Refractor baffling question

I have recently received a TAL 100R achromat (100mm f10) with loose baffles (it has been a tad abused over the years I think). Anyway, I am looking for help in figuring out the best baffle placement with this scope. I measured the ID of each last night. Here they are:

#1: 81.5mm
#2: 71.3mm
#3: 56.2mm

Objective diameter is 100mm; focal length is 1000mm. This scope will be used for visual use only.

Could someone help me out with the best baffle placement? The rear element of the objective is pretty much flush with the end of the tube, so I think that I can use the tube end as the reference point.

Thanks for any help!
Cheers!
Tim

If it's f/10 then the math is easy. The light cone narrows proportionally down the tube so:

The 81.5mm baffle should be 815mm from the focal point, or 185mm from the objective.

The 71.3mm baffle should be 713mm from the focal point, or 287mm from the objective.

The 56.2mm baffle should be 562mm from the focal point, or 438mm from the objective.

At f/10 a 1-cm error in baffle placement will only over/under-cut the light cone by 1mm or 1% of aperture (2% at the halfway mark)

Tim, the method for calculating stop diameter in a refractor is the same as that for calculating the secondary minor axis in a Newtonian reflector with no secondary offset. The formula is ususlly used to calculate the diagonal size, given the size of the fully illuminated image.

d=L(D-I)/f + I, where d is the minor axis diameter, or stop diameter, L is the distance from the stop to the focal plane, D is the diameter of the primary (objective), f is the focal length of the objective, and I is the diameter of the fully illuminated image.

In order to determine d, it is necessary to decide on the size of the fully illuminated image. Since you already are stuck with d (three of them) solve the formula for L to determine the distance of each stop from the focal plane of the objective.

L=f(d-I)/(D-I). Note that for a stop diameter that is equal to the objective diameter, L equals f, as you would expect, since the stop is the objective itself. Also for a stop diameter equal to I, L=0, again, as you would expect, since the stop is the same as the image diameter, and is located at the focal plane.

For your refractor, a fully illuminated image of 20 mm should be sufficient for visual use, although if you have a 2" focuser and want to use wide field 2" eyepieces for unvignetted low-power views, a larger fully illuminated field may be desired. FWIW, The stops in my 4" f/15 Unitron are positioned to give a fully illuminated field of 25 mm. I don't notice any vignetting with a 2" 32 mm f.l. eyepiece that has a field stop diameter of 38mm (67-degree apparent field of view).

For the stop diameters you've given, I calculate the following values for L with a fully illuminated 25mm field. (Please check my arithmetic.)

d=56.2 mm; L=416 mm; P=584 mm
d=71.3 mm; L=617 mm; P=383 mm
d=81.5 mm; L=753 mm; P=247 mm

Subtract these L-values from 1000 to get the distance of each stop from the objective. Those are the P-values I've listed. In positioning the stops what you really want is the distance of each stop from the objective end of the tube, so subtract from each P-value the distance of the objective from the end of the tube.

Hope this helps. Have fun with your project.

Dom Q.