A spreadsheet to convert Hocus Focus result in pratical move/turns of a manual tilt adapter plate

Frédéric Ruciak · Jan 7, 2026, 05:26 PM

Hi,

I recently got a wanderer astro ETA. It works great but I wanted to better understand the way it works and I asked OpenAI to prepare a spreadsheet to check what the ETA was doing and also to use old Hocus Focus results as the ETA works only for the real time Hocus Focus results. I checked the formulas and they are good.

I then asked ChatGPT to prepare a version for manual tilt adapter either the one of the camera (Zwo for example) or any manual tilt adjuster with 3 adjustment points in an equilateral triangle. I will use it in another setup where I do not have the ETA.

I thought it could be useful to share as I personaly struggled a lot to translate the Hocus Focus results in pratical adjustments on the mechanical tilt plate adjustment. You just have to input the size of the sensor (APSC or full frame or whatever size), the size of the equilateral triangle of the ajusters (with the pitch of the screws) and the orientation of the adjustment triangle to the base of the sensor (bottom). Once properly setup you just input the output of Hocus Focus and it gives you the number of turns of each adjustement screws of your adjustment plate or the displacement in microns (to be measured with a calliper).

I add the small text that I asked Chatgpt to prepare as an explanation of how it works and how to use it.

Hope this can help some of you.

CS and happy new year Frédéric

tilt_corrector_3points_variable_angle_EN.xlsx

Below directrly from chatGPT

3-Point Tilt Corrector Spreadsheet (NINA / Hocus Focus)

This spreadsheet converts Hocus Focus sensor corner measurements into exact mechanical corrections for any 3-point tilt adjuster (manual or motorized), without trial-and-error.

It works for:
• any APS-C / full-frame sensor
• any triangle orientation
• any 3-point mechanical corrector (manual screws or electronic actuators)

Principle

A tilted camera sensor is always a geometric plane:

z(x,y)=ax+by+c

Hocus Focus measures the relative focus offsets at the four sensor corners.
These four values completely define the tilt plane.

Your corrector has three actuators, which exactly control the three degrees of freedom of a plane:
• tilt X
• tilt Y
• piston (backfocus)

The spreadsheet mathematically converts the HF corner measurements into the exact height corrections required at each actuator position.

How to use 1) Enter sensor data

Enter sensor width and height (mm).

2) Enter Hocus Focus measurements

Fill the four corner values (µm):

• Top-Left

• Top-Right

• Bottom-Left

• Bottom-Right

Also enter the backfocus correction if HF provides it.

3) Enter your corrector geometry

Define your triangle:

• Triangle side (mm)

• Triangle rotation angle θ (degrees) relative to the sensor X axis

• Centroid offset (if the triangle is not centered)

This makes the spreadsheet compatible with any triangle orientation.

4) Read the actuator corrections

The spreadsheet outputs:

• Required correction at each actuator (µm)
• Equivalent turns / degrees (using your screw pitch)

Apply these corrections and your tilt is geometrically canceled.

Why this is better than trial-and-error

Because it uses the exact plane equation, the correction is:

✔ deterministic
✔ optimal
✔ repeatable
✔ independent of guesswork

It is mathematically equivalent to professional electronic tilt controllers.

Notes

• Adding the same backfocus correction to all three actuators moves the sensor without changing tilt.
• The RMS residual indicator shows if the measured surface is a true plane or affected by optical field curvature / aberrations.

This spreadsheet was developed from first-principles geometry and is free to use and share.

If you want, tell me which forum you’re posting on — I can adapt tone/format (CloudyNights, Astrosurf, solitaire bliss Stargazers Lounge, etc.).

This is incredibly helpful, thank you for sharing, Frédéric. The geometric explanation makes the process much clearer than trial-and-error. Have you tested it with different triangle orientations or asymmetric centroid offsets yet?

Hi Jean,

The initial purpose of the spreadsheet for me was to check that the moves of the ETA compared to Hocus Focus results were consistent and they are, and to be able to reuse previous hocus focus results as the ETA works automatically only for the very last (<30s) Hocus focus. Even with the ETA I had to go through some trials and errors to find the right combination (180° was the logical orientation but 0° works right).

I will test this week the spreadsheet with another setup that uses a tilt plate (a zwo camera). The math behind the spreadsheet is algebra applied to the plane and is fully deterministic and I checked some of the formulas (as I always do with chatGPT ;-)) and they are consistent. At the bottom of the spreadsheet you can input an offset (x0 y0) of the adjustment triangle compared to the sensor if necessary, the rest is just math but I suspect that you have to find the right x0 and y0 relative to the sensor and also the right sign for x0 and y0 otherwise the math will give wrong results (you have 4 combinations +x0 +y0, -x0 +y0, +x0 -y0 and -x0 -y0) I tried by putting an offset of 1mm and the output for the adjustment change in a way that I could not measure with a caliper. The effort to measure and find the right position for the displaced centroïd is not worth it unless you have something completely different (cf the right triangle version of the spreadsheet)

What is very important for me is to input the right parameters to the spreadsheet, the size of the sensor, the position of the 3 adjustment points relative to the sensor and the direction of adjustment relative to the sensor. You’ll most probably have to find the orientation by trial and error to link bottom right/bottom left from hocus focus to the adjustment points. But once this is done, it works.

Hope this helps, Frédéric