The question
Why this one is free
Everyone else charges forty euros for this. What is the catch?
A personalised star map usually costs somewhere between forty and ninety euros. This one costs nothing, and the reason is duller than a catch.
What free means here
It means the file. No account, no email address, no watermark, no reduced-resolution preview with the real thing behind a payment. The PNG that downloads is the same PNG at every size, at print resolution, and you own the use of it.
There is no trial that expires and no second, better version further along. What you can make on the first visit is everything the tool can make.
What the paid services are actually selling
This is worth saying fairly, because several of them are good at what they do. When you pay sixty euros for a star map, almost none of that is the chart. The astronomy behind it is a solved problem — the star positions come from public catalogues that anyone can download, and the mathematics of projecting them onto a disc has been settled for centuries.
What you are paying for is printing, packing, posting, customer service, returns, and the advertising that got the site in front of you. Those are real costs and real work. But they are the costs of a shipping company, not of a star chart, and bundling them together makes the chart look like the expensive part when it is the free part.
How this pays for itself
After a download, the sky tool shows one link to a print-on-demand service. If you follow it and order a print, this site earns a commission on that order. The link is marked as an affiliate link where it appears, because a disclosure that you have to go looking for is not a disclosure.
That is the whole model. There is nothing else — no data being sold, no mailing list you are quietly added to, no account holding your birth date on a server. The dates and places you type stay in your own browser.
Why that arrangement keeps the chart honest
A service that sells the file has a reason to make the free version worse: a watermark to remove, a resolution to unlock, a size withheld. Every one of those decisions makes the product slightly more annoying on purpose, and once that incentive exists it tends to win.
Here the file is the advertisement. The only version that exists is the good one, because a chart nobody wants on a wall earns nothing either way. That is not generosity so much as the incentives happening to point in a useful direction.
What this is not
It is not a horoscope, and the chart will not tell you anything about your character. It is a drawing of where the stars actually stood over a particular place at a particular minute, computed from measured positions, and its only claim is accuracy.
It is also not a promise that the site will exist forever. Which is a reason to download the file rather than bookmark the page — once the PNG is on your own disk, nothing here can take it back.