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I was able to find a drawing on his website of a trapezium dome 'gore' with the internal angles shown. I then subtracted these angles from 90 degrees and printed these in red.
These are the vertical angles I need to set on my compound miter saw for cutting the horizontal struts.
These numbers certainly explain the difficulties I had in getting the correct compound miter angles for each horizontal strut. I had falsely assumed that the angles would all be the same. Then found each rib was pulled in or out as I tightened the screws. I noticed that the lower struts needed hardly any vertical miter at all. While the upper struts obviously needed more. Though my choice of 5-6° was still more than required.
Tomorrow is wet again but I can still use the miter saw in the shed. This time I shall set the horizontal miter angles to 11.5°. Then cut new struts to the correct lengths and vertical miter angles.
I am most grateful for the continuing advice given on Cloudy Nights DIY Observatory discussion forum. Without which I would not have made so much progress. Nor made anything to a reasonable accuracy. Those who make sawdust for a living develop experience and tricks of the trade to succeed.
It is not simply a matter of saving time and materials but of simplifying seemingly difficult tasks and procedures. There are checks and working methods which ensure accuracy which are very unlikely to occur to the average hobbyist.
If carpenters had to reinvent their work practices every single morning then mankind would still be living in caves! Their willingness to offer free advice based on their long experience is an invaluable gift to a bumbling amateur like myself.
In that light I have been advised to mark the base ring to avoid nasty surprises from cumulative errors. Multiples of 16 soon add up. Using simple measuring jigs and sticks is another vital piece of advice. This will depend on my having dry weather so I can set out the complete ring, of course but that day must come eventually. Then I can use their practical advice to help ensure success.
And then there were 11 [ribs.]
Sooner said than done. I pinned the half lap joints of the base ring with screws and proceeded to scribe 16 segments using a batten and two nails as a divider. A couple of hours later I managed to find the exact division after repeatedly under and overshooting my start point. 60.5cm was the final measurement obtained by gently bending the nails in and out to change the divider's radius. It is easy to imagine one is marking off the circumference of a circle when in fact these are flat sided segments of the circle. A straight line [a chord] is shorter than the arc it subtends.
A forum member suggested using a long tape measure and I realised that it could be held in place by clothes pegs without needing to pin through it.
Friday is a still, sunny day with 60F, so I am cutting out more ribs. I job I can't do in the shed for lack of room to handle full sheets of plywood. Reached 11 ribs in total. Both sides of the rib have to be cut twice because of the difference in radius. Leaving a long thin ans curved, tapered piece of waste.
Saturday is another fine, still day and I just finished the last 5 of the 16 ribs required before morning coffee. Which makes me realise that I had better start building something soon before it starts snowing!
These are the vertical angles I need to set on my compound miter saw for cutting the horizontal struts.
These numbers certainly explain the difficulties I had in getting the correct compound miter angles for each horizontal strut. I had falsely assumed that the angles would all be the same. Then found each rib was pulled in or out as I tightened the screws. I noticed that the lower struts needed hardly any vertical miter at all. While the upper struts obviously needed more. Though my choice of 5-6° was still more than required.
Tomorrow is wet again but I can still use the miter saw in the shed. This time I shall set the horizontal miter angles to 11.5°. Then cut new struts to the correct lengths and vertical miter angles.
I am most grateful for the continuing advice given on Cloudy Nights DIY Observatory discussion forum. Without which I would not have made so much progress. Nor made anything to a reasonable accuracy. Those who make sawdust for a living develop experience and tricks of the trade to succeed.
It is not simply a matter of saving time and materials but of simplifying seemingly difficult tasks and procedures. There are checks and working methods which ensure accuracy which are very unlikely to occur to the average hobbyist.
If carpenters had to reinvent their work practices every single morning then mankind would still be living in caves! Their willingness to offer free advice based on their long experience is an invaluable gift to a bumbling amateur like myself.
And then there were 11 [ribs.]
Sooner said than done. I pinned the half lap joints of the base ring with screws and proceeded to scribe 16 segments using a batten and two nails as a divider. A couple of hours later I managed to find the exact division after repeatedly under and overshooting my start point. 60.5cm was the final measurement obtained by gently bending the nails in and out to change the divider's radius. It is easy to imagine one is marking off the circumference of a circle when in fact these are flat sided segments of the circle. A straight line [a chord] is shorter than the arc it subtends.
A forum member suggested using a long tape measure and I realised that it could be held in place by clothes pegs without needing to pin through it.
Friday is a still, sunny day with 60F, so I am cutting out more ribs. I job I can't do in the shed for lack of room to handle full sheets of plywood. Reached 11 ribs in total. Both sides of the rib have to be cut twice because of the difference in radius. Leaving a long thin ans curved, tapered piece of waste.
Saturday is another fine, still day and I just finished the last 5 of the 16 ribs required before morning coffee. Which makes me realise that I had better start building something soon before it starts snowing!
Click on any image for an enlargement.
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2 comments:
Hello,
I've an old AWR powered HEQ5 mount and set things up recently on a pedestal with temporary wiring to an adjacent shed control room. Initially I haven't gone the ASCOM route as Cartes du Ciel has an inbuilt driver for a Meade LX200 which works well. However, I find using the mount control paddles in the Cartes du Ceil is slow to react and continues too long and beyond my star. I would like to move over to Maxim which has a customable nudge feature. Unfortunately Maxim won't connect to my AWR mount. Do I have to buy the TiGna driver and powered serial cable? Or is there a method to control the local slewing more accurately to centre my star?
Thanks, Andrew
Hi Andrew
I wish I could advise but I'm afraid I haven't a clue beyond initial experiments with my own AWR IH2 set-up. Which I have hardly touched from new since getting involved in my dome build.
I had to buy the AWR ASCOM driver and serial cable to drive AWR from my laptop or PC. AWR do the serial cable at very reasonable cost. In fact I'd strongly suggest you contact AWR for their expert advice but suggest you hide your wallet first. ;-)
Stargazer's Lounge [or Cloudy Nights] are both tremendously helpful in answering your sorts of questions. Being more UK-based AWR is probably better known on Stargazer's Lounge.
There is also a Yahoo ASCOM forum [which is often rather technical on the software side] but could probably help to answer your queries in seconds. Search for ASCOM Initiative Discussion List and join to pose your question under a specific topic.
Personally, I much prefer Stellarium [with Stellarium Scope] to Cart du Ciel.
If you have the AWR IH2 paddle then you have four basic drive speeds available on that for centering. I found the centering remarkably accurate on my IH2 system. With about 45° per minute Goto Slews on my massive mounting with my big refractor aboard.
None of which answers your specific query. Do you have the latest drivers and firmware for your own AWR set-up? What about the LX200 driver? Is that up to date? That might help too.
Best of luck getting sorted.
Chris
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