Subs show zero as minimun ADU value after calibration?

[Terry Danks]

I recently tried to use Steve Cannistra's sub length calculator. It demands the sky background value of a sample sub AFTER calibration. But, after calibration, PI returns a minimum ADU value of zero?

[Ignacio]

A calibrated hot pixel may turn into a cold pixel, explaining your result.

How many pixels have zero value percentagewise? If too many, then you may have an issue with your calibration frames (bias and/or darks).

Ignacio

[NKV]

Set Pedestal = 3 * StdDev(in single Dark)

[MortenBalling]

Depending on what camera you use, you could have a problem with darks when they are calibrated with bias. On DSLR and cameras without temperature control, you might have lower ADU on your Darks than on your Bias, especially with short exposures. Also Canon uses some sort of scaling before the raw images are saved. Read the article at stark-labs.com, if you use Canon.

Calibrating a dark stack, normally uses the formula:

Calibrated darks = darks - bias

That will give a negative ADU after calibration, and negative values are truncated to zero by PI.

In ImageCalibration, set the output pedistal to 3x the std dev of your darks. You can find that using Statistics.

Be aware that this will change the measured value of your sky background as well, so you probably will have to subtract the pedistal afterwards.

When calculating sub lengths, you don't have to be to precise. The sky background normally varies across the frame, so it's hard to measure that precisely anyhow.

Morten