Lunar Imaging

I write these articles for two simple reasons. Firstly, the subject interests me. Secondly, it’s a diary of learning which I can refer back to as required and it’s published on this site in the event anyone else may benefit from that. This article is no different and here I’m going to look at how make a lunar image to be proud of.

According to NASA, the Moon is 238,855 miles (384,400 kilometres) away from Earth. That’s an average figure. The Moon’s orbit around the Earth is not a perfect circle so when it’s at its furthest point, called the apogee, it’s 252,088 miles away and when it’s at its closest point, the perigee, it’s 225,623 miles away. By all reasonable thinking, that’s a long way, and yet we can see it clearly, sometimes even in daylight, given favourable conditions. That’s because, with a diameter of 2159 miles, it’s a fairly substantial object. No matter where we are on Earth, it appears roughly the same size to our eyes. Deceptively, though, the Moon takes up very little space in the sky. It may come as a surprise but if you were to draw an imaginary circle in the sky right around the earth and divide that into 360 equal degrees, the angle of the Moon would only fill half a degree (there are 60 arcminutes in a degree and the Moon is 31 arcminutes wide). It’s not actually as big as our eyes make it seem, and if we try to discern much detail with unaided sight, especially from our light polluted towns and cities, we really are going to struggle to see much more than the largest craters, the dark patches (called mare or the plural maria – latin for “sea” or “seas”) and the lighter patches (highlands).

With reasonably economic camera gear, it’s very possible to see a lot more than this, and it’s a perfect winter project when the skies are clear and not subject to quite the same amount of atmospheric turbulence.

Before I start, I need to talk about focal length. If you want to discern any greater detail in the Moon, then you are going to need some fairly high focal lengths. Let’s first examine what you can see of the Moon with an APS-C camera from 10mm, all the way to 600mm.

I think we can agree then, if my purpose is simply to take a detailed picture of the Moon with standard camera gear, then I first need to have the largest focal length I can get. At 10mm the Moon is just a barely visible speck. And, unfortunately, a standard kit lens is just not going to cut the mustard. 18-55mm is going to be no clearer than the 50mm picture above which renders an image too small to see much detail – it’s really no better than what you can see by eye when viewed at 100%. Even at 600mm I’m still going to have to crop the final picture to really improve on things! This is why those who are serious about astro-photography spend a great deal of extra cash on telescopes and specialised equipment to really push that magnification (a financial black hole I have, for the most part, avoided)

Let’s see what can be done, however, with a modestly priced 600mm zoom lens, a sturdy tripod, a cheap shutter release cable, a clear night, and some free software.

Now, some will say that what I’m about to do requires a tracking mount, sometimes referred to as a star tracker. This is completely untrue, as I hope to demonstrate. A tracking mount, in case you want to know, is a specialised electric motor that counteracts the movement of the Earth; as the Earth turns on its axis, the stars and Moon appear to move across the sky. This is called the sidereal rate. A tracking mount counteracts the sidereal rate and ensures the camera will be pointing at the same objects in the sky for an extended period of time. The more expensive the tracking mount, the better it will track and the longer you can make an exposure. No such luxury here. And some very clever software means that this really isn’t necessary for what I want to do.

The second thing to know is that I’m not just going to be taking one picture of the Moon and working with that. I can greatly improve on clarity and reduce image noise by taking lots of pictures, and “stacking” them, much as I did in this earlier article. By doing this, the software can analyse all the different pictures and by averaging out the data between them all can enhance real detail, and remove camera artefacts such as noise.

So, with a clear night and my camera and 600mm lens secured to a sturdy tripod, I began the process. First things first, obviously, was to locate the Moon. This is where the flip screen on my camera comes into its own and I can very comfortably point the camera pretty much straight up and comfortably position the screen so that I can see what’s going on. Without a flip screen I would be somewhat hunched uncomfortably for this part of the process – of course it’s not a problem if you don’t have a flip screen on your camera, it just means you’ll have to contort yourself a bit.

I didn’t want to centre the Moon in the field of view  – as I take pictures over two minutes the Moon will appear to move across the screen.

Movement across the field of view over two minutes

I wanted to give the Moon room to move but still stay within the frame so I kept the Moon slightly to the left of the frame knowing it would appear to move to the right as the minutes pass. I was in manual everything – both focus and exposure. As you can see, I had plenty of space to play with and even though the Moon has drifted right and down, because I'll be cropping later this doesn't matter.

Now, the Moon reflects sunlight and because the regolith (a fancy word for dirt) of the Moon is a pale grey it reflects a lot of sunlight so consequently the Moon is reasonably bright. I was able to have a reasonably high shutter speed, low ISO and smaller aperture so I sat around 1/125th of a second, ISO 100, f/8. Different lenses and cameras will give different results but on this occasion those setting gave me a perfect histogram  – a nice number of highlight pixels (but none straying into white) and a good amount of black because at those settings there was no hope of discerning any other objects in the sky. If you are interested, I provide an explanation of the histogram here. The low ISO would minimise noise for a cleaner end result and the fast shutter speed negated any need to worry about tiny camera movements. Oh, and for optimum results, I always shoot in RAW.

Then to refine the focus. This is perhaps the most important part to get right and unfortunately it isn’t always as easy as it sounds – in Live View I always zoom in to focus but even on a reasonably sturdy tripod the very act of turning the focus ring causes the image to dance around the screen pretty wildly so it’s a case of making small adjustments to focus, then waiting for the image to settle, then adjusting some more and so on until I’m happy. Also bear in mind I’m looking through approximately 300 miles of Earth’s atmosphere as well which causes the image to appear to shimmer (this turbulence is generally worse in summer than winter but is almost always present to some degree) and this can affect whether I know if I’ve achieved focus or not. Persistence and patience are key here. If you do not get sharp images, the rest of this process is an utter waste of time so if it takes 10 minutes or more to nail it, then so be it. Of course by now the Moon has moved from its position on the left of my image and so I need to recompose before beginning.

As I said above, I was using a shutter release cable to control the picture taking, and by setting the camera to take “continuous” pictures, as long as the button on the release is locked down it will keep taking images until the battery runs out or the memory card fills up. As I was only taking pictures for a few minutes though, that was not going to happen. Really, the more pictures you take, the better the final output will be. There may be a point where it doesn’t really matter because there is no detail left to extract, but I’ve not experimented enough to know where that point is so I just take as many pictures as I can whilst I know the Moon is still in the frame. That worked out at around 250. These are my “light frames”. If I was using a 400mm lens, I would have a bit more room to play with and could take more but the 250 I get are more than enough to work with and get a very pleasing result. If I wanted to reframe and get more pictures, so long as I didn’t change the focal length, that would work out too. For the very best of results I should have taken a number of “dark frames” – these are pictures with exactly the same settings but simply with the lens cap on. This improves the ability of the software I was going to use to reduce camera artefacts. 20+ dark frames is recommended for best results. The more the better really. I didn’t do that on this occasion as it was already 1.30am, on a school night. Tsk.

Once I was done, I got back inside and warmed up with a hot coffee before downloading the images to the computer. I probably should have gone to bed at this point and done the rest of this the following evening but no… and I was hurting the next day.

If you want to follow my methodology precisely you will require two free programs (as far as I am aware, there is no single, all in one solution to this):

1) PIPP – Planetary Imaging PreProcessor
2) Registax6

What follows is not a guide to using either of these programs – the internet is filled with articles written by people much more informed than I and a very quick search will prove fruitful. Neither of these programs is particularly intuitive and so further research will be required on your part, but the gist of these is as follows:

PIPP is used to analyse all the images and sort them by quality and can also crop some of the extraneous black sky so that there is less useless stuff to process in the next step. Crucially, it also centres the Moon perfectly in each cropped image. It takes a few minutes to run through all the images so this is where you can enjoy that coffee you made earlier.

Registax6 then takes those cropped and centred images, and with a bit of configuration, will align and stack them. The stacked image will require sharpening and this can also be done in program.

Finally, and this is where personal taste and artistic interpretation comes in, you may want to edit the image in your favourite image editing software to enhance as you see fit. I always add some contrast to increase the difference between the marie and the highlands and because the Moon is basically a very boring grey, I push the colour balance a little bit towards the yellow and increase overall saturation a bit. The trick here is to keep all edits under control – I don’t want it to look edited – I just want it to look punchy and sharp.

Full stacked image

Let’s now compare a portion of this stacked image I’ve spent a great deal of time and effort creating, with a single snap of the Moon, both at 100% magnification. Was it worth it? Only you can decide if it’s something you wish to explore further.

Single image on the left. Stacked image on the right

Before I bring this article to a close, I briefly want to mention how difficult it is to get satisfactory sharpness in lunar pictures, even with all the effort described here.

From sea level, which was roughly my altitude when these pictures were taken, I was looking at the moon through 300 miles of atmosphere and that's if it's directly overhead. The atmosphere is not a uniform temperature and it is not completely still and both of these factors directly affect how well we can see through it. Light from space is bent by our constantly shifting atmosphere. This is why stars appear to twinkle and this is why despite my very best efforts, lunar pictures are never as sharp as I'd like them to be. Sometimes this turbulence is worse than at other times too, so it's definitely worth doing this multiple times through the year to maximise your chances of getting a night with good conditions and minimum turbulence.

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Cameras, Zoom and Focal Length

Here I want to talk about the different types of camera you can buy and why this matters when it comes to getting close to your subject. This is quite an involved article, so do give it some time and thought and please beware, the internet is awash with well meant but bogus information on this topic and it's taken me a long time to suss it all out for myself. I hope you find this helpful.

Before taking the leap to buying a DSLR or Mirrorless camera (we call these Interchangeable Lens Cameras, or ILCs for short), many people consider using Bridge Cameras, so let’s talk about them first. A Bridge camera is called a bridge camera because it bridges the gap between a compact digital camera (or mobile phone) and an ILC. Typically a bridge camera will have a large zoom (which I’ll talk about soon) but it will still be quite light. Bridge cameras are typically less expensive than ILCs and you can’t change the lens. Another thing to remember about a Bridge Camera is that it has a very small image sensor. Now, once again don’t worry if you don’t know what that means as I’ll explain that later. At this stage you just need to know that it’s the sensor size which is crucially important to how bridge cameras work. Bridge cameras are reasonably easy to use and are often all about their zoom range. You will hear people saying that “this camera has a 20 times zoom” or “50 times zoom”. I have a bridge camera which has an 83 times zoom but what does that actually mean? Well, once again, I will explain what it means later but for now all you need to know is that the bigger the zoom number, the closer the object you’re photographing will appear. Does it mean it will be 83 times closer? Well, sort of, but bear with me and all will become clear.

ILCs are what the serious hobbyists and professional photographers tend to use. They are expensive when compared with Bridge cameras and you have to buy lenses separately so that’s an additional expense. They are also much heavier than Bridge Cameras and when you add on the weight of bigger lenses with more zoom they just get heavier. However, the sensor inside an ILC is much bigger than inside a bridge camera. This means two things, firstly even with a huge lens a ILC will seem to struggle to get as close to your subject as a bridge camera, but the image quality you get will be much, much better.

When it comes to ILCs, Mirrorless cameras can be slightly smaller than DSLRs because, crucially, they do not contain the reflex mirror (hence the name) and they use different viewfinders. It is absolutely not the purpose of this article to explain the technical differences but mirrorless is newer technology and these cameras tend to be (but not always) smaller and lighter than DSLRs - that said, large lenses will always be heavy so the benefits of a scaled down Mirrorless camera may well be lost.

In ILCs there are also different sizes of sensor. Common sizes are "Medium Format", “Full Frame” and “Crop Sensor”. Cameras with Crop Sensors tend to be cheaper than Full Frame or Medium Format cameras and, as the name implies, have a slightly “cropped” or smaller sensor and this keeps the price down, consequently, these are the most popular ILCs for enthusiasts. Generally speaking, the larger the sensor the more expensive the camera. You cannot assume that the “bells and whistles” you get with your regular compact camera – like a swivelling touch screen for example will be present on the DSLR or Mirrorless you are interested in either but as time goes on and manufacturers have to compete with each other these features are slowly creeping into the DSLR market.

To be blunt: DSLRs and Mirrorless are for maximum image quality and Bridge Cameras are for convenience and economy.

Now let’s talk about zoom. Firstly let’s get Digital Zoom out of the way. All this means is that the software in the camera will enlarge part of the picture for you. It basically cuts out (or “crops”) part of the picture to give the impression of having zoomed in when all it’s really done is enlarge part of existing picture. Almost always this is results in very poor image quality (see the example in the last article here) and whilst it’s a handy feature for identifying a far off bird for example, it is usually best avoided for pictures you want to print.

Moving on to Optical Zoom. As I said above, Bridge Cameras tend to advertise themselves as having “86 times zoom” for example. In the world of ILCs they talk in terms of “focal length” which is measured in millimetres. Now these are just two different ways of describing the same thing but this is where things get a little bit complicated. I’ll try to explain it as simply as I can.

When the light shines into any digital camera it passes through the lenses which focus that light onto the sensor. Think of it like a projector: the lens is the projector and the sensor is like the screen. Let’s look at a very simple illustration:

Simplified illustration of light passing through camera lens

This is what happens when we take a picture of a tree. The light from the tree (red, green and blue lines) passes through the lens and projects the image (back to front and upside down) onto the sensor or film inside the camera (the electronics in digital cameras make sure you always see the image the correct way).

Now, in the picture above you can also see several rectangles all of which are representative of different film and sensor sizes found in different cameras and at their approximate scale. Let's take a much closer look at these relative sizes:

Large Format: Any film or sensor larger than 5 inches by 4 inches
Medium Format: Any film or sensor larger than Full Frame but smaller than Large Format.
Full Frame: The same size as 35mm film.
APS-C: Also known as a "crop" sensor
M4/3: Full name is Micro Four Thirds (MFT) and is four times smaller than Full Frame.
B: This is my abbreviation for the sensor found in bridge cameras and mobile phones

Thinking back to the picture of our tree, above. All things being equal, the large sensor can see the whole scene, whereas the smaller sensors can only see part of the scene. So even when everything else is identical, just having a smaller sensor in the camera gives the appearance of having zoomed into the tree - but as you can see, that's not what has happened, you are merely seeing a cropped version of the whole scene. So, depending on the camera you are using, and the size of sensor inside it, the image you can capture of exactly the same scene will vary. In technical jargon, this means that your Field of View (FoV), can vary depending on sensor size.

Now by moving the lens backwards or forwards I can make the image projected onto the sensor even bigger or much smaller. And that’s what optical zoom is. In technical jargon, that's the Angle of View (AoV). By Zooming in, I narrow the Angle of View and I see less of the whole scene, by zooming out I widen the Angle of View and see more of the whole scene. Different lenses will have a different Angle of View, and obviously the Angle of View of the lens will affect the Field of View of the camera: if a lens can see more, then so can the camera sensor. The following diagram illustrates the changing Angle of View with a zoom lens:

So, we can say that the Angle of View is determined solely by the lens you are using and this affects what your camera can see, but the Field of View is determined by both the lens you use (the Angle of View) and the sensor size in your camera.

In Compact cameras, Bridge cameras and Mobile Phones, magnification of the image is talked about in terms of "zoom factor". For example 3x zoom or 20x zoom or even 83x zoom. I posed the question above, does 83 times zoom on a bridge camera actually bring the image 83 times closer? Well that's what we're going to answer now.

Every lens has something called a Focal Length. Simply put, this is the distance, in millimetres, from the point in the lens where all the light from outside comes together (the convergence point) to the surface of the camera sensor where all the light is focused. In a lens with no zoom the focal length is fixed; it cannot move. However, in a lens that can zoom the focal length is variable; it can move. This means that the point of convergence will move back and forward inside of the lens, which has the effect of both lengthening or shortening the focal length and narrowing or widening the Angle of View. This simplified animation shows the process in principle:

A long focal length (say 600mm) gives a very narrow Angle of View (approx 3˚), and thus a lot of magnification. A very short focal length (say 10mm) gives a very wide Angle of View (approx 97˚) and thus very little magnification.

Important note. The focal length of a lens will always be the same: if a 50mm lens is on a crop sensor camera, then the focal length of that lens is still 50mm. If it's on a full frame camera, its focal length remains 50mm. All that changes is the Field of View: the crop sensor camera has a smaller sensor which equals a smaller Field of View. This introduces a concept called Full Frame Equivalency.

Full Frame cameras are basically the standard photographic format. If we deviate from that standard, by having a smaller sensor, it is helpful to create an equivalence so we can imagine how our images will be compared to the standard. To do this we need a "crop factor". The calculation for the crop factor is:

Full-Frame Sensor Diagonal (mm) ÷ Crop Sensor Diagonal (mm) = Crop Factor

For this explanation let's say this calculation comes to 1.6 for our crop sensor camera.

If I put a 50mm lens on my crop sensor camera, then the Full Frame Equivalency calculation is:

50mm x 1.6 = 80mm

Which means the Field of View of my image will look as though it has been taken with an 80mm lens on a full frame camera. Remember, the 50mm focal length of the lens has not changed. All we are doing is working out how the image would look if we pretended we had taken it with a full frame camera. This can work in reverse too. If I like a picture taken with a full frame camera at 16mm then with my crop sensor camera to get the same Field of View I need to use a 10mm lens:

16mm ÷ 1.6 = 10mm

So, with that understood we can finally give an answer to our question and that is: Yes, a bridge camera does magnify an image 83 times, but...

And the "but" here is that it only magnifies 83 times from the shortest focal length available on that camera. So, if you zoom out all the way with your bridge camera, this will allow the camera to see as much as it can - it's largest Field of View. Now if you actually look at the picture you are going to take like this and then look at the same scene with your own eyes, everything is much bigger with your eyes. This is because the smallest focal length on bridge cameras is somewhere around a full frame equivalent of 24mm whereas the focal length of your eyes is about 50mm. The larger the focal length, the larger the image seems. So, what 83 times zoom means is that an image will appear 83 times larger than how it looks at an equivalent focal length of 24mm, not 83 times larger than how you see it. It's really just a big number to draw peoples attention and means very little. Using focal length and millimetres to describe zoom is actually pretty sensible, as it's connected to a standard which is easily understood.

What’s the point in telling you all of this? Well, the point is to know roughly what a focal length is, how it relates to magnification (or optical zoom), how that’s affected by the particular lens and sensor size in the camera and how that will affect your photography. If you want to get close to a subject that is far away (or very small) you are going to need to have a large focal length. Even though bridge cameras are advertised as having say “83 times zoom” that same camera will also have the equivalent focal length information buried somewhere. In the case of that particular bridge camera, the lens only has a real focal length of 357mm but, because the sensor is so small, that gives it a Full Frame Equivalency of 2000mm. So, to get an image to appear as close with a full frame DSLR camera you would have to buy a 2000mm telephoto lens which would be, frankly, unfeasible for most people due to the breathtaking price, outlandish size and staggering weight of such a lens.

Given that, why would anyone buy a DSLR or Mirrorless camera? A couple of reasons. Those small sensors in bridge cameras (Marked "B" in the picture above) make distant things seem really close, but at the expense of image quality which can seem very artificial when looking closely (called "pixel peeping"). Because the lenses are of a small size, they don’t let in as much light as large telephoto lenses and less light means poorer image quality. These lenses are also relatively inexpensive and cheaper lenses usually means you will start to see chromatic aberration - don’t worry about that just now though, I have a separate article dedicated to Chromatic Aberration. More expensive ILCs also come with other features that cheaper cameras don’t, like the ability to take 20 pictures per second for example – when a bird is jumping around quickly, think how useful that would be to capture the perfect moment. The picture quality in DSLRs and Mirrorless is consequently much better and can also allow you to crop the final image to give a better final result.

Now, of course, everything I mentioned above about image quality is relative and depends very much on what you are going to be doing with your image. If you are printing large posters or selling to magazines then image quality is vitally important. If you are sharing amongst friends on social media and for your own use then image quality doesn’t matter quite so much – noisy images and pixelation (which I will speak about in another article) don’t matter all that much when you are looking at small versions of your image, either on the internet or printed out at standard sizes. Many people like to carry a bridge camera instead of carrying binoculars – they can carry a light and reasonably inexpensive camera to record what they see and or to identify something later if it’s very far away. As I said above, both are very good at what they do, and both have their place if you understand the limitations.

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