Telephotography of restricted ranges
Chromatic aberrations, or "Am I seeing things?"
Refractor telescopes have one big problem: chromatic aberrations.
In simple English, if the image was a pinpoint of white light on
a black background, what comes out of the telescope will not be a
pinpoint of light. Different wavelengths of light take different paths
and end up in different locations on the focal
plane. So the white dot has some color, generally on the edges. Glass
that has reduced chromatic aberrations is known as ED glass. [ED stands
for extra-low dispersion, and cannot be cured by Viagra.] The net step
up is APO,or apochromatic. This glass has very little "color", and of
course comes at a premium price. Usually the glass has a fluorite
element in it. Note that the chromatic aberrations are most
noticeable at points of sharp contrast. For astronomy, you really want
APO glass. For terrestrial applications, especially photographing Area
51 from Tikaboo, ED will get you there, though I'd suggest going for
the APO if you are going to be regularly doing such photography. While
we are at it, the barlow or eyepiece projection setup also has
chromatic aberrations.
"Baby Got Back-focus"
If Sir Mix-A-Lot was into telescopes, he would rap "baby got
back-focus." OK, what is back-focus? Consider the task of mating a
telescope with a camera. The distance from the lens mount of the camera
to the focal plane (film or sensor) is not a standard. Thus the
telescope manufacturer has to cut his tube a bit shorter than the focal
length to compensate for the distance from the lens mount to focal
plane. "But wait, there's more!" For
astronomical viewing through a telescope with an eyepiece, it would be
mighty uncomfortable to be looking out the back of the scope, given
that the telescope is pointed at the sky. To make viewing easier,
a "diagonal" is inserted in the optical path, bending the light
path 90 degrees. The diagonal has a physical length, and the telescope
must be cut shorter to compensate for it. The amount that the tube is
cut shorter is the "back-focus."
When attaching a camera to the back of the telescope, you don't have a
diagonal in the path. This means that an extension tube has to be added
to the back of the telescope before attaching the camera or inserting
the barlow. You may need an additional tube if you are trying to focus
on a nearby object.[The focal length is measured when focused at
infinity. Close-up work is macrophotography. Spacers are used with
cameras as well for this close focusing ability. Note that once the
spacer (tube) is installed, you can no longer focus at infinity.]
Takahashi sells threaded extenders for their
scopes. For other telescopes, an eyepiece extension tube can be used.
It is best
to use extension tubes are large in diameter as you can get. There are
two reasons for this. One, if the tube is right before the camera
mount, it may interfere with the light path to the focal plane, choking
it and causing vignetting. Even if the tube does not directly interfere
with the light path to the focal plane, some light will hit the sides
of the tube and could bounce around, reducing contrast. [This happens
in the telescope itself, but the manufacturer inserts baffles to block
such light.]
Hooking everything together
Most telescopes will have a 2 inch eyepiece holder on the back. If the
telescope only handles 1.25 inch eyepieces, look elsewhere. If you are
going to attach a Televue Powermate, one end will fit in the 2 inch
holder on the scope, and the other end terminates in T-threads. To mate
the T-threads to your camera, you need a T-ring specifically designed
for your camera lens mount. If you have trouble getting the setup to
focus, you may need a extension tube. Eyepiece projection hardware
should have the same fittings.
One drawback to standard T-rings is the threaded opening is too small
for full frame cameras. The reduced opening can produce vignetting
If you never studied French, this translates to the light
intensity will fall off at the edges. This is on top of whatever light
fall off the telescope produces. If you are using a DSLR that doesn't
have a full size sensor, there may not be much vignetting. If you are
using a full size sensor or a 35mm camera and must use a T-ring, then
keep the from the barlow to the T-ring short.
For Takahashi telescopes, Borg makes many accessories that can thread directly to the scope, including a wide T-ring and large diameter extension tubes.