The Binocular & Telescope store is well known for selling and supporting consumer telescopes, but behind the scenes research & government requests for custom-built optics are not unusual as Don Whiteman describes in this article.
“This one would mean some late nights in the workshop.”
An email comes into my inbox with a request for a custom scope. Not unusual as I get many requests for scopes for special applications. This one would mean some late nights in the workshop.
Based on a Bintel 16” Truss Ritchey-Chrétien, the brief was for gold coated mirrors and heaters fitted for the secondary mirror.
We contacted Wayne Sainty of Palmway Optical in Tuncurry NSW to see if he could do Gold coatings and he assured us that he could do it. There would have to be some test runs on the substrate to see that it would work. Our customer contacted Wayne and he assured them it would work, so the order was placed.
We had a lengthy conversation with Jim Sheng from GSO Optical in Taiwan who would make the mirrors and ship the scope components to us. It would take about 6 weeks. In the mean time we started to plot our path for the build.
The mirrors were the first to arrive as they came in by air freight. The mirrors were dispatched to Wayne for coating. He advised it would take some weeks to complete. About 4 weeks later the rest of the scope arrived in a shipping container along with our shipment of other scopes and accessories. It went into our warehouse until the mirrors were finished and we could commence the build.
About 6 weeks later Wayne drove the mirrors down to Sydney and brought them in to us. Cost was what we expected gold would cost. The amount of gold to coat both a 406mm RC Primary and 184mm Secondary is about ½ the size of a postage stamp and about the same thickness.
Gold offers 98%-99% reflectivity throughout the infrared, from 550nm to >5μm.
A few days later we got the scope bits and pieces out of our warehouse and began to start the assembly.
The truss tubes were assembled with the top end and mid section rings secured in place. The bottom truss poles were fitted and that’s as far as we got on the first day. Now to start getting optics installed.
First thing was to open the box with the mirrors in it. To say we were blown away by the sheer beauty of a gold primary just staring at us would be an understatement. The primary mirror cell was made ready to accommodate the primary, clips off and we were ready to really start.
The primary was fitted and blown clean with air and we took the first images of what we had done.
“We were blown away by the sheer beauty of a gold primary…”
The baffle was fitted to the cell and the mirror clips were fitted to hold the primary in place. We allowed some space under the clips, about the thickness of a postage stamp. Next we fitted the bottom tube section into the groove machined into the bottom plate that hold the primary cell. This went smoothly. The trusses were then raised and fitted over the top of the tube section and the minimum number of cell screws fitted so we could lay the whole scope down on the top of the work bench.
The rest of the screws were fitted and the whole truss/ tube section bolts were tightened. It all felt very stiff which means the serrurier truss was doing its job.
“Now the fun begins…”
The secondary was something that took some time to get around, firstly we had to fit a heater element to eliminate dew forming on the secondary mirror. We used a Kendrick 2080-GSO premium heater, fitted to the inside of the back of the cell so that even heat would be applied. A small exit hole for the wiring was made and wiring put in place.
Next came the mirror which had yet to be opened up. When we opened to wrapping it was super sexy looking mirror all in gold. First order was to centre spot the mirror. We cut a template and placed it on the mirror then made a mark with a fine felt tip pen. Then carefully a collimating ring was placed over the mark and set in place.
The spacing between primary and secondary optics is very important and critical to the millimetre. The secondary holder was measured so it could be subtracted from the calcs. We placed the Secondary Baffle over the mirror and inverted it so that the rear of the mirror was facing up. We placed 1mm cork pads in three positions 120 degrees apart on the rear of the mirror for the Locking ring to mount up to without actually touching the glass. Using a spanner wrench we tightened it down to the pads without undue pressure.
The fine adjustment locking ring was fitted and the whole assembly was screwed into the secondary spider. With that in place it was now time to fit the focuser, this was the easiest part of the job.
Now the fun begins, collimation.
The only real way to collimate an RC is to use the Takahashi collimating telescope, it allows you to firstly collimate the focuser tilt so that the line of sight is aimed at the centre spot on the secondary mirror. This takes about 20 minutes to get right, then comes the secondary tilt to ensure it is aimed back at the centre of the focuser. This took a little more time, about twice as long. Third is aligning the primary, this takes about 15 minutes, then some tweaking before we were satisfied with the collimation. The scope is going to be shipped over a 1000kms so it will require tweaking when it arrives at it new location.
Clean up and end of the day. Packing to come and then it ships.
This was a very interesting job and so different looking at gold coated optics. It is a job that can only be done after hours at Bintel when the staff have gone and the phones are on night call. It requires concentration and coffee, and as Mick says measure as many times as you want before you cut.
Education & Corporate Manager