 |
 |
Mounting Photographic Lenses
on the ST Imagers and
Mounting the Imagers on the
OTA.
There are basically two ways to mount camera lenses and especially telephoto lenses on cameras or imagers. One can consider the imager the base element and mount the lens on the imager depending on the mount thread, in this case a T-type thread, for the integrity of the imager to lens mounting. The other way is to consider the lens the base element and attach the imager using the same thread. For light weight lenses, the former is a good way to do the mounting provided that the integrity of the thread that mounts the imager to the OTA is good. For heavier lenses it is desirable and even necessary to mount the lens securely and attach the imager as an accessory to the lens.
The ST imagers have a standard tripod socket built into one side.
This is considerably off center from the axis of the imager chip and so
the lens. This effect is shown in the two photos below. The ST is
mounted on a Losmandy DCM and has a T-thread to Canon lens adapter screwed
into the thread on the imager. Notice that when you use the tripod threaded
socket Build into the ST imager you will have the imager rotated 90 degrees
from what I consider the nortmal position. It is always wise to reduce
stress on the cables by clipping them to the OTA in some way no matter
the orientation of the imager box.
|
|
The actual offset of the chip axis and thus the lens axis is 2 1/8 inches.
For lenses which can be held directly on the ST without added support,
this is no problem whatever. But for lenses that need some additional
support it might complicate mounting and supporting the lens.
That this is not generally a problem will be seen in the following discussion.
Even a rather husky lens like the 200 f 2.8 shown below fits firmly on
the ST imager and seem to the touch to be very solid. I would not
use this lens or a heavier one directly on the imager however. It
think the mounting a bit marginal. For this lens, I would use mounting
rings and attach the imager to the lens as an attachment.
 |
 |
Light lenses like those shown below are not problem at all, They
fit well and are very solid to the touch. I would not hesitate to
use any of then directly on the ST imager. Below are shown Canon
lenses, the 50 mm f 1.2 and the 24 mm F 1.4.
 |
 |
For longer and heavier lenses the mounting method should certainly be
to mount the lens on the OTA and mount the ST imager as an attachment to
the lens. This is shown below for the 300 mm F 4.0 Canon lens.
Even here, the lens/imager structure is quite heavy so it might be wise
to use an additional ring at the front of the lens to stabilize it. Such
mounting techniques are described elsewhere on this web site.
 |