Equipment |
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This section sets out more about the following equipment and
observatory structure details
1. Mounting Pier (custom made) and observatory (home made)
3. Automated focusing system (RoboFocus)
5. Newtonian fan cooling system (home made)
6. Newtonian cooling system sliding vents (home made)
7. AP155 automated focusing assembly (Robofocus)
8.AP155 Star spike mask (home made - really cool results!)
10. Focusing arrangement on the FSQ106N
The mounting pillar is custom made from welded steel. The intention was to create usable space inside the pillar to accommode the cables and power supplies. There are two steel sheves inside that support the AC to 12VDC transformer for the AP 1200GTO mount, the AC to 12VDC transformer for the CCD camera assembly and a separate set of 12VDC regulated connection sockets for other accessories. The transformer for this regulated supply is beneath the computer desk behind the left hand front wall in this photo. All the shelves, top flange and base flange of the pier have 40mm diameter holes in the centre to allow the free run of plugs and cables. The pillar has a 600mm square flange welded to the base. This flange is bolted down onto a 1 cubic metre concrete block set into the ground. Around this block the concrete base of the building is 150mm thick.
Power cables are routed under the wooden panelled floor to a fused distribution box on the observatory wall. Operational cables for the PC to mount and accessories are routed under the floor in the opposite direction to avoid interference. The floor is raised 25mm off the concrete base by wooden slats to reduce vibration in the mount and to raise the floor above the rainfall level outside.
The mount control electronics and hand control are currently on the chip kit "D". The only problem I have ever encountered with the mount is the physical connection of the 12VDC input jack. This needed to be tightened with pliers to avoid intermittent bad connection and the resulting loss of power.
With the risk of snagging and the inherent weight of the cabling needed to operate the focuser, camera and heater or cooler it was important to keep them tidy. However it is important to ensure that pulsing power supplies such as for a Kendrick dew remover or raw AC power are kept away from data cables as these can cause interference and data corruption. The rest were placed inside a cable tidy shown in the photo and this was loosely hung onto the mid-section of the scope or the mounting plate to enable freedom of movement of the telescope to any position.. This detail can seriously affect your financial health and imaging results and should not be ignored. On a couple of occasions the mount has set off to park and lost its bearings (due to my error) resulting in the whole cable assembly getting jammed between the telescope mounting plate and the fixed mount. This was very bad news!! Fortunately, no damage was done but it could have damaged the drive electronics.
Incidentally, the spare cable you can see running over the floor is a serial cable I was using to test out PulseGuide. If or when I decide to use it permanently this will go under the floor.
I decided to use the RoboFocus system for two reasons. First - it was relatively inexpensive to equip both of my telescopes with the stepping motors. When I change to the AP155 a second stepping motor is fixed on the rack and pinion and it only requires the motor cable to be unplugged from the Newtonian motor and plugged into the AP motor. The same controller is used throughout. Second - all feedback from existing users was very complimentary. I have never regretted the decision and the system works exremely well but with one proviso that I discuss below.
The next picture shows the RoboFocus motor fitted to a JMI NGF-DX1 crayford style focuser on the 12" Newtonian reflector. I discarded the aluminium brackets supplied with the RoboFocus Kit and made up an alternative one from 3mm x 50mm aluminium stock. This was extremely rigid yet easy to remove if necessary. It was finished with a spray coat of black Hammerite.
There is a major drawback in this setup. Whilst the RoboFocus motors and clutch collars can easily support the whole weight of the imaging camera in all positions, the NGF-DX1 focuser assembly can slip on the roller bearings. There is no means of adjusting this tension except by tightening the locking screw on the NGF-DX1 but of course this will potentially cause stress on the RoboFocus motors and wear on its clutch collar. I would not recomment the use of this type of focuser with an electronic focusing system. This problem highlights itself when any adjustments need to be made to the camera. If the camera needs to be rotated or a filter fitted, it is almost impossible not to disturb the drawtube position in the NGF-DX1 which can move relative to the stepper motor. This means that the absolute focus position has been changed with reference to the stepper motor and the focus routines have to be re-run. If it has moved too far the RoboFocus controller may need to be re-calibrated if you run out of travel which is a distinct possibility in such a low profile focuser.
The picture below is not well focused but it is intended to shown the clutch collar connecting the RoboFocus drive motor to the focus shaft of the NGF-DX1. This collar can be changed depending on the type of focuser you have and it is an inexpensive but effective way of attaching RoboFocus to different focusers. This collar has a different drive shaft diameter to the collar on my AP155 rack and pinion focuser. Both were supplied by Technical Innovations - the makers of the RoboFocus system.
When taking digital images of faint objects, it is important to get rid of the subtle variations in the distribution of light in the optical system. This is called Flat Field processing and involves taking a set of exposures of an evenly illuminated field with exactly the same, unmoved, camera and telescope setup that was used for taking the astronomical images. This will allow the effects of light obstruction in the optical path (vignetting), dust specks on the optics and filters and internal reflections (hot spots) to be removed from the final images.
To achieve this I use a light box that provides an evenly illuminated white field at the front of the telescope. As the largest telescope I use has a 320mm Newtonian tube diameter, the light box was designed to fit this and can also be used equally effectively with the 155mm AP155.
The light box is mounted on the observatory wall so that when the telescope is parked after an imaging session, the flat fields can be taken immediately by pulling the light box out on its extending swivel and covering the front end of the telescope. The flat field images can then be taken and saved in the same imaging files as the astronomical images for later processing. The positioning of the light box is shown in the picture below.
The light box is mounted on an extended TV wall bracket and it sits unfixed on the TV support plate. This allows the box to be easily moved forward onto the front of the parked telescope. The power to the light box is from a variable (by switch) wall socket transformer giving 1.5, 3.0, 4.5, 6.0, 9.0 and 12VDC outputs. The box is made from artist's foam filled card with a 9mm plywood front face. This front face has the 320mm aperture cut into it to fit over the Newtonian scope tube and a piece of 4mm white signmakers acrylic sheeting is fixed to the inside surface.
Inside the box and set around but shielded from the Acrylic screen are four white LED's linked in parallel to the power socket mounted on the front face. I use these LED's with the 3VDC switch from the transformer to achieve around 25,000 ADU level of illumination in 10 second exposures on my ST-2000XM CCD camera. There is no other form of baffling inside the light box and it works fine with the setup I have.
Whilst the secondary mirror rarely fogged during a night's imaging, the primary mirror of the 12" Newtonian was very susceptible to dew. It was also necessary to open up the observatory and allow the optics to cool for at least two hours before equilibrium was reached between the glass and air temperature inside and outside the tube assembly. To solve these problems I installed a cooling fan at the lower end of the tube and cut six 25mm diameter holes around the tube circumference at the same level to help dissipate the warm air and moisture.
This system works very well indeed. Equilibrium is reached typically in around 30 minutes and I no longer have the problem of how to remove dew on the primary mirror when I have started imaging. I can even leave the fan on without noticeable effects on CCD images. There were two main tricks in getting a successful design.
First - the 12VDC cooling fan, which I took from an old PC, was mounted just above the level of the mirror on the outside of the tube and on a thin foam pad before the retaining nuts and bolts were fitted. These retaining bolts were passed through loose rubber grommets set into the wall of the scope tube. These arrangements avoided vibrations when the fan was running.
Second - when imaging, it is bad news to have light falling on the primary mirror and illuminating the dust motes and grime. To avoid this I made up a hoop of black plastic around the circumference of the tube and cut out flaps that would allow air to circulate (especially when the fan was on). This hoop was snug fitting on the tube and was cut out so that it passed either side of the fan assembly to form a completed ring. This arrangement held the hoop in position on the tube but could be rotated to have the vents open or fully closed with just a gentle push. At first setup, the vents could be angled sufficiently open to allow circulation but to cut out 80% of the light falling on the primary mirror as shown in this picture. These open vent positions are not changed in practice but the hoop is just rotated to open or fully close the vent holes.
The picture below shows the hoop passing either side of the fan housing. To close the vent, the hoop is simply slid towards the fan housing.
Since installing this arrangement, dew on my Newtonian mirror does not happen.
7. AP 155 Automated focusing assembly (RoboFocus)
The AP155 refractor was first fitted wth the Astro-Physics RoboFocus Bracket. This is an inexpensive but neat and rigid method of fitting the stepper motor of the RoboFocus to the rack-and-pinion shaft of the AP155 focuser assembly. The instructions for fitting are supplied with the bracket kit and the bracket is made with the correct sized slots for the RoboFocus supplied nuts and bolts. This only involves the removal of one of the AP155 focuser knobs and slipping the clutch collar over the exposed shaft. The photo below shows the assembly.
When the RoboFocus has been fitted the clutch collar grub screws are tightened on to the rack-and-pinion shaft and the stepper motor shaft. This clutch pressure on the rack-and-pinion shaft is easily capable of supporting the camera assembly without slippage. To demonstrate how well engineered the Robofocus is, I show below the current load that it is supporting - in excess of 10 pounds weight. This is the AP Field Flattener, The SBIG ST-11000M CCD Camera and the SBIG ST-L (an 8 position x 2" filter wheel unit).
This mask is used on the AP155 refractor to create the traditional star spikes (diffraction spikes) of type that appear on systems that use secondary mirror supports. It took 30 minutes to make from a sheet of 1mm thick textured black plastic sheeting available from any signmaker and some dressmakers black elastic. The only tools needed were a cutting knife, cutting mat and some superglue. The total cost was under £1.50 ($3 at todays exchange rate)
The circular cut-out on which the diffraction cross-hair was mounted was made from a piece of plastic sheet 200mm square.The hole diameter is the inside dimension of the dew shield on the AP155. The cross-hair itself was made from 1mm diameter fine black elastic used by dressmakers. The cross-hair was located into 1mm wide slots cut into each corner of the square.
The webbed brackets were cut from the same plastic sheet and superglued to the mask at 90° intervals. Both the brackets and the mask were slotted to help strengthen the joint and each of the brackets were set back from the rim of the mask aperture by just the thickness of the dew shield - about 1.5 mm. This prevented the mask from slipping in to the aperture but positioned the mask itself exactly central on the dew shield.
The brackets were all drilled out with holes in the same position to allow another piece of black elastic to run through them.
The elastic cord shown passing through the bracket holes in the above picture fits around the AP155 dew shield and holds the whole assembly comfortably in place.
The final trick was to place the mask on the scope at the preferred position of the spikes relative to the CCD X-Y axes and place just one pair of insulating tape strips snugly on either side of the bracket but on the dewshield surface only. I was surprised that this simple method holds the mask firmly in the same position yet the mask can be repeatedly removed and replaced with exact repeatabiliy on the dewshield. Obviously, if the dewshield is taken off and then replaced on the telescope, then this will not repeat the positioning exactly relative to the telescope. The picture below shows the spike generator in place on the AP155 dew shield.
The Takahashi was mounted onto the AP155 tube rings using (a) AP mounting plate compatible hinged tube rings from Parallax (b) Home made aluminium spacer blocks to raise the Tak an extra 20mm clear of the AP155 (c) AP Losmandy sliding bar. The Losmandy plate into which the sliding bar was set was bolted to the top of the two AP155 tube rings.
A RoboFocus stepping motor was attached to the Takahashi rack and pinion mount. One of the focusing knobs was removed and a collar was supplied with the motor to fit the FSQ106N shaft exactly. The AP155 also has the RoboFocus motor setup and the same controller is used when imaging with one or the other of the Tak or the AP155 (or the Newtonian when it replaces the whole of this scope assembly from time to time).
The observatory PC is controlled directly from the keyboard/mouse below the shelf in this picture. However, once the observatory has been opened and (if necessary) the telescope re-synchronised on a star, a laptop computer is used in the (warmth of the) house to take over control of the observatory PC as if it were the PC itself. Windows XP provides this facility which is called Remote Desktop.
Remote desktop works whatever the method of connection. For example the connection can be Ethernet, Internet or radio. I use a Buffalo 54 Mb/s PC card in the observatory and a Buffalo 54 Mb/s Cardbus card in the laptop. The radio standard used is 802.11g and the configuration I use is a simple Peer-to-Peer link. To achieve a real life data rate of around 11 - 20 Mb/s a mini aeriel is needed in the observatory. Without this the data rate falls to 5 - 11 Mb/s. The aeriel is best positioned so that it has line of sight view of the laptop's Cardbus card. A better link speed could be achieved if a similar aeriel was attached to the laptop but I find that this setup works perfectly well. The distance from the observatory to the laptop is about 30 metres. However, if the line of sight was clear, the setup would work as effectively at 75 - 100 metres.
The following sofware systems are used on the observatory PC. The PC's operating system is Windows XP Pro. The indoor laptop operating system is Windows XP Home.
Telescope positioning - TheSky planetarium software V5.00.133
Camera control - MaxIm DL version 4.06 or CCDSoft V5.00.135
Image acquisition - MaxIm DL version 4.06 or CCDSoft V5.00.135
Focusing system - Robofocus/FocusMax
Mount alignment - Star drift on imaging CCD and TPoint
Periodic error correction - PEMPro
Telescope Control (whilst imaging) - TheSky, MaxIm DL or PulseGuide
Image processing - MaxIm DL V4.06, Registar and PhotoShop CS
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