Removing mirror clips to eliminate diffraction

I recently came across in a video from Dakota Starry Nights. If you’re into astrophotography and use a Newtonian telescope, you might have noticed some odd diffraction spikes on your images, especially around bright stars. Those weird off-center spikes with dark gaps in between can make your images look a little off, and they can be particularly distracting when shooting deep-space objects. Turns out, those mirror clips holding your primary mirror in place might be the culprit. Now, a common fix for this is using an aperture mask, but that comes with its own set of drawbacks. Dakota Starry Nights came up with an alternative that keeps the full aperture intact while getting rid of the unwanted diffraction spikes. The fix? Using Gorilla Tape to create a mirror sling.

Removing mirror clips to eliminate diffraction spikes without an aperture mask!

Now, before getting into how he did it, let’s talk about why this is even an issue in the first place. Newtonian reflectors, including the Apertura CarbonStar 150 he was working with, have primary mirrors held in place by clips. These clips, while necessary to secure the mirror, can cause diffraction artifacts. That’s because they stick out slightly over the mirror’s surface, blocking a tiny portion of the incoming light and creating those odd diffraction patterns. Some people deal with this by installing an aperture mask, which covers the outer edges of the mirror. This does help get rid of the diffraction effects, but at a cost: it reduces the telescope’s aperture, effectively slowing it down and slightly decreasing resolution and light-gathering ability. For astrophotography, where every bit of light counts, that’s not ideal. The problem becomes especially noticeable when imaging bright stars, where the effects of the mirror clips become quite obvious.

So Dakota Starry Nights started thinking about how to fix the issue while keeping the full aperture intact. He drew inspiration from Dobsonian telescopes, which often use a mirror sling to hold the primary mirror instead of clips. A sling supports the mirror from underneath rather than pressing against its front edge. He figured that a similar approach could be applied to his 6-inch Newtonian, but instead of a traditional sling made of wire or straps, he used something simple and effective: Gorilla Tape All Weather.

This tape is designed to handle extreme conditions, from freezing cold to intense heat, and it has a strong adhesive that makes it suitable for securing the mirror while still allowing it a bit of movement for temperature adjustments. The trick was to apply it in a way that would keep the mirror secure without pinching it, which could lead to distorted optics. So here’s what he did.

First, he removed the primary mirror from the telescope. If you haven’t done this before, it’s a relatively straightforward process, though you have to be careful when handling the mirror. He recommends wearing a mask or standing back while working to avoid accidentally spitting on the mirror surface, which can be surprisingly easy to do without realizing it. After taking the mirror out, he cleaned off any leftover smudges from the rubber clips, using a bit of alcohol and optical tissue. There’s usually a little residue left behind from where the clips pressed against the mirror, but it’s easy to wipe away.

Now, instead of getting rid of the mirror clips entirely, he reinstalled them in a reversed position. Normally, the clips slightly overhang the mirror’s edge to hold it in place. By flipping them, they still act as a stabilizing guide, but they no longer touch the mirror directly. This creates a small gap between the clips and the mirror, preventing any contact that could lead to diffraction artifacts.

With the clips reversed, he then moved on to the tape. He cut strips of Gorilla Tape about five inches long, making sure they were just under a quarter-inch wide. The idea was to attach the tape underneath the mirror and onto the reversed clips, essentially forming a flexible belt that would hold the mirror securely while still allowing a tiny bit of movement to accommodate temperature changes. It’s important not to make the tape too tight, there should be just a little bit of slack so that the mirror isn’t locked in place too rigidly. This prevents what’s called “pinched optics,” where an overly tight constraint on the mirror can introduce unwanted distortions in the image.

Once the tape was applied, he tested the modification. The first target was the Pleiades, which is a great choice for testing because of the bright stars in the cluster. If anything was off, it would be very noticeable in the diffraction spikes. The results looked really promising. The stars appeared clean and sharp, without the odd diffraction spikes that were present when the mirror clips were directly touching the mirror.

To further confirm the effectiveness of the mod, he moved on to the California Nebula, NGC 1499. Star sizes were tight, and focus was holding well even after a couple of hours of imaging. Without an aperture mask, he was able to use the full 150mm aperture of the telescope, maximizing light collection and maintaining the fastest possible focal ratio.

One thing he pointed out was the importance of using a Bahtinov mask for focusing. Without it, dialing in precise focus can be tricky, especially if you’re not using an electronic focuser. The Bahtinov mask helps eliminate guesswork and makes it much easier to achieve perfect focus in a short amount of time.

Another key aspect he tested was collimation and focus stability over time. After more than two hours of imaging in 18-degree weather, the modified setup held up well. There was no noticeable shift in collimation or focus, suggesting that the tape sling was doing its job effectively.

There was an interesting moment when he spotted what looked like a triple star system in his image of M33, the Triangulum Galaxy. He wasn’t sure if it was an actual system or something else, so he asked viewers to check their own images of M33 to see if they could confirm its presence. It was a fun little mystery that added a bit of excitement to the whole process.

One final tweak he made was to cover the now-exposed mirror clips with black gaffer’s tape. Even though the clips were already black, they still had a bit of a reflective sheen to them. For night vision work, where any stray light can be amplified and become an issue, reducing reflections as much as possible is crucial. He compared it to how the James Webb Space Telescope is placed in deep space with a sunshield to minimize heat interference, while not as extreme, the same principle applies when trying to eliminate stray reflections in a telescope.

Gorilla Tape All Weather is even better and can withstand extreme weather conditions

Now, a word of caution: this method works well for a 6-inch Newtonian, but if you have an 8-inch or larger reflector, you might want to think twice before removing the mirror clips. The weight of larger mirrors means they require more substantial support, and a tape sling alone may not be enough. For telescopes 8 inches and up, the aperture loss from an aperture mask is minimal compared to the overall light-gathering power, so in those cases, sticking with an aperture mask or dealing with the mirror clips might be the better option.

After a few weeks of testing, he was happy with the results. The modification held up well, with no unexpected issues arising over time. It maintained collimation, focus stability, and provided cleaner images without diffraction spikes. For those who are comfortable making minor modifications to their telescopes, this could be a great way to improve image quality without sacrificing aperture.

At the end of the day, it’s all about getting the best possible performance out of your gear. If you’re looking for a way to clean up diffraction spikes in your Newtonian telescope without reducing aperture, this tape sling method is definitely worth considering. Just take your time, make sure everything is secure, and test thoroughly before committing to any long imaging sessions. And if you try it out, let us know how it works for you!

Boost your primary mirror for astrophotography