To minimize drafts and humidity when the dome is closed, the ScopeDome manual recommends capping the top of the crown with a ring of sheet metal that extends horizontally to the
lower dome casing. This closes the gap between the crown and dome casing, and forms a horizontal barrier that blocks wind and rain from entering the dome through the open space between the
An astronomer who owns a four meter ScopeDome suggested a simpler weather-proofing alternative that does not require cutting an annulus. One can instead use an eleven meter strip of metal or other water-tight material in order to form a vertical barrier.
I obtained forty meters of 25 cm wide, 1 mm thick aluminum roll as offcut from a local aluminum foundry.
The drawing shows how the aluminum roll flashing extends above the observatory crown. Provided the outer diameter of the observatory crown is the same or slightly larger than the diameter of the
rotation ring, the flashing forms a tight fit. However, the flashing must not contact the upper rotation ring.
The inner casing is not shown in this illustration. The inner casing forms an additional weather barrier. With these two barriers in place, the four meter ScopeDome weather tight.
The photograph to the right shows the aluminum roll flashing held temporarily in place by a cargo strap. This holds the flashing securely while allowing vertical adjustments. The flashing must just clear the upper rotation ring by a millimeter or two.
The flashing is later fastened permanently using masonry screws.
The spider assembly is shown prior to mounting on the optical tube assembly (OTA). When a camera, spectrograph or other detector is mounted at the prime focus, the secondary mirror is removed, as shown in the photograph.
The photo shows the side of the spider that faces the primary mirror in the OTA. The flat field corrector is in the center of the spider. This corrector is similar to a Wynne corrector, and consists of three lens elements designed by Philipp Keller, an optical engineer at ASA Astrosysteme near Freistadt, Austria.
The photo below shows the optical tube assembly (OTA) configured for prime focus flat-field photograhy. The secondary mirror is removed so that the light path passes through the flat field corrector and filter wheel.
The prime focus option makes the OTA longer than an identical Cassegrain without an accessible prime focus. This is two telescopes in one: a 620 mm f/3.4 astrograph and a 620 mm f/10.4 Cassegrain astrograph.
Baffle and focuser
Astrosysteme Austria (ASA) designed and produced the entire optical tube assembly (OTA) minus the primary and secondary mirrors, but including the flat-field corrector for the prime focus. ASA calculated the OTA specifics for the parameters of my home-made mirrors. These specifics include the diameter and length of the primary and secondary baffle tubes.
Shown here is the primary baffle and intergrated focuser.
Click image to enlarge.
Primary mirror cage
The assembly of the optical tube is progressing well.
The primary mirror cage is the most massive part of the optical tube assembly. It is a truss-tube made of powder lacquered square bar steel. To minimize the mass during assembly, the focus tube, back plate and primary baffle tube were removed.