20+inch+F3.8+Gallery

The 20 inch F3.8 was originally completed in March 2012, but I was not too happy with the image quality at high power. Progress in testing optics using the Bath Interferometer meant that we could really improve the figure.

Primary is now 17.6 nm RMS on the surface and gives good images on axis of the planets too at high power. At very low power there is coma, but that is to be expected at F3.8 and well over 1 degree field! The Baader Coma corrector mark III works well with it. The pdf link is to the older model. []

Mirror cover flips up and is held in place with Velcro. It can be removed completely if needed. Below shows the secondary heater. 13 x 1 ohm resistors with 3 x AA batteries in the small black box. This gives nominal 1.5 W of power. An LED provides indication and if dim tells the batteries are running low. Secondary mirror cover is very simple plastic bottle but works well.

The primary and secondary were coated at Orion Optics UK. Both optics had the Hilux coating. Price was £300.