aim for skyfog to occupy lowest 1/3rd of image histogram with some gap to its left on all 3 RGB channels as using this regime will mean that when N images of T minutes are stacked they will give SNR comparable to a single image of NxT minutes.
SNR in a 2min integrated image at dark sky sites = 30min integrated image in outer suburbia with 15x brighter sky fog
a light pollution filter can improve SNR by 2x in outer suburbia ie. you can use half the integration time for same SNR.
in outer suburbs, sky fog means that you can use a lower ISO and 30-120sec sub-exposures according to histogram for the aperture, but you may need a total of 2-3 hours integration time for the best SNR.
measuring sky fog:
use 800ISO at f/4 and expose until peak of sky fog is in middle of histogram then use:
Mag/sq arc-sec = 13.93+2.5*log10(seconds to mid histogram at ISO 800 and f4)
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!
camera + telescope = $US5500, but to get this quality 15min exposures, may have been on a $US12,500 mount and the 3 special narrow band filters would cost an extra $US1500 and of course you need a computer.
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 $US12,500 plus cost - usually +/- 2 arc sec error; weighs 64lb; max. load 150lb; designed for robotic use;
if you are only imaging in the outer suburbs in light pollution, you do not need as good a mount as your individual exposure times will be limited to less than 60sec anyway due to light pollution.
a telescope optical tube with minimal aberrations, fast optical speed (eg. f/5.6-f8) and high contrast
examples:
ED or flourite APO refractor 3-6” diameter (60mm or more)
Canon 400mm f/5.6L vs Televue TV-60is f/6 - see here
NB. while SCT telescopes are great visual telescopes and for planetary/lunar photography (eg. with a webcam or similar and stacking images), it's focal length is generally regarded as too long for DSO's, even with a focal reducer (and this limits sensor size possible without getting vignetting), and the fork mounts they are usually sold with tend not to be adequate for the precision guiding needed.
low noise prime focus mounted camera capable of long exposures such as:
for galaxies, star clusters and some nebulae, an un-modified dSLR will do fine
BUT for emission nebulae, unmodified dSLR's have a IR blocking filter which prevents most of the H-alpha infrared emissions from being recorded, removing this filter allows ~3.4x sensitivity and thus allows ~1/12th the number of sub-exposures that need to be taken to get the similar results as an unmodified camera
SBIG CCD camera cooled to -45degC with in-built auto-guider and filter wheel
SBIG CCD cameras with built-in autoguiders (2004 prices):
ST-7XE ($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.
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.
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
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.
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”
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
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.
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
