Focusing with the Sesto Senso

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  • EQUIPMENT/SOFTWARE LIST
    • Mount: Skywatcher EQ6-R
    • Telescope: Skywatcher Esprit 120
    • Imaging Camera: ZWO ASI 533 Pro
    • Focuser: Primaluchelab Sesto Senso
    • Laptop computer
    • Sequence Generator Pro

I have been using this focuser with my Skywatcher Esprit 120 and ZWO ASI 533 Pro. The focuser was fairly straight forward to attach to the telescope following the instructions.

Once fitted and the relevant software installed on the laptop, the focuser connected via USB to the laptop and once powered up connected okay. Initially you need to calibrate the focuser, which involves measuring the full in/out positions of the telescope focus tube TIP: Put a couple of pencil marks on the telescope just before the max and min points so the focuser doesn’t hit the ends (also, it does take a good few minutes to do the full calibration).

It can be tricky trying to find rough focus using a dedicated astro camera. The focus steps on the electronic focuser are VERY fine and with the Sesto Senso even 50 steps barely moved the focus. TIP: If you can find the rough focus before you attach the electronic focuser and make a small indicator (I use a pencil), it can save you some time later. Once set up I attempted to get rough focus during the day on a distant tree using APT – then under the stars I set the focus steps to 100 and spent a few minutes getting it as close as possible.

Although I now use the Auto Focus routine in SGP, which is very good, I actually found the setup in Astrophotography Tool (APT) the easiest to getting the first accurate focus.

Focus using APT:

  • Connect equipment;
  • Point telescope towards distant object – the farther the better;
  • Focus in one direction to try and get focus;
  • TIP: try and find a closer object to focus on, then focus on the farther object – this will let you know the direction to focus past when you get under the stars;
  • Use the Focus Aid in Tools to try and get the HFR value.

Focus using SGP:

For this your telescope needs to be in as good a focus as you can. In the Autofocus settings you need to work out the Step Size for it to work correctly.

The formula to calculate the steps is move the focus from HFR 1 to 4 – Step number x 2 divided by focus points -1

  • Calculate your Focus Steps
    • Get HFR around 1-2
    • Note Focuser setting (e.g.
    • Move out of focus in steps of 100 until HFR=4-5
    • Note Focuser setting
    • Step difference x 2 / focus points -1

As an example, my Auto Focus settings are as follows:

  • Camera exposure = 6s
  • Bin = 2×2
  • Data points = 11
  • Step size 600
  • My focus setting is 96112

TIP: Another thing to consider – if you are using a power box / hub (I use the Pegasus Astro Pocket Power Box), the power cable needs to be different to the standard connectors that come with the Pegasus/ZWO products (2.1mm). The Sesto Senso needs to have the 2.5mm center positive power cable.