“How do you take those photos?”

Most of the content on this website is aimed at astrophotographers, but actually the most common question I’m asked comes from people with no background in it at all: “how do you take those photos?”

A very brief answer is: I use a telescope and a special camera to take lots and lots of photos of the same thing in space. Then using computer software I combine all those photos to make a single, high-quality image. I edit this to bring out the details and make it look good.

If you want a bit more info, but not lots of technical detail, then read on…

The process of taking astrophotos can be split into three stages: Planning, Data Acquisition, and Processing.

1. Planning

The first thing is deciding what to photograph. Different objects are visible in the sky at different times of year, so to help I use The Astrophotography Planner Custom Digital Edition (review here); and then use a free piece of software called Stellarium. I’ve input my location as my home city of Bristol, and with this information Stellarium can show exactly what’s visible in the sky above me.

My telescope and camera can photograph an area of the sky about half the size of your thumbnail held out at arm’s length. This is important because it affects what targets I can take good photographs of. For example, the planets (Mars, Jupiter, Saturn…) appear tiny when viewed through atelescope like mine. I can’t take good photos of them, unfortunately. Similarly, most galaxies appear small. I can photograph them, but I’ll need to crop in a lot during post-processing and the whole process is sub-optimal.

The good news is that there are lots of clouds of gas and dust in space — called nebulae — that appear large. Stellarium helps here too. After inputting my telescope and camera specifications, it draws a red rectangle to show the area of sky I can photograph. I then test out different objects and see what fits best. The images below give some examples. See how Jupiter is tiny, but other objects fill the frame?

I’ve narrowed down my choice of targets, but I’m still not there. I’ll want something that’s as high in the sky as possible, because that’s when you’re looking through less atmosphere and so the sky is darker and you get crisper views. The target should also be in a good position not just on one night, but ideally over the next month or so because I’ll be returning to image it night after night. More on that in the next section.

But we’re still not there! I use a particular type of camera and filter that’s optimised for astronomical objects that contain lots of hydrogen. Luckily, that includes a lot of the nebulae that also tick the wide-field box. It’s certainly possible for me to photograph objects not rich in hydrogen, but it’s more difficult to get good results.

All this just to choose what to image!

2. Data Acquisition

Once I’ve completed the planning stage I can move onto “data acquisition”, which is a fancy way of saying “taking photos”. When it looks like the skies are going to be clear, I’ll take my telescope and all the gadgets that are bolted onto it from its home in my shed and out to the garden and will attach it to a special motorised mount that’s permanently installed in my garden, under a waterproof cover when not in use. This only takes a few minutes.

If I’m imaging one of those hydrogen-rich nebulae, I’ll also attach a special filter called an Optolong L-Ultimate. This only allows light associated with hydrogen and oxygen through to my camera sensor, which lets me record them in more detail while also blocking out a lot of the light pollution that surrounds me.

At no point do I ever look through the telescope with my eyes. Instead, my astrocamera is attached at the back of the telescope. This camera, the mount, and all my other gizmos are hooked up to a little gadget called an ASIAIR Plus, or AAP for short. This is a computerised brain that controls everything. I connect to the AAP wirelessly using an app on my phone. I can then issue it commands.

The first command I’ll send is to activate my astrocamera’s cooling fan, lowering the temperature of its sensor to -10 degrees C. It’ll stay at exactly that temperature for the entire night. Being below freezing lowers the amount of “digital noise” that the sensor produces, which appears as coloured speckles in a photo. Lower noise = smoother images. Being at a set temperature also helps with calibration, which we’ll talk about later.

Now it’s time to actually find the target I’m photographing! I’ll tell the AAP what I want to look at, either by inputting an object’s official astronomical name, or its celestial coordinates (right ascension and declination). All this information is readily accessible in Stellarium. The AAP has a model of the sky built into its memory. It uses this to tell the motorised mount to swing the telescope around to where it thinks the target will be. Once it’s done that, the camera takes a photo, and the AAP analyses it. By noting the positions of the stars in the image and comparing them to its internal database, the AAP can work out exactly where in the sky the telescope is pointing. It’s probably close to the target, but not exactly on. So the process repeats again, and perhaps one more time after that, until it’s exactly on target. This technique is called plate solving and it’s fantastic!

If this is the first time I’m imaging a target, next I’ll take a test photo to make sure my camera is at the right angle. Its sensor is rectangular, so the angle can make a big difference to the framing. See the images below to see what I mean, noting how the red rectangle fits the galaxy best in the third version. If the angle isn’t optimal, I need to twist a part of the telescope that then rotates the camera, and then take another image and see if that’s better.

Next, I’ll issue the AAP a series of commands for it to carry out automatically over the night. It will first run an autofocus routine, where a little motor adjusts the focus of the telescope until the stars are as small as possible; this means they’re in focus. Then the camera will begin taking photos.

Astronomical objects like the nebulae I favour tend to be very faint, so a long exposure time is needed to get any detail at all. To explain further, let’s say you’re having a day out at the beach and are taking some photos with a regular camera. It’s a sunny day and there’s a lot of light. Your camera’s exposure time — how long the shutter is open, exposing the sensor to light — will be really short. Say, 1/500th of a second. Blink-of-an-eye. Then the sun sets and it’s twilight. To gather enough light now, your camera sensor needs a longer exposure time, ideally several seconds. You’d better have steady arms, or you’ll get a blurry picture!

Objects in space are really faint, and the amount of light we receive from them is tiny compared to even your twilight beach photography session. I tend to set my camera’s exposure time to 120 seconds. If you’re a regular photographer, you’ll appreciate that this is ludicrously long! In the realm of astrophotography, it’s actually quite short.

Here’s where things get really tricky… the target I’m photographing doesn’t stay still. Just as the sun moves through the sky throughout the day, the stars and other astronomical objects do the same thing at night. (It’s actually the Earth spinning in space that makes them seem to move). Just looking up you don’t notice, but the motion is very apparent through a telescope. Our target needs to be exactly and constantly centered in our astrocamera view to get a good long exposure photo of it, otherwise it will appear as a streak. To solve this conundrum, my telescope mount’s motors keep whirring away, moving the telescope at precisely the same rate as the Earth spins, so the target stays centered in the camera’s frame for the entire exposure.

There’s actually a bit more to it that than, even. The telescope’s motors aren’t perfect; sometimes gears slip, or other issues cause the tracking to get slightly out of sync. To solve this, I have a mini telescope and mini camera attached to the side of the main telescope: officially called a guidescope and guidecam. Their job is to stare at the stars, taking short one-second exposures. If the stars don’t stay still, that means that the mount’s tracking is slightly off. The AAP detects this right away and corrects the mount.

After all that, I’ve got one 120-second photo of the target. That’s called a sub-exposure, or sub for short. Alas, one sub isn’t nearly enough. To get proper detail I need to take lots and lots of 120-second subs — to be explained more in the next section. I generally aim for between 600 and 900 subs (20 to 30 hours). This needs far more time than is available in a single night, especially as clouds can cut an imaging run short. So, I repeat the process every clear night for as long as it takes for me to get all my subs. I can spend more than a month per target, but really it depends on how cloudy the weather is; and also the season, as winter nights are longer so I can get more subs per night.

Even then there’s a bit more to consider. To help get the best image possible it’s necessary to take calibration frames. The two main ones I use are called Darks and Flats. Darks record the background noise level of a camera sensor and are obtained by taking photos with the lens cap on the telescope so no actual light is being recorded; just the sensor noise. Flats let you remove the effects of any particles of dust that are in your telescope and perhaps on the camera sensor, as well as dark corners called vignetting. I obtain Flats by holding an electroluminescent panel right up to the telescope. Taking calibration frames is a bit of a hassle, but well worth it in the end. I can re-use my Darks, and a fresh set of Flats is only needed once per imaging project.

3. Processing

Ok, now I’ve got many hundreds of subs, and calibration frames. We can move onto processing! This is all done at the computer. Processing is as important as image acquisition, and is the difference between a good and bad final image.

I’ll put all of the subs onto my computer and then load them into specialist astrophotography processing software. I’ll then analyse the subs in order to delete any that are poor quality. My final image will be a combination of all these subs, so we only want the best. If you’re baking a cake, you want the best ingredients! First I’ll visually inspect the subs to see if any are obviously bad; typically because clouds rolled in. This often removes 10 – 20% of the subs. Next, I’ll use the software to analyse each sub and graph the results. I remove any subs that aren’t good enough quality. Maybe the stars aren’t sharp enough, or there was slight trailing because the mount and guide system had a glitch, or dawn was closing in and the sky was getting bright. Whatever the reason, if the sub isn’t good, it gets deleted. This tends to remove another 10 – 20%.

Next comes those Dark and Flat calibration frames. The software will subtract them from the remaining subs, which ultimately improves their quality.

The next step is a big one. I’ll tell the software to take all the calibrated subs and combine them together to make one image. This boosts the light from my target, while averaging out (so helping to remove) anything I don’t want — noise, satellite trails, light pollution… This process is called stacking, or integrating. It’s intensive, and might take my computer several hours to complete. The more subs that I put in, the higher quality the integrated image that I get out will be. This is why it’s important to take lots and lots of subs during image acquisition. Add up the times of all the subs and that gives you the total integration time.

I’ve spent a long time getting to this stage, but the freshly-integrated image invariably looks disappointing. It still needs a lot of work to really bring the data out, and also combat the noise that remains.

In general terms, the next processing steps involve taking an integrated image and adjusting lots of criteria such as brightness, contrast and saturation to bring out details. I’ll often split the Red Green Blue channels and recombine them in different ways to make interesting colour combinations too. There’s a lot more to it than that though, and if you’re interested than maybe check out notes on my workflow here.

I’lI typically spend a few hours processing an image. There are lots of different methods that can lead to images that look quite different, even though they’re based on the same original data.

After all that, the end result will be a single image that can be saved as a JPEG and posted online, or printed and added to the album!

Ta-daaa!

There are many more little things to consider when astroimaging, but that’s basically it in general terms. I keep the original data saved on my computer. Image processing is a skill that develops over time, so it’s always possible to re-process an image a year or two later and get a better result.

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