Originally published at: https://tidbits.com/2019/11/20/astrohqs-luna-display-dongle-turns-a-mac-into-a-second-display/
AstroHQ’s Luna Display dongle can transform one Mac into a secondary display for another Mac. This is similar to what Apple’s Target Display Mode does, but that feature works only with iMacs of a certain vintage as a second display. Luna’s Mac-to-Mac Mode works with a wider range of Mac pairings, such as side-by-side MacBooks.
This is indeed a very interesting tool.
I’d just like to learn more details about what the dongle actually does. Seeing as all communication is apparently over the network, what is the dongle required for? Is any computation going on there? Encryption? And how come there has to be two versions, what is DP required for here when you already have USB?
Would also be nice to hear how this compares with DuetAir. I know hardly anything about it, but it appears to offer PC compatibility which might be interesting. Using the PC screen for work on your Mac of course. 
My original Luna article got into its tech underpinnings a bit. Basically, it works as a monitor, just one without the glass.
Also, to clarify on DP vs. USB, the latter refers to USB-C. Some Macs have DP and some have USB-C; none, to my knowledge, have both.
Regarding Duet, I understand there is a major update that would presumably make it more competitive with Luna, though the latter in my experience works better because it’s hardware + software, not just software.
Thank you, @julio. That clarifies it. I dig the “screen without the glass” explanation. 
Just one last question. If the dongle is essentially capturing video output, how does it transmit that over the network? At first I though wireless straight from the dongle itself, but then I realized that this solution also works for Macs connected through Ethernet alone. Now if this were just a USB-C dongle I’d assume it’s getting video from the bus’ video layer and using the data channels on the same bus to get the data back to the Mac’s NIC. But how would that work with the miniDP dongle? Does DP offer some kind of data channel back to the host?
I recently bought a Luna Dongle and I am very impressed.
Installation was simple and intuitive. The performance on my iPad Pro with my MacBook 2015 is every bit as good as plugging in a hardware monitor. Now I can have a second screen while not at home or doing a presentation using the iPad for others to view.
It is rare to have things work this well.
Yes a little pricy but I found it worth it. And for your first purchase direct from the company they will give you a 10% discount.
OK, I have to take issue with some of your comments so please indulge me if I provide a little background.
I recall that Adam cottoned on fairly early to what some of us recognised even well before his article ‘Double the Fun with Multiple Monitors’, TidBITS-421. Flashing forward to the 21st century, in my computer room at home, until last year, I had a 2014 iMac 27 Retina with an Apple Thunderbolt Display as a second screen while my wife had a 2010 iMac 27 with an HP 27 AND an HP 24, with the second external monitor driven by a USB DisplayLink dongle. The 2010 was starting to get a bit long in the tooth so soon after the 2018 Mini came out I bought one. I shifted the Thunderbolt Display to be the main monitor for the Mini, with the HP 27 connected via a USB-C to DisplayPort cable and the HP 24 connected with HDMI. Then I connected the old 2010 iMac as a second monitor for the 2014 iMac Retina, using Target Display Mode. I bought a cheap little USB A-B switch for the keyboard/mouse, to invoke the Command-F2 when needed and to use the 2010 iMac when desired – and it has been working perfectly ever since.
The 2010 iMac runs in the background as a kind of file server but in fact its main purpose is simply to be the second monitor for the 2014. Did I mention that it has been working perfectly for nearly a year?
Thus, I have to quibble with remarks like
… only under limited circumstances.
Well, yes – but if one’s circumstances happen to match, as mine did, then hallelujah. And
… these setups tend to be temperamental.
Not at all in my experience. And
[for] Adam Engst … the experience wasn’t entirely satisfying.
Well, they always say YMMV … and I for one am completely happy. And then at the end you make the blanket generalisation, citing Adam Engst again, that
Apple’s Target Display Mode doesn’t work very well.
to which I enthusiastically reply ‘Hogwash! It works PERFECTLY for me!’
I was drawn to read this article because, it is true, Apple in their infinite wisdom have given up on Target Display Mode in more recent Macs and when the time comes to retire my 2010 iMac altogether and demote the 2014 Retina to ‘second monitor’ for a super-shiny new machine I will obviously need something like this Luna. Oh. But it doesn’t work with 27" Retina iMacs! And it is
… not yet fully baked …
anyway. Good, then, that I don’t need anything right now; maybe the hardware will catch up with my needs before I do need something new.
Glad it’s working for you. I wrote the bits about Target Display Mode in Julio’s article since I had experience with it, and as the article says, I found it annoying enough with a MacBook Air to be largely unusable.
Using it with an iMac that doesn’t move and get disconnected probably helps, and yes, buying a switch so you don’t have to have another keyboard and mouse cluttering things up would help.
But in the end, Target Display Mode works with so few iMac model that it’s just not an option for most people. I was sad that Luna Display’s Mac-to-Mac mode doesn’t work with the 27-inch Retina iMac, since my 2014 model is getting a bit long in the tooth, but I’d love to use it as a secondary monitor for a new Retina iMac. 5K Retina is vastly better than the 27-inch Thunderbolt Display I use now.
A few people used to ask me if you could use an iMac as a second monitor. This seemed to be an obvious question but the answer was usually “no.” This was before the advent of display port connectivity. So this has been an issue for a long time. That you can now do so under some circumstances, depending of the vintage of your iMac, is nice. But you can buy an inexpensive monitor for roughly the price of an AstroHQ Luna Display Dongle, providing a reliable alternative that you can hook up to any Mac, with the appropriate adapter if necessary. Using a Mac as a secondary monitor appears convenient under some circumstances, but rather a waste of resources. Of course, if you’ve got old iMacs just hanging around, doing nothing but collect dust, well the monitor on almost any iMac is superior to an inexpensive monitor if graphics quality is a factor. But given the caveats covered in this article, a cheap monitor might still be a viable alternative.
Correct me if I’m wrong in some way.
so sorry if i missed this but - the biggest detractor for me of sidecar - and why i’m delighted there’s this alternative - even if hardware is required - is the reqirement for iCloud - why oh why does apple require iCloud to operate sidecar?
anyone??
thank you for these great reviews. bummer about the 5k imacs but still cool for mac mini options.
m.c.
I’m guessing that Apple requires iCloud because you need some sort of authentication between the devices (so bad guy couldn’t hijack your video stream), and iCloud is pretty much guaranteed to exist for every individual Apple user. It’s required for a wide swath of Continuity features, so it makes sense that Apple would assume it for Sidecar as well.
ah right - and that’s the reason i’m not using them i suppose - does apple learn any more about us from icloud on or off if we’re using these devices? perhaps such rejection on my part is naive.
thanks again.
much obliged
No, iCloud really isn’t a privacy concern, largely because Apple has extensive technical and corporate policies in place that ensure that the company has nothing to gain by knowing more about you. It does increase your exposure to Apple knowing more, but Apple doesn’t make any money by knowing that information, unlike most other tech companies, so the information goes nowhere and generally does nothing. For more details, see
And Apple’s full privacy policy at:
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thank you Adam
that helps