Image Credit: Steve Richards
Click here to download a spreadsheet to simplify the maths for this project
How to add azimuth control of the dome
If you are a deep-sky astrophotographer in particular, you will benefit enormously from the greatly reduced set-up times as all your additional photographic equipment can be left in place, your mount is already polar aligned and your equipment balanced.
All you need to do is switch it on and perhaps perform a simple star alignment at the start of the session.
Automating the rotation of the dome, which is the subject of this article, is the icing on the cake.
Tools – Drill, drill bits, marker pen, masking tape, tape measure, metric Allen keys and metric spanners.
Drive gear – A Pulsar rotation drive system comprising USB control box, motor drive housing, encoder unit, 12V power supply unit, homing cam and all fastenings.
Hardware – A suitable computer running Windows (XP upwards) with a spare USB port.
Software – You will need to have installed the latest ASCOM platform (free; http://ascom-standards.org), Microsoft.NET Framework 4.0 or later (free; www.microsoft.com/net) and the LesveDome control software (paid; www.dppobservatory.net/downloads/downloads.php).
Many astrophotographers start an imaging sequence and then retire indoors to let their setup run by itself.
This works very well if you have an observatory with a roll-off roof, but a conventional domed observatory has to rotate precisely so that the telescope continues to point through the roof aperture.
This means that you have to step outside every 20 minutes or so to nudge the dome round a little.
Completing this project will fully automate this process.
There are various homebrew solutions to driving the dome in azimuth but these require engineering, electronics and software skills beyond the scope of this article.
Instead of designing our system from scratch we opted for an off-the-shelf package designed specifically for our Pulsar observatory; other dome makers have drive systems tailored for their particular dome design.
This well-proven azimuth drive was the start point for a fully automated imaging observatory.
All azimuth drive systems require a motor drive using either a friction drive or a cog drive of some kind, a control box, a position sensor (encoder), a home sensor and control software.
The system detailed here uses a friction drive and the popular LesveDome ASCOM drive software.
The motor housing, control box and encoder are attached to the wall of the observatory by carefully marking and drilling suitable holes, taking care not to crack the gelcoat on the outside.
Drill small pilot holes from inside first, then drill the main holes from the outside with the drill running in reverse to grind, rather than cut, the hole through.
Attach the three units using the supplied stainless steel nuts, Allen bolts and washers.
Decide where you would like the dome to ‘park’ and then move it manually to this position – you can then mark the position of the home actuating cam on the dome flange where it centres over the home sensor built into the motor housing.
Making light of the maths
For long-exposure astrophotography, an equatorial mount is required and in most setups this takes the form of a German equatorial mount.
These popular mounts follow the apparent movement of the stars in a smooth arc: ideal for imaging, but through the course of the imaging session, the telescope starts on the west side of the pier pointing east and ends up on the east side of the pier pointing west.
This non-linear pointing has to be accounted for to ensure that the telescope points through the dome’s aperture at all times, requiring some complex mathematics.
It is the job of the dome control software to do this for you.
However, to do this correctly, the software must know exactly where the telescope is mounted in relation to the centre of the dome, so your first task is to make some careful measurements to obtain the dimensions required.
Using the spreadsheet available at the link at the top of this article will make it easier to get all the dimensions correct and ready for insertion into your software.
Install the ASCOM software and enter the offsets from your spreadsheet, ensuring the correct signs (positive or negative), into your choice of control software – we used MaxIm DL and POTH (Plain Old Telescope Handset) but another popular choice is Sequence Generator Pro.
Once the above is completed, your telescope and dome aperture will be in sync.
Components – The Pulsar Shutter Drive system comprising an RF controlled motor drive housing, limit switches, 12V battery, solar panel, nylon chain, roller, chain guides and all fastenings
Tools – A variable speed drill with 2mm, 6mm and 8mm bits, a set of metric Allen keys, flathead and crosshead screwdriver, a set of metric spanners and a metal file.
Sundries – Sandpaper, a marker pen, masking tape and a tape measure.
The curved profile of these apertures makes motorising them a little problematic, but the chain drive system described here is both elegant and secure.
Because the dome must be free to rotate, it cannot be electrically connected to the rest of the observatory, so the aperture motor drive requires its own power supply.
In the case of the Pulsar system we are using, this comprises a small, motorcycle-style 12V lead acid battery, along with a solar panel to keep the battery topped up and ready for action.
For the same reason, open and close commands from the main controller must be sent over a wireless link.
In this project, a simple two-channel 433.92MHz radio frequency link connects to the main controller installed in the section above.
The latest version recently launched by Pulsar uses a bi-directional Bluetooth link.
Begin by removing the aperture lid to gain easy access for fitting the three guides and top return roller for the chain.
It is important that these are placed approximately equidistant from one another, from the drive cog built into the motor housing at the bottom, and from the roller at the top.
This ensures that the chain follows the contour of the dome aperture as closely as possible.
Once installed you won’t have to keep popping out to nudge your dome along
Image: Steve Richards
Connecting the chain
The position of the top roller is at the top edge of the aperture.
Place the motor housing onto the flat vertical panel under the aperture opening ensuring that the top is 10mm lower than the bottom of the aperture opening.
Mark the position of the cog by placing a piece of masking tape aligned with it on the dome, in line with the left-hand side of the aperture as seen from inside the dome.
Place each of the three side-chain brackets over the dome edge in turn and position them equidistantly spaced.
File the glass-fibre edge if necessary using a metal file and sandpaper until they are a neat, firm fit.
Mark then drill a 2mm pilot hole clockwise through the fixing holes from the gelcoat side (the outside) and then drill 6mm holes.
Do this with the drill running anticlockwise, so you grind a hole rather than drill it, to avoid damaging the gelcoat finish.
Bolt the three brackets in place using 6mm nuts and bolts and then loosely loop the nylon chain through each guide.
This will help you to assess whether or not the guides are in perfect alignment with one another and if necessary, you can pack them with washers until the alignment is spot on.
Again, using the nylon chain as a guide, position the top roller in perfect alignment with the side chain brackets, mark the mounting holes and drill them in the same manner as for the chain brackets.
Attach with bolts and check that the roller is carefully aligned and horizontal.
Pack with washers to ensure correct alignment.
Reposition the drive motor housing again, align the cog accurately with the nylon chain then mark and drill the mounting holes as before but using an 8mm drill bit. Attach it with 8mm nuts and bolts.
Mark, drill and attach the two limit switches and single limit switch actuator then mark, drill and attach the solar panel to the outside.
Finally, connect the top limit switch, battery and solar panel to the motor housing and complete the chain around the drive cog.