Friday, July 14, 2017

Putting the pieces together....

The new tripod and alt-az mount has arrived and look very nice - great workmanship and finish. The compactness of the tripod and dual-scope capacity of the mount will do wonders in reducing the amount of gear I need to haul along. I am using a simple aluminium plate to connect the Borg refractor to the mount - it needs to be offset pretty far from the mount to ensure proper balancing.
Berlebach tripod and TS AZ5 mount with the two refractors that I'll be using for the upcoming total solar eclipse.

Next step is to connect all the parts and do some field testing of the imaging setup. I need to find out if the setup suffers from vibrations and investigate if the optical quality of the 4" f/4 refractor is OK. Doing this is really simple: just set up like it's a total solar eclipse, point the scope at Vega and run the Eclipse Orchestrator script. This script runs through a number of exposures, ranging from 1/800 to 1.3 second. The short exposures will primarily sample the optical quality while the longer ones will be sensitive to vibrations.
In-focus stellar images taken with Borg 4", f/4 refractor. Scope was not collimated prior to this test.
I focussed the scope using a zoomed liveview display on my laptop. A bright blue halo appeared around Vega at focus and the highly out-of-focus image was not perfectly round. This is not indicative a excellent optical performance! I was in a hurry, so I pressed on running the script. Below are shown six images from the run; three at 1/320 second exposure and three with 1.3 seconds. Each series shows the same star which is near the center of the field. From these shots I conclude:
  1. The short exposures are generally sharper and more variable - both effects are due to atmospheric seeing.
  2. The short exposures reveal an asymmetrical halo - this is likely due to optical misalignment within the scope.
  3. The long exposures show no signs of vibration
The refractor used for this test consists of a Borg 4" f/6.4 ED lens coupled with field corrector and reducer lenses to yield f/4. Such a fast configuration does require careful attention to mechanical alignment of the optical elements - and I did none prior to this test. I have previously done tests at f/6.4 where I could reach FWHM=3.2 pixels.

Next step is to try and improve the collimation of the f/4 configuration - if I can't get it satisfactory I'll go for f/6.4 and the sharper images. Perhaps it would also be beneficial to use a filter to reduce the blue wavelengths - this is known to improve contrast with achromatic and ED refractors.

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