Digital Setting Circles
I find digital setting circles (DSCs) to be a very useful accessory from my light-polluted backyard. On some nights I'm lucky to be able to see 4th mag stars, so star hopping can be very tedious to even get to stars anywhere near the object of interest. With my DSCs it only takes about 30 seconds to perform a 2 star alignment and then I can find anything my sky will allow. Also, if I have limited time, I can quickly find half a dozen or so new objects and choose the best to return to another night.
I use 1024 encoders connected to my old Palm m105 running PalmDSC. This is an absolutely fantastic piece of freeware with lots of really useful features and I highly recommend it. I especially like the ability to sort objects by distance to the current sky position. After correcting for mount fabrication errors, I get a pointing accuracy of ~0.5 degrees over the whole sky. This means that, at worst, the object I'm aiming for might be just outside a medium power eyepiece. As an aside, my mount fabrication errors, measured using Toshimi Taki's method on ~10 stars, are z1=-0.5 and z2=0.8 degrees. I mention this as I'm really pleased at building a mechanical system to sub degree accuracy!
Mounting Hardware
It took me a lot of trial and error to work out how to build the
necessary hardware to mount my shaft encoders.
Several companies now sell mounting hardware for DSCs, but the prices seem very high and one of my main aims was to put together a very cost-efficient DSC system, just to see if I enjoyed using it. Also, I'd had several attempts at making the mounting hardware, and I wasn't going to let it defeat me!
The following describes my successful installation using only a [cheap] drill press, a length of 1/2-inch square aluminium rod, some JB Weld, and standard nuts and bolts, etc. – i.e. no lathe!
Fabricating the azimuth bolt is the tricky bit. The Kriege & Berry
book advises the reader not to attempt this without a lathe, but I
found this tip which suggested a method using just a drill press.
The same link also has good diagrams showing the basic anatomy of the mounting
hardware.
I
tried this and found that, unfortunately, my drill press didn't seem
to want to drill through a stainless steel bolt head and, after making
horrible screeching noises, the bolt just started spinning freely.
So, I came up with this modification. The photo on the left shows my
jig. This is basically the same as described in the above link – I
clamped some scrap wood to the drill press, drilled a hole the
diameter of my azimuth bolt, popped the bolt in and then switched to
a smaller drill bit to drill into the head. This ensures that the
second hole will be coaxial with the first. A couple of days
earlier, I had JB Welded a short length of 1/2"
square aluminium stock to the bolt head, figuring this would be much
easier to drill than steel.
Again, I found that the bolt would just spin
freely. So, I threaded a nut all the way onto the bolt and
clamped a strip of aluminium flat against one face of the nut to
prevent the bolt spinning. In this case the aluminium strip is the
bracket into which the azimuth bolt will eventually thread. It's a simple strip
of aluminium with a hole at each end to screw to the ground board. In
the centre there's a hole for the azimuth bolt to go through, and a
stainless jam nut JB welded around that. Note also the masking tape on
the drill bit to ensure I don't drill too far through the aluminium
block and risk parting it from the bolt head. The installed encoder is
shown on the right.
The altitude encoder bracket is much easier to make but slightly
harder to figure out where to mount (unless you have the centre of your
altitude bearings already marked, in which case it's trivial). Again,
the main component is a length of 1/2" square aluminium rod. A 1/4"
hole in the centre for the encoder shaft, a countersunk hole either
side for the screws to mount it to the bearing, and a hole
perpendicular to the encoder hole, tapped for a set screw. The image
on the left shows the altitude encoder mount (left) and azimuth bolt
(right).
To work out where to mount the altitude encoder, I clamped the arm
that was going to hold the encoder onto the side of the rocker box in
roughly the right place and put a pencil in place of the encoder.
Swinging the telescope tube up and down caused the pencil to draw small
arcs on the side of the bearing. By shifting the position of the
pencil slightly, I could find a place where the pencil only left a dot,
and hence had located the centre of the bearing. In order to
accurately mount the bracket around this dot, I stuck a 1/4"
wood-cutting drill bit into the encoder hole and stabbed the point into
the pencil mark. Then the bracket could be clamped accurately in place
whilst inserting the mounting screws. The images to the right show the
altitude encoder mounted (the yellow kevlar cord is part of my spring
counterweight system and nothing to do with the encoders). I have found a better nylon thumbscrew since the
previous picture was taken. The encoder arm is just a length of aluminium C
channel (left over from building my travel
scope). A steel bracket holds the encoder in place and a 1/4-20
eyebolt holds the arm to the side of the rocker box. In the rightmost
picture, the alt encoder arm is shown stored for transport.
Note: the arm attachment to the rocker box should be left slightly loose in use – I found that tightening
it up caused some hysteresis in the encoder readings, presumably from
some small misalignment.