Tilt analyzer
Check whether your camera’s sensor is tilted and fix it step by step, with a few sets of frames. Everything runs in your browser: your frames never leave your computer.
Plan on a couple of hours of clear sky and several sets of frames: measure, turn a screw and measure again. Do step 1 before you set up: that way you won’t have to take things apart in the middle of the night.
What’s wrong with your stars? Before you touch anything, check this
Take a focused frame and look at the center and the four corners.
- One or two corners worse than the others: the sensor is tilted. You’re in the right place: read on.
- All four corners equally bad, stretched toward the center or outward: it’s backfocus, the distance between the reducer or flattener and the sensor. The backfocus calculator tells you the spacing.
- The center isn’t right either: defocus a star in the center; if the donut is brighter on one side or the hole isn’t in the middle, the scope is out of collimation. With an RC, the collimation assistant. With a Newtonian or an SCT, the guide Colimar tu telescopio (in Spanish for now).
- The whole field equally bad, and it changes from frame to frame: it isn’t the optics. Check focus (with a Bahtinov mask), guiding, or wait for the seeing to settle.
Just want to know if you have tilt? Check it with 5 numbers
Take a 30 to 60 second focused frame of a rich star field and note the HFR (or HFD, or FWHM) of the center and each corner. ASTAP gives you these with its CCD Inspector, Siril with its sensor tilt tool, or any program that measures by zone. Use the same unit for all five.
What it doesn’t tell you: how far to turn each screw. And with focused stars, seeing evens out the corners a lot: a small tilt may not show, and a side-to-side one can pass for curvature. That’s what the guided session below is for, with defocused frames.
- Prepare
- Measure
- Correct
- Check
1. Prepare
Before you set up, tick what you already have:
There’s nothing else to measure: the analyzer learns how your camera responds from the frames themselves.
Step 2 · Measure
Take the first set
Which spacers to use for those millimeters
To collimate: with an RC, the collimation assistant; with a Newtonian or an SCT, the guide Colimar tu telescopio (in Spanish for now).
Drag your set here
FITS from NINA, SharpCap, ASIAIR and the like. All at once.
Test mode, no sky needed. A simulated sensor with the tilt from Fran’s frames and some hidden screws with plenty of play. Do what the step asks and press the button as if you’d taken the set. You can also drop in real frames.
Your sets
See an example: Fran’s frames
Your sensor’s focus map
How far focus at each point of the sensor is from the center, in microns, as you see the image in your capture software.
The donuts, corner to corner
A typical donut from each corner and the center, placed as on the sensor. On one side of focus, the tilted corner has the bigger donuts; on the other side, smaller ones.
The stars in focus
The corners and backfocus
Backfocus, step by step: the backfocus calculator · the full guide, Backfocus, sin misterio (in Spanish for now).
The frames
| Frame | Focuser | What it is | Stars | Tilt it gives |
|---|
How it works
In each frame it finds the stars and measures the radius that encloses half their light, using only the green pixels. Out of focus, that radius grows with how defocused each point of the sensor is: multiplied by 2√2 and by the focal ratio, it gives microns. It fits a plane (the tilt) and a term that grows toward the corners (the curvature) across the whole sensor. On one side of focus the tilted corner has bigger donuts; on the other, smaller ones: if both measurements agree, it’s real tilt and not an effect of the tube or vignetting.
It doesn’t need to know where your screws are or their thread pitch: when you tighten screw 1 by 1/8 of a turn, and then screw 2, it measures how much and in which direction the tilt moves, and from that works out how far to turn each one. The critical focus zone is 4.88 × λ × (f/)², with λ = 0.55 µm: tilt is good enough when it’s below half of that between opposite corners.