The next piece to be machined was the right ascension box top, bottom and sides.  I haven't purchased the metal yet to machine the bearing blocks(which go at the north and south sides of the RA box), so those will come later.  We started with the RA sides, which are two of the strangest shaped pieces on the mount.

 
 
The first part we machined was the top of the pier.  The pier top has 16 counter-bored 1/4-20 clearance holes to mount it to the pier(to be designed), 3 slots to allow limited rotation(about 20 degrees) for polar alignment, and a post for a screw to push against to allow rotation.  The three slots each have a 1/2-20 bolt in them, which lock the mount in place once azimuth is adjusted.

 
 
Today almost two years of design work began to take shape; the first pieces were rough cut out of 3/8" thick aluminum.  We cut the sides and top for the right ascension and declination boxes, and the bottom and sides of the polar fork.  I had a local print shop print these pieces at 1:1 scale so we could lay them out beforehand and make sure we maximized our use of the aluminum sheet.

 
 
Last month I wrote about a design I had been working on for a large German Equatorial Mount for astrophotography, to be machined this summer.  After I finished the design I called Ed Byers to order the right ascension gear, and we ended up discussing the design.  He gave me an amazing amount of advice, and I decided to redesign the mount to fully take into account what I learned from speaking with him.

Pictured above is a Solidworks render of the mount with an 80mm APO telescope pictured for scale.  During operation it will (hopefully) carry a much larger instrument.  Below is a summary of the features of the mount.

  • Mostly Aluminum and Steel construction.  Most panels are 3/8" aluminum, a select few are 3/4", and non-structural plates are 3/16".  Bearing blocks are 1.25" aluminum.  The aluminum plates are all machined from 6061-T651.  All fasteners are Type 18-8 or Type 304 Stainless.   The axis shafts are 7075-T651 aluminum. The worm-wheel cover sides are made from heat-formed acrylic, to display the beautiful Byers gears.
  • Swiss made Maxon RE-025 Motors with German made Maxon gearboxes on each axis.  These are from the same series of motors used on the Curiosity mars rover and the Astro-Physics German Equatorial mounts.
  • 11" "Star-Master" 440 tooth anodized aluminum worm wheel on right ascension, machined by Ed Byers in California.
  • 10" 360 tooth anodized aluminum worm wheel on declination, again machined by Ed Byers.  This worm block has a spring loaded worm to take up backlash.
  • Nylotron slip clutches on both axes, from Ed Byers.
  • 3 Thin-section Kaydon slewing ring bearings.  Two at the front of each shaft, and one on the back.  I am told this is the configuration used on the Byers Series I mount.
  • A Heidenhain sinusoidal encoder on RA for real time periodic error measurement and correction.
  • An Avago absolute encoder on each axis, so the telescope knows where it is (to within about an arc minute) upon start up.  This also helps it keep track of its position if the clutches slip, either due to a collision or manual adjustment.
My original design had incorporated 3.75" 6061-T651 aluminum shafts on both axes, each around 8" long.  In the newer design I chose to reduce the shaft diameter to 2.75", but increase the distance between the bearings to 12" on each axis.   The general consensus among bearing literature is that bearing spacing is significantly more important than bearing diameter(within reasonable limits).  Ed also recommended I use 7075-T651 aluminum for the shafts, instead of 6061.  At this point the design is done and I should be machining parts in the next few days.


 
 
    To augment the knowledge we gained through building mount control electronics and software last summer, and to (hopefully) construct a respectable imaging mount, we designed and plan to machine a large German equatorial mount this summer.  I have been working on the design since September 2012, and should finish the design this weekend.  After I complete it I'll post more pictures, the Soldworks files, and some more explanations of some design choices.  Our goal is to image with a 12-14" reflector.  Finite Element Analysis was used in some key places, but my version of Solidworks is relatively limiting in that regard.  The only major change that will happen between now and the final design is an increase in strength of the polar fork, and the addition of a plate on declination to mount a telescope to.  Here is a brief list of some design highlights:
  • 3.75" Aluminum shafts on each axis supported in Timken Tapered Roller Bearings.  Shafts have an internal bore to allow cables to run through the mount.
  • Swiss made Maxon RE-025 Motors with German made Maxon gearboxes on each axis.
  • 9" diameter 360 tooth aluminum worm wheels on each axis, machined by Ed Byers in California.
  • Avago 17-bit absolute encoders, with a resolution of 6.3 arc seconds.
  • Constructed mostly out of easily machinable aluminum plates.
  • Machined for the possibility of altitude and azimuth motors to test automatic polar alignment algorithms.
 
 
    In order to learn more about how telescope mounts work, and to save a significant amount of money, Izak McGieson and I set out to design and program our own mount control electronics.  We started this in June of 2011, after adding steppers to a very cheap Chinese mount carrying a 4.5" Newtonian.  We completed the majority of the electronics and software on that mount, but after realizing the following December that the accuracy of our electronics was outdoing the accuracy of the mount, we decided to upgrade.  We upgraded to a Japanese made Vixen GP-2, and also replaced the 4.5" Newtonian with an 80mm apochromatic refractor(Astro-Tech AT80EDT) with hopes of widefield astrophotography.  Our mount and software are capable of closed loop GOTOs using plate solving to accurately place the telescope, and guided tracking for astrophotography.  This post is about the mount electronics, you can find the post about mounting the motors here.  A post about the software will be written soon.  With the exception of Astrometry for plate solving and Xephem as an object database, we wrote all of the software.

 
 
This is my third of four flashlight builds, but the first one to be usable  The first one was more an act of desperation for lathe projects than a real build, and the second, while technically bright enough to be useful, was aesthetically disappointing.  Regardless, the third build is both bright enough to be usable(almost too bright), and something I'm proud to carry around.


 
 
I've always wanted to try terrestrial night photography with an actively cooled CCD imager and a regular SLR lens.  The combination of a regular(wide to an astrophotographer) field of view with the incredible noise characteristics of a professional imager seems quite appealing.  I recently acquired an SBIG ST-2000XCM CCD Imager for astrophotography, so this dream is only one adapter away from reality.  At only 2 megapixels, the resolution might be a tad low, but the light collection and low dark current are way beyond any dSLR, even the low noise Pentax K-5.  SBIG makes an adapter for their ST-2000 taper to fit Nikon and Canon lenses, but not for Pentax.  I could purchase the SBIG to Canon adapter and use a Canon to Pentax bayonet adapter, but that would increase the flange distance and prevent me from focusing on infinity; rendering the setup totally useless for outdoor photography.  Instead I machined my own adapter to go directly from the SBIG Taper to the Pentax K-Mount, properly spacing the lens according to Pentax's flange distance specification.


Front and back views of the completed adapter.  The first step was a rough Solidworks design. 

 
 
To fit a Vixen GP-2 mount with GOTO electronics from Vixen directly costs $999, twice what the mount itself costs. To buy just the motors with no control electronics is $430, only slightly less than what the mount costs. This was partially the motivation for adding our own off the shelf stepper motors, as well as the opportunity to design all control electronics and software to gain a deeper understand of how GOTO, tracking and autoguiding systems work.