Astro photography for beginners
Introduction
Astrophotography is a discipline dedicated to photographing the night sky in all its splendor.
For this exercise to be successful, there are a series of environmental conditions, equipment demands and photographic operational- plus post-production techniques that we will explore in broad lines in in this article.
Thus, people who are starting out in the discipline can have a general idea on how to achieve satisfying results and, in addition, can prepare properly for a night excursion taking the necessary precautions for their greater protection and comfort.
Environment
In Chile we are still fortunate enough to find fairly dark locations relatively close to the big cities, an essential condition for astrophotography.
Although there are many people in other latitudes whom have to cope with far worse conditions than ours, nothing beats a location with low light pollution to capture outstanding sky images.
In Santiago we can access relatively dark skies in the Cajón de Maipo, at distances between 90 (San José de Maipo) and 180 km (El Yeso Reservoir Lake), respectively, while those traveling to places like Vicuña – in the Elqui Valley – or better yet, to San Pedro de Atacama, can still find clear skies with very low levels of light pollution.
It is no coincidence that Chile’s second to fourth regions concentrate a large number of astronomical observatories, such as Tololo, Paranal, ALMA and E-ELT, among others.
The ideal nights are clear and moonless, while the best locations are as high up as possible.
My locations in the Cajón de Maipo are all well over 2.000 meters, above the flying dust and as far away as possible from major potential sources of light pollution.
The equipment
Although more sophisticated cameras offer certain operational advantages over the more basic ones, any recent DSLR is more than adequate for astrophotography.
In terms of lenses, the most appropriate are wide angles between 12 and 28mm, since they allow for longer exposures – up to 30 seconds – without much appreciable deformation of the stars into lines, so-called “trailing”, which results from the rotation of mother Earth.
As we increase focal length, the exposure times must become shorter and shorter. With a 300mm f/4 lens, for example, we ought not exceed 2,5 seconds to keep the stars more or less rounded, in my experience.
The faster a lens, the more suitable for our purposes, since it allows for shorter exposures, without even considering that it also helps to maintain ISO within an acceptable range.
On the D7100, I prefer not to exceed ISO 1600 to avoid that noise – and its eventual suppression – negatively affects image quality (IQ).
Thus, fixed focal lenses with a maximum aperture of between f/1.4 and f/2.8 are considered ideal, even though lenses of up to f/4 are still suitable enough.
Accordingly, the typical 18-55mm f/3.5-5.6 zoom that comes with almost all popular cameras today, is appropriate, at least at its shortest focal length; i.e., 18mm f/3.5.
Due to the long exposures, a stable, quality tripod is an inevitable piece in the set-up of the astro photographer.
Although relatively inexpensive cameras can be used on lighter tripods, as the weight of the equipment increases, so does the demand that is placed on the support system.
And with increased weight capacity, unfortunately, goes steeply increased price.
While a decent “supermarket” tripod can be had for as little as USD 45, for a professional model that supports 5 kilos or more, you may add a cero to that number.
Keep in mind though, that this is one of those rare one-time investments that last a life time.
Luckily, there are a number ways to avoid that the vibrations in the support platform – resulting from the lifting of the mirror (mirror slap) and/or the manipulation of the camera – negatively affect image sharpness.
First, the release modes self-timer (delay) or the "mirror up" or "mirror lock" (MUP) mode, which lifts the mirror a couple of seconds before firing.
Then there is Interval timer and Time Lapse shooting, which can typically be set to delay as well.
Finally with a IR remote control you may get options such as Delayed remote and remote MUP, which allow to fire the camera without even touching it.
The USD 20 ML-L3 IR control for Nikon cameras up to the D7K series is particularly attractive for our purposes.
A IR remote is an affordable accessory available for most popular cameras of other brands as well, and allows you to run several of the mentioned modes.
The availability of one or more of these modes depends on the model of the camera, but in most at least the Self-timer (delay) and Interval Timer modes ought to be available, even in the absence of a remote control.
Camera configuration
For astrophotography the camera is set to all manual: mode dial on M, manual focus and manual – or rather experimental – exposure.
Similarly, the ISO auto must be turned off, and it is recommended to also deactivate the noise reduction for high ISO and long exposures, in order to achieve the best possible detail rendition.
It is also highly recommended to photograph in RAW format (NEF, CRW/CR2, etc.), although for those who have no experience with this format or do not have the right processing software, now is not the time to start experimenting.
I suggest – if your camera allows it – to choose RAW + JPG maximum quality, although it demands a lot more space on the memory card than JPG alone.
Some cameras offer only RAW + basic JPG. In these cases it is preferable to choose large, high quality JPG, except if you already have experience in processing RAW files.
It is essential you know how to modify the camera's settings; and even so, I still recommend setting configuration as indicated, before leaving for the field.
The complete dark adaptation of the human eye takes about half an hour and is easily spoiled, which is why we always try to avoid the use artificial light on location.
If you cannot locate the main configuration buttons of the camera from memory and on touch, it can be very difficult to make mayor modifications in the dark.
However, minor and common modifications, such as shooting speed or aperture, for example, can be observed in the camera's viewfinder or – only when strictly necessary – on the monitor, and executed with the control dial or dials. Therefore, these mods ought not present major challenges at night.
Let's Shoot the Stars!
First we have to find – in the light of vehicle headlights or a flashlight – a flat and safe space on the location, identifying and avoiding any obstacle that we might bump into in the dark, to then set up the tripod and mount the camera.
Once set up, all sources of artificial light should be turned off, avoiding their use during the rest of the session.
A less intrusive way to illuminate the camera, if necessary, is a cell phone screen - as long as it is not too bright.
The use of any other means of lighting should be avoided as much as possible and, in the case of necessity, we must always illuminate downwards so as not to affect the night vision of others present on the location.
If you want to protect your night vision using your cell phone, there is an Android application that applies a red filter to the screen which I consider very useful. You can download it here.
To confirm correct focus, it is recommended to use direct view or live view mode, with the monitor at its maximum magnification, an operation that ought be repeated a few times throughout the session, to ensure that the camera maintains focus as initially set.
If the focus ring is very loose, it may be advisable to apply a piece of adhesive tape to ensure it stays in position.
Keep in mind that it is not enough to leave the lens at its infinity lock-point, because, unlike most manual lenses, AF lenses may manually focus past the point of maximum sharpness at infinity.
In astrophotography we use lenses typically at their maximum aperture, or eventually at up to two stops below their maximum aperture in the case of very fast lenses (f/1.4 - f/2) to achieve the best image quality they can provide.
Using a wide-angle lens, I recommend as starting point f/4, ISO 1600 and a 20 second exposure, so that we can establish the correct settings while leaving room to lengthen or shorten the exposure, open the diaphragm (if not already at max aperture) or raise ISO – although the latter only when absolutely necessary.
It is good to keep in mind that, in my experience, we can under expose down to 2 EV without this presenting major quality problems, especially when we work in RAW. The under exposure can be easily corrected in post-production and gives us a little more room for maneuvering when necessary.
Although the phrase "Eppur si muove" – However, it moves – is incorrectly attributed to Galileo Galilei, the claim about the rotation of the Earth is correct, obviously, although many people do not realize its amazing velocity.
Thus, to avoid the deformation of the stars or so-called "trailing" we should always try to achieve the shortest possible exposures, without compromising the image quality too much with a very high ISO.
I recommend using the self-timer with delay mode and, if the camera allows it, series not inferior to 3 takes; I usually take between 6 and the maximum allowed by the camera (9).
The latter is recommended not only because noticeable differences in sharpness can occur for a number reasons – such as air turbulence, for example – but also because there is a post-production technique called stacking that combines a series of images into one in order to considerably reduce random noise in the individual takes.
Photoshop offers stacking in the layer menu> smart objects, while there are also several free programs available for this purpose.
The best known is Deep Sky Stacker (DSS), which can process RAW, JPG, TIFF, BMP and FITS images automatically, saving the result in different formats as well, including 16-bit TIFF.
The program produces excellent results, although it can be a little slow if many images or heavy images are processed. You can download it here.
For more information about stacking in Photoshop, check out this tutorial.
If you have a remote control, and depending on the model of the camera, you can also use the mirror up mode, even though most cameras cannot shoot in series in combination with MUP.
In the absence of a remote control, this mode is recommended only in combination with long (100mm or more) and/or heavy lenses.
Recommended initial configuration
Mode Dial: M(anual)
Focus Mode: Manual (once focused on a bright sky object)
Shooting Mode: Self timer
Self timer delay: 2-5 seconds or whatever the camera allows
Noise Reduction (ISO and long exposure): Off
Lens type: wide angle
Aperture: maximum
Fast lenses (f/1.4 to f/2): between ½ and 2 stops below maximum aperture (depending on conditions)
ISO: ISO 1600
Initial exposure time: 10-20 seconds
IR remote control (opcional)
Final recommendations
In the early hours of the morning, the temperature can drop sharply, even in summer and especially at altitude.
Therefore, it is very important to have the appropriate clothing and footwear available which, at least in spring and fall, should include a hat and gloves – even if you may think that's exaggerated.
Do not forget to bring some snacks, plenty of water and eventually a thermos with tea or coffee; once the temperature drops, you will appreciate a hot drink.
Also keep in mind that alcohol is the worst possible companion on outings such as these!
Security is Key!
Humans have considerable limitations when it comes to seeing in the dark, which makes it essential to take the necessary precautions to avoid accidents.
It is key to make an initial inspection of the location where the activity takes place and to remember where the eventual obstacles and/or the external limits of the site are to avoid falls or other mishaps.
Always move with purpose, care and without hurry and, in case that you are not sure where you are, turn on a light; it is far preferable to temporarily lose your night vision than to risk a fall or worse.
For this same reason, you need to have a light source handy always; if your cell phone does not have a "flashlight" function, you should bring one; even a small flashlight will do.
I hope you will find this guide useful.
I have placed some terms in parentheses to represent the differences in terminology of the Nikon and Canon manuals and to facilitate their search.
Other brands may use different terms to refer to operations or configuration options.