You want to take pictures of the night sky?

In this article we explore some of things that you need to think about if you want to take pictures of the night sky.

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We’ve got a bit of a theme going at the moment about getting started in some of the different aspects of astronomy. Just before Christmas we made some suggestion on what to buy someone interested in astronomy to get them started with, “Panic, panic, what should I buy for Christmas”. Then we gave some suggestion on what to look at, for the Southern Sky observer with our pre-Christmas “Preparing for Monday” article. Those articles give some advice on where to start but if you’ve been observing the night sky for a little while then at some point you’ve probably thought about taking pictures of the night sky. At Milky-Way.kiwi we think all aspects of astronomy are for everyone so we really support getting people to take photos of the night sky and share them with their friends and family, you don’t need expensive equipment as we explored in “Astrophotography is for everyone” as shown by this picture I took with my iPhone of Orion Nebula:

With this article we’re going to go a little deeper and give some advice from our own experiences with dipping our toes into the deep deep ocean of astrophotography. So get ready, check your bank balance, look for things to sell and ask yourself, do you really need that other kidney? Seriously though, the world of astrophotography can be astronomically expensive but you can start with very modest equipment.

The equipment list

Disclaimer: This article assumes you’ve got a powered mount (EQ or Alt/Az with wedge), a telescope and a DSLR camera with an adapter to put it in the telescope’s focuser

Aligning the telescope

First the basics, the Earth spins, that’s why the Sun appears to come up everyday. And that is the explanation as to why astrophotography is not a simple point and click activity for deep sky objects that require long time exposures. So without something that compensates for Earth’s rotation you’ll end up with star trails. Which are quite pretty for when you do landscape photography but not that useful for deep sky. To compensate for the spin of the Earth and keep the camera pointing to the same spot in space we use an equatorial mount, which moves the telescope and camera with the sky. Equatorial mounts have one axis that is parallel to the Earth’s axis, and it’s perfectly aligned South-North. One end of it points towards the South Celestial Pole, which is the extension of Earth’s South Pole in the sky and the other end points toward the North Celestial Pole, which is… you guessed, the extension of the North Pole in the sky. Our telescope’s fancy mount in Wellington, for instance, points its south end at a 41 degrees angle above the horizon (the latitude here is 41 South). If we were living at the South Pole, which we wish we could visit one day, it would point straight up, at Zenith (the latitude there is 90 degrees South). If we were at the Equator it would be parallel with the ground, pointing at the horizon (the latitude there is 0 degrees). Think of that axis as the axis of a clock (or watch) that is permanently aligned South-North. That’s the most important alignment you must do. Then once the telescope’s mount “knows” where is north and south then it can start “reading” the rest of the sky. The telescope tube and all the other equipment on it would be acting like one giant clock handle that rotates once every 23 hours and 56 minutes (that’s the length of the sidereal day, which is 4 minutes shorter than the Solar day, long story short)

The coordinate system, so you can find stuff to photograph

Here is another thing to know: during our life time stars will seem to be stuck to the same position in the sky. We actually call the sky the celestial sphere as we can observe it from Earth it seems like a giant sphere that surrounds us. In reality, stars change their position all the time, they orbit around the galactic black hole, galaxies shift away from us, other galaxies are on a collision course with ours (like Andromeda) yet our lifespan as humans is so short that this Universe, that nobody even knows what shape it has, it’s seen by us as a frozen moment in time. Our ancestors about 4-5000 years ago were looking at almost the same constellations as we are today. Earlier than that they would have been a bit skewed than now to but still recognisable. That’s why a lot of star maps always use “epoch” as term of reference. That means the position of the stars in the sky relative to each other at a given time. The one most used nowadays is epoch -J2000. So for the sake of the argument, from our observer position here on Earth the stars will seem like little dots poked on a giant balloon, the Celestial sphere, from our perspective we are at the centre of it. That’s good because we can find /assign things in the sky just like we can on Earth, by using latitude and longitude since we are using the same principle. For instance Wellington is at 174 longitude and -41 latitude or 41S and it will always be here no matter how much some people would like to move our capital to Auckland.

Countries in the sky

The equivalent of longitude in the sky is called right ascension, for short “RA”, the equivalent of latitude is declination, for short “dec.” So in the sky like on Earth. Wellington is in New Zealand, that is a country on Earth which has borders, a capital, and many other cities, towns and villages. In fact at night it’s quite cute to look at them, they all look like stars. My favourite star, Canopus is in Carina. Carina is a constellation and that’s the analog of a country. There are many stars in Carina, some brighter than others just like towns are to cities or villages. The brightest of all is Canopus, that’s the main star in Carina, called the Alpha star just like a capital is on Earth, by administrative convention, the alpha city in a country. The analogy with Wellington is not the best here because in NZ Auckland is the biggest city but for most countries the capital is the biggest city but you get the point. Canopus is at Right ascension 06 hours, 23 minutes and 57 seconds and -52 degrees of arc, 41 minutes and 44 seconds of arc from the celestial equator.

All we need to do now is find those stars and deep sky objects, make sure the Telescope is perfectly aligned and photograph the … out of them.

How to align everything

The key to astrophotography is removing all of the hard work, you want the mount, camera and computer controlling everything to do as little as possible. The first step in achieving this is to get the setup aligned as properly as possible. In the Southern Hemisphere that means having the mount pointing at the South Celestial Pole (SCP) and ensuring that the mount is as flat as possible. And no, we cannot see the north celestial pole from the surface of earth if we are in the Southern Hemisphere, unless the earth would become all of a sudden transparent.

To point at the SCP you must level the mount with a spirit level and then angle the altitude so it matches your latitude, normally the mount has a graduated scale for that. Then you have to point it in the right direction and there’s a bunch of ways of doing that. The general rule is that the better the alignment is the longer the exposure time you can take with the camera for each shot, up until the mechanical errors compound and you have to think about guiding. Polar alignment is an art within itself but well worth the effort. So to do this you’ll need a equatorial mount or and altitude/azimuth mount with a wedge, the wedge to align the altitude with celestial plane. A really well aligned unguided setup should give you 120 second exposures without much trouble, you might lose the odd one depending our bad the periodic error is.

Taking photos

Next you need to point the telescope and camera in the right direction and compose the image. It’s important to remember that often with astrophotography, the image that you see through the camera optics is significant less obvious than the image that will eventually appear on the screen, so you have to take your time and take plenty of test shots. The other challenge is focus, you want the best focus you can possible get. Use a Bahtinov mask if you can, if you’ve got a reflector as the imaging telescope then try and get the diffraction spikes as sharp as possible. Focus is like alignment, getting it right will make a big difference to the final image.

Next we take the images. Assuming it’s an unguided setup and we’re doing 120 seconds exposures, then try and get about 30 to 40 images in RAW mode