Practical Comet Digital
Exposure guides for comets:
- "naked eye" comets (magnitude 4.5):
- a 15sec exposure at f/4.5 using a 100mm lens or more at 400ASA on a
digital will show the coma as a small dot
- to reveal the tail requires at least 90sec (preferably 180sec or more)
at f/3.5 400ASA on a digital
- the maximum duration of the exposure will be limited by:
- digital camera:
- most compact digital cameras only allow maximum 15 sec
- most prosumer and SLR digitals have "Bulb" which
allows manual exposures usually up to 8 minutes or so.
- light pollution:
- in the suburbs at 45deg altitude, max. exposure is ~
90sec and tail will not be visible
- 30km from suburbs, max. exposure may be significantly more
than 3 min & tail will be visible
- a nearby moon will have a severe impact on ability to show
- the closer to the zenith, the less light pollution
- light pollution will obscure the tail in particular and
this is difficult to fix so best advice is to choose a
location away from significant light pollution
- light pollution filters do not work well for comet tails
as the gas trail is mainly the cyanogen line which few LPS
filters transmit (except the Comet filter) while the dust
trail is reflected sunlight and thus will only get dimmer
with a filter rather than have its contrast enhanced.
- unguided motor driven with no wind buffeting will allow up
to 3min exposure but there will be some star trail at 400mm
focal lengths or more, but hopefully acceptable
- 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.
- non-motor driven on tripod, star trails are evident at
15sec on 140mm focal length
- presence of wind, clouds, car headlights, etc.
- stacking photos:
- stacking shorter exposures of 60secs may be helpful, but the more
pics, the greater the comet travel in relation to the stars over the
time taken to take the pics, and thus the greater the apparent star
The importance of subtracting dark frames
from digital camera exposures:
- all CCD sensors have dark currents that create an image on long exposures
even when the lens cap is on and completely dark, such an image is
called a dark frame and must be taken at the same exposure duration as your
subject photo and then subtracted from it - this may be done in-camera or
manually by taking a number of dark frames and using software to take the
median of these to make a master dark frame.
- seeing is believing! - here is a 10% resized dark frame of a 90sec exposure in a Olympus
C8080 digital camera at 400ASA - you don't want this artefact added in every
image you take, fortunately, the camera will automatically remove this for
you if you wish, although it doubles the time the camera is locked.
Examples of photos of Comer LINEAR T7:
- these were taken in May 2004 when it was magnitude ~4 at altitude of 45deg
from the west horizon, taken with the Olympus C8080 with a teleconverter
making it 200mm focal length f/3.5 at 400ASA with camera on a motor-driven
equatorial mount but unguided (ie. there was no drive corrections made to
allow for comet drift or inaccuracies with mount set up).
- the images are with in-camera dark frame subtracted and cropped from the original 8 megapixel jpegs and resized to
70% of original. No image processing has be done.
- the coma of the comet was just visible in binoculars in Taylors Lakes and
just visible to the naked eye from Mount Aitken. The tail was difficult to
see even through binoculars at Mount Aitken.
- the difference in apparent size of the comet's head in the images is due
only to the differences in exposure and the difference in contrast with
background light pollution, not to any change in magnification.
Firstly, from Taylors Lakes where there is moderate light pollution
looking west at 45 deg altitude:
Now, how much better it can be - from only 30km north at Mount Aitken where
there is only mild light pollution:
|240 secs but taken when only 20deg above horizon, thus more
light pollution and atmospheric extinction