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The slit mechanism at our observatory is opened and closed by means of a crank handle which acts through a pair of bevel cogs to rotate the blue shaft in the illustration which rotates the orange cogs which move a pair of Renold chains. These chains go over sets up wheels up the side of the slit, and back down, and both ends of the the chain attaches to the base of the shutter. Winding one way opens the shutter, and widner the other way closes the shutter. There is no counter balance system so to hold the shutter in any position other than fully open or fully closed requires wrapping a rope around the crank handle.
We’d like a ratchet system so letting go of the handle, wherever the shutter was, would just lock the shutter in place, whether opening or closing. We need some idea of how potentially to implement this.
The other option is a counterweight system, but my feeling is that would be harder to design / implement. Nothing can dangle or pertrude lower than the bottom of the dome on the outside of the dome.
I’d value any thoughts on these ideas, or other ideas.
James
[The illustration is neither to scale nor precise in any way]
And posted this on SLG: https://stargazerslounge.com/topic/427185-ratchet-mechanism-for-opening-slit-in-dome/
First thought.
You have a square toothed wheel at the end of the shaft.
How about a simple arm with a matching tooth.
Pivoted above the wheel, lift to disengage, lower to lock.
Would need a spring in-line with the pivot to hold it open/closed
Not as nice as an auto ratchet but very easy to make.
RoyJames,
Would it be possible to replace the two bevel cogs with an ordinary cog and a worm gear?
The worm would be attached to the handle and provide such mechanical disadvantage that the slit would not have enough energy to spin the handle. The downside of this arrangement would be a very great increase in the time it took to open/close the slit.
Bill.
I think it would be too slow Bill unless we motorised it but getting power to the dome at any position in its 360 degree rotation would be difficult.
I think it would be too slow Bill unless we motorised it but getting power to the dome at any position in its 360 degree rotation would be difficult.
I have been considering the same problem for my dome.
Front-runner idea at the moment is a UPS (i.e. a controllable and rechargeable battery) attached to the inside of the dome. Attached to it, also on the inside is a Raspberry Pi which switches power from the UPS to the dome motors – slit and dome azimuth – as required, the Pi being controlled from the main TCS over WiFi. Attached to the UPS on the outside of the dome is a solar panel which keeps the UPS fully charged.
In my case the dome azimuth is already motorized and controllable by the TCS so I need only the slit control but you may need both degrees of freedom to be upgraded.
Perhaps the same idea may work for you.
Paul
Hi James. I have studied your photo of your winding mechanism and seen that below the cranked handle there are two square section tubes that have bolts passing through. If you made a U-bracket that slides into these then with the handle down it will pass either side and hold the dome slit open at any height you wish. Hope this helps. Mel Joslin.
Mel, thanks. We want something which will hold it whenever someone takes their hand off the crank handle; if they let go it runs away as described above. The U bolt idea is good though for holding it; we use a rope at present which does something similar. But thank you.
Paul, that is a good idea. Our shutter is pretty heavy so it may need to be a chunky battery. How do you go about charging up that battery? We’d need someone to remember to connect it to the charging circuit unless we always parked the dome in azimuth in the same position and set up a positive and negative contacts at specific locations on the dome and the on the rim of the supporting wall. But an interesting idea.
James
Mel, thanks. We want something which will hold it whenever someone takes their hand off the crank handle; if they let go it runs away as described above. The U bolt idea is good though for holding it; we use a rope at present which does something similar. But thank you.
Paul, that is a good idea. Our shutter is pretty heavy so it may need to be a chunky battery. How do you go about charging up that battery? We’d need someone to remember to connect it to the charging circuit unless we always parked the dome in azimuth in the same position and set up a positive and negative contacts at specific locations on the dome and the on the rim of the supporting wall. But an interesting idea.
James
The battery charges automatically from the solar panel. No human intervention required and there are no electrical contacts between the wall and the dome. All of the slit motor, battery and solar panel are attached to the rotating part of the dome so nothing moves relatively to anything else. The purpose of controlling them all through a small computer such as a Raspberry Pi, which is itself fixed in position relatively to the UPS and takes its power from it, is that WiFi works regardless of the position of the computer as it rotates with the dome.
As for the chunkiness of the battery, it only needs to be able to open and close the slit once or twice before it is recharged from the solar panel. The power drain of a Raspberry Pi is tiny and any reasonable UPS could keep it running for weeks if not days. For a reason which I hope is obvious, the panel should be on the south side of the dome when the latter is in its parked position. If the azimuth of the dome is also automated, as is the case with my dome, it can be parked automagically in the correct position. Assuming that the azimuth motor is fixed to the wall, powering and controlling it should be straightforward.
The size of the panel and the capacity of the UPS will need to be calculated from power of the motor used and for how long it needs power applying.
Of course, with all that in place the dome can be fully controlled over the interweb thingy given an connection to the TCS, which is the rationale behind my plans. It’s all I need to make my observatory fully automatic and available whenever the sky is clear, wherever I happen to be located at the time.
Incidentally, I was brung up in Long Eaton, near Nottingham, and a younger bother lives nearby. I could in principle pay a visit to your observatory and explain things in more detail.
Paul
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