Eyepiece focus requirements

[hunterknox]

I'm planning on building a suitcase Dobsonian (thanks to Moonshane for the inspiration) and am trying to figure out how much focuser travel I need. If I measure the range of focus needed for my eyepieces on my current scope (80mm f7ish refractor) is it likely to be the same for my f6 Dobsonian or does the range of focus vary with the f ratio?

[Merlin66]

No, the eyepiece focus position, or the variation between them is independent of the focal ratio and the focal length....

To reduce this variation most new eyepiece sets are made par-focal - this means being able to switch eyepieces without touching the focuser (well, in theory at least!)

[hunterknox]

Thanks Ken - so measure the difference on one scope, same applies for all scopes. That makes my life easy, thanks!

[Moonshane]

what I would normally do is in any old scope, just establish the eyepiece with max in focus and then max out focus and gauge the difference. In mine it's 18mm. I then look at the focuser range and that's 30mm so I aim to get the mid point of my range (9mm) on the mid point of the travel (15mm) and this means I have 6mm slack either side which is plenty. if in doubt push the focus point out more as you can always add extension - but get it right and you won't need one.

when establishing the correct truss length, I made an adjustable set of four pieces of ply which I can clamp to each side of the mirror box, and enabling me to move the UTA up and down.

cheers

Shane

[hunterknox]

what I would normally do is in any old scope, just establish the eyepiece with max in focus and then max out focus and gauge the difference. In mine it's 18mm. I then look at the focuser range and that's 30mm so I aim to get the mid point of my range (9mm) on the mid point of the travel (15mm) and this means I have 6mm slack either side which is plenty. if in doubt push the focus point out more as you can always add extension - but get it right and you won't need one.

when establishing the correct truss length, I made an adjustable set of four pieces of ply which I can clamp to each side of the mirror box, and enabling me to move the UTA up and down.

cheers

Shane

Good tips, thank you Shane. I may pick your brains some more as I'm going along if that's OK? I would have commissioned you to make it for me but I've a friend who's a carpenter who could do with the work so I'm busy reading Kriege & Berry and every Sky and Telescope article I can find on Dobsonians to figure out what needs doing. Your threads have been invaluble in getting me up to this point and will doubtless continue to be so as I go along.

Thanks again,

Tom

[cs1cjc]

Focusing a telescope can be thought of as making the focal point of the telescope coincident with the focal point of the eyepiece. With this in mind, it is easy to see why you need move focus the same distance as you change from one eyepiece to another, whatever the telescope.

[Moonshane]

Good tips, thank you Shane. I may pick your brains some more as I'm going along if that's OK? I would have commissioned you to make it for me but I've a friend who's a carpenter who could do with the work so I'm busy reading Kriege & Berry and every Sky and Telescope article I can find on Dobsonians to figure out what needs doing. Your threads have been invaluble in getting me up to this point and will doubtless continue to be so as I go along.

Thanks again,

Tom

no sweat mate - just ask

not sure where you are but if close to NW England feel free to visit.

[hunterknox]

no sweat mate - just ask

not sure where you are but if close to NW England feel free to visit.

Very kind of you. It'll be a few months before I start properly but I might just take you up on that.

Best wishes,

Tom