Author: Tom Droege Date: 980302 Revision: #0 980302 Key Words: hardware instrumentation
Table of Contents
This note will describe some of the mechanical hardware for the Mark IV. It will be particularly useful to those planning to use the camera head and electronics on other than the standard Mark IV telescope and mount.
The Mark IV system consists of a mount, one to four camera heads, control electronics, and a control computer. Figure 1 shows a single camera head on the prototype mount. It is shown with a 135mm focal length, f/2 lens. Multiple cameras are to be mounted on rails which move in declination driven by a motor which can barely be seen at the left in the picture. The declination mount is mounted on a barn door mount which is hinged at the left in the picture. At the right is the linear motor drive for the barn door. The linear drive moves the barn door in RA by means of a stainless steel tape which winds around a barn door radius cam.
This prototype mount has space for three cameras. The next version will have space for four cameras and will be pivoted in the middle.
The Mark IV camera head contains a Lockheed 2048x2048 pixel, 15 micron pixel size, CCD. Figure 2 is a close up view of the Mark IV camera head installed on the prototype camera mount. The shutter brackets can be seen sticking above and below the camera head body. The production shutter brackets are somewhat larger to allow longer shutter motion.
Figure 3 is an exploded view of the camera head. At the bottom is the Base Ring Plate that seals off the cooling channel. Next comes the Base Ring which is the mount for the Thermo Electric Cooler (TEC). It contains a cooling water channel which can be seen in Figure 4, penetrations for two connectors, and a 1/8"NPT tubing connector for an external desiccant container. The base ring contains a groove for an "O" ring with seals the inside of the camera head from the environment to prevent condensation on the cooled parts.
The Middle Ring (Figure 3) is a spacer which can be used to bring a particular lens within focus range. The one shown is minimum length and results in the surface of the CCD being about 0.8" behind the face of the Lens Ring. The Middle Ring is 4 1/2" in diameter. Except for the Shutter Mounting Brackets, this is the maximum diameter of the camera.
The Lens Ring (Figure 3) contains the shutter, and a well in which the Filter/Window resides. The Lens Ring is glued to the Middle Ring. The Filter is glued in the well behind the shutter and cannot (for practical purposes) be changed. We are presently "gluing" parts together with wax. When everything is proven to work the parts will be permanently glued together. Mounting holes on flats on each side of the Lens Ring hold brackets for the shutter mechanism. A plan view of these is shown in Figure 5.
The lens ring contains a groove which can be used with "servo mount" clamps to mount the camera head to a flat surface such as the back of a telescope. There are also four screw holes in the back of the Base Ring which can be used to mount the Camera Head from the back.
Screwed to the front of the Lens Ring is the Lens Mounting Plate. This mounts to the Lens Ring with 4 #6 screws. The Lens Mounting Plate contains a 40mm diameter "T" thread hole. Adapters are available for this threaded hole that take most 35mm format lenses. With the dimensions shown, the Lens Mounting Plate and adapter will cause most 35mm format lenses to focus in the middle of their range.
Figure 11 is a head on view of the Camera Head with the lens snapped out of a Konica adapter. You can see the ccd through the hole in the adapter, and can see that the corners are not properly covered. Figure 12 is the front view with the Lens Mounting Plate removed. The shutter is full open. The shutter motor is to the right. Dial cords move the "parlor door" shutter open when the motor rotates. The shutter servo motor is rated to move the shutter from full closed to full open in 0.15 seconds. Figure 13 shows the shutter half way open. This is the prototype and is made from paper and glue. The design has also been changed somewhat from that shown to produce a better light path and fewer leaks. Figure 14 is a close up with the shutter full open to show that the shutter opening is larger than the CCD.
Figure 6 is an assembly drawing for the head. The TEC is glued to the base ring. Next is a spacer which provides a place for a thermistor to measure the CCD temperature. It also provides enough space between the CCD socket in its printed circuit board (not shown) and the wiring to the connectors. We are presently gluing the thermal path parts together with silver filled epoxy.
The CCD (Figure 6) is mounted directly behind the Filter/Window. This is a tight package, and there is not much room anywhere.
Not shown in the assembly drawing is the printed circuit board which contains the direct drive electronics for the clocks, and the first level pre-amplifier and base restoration circuit.
Figure 7 is an assembly drawing (somewhat obsolete) showing water cooling tubing and connectors. The Camera Head uses standard DB-25 and DB-9 connectors. You can get a good idea of how much space they will take by looking at a parallel printer cable (the connector end that plugs into the back of the computer). Figure 8 is a photo of the prototype head with a short focal length lens as used for tests. The necessary water tubing and connectors are clearly visible.
The present plan is to put four cameras on a barn door mount in a little "house" with everything needed for operating the system. This is shown in Figure 9. (Full size version of Figure 9) The "barn door" mount is shown in the lower right corner of this side view. North would be the right face in this view (in the northern hemisphere). The dotted lines show a +/- 8 degree travel. The declination drive can move the cameras from the north horizon as shown to the southern horizon (through a south wall cutout if desired). Figure 10 shows the roof open and the telescopes pointing at the north pole. (Full size version of Figure 10)
When the roof is slid off on rails to the North, the camera can be positioned in declination and clocked in right ascension. There is no intention of operating this mount in a way that it it pointed to any particular object. Instead, it is planned to move the camera set to a particular declination, and track the available sky as it passes. The lens tube itself is shown in red, as is the camera head. There should be a gap shown between the lens tube and the camera head to allow for focus motion. The shutter brackets are shown sticking up above and below the lens tubes. The lens tubes (four in depth) are driven in focus by linear stepping motors shown above the lens tubes. The tubes are mounted on linear bearings on 1/2" stainless steel shafting. Experience indicates that this will allow moving the lens tubes in 0.001" steps with little backlash. Standard mechanical design practice should allow positioning the lens axis perpendicular to the CCD to sufficient accuracy. A number of limit switches will allow establishing a "home" position for most motions, and counting steps will allow positioning the cameras any place in their range. All four cameras reside on the same platform and move together in RA and declination but separately in focus.
The lens tubes are shown with large light shields to improve performance. The drawing also shows a built in Flat Field Box which would allow establishing some sort of flat field. There may be better schemes.
The drawing dimensions are hard to read and in any case are not final. The left dimension from the base to the roof peak is 36". The top dimension - the width of the roof base is 42". The base width is 27". The depth (for a 4 camera mount) is about 40". Rails mounted at the top of the enclosure stick out to the right of Figure 9 (full size Figure 9) to allow the roof to be moved to the open position. It is not planned for the enclosure to be weather proof, but rather it is designed to be a temporary protection until it can be covered with a tarp.
The present plan is to mount most of the electronics on the declination carriage above where the Focus Stepping Motors are shown in Figure 9. (full size Figure 9) The boards are 10" wide in the view shown, and would extend 26" in depth above the four lens tubes. Power supplies would be mounted approximately where shown for electronics.
Where absolute minimum obstruction caused by the Camera Head is desired as when mounting the Camera Head in a Schmidt camera, the Shutter Mounting Brackets can be removed and the shutter strings pulled from outside the camera. Other shutter schemes can be implemented if desired.
Just to show that everything sort of works, Figure 15 is a full scan exposure taken with the Mark IV prototype (full size version of Figure 15) on Oct 29, 1997. Exposure time was 100 seconds. The lens was a Soligor 135mm focal length, f/2.0 with a Konica mount. The camera was pointing approximately south, and roughly at the meridian. North was probably to the left, but I don't have proper notes to tell. If you look closely, lots of things will be found wrong with this exposure. There is low level electronic noise. This is one of the Ford chips, and it clearly has lots of defects. The dynamic range is very limited.
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