been working on this for 2 days now and I give up. Had all the blue doors on the outline - door wouldn't open. Any suggestions or savegame???
Thanks!

Be sure to look again through the telescope AFTER you have turned the gates to the correct colors. 2 gates are different from the regular screen view than through the telescope. Check all of them again.

Be sure also that you are using both filters when you look through the telescope.

trouble is, I have no clue what to do with the filters. How can I use both at the same time?
I'm lost. Everytime I look through the telescopes the colors would change, so I start anew. And what function has the upper button?
I just don't see the logic behind this puzzle!

Remember what the companion who gave you the two filters said - two filters for tower security system: one shows the correct colors and the other shows them in reverse. Observe all portals and go through the red halves. Only the red halves.

So the logic of the puzzle is to arrange all the gates to show you which ones are the red gates.
But you have 2 filters one that shows the true colors and the other one that shows the wrong colors.So it's up to you to deduce which one and that is the puzzle.

The telescope setup:
there's 2 side holders for the filters. If you click one or both the filters goes over the telescope lens.
top button is for removing the filters from the telescope lens.
bottom filter is for turning the gates.

Solution:
Place BOTH filters over the telescope lens - so that whatever you see are the WRONG colors.
Then rotate if necessary for you to place the BLUE (since it should be the wrong color) at the outward gate. The reason for this will be obvious when you walk the gate path.
Check whatever you did on the gates by looking through the telescope. One or more are different from the screen view as oppose to the telescope view.

Good luck!

You use both lenses at once.

You look through the telescope in close up so you can see which color is in the back, farthest away from you. It isn't always the door on the left or the door on the right that is in the back.

You set the doors so the blue door is in the back.

If you have any of the doors set wrong, the portal at the end of the walkway will not open.

There are two telescopes on one of the platforms. I made the mistake of not noticing and couldn't get the gate open until I went back and set the second telescope.

Marillion,

As I posted above - use both filters on the telescope lens - by clicking on the filters at either side of the telescope. You will see that they move to the telescope lens and there will be no holders seen on either side if you have done so.