EbolaBooze’s Fortnightly Astro Corner – So You Wanna Be an Astrophotographer

Welcome to the first instalment of EbolaBooze’s Fortnightly Astro Corner! I was planning on jumping straight into the deep end and posting awesome pictures of nebulae, but I figured a guide to basic astrophotography would be more appropriate as an introduction.

Here and now I’m gonna make a few assumptions. Okay, more than a few.

Postulate 1:

You have a basic grasp of the English language.

Postulate 2:

You have a basic understanding of photographic principles, like f-stops, aperture, focal length and white balance.

Postulate 3:

You are in fact, interested in astronomy, and have owned at least one of the following at some point in your life and used them to stargaze:

Binoculars (Any magnification)

A refractor telescope of aperture 60mm (~2.3 inches for you barbarians not using metric or SI units)

A Newtonian telescope of aperture 152mm (6”)

Any telescope of greater than 120mm aperture (~4.7”)

Postulate 4:

You have more money than sense, or have meticulously planned out your initial purchases and upgrade paths for the next three years or so.

Postulate 5:

You have friends who are also interested in astronomy, a very tolerant set of friends, little to no night-life to speak of, or absolutely no-one that would actually give a damn about you disappearing into the black for the better part of the night.

Postulate 6:

You’re able to learn quickly, and you have patience. Lots and lots and lots of patience.

Remember, 50% is a passing grade!

Now, the easiest way to actually start getting into astrophotography is with a camera that has a good manual control mode (high-end point and shoots, DSLRs) and a sturdy tripod.

For beginners, the moon is one of the best targets you could go for as it’s very bright, easy to find, and has enough interesting surface features that you can tell whether you have good focus or not.

Recommended focal lengths for taking pictures of the moon with a digital camera are between 55mm and 400mm. F-stops don’t matter hugely with the moon due to its high luminosity, so if you want to have some foreground objects in focus as well you can comfortably go to f10 while keeping reasonable exposure times. Experiment for the best results with your camera/lens setup!

55mm Moon

This particular photo was taken with a Canon 550D and 18-55mm kit lens, at 55mm, ISO-200, f7.1, 1/400s exposure time. As you can see, still very good surface detail!

For taking pictures of the Milky Way you really want to be in an area where light pollution is zero, or at a minimum for best results. Stars are orders of magnitude less luminous than the moon, so light from cities and towns can easily wash out the fainter features of our home galaxy. You can still take pictures even from within city limits, but you will be limiting your exposure times, and the detail you can see as a result.

Set your focus to infinity, stop down your short focal length lens (anything up to 55mm, I recommend) down as far as it will go, set your white balance to daylight, your ISO at whatever you feel comfortable with (I use 1600 at most), aim at an interesting point in the sky and expose for 10-30 seconds.

Bam, we have a Milky Way widefield.

18mm Southern Cross 10s

This one was a 10 second exposure of the Southern Cross at 18mm, f10, ISO-1600. There’s star trailing, but that’s because it was a windy night. And yes, I didn’t use a fast f-ratio, because I forgot to reset it from the photos I was taking earlier in the day.

Why only 10-30s worth of exposure and a short focal length lens, you ask? (I hope you asked yourself this.) It’s because stars move across the sky as the night goes on – it’s a consequence of the Earth’s rotation. The longer you expose for, the more the stars will move, and the more trailing you get. Same with longer focal length lenses, except the greater apparent motion of the stars will lead to longer trails in shorter periods of time.

You know those cool circular star trail pictures? The photographer has simply set his camera to long-exposure mode, left the shutter open and had a beer or ten while distant suns did the hard work and left their burning trails across the firmament.

If you want to actually have nice detail, or use a longer focal length lens, you need a tracking mount.

The simplest type of tracking mount is here: http://en.wikipedia.org/wiki/Barn_door_tracker

It’s cheap, easy to make, and the simplest ones will enable you to increase your exposure times to 5-10 minutes with a 50mm lens, depending on how well you polar align the mount. Of course, the longer your focal length, the quicker errors will become apparent, and the quicker your stars will start to trail. Once you want to take exposures longer than that, it’s just easier to get a motorised German equatorial mount for your camera and image that way.

But wait! I can hear someone say: “But EbolaBooze, as cool as widefield is, I wanna take photos of nebulae and star clusters!”

Here’s where I stick a disclaimer.

This is where the slippery slope starts.

Everyone who starts says “Oh, but it’ll just be a one-off purchase, I’m sure I’ll be happy with that setup!”

You’re wrong. Trust me.

I started reading about astrophotography in 2010, bought my second telescope in 2011, and since then have spent more than AUD $12000 on this hobby. Expensive? Definitely. Rewarding? You bet your arse it is! Also, it’s not just the expense you have to worry about; it’s the learning curve as well.

Still interested? If you want decent equipment that you can actually grow into getting the best out of, you’re looking at an outlay of a minimum $2000, just to get started. This is the cost of a motorised, heavy-duty equatorial mount, a telescope of sufficient aperture (80mm minimum) and a good camera.

I started off using a Canon EOS 550D attached to a 254mm aperture 1200mm focal length Newtonian telescope, mounted on a Skywatcher NEQ6 PRO computerised German equatorial mount. A damn good setup for visual astronomy, which is what I started off doing, but also a very nice jumping-off point for astrophotography. It was a decent mount, a nice fast (f4.7) optical system and a good camera. This setup set me back around $2500.

I did all the reading I thought I needed, researched the crap out of the limitations and benefits of my telescope type, and generally thought I had done a pretty good job educating myself.

My first pictures were shithouse.

I could barely get 5 second exposures without getting smeared, blurry stars and absurd amounts of background noise.

And here we come to the first lesson of long-exposure deep-space imaging: knowledge alone does not successful execution make. I quickly found out that I wasn’t polar aligning my mount accurately enough for the focal lengths I was working at.

The second lesson is: Polar align your mount accurately!

You can get very good pictures with a stripped-down webcam attached to an 80mm aperture refractor, on a basic motorised equatorial mount, as long as you are accurately polar aligned.

I won’t go into detail on how to accurately do this, so I’ll post a few links to the better guides. This is the most important skill will learn as an astrophotographer, bar none. Everything else is secondary, polar alignment skill is mandatory.

http://starizona.com/acb/basics/using_polar.aspx

http://www.backyardastronomy.net/drift_alignment.html

If you’re one of those lucky ones that can control their mount through a computer interface, and have a webcam that is easily attachable to our telescope, there are a number of programs to simplify polar alignment available to you. This is the one I use:

http://eqalign.net/e_eqalign.html

With practise, I got better steadily over the years, until I could justify to myself (lie sufficiently) that I had outgrown my equipment. I’ll go into detail about my equipment and more equipment choices at a later date, but for now I’ll leave you with one of my works-in-progress.

NGC2020 Hubble

Clear skies!

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