Re: Projection Formula

Hugh, the formula is as follows:

f/ = (telescope f/) X [(projected distance* - eyepiece F.L.)/eyepiece F.L.]

* please note that this distance is from the focal point of the eyepiece to the film plane.

It may be stated using other terms in the books, but this nomenclature is a way for me to understand it. Below is an example for my Meade SCT using a 25mm eyepiece and my variable tele-extender:


f/ = 10 x [(185mm* - 25mm)/ 25mm] = 64 or f/64


with a 9mm eyepiece:


f/ = 10 x [(185mm* - 9mm)/ 9mm] = 195 or f/195


I hope this helps you.

Clear skies,

Martin Boys

Hugh,

The focal plane of the eyepiece should be at or very close to the shoulder or step of the barrel, (i.e. eyepiece theoretical parafocal point) for whatever eyepiece you use. To keep things simple, use the visual back as your reference. Please note: If you use parafocal rings you will need to take the added distance into account. By normalizing the eyepieces to each other, the focal point of the scope is closer to the film plane to allow for the extra distance required. This will result in lower f/ values and smaller projected images.

For the steps below, I will not include the parafocal ring ring issue.


First, measure from the very back edge of the visual back to the step where the Tele-extender will bottom. Second, measure the length of the Tele-extender. Third, subtract the first dimension from the second dimension. Fourth, add the the T-ring to film plane measurement and you will have your number. Absolute, to the nearest .1mm, accuracy isn't necessary as you will be approximating the f/values typically anyway and bracketing exposure times.

I highly recommend that you get Michael Covington's book Astrophotography for the Amateur. It has the formulas you are looking for, exposure tables worked out for the solar system as well as a wealth of other usefull information.

Let me know how you do.

Martin