what length extension tube

[chemtom24]

what length should i make an extension tube for a neximage on a 200p f5 newt.

Trying to use a tube instead of a barlow

Is there any formula for working out where the focus point is?

[chemtom24]

well after some awkward attempts to hold everythign in the optical train, i9 managed to get focus. tube is 10 inches long

my initial tubes of 3-4 inches were way out lol

just my luck jupiter will probably be TOO magnified to fit on the chip!

[Psychobilly]

Not doubting your testing but That's crazy

[chemtom24]

it is, couldnt get focus with 3,4,6 inches. so pulled the cam out of the tube and tried to keep it in line, pulled it out so far and got focus. ended up 10 inches.

Is this too much? will it be too high a mag now? should i be able to achieve focus with a shorter tube???

[Psychobilly]

I'm still trying to get my head around it at the moment as its an "unexpected" result

[RogerTheDodger]

Hmm, on my 200pds with no barlow, the webcam comes into focus about 3" from the outside of the OTA, the 200p should be closer in...

[AztecastroMcJ]

cant remember exact calculation but i'm sure 5cm or 2" ext was like x1 -x2

u should still use ur x3 barlow then use smallish ext say 2" after barlow

rather than one long tube,

that will give u at least x4-5 so should be ok for ur scope

HTH

James

[chemtom24]

let me try again

thanks for the comments so far

[RogerTheDodger]

I can give a better measurement a bit later if you need it.

[chemtom24]

ah i think i see whats up.

i got focus on a flower branch which was pretty near the scope.

tried it with a tree further away and nothing. also, triued 3 inches and nothing. should i try 2? im running out of pipe here!!

[steppenwolf]

You really do need to do this on a VERY distant object.