Wedged or Perpendicular Back version                  Parallel Back option

Wedged or Parallel Back?

We make off axis paraboloids by two distinct methods, giving rise to a choice of physical form shown at left.

Parent method

Generally, OAPs up to 152mm diameter are manufactured in a “parent” or “mother” paraboloid – an on-axis paraboloid. The customer’s require off-axis segment is then drilled from this parent. Standard parents are stocked in 356mm diameter or 406mm diameter (dependent on focal length) although specials can be made to >600mm diameter. We can trade off diameter and off axis distances anywhere within the clear aperture of the parent.

The result of this production method is that the “wedged back” form is the natural choice since the OAP can be directly cut out with a core-drill axis perpendicular to the back.

This form has some advantages: since our parents are made accurately centred, the back can be polished and used to define the optical axis direction. This helps with alignment, and the front of the mirror is already tilted in the correct direction when the back is running square to the customer’s optical table. A mounting groove can be generated in the edge which is often useful to mechanical designers solving the problems of mirror mounting.

Stand alone method

Mirrors above 152mm diameter are generally made as single items- “stand alone” OAPs. This has the advantage of flexibility of focal length beyond the choice of stock parents. This method naturally results in the parallel back form of mirror, nominally equal in thickness round its perimeter. We do make large mirrors in a wedged back form, but this is an additional cost due to the amount of machining required. Summary: mirrors up to 152mm diameter can generally be supplied wedged or parallel back at the same price. This option should be discussed and clarified prior to order. Above this size, the standard form is parallel back, with wedged back available only at an extra cost.