Use an iMac as a Monitor: 3 Ways, One You Can't Undo

A working 27-inch 5K panel is a terrible thing to throw away, which is why so many people end up wanting to use an iMac as a monitor. The computer inside goes obsolete years before the screen does, and macOS drops support for a machine whose display is still better than most monitors you can buy today.

I solved this the stupid way. I took a screwdriver to a working 2015 iMac 5K, pulled the logic board out, and wired the bare panel to a third-party driver board. It worked. It also cost me a day, a controller board, a power supply, and a functioning computer I can never get back.

There are three ways to do it, and only one of them is reversible. Here is what each actually costs, what Astropad's Luna Display can and cannot do according to its own published figures, and which one I would pick if I were starting again.

One thing up front: I have done the conversion, but I have not run Luna Display myself. Everything below about Luna comes from Astropad's published specifications and its public review record, and I have said where each figure comes from. Half the pages ranking for this query imply hands-on testing they did not do, and I would rather just tell you.

The three ways to use an iMac as a monitor

Most guides cover one method and quietly pretend the other two do not exist. Picking the wrong one costs you either a weekend, ninety dollars, or a working computer.

1. Target display mode: free, if your hardware qualifies

Apple's own method. You press Command-F2 on the iMac acting as the display and it accepts video from another Mac over Thunderbolt or Mini DisplayPort. It costs nothing, and it is a real cable rather than a stream.

The catch is compatibility, and it is severe. Target display mode works only on a narrow band of older iMacs, driven from equally specific older Macs. Apple's support page lists the exact combinations, and if you are running an Apple silicon Mac you are not on that list. No adapter or dongle changes that. Check before you get your hopes up, because if you do qualify nothing else here comes close on price.

2. Luna Display: the reversible middle ground

Astropad's Luna Display is a small hardware dongle plus free software on both machines. The dongle is the part people misunderstand: it is not a video adapter. It presents itself to macOS as a virtual GPU, which is why the second machine behaves like an actual display rather than a screen-sharing window.

The property that matters is reversibility. Your iMac stays a working Mac. It still boots, still runs its own apps, and volunteers as a display when you want one. Unplug the dongle and nothing has been lost. That sounds obvious until you have done the alternative, which brings me to the third option.

3. Driver board conversion: permanent, and the best picture

This is what I did. You gut the machine, discard the logic board, and drive the bare panel from a replacement controller board. I wrote up the full conversion here, including the parts I would buy differently.

What you end up with is a genuinely dumb monitor: a real cable, no network, no encoding, no software in the path and therefore no latency at all. What you give up is a computer, permanently. It is a one-way door, and you only find out whether it was a good trade after you have walked through it.

What Luna Display costs in 2026

Astropad sells two versions, and the difference between them matters more than the price does. The USB-C unit is $89.99 and is the one for modern Macs and PCs. The HDMI unit is $59.99 and is listed as PC only. So if your plan is driving an old iMac from a modern Mac, the $89.99 USB-C unit is your only option, and the cheaper figure you may have seen quoted is not available to you. It is a one-time purchase with no subscription. Older write-ups quoting $129.99 are out of date.

Two practical warnings before you order. First, Astropad states plainly that Luna does not work with adapters of any kind and has to be plugged directly into the machine. If your ports are already occupied and you were planning to route it through a hub or a dock, that plan does not work.

Second, if you are buying from the UK that dollar price is not what you pay. Astropad's shop calculates VAT and duties at checkout based on the shipping destination, and lists standard international delivery at 14 to 21 business days once dispatched. Budget for meaningfully more than the sticker price, and do not expect it next week.

On requirements, the primary machine needs to be a 2011 Mac or newer running macOS 10.13 High Sierra or later, and the iMac acting as the display needs High Sierra or later too. Anything modern is comfortably fine here, whether that is a MacBook or a Mac Studio. If you are using an iPad as the secondary screen instead, it needs iOS 13.1 or later.

The latency question, answered with Astropad's own numbers

This is what everyone actually wants to know and what most reviews are vaguest about. Luna streams your desktop over Wi-Fi or USB, so there is real delay between your Mac rendering a frame and the iMac showing it.

Astropad publishes figures, which is more than most companies manage. Over a wired connection it quotes 1 to 4 milliseconds. Over Wi-Fi, and this is where it gets interesting, the same product page gives two different answers: the marketing section claims 16 to 32 milliseconds while the FAQ further down says 7 to 25 milliseconds. Both are manufacturer claims rather than independent measurements, so treat them as a best case on a good network. Astropad also recommends an 802.11ac router on the 5GHz band, which tells you how sensitive the wireless path is.

The more useful signal is what Astropad tells you not to do. In its own words, Luna is intended as a side reference screen for content that does not change frequently, and it is not recommended for gaming, video playback, or other high-motion content. Note that video playback is on that list, not just video editing. That rules out a whole category of second-screen use that most reviews never mention. Believe a manufacturer when it limits its own product.

What 1,399 customer reviews actually say

Astropad hosts customer reviews on its own product page, which makes the distribution more interesting than the headline number.

Across 1,399 reviews, 44 per cent are five star. That sounds strong until you look at the other end: 14 per cent are one star and 9 per cent are two star. Almost a quarter of buyers rated it two stars or below on the manufacturer's own website, where you would expect the sample to skew favourable. The weighted average works out at roughly 3.7 out of 5.

That spread tells you something specific, and it is more useful than an average. This is not a product that is mediocre for everybody. It is a product that works beautifully for some setups and badly for others, and the variable is almost certainly the network and the expectation. The glowing reviews describe exactly the scenario Astropad recommends: an old 27-inch iMac parked beside a modern Mac showing reference material. The angry ones read like people who expected it to behave like a cable.

What I got wrong when I gutted my iMac

Here is the part I did not expect.

The driver board gives a better monitor. That is not in question. It is a real display with a real cable, no software dependency and no network, and it will still be a monitor in ten years whatever happens to Astropad. That is Luna's biggest structural weakness: you are betting on a small company shipping macOS compatibility updates indefinitely. A controller board does not care what Apple does next.

But I destroyed a working computer to get there, and I did not need to. That 2015 iMac still ran. It was slow and abandoned by macOS, but it worked. If I had spent ninety dollars first I would have learned within a week whether a streamed 5K second screen was good enough for what I actually do, and I would still have had the screwdriver option afterwards.

The order matters. Reversible first, permanent second. I did it backwards.

Which method should you use

• Check target display mode first. If both machines qualify it is free and it is a real cable. Nothing else here beats that.
• Buy the USB-C Luna if your iMac still works and you want a second screen for documents, browsers, Slack and reference material without killing the machine.
• Buy it if you want the option to change your mind. This is its real advantage over a driver board, more than picture quality or latency.
• Skip it if you want to play video, game, or grade colour on that screen. Astropad says so itself, and no network tuning fixes the architecture.
• Go for the driver board if the iMac is already dead, or you have tried a streamed display and decided it is not enough.

FAQ

Can I use an old iMac as a monitor for an Apple silicon Mac?

Yes, but not with target display mode, which Apple silicon Macs cannot use for this at all. Your options are Luna Display or a driver board conversion. If you go the Luna route it is the $89.99 USB-C unit you need, plugged directly into the Mac rather than through a hub.

Does Luna Display really do 5K?

Astropad advertises high-resolution, pixel-perfect rendering, and its customer reviews include people running 27-inch 5K iMacs as second screens. What it does not do is deliver that resolution at cable-quality responsiveness, because the image is compressed and streamed either way. Treat 5K here as a resolution claim rather than a performance one.

Is it cheaper to just buy a monitor?

Often, yes, and it is worth being honest about that. The USB-C unit is $89.99 before UK VAT and duty, which does not buy you a monitor but does buy you a second screen if you already own an idle iMac. If you do not already have the iMac, buy a monitor instead. If you do, the question is only whether a streamed screen suits the apps you would actually park on it.

My take

If you have an old iMac in a cupboard, do not do what I did. Try the reversible option first.

Luna Display is not the best monitor solution on this list, and I would not pretend otherwise. The driver board gives a better picture with no latency and no dependency on anyone's software roadmap. But Luna is the only option that lets you find out whether an old iMac as a second screen is genuinely useful to you before you commit, and that information is worth ninety dollars.

I would still recommend the conversion for a dead iMac, or for anyone who has already established they want a permanent 5K panel and is willing to lose the computer to get it. For everyone else: spend the smaller amount, keep the machine, and reach for the screwdriver only if the streamed version really is not good enough.

Pricing, latency figures, system requirements and review data from Astropad's Luna Display product page, checked 3 August 2026. Target display mode compatibility per Apple Support.