T-thread: 42 mm x 0.75 mm pitch
(sometimes referred to as
M42 x 0.75)
M42 thread: 42mm x 1.00mm pitchThe eyepiece barrel should have a matching male T-thread which can be
screwed into the camera adapter. The end result is a camera body which
can be slipped into the telescope eyepiece holder and locked into position.
For normal DSLR use and if your telescope supports it, a 2-inch barrel
is to be preferred over a 1.25-inch one so that the imaging chip can be
properly illuminated without light being clipped by the edge of the barrel.
Webcams normally come with a lens pre-fitted. For those models
which can be used for astrophotography, this lens needs to be removed
and replaced by a 1.25-inch eyepiece barrel. Astronomical stockists
carry adapters designed to insert into certain webcam bodies. It’s
recommended to purchase an adapter which is pre-threaded to accept
1.25-inch filters. In this way, it’s easy to add an IR-cut filter, sometimes
known as an infra-red blocking filter, for better end results. A 1.25-inch
adapter is normally fine for a webcam chip which will typically be
much smaller than that used in a normal photographic camera. High
frame rate planetary imaging cameras normally come with their own
1.25-inch adapters.
Cooled CCD cameras typically come with pre-threaded bodies
presenting either female T-threads or the slightly different pitched M42-
thread. Although it’s possible to partially screw one of these threads into
the other, care has to be taken not to go too far as the pitches used (that’s
the number of threads per unit distance) are different and damage can
occur. Telescope eyepiece barrels, normally supplied with the camera,
may then be screwed into these threads, allowing the camera to be
inserted into the telescope’s eyepiece holder. It’s also possible to screw
correctly threaded camera lenses into CCD cameras, giving access to
large areas of the sky. The M42 thread, for example, was popularized by
Pentax and there are many inexpensive Pentax M42 threaded lenses on
popular online auction sites which can be used with M42-threaded CCD
camera bodies.DSLR cameras offer much greater flexibility because, as well as being
able to support a range of different lenses, they can also direct-couple to a
telescope, effectively using it as a large telephoto lens. A DSLR is normally,
but not exclusively, attached to a telescope using prime-focus coupling.
This method uses the telescope as the main lens for the camera and no
intervening camera lens or eyepiece is used.
All that’s required for prime-focus coupling a DSLR camera to a telescope
is an adapter ring specific to your camera make and model, together with an
eyepiece barrel; both should be readily available from astronomical stockists.
The ring locks into the camera body as would a normal lens and presents
a female T-threaded aperture. The T-thread is a commonly used thread for
joining various optical devices together in astrophotography.
[10] DSLR camera prime-focus coupled to a telescope.
[11] A beautiful photograph of the Elephant’s Trunk Nebula (VdB 142) taken by amateur
astronomer Ian Sharp.
1011Astronomy with your camera