Bill wrote:
I've heard of that...I think it's just too much signal gain at high ISO
settings in dark shots. Lower the ISO to a more appropriate setting.


It's because of heat coming from electronic circuits near the sensor.
Not entirely surprising after an exposure of 3 minutes at ISO 1600.

It is removed, as you say, by dark frame subtraction.


You should already know this, but those are called "hot pixels" and it's
actually normal. The longer the exposure time the more hot pixels will
show.

You have a noise reduction (NR) feature in the camera called dark frame
subtraction for long exposures. When enabled, it does a secondary
exposure without opening the shutter to get a completely dark comparison
for the previous shot. It does a decent job of removing the hot pixels
and any other anomalies.

The ISO setting has an effect on it too. For long exposures you should
be using ISO 100. Higher settings are not needed and introduce unwanted
noise. Limit high ISO shots to low light, such as indoor shots. For dark
or night shots where you have to use a tripod anyway, set the camera for
ISO 100 and get the best image quality.

Hot pixels happens with all digital cameras. It's nothing new.

The only thing of concern is the dead pixels. You shouldn't have that
many after only 8 months.