Using iMac as second monitor

[pwm86]

How can I use an iMac8.1 (from 2008) running High Sierra as a second monitor for an iMac9.1 (2009) running Mojave?
What cable? What do I have to do on the iMac8.1? Or do I have to install an older version of macOS on it?
Many thanks.

 

[MultiFinder17]

Long story short, you can't.

https://support.apple.com/en-us/HT204592

Only the 27" late 2009, mid 2010, and mid 2011-2014 iMacs support it. The 8,1 is too old to support it.

 

[mikehalloran]

Fortunately, inexpensive 20"–24" monitors that will work are readily available. 22"–23" seems to give the best bang for the buck.

Those that are backlit LCD are brighter (like your iMac) than those that aren't. Don't worry that the connections are HDMI and VGA as inexpensive adapters are readily available. If you can get one with a cable, you'll save money over buying it separately. The Apple adapters are expensive and unnecessary—the generic ones work just fine.