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Old 10-05-2021, 02:32 PM
Stefan Buda
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Stefan Buda is offline
Join Date: Jul 2013
Location: Melbourne, VIC
Posts: 700
I think it is high time for an update in case anyone is wondering whatever happened to this project.
Well, serial number 001 has been capturing photons in Tom's automated observatory for over a year now and I've been getting a lot of feedback that has enabled me to finalize the design.
The thermal control worked well in all seasons and no observing time was lost due to mirrors getting fogged up, but one of the fans failed after about a year and I replaced all four at that stage. The chrome plated fan grilles started to show signs of rusting after a year and SN002 will get 3D printed ABS grilles.
The secondary focuser has been generating lots of nice V-curves every clear night and caused no trouble, but Tom said that it took a bit of time to get the autofocus parameters optimised because existing software was written to work best with very small step sizes. My secondary focuser has a step size of about 12 microns and that means only 2 or 3 steps will fall in the critical focus zone. In theory that should be enough and it did turn out to be so once the software parameters were right.
SN002 will use a conventional rack and pinion focuser but not because of the issues with the step size. The main reason is that I want to free myself from software development. Simply not enough hours in a day. When I wrote the Arduino software for SN001 I used the same pins for the motor drive and the ambient temperature sensor as Rod Brown's open source project, so that his focuser driver could talk to my shield and make it ASCOM compatible. This was going to be just a temporary fix until I got round to writing my own driver. Since then I decided that I can't afford the time needed. Software also needs updating from time to time, to maintain compatibility with the evolving ASCOM platform. By using an off the shelf controller, such as the AEF, my software problems should be solved. However I will have to design a new smaller PCB, based on Arduino Nano, for the thermal control. That should be no problem as it is a stand alone unit that does not need to interact with other software.
Last but not least is what I learned about the optics. While SN000 had a Pyrex primary and Borofloat secondary, SN001 had both mirrors made of Schott Borofloat glass. I had a lot of trouble making this optical set. It was very difficult to get consistent readings during the figuring process and in the end the Roddier test came in at about 0.9 Strehl. That is a bit better that what is called diffraction limited, but in practice, for a high resolution instrument, that means somewhat larger stars than theoretically possible in a given seeing. So I made another Borofloat set and I got the same result. In the end I figured out what the problem is with what I now call Gremlin Glass. When examined between crossed polarizers, there is no sign of internal stress, as long as one only looks trough the disk in an axial direction. When examined from the side, there are very strong parallel stress layers that cause problems once one cuts a deep curve into then.
So in the end I made a mirror set using old Pyrex stock that came in at Strehl ratio of about 0.97 and installed it in SN001 about a month ago. The weather has been bad but Tom is reporting good performance and he is getting closer to producing a first image with the new mirror set.
Meanwhile I'm working hard at making an exorcising kiln for Gremlin Glass.
The attached photo shows the new focuser attached to SN000 and the new fan grilles.
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Click for full-size image (New-Focuser.JPG)
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