Kriege and Berry: Poles 2" longer?

[gooseholla]

Hello,

I am having trouble working something out. My eyepieces focused last week when the poles were at a specific height. The K + B book tells me to add two inches to this and cut them - for future use if I get new eyepieces... Now, the poles were moved down in 2" stages, and it didn't focus at 2" longer. Why then, would I want to add 2" and never be able to use it?!

Thanks

John

 

[swamp thing]

I think he says that so people aren't tempted to try and cut the poles to exact length straight away. If you know the exact length you need thats fine John. Its just cut them too short = New truss poles. Rather than the simpler job of trimming them to length. Thats all I think.

[gooseholla]

Well I have no problems if they get whittled down before final use. But the book seems to suggest you leave them that way incase you get an eyepiece in the future! I am going to cut them a bit longer before final use - after all the mirror box might need cutting down to balance. No way am I leaving them longer for life and never focusing!
Thanks

John

 

[swamp thing]

Yeah it is kinda strange in a few places that book. He does mention the truss pole trimming further on in the book (p313) but it's all a bit up in the air isn't it.

[gooseholla]

The book is strange in places yes. Like he says install the dust cover onto the primary mirror box baffle with cupboard clips... Then never bothers to explain how, or what type, or even show a picture of what it should look like! And all this talk of inches...

John

[swamp thing]

 And all this talk of inches...

John

Thats where we are going to disagree me old china. 

Good old British feet and inches any day over your French metric stuff.

[Moonshane]

you can always use an extension tube if slightly too short but not if slightly too long.

[gooseholla]

I am a strange person. I like mm and cms for small work, but not inches. Yet I'll work in feet not mm for large numbers. I use pounds for weight and don't like stone or kgs. I guess I just use whatever is easiest for the situation and don't subscribe to one complete system over the other...

I agree about the extension tube moonshane, which makes it seem strange they would make you cut the poles longer in case you couldn't rack an eyepiece out far enough.

John

[Moonshane]

the ideal scenario is that your focuser should be at about the mid position for your mid focus point eyepiece (include any coma corrector adjustment) - there is about 18mm between my eyepieces 

as you say though, in some ways, cutting and inch or two off the top of the mirror box is in some ways easier than adjusting all the tubes depending on construction. cats with skin beware!

[Moonshane]

ps

don't forget that length is not length as such - i.e. the tube forms the hypotenuse so the shortening will need to be a bit longer than the required reduction in focal length.

[gooseholla]

Well the way my build has been going, i'll no doubt be ordering new poles and plywood

John

[swamp thing]

Well the way my build has been going, i'll no doubt be ordering new poles and plywood

John

Always look on the bright side of life  

[Moonshane]

everyone getting to the end of the build feels like this and even after it's a working scope starts to think, "I could do that better....if I only change this.....etc"

[mapstar]

Well the way my build has been going, i'll no doubt be ordering new poles and plywood

John

You wont as your nearly there john.

Metric or imperial the debates roll on, although they couldn't decide with eye pieces either stating mm for the focal lengths and inches for the focuser fitting!

[Moonshane]

I like using my 12" dob with my 2" 26mm Nagler.  

[gooseholla]

Yes it will feel good when sometime in the next couple of days the final large bit is done and then it is just tweaking and testing. It'll also feel good when in about a months time I might get some clear skies!!!  

What you talking about... I often use my 2 inch, 0.78" Nagler T5 .

John

[ollypenrice]

Thats where we are going to disagree me old china. 

Good old British feet and inches any day over your French metric stuff.

Seven sixteenths. You kn w it makes sense...

lly

[Sporadic Dobstronomer]

Seven sixteenths. You kn w it makes sense...

I too enjoy the odd things that happen with metric and imperial like how a 2" eyepiece takes a 48mm filter and metric plywood comes in sheets 8' x 4'!

I notice that German telescopes are measured in inches, as are Polish televisions.

And my favourite measurement factiod:

Napolean was not short despite being 5' 2" because that's in French feet.  In shorter English units he was 5' 6".

Sadly, the inch is really a metric unit.  It is exactly 25.4mm.  Except when it isn't...

[gooseholla]

Perhaps I should of titled this post K + B: Poles 1inch and 25mm longer?

John

 

[mapstar]

Actually 1inch or 25.4mm longer ha ha

[gooseholla]

What's .4 of a mm between friends?

[Sporadic Dobstronomer]

What's .4 of a mm between friends?

1/64 inch of course.

[Soupy]

1/64 inch of course.

Come on guys it's 1/63.5 inch... If we can't agree on metric or imperial let's at least get it accurate.

[gooseholla]

I'd prefer to say 0.0157 inches...

John