Telescope Setting Circles

I have been considering the best way to improve my telescope viewing activities and Setting Circles seem to be an improvement I could make to my two Dobsonian Skywatcher telescopes that provide a quick win.

Here is a view of the Setting Circle on my smaller 130p telescope, there are more images of this and my larger 305p telescope later in this instructable.

• The Smaller Telescope is a Skywatcher Heritage 130p D130mm F160mm on a Dobsonian Mount
• The Larger Telescope is a Skywatcher Skyliner 300p Flextube D305mm F1500mm on a Dobsonian Mount
• The wooden annular rings were sourced from this eBay company who were excellent and helped me work through the design, size, thickness, etc. I highly recommend them

  https://www.ebay.co.uk/itm/Circle-MDF-400mm-diamet...

• I used the following website to prepare and print the scales to go on the annular rings https://www.blocklayer.com/circle-divider.aspx
• I also looked at a number of other people’s designs on a variety of websites and forums to get ideas

In planning my design I worked with the following ideas and principles:

• Each setting circle will be placed over the appropriate telescope and placed around the top base disc:
  • For the smaller telescope the annular ring will rest on the lugs of the lower base disc
  • For the larger telescope suitable supports will be fitted to the lower base disc, protruding out, that the annular ring will rest upon
• In use the telescope is aligned North – South (using a compass) and the annular ring rotated until the 0° is under the pointer
  • The pointer will be fixed to the base using a magnet so that it can be removed and replaced easily whilst maintaining pointer alignment - I didn't actually do it like this as I found a simpler solution
• When the telescope is rotated the annular ring remains in situ hence showing the angle that the telescope is now pointing (Azimuth)
• The annular ring will have 360 degrees marked on it so that the telescope can be aligned to a single degree accuracy
• The inside diameter of the annular ring quoted for each telescope is the actual measurement therefore some tolerance is required to allow the telescope turntable to move without catching on the annular ring
  • I’ve allowed 2 mm all round for clearance
• The actual width of the annular ring is not vital as there is sufficient clearance once the ring is laid over the base
  • I have worked with 30 mm as that will allow sufficient space for the angle graduations to be placed easily – probably using a paper template
• 25 mm thick MDF for both annular rings is probably best to maintain strength
• Minimise any permanent changes to the telescopes or their mounts by reusing existing holes, mounts, etc.

This instructable will show the steps primarily for the smaller 130p telescope, but the larger 305p telescope steps and principles are basically the same. I have attached a pdf at the end of this instructable which have all of the steps and a few more pictures which might help in understanding why I did what I did and what you might want to change/adapt.

There are a number of scale production websites out there but I found this site gave me a good range of options to parameterise to my needs the scales to go on the annular rings

https://www.blocklayer.com/circle-divider.aspx

There are more photos and details on producing the paper scales in the pdf at the end of this Instructable.

Ideally the scales should be printed on a single page but I was limited to an A4 printer. Under normal times I might have considered using a printing service but as it turned out separate arcs printed on A4 then pasted on the annular ring works very well.

• The paper scale was glued onto the wooden annular ring with PVA glue
• The scale was sealed with Jigsaw Puzzle Conserver as I happened to have some handy. I believe an alternative would be Modge Podge although I’ve never used it myself
  • Although I am not planning to have the telescopes out in the rain there is a risk of damp when in use

Although the Setting Circles sit well on the telescopes, I wanted to ensure that any errors were minimised so the Setting Circles should be centralised around the telescope base and ideally limited in its movement once aligned.

The same process was followed for the 305p telescope. As it is a significantly larger and heavier unit, I have built a wheeled trolley and I was able to make Setting Circle supports that could be retained between the base of the telescope and the trolley thus avoiding a need to attach something permanently to the telescope.

Having solved the Azimuth calibration I chose to use a magnetically attached inclinometer to the tube. To make it easier to read at night I fitted a red LED rear cycle light in a shroud. It's a little bulky and at some point I may refit the lights into a smaller shroud.

Another spare red LED rear cycle light (I wonder what happens to the front ones??!!) was mounted on a small plastic slope so the Azimuth scale could be read at night.

In case there are no suitable places for the 130p telescope to stand when in use I repurposed an old Meade tripod with a 40 cm wooden circle (also from Applebridge)

What I might do differently:

• The larger setting circle paper scale glued on the annular ring more easily and evenly. This might be due to the arcs being smaller and so I’d probably print the smaller setting circle scale in six sections of 60 plus appropriate overlaps rather than the four 90 sections that I did
• Measure at least three times and then get someone to check your designs! I would have ordered the wrong size annular rings without the help of Applebridge who noticed my error of measurements despite my checking many times!!

Please feel free to download the pdf and I hope it might help you in what you may design/produce.