Advertising is a tricky business, especially at times when budgets are tight, but ad prices is not the only issue... The distribution is also important. Also, advertisers LOVE statistics. How many people (unique readers) downloaded the first issue? How many are expected to read the second? Any demographics? How often they will be issued? If there's one thing you CAN'T tell an advertiser is "our next issue will be out POSSIBLY in 2-6 months"... Timing is very important for advertisers as I'm sure you know. Depending on who you deal with, if you don't speak their language, they'll ignore you - in the worst case, might even mock you (although that's just saying they're worth your time as much as they say you're worth theirs)...
If PixInsight Magazine was a work of love - it surely seemed that way - and you just don't have the resources or time to produce a second issue for now... that's one thing. If you want to sell ad space in an electronic publication, even one so specific as PixInsight Magazine, in order to support the product... That's a whole other ordeal, and perhaps you weren't ready? Maybe you were, I just don't know. Sending a mass-email to a bunch of vendors - even if the messages were personalized and sent one by one - is probably not the formula I would have recommended as a first step FWIW.
I'm just saying this because although it's very easy to conclude "OMG such an excellent software, how come nobody wants to advertise?" I wouldn't go on a vendor witch hunt after reading Juan's email unless I had all the facts. In the end, it's also true there are many idiots out there anyway.
In any case, Juan, the fact that you just said you don't have the time needed for this project, tells me that advertisers and some extra money aren't going to solve that problem anyway.