Aggghhhhhhhhhhhhh.Noooooooooooooo!

[G4YVM]

My 16" Dod is progressing well and tonight i hope to lash up the various compnents and sset the length of the main truss tubes by achieving focus.

However, during the lash up procedure i have discovered that my focuser board, loving crafted and glued, puts the eyepiece / focusser about 3" too high for the secondary mirrror! I cant adjust the secondary anymore, there's nor more slack ion the system, so I have to build another focusser holder! Agh!!!!!

There should be just enough secondary visible to achieve fodus of sorts and start cutting the trusses though. Fingers crossed!

David

[Doc]

Best of luck with David.

Did you muck up your calculations for where to position the secondry.

[adamsp123]

ooops sorry to hear that, hope you can fix Ok

[G4YVM]

Yes I mucked up the calcs alright!

Ive just spent a very frustrating and cold time trying to find stars to achieve focus. All I can find is beige! What the heck is beige? A new form of matter???

When i look through the focusser without the eye piece I can see secondary and primary reflections, when i shine a torch at the primary, or from it, I get dazzled as I can see the torch, yet when I put the eyepiece back in...nowt.

My poles are too flexible as well, causing the thing to wobble (b and Q black pipes, just intended to use as a length finder)

I am surprised I cant find any stars at all - I have slid the focusser ring up and down well past the nominal focal length, yet i can see absolutely nothing. What surprises me is that I cant even see the blackness of space!

The focal length of my primary is 71.2", so as I understand it there should be 71.2" between the surface of the mirror and the place where I bung my eyeball, but in an "L" shape as it bends through 90 degrees at the secondary.

I think the thing must be so badly out of alignment it just all adds up to rubbish: I'll recut the focusser board, tighten up the poles and start again. Someone told me it would take a long time. They weren't kidding.

david

[barkis]

A 16" f4.5 requires spot on collimation as you probably know. An impossibility in view of what you have been saying about your focuser.. I am not sure about the beige you mention. An extremely out of focus star could cover the mirror with a faint beige colouring, but you say focusing does not change the intensity of the faint light, if that is what you are seeing of course.

Did you check the mirror focal length in a practical way, before you placed it in the OTA.? or have you taken it as gospel that the focal length is as stated from where you acquired the mirror.

72.2" seems a very precise figure, which gives a ratio of f4.5, or very close to it.

Ron.

[G4YVM]

The figure of 71.2 was pencilled on the back of the mirror (its a Meade) and the figure of f4.5 was quoted by the seller, so I am sure it's pretty close.

I didnt appreciate that a single star would fill the thing, so thanks for that. I don't think thats what it was mind you, more like stray light or even a close up of the focusser ring!

I'll pack the mirrors away and recut the focuser board; disappointing but inevitable.

Oh well, I just love the smell of sawdust in the morning!

david

[The Warthog]

You are worrying me, as my 11.75" mirror has almost exactly the same f/l. Don't worry, though, we'll both end up OK.

[DanDare]

That's a shame David, hope you get it sorted OK. I read that you measure to the top of the focusser (with it fully racked in) + 1".

Now you're making me nervous about cutting the hole for my focusser.

Roy

[Doc]

71.2 is about right. Just measured my 16" Meade and I made it a tad over 71.5 but these are not exact measurements but pretty darn close.

[G4YVM]

Thanks Doc. I havent worked on it today but will resumee work over the next couple of days. I may end up cutting a new couple of rings to produce a secondary cage since it has to hold a longer focuser board now. We'll see.

david