Prime focus digital imaging
- prime focus is when a camera is attached directly to the optical path of a camera lens
or telescope without an eyepiece although a camera tele-converter lens or a Barlow lens may be added to the system to increase
- most telescopes use a T2 mount which is a screw mount similar to the
original T mount (the Pentax M42 mount) but the thread is 0.75x that of the
T mount thread and the flange to image plane is 55mm compared with 45.5mm on
- you can buy T2 adapters for practically all 35mm and digital SLR mounts
- the main advantages are:
- image quality as there less optical elements to add to aberrations as
with eyepiece projection
- shorter exposure durations needed as faster f-ratios
- exposures longer than 1/30th sec will need tracking with a motorised RA
- images will be improved further if there is some form of accurate
- via a 2nd telescope system attached to the mount:
- manual guiding via an illuminated reticle eyepiece
- auto-guiding via a dedicated auto-guider
- via the same telescope optics:
- manual guiding or auto-guiding as above but via an off-axis
guide that splits the beam to some is visible in an eyepiece
right angles to the camera path
- some dedicated CCD cameras have a built-in auto-guider
- NB. auto-guiding systems require a telescope mount that will
accept the auto-guider's input
- focusing can be problematic see focusing a telescope
- digital options are:
- digital camera - for a digital camera to be able to do this,
it's lens must be removable & thus it must be a digital
- eg. Canon Digital Rebel 300D
- NB. need to ensure you can set the mirror to be up before the
self-timer starts to avoid camera shake from the mirror.
- Canon EOS cameras have a reputation for noisy mirror slap and
apart from the 1Ds, what is more difficult to deal with is their
shutter shudder even with mirror up (may need to use a manual
shutter such as a cover over the telescope). This is why the Olympus
OM-1 was so popular.
- webcam - usually modified but tend to have low pixel counts
- great for high power imaging of moon & planets with eyepiece
projection as take hundreds of images for stacking & the
resulting images rival if not better than most DSLR images eg. 500
frames at 5fps although total duration is limited by object rotation
esp. jupiter as the clouds will appear blurred in > 90sec.
- eg. ToUCam $US150
- eg. Quickcam 2000 $US40
- dedicated astrophotography CCD camera
- these cameras are especially designed for high sensitivity, high
dynamic range (often 14-16bit) and low noise via cooling to minus
- eg. S-BIG's range
- the best set up for deep sky objects if you have lots of money is a
high-performance mount, CCD plus autoguider with adaptive optics, and
- maximising the signal to noise ratio:
- readout noise is not constant. By summing images this non constant
noise (poisson distribution) is averaged out over the entire image. Yes
it will bring up the background brightness if stacking many many images,
but for the most part summing many images increases the signal to noise
ratio by the square root of the number of images summed. There is a
point of diminishing returns as the background level increases but for
those who do not have mounts that can track for 3 to 4 minutes or do not
autoguide, summing images works quite well. Noise that is constant such
as bad/hot pixels or readout defects due to charge transfer efficiency
will however degrade the image if summing. This is where darks, flats
and bias frames come in.
- whichever option is used, you will still have to weigh up multiple
short exposures vs one or a few long exposures:
- This debate over 1 long exposure vs. summing has been going on for
quite sometime. They both have there benefits and downsides. There
is a sweetspot that every image train and setup has where you are
taking the longest exposure you can and have to sum images beyond
that. You just have to find that spot.
- when effective focal length is long such as 2000-4000mm, most people
will not have the equipment to track accurately to allow more than
- bright globular clusters such as M13 and Omega Centauri can be
imaged at 1600ISO, 14" f/11 4000mm with stacking of approx. 30
frames of 10-15sec exposures.
- some maths for digital prime focus:
- image scale = 206 * (pixel size in microns) / (focal length in mm)
- the Canon 300D pixels are about 7.2 microns so assuming an LX200
10" f/10 telescope you would get:
- 206 * 7.2 / (25.4 * 10 * 10) = 0.58 arcseconds per pixel.
- So your field of view would be:
- 3072 * 0.58 / 60 by 2048 * 0.58 / 60 = ~ 30 arcminutes x 20
- If you put your 26mm eyepiece in the scope and assuming an
apparent field of view of 50deg and again an f/10 telescope:
- Magnification is: 10 * 10 * 25.4 / 26 = 97x
- The actual field of view is the apparent field of view divided
by the magnification:
- 50 / 97 = 0.51 deg or about 30 arcminutes
- So the long side of your ccd chip is going to give you about
the same field of view as your 26mm eyepiece assuming its a 50 deg APOV
SLR vs dedicated CCD camera:
- digital SLR's:
- see also http://www.pbase.com/mataylor/image/39066227
for a method of using a modified 300D with filters for prime focus.
- versatile & can be used for other photography
- cheaper (although still cost $A2000) => great bangs for bucks
- easy to use
- noise is becoming less:
- you can shoot with a Canon 10D at 10°C for 10 min and get
almost zero noise.
- easy one-step colour photos but not as good a quality as more
complex 3 or 4 photo colour-filtered B&W CCD's
- great for bright objects such as moon, jupiter, sun even in light
- good for the brightest 10-20 Messier objects (eg. M45, M42, M31)
away from light pollution, but not as good for the dimmer ones as
these need long exposures (eg. several minutes) for each frame and
- not good in light polluted regions for deep sky objects
- dynamic range is still not as good - usually only 12bit per
- with only 12 bits, and with gains dialed in optimized for
daylight, it is incredibly difficult to overcome light
pollution, which shortens how long one can expose. The
dedicated astro camera has electronics optimized for scraping
the electrons from the dimmest signal off the noise floor.
That's what you're paying for.
- with dark skies and a fast scope, you gain signal as fast as
possible before noise can take over to begin with. In
these conditions, the differences narrow significantly and come
down to more RBG Bayer Matrix limitations vs. color filtered
Mono with Luminance ability.
- in-built fixed IR cutoff filter limits sensitivity:
- you will always have the limited sensitivity due to the Bayer
filter and will have the red-cut off filter to preserve a
natural color balance in daytime use.
- it captures H-B (which occurs with H-alpha, but in smaller
amounts) quite well, which produces those very lovely magenta
emission nebula (like most 10D lagoon images).
- the purpose in trying to hit the h-alpha line, and the sulfur
line above it, is to bring in detail that wouldn't ordinarily be
seen by the eye. This is largely why all the great
astrophotography films are disappearing, because the wavelengths
that make for detail in emission objects are NOT the same as
those in portrait photography, for example.
- these can be removed but then may
make camera unusable for daylight photography
- the problem of colour sensors vs B&W CCD's:
- D-SLR will be close to cooled one shot color cameras, but
never close to b/w CCD cameras in performance. This is a
physical principle from the design - getting all photons to all
- imagers in light polluted environment, the narrowband imaging
gets more and more essential and this is a specialty for b/w
- dedicated astrophotography CCD cameras:
- can produce the best quality images of deep sky objects, esp. in
light polluted regions, in experienced hands
- can image dimmer objects
- use of narrow-band filters allows imaging even in light pollution
or the full moon
- low noise - cooled to minus 45degC
- B&W sensors give higher resolution and sensitivity
- high quantum efficiency (QE) - but lower if ABG or if have
color filters placed in front of the sensors as is the case in
most one-shot colour CCDs
- higher dynamic range - usually 14-16bit
- sensitive to H-alpha region of IR light
- often incorporate auto-guiding correction - but need compatible
- often are designed to limit "blooming" effect of bright
stars ie. have an anti-blooming gate (ABG)
- the better ones use Kodak KAF scientific sensors
- tri-colour imaging using a monochrome CCD minimises chromatic
aberration in refractors so may be able to go for a cheaper non-APO
- expensive, especially the larger pixel versions
- require use of a laptop to focus and capture image
- use water to cool them down to minus 30-45degC - tubing, pumps,
etc and when get cold, are subject to dew condensation if high
- older ones are small image size and slow image transfer to
computer via parallel port - avoid these - choose USB2.0 for better
- can only be used for astrophotography
- for colour images:
- B&W sensors:
- need to take 3 photos ("tri-colour imaging") - one with each of the 3 colour
filters in place:
- the normal practice is to take tri-color images with each
filter at a specific length given a particular ratio.
This ratio is determined by the length of three images with
RGB filters that produces TRUE color in an object we KNOW to
be a specific color, in most cases, on a true white G2V
star. From there, we just let the colors fall as they may.
This balance is indeed affected by the extra information
from h-alpha sources, for example, but it's also affected by
light pollution and atmospheric dispersion. So, when
this happens, the idea is to balance the image on a known
constant, where white stars are indeed white in the image.
- colour one-shot sensors:
- all Kodak color sensors have an IR blocking coating on
them (that makes them TRULY ONE SHOT!), however this coating
can be polished off.
- consider as a starter kit:
- astronomical filters + filter holder, and either:
- used SX HX-916 camera - nice field of view, great sensitivity,
doesn't need dark frames & is very lightweight
- SX V-H9
telephoto lens vs Refractor telescope:
- the main issues:
- if your mount and guiding is not high-end & critical then you must
use short exposures and this then needs fast, flat field optics such as
- optics must be high quality to ensure minimal aberrations and a flat
field (ie. stars all over the image are sharp) as any problems become
very obvious in astrophotography. Zoom lenses are not usually adequate.
- temperature instability may change the focus point during a long
- does the image circle cover your camera sensor - this can be an issue
with larger sensors such as the 35mm full frame sensors.
- the more elements in a lens, the lower the contrast and the poorer the
- for colour images, the lens must have minimal if any chromatic
aberration, and thus all glass should be APO not just ED.
- ability to use for visual use &/or cooled CCD cameras with filters
- cost and versatility
- camera telephoto lens:
- the resolution problem:
- lenses with f-ratios smaller than f/8 result in poor resolution on
- if you put the x2 teleconverter on the 400f5.6L you cannot get
even the Canon 10D resolution out of it, while the 300f4L is a
borderline case, with barely 10D resolution
- only the f2.8 telephoto lenses seem to have more resolution with a
x2 teleconverter than 10D has.
- a 105mm aperture f/5.8 Traveler refractor w/flattener outperforms
a Canon 500 f/4 and 300 f/2.8 for astro imaging with a 1Ds.
Stars are smaller/cleaner and contrast is greater in the scope.
The 500 and 300, however, focus much easier and are easier to use
under the stars than the scope.
- Canon L series IS lenses tend not to be as sharp as non-IS lenses
for astrophotography but still acceptable.
- see super tele
- Canon 600mm f4.0 L IS USM - 133mm aperture:
- Samir Kharusi feels that this lens competes well with modern
APO refractors in 2007 see here
- Canon 500mm F4.0 L IS USM - 125mm aperture:
- lighter than the 400mm f/2.8
- approx. $US5000 ~ same price as Takahashi TOA 130 f7.7 but half the weight
- used with the Canon 10D and the 2x + 1.4x teleconv. results in
an effective focal length of 2240mm.
- Canon EF 400mmf2.8L II USM IS - 130mm aperture:
- heavy for a telephoto (6kg) - perhaps too heavy for wildlife
- 11 lenses incl 1 flourite and 2 SD lenses
- high resolution, perhaps not as much contrast as a APO
- equates to 142mm aperture & gives good results with 2x
teleconverter (ie. similar to 5.5" APO with focal reducer,
- can be stopped down to 100mm aperture at f/4 for even sharper
images, thus similar to a 4" APO with focal reducer
- autofocus works on bright stars
- even at f/2.8, it is sharper than Borg 101EDf4 with focal
reducer, but the Borg costs $US2490 while the lens, second hand,
is around $US4300. The IS model costs about $US6500 but is
still not as sharp as the Borg refractor.
- Canon 300mm f2.8 IS:
- great for wildlife as only ~3kg
- Canon 300mm f4 IS:
- slightly less sharp but cheaper than the Borg 77ED
- Canon 200mm f/2.8 II:
- a favourite for many astrophotographers
- slightly softer than a good APO but is slightly better color
- Olympus C8080ZW with 1.4x teleconverter = 196mm f/3.5:
- cheap, great quality optics, no problems with dust getting on
- lighter, cheaper, more versatile and more readily available than
APO refractors at focal lengths 300m and less.
- auto-focus removes headaches
- can be used for general photography
- for most amateur astrophotography these are adequate as seeing
conditions and guiding problems usually cause more problems than the
L series optics, particularly when talking about focal lengths of
- very few have the diffraction-limited optics of APO refractors (which are designed to work mainly at infinity focus),
unless stopped down.
- cannot use for visual use with eyepieces unless get special
adapters, and even then you cannot use a diagonal with the lens as
there is not enough back focus available (although medium format
lenses do have sufficiently long back focus). Hence, for
visual use a scope is always much more versatile.
- zoom lens are not usually good enough and an IS lens is not as
good as a non-IS lens.
- can only be used with one style of SLR camera mount
- stopping down the lens, while producing sharper images and perhaps
less aberrations, loses resolution and light-gathering as the
effective aperture is reduced.
- for critical use, even the best camera lenses are NOT adequate as:
- limitations on back focus
- inability to use modern filtering techniques & cannot use filters as too expensive to fit on the front aperture,
although fortunately the non-IS supertelephotos take 48mm
filters - exactly the same as 2" eyepieces take.
- cannot use cooled CCD cameras with them
- lowered contrast because of the large number of optical
elements than the optimal 3 or 4 that are in an APO refractor
- focusing problems
- temperature instability affecting focus
- depending on the scope, it either has been designed for
astrophotography or not, and it might need an additional field flattener for that, and it might not be diffraction-limited all
the way to the corners of a full-sized sensor (NP-101 is not, while the
FSQ-106 has been designed for medium format photography)
- in general, APO refractors are less expensive than high-quality L
series Canon lenses when considering focal lengths greater than 300mm
- the shortest APO refractor is 300mm
- the best refractors for astrophotography are those with true triplet
or Petzval (4 element) designs are true APO and include:
127 mm aperture 660 mm focal
length f 5.2 $US6500
101 mm aperture 540 mm focal
length f 5.4 $US3600
- Takashi FSQ 106 mm
aperture 530 mm focal
length f 5.0 $US3700
- Vixen APO
130 mm aperture 860 mm focal
length f 6.6 $US4400
- TMB APO
105 mm aperture 650 mm focal
length f 6.2 $US4400
- TMB APO
130 mm aperture 780 mm focal
length f 6.0 $US5500
- StellarVue APO 115 mm
aperture 805 mm focal
length f 7.0 $US4000
- StellarVue APO 130 mm
aperture 780 mm focal
length f 6.0 $US6000
- William APO 110 mm
aperture 715 mm focal
length f 6.5 $US3300
- William Megrez 80 Triplet APO 80mm 480mm f/6.0
- see william
- other options:
- the new Takahashi Epsilon 180 mm aperture
500 mm focal length f 2.8 $US3900
covers 35mm full frame DSLRs well.
- hyperbolic astrographs:
- Newtonian-configured reflector telescopes with hyperboloid rather than
parabaloid primary mirror.
- 3 or 4 element Ross corrector element at the output end.
- Imaging at f/3.3 is really great! One can take centering shots
and bin 4x4 or 6x6 giving another factor of 16 or 36 in sensitivity.
If one can't see a comet or galaxy or nebula in a 5 second exposure, it
simply isn't there! Even with an H-a filter that cuts down the
transmitted flux by a factor of about 2000, one can usually see the
target with a ten-second binned centering shot.
- not useful for visual use as very few eyepieces will come to focus
but excellent for prime focus astrophotography.
- central obstruction from the diagonal secondary mirror reduces
- thick spider arms result in square stars and fairly prominent
- Collimation is exceedingly difficult, and most owners send the
scopes to TNR if they have to be collimated. If your new scope
is out of collimation from the factory, you may have additional
costs to get it right. Finally, Tak accessories are expensive.
- see http://dpersyk.home.att.net/
- Takahashi E-160 160mm aperture, 532mm focal length hyperbolic
astrograph with its f/3.3 and no perceivable CA from 420 nm to 1060 nm,
meaning one can take luminance frames without killing off the NIR with a
blocking filter. Has a 43 mm image circle and costs $US3300.
180ED hyperbolic astrograph that is 500mm focal length and f/2.8,