This is the way I would do it.

You need to copy two files from your GalCiv directory, assuming you have a Steam install.

...\Steam\SteamApps\common\Galactic Civilizations III\data\Game\ImprovementDefs.xml

Copy to

...\Documents\My Games\GalCiv3\Mods\ExampleMod\Game\

assuming English

...\Steam\SteamApps\common\Galactic Civilizations III\data\English\Text\ImprovementText.xml

Copy to

\Documents\My Games\GalCiv3\Mods\ExampleMod\Text\

Open the newly copied ImprovementDefs.xml and paste the following near the end of the  file.

NB. You must paste it before this line </DefaultImprovementList>  This has to be the last line in the file.

<!-- Begin Improvement -->
 
  <Improvement>
    <InternalName>HubbleArray</InternalName>
    <DisplayName>HubbleArray_Name</DisplayName>
    <ShortDescription>HubbleArray_ShortDec</ShortDescription>
    <Description>HubbleArray_Dec</Description>
    <Icon>EleriumDefenseShield.png</Icon>
    <BuildIcon>EleriumDefenseShield_Build.png</BuildIcon>
    <ListIcon>EleriumDefenseShield_Icon.png</ListIcon>
    <ImprovementType>Military</ImprovementType>
    <PlacementType>Military</PlacementType>
    <IsColonyUnique>true</IsColonyUnique>
 
    <!-- Stats -->
 
    <Stats>
      <EffectType>ManufacturingCost</EffectType>
      <Scope>Queue</Scope>
      <Target>
        <TargetType>Improvement</TargetType>
      </Target>
      <BonusType>Flat</BonusType>
      <Value>150</Value>
    </Stats>
 
    <Stats>
      <EffectType>Maintenance</EffectType>
      <Target>
        <TargetType>Improvement</TargetType>
      </Target>
      <BonusType>Flat</BonusType>
      <Value>1.5</Value>
    </Stats>
 
 <Stats>
      <EffectType>SensorPower</EffectType>
      <Target>
        <TargetType>Colony</TargetType>
      </Target>
      <BonusType>Flat</BonusType>
      <Value>32</Value>
    </Stats>
    <!-- Triggers -->
 
    <!-- Level Effect Trigers -->
 
    <!-- Level Effect Stats -->

 
    <!-- Adjacency Bonuses -->
 
    <NeighborBonuses>
      <GiveBonusToNeighborType>Military</GiveBonusToNeighborType>
      <NeighborBonusValue>1</NeighborBonusValue>
    </NeighborBonuses>
 
    <!-- Prerequisites -->
 
    <Prerequ>
      <Techs>
        <Option>InterstellarSurvey</Option>
      </Techs>
    </Prerequ>
 
  </Improvement>
 
<!-- End Improvement -->

Open the newley copied ImprovementText.xml paste the following near the end of the  file.

<StringTable>
    <Label>HubbleArray_Name</Label>
    <String>Hubble Array</String>
  </StringTable>
 
  <StringTable>
    <Label>HubbleArray_ShortDec</Label>
    <String>Allows us to see further into space</String>
  </StringTable>
 
  <StringTable>
    <Label>HubbleArray_Dec</Label>
    <String>Combining multiple sensors into large arrays allow us to monitor more of our local space more efficiently.</String>
  </StringTable>

NB. You must paste it before this line </StringTableList>  This has to be the last line in the file.

You should now have a Hubble Array available when you research Interstellar Survey.

A sensor power of 32 gives approximately a 10 hex radius. A normal Capital has a power of 16, a 5 hex radius.

Edit.

If you want a 25% increase change the word Flat in <BonusType> to Multiplier and change the <Value> to 0.25

I'd say give it to everybody for balance reasons.

Sounds like you may have already got this figured out, but here's another thought.

If your custom faction has a custom race trait, you could use that as a prerequisite and avoid having to change any techs (which effects the ability to trade those techs).

In the prerequisite section of the improvement, I think you just have to add the following:

<Prerequ>

 <RaceTrait>
  <Option>AbilityName</Option>
 </RaceTrait>

</Prerequ>

The Torians use this for special terraforming improvements using the Amphibious race trait.

Way, way back I created and posted a sensor improvement mod--I thought I lost my mind because I couldn't find it, and so I didn't share earlier... anyhow. It was from back before sensors were revamped and sensor power was added, so it wouldn't really be usable now.

I think I've been inspired to give it new life--and with what I've learned about modding the game in the almost two years since that post, who knows what I might make!