Workshop time ...

[ZeroID (Brent)]

Thanks Rolf, it's also bloody big!!
It seems surprisingly rigid even spaced off the bar, quite stable and still quite light. Bit forward heavy at present without the mirror but I have a 300 mm bar so will be able to slide it to get some adjustment.
I only have a single weight for the mount, the other disappeared in the post during delivery but so far it's seems quite happy with the single weight not quite at full extension.
I did some swing tests to see how much clearance I would have inside the planned Ob on the sewer pipe pier and looks like the park position would have to be an east west horizontal and it will still have a good horizon angle for most of the sky I have available to me. Even on the tripod it would reach about 300mm higher than the roofline for zenith views..
Looks like I will need a small stepladder for visual unless I use it in Dob mode.

[alistairsam]

Hi Brent,

That's really impressive work. look forward to seeing the finished scope.
but had two quick questions.
have you thought about using a broader mid section between the upper and lower truss segments for the EQ6 dovetail to reduce axial flexure?

not sure how to explain it, but just as an example, for a normal reflector OTA, the tube rings are around 30cm apart for a length of around 80 to 100cm. as you reduce the gap between tube rings, axial flexure becomes more pronounced.

here, the length of the member from primary mirror to secondary looks well over 120cm as a rough guess but the cross section in the middle that's held by the dovetail looks like its around 15cm.

just wondering how you overcame axial flexure especially once its loaded with the primary mirror, secondary, focuser and other elements.

I recall you mentioned this design ensures that sag is equal at upper and lower halves, but how about axial stiffness when loaded?
how quickly are oscillations dampened?

secondly, your rotating upper cage concept is really good, but how does it affect balance of the scope say if you have a dslr or heavy eyepiece attached and you rotate it. do you have variable counter weights at the opposite end? does the upper cage ride on roller bearings?

Thanks

[ZeroID (Brent)]

Hi Sam, (or Alistair ?).

The 'bottom' section of the central cage is a sandwich of 17mm ply, 2 x 5 mm spacer strips, 2 x 25mm hollow square alum and then 2 x right angle brackets( ~ 130mm ) all bolted together and is very rigid as is the rest of the OTA. Also the whole assembly is extremely light for it's size and so far the oscillation factor is less than a second when I give it a tap. I imagine there will be some flexure as it changes over but most imaging would be done in a single pass so once it is loaded in any direction it will to a large extent hold that position. Also once it gets the rest of the hardware I will do some balance tests and adjustments to minimise any tendency to overload the drives or mount points. The 300 mm mount bar will let me slide the whole OTA rather than add weight. Rolf has a very good explanation of the dynamics of the design.

Hadn't thought of weight with the rotating cage yet but good point especially when the DSLR gets attached. I want to minmise any extra weight addition so we will see how it goes and figure it out as necessary. My object now is to get it to first light and then refine any problems that show up.

No roller bearings, just a very smooth plastic surface between the two ply rings and a careful build to ensure it is all flat and smooth. Simplicity is my mddle name. I anticipate that most nights it will be fixed for the duration of the session.

I have to reiterate though just how surprisingly light and rigid the whole thing is. I'd almost be brave enough to stand it on end and stand on top of it. So far it has been a fascinating exercise in design and construction.

[ZeroID (Brent)]

Friday and Saturday, first two days of the Rugby World Cup were absolutely brilliant, sunshine, warm, gentle breeze. Then we must have said something and upset the Australians so for Sunday they sent us all the above.. !!
But every cloud has a silver lining and being unable to work outside due to adverse conditions I got some serious workshop time in.

So Skeletope I is no more, say hello to Skeletope II in it's DOB configuration. Completed the UTA with light sheilding, focusser and spider from SK I, built the light sheild for the mirror cell. Added the dob bearings to the Centre cage. Then rather than build all new components for the mirror cell I decided enough was enough and stripped SK I for it's hardware and fitted to the new cell base. Replaced all the industrial strength velcro I use to attach the mirror, put it all together and lowered it into place.

Discovered several things while pulling SK I to bits. It's amazing how much dust and spiders get inside a scope and where they go. And SK I wasn't as well constructed as I thought it was, there was a lot of wobble in those 4 aluminium rods supporting the UTA.

So guess I can't really blame you guys over the ditch for all the rain, I brought it upon myself really. Sorreeeee

Oh, .... you want some pix then ? OK, here you go.

And btw this thing is light !! OTA as shown is 11.3 kilo. The new strut braced DOB base is about 4 kg compared to about 15 or more for my old construction. The OTA is currently very mirror heavy but there is more to be added at the UTA end, RACI, Camera, Guiding etc eventually.

Now I better go and modify my sig ...

[ZeroID (Brent)]

Not quite first light but managed to shoot it out the garage door between showers and line up on the houses etc about 4 km away. Couldn't get focus because as usual I am always a bit generous in my calculations and the focal point was about halfway down inside the focusser. Out with the hacksaw and chopped off 50 mm from each of the bottom truss tubes. Let me tidy up some not so pretty work anyway and now got focus about 20 mm of travel above the the bottom stop. Centred the spider, did a visual down the OTA to the mirror and aligned that and bingo, I could count the stars on a flag across the valley.
Chopping a couple of inches off the bottom has improved the balance point nicely but now I need another Counter balance weight for sure. Did a test rotation on the UTA assembly to see how alignment held and not bad at first glance but more testing to be done there.
Wiring fan, secondary heater and building the power control system is next while the clouds hang around.
I'll do a first light in the DOB base as soon as they give me a break.