.
But that really begs the question:
What else could we see in the sky, if we had some more help from technology?
Specifically, modern digital cameras can be set up to be more sensitive than our own eyes, and collect photons of light from objects too faint to be seen otherwise. There are other techniques we can apply to these digital images to further enhance the view, such as combining multiple exposures together in software in order to detect and enhance the view of objects which are barely even visible to the camera, let alone our eyes. With these techniques, we can create images of faint objects such as galaxies and nebulae.
There are also some practical problems with trying to take long exposure photographs of the sky, such as the fact that the earth is constantly rotating. In this blog post I am going to explore some solutions to the issues involved in collecting enough photons from distant objects to create a viewable image on a practical engineering level rather than a theoretical or mathematical level.
Light and visibility
In order for us to see anything, our eyes need to receive a certain amount of light which has been emitted by, or reflected off an object. Our own eyes are calibrated to roughly deal with the levels of light present in environments found on Earth, meaning we can see clearly during the day when there is light coming from the Sun, and more or less just enough at night depending on the moon phase or other more terrestrial light sources.
But if we turn our head to look at the sky, we can see some things - the Sun itself (NOTE: it is not advisable to ever look directly at the Sun, it is in fact too bright for our eyes. Precautions must be taken when pointing anything at the Sun itself, it can cause blindness, damage to equipment and start fires), the Sun's light reflecting off the moon and large planets, and some stars which happen to be emitting enough light to be visible.
But that is not the full story. There is a lot more going on in the space around us; objects which are simply not emitting (or reflecting) enough photons of light for us to see. If we were able to somehow adjust or adapt our eyes to be much more sensitive, we would in fact see that the night sky is almost completely filled with various light sources, coming from a vast array of interesting objects. Our eyes unfortunately have a fixed and limited range of sensitivity, so we will have to substitute in some technology to help us see further into the sky.
Deep Space Objects
My aim with this hobby so far has been to try and take photos of both galaxies and nebulae. Without getting into requiring powerful optics (telescopes and lenses), and making use of what I already have, there are 2 objects which are obvious targets for getting started: The Andromeda Galaxy and the North America Nebula. The reasons for choosing these targets are that
They are relatively large in the night sky. They are obviously absolutely huge in absolute terms, but also due to their distance from earth, they make quite large targets in the sky, so can be seen with relatively small and low power optics, such as a medium (135mm) focal length camera lens.
They are relatively bright. Despite this however, they can only be seen with camera equipment and are not generally visible to the naked eye.
Both of these appear to the East-to-South direction, which is convenient for viewing from my south-facing garden.
I will be trying to explore other similar objects in due course, however most of them are too far away (too small) and/or too dim to be detectable with the equipment I have currently on hand.
Digital Cameras
Digital cameras work on the principle of using sensors which convert incoming photons of light into a small electrical charges. The typical image sensor consists of a 2D plane of individual "cells", each of which can build up its own electrical charge depending on the number of photons which have hit it during the exposure. By reading back these electrical charge values as digital numbers, we can interpret the result as an image. We can also control how long we allow each cell to charge for, and therefore how many photons it may be able to capture before we declare the image ready to read. By taking longer exposures, we can capture more photons, and it is possible now to capture more photons in a digital exposure in this way than our own eyes are able to detect.
There is also another parameter in play with digital cameras, that of the "ISO" value, or sensor gain. By using a higher ISO/gain value, we are essentially amplifying the electrical charge signal in the sensor, meaning that the resulting digital value we read out is higher for given number of photons which have arrived. However, this is not a magical solution to being able to see anything we want - as we increase the gain, we also increase the amount of random noise in the sensor. So, whilst we may be able to start to see fainter objects, as the gain increases, the image itself becomes covered in random noise which will eventually completely take over and destroy the image.
We can work around this though. Because the photons we are collecting are arriving in a constant stream at a constant intensity and the noise is random and constantly changing, by averaging many high gain images together, then the noise will start to cancel out, but the incoming photon signal remains constant.
The goal then, for astrophotography is to take:
- Long exposures - so that we can collect enough photons from dim objects that we are able to measure some degree of "constant" signal from that object
- Many exposures - so that the random noise in each exposure can be averaged out
Problems with long exposure
Long exposures are only going to work if the camera and the thing you are looking at are not moving relative to each other. If there is any movement, then the light coming from the object is going to be moving across the image sensor as time passes, turning the image in either a series of streaks of light, or just a big blurred mess.
Unfortunately for astrophotography, the Earth is spinning. This means that if we have a camera in a fixed position on Earth, and point it at the sky, then taking a long exposure is going to result in "star trail" images, and we won't be able to see any details in the objects we are looking at. This is a nice artistic effect, but it is not what we want if we want to marvel at the fine structures present in galaxies and nebulae. Thankfully however, the stars, galaxies and nebulae we can see in the sky are far enough away that we won't have to deal with any perspective or parallax issues, in effect the night sky on any given night is a flat image we can pan around. Note that this is not true for the Sun, moon and planets - these are moving relative to the background image of the stars. Nor is it true if we take exposures at different times of year when the Earth itself has moved significantly in space.
How do we solve this for astrophotography? We need to first tilt the camera at an angle so that we can remove the tilt offset of the Earth's axis of rotation. Then we need to slowly rotate the camera around that same axis, at the same rate as the Earth is spinning.
Mounts
Equatorial
The recommended solution for the first problem of the tilt, is to use an equatorial mount. The amount of tilt we apply on the mount is related to the latitude position we are observing from on Earth. I have an equatorial mount for my telescope, it was included in the package I bought along with its tripod. However, it is manual only, meaning in order to locate and view an object in the sky, I have to move the axes of the mount by hand. The second issue can be solved using electric motors and gears set up to control the equatorial axis of the mount, so that the telescope or camera tracks the apparent motion of the stars in the sky. The other main attraction of using an equatorial mount is that it allows us to reference the positions of objects in the sky using astronomical coordinates which are independent of the observer's location and current date and time.
I could very well have bought an off the shelf motorised equatorial mount for my camera, but I wanted to both avoid the expense (most astronomy equipment worth having is really expensive) and also to really understand how these work and how to locate objects in the sky using astronomical coordinates.
Altazimuth
An alternative to equatorial mount, is an Altazimuth mount - this is what you'd come up with if you were to think about panning and tilting a camera relative to the Earth's surface, such as you would do for film-making or CCTV cameras perhaps. This has the advantage of being somewhat simpler to make, as it consists of a flat turntable (Azimuth) upon which you mount a cradle which can tilt (Altitude, also sometimes called Elevation).
It is possible to convert astronomical coordinates to local Az/Alt coordinates so that we can locate space objects in this coordinate system relative to the Earth's surface at the observer's position. However, as we track an object across the sky, the image will be seen to rotate in the frame, because we are not aligning the camera to the Earth's axis of rotation - the camera is aligned to the Earth's surface.
Fortunately, there is a solution also to the frame rotation problem, we can incline the Altazimuth mount itself by an amount proportional to the observer's latitude and point the Azimuth axis of rotation towards the celestial pole ("North") - this effectively converts the mount into an equatorial mount! The Azimuth axis becomes Right Ascension and the Altitude axis becomes Declination. This works well for most of the positions we might want to photograph, but there may still be alignment and tracking issues with this arrangement when looking at objects close to the celestial poles.
My camera mount
I started out looking at designs for camera mounts which I would be able to make at home, using 3D printed parts and commonly available electronic components.
I was inspired particularly by the mount system designed by "isaac879". However, in the spirit of wanting to understand every detail of the design and working of the device, I actually made my own design from scratch. Also, the design of Isaac's mount is too small for my camera.
I also didn't want to spend too much time writing software for the mount, both on the embedded motor control side and also on the PC side. It is worth noting also at this point that the motors in this design are not only for compensating for the Earth's rotation, but we have control over the absolute direction we wish to point the camera - so we can use PC planetarium and astrophotography software to automatically point the camera at objects of interest.
Designing and building this mount has been a multi-month spare-time endeavour and is documented in much more technical detail in the next blog post.
Setting up a shoot
For an astro shoot, I currently need the following equipment:
- Camera - Sony a7r2 or Nikon D700
- Lens - Nikon 135mm f/3.5 or Nikon 500m f/8
- Mount
- Laptop
- Mains power (for mount power and laptop)
The lens gets mounted on the camera, and the camera mounted on the mount. The mount is oriented such that the RA axis is pointing North (here in the northern hemisphere). The mount is set to its home position.
Connect equipment
The PC connects to the mount over Bluetooth serial and the camera is connected via USB. Everything is then powered on.
Alignment
I use Stellarium to find the object I am going to shoot, and then using the Telescope Remote control, tell the mount to rotate the camera to point to it.
Next, using N.I.N.A. I take an exposure, and using plate solving it tells me how far off the camera alignment is. I can now re-orient the mount on the table, or turn its axes by hand to get as close as possible. It is not strictly necessary to get the target object exactly in the centre of the image - my camera has enough resolution to be able to crop the final image to centre the object. Once tracking is enabled, the object should at least be in the same place in the image for the duration of the shoot.
Starting the shoot
I enable sidereal tracking for the mount, and set up a sequence in N.I.N.A. to take multiple long exposures. I am still trying to find the ideal exposure time and gain settings for shooting, depending on my camera and lens combinations.
Image Processing
Once we have taken all of our exposures, properly aligned and tracked, we need to combine them into a single image, hopefully bringing out the details of the object so that we can marvel at its beauty.
There are a great number of software applications designed to do this and as of right now I am no master in using any of them. I am still trying to capture the ideal exposures to feed into these apps and also still trying to find the ideal processing parameters to get the best results.
Some options for stacking using free software include:
Next Steps
There isn't much of a conclusion to this post, because this is the start of a journey to find out what works and what does not. I also haven't really been rigorous in documenting the processing methods I've tried so far and the results those methods produced.
I will follow up with some photos and explanation of how they were made if I can get some good results that I am happy with. For now, I'm letting this all sink in and planning to put in the hours required to get good at this.