Calculating focus distance

[virtualpilot45]

I'm drawing some plans for a small home built refractor. The lens I will be using is an 80mm dialyte with a focal length of 900mm.

MTD 80MM DIA OBJECTIVE, 900MM FL - Surplus Shed

However, for the focuser I need to be able to know where my eyepieces will reach focus in relation to the focal point. So:

Does the measure of focal length begin from the first or second lens in a doublet?

and

Is there an equation to work out how far from the focal point an EP of x focal length must be to focus?

Many thanks

[great_bear]

Is there an equation to work out how far from the focal point an EP of x focal length must be to focus?

No.

Unfortunately eyepieces vary considerably in this regard - even ones of the same focal length. A long focal-length eyepiece might have its field-stop right at the bottom end of the barrel, just above the filter thread, whereas a short f/l one might have it just beneath the lenses at the top.

Then of course, you've got to consider that you might be using Barlows (which throws everything out) and/or binoviewers or cameras.

[virtualpilot45]

No.

Unfortunately eyepieces vary considerably in this regard - even ones of the same focal length. A long focal-length eyepiece might have its field-stop right at the bottom end of the barrel, just above the filter thread, whereas a short f/l one might have it just beneath the lenses at the top.

Then of course, you've got to consider that you might be using Barlows (which throws everything out) and/or binoviewers or cameras.

Thanks for the answer, I understand now

I think I'll do the focus thing by sight then, next simplest thing!

[Peter Drew]

The focal point of a doublet is usually measured from the rear surface of the second component. In general principle, a 32mm focal length eyepiece will have its field stop 32mm from the objective focal point as an eyepiece is really just a magnifying glass examining the image. However, as already posted, there are many variables in eyepiece construction, as long as you have a focuser with a reasonable adjustment range you should be fine. .

[virtualpilot45]

The focal point of a doublet is usually measured from the rear surface of the second component.

Thank you, that should help me get the best range for the focuser, as I know where the focal point is

[great_bear]

In general principle, a 32mm focal length eyepiece will have its field stop 32mm from the objective focal point as an eyepiece is really just a magnifying glass examining the image. However, as already posted, there are many variables in eyepiece construction

As an excellent example of this, consider the ubiquitous Meade 4000 26mm Super Plossl. Although it's a simple eyepiece and thus follows the rule above, the lenses themselves are mounted deeply recessed into the barrel to keep the user at an optimum distance. Consequently, the field stop ends up about 1/3rd into the chrome barrel. However, there are several different variations of the S4000 Super Plossl, and the field stop ends up in a slightly different place in each version..