RoboDIMM is an automatic monitor measuring the atmospheric seeing throughout the night,
with minimum human intervention. It is a DIMM-type seeing monitor, i.e. using the Differential
Image Motion technique [link either old DIMM page or new calibration page].
It consists of mainly off-the-shelf
components, i.e. devices available on the consumer market, with the crucial addition of custom-made
software [link to software page]. Here is a list of the "essential" hardware items, roughly in
order of cost, omitting items like cabling which depend on the particular installation:
A specially-designed 5m tower with observing platform,
A fibreglass "clamshell" dome,
A 15cm (11 inch) Meade telescope (LX200 model),
A CCD camera from SBIG,
A desktop PC, running linux,
An electronic focuser (spacer) between telescope and camera,
Four specially-ordered optical glass wedges (prisms),
An opaque and rigid mask plate, with apertures for the prisms.
These items are discussed in detail below, describing the particularities of their installation,
advantages and disadvantages, alternatives and operational characteristics.
Observing Tower
This item is considered essential because it raises the point of measurement above the strongest
component of ground-layer turbulence, which we we do not wish to contribute to the measurement
of the seeing. On a high altitude mountain site such as the ORM (Roque de los Muchachos observatory) this
ground layer turbulence is considerably weaker than at sea level, but it is still generally considered
necessary to measure seeing at a minimum height of a few metres.
RoboDIMM's tower was installed for the previous DIMM, in 1994 and only
required minor adaptation to accomodate the new monitor. It is located to the north and east of the
WHT, at a distance of some 75 metres from the telescope building. It has a clear and unobstructed
view over the steep convex northern side of the mountain, which is where the prevailaing trade winds
come from. It stands on ground some 3m below the earthwork platform around the WHT building, this
location being determined the rock type.
The tower is made up an inner vibration-proof truss and a surrounding, independent platform
on which the dome sits. The truss has a Serrurier-type structure designed to provide maximum stability
for the telescope which is bolted on top, at a height of 5m above ground. The access platform is a more
conventional structure and stands completely independent of the truss, joined to it only by a waterproof
canvas at the top. Thus no vibration from the access tower can transmit to the telescope.
The tower supports form a completely open structure through which the wind blows.
Originally, the platform floor was also an open grille, but this has now been sealed except for three
louvred ports which open automatically during observation and cover perhaps 50% of the floor space.
The tower was originally designed for use with a DIMM by Dario Mancini and acquired from Capodimonte
Observatory in Italy. It cost, very approximately, some EUR 25,000, including installation.
Telescope Enclosure
The telescope must somehow be protected from the elements, so that some kind of enclosure becomes
essential. In order to be autonomous, the RoboDIMM required a motorised
hard-shell enclosure and ING opted to buy a dome on the amateur astronomy market.
Following an internet search, the Astrohaven clamshell domes were identified as the best option.
ING purchased a 12 foot diameter (3.7m) dome to allow extra space for working inside the dome.
The tower access platform (see above) had to be widened slightly to accomodate this dome. The clamshell
has four sections which, under belt drive motors, slide down to open the dome, leaving a "wall" of 1.5
to 1.6m tall all around. This presents a solid barrier to the wind, which is undesirable, since in
the presence of temperature gradients, it can induce turbulence that would contribute inappropriately
to the RoboDIMM measurement. However, temperature gradients are expected to be minimal and ventilation
is helped by floor panels that open up (see above), so that such contributions are unlikely.
In any case the telescope stands above the height of this barrier and is exposed to the wind.
Indeed on nights when windspeed rises above approximately 40 km/h, the telescope is clearly buffeted,
and the image centroids move so much that some samples are discarded
by the program [link to software]. In this case the clamshell dome, by partially closing it, provides a
useful wind shelter and allows measurements to continue without interruption.
But anybody thinking of buying an Astrohaven dome should be aware that it does not satisfactorily
exclude humidity. While the dome shutter does indeed
appear immune to rain and completely drip-proof, there are some significant gaps even when it is
completely closed. Conditions on the ORM in winter can be quite demanding, with high winds and
fog pushing humidity inside the Robo-Dome to over 95% for days on end. When this happens, extra
protection for electrical equipment must be provided. To date, we have experienced no
electrical problems due identifiably to humidity.
Currently the dome must be opened and closed by push-button control by the Telescope Operator of the WHT,
which is done remotely from the control room. Our installation does not preclude installing automatic
opening and closing in the future.
The Astrohaven 12" dome cost some EUR 6,000 with an even larger transport bill.
Telescope
The RoboDIMM telescope is a Meade LX200 (non-GSM model) with 15cm entrance aperture. The main
capabilities of this model availed of by RoboDIMM are PC control of slewing through an RS232 serial
link and a reasonable pointing accuracy. The telescope is mounted
in equatorial mode to obtain the best pointing performance, using a "wedge" (purchased separately).
The telescope is normally left powered on permanently, since on power up, the Dec "worm" drive
(or infinite thread) in order to enter into contact with the gear always moves, and by an
indeterminate amount. This means that an operator must correct the pointing
on the first target using the finder telescope.
The main disadvantage of this telescope, and of "amateur" telescopes generally, is that the
mechanics of the rotation axes are rather lightly built. This makes them more vulnerable to wind
shudder and mechanical wear (some gear parts are plastic and the motors are reminiscent of toy
cars).
We have already replaced the whole drive unit (control electronics + motor) for the RA axis
due to a fault that was probably electronic. Fortunately this was quickly diagnosed and obtained through
an experienced Meade contact in the UK.
Another disadvantage is flexure, most noticeably that the primary mirror tilts during tracking and
flops considerably during slewing. This has been observed to stretch the image by up to 15% following
a slew, which probably alters the image scale and thus the accuracy of the FWHM estimate. We are
working on automatic compensation of the focus error (see section on focuser) and considering
mechanically fixing the position of the primary mirror.
Reportedly, the new version of the Meade LX200, with GSM, is more mechanically robust, but also
more expensive.
Camera
Whereas the previous ING DIMM had an intensified MCP camera, CCDs are nowadays a much more affordable
and preferable alternative than they used to be. However, a video-type, interlaced readout should most
definitely be avoided. ING purchased an ST-5C camera from SBIG (Santa Barbara
instrument Group). This has 320 x 240 pixels with 10x10 micron size and is thermoelectrically cooled.
This model is now obsolete and has been superseded by new SBIG products with more and smaller pixels.
Pixel scale in our setup has been measured (using a binary star) at 0.72 arcsec per pixel, giving a field
size of nearly 4 x 3 arcminutes. The pointing of the Meade telescope is accurate enough to acquire the
star within a square of 3x3 such fields almost every time.
The original firmware is still used on this CCD, which gives a minimum exposure time of 10ms. During a
sample of image motion, a single window of typically 80x80 pixels is read out. It may be possible to push
the minimum exposure time to 5ms by altering the firmware.
Computer
It is clear that some kind of processing power is required in order to coordinate the the CCD exposures,
compute results and store them. In RoboDIMM's case, this is performed by a desktop PC
housed in a water-sealed cabinet inside the dome. Alternative computers such as Notebook PCs or IMacs
are perfectly possible, but are probably not as robust in cold or humid conditions.
Focuser
The electronic focuser is not an intrinsic part of the automated seeing monitor but it is certainly
very useful. It is an adapter fitted between the telescope Cassegrain port and the CCD camera that,
using a small motor, changes the distance between these to move the detector onto the focal plane
of the telescope.
