[ATM] infra-red

["John Sherman" <atmer@johnspics.com>]

Hi Mr List,

I have made an observation that might be of interest to those building 
infra-red telescopes. Since ccd's are sensitive to the near-IR, there may 
already be many people taking IR images without knowing it.

My visual scope has the inside lined with velvet. When illuminated with a 
red light the velvet is the darkest part of the telescope to my eyes. But in 
the near-IR it is the brightest part! I discovered this while playing with a 
Meade NiteView scope. That uses a bank of near-IR leds for illumination. The 
light reflected from an object is focused by a lens onto a ccd. Electronics 
convert it to visible light on a lcd, which is viewed with an eyepiece.

I have no way of explaining why velvet can be dark in red light and much 
brighter in the near-IR. But if I was building an IR camera, I would take 
that into account. Even a visible-light camera with velvet inside should 
probably use an IR-blocking filter.

Have fun,

John




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