dslr - astrophotography - ashford ?? image processing . my first astro-image . ... • cameras are...

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