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Digital SLR Astrophotography
Ashford Astronomical Society Fri 27 Feb
Mark Shelley
Overview
Types of Astrophotography
Types of Equipment
Hints and Tips for imaging
Image Processing
My first astro-image
My first telescope
Types of Astrophotography
Telescope not always required!
Through the telescope eyepiece
Camera with lens on tripod
Camera with lens on motorised mount
Camera attached to Telescope
Webcam imaging (planetary imager)
Guided imaging on motorised mount
Through the telescope eyepiece
Mobile phone or compact camera
Hand held or attached with bracket
Eyepiece brackets can easily be bought
Technique:
Switch off flash!
Carefully line up lens with eyepiece
Adjust exposure if necessary (prevent overexposure)
Remote control prevents shake
My second astro-image
Camera taped to the eyepiece!
On a Tripod
Mobile phone, compact camera, DSLR
No laptop required
Intervalometer for multiple exposures
Technique: Switch off flash!
Carefully focus
Adjust exposure if necessary (prevent overexposure)
Remote control prevents shake (pressing button)
Remote Control Lead
Lens & Tripod Imaging
No telescope required use standard lens
Short exposures prevent star trailing
Multiple images can be stacked
Saturn, Moon & Venus
75mm lens 3 sec exposure
Moon and Venus
145mm lens 5 sec exposure
Milky Way
18mm lens 40 x 30sec
Problems and Solutions
Problem 1: Focusing
Difficult to focus in darkness!
For bright objects use autofocus
Liveview can also work
Otherwise use manual focus
Make a series of test shots
Lens with focusing scale
DIY Focusing Scale
Problem 2: Lens Aberrations
Stars in image centre may look fine
Stars nearer the corners may be squiffy
Depends on lens quality
Experiment with different focal ratios
F/2 is fast lens wide open
squiffy stars in corners
F/8 is slow lens stopped down
stars less squiffy
Focal ratio is set in camera or manually set on lens
Look at centre and corner when focusing
Problem 3: Dew
Dew on lens optics ruins photos
Headlamps in the fog effect
Solutions
Hairdryer! Dee bought me one for Christmas
Heated dew strip (needs 12 volt power)
Dew Heating Strip
Problem 4: Star Trails
Longer exposures = longer star trails
Keep exposures short
Later well look at motorised mounts
Orion at Kelling Heath
20mm lens 30 sec exposure
Zoomed in
20mm lens 30 sec exposure
Rule of 600
Rule of 600 is often quoted for a DSLR
Multiply:
Focal length in mm
Exposure in seconds
Keep this less than 600
Other cameras would have similar rule
I prefer Rule of 300 !!
Summary of Problems
Focusing
Manual with test images
Lens Aberrations
Experiment with focal ratio
Dew
Dew heating strip
Star Trailing
Rule of 300/600
Deliberate Star Trails
With Intervalometer
2 minute exposure
Startrails Software
Final Result
30 x 2 minute exposures
Star Movement
Stars rotate around the North Celestial Pole
Pole star is near the NCP
Rotation is 15deg/hour (approx)
360 degrees (approx!) in 24 hours
North Celestial Pole
Motorised Mount
Camera attached to motor driven axis
Axis points towards the NCP
Axis rotates at 15deg/hour
360 degrees (approx!) in 24 hours
Motorised Mount?
If you already have a motorised telescope
one solution is piggybacking
Mercury & Moon
300mm lens 13sec exposure
Motorised Mount
DIY barn door device
Commercial devices
iOptron SkyTracker
Sky-Watcher Star-Adventurer (more versatile)
Axis points towards the NCP
DIY Barn-Door Tracker
Not motorised!
iOptron SkyTracker
SkyTracker with camera
Sky-Watcher Star-Adventurer
Orion 5min exposure
Orion zoomed in
Camera Types
Film?
Mobile Phone or Compact Cameras
Small sensors
Lack night time sensitivity
DSLR and other mirrorless cameras
Large sensors with good sensitivity
Interchangable lenses
Adapters for telescope attachment
Astro-CCD
Mono or colour. Good response to H-alpha
Colour or narrowband filters needed for mono camera
Cooled sensor
Why DSLR or Mirrorless?
Large sensor creates great images
One shot colour
Cheap vs equivalent astro-CCD cameras Manufacturing economy of scale
Second hand on eBay!
Excellent start to astro-imaging
Which DSLR? Canon cameras have best astro-community support
This is slowly changing
Nikon and Sony have better sensors at budget end
DSLR Disadvantages
Sensor has no cooling
Can affect quality of Summer imaging
Astro-CCD has cooling
But cooling increases price!
H-alpha wavelength insensitivity
Manufacturer internal filter
This filter can be removed (see later)
Mono astro-CCD is much more sensitive
Tips & Tricks
Always take RAW images JPEG compression causes artefacts
Carry spare camera batteries Or power adapter (e.g. from 12 volt)
Remote control lead or intervalometer
Focusing tricks On lenses and telescopes
Astrophotographer Pet Hates
Clouds! The UK is in the wrong place
Cold Nights Wrap up warm!
Temperamental hardware and software Wastes time on precious nights
Dew Heated dew bands and dew shields
Light Pollution Muddy brown or bright orange background
Light Pollution (Sidcup)
F3.5 30sec ISO100
Astronomik CLS filter profile
Before and After
Telescope Imaging
T-Ring bayonet adapter attaches DSLRs
Focal ratio is fixed (unlike lenses)
Rule of 600 gives what exposure length?
Motorised mount essential
Telescope Types
Refractor
What the general public recognises as a telescope!
Lens at front (objective)
Eyepiece at back
Newtonian
Big mirror at far end of tube
Eyepiece of side
Schmidt Cassegrain
Glass corrector at front
Big mirror at far end of tube
Eyepiece at back
Long focal length with folded lightpath
Refractor on motorised mount
Motorised mount aligned with NCP
Camera attached
With Annotation
Pleiades (Seven Sisters)
24 x 5minutes
Andromeda Galaxy
60 x 5minutes
Guided Imaging
Motorised Mount Exposure length limited by motorised mount quality
Guided Imaging Overcomes tracking inaccuracies
Guide scope is mounted parallel to imaging scope Guide camera linked to laptop
Locks onto a guide star
Mount corrections sent every 1-2 seconds
Stand-alone guiders also exist No laptop required
Guided Imaging
Guided Imaging
Free guiding software available
e.g. PHD Guiding Push Here Dummy
Old telephoto lens can work very well
Lens, Milk Container, Webcam
Ready to Assemble
Ready for Guiding
Mount Types
Equatorial
Main Axis point at North Celestial Pole
So it follows motion of the stars
Alt-Azimuth
Twisting base (azimuth)
Up and down (altitude)
Main problem is field rotation
Alt-Azimuth Mounts
Field Rotation
What is field rotation? Think of Orion
Cue volunteer from audience
Limits the length of exposures
Makes stacking more difficult
Can buy a wedge
Telescopes What they dont tell you!
Refractor, Newtonian, Schmidt Cassegrain
All suffer from field curvature i.e. the focal plane is curved
Observatories used curved film
But camera sensors are flat
All suffer from other optical aberrations
e.g coma
Often have matched corrector lenses
Astrographs have correcting lenses built-in
Expensive!
With Annotation
Refractor
Chromatic Aberration
The objective splits the light
Colour fringing on stars
Hence buy Apochromatic (not achromatic)
Often have matched corrector/flatteners
Lens hood give some dew protection
May need heated dew strip as well
Newtonian
Rarely has enough focal travel for DSLR
Unless designed for astrophotography
Sometimes matched corrector/flatteners
Otherwise buy generic coma corrector
May need dew shiel