iPad as Touchscreen on Mac

I'm sorry you have had problems mirroring the display to your iPad. I had an old iPad for many years, which connected well using TouchOSC to Hauptwerk.

That is strange that you have not had any replies from the makers of Duet display, but it may be because they don't officially support your operating system (Mac OS High Sierra 10.13). I agree that it is cryptically arranged on their website - I had to find it in their poorly organized FAQ section. They say that in theory it should work, however. Have you tried upgrading your Mac Pro to Mojave? If you were able to do that, then they should have to provide support, since it is officially supported. However, the stand alone app also seems to require a thunderbolt 3 port (again, cryptically only mentioned in their FAQ section), which I don't think a 2013 Mac would have.

I am guessing that you also paid for the 1 year Duet Air subscription? I'm guessing this because the desktop app is normally free. However, I am guessing that Duet Air would probably would only work if you have Mojave (10.14). If you did buy it, the signup process is quite long and complicated, but I'm assuming you already went through the process? I provide the FAQ link here in case it would be of help:

https://www.duetdisplay.com/help-center ... r-duet-air

If you haven't signed up for Air already, then I wouldn't - no guarantee it would work on High Sierra. They don't seem to mention the requirements on the website, at least I couldn't find them. I seem to recall there were some important changes to Desktop mirroring in Mojave, and it is possible that Duet Air requires them to work as well.

If it is any consolation, TouchOSC would probably still work with your setup, and I find the process isn't too difficult. I will be following this thread, and can help you with any problems setting it up if you encounter it. It's extra money, but you could also consider Luna Display. It will certainly work on High Sierra. https://lunadisplay.com/pages/support.

Thanks, very helpful. TouchOSC looks a bit complex for me.
I can only assume that iDisplay have disappeared. Duet is disappointing because I feel that I was conned into paying for the sub on both Mac and iPad because the trial didn't look as though it was going to work; and their website is so confusing; and I have had no reply from them. I may give it a little longer, but in principle I would like my money back, but getting that from them or Apple Store seems unachievable. Mojave is as far as I could go with the Mac Pro, but if they do not respond I am not comfortable with wasting more time on it.
I was not aware of Luna, so that is a promising suggestion, thanks. My Mac Pro does have mine Displayport, so it may be my best line of attack now.

Ok, though I would be careful with the display port. Older macs with the DisplayPort usually used the Thunderbolt 2 technology, whereas new macs use thunderbolt 3. You can buy an adapter from thunderbolt 2 to thunderbolt 3, but it is expensive, and it doesn’t work in every instance. Duet display seems to require Thunderbolt 3, so there is no guarantee it would work if you bought the adapter. I ran into this problem because my old Mac has a thunderbolt 2 port, and my new one has 2 thunderbolt 3 ports. I ended up buying a new HDMI adapter for my new Mac, even though I had one for Thunderbolt 2, because it seemed from the reviews of the adapter that it wasn’t going to work to connect a TV to my new computer.

Thanks Larason.
I eventually heard back from Duet - you are right that Duet Air will not work on High Sierra. They say it will work with Mojave, but I cannot install that without buying a new Metal graphics card. Better to write off the Duet costs incurred.
Luna have been helpful, and assure me that it will work on High Sierra, and that the Mini Displayport on my Mac Pro 5.1 will be fine. They strongly advise against any form of port adaptor. So I think I will give Luna a try. Not sure how long it will take to arrive though, in these strange times!?

Ok, though I would be careful with the display port. Older macs with the DisplayPort usually used the Thunderbolt 2 technology, whereas new macs use thunderbolt 3. You can buy an adapter from thunderbolt 2 to thunderbolt 3, but it is expensive, and it doesn’t work in every instance. Duet display seems to require Thunderbolt 3, so there is no guarantee it would work if you bought the adapter. I ran into this problem because my old Mac has a thunderbolt 2 port, and my new one has 2 thunderbolt 3 ports. I ended up buying a new HDMI adapter for my new Mac, even though I had one for Thunderbolt 2, because it seemed from the reviews of the adapter that it wasn’t going to work to connect a TV to my new computer.

Thanks. The minidisplayport on my Mac Pro turned out to be compatible and very easy indeed. The Luna product was swift to arrive and very easy to install. Good support too. The main thing to watch out for is the version of iPad hardware. I had been using an iPad 2, but this was not compatible (on a hardware criterion, even though the iOS version was OK), so I had to use a more recent iPad Pro. However this has the benefit of being a larger screen.
Luna - recommended!