Focus Travel - How Much Needed. Help!

[Stub Mandrel]

I'm making a scope.

How much focus travel should I provide in total.

How far in front of/behind the focal plane should these points be to work with normal eyepieces and diagonals ?

I have already worked out that I need the end of the tube to be exactly 100mm in front of the focal plane to reach focus with my DSLR and focus flattener (1:1) and it does not affect the focal distance).

I have been playing with CAD and working out sums for days, and its proving challenging

[wimvb]

Will you be making your own focuser as well? I always thought that eyepiece-mirror distance = eyepece_fl + mirror_fl, whereas for camera you use prime focus = mirror_fl.

You could do as TS in their ONTC customised Netwonians, and have the primary adjustable.

Or play it safe, it's always easier to add spacers to get to focus, than it is to have too little backfocus.

Sorry, can't be of any more help than that.

[Stub Mandrel]

I'm making a refractor.

For a camera with field flattener the solution is simple. The problem is that I have (at least) two 1.25" diagonals with different lengths, plus I need to allow for a 2" daigonal with no idea what length they require, plus allowing for using it without a diagonal, plus EPs have a wide range of positions for the field stop above and below their 'shoulder' which greatly adds to the travel needed.

A moonlight has 4.5" travel, but to allow for all possible permutations I probably need even more!

I would like to use rather less to avoid vignetting by the inner end of the focus tube.

It would  be helpful to have some idea what sort of travels some similar ~400mm F/L scopes have.

[Art Gecko]

I would make up a dummy frame first, mount your lens and drawtube on a piece of spruce or plywood around the same length as your primary FL.. test for focus and adjust as needed... that way you can figure out your measurements before you cut your tube to size and start creating an OTA

[Stub Mandrel]

5 hours ago, Art Gecko said:

I would make up a dummy frame first, mount your lens and drawtube on a piece of spruce or plywood around the same length as your primary FL.. test for focus and adjust as needed... that way you can figure out your measurements before you cut your tube to size and start creating an OTA

I have found the exact focal point. The problem is I want to allow for things I don't have like a 2" diagonal or decent eyepieces...

[Art Gecko]

If you've got a particular diagonal make in mind, a 'to scale' photo could help work out the dimensions?! 

As for the eye pieces... no idea! To be fair I'd wait until you've got all the bits you need and then mock up a jig like I said before.... a visual test is worth a thousand calculations!! 

[wimvb]

6 hours ago, Stub Mandrel said:

I'm making a refractor.

 

Rereading your original post, I can't see where I ever got the idea that you were making a reflector. Moving the objective lens is hardly an option then. 

6 hours ago, Stub Mandrel said:

The problem is that I have (at least) two 1.25" diagonals with different lengths, plus I need to allow for a 2" daigonal with no idea what length they require, plus allowing for using it without a diagonal, plus EPs have a wide range of positions for the field stop above and below their 'shoulder' which greatly adds to the travel needed.

Would using spacers make sense? Make the tube/focuser to fit the longest diagonal, and add spacers for shorter diagonals. That way, you would avoid the focuser drawtube moving too far into the tube. (No idea what I'm talking about really, just brainstorming.)

 

[Alien 13]

You could make the scope tube modular like the Borgs that way you can even accommodate bino viewers..

Alan

[Art Gecko]

On 20/01/2018 at 13:15, Stub Mandrel said:

I'm making a scope.

How much focus travel should I provide in total.

How far in front of/behind the focal plane should these points be to work with normal eyepieces and diagonals ?

I have already worked out that I need the end of the tube to be exactly 100mm in front of the focal plane to reach focus with my DSLR and focus flattener (1:1) and it does not affect the focal distance).

I have been playing with CAD and working out sums for days, and its proving challenging

How many mm's does the drawtube travel? If you set it around 75% extended and then use Barlow tubes with the lenses unscrewed to make up the 100mm to gain focus... or close to it. Then you know you've got movement either way in case the diagonal is bigger or smaller than the 100mm you need... if you have a 2" Ep try it as well and see if both Ep and camera can gain focus

[Stub Mandrel]

OK, lets assume field stops can be +/- 12.5mm either side of an EP's 'shoulder'. Lets allow just 2.5mm extra to allow for adjustment & errors:

Maximum extension is for a 2" EP with the field stop in front of its shoulder, this needs an extension to focal plane +15mm.

Minimum extension is for 2" to 1.25" adaptor (38mm) + my 90-degree diagonal (70mm) + EP with field stop 15mm behind its shoulder. This needs inward travel of ~125mm.

That's a total travel of 125 + 15 = 140mm.

My 700mm scope has travel of 130mm, but not practical with a 400mm long scope.

Now I have a 2" - 2" extension that is 48mm, so assuming that either that or the 2" to 1.25" adaptor is used, then total travel reduces to 115mm.

The greatest inward travel that is practical is 110mm, due to the baffle fitted to the rear of the lens cell (which is widely celebrated as one of the great features of the 66ED so I want to keep it).

>Moment of inspiration< - I've realised I am obsessing about using the 2" to 1.25" adaptor. I can make a shorter one with 5mm or less extension which means I can use the same maximum inward travel as for the DSLR, basically just a ring with a 5mm flange for an M3 locking screw.