how to decide on draw tube size in focusers?

[irishbloke]

hi all,

i'm talking myself around to a new focuser and am wondering how to measure the focal point for the different eyepeices i have.

Do i just measure from tube to edge of eyepiece in focuser? Then compare the difference to find out the range of travel needed and buy one that covers this range.

Is there more calculations involved to do with the number of spacers needed etc.

Thanks for any help. I just like understanding what's involved.

Brian

[simon84]

Sounds like a plan to me, if your scope has 50mm of travel from fully in to fully out and you can focus all your EP's in that range then thats what you need, plus then any height difference in the focuser body.

From OTA tube to edge of drawtube fully racked in then add however much you rack out the drawtube to focus the EP, then if the new focuser has a OTA tube to drawtube fully racked in less than the other focuser then you will need to add that amount to drawtube travel.

My moonlite focuser ended up being the same height as the old Skywatcher focuser but is acutally 10mm less in drawtube travel so I now need an extention tube or pull the EP out of the drawtube a bit.

HTH, but now I've just read my post again I'm really confused.

[G4YVM]

When I made mine I used the bog standard empirical method you advocate (i.e. if all the EPs focus then the range is good enough). Where it failed is that it wont bring the SLR into focus! I didnt think of that in time!

David

[irishbloke]

i suppose i have to ask myself do i want to be able to use my dslr on my dob.

[Bizibilder]

If in doubt you want the shortest (lowest?) possible focusser. 'Tis easy to add an extension but a little more difficult to remove a bit!!!