Long awaited, still partial, solution.
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After a long time in space I started to forget what N and E stand for here, on Earth. Well, now it's clear to me: East and North are directions. The first time I tried to get through those Space Grids, I mixed them up and also tried to take clues from the spectrum... Oops, let me get the spoiler glasses on. I might talk something dangerous.

Allright, so the third slide is a classical

>! Space radar calibration table. The things beneath are linear equations with color-coded numbers. A linear equation describes a straight line and a dots beneath must be a number of stars in its vicinity (that's what I made out from classical d<1/2 expression - stars are counted if they are less than 1/2 spacemile from the beam). <br> <br> Yet, not only you expect six stars on a line, not only you can assume that BLACK codes 0. There is also a nice twist: lines described as [ax+by=? and bx-ay=?] are _perpendicular_. This lets us calibrate a picture:

Beautiful.

>! [![Wonderful.][1]][1]

After that (if you didn't mix up the axis and spend an hour crying of helplessness) calibrating three radars on the second slide is relatively easy. I attach the image here:

>! [![Fascinating.][2]][2]

Now I'm pretty stumped, because radar calibrating data is about the half of your friend's transmission, and I can see no connection to the star-maps (or that little hint with arrows). Basically, the data I gathered, condenced, would be:

>! 1)BLUE, 2)PURPLE, 5)GREEN, 7)ORANGE

There is one more thought on the main map, however, I don't know how to evolve it (also, what is the line across all the star-map? Is it a route of an ice-cream van? If so - is your friend an ice-cream merchant?)

>! I thought that six pieces on the minimap represent six different places on the star-map - like when you take a photo of a special crossing so you could be found later. Nevertheless, this seems not too probable - some pieces I can put to different places on the map, yet some of them don't seem to fit.

That's all I got,

_SEE YOU SPACE COWBOY_


  [1]: https://i.sstatic.net/yJC0f.jpg
  [2]: https://i.sstatic.net/cTMCn.jpg