Technical Data - Mechanical
The Basic Stucture
From the very outset of this project we had several objectives in mind:
- To make Lhires III the most economical spectrograph possible, so that
it would be affordable to the largest possible audience and consequently
distributed as wide as possible in the amateur community.
- To have available an instrument that would be compatible with the widest
possible range of telescope types and their accessories: reflectors and
refractors and catadioptrics, different CCD cameras, different guide
cameras.
- To have a compact and lightweight instrument so that it could be mounted
on telescopes of modest size (typically a C8 on a Vixen GP mounting).
- To construct a modular instrument: - a spectrograph that would be capable of
evolution and customisation in the hands of amateurs wishing to modify the
general design.
- To have an instrument that was quick and simple to set up for making
adjustments, for aiming, for guiding, etc.
With these criteria in mind, Christian opted for a Littrow configuration for its
compactness (a folded optical axis) and also for cost
(the dual role of the collimator/focusing lens eliminate the need for 1
component). From that basic premise,
we also made other important choices at an early stage:
- The spectrograph body is in folded sheet aluminium.
This method of fabrication is well adapted for short series mass production,
with the various pieces manufactured by CMC methods from CAD data files,
resulting in a good compromise between production cost and component
functionality. Assembly of the differnet pieces of the external body panels
achieves the required high rigidity whilst minimising weight.
Sheet metal screws are used to fix the various body panels in order to avoid
the problem of threading holes through the aluminium panels which are only
1.5mm in thickness - this would have been too difficult a task with many
stripped threads at risk!
The core structure of the instrument, based on sheet metal, means that the
different modular components (camera mounts, slit and grating supports, the
lens mount, etc.) can all be designed independently, thereby creating the desired
modularity.
In order to limit the types of screws used, we decided to use M3 metric screws for
the assembly of the basic chassis and M4 screws whenever any additional
mechanical strength was required or whenever frequent screwing and unscrewing
would
be needed.
- The choice of Reflective Slit:
Due to the narrow slit widths to be employed with this spectrograph,
some mechanism for guiding become quickly apparent. A reflective slit is
more complex to manufacture (especially for mass production) but it has the
advantage over the semi-reflecting type that no light flux is lost. We
therefore preferred and selected this option. André Rondi performed the
initial critical trials on a brass slit, and his approach guided the way. Then
Patrick Fosanelli took up the challenge of making stainless steel slits
(harder, more durable), taking particular care with the flatness and
smoothness of the jaws.
It is thanks to the talent and ingenuity of André and Patrick
that we have been able to achieve an instrument with these performance
characteristics!
- The possibility of including a DSLR
Quite quickly Christian was adamant that Lhires III should be
capable of being used with large format digital single lens reflex cameras.
Although it is true today that cooled CCD cameras are significantly more
sensitive than DSLRs, this consumer-driven market is driving the cost of
DSLR's down and improving the technology to the extent that more and
more amateurs will be using these cameras in the future.
Providing support for DSLR's did not turn out to be as straightforward as we
thought. The
dimensions of these cameras and the fact that the CMOS sensor is situated
towards the rear of the body and about 50mm from the camera adapter at the
spectrograph requires
a telescope with plenty of back-focus. In order to keep the system compact
still, we had to be more than a little cunning and get the telescope beam to come very
close to the main faceplate of Lhires III (where the webcam / CCD guide
camera are mounted). This explains why there is a shift in the position of
the mounting plate on the telescope (see the photo of the prototype with a
DSLR attached on the "Prototype" page). Despite
this, we got there in the end and with no regrets since it all works !
- An Integrated Neon Calibration Lamp
A small Ne lamp for the wavelength calibration of spectra is
integrated in the design. Neon has the great advantage of several several
strong emission lines around H-alpha. Initially, this lamp was supplied with
mains voltage (220V). Safety considerations, however, led us to design a
small circuit to drive the lamp from a 12V source. Fortunately, there is
always a 12 volt source around a telescope these days!
3D Schematics!
Technical drawings are available in PDF, DXF and SolidWorks
format. There can only be used for personal, non-commercial purposes. Here is an extract in JPG format:
You can also download this programme (Windows, 2.4MB) that displays the 3D
schematic drawings and allows you to rotate the spectrograph at explore the
internal design.
Optical Schematics & 3D schematics
Mechanical Interfaces
Lhires III has been designed to be easily connected to a wide variety of
telescope configurations. Let's have a look in some detail at the
different possibilities.
Schematic drawings of the Adapters
(pdf file still in French)
1 – Coupling to the Telescope
Lhires III comes with two coupling adapters: one is 50.8 mm ring for direct
connection to the rear thread of the popular SCTs by Celestron (C8, C11, C14,..)
and Meade (LX200, LX200 GPS,..). The other is a 2 inch (50.8 mm) tube
adapter. You can also make your own adapter tube or ring which is then
attached to 6 countersunk M4 screws (refer to the drawings below - the notes are
still in French, but a picture says a thousand words!).
2 – Coupling to the CCD Camera
Several camera adapter rings are also supplied with the kit for
connecting to the following CCD cameras:
- Audine Camera (M42 x 1mm thread).
- SBIG ST7/ST8 (M42 x 0.75mm thread)
- DSLR cameras. In this case the user must supply the T-ring
specific to his or her brand of camera (Canon, Nikon, etc.) and so this could
not be included in the kit.
- Any other compatible camera that has a 1mm or 0.75mm M42
thread, and with enough back-focus (of about 20-25mm from the faceplate to the
CCD sensor) can also be used. Note also that lateral adjustments to the
collimator lens by +/- 5 mm can also be made - this has little or no effect on
final image quality.
- For any remaining configurations, you can of course remove the
M42 ring and replace it with a ring of your choice, if desired. The total
available back-focus is 60.5 mm between the body of the spectrograph and the
position of the CCD chip. This distance is sufficiently large to accommodate the
great majority of amateur cameras, and even the more demanding DSLR's now coming
onto the market.
- Finally, a simple 31.75 mm eyepiece holder is included in
the kit. This will prove useful for you during the testing phase of your
instrument and for making any fine adjustments. When observing the solar
spectrum, any changes you make can be instantly seen. And a high resolution
visual spectrum of the Sun is a truly magnificent sight!
CCD Camera adapters (still in French)
3 – Connecting a Guide Camera
Several kinds of guide camera can be employed. The initial tests were
performed with a webcam (Vesta Pro), with a tiny security video camera and also
with a high sensitivity integrating video camera (Watec 120). All these cameras
use a 1¼ inch (31.75 mm) thread and so this is the only one supplied with the
kit. For webcam's such as the Philips ToUcam, an adapter is readily available
commercially. For any other type of guide camera, you must provide your own,
noting that the distance to the video chip in most cases is 53 mm.
Neon Lamp Power Supply
Here you can see the small PCB that is used to convert 12V dc to ~100V AC to
drive the Neon calibration lamp. It is mounted on the switch assembly.
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