can I use mac book air super drive to back up my desk top

You can burn pretty much any type data to CD , DVD or BT. But I'd imagine to back up your full Mac to a set of optical disks, and maintaining the backups, is going to be a real interactive nightmare compared to using an external HDD you "set and forget".

Depends on how much data you're backing up; that's your call. External drives are not that expensive. As long as you're using Time Machine and/or Carbon Copy Cloner automatically, you should be good.

I've seen a few complaints about Time Machine on these forums. I run both Time Machine and CCC, and have CCC make a bootable backup for me. I get by with a 100 GB drive, but you may need more (or less). I also have a 2 TB drive on my network, but that handles more than one computer, and it's way more than we need.

Using an external hard drive is a much better idea.

Once you burn a DVD, you cannot change the content, so you would need a lot of DVDs to keep burning backups to it.

I like these Mercury Elite Pros froma recommended vendor:

https://eshop.macsales.com/shop/external-storage