Astrophotography with
Digital Cameras
Choosing a dSLR for deep sky astrophotography:
- see also:
- in the days of film, the legendary Olympus
OM-1n was king of astrophotography as it was light, had mirror lockup,
bulb time exposure with cable release, worked without batteries and you
could buy focussing screens that enabled easier focus when used on long
focal length lenses. But today digital is king, although battery life can be
an issue, particularly if you want to do those star trails over several
hours!
- most beginners should aim to take multiple 60sec sub-exposures at ISO
800-1600 and at f/2.8 in dark skies with focal lengths no longer than 300mm
in 35mm terms using a tracking mount (unless you have a really good mount +/- autoguiding gear).
- you can use longer focal lengths (eg. prime focus
with camera attached through a telescope) for bright subjects such as
the moon and Jupiter, but for other subjects
you will need a VERY GOOD mount probably with auto-guiding system to get
good results.
- see Samir's
guide for beginner astrophotographers
- ideally the camera should have:
- low noise at high ISO (most currently use ISO 800-1600)
- Canon wins here but the latest Olympus models are OK at ISO 800
- for dark sky sites, using a camera with low noise at high ISO like
the latest Canon dSLR's, it seems that using 1600ISO to give you
shorter sub-exposures (60sec at f/2.8) and thus less guiding error is the way to
go.
- in light polluted sites, sub-exposures are limited by the light
pollution anyway so lower ISO's are better (eg. 400-800).
- see http://www.pbase.com/terrylovejoy/optimal_isos
for details on the best ISO to use for DSO's
- see Roger
Clark's many articles on sensor performance in low light
situations
- true RAW mode that doesn't obliterate faint stars
- ie. not Nikon unless you want to use Mode 3 controllers
- NB. there are concerns that the RAW file is not a true RAW
file in Nikon dSLRs as Nikon applies a blurring noise reduction
in-camera process even to the RAW files which removes faint
stars, thus they need to be used in "Mode 3" which is
a bit of a pain.
- see here for more details.
- its IR filter replaced with UV/IR Ha-enhanced filter to give good nebula images via improved Ha
infrared response
- modern dSLRs block the H-alpha region allowing
only ~27% transmission, removing the IR blocking filter increases
transmission to nearly 98%, thereby giving 4x more sensitivity or
2EV for H-alpha nebulae.
- ie. Canon & Nikon dSLRs or Olympus E410
- see Hap
Griffin's modified dSLRs and Hutech
- Live Preview to
accurately manual focus
- most new dSLRs, although in addition, Olympus models have Live Boost for seeing even
fainter stars
- see also: focusing a telescope
- preferably an articulating LCD so you don't get a sore back
- high quality prime lens (not a zoom lens) with good performance at aperture f/2.0-f.2.8
at 35mm effective focal length 200-300mm, preferably without optical IS
which degrades star images.
- lenses wider than 100mm in 35mm terms have limited utility as
light pollution gradients are problematic and the lens preclude the
use of interference type filters such as most narrow band light
pollution filters, while the only objects for a wide angle are the
Milky Way, aurorae and perhaps meteor showers, but if you do wish to
try them out, be aware that you will need to stop them down for good
star images: eg: 50mm lens at f/2.8-4, 28mm lens at f/5.6.
- see also super tele lenses
- for Canon:
- most get the Canon EF 200mm f/2.8 L non-IS lens with the
tripod mount ring (buy separately as not supplied), better star
shapes at f/3.2
-
Canon 100mm f/2.8 USM macro gives excellent results
at f/2.8 (don't use the older non-USM one) as does the 100mm f/2.0
when used at f/2.5.
-
one might also consider the Canon 135mm f/2.0L
although there seems to be little experience on the web
forums.
- for Olympus:
- the expensive but very nice ZD 150mm f/2.0
- the cheaper but still high quality ZD 50mm f/2.0 macro lens
+/- 1.4x teleconverter to give an effective 140mm f/2.8 lens
although lacking in telephoto reach.
- legacy manual focus 135mm f/2.8 lenses such as Nikkor,
Leica R or Carl Zeiss lens may give good results at effective 270mm
f/2.8
- be light, compact with good battery for cool conditions
- mirror lockup:
- this is to avoid vibrations of the camera mirror blurring image - this is
essential for exposures from 5sec to 1/250th sec
- Olympus - mirror lockup can be achieved through setting a timer in
the menu under "AntiShock" which will put mirror up for
that length of time prior to shutter being opened.
- Canon - seems there is a problem that you cannot set mirror lockup
when using the Timer Remote Controller!
- timed 60sec exposures as this is the most commonly used sub-exposure
duration
- Olympus has this but you need a TC-80N3 Timer Remote Controller for Canon
dSLRs (but not available on the Rebel dSLRs)
- the latest Nikon or Fuji dSLRs can use the Nikon
MC-36 Multi-Function Remote Cord
- You can't remote release many Nikons (eg. D40) using Camera
Control Pro when the camera is in Bulb mode, you need to buy 3rd
party tools like Shoestring Astronomy's DSUSB-IR and download DSLR
Shutter from Stark Labs.
- time lapse facility so you can do 20-100 sub-exposures easily
- for Canon, use the TC-80N3 Timer Remote Controller (with
N3-capable dSLRs)
- Nikon D300 has a built-in intervalometer
- for Olympus, use their software or 3rd party software but avoid
using AF lenses as software will try to AF it unless you use a MF My
Mode setting which is not available on the E410 so you may need to
resort to destroying the AF pin.
- see remote control of cameras
Example dSLRs for astro work with Live Preview:
- if you are prepared to work using a tethered laptop, then Live Preview
is not so important, and the laptop has advantages of bigger screen and
you don't get a sore back trying to see the image on the camera in an
awkward position, BUT the laptop method is much more cumbersome,
particularly for the beginner.
- However, even when using a laptop, live
preview is useful as it allows real time assessment of images rather
than having to take a shot then assess it, but be aware that live
preview:
- is limited to brighter stars using a fast aperture lens (eg.
f/2.8-4)
- stops displaying during the exposure
- is usually output via a video cable at low resolution - often the
resolution of the camera LCD screen
- at high magnification, the slightest movement will cause trailing
of stars so you may need to aim for the narrowest trail to get focus
- Canon live preview dSLRs:
- Canon has long been the favourite for astro-photographers with its
low high ISO noise and ready availability of 3rd party software to
control it, although this is not quite as important now that Live
Preview is available to focus.
- Canon 1D Mark III:
- has best low light performance of the Canons but is big, heavy
& too expensive for most to replace its IR filter
- a better compromise is a modified Canon 40D with Timer Remote
Controller.
- a cheaper alternative is a modified 450D but it does not have a N3
connector so cannot use the TC-80N3 Timer Remote Controller without
modifications which makes
timed 60sec exposures and repeated sub-exposures more difficult and
requires a laptop with a 3rd party cable and software.
- if you don't need live preview, or can't afford it, a modified
350D is better than a modified 400D for astro work see here
- Nikon / Fuji live preview
dSLRs:
- Hutech sell modified Fuji S5 Pro, Nikon D300
- Pentax live preview dSLRs:
- Pentax 20D
- Pentax have some nice prime lenses such as the 200mm f/2.8 ED IF
which would be perfect
- currently Hutech do not offer to modify Pentax
- Olympus live preview dSLRs:
- for many, the inevitable increased noise and lower dynamic range
of Four Thirds sensors compared with Canon sensors is too limiting
for serious astro work where ISO 1600 is preferred, but for wide
field use or in light polluted skies where one only needs ISO
400-800, then these can make good astro cameras.
- furthermore, the 2x crop can be of use in prime focus work where
magnification is important and when the telescope is not able to
give good images at the periphery of full frame sensors.
- maybe the articulating live preview LCD of the Olympus E3
will save some sore backs, while its timed 60sec exposures is
perfect for most deep sky work although you really need to get its
IR filter removed for nebulae work.
- the Olympus E410 with IR filter modified seems to be a reasonable
choice if you are an Olympus fan:
- its relatively cheap, light, compact so it will not add much
strain to your telescope mount
- it has Live Preview to assist manual focus although not
articulating LCD as with the E3.
- Hutech will replace its IR filter for you
- IS is not needed for astro work so the added expense of an
E510 is not worth it.
- BUT as there is no My Mode facility to stop Olympus Studio
insisting on doing a autofocus and stuffing up your careful MF
with ZD lenses, you may wish to further modify it by disabling
the AF pin permanently!
- Olympus Studio software is useful for Time Lapse control.
- other accessories to consider:
Using a
digital camera at higher magnifications:
- for bright planets and the moon:
- a SCT telescope is probably best for this high magnification work
although a Newtonian will do but need a good, big mount.
- APO and prime lenses do not usually have sufficient aperture for
the required magnification
- via prime
focus on a telescope:
- removable lens to allow direct attachment to the telescope
eyepiece mount ie. must be a DSLR
- generally a webcam with eyepiece (see next) is a better choice
than a dSLR as can take many photos in 1 minute and then stack them.
- for optimum resolution & brightness then aim for a f/ratio = 4
x pixel size of your sensor
- via eyepiece on telescope (afocal
method):
- ability to be attached to the eyepiece via an adapter
- small lens where its diaphragm is near the front of the lens to avoid
vignetting
- not too heavy camera
- many use a webcam or the Nikon 4300 for this
- I have used a Canon S30 point and shoot with my 10" Newtonian
via an eyepiece - see astrophotography
- when purchasing a camera to use, consider:
- can it be easily mounted to the telescope eyepiece
- cameras with a thread on the front of the lens can usually be
easily adapted to fit eyepieces via adaptors
- options include:
- all-in-1 DigiT kit specific to a camera style: $A179-209
- DigiT adapter
(Pentax T screw mount) clamps onto eyepiece
($A60) + specific camera T adapter ($A130-170)
- MaxView 40 eyepiece with inbuilt T thread ($A279) +
specific camera T adapter ($A130-170)
- better for wide aperture lenses to minimise vignetting
- cameras with removal lenses (dSLRs) can also use either:
- prime focus if a T-mount
adapter is available
- eyepiece projection method using an eyepiece and an adapter
but no camera lens:
- afocal method using an eyepiece and an adapter and a camera
lens
- with
"fast" fixed focal length lenses or telephoto lenses for
better piggy-back pics
- photos of constellations, aurorae, or meteor showers can
be done with almost any lens
- comet photos ideally need a 200-500mm lens, preferably
with aperture f/3.5-5.6, but will need to be piggy-back
mounted on a telescope which is guided on the comet for good
results as exposures at 400ASA need to be 15sec - 2min on
digital cameras, to get image of the fainter tail, it is
important to have a dark sky away from light pollution
otherwise the whole image will be light & obscure the
tail.
- other cameras can by mounted with a EZ-PIX universal camera adapter
($A109) although this require some time to align properly as there is no
direct connection to the eyepiece axis.
- issues with non-dSLR digital cameras using afocal method:
- weight may be an issue as heavy cameras may:
- create balance issues with telescope mount
- digital camera lens attachments may stress the lens filter or
the lens itself, permanently damaging the camera - may need to
use an attachment system that clamps onto camera body or via the
tripod mount.
- the larger zoom lenses usually have a very proximal iris diaphragm
which is impossible to get sufficiently close to the telescope
eyepiece to avoid vignetting making the afocal method impossible
- may need to use 2" eyepieces &/or eyepieces with long
eye relief
- cameras with larger lenses are best suited to piggy-back photography
only.