Eyepiece cheat codes: How to use setting circles on an alt-az mount

I remember when I got my first telescope over 30 years ago, a Tasco 11TR 4.5 inch Newtonian reflector on a cheap equatorial mount, I looked at the setting circles and then ignored them, never bothering to polar align the scope and use them. Probably a good idea at the time, because the mount was not very sturdy, and I was able to quickly find things with a straight through finderscope and starhopping, without polar aligning the scope.

Fast forward to today, and manual setting circles are my go-to method for locating objects. 

What are setting circles?

Setting circles can be used on an equatorial or an altitude-azimuth (alt-az) mount to find objects in the sky. As noted, I don't have any experience with using them on an equatorial mount, but the concept is similar, only you use coordinates of declination and right ascension that don't change for an object. In this article, I am not going to get into equatorial or digital setting circles, but rather those that the observer lines up manuallly by eye on an alt-az mount.

Each axis, in this case altitude and azimuth, moves in a two-dimensional plane: altitude up and down in the sky from the horizon to the zenith, and azimuth in a 360 degree circle parallel with the horizon.

As in the diagram at right, it is convenient for us to look at the sky as a celestial dome, with altitude, graduated from 0-90 degrees, and azimuth from 0-360. A setting circle is a circular scale placed on each axis with the mount leveled and aligned so that 0 corresponds to the actual horizon (for altitude) and 0/360 corresponds to true north (for azimuth). 

Once the mount itself is aligned and leveled, you can move the scope to the coordinates of an object, obtained in real time from an app, and your telescope will be pointed at it. 

The accuracy depends on the construction of the mount and setting circles, and the accuracy of your alignment and leveling. You might get the object right in the center of your eyepiece or it might be out of the field of view but within your finder's field of view, and you'll have to starhop or adjust a little to find it.

For an alt-az mount on a tripod, the azimuth circle will be physically located at the point where the upper part of the mount rotates against the fixed base that is attached to the tripod or pier. 

The altitude circle will similarly be mounted at the point where the movable arm that holds the telescope tube rotates against the fixed part that holds the arm to the mount. 

On a Dobsonian, you typically have to create your own azimuth circle, as shown at right, because manufacturers haven't caught on to the usefulness of setting circles and would rather sell you fancy go-to or plate solving systems. See this post on creating your own azimuth circle. Instead of an altitude setting circle, most people use a digital angle gauge, like this one that I use, sitting on the tube.

Typically, a manual alt-az mount that is designed to be attached to a tripod, such as the SV225 that I use, will have setting circles on it from the factory. However, these may be quite small, making them less precise, and depending on the telescope tube you mount on it, may not be easily visible for the observer. In the case of the SV225, I had to take a few pieces apart to loosen the setting circles enough to be able to rotate them to line them up accurately for each observing session. I put my own pointer marks using blue painter's tape where I could more easily see them instead of the light gray, hard to see, markings that came from the factory. Sometimes I think manufacturers put these things on just for looks and marketing, but you can actually use them!

You can also add a vernier scale to smaller setting circles in place of a pointer mark. This allows you to accurately set them in smaller increments. In the case of the SV225, the circles' smallest increment is 5 degrees, but adding the vernier scale allows you to set to single degrees. I was skeptical that it would help, but found it actually does—a little. Plus it looks more scientific and makes me look like I know what I'm doing! (The setting at left is 273 degrees.)

Adjustable for accuracy

The mount must be leveled as accurately as possible and lined up so that the azimuth circle is aligned with the proper compass directions. You can either make the setting circle rotatable to line up with a pointer, or make the pointer movable. The pointer will show you what the current setting is, for example, once aligned, if the pointer on the azimuth circle is at 270, the scope is pointed due west. 

Regardless, you want the pointers to be within easy view from your observing position. A mount with setting circles built in should have the pointers already well-placed, but as noted, the type of scope tube you use on it may require moving the pointer. 

With a rotatable setting circle, you can position your mount close enough that you only have to rotate the setting circle slightly to get it as accurate as possible. With a movable pointer, you also have to place your mount as close to the correct position as possible and then move the pointer slightly to improve accuracy.

Sequence for alignment:

Here are my recommended steps for aligning your setting circles. Details below.

1. Rough align the mount for azimuth

2. Level the mount for altitude

3. Do a fine alignment on azimuth

4. Rinse and repeat

1. Rough align the mount for azimuth

It's better to do the rough azimuth alignment before you level the scope, because if you have to move the mount it may change the level adjustment needed and you'll have to do it over. If your scope tube is heavy, do your alignment and leveling before mounting the tube.

Because altitude and azimuth coordinates of a given object are continuously changing as the Earth rotates, you will need a charting app, such as Sky Safari, that will tell you the coordinates of objects viewed from your specific location and time, updated continuously. Make sure alt-az coordinates are selected in the settings. Your phone does not have to be connected to a network or wifi.

In Sky Safari, go into Settings > Coordinates and select "Horizon." Rather than futzing with degrees/minutes/seconds, I like to have them set as decimals. Go to Settings > Formats, then under "Azimuth, Altitude" select "DDD.DDDDDD, DD.DDDDDD." Now the alt-az coordinates of any object you select and center will show up in the upper left of the screen. You have to center the object or you won't see its correct coordinates. See the screenshot at left. In the example, 59.2 is the azimuth (toward the northeast) and 68.3 is the altitude of the centered item, M51, the Whirlpool Galaxy.

In Stellarium Mobile, you just tap an object, tap the info box at bottom for details and you'll see the alt-az coordinates.

Once it is dark enough, pick a bright object that's easy to find by sighting along the mount or tube by eye, such as the Moon, Jupiter, Saturn, or one of the brightest stars. Look up the azimuth of the object and move the mount so that the azimuth pointer is on the correct number, as close as you can eyeball it when the mount is lined up as if you had the scope on it. It won't be exact, but close enough that you can adjust the circle or pointer for more precise alignment later without moving the mount. Now you can go ahead and level it.

2. Level the mount for altitude

Leveling the mount will take care of the altitude alignment. The idea is to have the pointer at the 0 mark on the altitude setting circle when the telescope is exactly horizontal, and at 90 when it is pointed exactly at the zenith. Any bubble level will get you there. I use a phone app and it's close enough. Just put it on a flat horizonal surface somewhere on the mount. 

Some tripods have a small bubble level built in or you can add one. With a tripod, you can adjust the length of the legs until it reads level. 

Left: The Sky-Watcher Star Adventurer tripod, like many, has a built in bubble level.

With a Dobsonian, place the bubble level somewhere on the base and level it before you place the tube on it. Inside the box works if you can see it well enough. The simplest method is to use a set of shims under the three feet to level it. You can buy plastic or wood shims. I also have 4-inch squares of 1/2 inch plywood for more uneven ground. You can stack them as needed. Just don't forget to pick them up when you pack up for the night. White tape on them will make them more visible on the ground. You can check the level again once the tube is placed on the base, but I've found it doesn't usually change.

Above: Leveling a tabletop Dob table/base using a bubble level app and a piece of plywood. The plastic shims on the table are for finer adjustments. I put a piece of tread tape on the plywood for better grip on the feet.

3. Do a fine alignment on azimuth

Now that you have the mount roughly aligned in azimuth and leveled, you can mount the tube if it's not already mounted and do the fine azimuth alignment. This is where it's important to either have a movable azimuth setting circle or movable pointer.

Below: My DIY tabletop Dob design uses Velcro for a movable pointer. Most use a magnet, but in this application Velcro works better for me so I don't knock it out of place with my hand when I'm fumbling for eyepieces in the dark.

Again, find a bright object and look up its azimuth and altitude. Usually something about 30-60 degrees up will give you a good calibration. It really doesn't matter what direction it is. It's not necessary to use Polaris for an alt-az mount. Move the scope until the circles show the correct azimuth setting and then the altitude setting, without disturbing the azimuth position. Look along the tube to see that it's roughly pointing at the object. Now look in your finderscope. If you were pretty accurate in your rough alignment, you should see the object in your finder. If not, move the scope around until you do.

Put in a low power eyepiece, find and center the object. Next, line up your finderscope so that it matches the eyepiece view, with the object in the center of both. Adjust the finder with the adjustment thumbscrews to match the eyepiece. You should perform this alignment at the beginning of any observing session regarless of whether or not you are using setting circles. 

Next, look at the alt-az coordinates of the object again in your app and compare them to those on your setting circles. They will likely be a little off. Just move your circle or pointer to match the coordinates from your app while keeping the object centered. The higher power the eyepiece you use, the more accurate it will be, but that level of accuracy is usually not necessary. The closer you can get the match between the listed coordinates and those on your circles, the more accurate your subsequent pointings will be.

Your altitude might be slightly off, too, so adjust that as necessary. 

4. Rinse and repeat

Now you can look up the coordinates of any object and dial them in on your mount. Depending on how well you aligned everything, you may see the object immediately in a low power eyepiece. However, don't be surprised if it's off enough that it's not in the field of view, and you can only see the object, or the correct location, in the finderscope. If that's the case, just use your charting app and starhop to the correct location. You'll be close enough that you should be able to find the object every time. 

I find the greatest challenge when I have to starhop to the exact location is when the sky is either too light polluted to see many stars in the finder, or if the star field is difficult to match to the chart. This is often the case with Sky Safari, as I have to have it rotated correctly to match what I am seeing, it's often very cluttered with objects, and the star magnitude settings don't really make the brighter ones stand out enough from the dimmer ones, making the patterns somewhat confusing. Don't worry, have patience, and you'll find your object. You'll get better with practice.

An added benefit, and the reason I personally went with setting circles, is you don't have to crane your neck to look through a straigh-through finder. You can use one with a 90 degree diagonal (right angle correct image, or RACI). Occasionally I'll try to look through the red dot to get an initial fix, but I usually can't even manage that anymore. Getting old ain't for rookies, as my brother likes to say!

Additonal tips:

• Have a red light handy so you can read the setting circles
• Use a low power eyepiece when you are first locating an object, then move to higher power as desired
• It's not uncommon to have to re-calibrate if you find the settings are off a bit, especially in a different part of the sky. Just adjust the circle or pointer to match the coordinates of a centered object.
• If you're having trouble finding a faint object, look up a nearby bright star and see if you perhaps moved the circle or pointer by accident, then re-calibrate on the star and try again. You can also just starhop from that star if it's close enough.
• If you don't like where the pointer is placed on a commercial mount, simply put a little triangular piece of tape or other marker in the location you prefer.
• If you find you don't have setting circles, don't want to make them, or just don't like using them, try the free AstroHopper phone app. I use both, and I find I like setting circles better. But you're not me.

Below: An alternative solution, the AstroHopper app, in use.