Flats made it worse

A calibrated frame beside the raw one. A donut that now appears twice, or corners lighter than the middle.

1. 01

   Focus or rotation changed between lights and flats

   Flats calibrate the dust and vignetting of one optical setup. Move the focuser or turn the camera and they calibrate a different one, so the dust shadows land in the wrong place.

   FixShoot flats before touching focus, rotation or the camera. With per-filter focus offsets or a rotator, match the positions the lights used.

   Session Checklist
2. 02

   Flats over- or under-exposed

   Overexposed flats run into the sensor's nonlinear range, and underexposed ones are dominated by their own noise. Either way the correction comes out wrong.

   FixExpose flats to a measured ADU target, counted from the offset up, rather than by eye. On a color camera, read the green channel.

   Session Checklist
3. 03

   Panel flicker

   An LED panel's PWM flicker beating against short exposures makes the flats uneven, and that unevenness is divided into every light.

   FixDim the panel so the flat exposures are long enough to average the flicker out.

   Session Checklist
4. 04

   Bias used instead of dark flats

   Flats have to be calibrated by something, and on CMOS the short-exposure frame is where bias and dark behavior part company.

   FixShoot dark flats at the flats' exposure, gain and offset, and at their temperature on a cooled camera.

   Sub-Exposure & Calibration Planner

The atlas lists no look-alike for this one.

A calibrated frame shows the fault. Which cause made it needs the flats themselves: their exposure level, and whether focus or rotation moved before they were shot.

Calibrated frames with no doubled dust, and corners that match the center under a hard stretch.

Bias versus dark flats on CMOS is argued both ways. Testing reported by AAVSO found short CMOS darks carry structure bias frames miss, which is why we default to dark flats. Many imagers calibrate flats with bias on sensors whose short exposures are clean, without trouble. Compare a bias with a dark flat from your own camera; the answer is per sensor.

• Astrocamp, “How to calculate the correct ADU target for flats”
• AAVSO, “Bias frames and CMOS cameras”