BAA Lunar Section
Dedicated to amateur research and observation of the Moon
The Moon: Notes and Records of the BAA Lunar Section
The New Moon journal (1983-2010)
Lunar
photography
Advisor:
Bruce Kingsley
Introduction
to lunar photography
by
Peter Grego
Until
the advent of photography in the mid-19th century, drawing the Moon
at the eyepiece was the only way to capture the splendour of the
lunar landscape. The arrival of CCD cameras and their increasing
availability to amateur astronomers since the 1980s enabled lunar
imaging to take its next great step - high resolution imaging that
captures all the fine detail that the eye can see. Image processing
and computer enhancement further refines images to bring out more
detail, and our understanding of lunar topography is aided by placing
features in fresh perspectives (such as creating alternative views
based on 3D modeling or producing anaglyphs of the sort shown below).
The
Moretus area on 2008 September 21, imaged and processed into a 3D
anaglyph by Bruce Kingsley (requires red/blue glasses)
Conventional
photography
The
Moon is such a large, bright object, that any film camera pointed
through a telescope can capture a lunar image. With luck and
experimentation, pleasing images can be obtained by holding a camera
up close to the eyepiece of an undriven telescope and clicking away.
But this is very much a hit and miss approach - costly, too, if out
of a whole roll of film you're only going to produce just one or two
reasonably good shots!
There
are a number of important guidelines that all successful
conventional lunar photographers follow, but there is also a degree
of art to it - the products of experience and intuition that can't be
explained clearly in writing. For the very best results, a sturdy,
driven equatorially mounted telescope is required, since even with
exposures of fractions of a second, the Moon's drift through the
field of view in an undriven instrument will produce a degree of
image blurring. To obtain the sharpest image possible, a camera
should be secured to the telescope so that it is in line with the
optical axis and unable to wobble around.
Afocal
photography with a fixed-lens camera
The
techniques of afocal photography apply to both conventional and
digital fixed lens cameras alike. However, the simplest and cheapest
types of film camera don't have any means of focusing the image -
they have a set focus range from several metres to infinity - nor can
the exposure time be adjusted. In spite of their severe limitations
such basic cameras are sufficient to capture a pleasing image if the
Moon is first focused in the telescope eyepiece, and the camera is
then positioned close to the eyepiece (but this has to be the right
kind of eyepiece, as explained below). The photographer will usually
have to fashion a makeshift adapter for the camera to fit onto the
telescope, since the cheapest cameras won't be threaded to accept
adapters or attachments. All amateur astronomers ought to have in
their toolbox some blu-tack and electrical tape - adequate materials
to temporarily fix a lightweight camera to a telescope.
Even
though an eyepiece can give a large apparent field of view when
viewed with the eye, vignetting can occur on the finished photograph
- that's where the image of the Moon is surrounded by a dark circular
border. The degree of vignetting depends on the size of the camera's
fixed lens in relation to the eyepiece lens, the type of eyepiece
used and the distance of the camera lens from the eyepiece. The main
cause of vignetting is due to the camera lens being much larger than
the eyepiece lens. In addition, the eye relief of an eyepiece affects
the degree of vignetting, and very short eye relief eyepieces
(usually short focal length eyepieces that give high magnification)
are difficult to use afocally with a fixed lens camera because it is
necessary to position both camera lens and eyepiece lens as closely
as possible (so close that they are almost touching). Usually the
result is highly unsatisfactory, with only a small central image
surrounded by a dark border, like viewing through a long tunnel.
Eyepieces that deliver small apparent fields of view also produce
vignetting, so basic types like the Huygenian design are to be avoided.
Finally,
make notes of your experiments! There's nothing more frustrating
than running off a roll or two of lunar exposures, discovering
several lunar images that appear to be perfect, only to scratch one's
head and wonder exactly which eyepiece was used and what exposure employed.
Advanced
lunar photography
The
most popular camera used by lunar photographers is the 35 mm SLR
(single lens reflex), a design that has been in common use since the
1960s. Some of the older and lower end SLRs are very basic in design,
with exclusively manual controls. The more expensive SLRs have full
electronic controls that can adjust every aspect of the camera's
functions. A basic SLR is all that is required for lunar photography,
and it can deliver just as good a lunar image as a camera ten times
its price. The image entering the SLR is directed by an internal
mirror into the viewfinder, allowing a direct view of the subject
being photographed. When the shutter is released, the mirror flips
up, allowing light to fall directly onto the film. The mechanical
vibrations introduced by 'shutter slap' can affect the quality of the
image, and the mirrors of some cameras can be locked into the 'up'
position before the image is taken. To minimise vibration that might
blur an image when pressing the shutter button, lunar photographers
use a cable release, a function that can be controlled electronically
on high-end SLRs.
Telephoto
lens photography
Telephoto
lunar photography has several advantages - the equipment is
relatively lightweight and portable, allowing quick setup and access
to the Moon from areas in the backyard that might not be possible
with a bulky telescope and its mount. Used with fast film (with a
higher ISO rating, hence shorter exposure times) the relatively low
magnification of the Moon will not produce a great deal of image
drift and blurring. Provided that the camera is stably supported
(preferably on a tripod), very nice images of the whole Moon that
show detail along the lunar terminator can be obtained with a 35 mm
SLR using a telephoto lens with a focal length of 800 mm to 2,000 mm.
To find the size of the lunar image on film, simply divide the focal
length of the lens by 110. A 50 mm lens will give a tiny lunar image
size of less than half a millimetre; an 800 mm lens will give an
image more around 7 mm across, and a 2,000 mm lens will produce an
image of 18 mm. A x2 teleconverter will effectively double the focal
length of any telephoto lens, extending a 500 mm focal length into
1,000 mm, although the resulting image will be slightly dimmer and
inferior to one taken with a 1,000 mm telephoto lens. However, the
larger image size resulting from the use of a teleconverter will
produce a far better image than a lab-enlarged image of the Moon
taken with the same telephoto lens without the teleconverter, since
the enlargement will be grainier and less distinct due to the
limitations of the resolution of the film.
Prime
focus photography
Using
a T-adapter to attach the camera body to the telescope, the
telescope effectively becomes a large telephoto lens. Using prime
focus, a telescope with a focal length under 2,000mm can project the
entire half-degree wide lunar disc onto a 35mm frame. Prime focus is
the best method to capture a sequence of lunar phases. It is quite
easy to focus the Moon through the camera's viewfinder, and exposure
times must be judged according to the focal ratio of the telescope,
the ISO (speed) of the film used and the Moon's phase.
Eyepiece
projection
Prime
focus photography is great for capturing the entire lunar disc, but
for closer, higher magnification views, an eyepiece can project the
image directly onto the film. Eyepiece projection adapters are
available - these fit into the eyepiece holder, the eyepiece is
inserted and the camera is fixed to the adapter. Standard Plössl
eyepieces can be used to deliver nice crisp images with flat fields
of view. Experiment with a variety of focal length eyepieces and
bracket the exposure times to find the combination that suits your requirements.
Film
types and exposure times
When
beginning in lunar photography, generic 200 ISO colour film is ideal
to experiment with, since it is cheap and widely available, although
the quality will vary from brand to brand (and even in different
batches of the same 'budget' brand). The ISO rating of a film denotes
its 'speed' - the higher the ISO, the faster the film and less
exposure time is required. The Moon is so bright that it is possible
to use very slow film to capture it - even as slow as ISO 25. Slower
films are less grainy and as a result can withstand higher
enlargement than faster films without the grain showing through.
Formulae
for calculating lunar exposures take into account the aperture and
focal length of the lens/telescope, the camera f/stop employed, the
film speed and the Moon's phase. The brightness of the Moon and hence
the exposure time required is also affected by the Moon's altitude
and the presence of cloud.
Digital
cameras
Most
digital cameras possess non-removable lenses, so the principles
involved in photographing the Moon through them are identical to
fixed-lens conventional film cameras and the afocal method (see
above). Digital cameras are versatile, with a variety of functions
that can be adjusted to suit the conditions, and images can be
instantly reviewed on the camera's TFT screen to determine whether
they are worth keeping. After manipulation with an image program on
computer, the end results, on screen and in print, can be superb.
A
digital camera's resolving power depends on the size of its CCD
(charged coupled device) - a tiny electrical chip with an array of
minute photosensitive squares (pixels), immediately behind the camera
lens. Cameras with the highest pixel rating will capture the highest
resolution single-shot images of the Moon. Digicams can store images
in a range of resolutions - the lower the resolution, the smaller the
image file and the more images can be stored in the camera. Digicams
of 3 megapixels or higher can capture impressive single-shot views of
the Moon, while images secured with high-end 8 megapixel (or higher)
digicams can produce crisp images capable of being enlarged. Many
digicams are capable of taking video clips which can be computer
processed to produce high-resolution still images, overcoming the
effects of poor seeing.
Focusing
can be a problem with digicams, as their TFT screens are usually
quite small, some measuring less than 4 cm (diagonally). First, focus
the image with your eye through the telescope eyepiece and then
attach the digicam to the eyepiece; use zoom to zero-in on the lunar
terminator, where fine focusing will prove easiest. If your camera
can be hooked up to a TV monitor, the larger image will be far easier
to focus by.
The
problems of image vignetting may be combated by using the zoom
facility. If the edge of the field appears dark or hazy, an
adjustment in zoom can remove it altogether. High degrees of zooming
brings into play the camera's 'digital zoom' and should be avoided,
as in this mode the image is usually of low resolution. It's better
to enlarge a high resolution picture than to try to capture the same
area zoomed in at low resolution.
Gauging
exposure can prove troublesome. The camera's inbuilt autoexposure
meter may overexpose the Moon, particularly if the whole Moon is
being imaged and the Moon is not centred in the field of view.
Autoexposure usually works best if there is a uniformly bright field,
and your digicam will probably judge exposures well with lunar
close-ups. Many digicams have exposure variation capabilities
equivalent to two f/stops above and below the optimum, so use this if
you're having problems with overexposure or underexposure. The type
of light conditions selected can affect exposure too, so experiment
with various combinations and note which ones appear to work best.
Digicams
can produce some vivid colours that bear no resemblance to the view
through the eyepiece. Many digital photographers mute these tones in
the processing, or decide to convert the images to black and white,
which can appear much more pleasing to the eye than a multicoloured
Moon. If your camera has the ability to photograph in black and
white, give it a go - black and white images can appear much sharper
than colour images, and the smaller image files take up less space in
your camera's memory.
Camcorders
Although
the Moon is not a dynamic world, its shadows appearing to creep at a
snail's pace across the surface near the terminator, video footage of
the Moon is fascinating to watch, and conveys a sense of being at the
eyepiece far more than a still picture can ever do. Video captures
the atmospheric shimmer as it momentarily distorts the view, and a
sweep at high magnification along the terminator using the
telescope's slow motion controls allows the viewer to experience what
real observing is like. Camcorders are also ideal to record lunar
eclipses and planetary occultations.
Camcorders
are heavier than digicams, and the coupling between camcorder and
eyepiece must be rigid. Adapters are available that slot into the
eyepiece socket and hold the camcorder on an adjustable arm so that
it can be brought up close to the telescope eyepiece and locked into
position. Digital camcorders are the lightest and most versatile
available, and their images can be edited on the computer.
Camcorder
imaging uses the same afocal principles as conventional or digicam
photography. The problems of eyepiece selection and vignetting are
present, and their solutions are the same as with digital photography
(above). With experience, it is possible to produce a wonderful tour
of the Moon and its terminator, taking time to zoom in on interesting
features. These make fantastic presentations at any astronomical
society meeting - but think twice before showing all of your hard-won
camcorder footage to visitors and relatives, as they may not
appreciate half an hour of wandering around the crater-crowded
southern uplands as much as you!
By
downloading digital footage onto your computer, images can be
grabbed individually (at low resolution) or digitally stacked to
produce detailed, high resolution images. The same method is used to
produce high resolution webcam images (explained below). It is
essential, when embarking upon digital video editing, to have at
least 5 gigabytes free on your computer's hard drive, as the process
temporarily eats away at the available hard drive space. The
resulting edited videos can be transferred to CD-ROM or DVD-ROM and
then removed from your hard drive to free up more space.
Webcams
The
use of webcams to capture high resolution lunar and planetary images
has been growing in popularity since the early 1990s. Webcams are
only a fraction of the cost of dedicated astronomical CCD cameras,
they are lightweight and versatile - indeed, many experienced lunar
and planetary imagers prefer webcams because their high frame rate
allows ease of focusing and the opportunity to capture more images
per unit of time. Any off-the-shelf webcam attached to a computer at
one end and a telescope at the other can be used to image the Moon
and the brighter planets.
Webcams
are usually positioned at the telescope's prime focus in place of
the eyepiece. Several makes have removable lenses into which can be
screwed commercially available adapters, such as those for the
Philips Vesta Pro, Philips ToUcam Pro and the Logitech QC 3000 Pro.
CCDs are highly sensitive to infrared (IR) light, and the lens
assembly contains an IR blocking filter. Without an IR filter, a
perfectly clean focus through a refractor is not possible as IR is
focused differently to visible light. IR blocking filters are however
available, to place between the telescope and webcam, allowing only
visible light to pass through to a sharp focus.
Although
webcams are used at prime focus, the image of the Moon they produced
is at quite a substantial magnification, and only the shortest focal
length instruments will allow the whole Moon to be viewed in the
field. The CCD chip itself is quite small, and only part of the image
is being projected onto it. For example my short focal length (f/5)
80 mm refractor requires the use of a x2 focal reducer in order to
fit the entire Moon into the field of view.
Since
they don't have a viewfinder or a TFT screen, focusing a webcam can
be a trying experience. There's no point focusing the Moon through
the eyepiece and replacing the eyepiece with the webcam, as there
will be a substantial difference in where the focal point lies. It is
best to focus the webcam on the lunar terminator whilst having a
clear view of the computer monitor, best done with a laptop in the
field, next to the telescope. Even a slight knock can push the Moon
out of the small field of view - this is where a really well-aligned
finderscope comes in extremely useful! If your webcam is connected by
wire to a computer indoors, focusing may entail some rushing to and
fro until a good focus is achieved. Once a good focus has been found,
carefully mark the position of the focuser tube with a chinagraph
pencil so that you don't have to go through the whole process again
the next time you want to do some imaging. Of course, the focus will
need tweaking slightly each time you set up, since a fraction of a
millimetre can be all the difference between an acceptable focus and
a razor sharp one. Electric focusers are a highly desirable item in
webcam imaging, and being able to adjust the focus at leisure from
indoors at the computer can make a great deal of difference to your
enjoyment and the quality of your images.
The
webcam manufacturer's own software is used to record image sequences
as AVI files from the webcam to the computer. The image download rate
will vary with the type of equipment used. Using an older USB-1
camera of 640 x 480 resolution, the rate should be limited to a
maximum of 5 frames per second to avoid lossy compression. Much
higher rates of up to 30 fps with no compression are possible with
USB2 enabled high-end webcams. The image can be adjusted to produce
the best balance of exposure, brightness, contrast and colour, but
often the Moon can be successfully imaged in full auto mode in short
bursts. As with digicam imaging, the use of black and white
(greyscale) can simplify matters, especially if you're using a short
focal length achromat prone to producing a degree of false colour.
Image
editing software, such as the freeware Astrostack, is then used to
extract individual images from the AVI. A well-aligned and smoothly
operating equatorial mount is essential to good webcam imaging, as a
misaligned mount will produce image drift even in short capture
sequences - this produces problems when attempting to align and stack
the images. AVI files are chopped into individual BMP frames by the
processing software, and each BMP can be checked visually (a
laborious process) or by the software itself for quality. The best
images are stacked and combined to remove noise (improving the signal
to noise ratio) and eliminate artifacts. The stacked image is then
processed to bring out detail, unsharp masking being one of the most
useful tools for achieving a clean-cut image. Beware of applying too
much image processing - spurious artifacts can, and do, arise as a
result of overzealous manipulation, and fine tonal detail can be obliterated.
Beginning
lunar imaging
Introductory
notes by Maurice Collins
I
have recently been contacted by someone who is interested in getting
into lunar imaging, so I have been thinking about how I started, and
giving him some tips. I thought may be worth sharing with a wider
audience also in case there is anyone out there who may be under the
impression that it is something way too difficult or expensive for
them to do.
Firstly,
do you have any sort of still digital camera that you use for family
photos, holidays etc? Even a mobile phone camera will work. If you do
have a digital camera and a good eye relief eyepiece of fairly long
focal length, you are all set to make a start. To begin with, a 25 mm
to 17 mm eyepiece works well, but you can use up to the limit of your
telescope as long as the camera can see through it.
To
take images, simply point the camera through the eyepiece, holding
it by hand. To find the Moon with the camera lens, which can be a bit
tricky, back off a bit until you see the bright spot of the
Moon’s image on the cameras LCD screen, then bring it down
closer, but never actually touching the two lenses together (though
no harm seems to be done if they do briefly touch). Put one finger on
the side of the eyepiece to steady the camera in place. It does not
cause as much vibration as you would expect and helps keep the Moon
in the field of view while taking the photo.
Next,
focus the telescope, as the focus of your eye at the eyepiece is
different to what the camera sees — for me it is a reasonably
large change of telescope focus. Then use the camera’s zoom
function (either digital or optical zoom is okay) to frame out the
black eyepiece field stop that vignettes the image so that you only
see the lunar surface on the LCD of the camera, or at least only the
edges of the frame showing black. Gently press the shutter while
holding your breath so as not to giggle anything. Oh yes, and
remember to disable the flash! And voila, you should get a very nice
image of the whole disk or part of the Moon. If it looks too bright
and saturated, or too dim and grainy, check the EV settings on the
camera and go up or down a couple of settings to see what works best
for that phase of Moon.
Take
lots of shots. Some will be blurry, some will be sharp, and the more
you have the better you will get a good one out of it. You can then
process the images on your computer to reduce the size to sharpen it
and run the other image processing functions over it to get it
looking good. Sometimes the images look better in grayscale,
sometimes the good ones look nice in color.
The
technique described above is the easy way into lunar
astrophotography. If your camera has an AVI movie mode, you are even
one step up and can just record a video sequence which you can then
stack using a program like Registax to get really sharp images that
you can mosaic together if you like. This works for planets too. This
way, I recently obtained a really nice shot of the crescent Mercury
showing its phase. You can even use a video camera, and convert the
camera files to AVI and then do the stacking. I have only used this
for a lunar eclipse and thin crescent Moon but it works, though a bit
more work is involved.
The
next step up (and easier way) is with the a modified webcam or
astronomical CCD camera such as the Meade LPI (Lunar and Planetary
Imager). No eyepieces are needed, just the camera at prime focus and
you can add a Barlow lens if you wish to increase the image scale.
Think
simple, and it usually works. Also if someone says to you: “You
can’t do it that way!” ask “why not?” Break the
rules (if there are any hard and fast ‘rules’) and
experiment to see if it works. You don’t you need brackets and
fancy CCD cameras to do lunar astrophotography. If you have them, all
the better, but if you don’t, lunar imaging is still something
you can do.
I
now use the Meade LPI to do my lunar imaging, and much of it is done
remotely, but it wasn’t too long ago that I was using the
simplest methods outlined here to take pictures of the Moon, so
before I forget my beginnings I thought it worth sharing with others.
I hope this helps someone in getting started who would otherwise not
think of trying it, using gear they may already have around home.