Night Sky Photography – Capturing Stars Without Trails – Chris Schultz

Stars-1 Introduction

The problem

  • The Earth is moving relative to the stars
  • long exposures of stars results in lines of stars rather than pinpoints
  • though star trails can be stunning!

Solutions

  • Equatorial geared mounts that move at the same rate as the Earth
  • cameras such as the Pentax K-3 and K-1 with astro-tracer function (sensor moves with GPS referencing) – giving up to 5 minute exposure!
  • Expose to minimise visible motion – exploit ISO, aperture & focal length

Know your subject

Picture2

  • Dark skies do exist – and Australia has a lot of them compared to our Northern Hemisphere cousins

Picture1

Planning tools

Use software and internet tools to assist you

Equipment

  • Camera with bulb (lock shutter open) or manual mode you need to control shutter speed, aperture, iso
  • sturdy tripod – one you can weigh down is useful too
  • wide angle lens
    • Micro 4/3 sensor : between 7 and 14mm
    • APS-c/APS-h sensor : between 10 and 17mm
    • 35mm sensor : between 14 and 20mm
  • preferably fast (eg f2.8 or better)
  • intervalometer to time exposures if longer than 30s
  • remote release for the camera (reduce vibration)
  • warm clothing – it can get cold out there!

The capture –

Focus

  • During the day
  • Turn off autofocus and focus at infinity (trees on the horizon are handy)
  • Set your focal length
  • Fine adjust focus with live view if you have it
  • Take a test shot at f8 or f11
  • Check it is sharp at infinity
  • Adjust focus manually if required
  • Repeat until focus is acceptable
  • Tape the lens barrel so it doesn’t move either the zoom or the focus rings

Exposure  – Theory

  • Exposure is the light level you are exposing for measured in EV based on the ISO, aperture, and shutter speed
  • negative EVs are often used
    • -6 to -7 EV is a good range for detailed images of the Milky Way, assuming no light pollution and no moonlight
    • -8 EV or lower (-9, etc.) Will probably be a little over exposed, (lower exposure slider when processing to hide noise)
    • -5 EV or higher (-4, -3, etc.) Will probably be a bit too dark, (raise the exposure slider when processing at the risk of introducing more noise)

Camera settings

  • Manual (or bulb)
  • RAW capture (preferable to JPEG for exposure recovery)
  • image stabilisation off
  • matrix/evaluative metering
  • white balance 4000-5500k
  • aperture f2.8 (preferably not lower)
  • ISO 2500-6400
  • mirror lock up (if it’s not automatic)
  • exposure time usually 25s

other considerations

  • Dark frame subtraction
    • ON or OFF?
      • If ON this will result in an exposure after the image you take of the same duration
    • Just one dark frame for all?
  • Image stacking – just like a macro!  (http://deepskystacker.free.fr/english/index.html)
  • batteries – the fresher the better, and several of them for lots of exposures
  • dew and condensation on your lens
  • scenery

Exposure time – the simple one

  • From film days the 500 rule exposure time at night is
    • shutter speed =
      500 ÷ (focal length x crop factor)
    • based on grainy film – and it’s a bit inaccurate
  • Sensor size has variations:
    • for a 35mm sensor 500 ÷15mm = 33s
    • for an aps-c sensor 500 ÷ (15mm x 1.5) = 22s
    • for a micro 4/3 sensor 500 ÷ (15mm x 2) = 16s
  • it’s a rule of thumb – so it may require some adjustment
  • find the best exposure to stop star trails

Exposure time – the complex one – the NPF rule

  • NPF rule accounts for pixel pitch & aperture & focal length
    • Pixel pitch = physical width ÷ pixels across sensor x 1000
  • This is important as it allows for the sensors ability to capture more photons (think of a deeper or larger bucket)
  • shutter speed = (35 x aperture + 30 x pixel pitch) ÷ focal length

Examples for common sensors (I won’t even mention 645 cameras!):

35mm sensors examples

Nikon D810

  • 14mm @ f2.8 = 17.4s
  • 24mm @ f2.8 = 10.2s

Canon 5d Mk III

  • 14mm @ f2.8 = 20s
  • 24mm @ f2.8 = 11.2s

APS-c/APS-h sensor examples

Canon EOS 7d

  • 11mm @ f2.8 = 20.3s
  • 20mm @ f2.8 = 11.1s

Sony a6300

  • 11mm @ f2.8 = 19.5s
  • 20mm @ f2.8 = 10.8s

Micro 4/3 sensor examples

OM-D E-M1 Mk ii

  • 7mm @ f2.8 = 28.23s
  • 14mm @ f2.8 = 14.2s

This link in PetaPixel allows you to view the spreadsheet with the calculator and adjust for your camera

Processing

The Basics

  • Set black level to not clip blacks
  • enable lens profile corrections for lens used (or choose a similar lens)
  • remove chromatic aberration
  • perform luminance noise reduction – zoom in on the image where some noise and fine detail show. Adjust the noise reduction sliders to reduce noise without affecting detail
  • optional: modify curves to boost faint astro signals
  • sync all exposures (if you have performed multiple exposures)
  • stack

Example – Lightroom

  • Basic tint and temperature adjustments to neutralize color
  • increase brightness globally:
  • Slightly increase exposure slider targeting mid-tones
  • increase brightness of stars:
  • Increase whites slider, enhancing white tonal range
  • retain dark details for contrast adjustment:
  • Increase blacks slider to enhance details in black tonal range
  • increase contrast:
  • Use contrast slider to increase contrast
  • increase vibrance

Here’s one I took with a near full moon just rising

Chris Schultz_Astrotrace_example

An example of Astrotracing Aldgate, 12-Apr-2017 – Pentax K-1 using Pentax 15-30mm f2.8 Exposure at 15mm @ f2.8, 60s ISO320 © 2017 Chris Schultz

 References

http://www.popphoto.com/how-to/2013/06/how-to-shoot-epic-landscape-photos-night-sky

http://www.astropix.com/html/i_astrop/toc_ap.html

https://www.pentaxforums.com/reviews/shooting-a-starscape/intro.html?src=all

http://geartacular.com/how-to-guide-astrophotography-with-dslr/3/

http://www.photosbykev.com/wordpress/2014/01/18/star-trail-photography/

http://www.davemorrowphotography.com/p/tutorial-shooting-night-sky.html

http://galleries.aaronpriestphoto.com/articles/night-sky-photography

http://www.clarkvision.com/articles/astrophotography.image.processing/

http://intothenightphoto.blogspot.com.au/2013/05/my-450-rule-to-stop-star-trailing.html

https://darkskydiary.wordpress.com/tag/bortle-scale/

https://www.youtube.com/watch?v=zlconcy-5wy

And this presentation as a YouTube video as well as one from Martin Lewicki from the SA Planetarium who presented to the club a few years ago

 

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