8" Dob on a barndoor mount.

[eqcradle]

I just finished making a (rough) barndoor mount for my new 8" Dob:

http://cluton.googlepages.com/IMG_5184.JPG
http://cluton.googlepages.com/IMG_5185.JPG
http://cluton.googlepages.com/IMG_5186.JPG
http://cluton.googlepages.com/IMG_5187.JPG
http://cluton.googlepages.com/IMG_5188.JPG

Unfortunately, the weather is crap, so I can't actually see whether it works tonight.

I mainly wanted to see whether it would be possible to build one sturdy enough for a Dob. It does wobble a little when adjusting the threaded rod, but not as much as I expected (and it settles fairly quickly). I'm sure it could be made sturdier by using stronger hinges and attaching them to bigger pieces of wood for the vertical bit.

The top of the platform can move from about -10 to +20 degrees (much more than necessary).

The 3 plastic feet on the base of the Dob simply slot into 3 holes drilled in the 'T' platform. This seems to secure the scope quite firmly in place.

The bottom of the threaded rod sits in a small cup drilled into angled piece of wood. With 2 nuts locked at the other end, I can turn the rod by hand. Underneath the top angled piece is just a nut and washer (that I plan to hold in place with a spanner and some tape).

It was fairly easy (and cheap) to make (it had to be, given my skills and equipment). The hardest part was cutting the angles to match my latitude (one day I'll get that electric mitre saw). It seems crucial to attach the arms directly to vertical part with long screws.

It weighs about 6.5kg (which is less than the base of the Dob), and it conveniently raises the eyepiece to eye level.

I considered other ideas for the hinge, such as a plain axle mounted in bearings at each end, or the rod ends (ball joints) I discussed in an earlier post. The axle idea might be a bit sturdier overall, but the hinges are cheap and easy.

I also considered other ideas for raising and lowering the platform. I started with a car jack but the lateral movement of the platform made it impractical.

Anyway, it should be fun trying to set it up correctly and actually get it tracking. Hopefully it'll work (I'll let you know)....

[davewaldo]

Great Work! I'll be very interested to see how this one progresses!

Any plans to motorize it?

[eqcradle]

Thanks. Motorising is probably beyond my abilities unless there's a very simple way to do it. But first I need to see if it's even capable of keeping something in view after an hour...

[eqcradle]

Now that I've played with this one today, I'm already having thoughts for a Mk II (see attached). It's similar, but has a slightly different arrangement. The one I've sketched would have the COG directly above the hinges, which would reduce some of the stresses on the tangent arm. I think it would be more rigid. But it would have a 'tipping' point - which might complicate it. It might be easier to motorise though, because the tangent arm wouldn't actually be taking the weight ...

// Just thinking out loud ...

[rmcpb (Rob)]

Great idea but when you are finished you will end up with a form of an equatorial mount.

If you are into ATM then have a look at some plans for equatorial platforms, the principle is the same but they are more stable.

Great work

[h0ughy (David)]

that is an excellent idea and a lot of thought has gone into that.

[eqcradle]

Well, I think it works! (But I need to learn how to fine tune it.)

I was able to do about 45min of testing tonight, and Jupiter was my subject. It took less than 5 minutes to set it up. I used a compass to aim it at true South (taking into account magnetic variation for my location). I folded the hinge open and put an inclinometer on the lower angle and then raised one of the legs until I got it showing my latitude.

Next, I put the scope on it and lined up Jupiter in the 10mm eyepiece. Sure enough, twisting the threaded rod re-centred Jupiter in the eyepiece after it moved. However, after a while it was drifting in one direction. It was able to make about 3-5 passes across the eyepiece before drifting out of the FOV at the top. It was moving left to right and, since that's east to west, it must have been drifting south?? It was taking about 1:40 to cross the width of the eyepiece and about 10 minutes before it drifted out the top. I tried raising the southern leg a little. This seemed to slow the rate of drift, but it was hard to judge without more testing.

Overall, I was pretty happy with the first test. I don't see why it shouldn't work more accurately, so I'm hoping the drift is just a matter of fine-tuning the setup (which I intend to research tomorrow - there's a great post over on CN at the moment). I'm especially happy with how it performed considering it cost less than $50 in materials and only a few hours to make (and my carpentry skills are lousy).

Adjusting the scope certainly caused a little bit of wobble, but as i noted earlier, it didn't seem too bad and it would settle pretty quickly. I'm certain it could be made more rigid. Motorising it would obviously improve it, but even a more convenient hand crank might make the small investment worthwhile.

I think the 'Mk II' version I described earlier would be *much* more rigid. Incidentally, I had the angle backwards on the one I posted earlier. The one I've attached here would allow the use of braces to make it even more rigid. The shaded bits could be pieces of ply.

Cheers,
CL.

[digby dart]

Well done. Very ingenious.