Astrophotography

Astrophotography Tutorial – Step 1 : The Captures

Intro

This part of the tutorial will guide you through the first step of each technique, gathering the images.  For everything but basic single images you’ll need to take a number of shots of the same scene, here are some basic guides:

  • Stack images ~ 10 images
  • Time lapse ~ 25 images per sec

I took 180 images for the various videos and trails I create later in the tutorials

Location

For best results you should take your images in ta dark sky away from civilization and light pollution, ideally with no moon.  But there isn’t much more annoying than driving an hour out to a dark location, spending 2 hours in the cold taking pictures and driving an hour back only to realize that you messed up the technique and its all wasted.  With that in mind I’d recommend you do a few practice runs from home or somewhere close particularly if you are taking time lapses – that way you can do about your normal business while your camera spends over an hour taking pictures by itself.

Framing the shot

Generally you’ll want an interesting foreground, if you want swirling circular star trails you’ll want to shot in the direction of Polaris (north star if you’re in the northern hemisphere) or the south star if your in the southern hemisphere.  If you want to do milkyway photography you will need to frame a shot with the milkyway in it which is generally in the southern sky

For the record the results I’ll be posting in these examples will be taken from a south suburb of Dublin, Ireland which has a moderate level of light pollution, the example shots I’ll be using are pointing north in the direction of the north star Polaris because I want to demo star trails which is unfortunately the direction of the main light pollution from Dublin city.

I’ll use the same example shots for all examples, they were taken with Canons light DSLR the which is called the 100d in Europe, the SL1 in the states and the Kiss X7 in Japan.  The shots were taken with the 18-55 STM kit lens at 18mm shot wide open at 3.5, 20 sec exposure with a 1600 ISO.  My example shout could do with being a little brighter, ideally they be exposed a little further to the right so I’d recommend 3200 iso instead of 1600.

Noise Reduction

If you are taking single shots you can turn on long exposure noise reduction in camera, this attempts to reduce noise by taking a 2nd exposure after your main exposure without letting light at the sensor using the same ISO and shutter speed,  Since ISO (the higher the more noise) exposure time (longer timer lead to more noise) and temperature all contribute to noise the 2nd identical exposure is used to detect hot pixels and make your image cleaner by removing them.   This doubles the amount of time needed to capture an image so If you are taking time lapses or stacking make sure you turn in camera noise reduction off

Finally if you are stacking you can put the cap on the lens and take a few extra shots before and after your main exposures – this will give you a bunch of dark frames which you can use later for noise reduction meaning you wont need to set in camera noise reduction, it gives a better results and saves you a chunk of time.

Focus

Focusing in the dark is difficult, auto focus can have issues, and Canon STM lenses don’t help too much in this regard either.  Lonely Spec have a great tutorial here but its aimed at slightly different types of lenses, the STM kit lens doesn’t have a focus distance scale and the focus itself is electronic so turning the ring instructs the camera to change the focus – it doesn’t do anything itself.  This means there is no stop  to tell you you have past the infinity mark so there are 3 basic ways to focus:

  • Cheat – rather than worry about focusing in the dark simply focus on distant object during the day at 18mm using AF then set the focus to manual and keep that setting until you go out at night
  • Set a single AF point (the center is generally best) then focus on a bright star or distant light and switch to manual focus
  • With the camera on, and set to a long exposure, set the lens to manual focus, turn on live view, zoom to 10x until you can find a star, then adjust the focus until the stars are smallest / pure white.
  • If none of the above work, walk to a distant object, illuminate it with your touch and then walk back to the camera and use AF to focus on it.

There is one benefit to using STM or more accurately electronic focusing: the camera needs to be on to change focus so once you have your focus dialed in you can turn off the camera – put it in a bag, even turn the focus ring a few times and you’ll keep the focus point you had before turning off the camera.  With manual focus lenses you need to make sure that once you get your focus dialed in you dont touch the focus ring at all so transporting the lens gets a lot more error prone.

Making the exposure

The goal here is to capture the most light possible, there are 3 things that control how much light we can gather:

  • Shutter speed (how long we gather light for)
  • Aperture (how big the hole which we gather light through is)
  • ISO level (how sensitive the sensor is)

Of all 3 options ISO is the worst thing to increase because higher ISO generates noise however we are limited by the lens max aperture and how long we can expose for without generating trails.  Wider focal lengths mean you can expose for longer without trails but if you use on a APS-C sensor @ 18mm the magic number is around 23 sec, I generally use 20

I used an intervalometer to take a 180 captures of the same scene.  Other settings i used were:

  • White balance set to tungsten in camera to adjust for light pollution (can be modified in post so its not all that important)
  • I also set the LCD brightness down so it doesnt mess with my night vision too much
  • I also turn off image preview if I am using an intervalometer to save battery power
  • Set noise reduction as you want.
  • Turn off lens stabilization
  • Set up a 2 sec timer in camera (if you are not using an intervalometer or wired / remote shutter release) so vibrations when pushing the shutter wont mess with the image
  • Set manual focus
  • Set camera to manual mode as above (shutter to 20 sec, Aperture to its highest value, use ISO to adjust your exposure, 1600 / 3200 ISO is probably what you want.
  • Take a few test shots and look at the histogram – you want to expose to the right if possible but don’t overexpose

My Results

Here’s the first one I captured:

Unprocessed Light Pollution
20 sec exposure @ 18mm, F3.5 and 1600 ISO

Full image @ https://www.flickr.com/photos/129291513@N05/24193948123/in/dateposted-public/

Obviously this is not ideal so now its on to post processing to try to improve our capture

Other links

There is a similar tutorial @ Lonely specs how to photograph the milky way, included is a exposure calculator which is really useful if you want to use other lens other than your lenses

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s