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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

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