photo:ast_photography_dso
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.
examples of results:
Equipment required:
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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
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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:
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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:
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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.
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consider a nebula filter:
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.
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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:
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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”
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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
image processing
photo/ast_photography_dso.txt · Last modified: 2022/01/08 06:13 by gary1