24 inchteelscope - foacl length of nulling lens

[Chanlunlun]

Dear ATMs,

In order to accomplish we need well preparation. Hence it is my desire to prepare as much as possible before we really start work on the 24 inch mirror making.

Of the most important is the construction of the Offner null tester. As mentioned before the focal length of the plano convex lens has to be measured to a very high degree of accuracy before any meaningful design work could possibly start. Hence I』ve made a focal length measuring jig which can provide very precise measurement of the paraxial back focal length of the plano convex lens concerned.

The heart of this measuring jig is a rack and pinion focuser with the lens under test fixed inside a black barrel located at right hand side of the focuser and the ground glass is located at the left hand side. The ground glass is stick onto a Or-25 mm eyepiece and very accurate focusing can be achieved by viewing through the eyepiece onto the ground glass.

The focuser is adjusted under a sharp image is obtained onto the ground glass under inspection through the or-25 mm eyepiece. Notice that this is not the true focus. Very carefully the ground glass is shifted backward until we get the maximum possible focal length with a still sharp image. In this way we can alleviate the spherical aberration of the lens and get the true focus. The measurement has to be carried out several times and the mean result is taken.

The results are as follows :-

Compensating lens – paraxial back focal length = 50.00 mm
Central thickness of lens = 5.55 mm

Field lens – paraxial back focal length = 128.65 mm
Central thickness of lens = 4.30 mm

With such parameters on hand I』m able to design the Offner null tester by computer ray tracing program such as OSLO.

Best regards
Chan Yuk Lun
1-2-004