If you’ve ever popped the dust cap off your telescope and noticed a smaller hole staring back at you, it’s not there by accident. Those openings are a built-in feature that can change how your telescope performs, depending on what you’re looking at.
Many Celestron telescopes, like the AstroMaster, PowerSeeker, Travel Scope, and Omni series, include objective covers with one or two small openings. These are called aperture stops. Let’s break down what they do and when to use them.
What Is an Aperture Stop?
An aperture stop reduces the effective diameter of your telescope’s objective lens. In simpler terms, it limits how much light enters the telescope.
Think of it like sunglasses for your optics. You’re not changing what you’re looking at, just refining how the light behaves before it reaches your eye.
Why Use the Hole in Your Telescope Cover?
Using a reduced aperture can improve your view under certain conditions. Aperture stops can:
-
Tame Bright Targets
The Moon and planets can appear overwhelmingly bright, especially in smaller telescopes. An aperture stop dims the image slightly, making details easier to see without glare. -
Improve Contrast and Sharpness
By allowing light through the central portion of the lens, an aperture stop can reduce optical imperfections. This often results in a cleaner, crisper image. -
Reduce Chromatic Aberration
Refractor telescopes can sometimes show color fringing (a colorful halo around bright objects). A smaller aperture helps minimize this effect. -
Help in Poor Seeing Conditions
When the atmosphere is turbulent, a reduced aperture can stabilize the image slightly, trading brightness for steadiness.
Why Do Some Covers Have Two Holes?
On models like the AstroMaster and PowerSeeker, you’ll often find two different-sized holes.
This gives you options:
- Smaller hole → Maximum reduction, best for very bright conditions
- Larger hole → Moderate reduction, a balance between brightness and clarity
What About Travel Scope and Omni Telescopes?
The Travel Scope features a single aperture stop for quick brightness control during lunar, planetary, and daytime viewing.
Omni refractors include a center aperture stop, specifically designed to use the sharpest part of the lens. This can noticeably improve contrast and reduce color fringing on bright objects.
When Should You Remove the Cover Completely?
Deep-sky objects like galaxies, nebulae, and star clusters are extremely faint. To see detail, light-gathering ability is everything. Always remove the entire cover and use your telescope’s full aperture for the best views of these objects.
The Bottom Line
That little hole isn’t an afterthought. It’s a simple but powerful tool that lets you adapt your telescope to different observing conditions.
- Use it for the Moon, planets, and daytime viewing
- Skip it for faint deep-sky objects
And once you start experimenting with it, you’ll notice that sometimes, seeing more of the universe starts with letting in a little less light.