Aluminium roll flashing

Aluminum flashing
Aluminum roll, one millimeter thick. Fastened with masonry screws, this forms a barrier to rain and drafts under the base of the dome.

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 rotation rings.


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.

Aluminum roll flashing
Illustration showing aluminum roll flashing used to weatherproof a four meter ScopeDome.

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.

Four meter ScopeDome
Photograph of the outer dome casing and aluminum roll flashing. The latter is first mounted temporarily by cargo straps, and adjusted so that it just clears the upper rotation ring.


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.

From the top of the dome

4m ScopeDome

ScopeDome 4m
As is the norm in observatory building, there are commissioning delays: mainly malfunctions in the electrics.

Fixed-post grinding maskin

Fixed post grinder
Fine-grinding and figuring will soon commence on the 62 cm lightweight cellular primary. More to come about the fixed-post machine and techniques. (click to enlarge)


The floorboards are lain in a pattern of eight fields oriented to cardinal points. The floor area is only seven square meters (75 square feet).

The spider

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 spider with the secondary mirror removed. The three-element flat field corrector is visible in the spider’s center gives an 80 millimeter flat focal plane. The entire assembly as pictured is 13 kilograms.

Camera and filter wheel at prime focus

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.


OTA for prime focus photography
The optical tube assembly configured for prime focus photography

Primary baffle tube

Primary baffle tube
Primary baffle tube and focuser made by Astrosysteme Austria.

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.

Ceiling Cat

Ceiling cat
Ceiling Cat suddenly materialized. There is a gap in our knowledge as to why this being is attracted to large telescopes.

Assembly of the optical tube

Truss tube
The overhead crane lifts the primary mirror cage in place.

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.