Back focus question

[JeffBevan]

I have a rather dated Meade Focal reducer. I am trying to determine the back focus for it. I have found various numbers online and have resorted to my own connivance. If I shine a light through the lens and move a paper back and forth beyond it I should be able to determine the back focus by measuring the distance at which that beam of light comes to a pinpoint. Unfortunately this measurement doesn't coincide with any given distance i find on line. While close not within 1 or 2 cm. I am inclined to just run with that distance.

[KathyNS]

You are measuring the focal length of the focal reducer. There are two problems with that. First, the focal length does not necessarily correspond to the required back focus. Second, to measure the focal length this way, you need to have a light source at infinity, not a nearby source.

The way to measure the back focus is to attach the reducer to a telescope and take pictures of stars at various spacings. You want the spacing where stars are round is the correct back focus distance. If the stars are elongated radially or tangentially, the spacing is wrong.

[JeffBevan]

Thank you, good to know the difference. I actually used the Sun, fiery, little thing.....

[KathyNS]

Thank you, good to know the difference. I actually used the Sun, fiery, little thing.....

I don't recommend that. Not only is there a danger of incinerating anything at the focal point, but also the Sun is too large for the kind of image analysis that would tell you if you have the right back spacing. You need something that ought to be a point source or a very tiny disk. Like a star.

[JayTee]

There is an alternative to doing this at night, there is a daytime method you could use, not quite as exact but it'll get you close for night time refinement.

Find a fence post more than a hundred yards away put a glob of modelers clay on top of it. Stick a chromium ball bearing onto the modelers clay, do this on a sunny day. The reflection of the Sun from the ball bearing is reasonably close to a point source of light. Watch out, it is still bright.