A change of plan...

[JamesF]

Some time ago I picked up a primary, cell and secondary from a 250P for not very much and had a plan to build my own truss-based dob around them. This weekend however, a 250 flextube tube (no optics, focuser or base) came up on astroboot at a price that meant I could justify buying it and fit all the bits I've collected so far leaving me with just a base and tube cover to make, thus getting the scope into use this autumn which was, I have to admit, looking pretty unlikely otherwise.

So that's what I've done. I intend to build a new base for this one following the instructions in Kriege & Berry and then revert to my original plan which was to build a smaller dob (either 6" or 8" based around a home-made mirror) before moving up to something a bit bigger, probably in several steps.

James

[nmoushon]

Well grats on the nice grab there! Hope everything goes as planned. *knock on wood*

[nytecam]

Two good finds = well done = scope for autumn then!

[JamesF]

I'm hoping so. I have (I hope) most of the bits I need to finish it on order now. Given time this evening I shall make some "lids" for the secondary cage and the primary tube. I was thinking about making just one lid, for the secondary cage, but it doesn't collapse down all the way, leaving a gap of about 50mm between the top of the primary tube and the secondary which obviously can allow dust in. And if I make a lid for that then it will protect the primary if I forget to put the scope horizontal before working on the secondary cage

I'm not sure what I'm going to do for alt bearings yet. Given that the OTA only weighs 15kg I can't believe the bearings will need to be that big, but I want to get them fitted to the tube before I put the mirrors in to avoid "accidents".

James

[swamp thing]

For alt bearings pay careful attention to the advice in the book James, nothing ruins the way a Dob moves faster than too small an altitude bearings, they cause no end of headaches. They also necessitate the need for counterweights all the time, basically a right PITA.

A nice large alt bearing can sometimes do away with the need for counterweights altogether. As long as the Teflon pads are kept reasonably small it should still move very nicely indeed.

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[JamesF]

They're already causing me headaches and I've not even started them yet

I don't know if it's just my particular flextube tube, but it's really quite stiff to open. No way is the weight of the mirror going to give the bottom end of the OTA enough weight not to lift off the ground when the tube is opened up. On the standard flextube that's probably not an issue as the entire mount is physically attached to the OTA, but with the B&K style bearings it isn't and the OTA will therefore just lift off the mount when I try to open it.

At the moment I'm thinking I might try 6" or 8" circular bearings in a full enclosure that hinges or splits open in the event that I wish to remove the OTA. That's smaller than B&K recommend (or rather, smaller than the figure I have by worked out by extrapolating B&K's figures, as they don't give any for a 10" mirror). I should perhaps go back and read the section towards the end where they talk about "small" dobs and see if there are any hints there.

James

[JamesF]

Actually, what's really giving me headaches is that I have an entire OTA and optics ready to roll, with what appears to be several days of clear sky ahead, and no mount

James

[swamp thing]

Dumb question I know but have you tried spraying the trusses with some dry lube. Obviously making sure it can't get anywhere near the optics.

Not a huge amount of hands on experience with opening and closing flextubes myself. I do remember that they're a little stiffer than you imagine they're gonna be.

Could you not hold the lower tube assembly between your knees then grab the UTA and extend the Flextube before placing it on the mount each time?

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[JamesF]

That may be the approach I have to take in the end. I was hoping to keep keep it all in one piece as much as possible as it's not that heavy and I can certainly move it around all in one, but perhaps that just isn't the way it's going to be.

James