Last Friday Night - Simple Pleasures

WAITING FOR THE SHADOW

Astronomical Observing and Photography

Last Friday night - simple pleasures
Friday 15 February, 2019

When I was fifteen, I ground and polished my first telescope, a 6" f7. I bought the United Lens Corporation pyrex blank from a local astronomy shop as part of a $25 mirror kit complete with grinding powders and another 6" diameter glass grinding tool.

I had an incredible mentor in the late Cliff Duncan, a mentor who taught a young impatient kid how to stick at a task until it was perfect, or damn close to it. I turned down the edge of the mirror. Instead of going back to fine grind, he had me spend a couple of hundred hours polishing it out. It was a true, "polish on, polish off" moment between master and apprentice and a great life lesson although I didn't know it at the time. He pushed me until that mirror was damn near perfect then he added his masterful touch to the final parabolising and figuring. After the mirror was finished, I spent several more years with him and members of the ATM club of Queensland learning and practising casting aluminium and zinc, metal machining, welding and brazing, and using hand tools. It was a mini/amateur apprenticeship in fitting and turning until I left Brisbane and moved to Canberra. The skills and patience I learned are skills that I've used my entire life.

The scope in its heyday was a remarkable instrument. I used it very frequently for many years. When the seeing was rock solid, I could use it up to 80-90x per inch of aperture. In 40 years of astronomy, I've only encountered 3 nights when conditions were that steady. After ten years, I had the mirror recoated. It was never quite as good after that. I thought it was poor seeing but I now think the coating was uneven. During the next 2 decades, life and work got in the way, the scope languished in my storeroom, observational astronomy was limited to solar & lunar eclipse observing, bright comets etc and very occasional visual sessions.

I've returned to frequent visual observing over the past 5 years. Late last year, I dusted it off, sent the mirror & diagonal for enhanced aluminium coating with a quartz overcoat. Last week I machined an adaptor plate to attach a Vixen dovetail bar for the old rotation ring saddle. The OTA has rotation rings which make it very easy to always have the eyepiece at a comfortable position.

Last Friday night was predicted to be clear. Moonset at 2:20 am, 3 hrs of darkness before twilight. I got home from work feeling pretty tired. Before dinner, I removed the mirror and diagonal from the aluminizers packaging. The mirror has two slivers of the aluminizers fingerprints on the edge and a smudge in the centre - disappointing but not likely to affect the performance. I put all the OTA components back together hiding the biggest edge fingerprint under the mirror clip and reassembling the spider. Finally, I assembled the new dovetail cradle. With moonset so late, I decided to grab a few hours sleep then woke at 11pm, packed the car and drove the 70km out to my dark site arriving at 1am. I set up and polar aligned the EQ mount, calibrated the GOTO, and collimated the scope.

The 6" f7 has a 1050mm focal length, and is set up purely as a visual scope not as a photographic scope or an in-betweener. The diagonal is only 3.3% obscuration by area. The 100% illuminated area is only 1mm across, the 75% illuminated area is 18mm diameter. I only use 1 ¼" eyepieces in this scope. At f7, coma isn't an issue.

By the time I had set up, the moon was low and hidden by trees. As the sky darkened, I started looking at some double stars, Alpha Crux and Alpha Centauri. Looking at the diffraction patterns, I made a few adjustments to the collimation. An f7 scope is very easy to collimate. As the sky grew darker I moved onto some of my favourite open clusters, the Jewell box with its many coloured stars, the Pearl Cluster, and NGC 3532. As the Moon was setting, I moved onto the omega centauri globular cluster.

In recent years, I've been using wide apparent field of view 2" eyepieces in larger aperture long focal length instruments. I had forgotten the simple pleasures of such an uncomplicated concentrated view afforded by a quality orthoscopic eyepiece in a shorter focal length instrument. I was transported back to my youth, to those first views through this instrument. Yes, this was MY scope performing as I remembered it could. "Hello old friend!"

The moon had set and comet 2018 Y1 Iwamoto near Iota Cancri was getting low in the northwest. I located the comet in my 9x63 binoculars. Even in binoculars, the coma was large and nucleus diffuse and uncondensed. I quickly swung the scope over there and centered the comet. Large with low surface brightness and a diffuse nucleus.

With the Moon set, the sky exploded in a sea of stars, the emu was sitting on the horizon and the Milky way was mine. I reacquainted myself with views of my favourite bright nebulae, the Tarantula, Eta Carina.

The few hours of sleep I caught after dinner had done the trick. I wasn't tired at all. Usually by this time of the morning my back is complaining loudly and I am falling asleep.

A Newtonian on an equatorial mount with rotation rings is a sublimely comfortable instrument to use. The tube spins effortlessly in its cradle allowing the eyepiece to be rotated into a comfortable position that can be viewed from a straight-backed standing position. I need to shim the new cradle assembly, there is quite a bit of cone error as the object does displace out of field with a 90-180 degree rotation. When using my Vixen cassegrain, the eyepiece and finder are frequently very low to the ground and after several hours my back really doesn't like it.

As twilight approached, Antares had climbed to a respectable altitude and so I went looking for the Antares B companion. After rotating the tube the 5.5 magnitude companion with it's green colour popped out between the diffraction spikes easily visible although supposedly a difficult target for a 6" scope.

It was a glorious night. I extracted a lot out of my 3 hrs of darkness after moonset. The local birds signalled the onset of twilight with a cacophony of calls and song as if they were calling to the giant emu that stood on the horizon.

I packed up and started the hour-long drive home. I poured myself into my bed at sunrise and slept-in until late Saturday morning.

Joe Cali

Part 2 – six shootout – Takahashi TOA150 compared to 6” bespoke Newtonian