DSLR - Astrophotography - Ashford · PDF file• Image Processing . My first astro-image ....

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Digital SLR Astrophotography Ashford Astronomical Society Fri 27 Feb Mark Shelley

Transcript of DSLR - Astrophotography - Ashford · PDF file• Image Processing . My first astro-image ....

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 we’ll 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 don’t 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 shield and heated dew strip

• Good Collimation required for imaging

– Offputting for beginners

Takahashi Epsilon 180ED

Bananascope with dewshield

Schmidt Cassegrain • Long focal length

– Excellent for planetary imaging

– Very difficult to guide accurately,

unless you have expensive mount

• Generic corrector/flatteners available – Not needed for planets

• Needs dew shield and heated dew strip

– Corrector plate is dew magnet!

• Needs frequent collimation for imaging

– Offputting for beginners

Focal Length 2800mm

DIY Focusing Dial

Focusing Trail

SCT Unguided Imaging

• Telescope is used on a tracking mount

• Limit exposure to 30sec

– Discard bad images before stacking

SCT without guiding

• Moon

Moon 1800mm F6 1/800s ISO 800

SCT without guiding

• Moon

• Globular Clusters

SCT without guiding

• Moon

• Globular Clusters

• Brighter Nebulae

M27 Dumbbell Nebula 1800mm F6 40x30sec ISO 800

SCT without guiding

• Moon

• Globular Clusters

• Brighter Nebulae

• ISS

ISS 2800mm F10 1/2000sec ISO 800

Image Processing

• Stacking

– Adding multiple exposures reduces noise

– This enhances faint details and nebulosity

• Software aligns the stars in each exposure

– Adds all the exposures together

• Deep Sky Stacker is the most popular software

– Free!

Why Stack?

• Adding multiple exposures reduces noise

• Sources of noise:

– Sensor read noise

– Sensor dark current (thermal noise)

– Shot noise – random arrival of photons

– JPEG compression

• Shoot RAW images!!

Cave Nebula (Mar 2011 Sky at Night)

Canon EOS 350D 500mm 64 x 5min ISO 800

Stack 1 frame

Stack 2 frames

Stack 4 frames

Stack 8 frames

Stack 16 frames

Stack 32 frames

Stack 64 frames

“Webcam” Imaging with SCT

• Why called “webcam”

– Philips Toucam was the popular tool

• Planetary Imaging Cameras

– Available < £100

• Insert instead of eyepiece

• 5-30 frames/sec

– Connected to laptop

Jupiter 21 Feb 2015

Jupiter Processing Sequence

• 1000 Frames stacked in AutoStakkert

• Deconvolution performed by Registax “Wavelets”

Jupiter 21 Feb 2015

1000 webcam frames @5 frames/sec

Jupiter and Ganymede

(Astronomy Now)

Why deconvolution?

• High magnification images are affected by

atmospheric turbulence

• Turbulence blurs the image

• Blurring can be treated by deconvolution

• Deconvolution = Deblurring

• Very mathematical

– But software makes it easy!

Crater Copernicus 20 x 1sec ISO 100

Crater Copernicus after deconvolution

Hydrogen Alpha

Horsehead Nebula 10 x 10min ISO 800

Astronomik CLS filter profile

Modding a Canon

• Cameras are made to match human vision

• Internal IR Filter cuts 80% H-alpha

• Can be replaced with

– Glass

– Alternative IR/UV filter e.g. Baader “pre-cut to fit”

The inside revealed

Sensor Revealed (top of picture)

Sensor Removed

New glass & old IR filter

First light is a TV remote

M27 – Before & After

Image Processing Techniques

• Calibration frames

• Stacking - combining sub-exposures

• Background subtraction

• Intensity scaling – “curves” or similar

• Deconvolution

Processing Sequence

• Free Software

• Deep Sky Stacker

• I use IRIS – but same principles apply

• PixInsight is the “Rolls Royce”

– Costs money and difficult to drive

– But almost perfect

How to turn this:

Into this:

Processing Sequence

• Create the following:

– Bias Frame (RAW)

– Flat Frame (RAW)

– Dark Frame (RAW)

Master Flat

Master Dark (5 min)

Processing Sequence

• Create the following:

– Bias Frame (RAW)

– Flat Frame (RAW)

– Dark Frame (RAW)

• Apply the above to each raw image frame

Raw Frame (5 min)

Pre-processed Frame

Convert to RGB

24 Frames Stacked

Background Subtracted

Range Scaling

Final Cropping

Summary

• Camera Types

– Mobile phones to DSLRs and astro-CCD

• Telescope types

– Refractors, Newtonians, Schmidt Cassegrain

• Mount Types

– Tripods, Motorised, Equatorial, Alt-Azimuth

• Imaging

– Through eyepiece, planetary “Webcam”, DSLR

• Processing

– Stacking, Deconvolution

Favourite Images

Pleiades

90 x 5minutes

Horsehead & Flame Nebulae

130 x 5min

VdB 126 Dark Nebula in Vulpecula

150 x 5min

Markarian’s Chain

50 hours over 8 nights

Auto-Annotated

Camping

Final Remarks

• I hope you are now inspired to try!

• At least you have a better understanding

• Don’t be afraid to ask advice

– It’s why societies like AAS exist!

• Imaging Night: Saturday 14th March

Further Information

• www.markshelley.co.uk

• Jerry Lodrigus

– A Beginner's Guide to DSLR Astrophotography

• Michael Covington

– Digital SLR Astrophotography

• Steve Richards

– Making Every Photon Count