To put it shortly, if I want to use a seismic torpedoe:

1) I declare action: seismic torpedoe and it turns out, that an obstacle is beyond range 2, do I lose action or am I free to pick another one?

2) may I premeasure range to an obstacle before declaring on picking the action?

3) may I premeasure range FROM the obstacle, that I want to destroy to see whether or not it will hurt somebody before declaring the action?

I guess the answers are:

1) I may pick another action

2) I may not, although I should be allowed in my opinion,

3) I may not.

I'll be glad for any piece of advice.

You have it right. You declare your action then check to see if you're in range. If you're not, pick another action. At no point can you pre-measure.

Same case as with any illegal action - you pick another one or another variant of the illegal action that is legal.

In this case if somehow you can double action you could check for Seismics that turn out illegal then boost or barrel roll and try seismics again.

You should be able to declare Seismic Torps any give obstacle, check the range/arc to that obstacle, and then if found to be out of arc/range rewind and start all over again. Avoid anything you are 99% sure will be in range and arc and you should be able to "ping" every obstacle on the table and if that turns up other information you find useful then you've gotten lucky.

1. You pick your target and if it is invalid you rewind.

2. You don't get to measure before declaring; now if you declare a target beyond something it may turn out to be out of range but you may "accidentally" discover something that is in range.

3. You couldn't see how close another ship is to a given obstacle until after you've had the action work.

So here's the golden rule for measuring: you cannot pre-measure.

There is one exception to this. When a ship activates during the attack phase you can measure range and check firing arcs from that ship and that ship only to any other ship in play before you choose a weapon and a target.

In ANY other scenario if the question "Can I pre-measure?" comes up, the answer is no. Always.

Now, it is also important to remember that any time you declare an action or effect that has a range limitation and then discover that you are not within that limitation you back up and can either choose a different target or a completely different action. If you check and DO have the correct range you must complete the declared action or effect.