Guide · updated 13 November 2026

How to use a 5K iMac as a monitor for an Apple silicon Mac

The 27-inch 5K iMac has one of the best screens Apple ever made. Here is every way to keep using it as a second display for a newer Mac, what each one costs, and who it suits. I make one of these options, SecondGlow, so I’ve tried to be fair, and every claim about the others links to their own site.

In this guide

Why there’s no Target Display Mode
Check these two things first
Software: AirPlay, Luna, RetinaRelay, Mirage, SecondGlow
Free and open-source apps
Hardware conversion kits
Which one should you pick?

Why there’s no Target Display Mode

Older iMacs had Target Display Mode: plug in a cable, press Command-F2, and the iMac became a monitor. Apple’s own page lists the iMacs that support it, and the list stops at models “introduced in 2011, 2012, 2013, or mid 2014”, running macOS High Sierra or earlier. The Mac on the other end must be from 2019 or earlier, on Catalina or earlier. The 5K iMacs aren’t on the list, and no Apple silicon Mac can use it.

So for a 27-inch 5K iMac and an M1 to M5 Mac, you have two kinds of answer. Software sends the picture over a cable or Wi-Fi to an app running on the iMac. A conversion kit replaces the iMac’s insides with a display board, so it becomes a plain monitor for good.

Check these two things first

Which iMac you have. Apple menu → About This Mac. The 27-inch Retina 5K models are from late 2014, mid 2015, late 2015, 2017, 2019 and 2020. The first two stop at macOS Big Sur; the rest run Monterey or later. Their ports differ: the 2014 and 2015 models have Thunderbolt 2, while the 2017, 2019 and 2020 models have Thunderbolt 3 (USB-C). All of them have USB-A ports on the back. The iMac Pro (2017) has the same 5K screen and Thunderbolt 3, and the 21.5-inch Retina 4K iMacs (2015, 2017, 2019) have a smaller 4096 × 2304 screen. Note the newest macOS your iMac can run, because each app on the iMac has its own minimum.

What you want from it. Sharp, smooth everyday work at full 5K? Sound from the iMac’s speakers? Wireless? An Intel Mac or a PC as the source? The answers point to different options below.

Software options

AirPlay to Mac (free, built in)

Since macOS Monterey, a Mac can be an AirPlay receiver. Apple lists the Macs that can receive as the iMac Pro, the Mac mini from 2020, and “all other Mac models introduced in 2018 or later”. That includes the 2019 and 2020 5K iMacs, but not the 2015 or 2017 ones. It works over Wi-Fi with both Macs on the same network; Apple doesn’t state a maximum resolution.

Suits: owners of a 2019 or 2020 iMac who want to mirror or extend now and then, for free, and can live with Wi-Fi.

Luna Display ($89.99, hardware unit included)

Astropad’s Luna Display is a small hardware unit that plugs into your main Mac, plus an app on the iMac. It’s a one-time purchase with no subscription. According to Astropad’s support pages, any Mac from 2011 or newer on macOS 10.13 or later can be the display, it works over Wi-Fi, Ethernet, Thunderbolt or USB, and on a Mac it lists 5K at 45 Hz or 30 Hz, and 4K at 60 Hz (5K needs the USB-C unit and macOS Big Sur). Sound stays on the main computer. It also works with Windows PCs.

Suits: people who want a long-established product, wireless use, a PC as the source, or an iMac older than 2015. SecondGlow vs Luna Display.

RetinaRelay ($49 once)

RetinaRelay does what SecondGlow does: two apps, Host on your Mac and Viewer on the iMac, a cable between them, and the picture compressed on the GPU instead of sent as video. It costs $49 once for one Host Mac, with a 7-day trial. The Host can be an Apple silicon or Intel Mac on macOS Ventura or later, and the iMac can be a late 2012 or newer on Catalina or later. It runs over USB 3 (Apple silicon Host only), Thunderbolt or Ethernet, and plays sound through the iMac’s speakers after a one-time audio device install. Its site lists native 5K at 60 fps.

Suits: anyone with an Intel Mac or an iMac older than 2015. SecondGlow vs RetinaRelay.

Mirage – Remote Display (free tier, Pro by subscription)

Mirage streams a Mac’s screen to other Apple devices over Wi-Fi or USB-C. The free tier is 720p at 30 fps; Mirage Pro adds Retina resolution for $7.99 a month, $59 a year or $199 once. Its App Store page says it requires macOS 27 and a Mac with Apple M1 or later (its support page says macOS 26). Every 5K iMac has an Intel processor, so it can’t be the display.

Suits: using an iPad, a Vision Pro or another Apple silicon Mac as a display. Not a 5K iMac.

SecondGlow ($39 once)

SecondGlow is the app I make. The same app goes on both Macs: on any Mac with Apple silicon on macOS Ventura or later it sends the picture, and on a 27-inch 5K iMac from late 2015 to 2020, or a 21.5-inch Retina 4K, on macOS Monterey or later it shows it, free. One cable, no dongle: USB-C to USB-A with USB 3, or Thunderbolt on 2017 and later iMacs. The iMac shows its native 5120 × 2880; the GPU compresses every frame with adaptive quality, so text stays sharp, and I measure about 50 to 60 fps on full-screen 5K video over USB 3. The link is encrypted. It costs $39 once for up to 7 of your Macs, with a 7-day free trial.

On macOS Sonoma 14.2 or later it plays your Mac’s sound through the iMac’s speakers, with your volume keys. What it doesn’t do: no Intel Macs as the source.

Suits: Apple silicon Mac owners who want a sharp, wired 5K display with nothing extra to buy but a cable.

Try SecondGlow free Check compatibility

Free and open-source apps

Three free projects do this job too. They’re worth trying first if you don’t mind some setup, and their READMEs are honest about what works.

OpenDisplay (GPL-3.0): the sender needs macOS 14, the receiver macOS 12. It runs over Thunderbolt, USB4, Ethernet or, on recent macOS, a USB-C data cable. It sends a video stream; its own measurements show about 30 fps at full 5K sharpness from an M5 Pro to a 2017 iMac, and around 58 fps in a softer mode. No audio.
TargetBridge (MIT): Thunderbolt, with an experimental network link. It recommends a 5K 48 profile, with 5K 60 experimental, and has an audio add-on. The pre-built receiver needs macOS Sonoma on Intel, which rules out the 2015 and 2017 iMacs unless you build it yourself. Its maintainer has posted that health issues have paused new features.
Sharp (GPL-3.0, beta): Ethernet, Thunderbolt or USB; the receiver needs macOS 10.15 or later and a 5K iMac starts at native Retina. It plays sound through the iMac. It sends anonymous usage data by default, which you can turn off.

Suits: anyone happy to tinker who would rather pay nothing.

Hardware conversion kits

A conversion kit turns the iMac into a real monitor: you open the iMac, remove the logic board and power supply, and connect the panel to a driver board with DisplayPort, USB-C or HDMI inputs. After that it works with any computer, with no app at all.

Boards from writeups cost roughly $170 to $350, plus a power supply. Cult of Mac did it for under $200 in parts in late 2024. David Green’s build, covered by Hackaday, came to under $600 after a fried board, and the cheaper of his two boards only drives 8-bit colour. Australian retailer Macfixit rates the job 60 to 120 minutes and moderate difficulty, warns that the power supply holds a dangerous charge after shutdown, and notes that the camera, microphone and speakers stop working. You need the right board for your panel’s part number, and the iMac can’t be a computer again unless you rebuild it.

Suits: confident DIYers who want a permanent monitor, and anyone using an Intel Mac or a PC.

Or replace it: a new 5K display

The simplest answer is a new monitor. Apple’s Studio Display has the same 27-inch 5120 × 2880 resolution, a camera, speakers and microphones, and one Thunderbolt cable that also charges a laptop. It starts at $1,599. It’s a plain display, so there’s no second computer to keep switched on. If your iMac is showing its age, or you’d rather sell it, this is the clean option. SecondGlow vs a new Studio Display.

Suits: anyone who wants a new display with a camera and microphones, or no longer wants the iMac on the desk.

Which one should you pick?

2019 or 2020 iMac, occasional use, free: try AirPlay to Mac first.
Free and willing to tinker: OpenDisplay, TargetBridge or Sharp.
An Intel Mac as the source: RetinaRelay, Sharp or TargetBridge.
Wireless, a PC, or an iMac older than 2015: Luna Display.
A permanent monitor for any computer: a conversion kit.
A new display with a camera and microphones: a Studio Display, from $1,599.
An Apple silicon Mac, one USB cable, sharp 5K every day: SecondGlow, free for 7 days.

One more thing to weigh: Apple said macOS 26 Tahoe is the last version for Intel Macs, so every 5K iMac has had its last big update. Its screen hasn’t aged the same way, which is why all of these options exist.

Whatever you choose, most of these have a free trial or a free tier. Try it on your own desk before you pay.

Sources

Checked on 13 November 2026, on each product’s own site. Prices and features change; follow the links for today’s details.