Add an azimuth circle to a your Dobsonian and ditch that straight-through finder

A couple of years ago I added azimuth circles to the bases of my two Dobsonian telescopes, and recently added one to a go-to tabletop dob to replace the often unreliable go-to system. Coupled with a digital angle gauge, available in hardware stores or online for about $20-30, this allows me to dial in the altitude and azimuth coordinates for any object, creating a "push-to" system. I can literally find anything anywhere now without straining to look through a straight-through finder, as long as I can see it in my scope and it's included in my sky charting app. 

The main advantages are:

• No neck strain looking through a straight-through finderscope or red-dot finder (this was the impetus for me)
• Ability to find objects in areas of sky without a lot of bright stars for starhopping, or in light pollution
• Quick and easily repeatable
• No finicky and power-hungry electronics (the angle gauge takes two AA batteries that last a long time)
• Inexpensive

What you need and how you use it

You will need an app to look up the alt-az coordinates for an object in real time. As the earth rotates, these coordinates constantly change, and are based on your location and time. As always, I recommend Sky Safari Pro (Android or iOS) as a great all-round app that will list the coordinates and show you the star field once you've gotten close to an object. Even the Basic version has the alt-az coordinates, but for a smaller database of objects.

In the Sky Safari Pro screenshot at left, I have selected galaxy NGC 7331, centered it, and the current azimuth (88.5) and altitude (62.4) are shown in the upper left. Make sure you center the object. If you don't, it will not show the correct alt-az coordinates. Then move your scope tube so the pointer on your azimuth circle is set on 88.5 and your digital angle gauge shows 62.4. Look in the eyepiece and, if you have properly leveled and aligned the scope, the object should be in there somewhere. If not, check the wider view in the RACI finderscope if you have one, find the object, and adjust the pointer as needed.

The following are the steps required to find an object with the azimuth circle/angle gauge method. Steps 1-6 are done at the beginning of each observing session. Step 7 is repeated for each object you want to observe.

1. Set the telescope base so that the azimuth circle is roughly aligned with either the Sun or Moon during daylight, or any bright object at night.
2. Level the scope. A cheap bubble level will do fine. I use an app. I made some plywood squares with tread tape on them for rough leveling and use composite shims for fine tuning.
3. Put in a low power eyepiece and find a bright object that's easy to align on without a finderscope. Just sight along the tube at something not too high in the sky. Once centered in the eyepiece, adjust your RACI finderscope, if you have one, to match.
4. Look up the alt-az coordinates of the object in Sky Safari or your preferred app. The altitude should match your digital angle gauge plus or minus the accuracy of the gauge. Make sure your gauge is sitting evenly on the top of the scope tube.
5. Adjust the azimuth pointer to match the azimuth shown in the app. Don't wait too long, as this will be constantly changing.
6. Look in the eyepiece and you should see the object, or at least the star field around or near the object. Identify the exact location within the field by comparing your view with the star chart.
7. To move to another object, look up the new object's coordinates and move the scope until they show on the gauge and circle. You may have to adjust the azimuth pointer slightly for inherent inaccuracies if you are in a different part of the sky, but you will be close.

I added right angle correct image (RACI) finderscopes to my scopes to verify I dialed the coordinates in correctly, help identify dim objects among star patterns, or move around an area to look for other nearby objects. You can get by with just having one RACI finderscope and putting a shoe on each telescope, then moving the finderscope between scopes. I do that with a 6x30 finder for my 4.5 inch and 6 inch scopes. I prefer an 8x50 for my 10 inch, and it can handle the extra weight of the bigger finderscope better.

Get a digital angle gauge

This is the easy part. If you have a telescope with a metal tube, pretty much any digital angle gauge will have a magnetic base that will work well with it. If you don't have a metal tube, you can stick on a metal plate or design some other system to attach the angle gauge. You'll need to cover the display with transparent red tape or something to dim it down to acceptable levels.

I chose a Klein Digital Angle Gauge because it has white numbers on a black background, so minimal light, and all I needed to do was cover it with a tranparent red material. I used the plastic pack that the gauge came in as a holder for the red material, and duct taped in a scrap piece of red acrylic I had leftover from resizing a laptop shield and some craft foam. It slips over the gauge with a friction fit. Just make sure the red material doesn't blur the display making it unreadable. The Wixey is another popular digital angle gauge. You can try to find one without a backlight if you are just going to use a red flashlight to look at it.

Making and installing an azimuth circle

There are many variations on the azimuth circle because telescopes are different and observers are different. Check out the megathread Degree Circles on Cloudy Nights for ideas and pictures. The standard way is to make the azimuth pointer movable, usually using magnets. You can also make the circle movable, but that's usually more complicated. You decide how you want to do it, but here's what I did.

For my 10 inch, I cut a notch in the round bottom of the rocker box and glued a paper azimuth circle to the round ground board beneath that. The azimuth pointer rides on a magnetic strip in the notch so I can adjust it during initial alignment and make subsequent fine adjustments.

For my 4.5 inch, my design of the base did not lend itself to simply gluing on a paper circle and cutting a notch, so I cut a circle out of a 1/8" thick sheet of FPVC, which is a light, semi-flexible vinyl, using a craft knife. I made the cut slowly and wore leather gloves for protection. I had to go over the cut mark multiple times until it cut all the way through. Then I glued a printed paper azimuth circle to the FPVC circle and assembled it below the bearing material disk. I drilled a hole in the center through which the bearing bolt passes. Here's my post on Cloudy Nights about my 4.5 inch project, with additional pictures.

For the 6 inch, I couldn't separate the round bottom of the rocker box from the triangular ground board for fear of messing up the electronics, so I cut the FPVC into a ring shape, glued on the paper azimuth circle, then sliced the ring in two places and attached it to the ground board with some double sided foam tape.

The cuts are next to 55 degrees and 295 degrees so I could attach the ends of the pieces to the "ears" of the ground board that you can see sticking out slightly from below the azimuth circle. I used small pieces of double-sided foam tape. You only need to make two cuts, 120 degrees apart, so you can position the bigger ring piece and then the smaller one to complete the circle.

The azimuth circle added 3/4" to the radius all the way around the base. I had to make a new, larger table for the scope because the circle now blocked the eyepiece holders. This new one is 20" in diameter. The original was 18". I took the opportunity to eliminate the unused 2" holes that I had on the old one and make four 1.25" holes on each side, so no matter where I am sitting, I have lots of places to store eyepieces. I also used 3/4" plywood. White paint makes it easy to see where you're putting stuff and makes it less likely someone will walk into it in the dark. See my post on making a table for a tabletop telescope .

Use the website blocklayer.com to design and print an azimuth circle that fits your telescope. Some people take it to a FedEx or another store that will print it for you. I tried that and they printed it slightly oversized, so I just printed it in several pages on my home printer and fit them together. That introduces a tiny bit of inaccuracy, but you're likely not going to get it perfect anyway. It'll still work fine.

The Blocklayer site has a huge number of templates of all types, and it's fun to browse. But for this project, I used Circle Divider templates. There is a green "Metric Version" indicator at the top, which is actually a button to change it to Metric from the default "Inch Version." Leave it showing Metric.

Due to the popularity of creating azimuth circles for telescopes, Blocklayer has added a template for this specifically: Protractor - Setting Circle. It does essentially the same thing as the Circle Divider template, and you could use that instead. It appears they have removed the option to set the scale counterclockwise, which you would need if you had a movable circle and a fixed pointer.

You have many options, including having the numbers on the inside or outside of the scale, black-on-white or white-on-black, size and length of tick marks, numbering of every 10 or every 5 degrees, etc. Choose what you like, but think about readability from where you are observing and using a red light to see it. Change the "Diameter inches" setting to what will work for your scope, then hit "Calculate" or use the slider. The circle needs to fit on your lower ground board or fabricated circle or ring.

These are the settings I prefer:

• Black print on white background
• Tick lines (default)
• Primary increments 10 degrees (default)
• Number orientation = Radial -90 (so you can read the numbers correctly at the eyepiece)
• Outer marks - note that if you choose Outer marks, the diameter you chose becomes the inner diameter, so you need to adjust the size so the outer diameter is the diameter you need (e.g., your ground board is 22 inches, and so you need a 22 inch outer diameter circle, or a tiny bit smaller). Font size, tick thickness, etc. will affect this, so check the info in the center of the circle on the Blocklayer page and adjust everything with the sliders until you have it the way you want it and your outer diameter is the correct size.

If you like my suggested settings and have the same scope, you can download the azimuth circle PDF that I used for my Sky Watcher Virtuoso GTi 150P here. If you need a 22 inch outer diameter azimuth circle, here is the one I created for my 10-inch Hardin Deep Space Hunter. The Cloudy Nights Degree Circle megathread has a bunch of other files created for different scopes.

Once you have the circle the way you want it in Blocklayer, select "diagrams to PDF" at the top, and in the page that comes up, select the paper size you will be printing on, put in the file name, and hit the "Trim" button. Full printing instructions are at the bottom of the Blocklayer page. Hit the "PDF 1" button in the lower right below the circle (to exclude printing the tape that otherwise would also print out). 

Your own computer's settings will determine how you print it once downloaded, but make sure you are printing at 100% and select "tile large pages" or a similar setting that will print the circle over several pages. If you have it commercially printed, make sure they print at 100%. If it doesn't come out right, just adjust in Blocklayer and try again. I like to print a little smaller than the diameter of the ground board so the edge doesn't peel up.

Once printed, check the fit against your FPVC circle or ring. If it's good, glue it carefully onto the circle or ring using contact cement, making sure you get complete coverage with no bubbles or bare spots. Then spray the paper with several coats of a fixative (I use Aleene's Acrylic Sealer - Matte Finish) outdoors because these often have really bad fumes, especially Aleene's. 

Once dry, mount the circle or ring between the ground board and the lower rocker box. For my 4.5 inch, I drilled a 1/4 inch hole to fit the 1/4-20 center bolt, and the circle sits underneath the azimuth bearing plate. Yours might be different. For the Sky Watcher Virtuoso GTi 150P (6 inch), I had to make two cuts to remove an arc 1/3 of the circumference because I couldn't separate the ground board and rocker box. I then reassembled it into a ring and attached it to the ground board with a few small pieces of double sided foam tape. I tried larger pieces of foam tape, but fitting them under the rocker box board was a mess because they would stick before I could get the pieces in position. Smaller foam tape pieces worked much better and it still holds well.

You'll need to make an azimuth pointer. I made mine from a scrap of thin aluminum flashing material I had from a roof job, but you can pretty much use anything. I attached a tiny rare earth magnet to it using duct tape. I couldn't find any glue that would hold permanently- duct tape to the rescue again! Then I took a piece of magnetic tape and attached that to the rocker box board, so that the pointer will move with the rocker box. The azimuth circle is fixed on the ground board and the pointer rotates with the scope. 

For the Sky Watcher Virtuoso GTi 150P, I switched to using a strip of Velcro instead of magnets, because I kept knocking the pointer when reaching for the azimuth bearing lock knob. You can use anything as long as the pointer can be moved over an arc of about 30 degrees. Any less and it will be harder to rough align the scope when you first set it down and still be able to put the pointer within range. Put the pointer where you'll see it easily from your normal observing position. 

The Sky Watcher Virtuoso GTi 150P with new azimuth circle and larger table. The digital angle gauge sits on the top front of the metal lower half of the tube.