The making of 26 inch Cassegrain-16, Star Test

[Chanlunlun]

Dear ATMers,

Our telescope under construction is a classical Cassegrain, which means that it has two mirrors – the parabolic primary and hyperbolic secondary. In order to test the hyperbolic secondary mirror we must first to make sure the primary is a true parabola. Although optical shop test had confirm the primary is a true parabola we still need a star test to make sure.

We also need to put the mirror cell into test to see whether it satisfactorily support the mirror without wrapping it. Furthermore we can see whether the structure of the struss tube is being strong enough to hold everything in place.

The focal length of our primary is 195 inches. We need to make an attachment to the secondary ring to increase the total length of the telescope from 120 inches to 190 inches. The length of the attachment is already 70 inches. The completed test rig has a length of nearly 200 inches, which is about half the length of a bus.

We have been waiting for a week for a non-raining school day to carry out the star test. Of course we didn’t use real star but a quartz lamp which is about 350 meters away, sufficiently distant to act as an artificial star. Test result showed that the primary mirror is a good parabola, in agreement with our optical test. The real focal length is 204 inches at f/7.8, instead of the original 195 inches at f/7.5.

The star test is really an amazing activity. The students, who took part in grinding, polishing and figuring the mirror, were amazed to see that their work produced sharp image under high magnification ( we use about 210X power ). They were stunned by how good a mirror could be made by their own hands.

The is the beginning of the end. We are all very excited.

Best regards
Chan Yuk Lun
9 March 2015

[Chanlunlun]

More pictures......