M25 Star-Hop Guide for 200-300 mm Dobsonians

Before you start

Use a Dobsonian with a finderscope or a red-dot finder, aligned on a distant star before the session. Plan to use two eyepieces. A low-power, wide-field eyepiece in the 25-32 mm range gives a field of roughly 2 degrees and is best for the hop. A medium-power eyepiece in the 12-15 mm range is better for resolving the cluster. These are typical ranges, so use whatever your instrument provides.

Sagittarius is a summer constellation in the Southern Hemisphere sky and is best placed in the evening around August. Check the time you plan to observe, because the cluster's position in the sky changes through the night.

Choose a session with a clear view toward the south. The cluster is never high from the Northern Hemisphere, so buildings, trees and haze near the horizon will affect your view more than they would for an object overhead.

The star-hop route

The route starts at Polis and uses bright stars that are easy to find against the Milky Way. Polis is also called Mu Sagittarii. From the cluster's position it lies about 3 degrees to the southwest, which is close enough to make the hop short.

Step 1: Find the Teapot and Polis

Locate the Teapot, the asterism that forms the most recognizable part of Sagittarius. Polis is one of its brighter stars, at about magnitude 3.8, and sits near the lid of the pot. Use your naked eye or finder to confirm that you have the correct star before you move on.

Center Polis in the wide-field eyepiece. Keep it centered for the whole hop so you can use it as a fixed reference.

Step 2: Move northeast from Polis

M25 lies about 2.5 degrees east of Polis and about 2 degrees north of it. The total distance is roughly 3 degrees, or about one and a half wide-field views. Move the telescope slowly toward the northeast. Use the finder to keep track of the direction, and stop once the star field becomes denser and more uniform.

Do not rely on the finder alone for the final approach. Switch to the wide-field eyepiece and scan slowly. You should see the star density rise in the area of the cluster.

Step 3: Confirm with a medium-power view

Once you are close, move to the medium-power eyepiece. At this magnification the cluster should fill a noticeable part of the field. Center the middle of the group, not an individual bright star, and use the surrounding stars to judge the shape of the cluster.

Alternative route from Kaus Borealis

Kaus Borealis, also called Lambda Sagittarii, is another bright star in the Teapot region. From Kaus Borealis, M25 lies about 6 degrees almost directly north, with less than 1 degree of eastward offset. This is roughly three wide-field views.

Move north in steps of about 2 degrees. After each step, check the star pattern against the view you saw in the previous step. The star density should rise as you move toward the cluster. This route is useful if Polis is blocked by a tree, building or cloud, or if you prefer to start from a star that is easier to find in your part of the sky.

Confirming you have arrived

The first sign that you are close is a change in star density. The field around the cluster will contain many more stars than the sparse areas that surround it. Look for a group of stars with varying brightness rather than a uniform spread.

Next, check the size. M25 spans about 14′, which is about one-quarter of a degree. At medium power the group should occupy a noticeable area of the field, and the individual stars should appear to belong together rather than sit at random across the view.

Finally, check your work by returning to Polis and repeating the hop. If the same group of stars appears in the same part of the field each time, you have found M25. If you are not sure, compare the pattern of stars with the layout shown on a detailed star chart for the same date.

Keep in mind that the Milky Way in this part of Sagittarius contains many star groups. A dense field does not prove that you are on the cluster. The shape of the group, its approximate size and its position relative to Polis are the most reliable indicators.

What you will see

From a suburban sky, M25 appears as a group of individual stars. In a 200-300 mm Dobsonian, the brighter members stand out clearly against the background, and the group has a loose, open shape rather than a tight core.

At low power, the cluster can look like a faint haze with a few brighter points. At medium power, which is typically around 60 to 80 times on a 200-300 mm instrument, the individual stars become easier to count. Expect a few dozen stars as a typical guide, with the brightest members standing out and the fainter ones near the suburban limit at the edges of the group.

The cluster is not a deep-sky object that rewards extreme magnification. Higher powers reduce the field and make the group harder to frame, and they do not add much contrast. Use the highest power that keeps the whole group in view with a steady image.

Do not expect the cluster to look like a dense ball of stars. It is open and spread out, and the gaps between members are part of its appearance.

Observing from different latitudes

The cluster's maximum altitude depends strongly on your latitude. At 35 degrees north it reaches about 36 degrees above the horizon, so it is low and must be observed through the thickest part of the atmosphere. Clear a southern horizon as much as possible, and avoid starting the hop when the cluster is near the horizon.

At 50 degrees north the maximum altitude drops to about 21 degrees. The cluster is therefore close to the horizon for most of its visible period from this latitude. Trees, buildings and haze can block it entirely. Plan the session for the period when Sagittarius is highest in the evening, and check the view for obstructions before you begin.

At 35 degrees south the cluster reaches about 74 degrees above the horizon, so it is high in the sky and easier to observe. The route from Polis works in the same way, and the cluster is more comfortable to view because the atmosphere is thinner at that altitude.

In all cases, the star-hop method is the same. Only the altitude changes, and that change affects how much sky you must scan and how steady the image will be.