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Ant Smith
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Photography

Shooting for the moon...

...my self-directed introduction to astrophotography

I've always loved the rare opportunity to have a peep through a telescope. I say rare because most people I've known that had telescopes, didn't really know how to use them; or only had little toy 'scopes. It's the kind of hobby a lot of people buy into, then quickly give up after realising it's not as straightforward, or as cheap, a hobby as they may have imagined. So even though the idea of astrophotography was always appealing, I had this sense that it was somehow unreachable; the barrier to entry just too great.

Then my wife bought me a 'scope for my (early) retirement birthday present - a Sky Watcher Star Travel 102D (a so-called 'short tube refractor' giving 500mm focal length at f/5). So whatever that 'barrier to entry' might be, I just got the excuse to cross it!

I quickly found that there's a real community of engaged amateur astronomers and astrophotographers. I guess like the early days of photography itself, there's an army of keen enthusiasts out there sharing knowledge and creating solutions. On the hardware side of things, the market certainly offers everything needed, but there aren't really any 'complete solution providers'. Telescope vendors only speak vaguely about the camera side of things, and the world of photography is just too broad to help in getting to the heart of the matter - so that means getting up and running really isn't straightforward.

Which is why I thought I would write up my own experiences after owning a 'scope for some 6 months now. Without doubt, what I'm trying to achieve and the kit I am using will be very different to that of any readers - but hopefully, there will be bits and pieces along the way that elucidates 'something' useful...

With much to learn, and only limited opportunities to do so (a few hours for only a few nights month by month) I felt I had to take a disciplined approach - I needed to be sure that every opportunity to shoot would deliver, if not a decent image, at least a bite of learning. So I set on 3 main objectives:

  1. Become familiar with the scope and the tracking mount it came with

  2. Learn how to capture 'simple' images

  3. Create an initial deep sky stack shot (I want to shoot some nebulae)

For the first couple of months (from January into March) we didn't have so many good night skies. Or I wasn't in the habit of looking for them. Or I wasn't awake and sober enough to make use of them. Certainly I wasn't experienced enough to understand how fundamentally different astrophotography is; how actually the opportunity to photograph what you want is pretty limited. Which meant the first few months were about just using the scope, a bit. My first view of moon craters was pretty enthralling, and I loved looking at the tiny dots of two of Jupiter's moons in the same field as Jupiter itself when my brother came visiting and impelled me to set the 'scope up. It was a slow start for me, but at least I got used to calibrating the tracking mount and assembling and focusing the scope. It also took me nigh on three months to discern and acquire everything needed to connect the camera.

Three months! Whatever I read and whoever I asked I was told the same thing - "You need a T mount, oh and an eyepiece projection tube depending on what type of photography you want to do.

I knew of three ways in which I could dabble in this field: I could simply use my camera and its telephoto lens; I could use the telescope attached to the camera without an eyepiece, as though it were just a cheap telephoto lens (prime focus); or I could use the camera to peer through the telescope's eyepiece (eyepiece projection).

With a relatively short refractor (short for a telescope at 500mm) prime focus is of little help to me. I have a 500mm lens for the camera so that's always going to be optically superior to any telescope (I think). I haven't used the lens for astrophotography for two reasons: even 500mm isn't long enough for lunar or planetary shots; and, I never had a 'sky tracking mount' for the camera. The Sky Watcher came with an AZGoto tracking mount which I can use with the camera (instead of the 'scope) if I want to shoot at 500mm. If the scope was much longer then prime focus would be more useful (and so might be for you) but the greater magnification from eyepiece projection is what I need and is therefore what I'm mostly talking about here.

But hang on, I'm saying 500mm isn't long enough to shoot the moon when there's loads of info online about how to shoot the moon with even 200 or 300 mm lenses... Well, if you're only looking at images on-screen you can get away with that since most screens are no more than 1080px tall, giving plenty of scope for cropping right into the image. But a 1080px image will only print at around three and a half inches. In fact I did get a shot of the full moon at 1000px diameter on January 11th 2009 using a 300mm f/2.8 lens with a matched 2x teleconverter on an APS-C format sensor (effective 900mm focal length); such a beautiful but absolutely tiny image! I never bothered again since I just didn't value an image that could only print at a little over 3 inch.

So if you have a short-refractor and you want to make big pictures, you're going to need eyepiece projection from the 'scope to the camera - well look, there's way more to it than that. Prime focus can use additional bits of kit (like field reducers, flatteners and barlows) and you can even introduce camera lenses with eyepiece projection (afocal photography, which is what people do when they hold their phone up to the eyepiece!). But heck, we gotta start somewhere right? You can read more about other set-ups on a decent site I found called AstroPix: https://www.astropix.com/html/astrophotography/imaging_setups.html

I bought a Variable Eyepiece Projection Adaptor with a T mount for Nikon (~£60). Once it arrived I noticed 2 things: It connects to the telescope in the same way as the eyepiece assembly (with a 'nose' type push connection); and, it just didn't work.

Here's an image that shows how my telescope's eyepiece assembly fits together:

Eyepiece assembly

A short adaptor screws solidly onto the M56 barrel of the 'scope's focus drawtube. This allows the coupler to be attached by a pair of clamping screws - not as solid a connection as screwing into the barrel, but the whole eyepiece assembly is lightweight so that's fine. The other pieces (the diagonal and the eyepiece) then push together and are secured by single clamping screws. These connections are very easy to change in the field, and the pieces are super lightweight so it all works fine - although it is easy to find yourself adjusting the wrong clamping screw as there are just so many of them!

And here we can see the set-up that was recommended to me for eyepiece projection photography:

Recommended assembly

As you can see the whole weight of the camera (about 1kg in my case) is supported by a single clamping screw in the coupler (which itself is only supported by two clamping screws in the adaptor). I think I shuddered at the thought of connecting my camera like that (in fact I only used my older camera in the beginning out of pure fear).

Despite these reservations, I persevered and it took me a long time to decide it didn't work. Everything I read, and the original suppliers of my telescope told me this is the way it is done. It seemed ridiculous to me. Nikon are one of the most common dSLR brands and the telescope isn't in any way exotic, so I've got to be doing something wrong! Fortunately I chose a decent supplier who had a telephone conversation with me where they advised: "waving the camera about behind the telescope until you can see something". And that worked! With a ruler in one hand and the (heavy) camera in the other, I was able to work out that I needed about 40mm of extension in making the connection - with the telescope focus barrel fully extended.

I discarded the push-in nose piece that came with the eyepiece adaptor which then allowed me to screw in the extension barrel to both the eyepiece adaptor and the coupler (which has an M42 barrel thread as well as a nose-piece clamping screw). But that left me with two more concerns, leading to my final arrangement:

Final assembly

Firstly, even though the coupler attaches with dual clamping screws I still didn't like that arrangement. I think it's unlikely that the camera would fall off, but the possibility of getting tremor in the connection, or slight misalignment in the optical train, seems real.

Secondly, I had measured the extension required with the focus drawtube of the 'scope fully extended. I was doing these trials in daylight looking at (relatively) close subjects (a church steeple) so I wasn't sure that a 40mm extension would give me enough leeway to achieve more distant focus. I have 60mm of leeway in my focus drawtube, so I reasoned an additional 40mm or so of extension would mean I can work with the focus drawtube somewhat closed-in.

So I decided on a replacement adaptor providing an M56/M48 step down with a long M48/M42 extension tube: 70mm, the additional 40mm extension I wanted plus the length of the coupler I was replacing. Plus an M42 extension tube of 40mm - which again, is so I don't need to extend the focus drawtube fully.

At the camera end there is the T-Mount. People mostly think of these as just something that converts the camera's bayonet lens mount to an M42 (or more recently M48) female thread accepting M42 male threaded barrels of optical components (like an eyepiece projection adaptor). But the T standard also specifies a flange distance of 55mm. Meaning the T-Mount is long enough to achieve that. So my Nikon T-Mount adds 8.5mm to the native 46.5mm flange distance; a Canon T-Ring would be slightly deeper. Mirrorless cameras have much shallower native flange distances, which is why their T-Mounts are so deep. The point is though, when measuring or calculating the optical path length you know that at the front face of the T-Mount you are 55mm from the camera sensor.

They are variously called T-Mount, T-Ring, T-adaptor, or just T2. They comprise an inner and an outer ring. The inner providing the female M42 and the outer providing the camera bayonet. Be aware, sometimes the inner ring isn't provided and the outer ring connects directly to the optical component. This is the arrangement on my Svbony eyepiece projection adaptor. When I attach it to my dSLR I can almost hear the metals tearing at each other. I only put it on a couple of times before deciding to get a better quality T-Mount. Only to find what looked to be a weird arrangement, like it has half a T-rng (wikipedia informs me it isn't an unknown way of making T-Mounts). I wasn't getting it to work anyway so I decided to start again at square one. This time I went back to the people who sold my wife the telescope and asked them, what did I need. Tell me everything I need to connect a Nikon to this telescope you sold us...

They said what everyone else said, an adptor and a T-Mount. My thinking was, well if I buy it here I can at least talk to them when it doesn't work. Which it didn't. And I did.

Svbony I don't believe are bad, but they're at a quality you would expect given the pricing. But in this field of endeavour, my advice is to forge a relationship with a decent supplier. At the very least they can point you in the right direction when you're wanting to buy a solution... I'm using Rother Valley Optics (RVO). Others are available.

I'm actually not happy with the RVO T-Mount either though. It's very difficult to find something that fits as smoothly as your camera lenses fit. I've just bought a Celeston T-Mount and on first try it seems to be next level, if still somewhat behind the OEM's own lenses.

Some T-Mounts provide an M48 female thread instead of an M42. This helps reduce vignetting on full frame sensors and so would be better for me. I couldn't find an eyepiece adaptor in M48 though so had to step down to M42 on my longer extension. If I'd been more savvy while collecting the kit I would have stepped down later, on the second extension - I may have to replace the whole train at some point! It's working fine for me now though as I've limited the subject to the central part of the frame as much as possible.

I now had two assemblies that screw onto the M56 barrel thread of the telescope focus drawtube:

Both assemblies

Changing these assemblies over is really quite awkward, but I don't feel the need to. I either set up with one or the other for a given session.

Now here's a thing... with my final assembly I can now measure the distance between the eyepiece and the telescope's main objective, and of course it is about 500mm - exactly as you would expect for a 500mm focal length! I didn't apply this knowledge sooner because I just wasn't sure where exactly to take the measurements. I was also pretty confused because the viewing assembly seemed unreasonably short - in fact it is 10mm shorter than my shooting assembly. But 10mm is neither here nor there considering I have 60mm of play in the focus drawtube. The upshot is, the amount of additional extension you will need is absolutely defined by the focal length of the telescope. The precise amount of extension depends on how much of the scope assembly you are replacing with the shooting assembly. You also don't have to be super accurate, as the focus drawtube will supply a degree of extension as needed.

With prime focus you will aim to position the camera sensor 500mm from the front objective of the 'scope (for a 500mm telescope). With eyepiece projection, it is the eyepiece itself that must be at that (500mm) distance. The camera sensor will then sit further back, and how far back it sits determines the ultimate magnification achieved.

When looking through the scope the magnification we achieve is given by dividing the focal length of the 'scope by the focal length of the eyepiece. So a 25mm eyepiece on a 500mm 'scope gives a 20x magnification. I'm also given to understand that the maximum magnification we can reasonably achieve is twice the scope's aperture (in mm). So the most I can hope for with my Star Travel 102d is 204x (it has a 102mm diameter) - which would require an eyepiece of about 2.5mm.

But the eye is very close to the eyepiece, much closer than we can get the camera sensor (which needs at least to sit far enough away that we fill the frame with the projected image). When shooting the effective focal length depends on how far away the sensor is from the eyepiece, in accordance with the equation:

Effective Focal Length = Tf * [ (Ed - Ef) / Ef ]

where

Tf = focal length of the telescope

Ed = distance from sensor to eyepiece

Ef = focal length of the eyepiece

The T-Mount ensures 55mm from the sensor to the front face of the mount and my variable eyepiece projection adapter provides an additional 25 to 60mm of distance.

With my setup then, Ed ranges between 80mm and 115mm (variable adaptor length + T-Mount flange distance). With my 'superior quality' 25mm eyepiece I get magnification of between 2.2 and 3.6x - or an effective focal length for the whole setup of between 1100mm to 1800mm.

At the moment I am prefering to control this magnification by selecting an eyepiece of suitable focal length, and keeping the eyepiece projection adaptor at its shortest setting. I'm not yet sure but I think that the image quality degrades somewhat when I increase the distance from the eyepiece to the sensor.

I can achieve decent eyepiece projection with 25mm and 20mm eyepieces, but am yet to manage to get anything at all with a 10mm eyepiece. I don't know if that's due to the quality of my kit, my handling of the kit, or else some absolute optical limit.

That's quite enough on magnification for now..!

You'll remember I said above how easy it is to operate the wrong screw out in the field, maybe cold, certainly tired - that's kind of important. The eyepiece is held in place by a single screw, and IF it were to be accidentally loosened then there's a real threat of the eyepiece falling free and smashing into the camera sensor. Especially as the scope is normally pointing somewhat upwards and the sensor is normally exposed (either for live view or else for long exposure). I don't want that to happen.

I tried to buy a baffle ring I could screw inside the eyepiece projection adaptor to act as a physical barrier (I drew trigonometry diagrams to calculate the size and position needed so it would physically restrict the eyepiece without invading the optical path) - but nobody makes such a thing. For Canon it is possible to buy a T Mount with a physical filter installed that would also help shield the sensor from a rogue eyepiece - but they're only for Canon and they're really expensive. I tried to get a baffle manufactured but the world price of aluminium means that fabricators won't touch such a small job... so I'm left with that risk. At least I have eliminated most screws in the assembly so the chance of loosening the wrong one is less, but still my heart beats a flutter when attaching the camera. To help soothe my nerves I got my wife to make me a mini bandana which I use to cover the eyepiece retaining screw once the eyepiece is in place, as a tactile reminder not to touch the bloody thing.

Here's what it all looks like when I am set up for astrophotography:

The full shooting setup

All the extensions and adaptors meant that the weight of the camera was hanging a good 16cm off the back of the telescope, which gave a very unstable setup. So finally I also bought a 25cm mounting bar (in a lovely bright orange sat next to the purple sparkles of the telescope!) allowing me to shift the centre of gravity closer to the tripod axis - although I'm yet to use this in anger, I should probably have gone for an even longer one... Stability in the setup is absolutely critical. I try to keep the tripod leg extensions as low as possible and I use a cable release to fire the camera to help with this.

By early May I was able to take my first (full frame) shot of the moon, at 45% (first quarter) captured at about 4000px diameter:

45 percent waxing crescent

Although new to astrophotography I've been taking images for over 40 years now and over time have been able to step-up the quality of my kit, always ensuring my hobby was paying for itself. Consequently I'm in the grand position of owning a 46MP full-frame camera now (Nikon D850). I think this is a major boon, but probably out of reach for many amateur astrophotographers. I will say though that pixels matter - the more of them you capture in your image the less 'clunky' you will find the noise and sharpening tools behave down the line. That said, at this stage I'm not doing any of that; the image above is pretty much how it came out-of-camera.

Most of the advice I read talked about using an ISO of 100-400 and a shutter speed of say 1/100th. Which just made zero sense to me and my setup. I'm on solid ground here though and I understand these things really well - so instead of spiraling into a pit of disbelief in my own experience (as happened when it came to simply connecting the camera!) I just set about shooting what would work. And I think this is an important lesson - other people's advice on things like 'how to connect the camera', or 'what settings to use' are only vaguely helpful. There are just too many variables to expect a single, silver-bullet, recipe that works. Your world is different to mine!

I'm happy with an ISO of 6400, on this camera. I'm keen to keep the shutter speed above 1/500th, in the first instance, to minimise the effect of camera shake with this setup (I believe I can go slower, and so reduce the ISO, but I need to do more shoots to confirm that). Advice on shutter speed normally hinges around how fast the moon moves through the frame but frankly managing shake and vibration is probably a much greater concern. The less stable your setup, the higher the shutter speed you need.

Use whatever settings you discover are needed to make it sharp and well exposed. Get those 2 things right and everything else can be fixed in post!

When reviewing your images on the camera, zoom in. Not only does this help in assessing sharpness, but you will see the histogram becomes more meaningful as more of the dark sky is excluded from the zoomed view - it is easier to assess the histogram as well as the sharpness when zoomed in; it becomes easier to tell if you have 'burnt-out' the delicate highlights of the debris trails radiating from the major craters.

For a first shot, with a beginner's telescope, I'm pretty pleased. The main areas that require attention are: The edge definition and chromatic aberrations are extreme; The overall definition could be better; the noise is ugly as sin!

I did two things to battle the first concern (edge definition and colour fringe/chromatic aberation). The native aperture of my 'scope is f/5. I made a stiff cardboard baffle with a 30mm aperture in it (500mm focal length divided by 30mm aperture equals f/16). The optical quality of any lens improves when stepped down (within limits), so I did trials at f/8, f/11 and f/16. Secondly, I had been using a 20mm eyepiece (to give 25x magnification with a 500mm telescope) which came with the kit. I reasoned that camera kit lenses are rarely the best quality and that the same would be true with my scope. I also thought slightly less magnification would utilise more of the centre of the optic, the sweet spot, so I spent £24 on a mid-range 25mm eyepiece of superior design. I then did all of my aperture trials again! These two changes had a dramatic effect on the achieved image quality.

Full moon

To be honest, the edge definition is still not up to the image quality I get in my terrestrial work - but remember this is a ~£250 scope whereas my camera lenses are all £1,000+ optics. I could upgrade the scope, but even spending thousands of pounds the sky IS the limit with astrophotography. No matter what I do it's impossible to achieve greater results than the best observatories (or even space telescopes) are already delivering - so what would be the point? It's tremendously liberating to aim for getting the best possible result, without having to also get the best actual result. There will always be someone else with better kit in this field. So the point absolutely isn't perfection. It's like at last I have learnt what 'good enough' truly means.

But of course, these images here are still not quite 'good enough' for me... There remains the question of noise management and image sharpness, which requires the intervention of software to stack multiple shots. I've made headway down this path, but am yet to be able to reliably and repeatedly achieve results. This is what I will be looking at over the next moonphase (skies permitting), and I'll write that up in due course...

Photography