Deep Sky Astrophotography
Introduction:
- good quality photography of deep sky objects is extremely demanding
requiring relatively expensive equipment (at least $A7,000) and an
obsessive-compulsive personality that can cope with managing the many fine
details that need to be addressed.
- it is one of the most technically demanding forms of photography but has
become accessible to amateurs with the advent of CCD cameras and the digital
SLR, and particularly so with the advent of APO refractors in the 1990's
which have eliminated the chromatic aberration that has plagued refractors
and allowed the development of high quality, minimal aberration fast
refractors highly suitable for astrophotography.
- a modest six inch aperture telescope
equipped with a CCD camera will detect fainter objects than a 36"
telescope with a standard photographic camera!
- examples of results:
- Orion nebula M42:
- Rosette nebula NGC2244:
- Omega Centaurus globular cluster:
Equipment
required:
- see http://www.atscope.com.au
Australian distributor of high end equipment
- high quality German equatorial mount
(GEM):
- a robust mount with minimal vibration, excellent tracking with minimal
periodic error and ability to autoguide are essential.
- examples:
- Meade LXD75 for those on a budget
- Losmandy G8 $US1500 for small scopes to 8" SCT or 6"
refractor & max. load 30lb
- Losmandy G11 $US2100 for larger scopes & max. load 60lb (NB.
tripod 35lb, head 36lb) 0.5arc-sec;
- Astro Physics AP900
- the heavy weight mounts:
- Losmandy Titan - 4.5arcsec error; can transport in 7 parts;
- Astro Physics AP1200
- Mountain Instruments
MI-250 US-made
- Tak NJP Temma 2 -
NJP has a very expensive optional hand paddle
- Millennium Mount MMII German-made mount with specs approaching
the ME, at $US7000 is a good investment and certainly worthy of
consideration - +/- 4arc sec error; weighs 72lb; 120lb max.
load;
- SoftwareBisque
Paramount ME is the best mount that money can buy even at the
$US12,500 plus cost - usually +/- 2 arc sec error; weighs 64lb;
max. load 150lb; designed for robotic use;
- if you are only imaging in the outer suburbs in light pollution, you
do not need as good a mount as your individual exposure times will be
limited to less than 60sec anyway due to light pollution.
- a telescope optical tube with minimal aberrations, fast optical speed (eg.
f/5.6-f8) and high contrast
- examples:
- ED or flourite APO refractor 3-6"
diameter (60mm or more)
- Canon 200mm f/2.8L II lens for Canon dSLRs
- Canon 400mm f/5.6L vs Televue TV-60is f/6 - see here
- NB. while SCT telescopes are great visual telescopes and for
planetary/lunar photography (eg. with a webcam or similar and stacking
images), it's focal length is generally regarded as too long for DSO's,
even with a focal reducer (and this limits sensor size possible without
getting vignetting), and the fork mounts they are usually sold with tend
not to be adequate for the precision guiding needed.
- low noise prime focus mounted camera capable of long exposures such as:
- digital SLR
camera:
- for galaxies, star clusters and some nebulae, an un-modified dSLR
will do fine
- BUT for emission nebulae, unmodified dSLR's have a IR blocking
filter which prevents most of the H-alpha infrared emissions from
being recorded, removing this filter allows ~3.4x sensitivity and
thus allows ~1/12th the number of sub-exposures that need to be
taken to get the similar results as an unmodified camera
- the Canon dSLRs are the most popular and the most commonly
modified although Hap does modify the Olympus E410.
- dedicated astrophotography camera:
- examples:
- SBIG CCD camera cooled to -45degC with in-built auto-guider and filter
wheel
- SBIG CCD cameras with built-in autoguiders (2004 prices):
- ST-7XE ($US2700) 0.39megapixels x 9micron
- ST-2000ME ($US3500) 2 megapixels x 7.4micron
- ST-8XE ($US6000) 1.6 megapixels x 9 micron
- ST-10MXE ($US7000) 3.2 megapixels x 6.8 micron
- monochrome CCD cameras require a colour filter wheel to allow
multiple exposures in each colour to generate a final colour image:
- some use H-alpha (green), OIII (blue) & SII (red) filters
- consider a nebula filter:
- high quality H-alpha (green) or OIII (blue) filter to minimise light pollution &
maximise emission nebulae
- autoguider:
- if using a webcam as an auto-guider, must attach it to a telescope
with at least 70mm aperture to allow guiding on stars down to
magnitude 6. An off-axis guider (OAG) will only allow guiding on
Sirius unless the webcam is modified to allow longer exposures.
- many CCD cameras have inbuilt autoguiding.
- autoguiding software:
- image manipulation software:
Imaging quality:
- imaging quality can be measured by graphing the image intensity of a star
against the diameter of the star's image, the width of this graph in
arcseconds where the intensity is half the maximum intensity for the star is
called full width half maximum (FWHM).
- professional ground-based astrophotographers strive for the best imaging
quality possible and this usually equates to a FWHM value of 1.5-2.25 arc
secs.
- values higher than 2.25 arc secs rapidly result in loss of contrast in the
image and thus loss of resolution and detail.
- see Richard
Bennion's online video on imaging quality whose experiments show effects
of:
- atmospheric seeing:
- great seeing will allow reaching 1.5 arc secs, less good seeing
will restrict one to 2.5 arc secs or worse
- a star at zenith may allow 1.9 but on the same night in same
conditions, a star at 45deg to zenith will restrict to 3.1 arc secs
- see astronomic "seeing"
- collimation:
- field flatness of optical system:
- Schmidt-Cassegrain scopes may have a 50% curvature in the field
which results in stars at the edges being out of focus, this means
that these stars may have 1 to 1.5 extra arc secs in FWHM
- so use the central part of the image or use a scope with a flat
field to strive for a curvature of < 10% - eg. Tak 106, or RCOS
use of a field flattener
- focus:
- focus changes with change in temperature
- a SCT focus may change by 350 microns over a 2deg C temp. drop
which is enough to add 4 arc secs to FWHM!
- re-focus every 30min during a session or ensure temperature
remains stable such as use of a RC
- optimise focus for a star in the centre of the field - may need to
use off-target focusing where software temporarily slews telescope
to a suitable star to allow focus then returns to original target.
- see focusing a telescope
- tracking:
- polar alignment:
- the greater the error, the more rapidly the star will drift
and impact on FWHM, even alignment errors of 30 arc secs is too
much - check drift alignment, and aim to be within 0.3 arc secs
error
- see telescope polar alignment and balancing
- periodic error due to imperfectly round worm gears:
- failure to employ error correction will cause drift and wider
FWHM
- aim for less than 1 arc secs periodic error
- autoguiding:
- aim for 3-5 seconds guided exposure duration to decrease error
by 0.2 arc secs in FWHM
- software settings to minimise unnecessary mount corrections
and set max. move to 0.5 arc secs to minimise effects of cosmic
ray hits
- wind and vibration:
- obviously this will really impact poorly
- use lower pier and stabilise with shock absorbers and increase
weight
- flexure and dragging:
- scope flexure needs to be minimised - consider avoiding
dovetails
- avoid cables causing dragging
- ensure scope is well balanced to minimise tracking errors
- now you can see why astrophotographers prefer to spend most of their money
on the mount and buy an optical system that has a flat field such as a Tak
106 refractor.
Getting
ready for a session:
- ensure telescope optics are collimated
- some people "hypertune" their mounts in an attempt to minimise
periodic error, for example with a LX55:
- "I have disassembled my mount and performed the following
procedures.
1.) Remove old grease and clean all metal parts with degreaser.
2.) Clean all plastic parts with detergent. I wasn't sure what the
degreaser would do to them.
3.) Polish all metal on metal surfaces with a Dremel tool, buffing pad
and polishing compound.
4.) Relube all weight bearing surfaces and metal on metal surfaces with
a light coat of white lithium grease.
5.) Adjust end-play and depth of worm gears for free operation with
minimal backlash.
6.) Upgrade Autostar Firmware to Version 32Ea for PEC and 3 start
alignment enhancements"
- or at http://www.astronomyboy.com/cg5/index.html
or at www.LXD55.com
- avoid windy nights that will blur the photos and contribute to poor seeing
at high magnification
- avoid poor seeing nights if high magnification is to be used
- go to a dark sky site if possible or early morning when there is less
light pollution and better seeing
- hopefully time it to ensure target is near zenith +/- 30 degrees to
minimise atmospheric problems
- as long exposures (usually 40sec to 5min) are needed, critical attention
to accurate setting up is essential to achieve good tracking:
- ensure telescope is well balanced and mount is level
- accurate polar alignment is important as although it doesn't effect
tracking it will cause field rotation if not accurate
- NB. comets should be tracked themselves not adjacent stars if exposures
are longer than ~60secs
- a well aligned, level, balanced mount should enable unguided images of
stars using 100mm lens for indefinite time, but using a 400mm lens, the
periodic error of the drive becomes visible at exposures greater than 5
min.
- train the mount to minimise backlash:
- backlash is when on starting a motor drive correction, the mount
actually reverses, this can be compensated for such as on the Meade
Autostar by performing the motor calibration and training procedure
to teach Autostar how much to compensate for RA and Dec backlash.
If you adjust the mechanicals to tighten up backlash, or change OTAs,
repeat the procedure.
- ensure light pollution is minimised
by selecting an appropriate region & consider using the nebula filter
Taking the
photos:
- focusing can be problematic, ensure it is perfect - see
focusing
a telescope
- minimise vibrations:
- don't touch telescope or camera for 10sec prior to and for duration of
exposure
- use self-timer on camera or remote control shutter release
- consider placing cardboard in front of telescope for 1st 10secs
- usually need to take quite a few photos of the same object in RAW mode
to get good results:
- adequate total exposure duration for nebulae:
- the aim is to maximise the signal:noise ratio, and for images that
have not become saturated and for which motion blur can be
controlled:
- signal:noise ratio improves with the square root of total
exposure time
- readout noise is proportional to the square root of the number
of exposures taken, thus 4x5min exposures should give better S:N
ratio than 20x1minute exposures
- using narrowband filters will increase contrast and visibility of
emission nebulae while reducing noise from light pollution, but at
the expense of reducing the signal as well and thus a longer
exposure will be needed.
- typical total exposure times:
- f/6 with CCD camera 5min to 120min depending on detail
required and brightness of nebula
- individual exposure duration:
- this is often trial and error
- minimum individual sub-exposure duration:
- subject cannot be too under-exposed as you will never be able to
create sufficient signal:noise ratio for an optimum picture
- increase length of exposure - but do not go past the tracking
limits of your system
- use a faster lens eg. f2-5.6
- use a higher ASA rating - if using film, consider gas
hypersensitisation
- as long as the subexposure is long enough to register the skyfog
at some 30+ times the Read Noise of your camera, then your 30*1min
exposures = 1*30min exposure. This is the so-called Skyfog-Statistics-Limited
regime.
- in digital cameras, many people aim to get the sky glow histogram
midpoint at 10% of maximum if the camera has very low noise (eg. Canon 1
dmk2, 20D, 350XT) whereas those with noisier cameras at ISO 800-1600
(Canon 300D, 10D & Nikon D70) aim for this mid-pt being at 25-50% of
maximum.
- suggested sub-exposure durations at ISO 1600 for dark skies:
- 1min at f/2.8;
- due to most people's mount limitations, this is what most
aim for, hence they use the EF 200mm f/2.8L lens.
- 2min at f/4;
- 4min at f/5.6;
- see http://www.starrywonders.com/snr.html
and http://www.samirkharusi.net/sub-exposures.html
- maximum individual exposure duration is limited by either:
- image saturation due to either the object (signal), light
pollution or noise.
- motion blur due to inaccurate guiding or inadequate mount
- film reciprocity failure - exposures beyond 1hr on film are
unlikely to yield significantly more detail
- subject cannot be too over-exposed as all stars will become fully
saturated and lose any color, and if there is light pollution, ensure
this does not saturate the background which would make gaining a
satisfactory signal:noise ratio impossible
- better to use multiple short exposures and combine them (see
below)
- consider a filter to minimise light pollution (eg. red H-alpha,
OIII, or other red filter) bearing in mind that if filter is not
perfectly flat, it may introduce optical aberrations which may
require fixing before the image can be stacked with images taken
without the filter
- thus a number of photos of the object for stacking purposes
- if using a monochrome camera, will need to take LRGB frames - a
number of frames with the various colour filters applied which will
be used to recreate a colour image often with exposure ratios of
4:1:1:1
- see http://www.ewellobservatory.com/gallery/
for examples of high quality images and how they were taken
- dark frames (with lens covered) to remove sensor noise
- flat frames (of an evenly lit subject) to remove uneven light density
caused by the optical system
Image
processing:
