Re: [ATM] How dark is dark?

[John Sherman <atmer@johnspics.com>]

Hi All,

Ken,

> Given the same emulsion, a matte  black surface will read about
> 1.7 while a glossy black surface will read 2.0.

This is to be expected. Now try the same glossy surface with the light
reflecting directly into the sensor. Using a ring light on a glossy surface
means the light is reflected back onto itself, instead of into the sensor.
The matte surface will scatter more light into the sensor.

> Basically this says a glossy surface will  appear much darker
> to the eye.

If you shine a single light onto both surfaces, and measure the direct
reflection, the glossy will of course be much brighter. This is because the
matte has scattered much of the direct reflection in other directions. And
so now the matte will appear darker.


Nils Olof,

> So why not make the parts of the OTA visible from the focuser
> glossy black and placed as to

You said this years ago. And I tried it. And I like it! Having a matte
surface visible from the focuser means that light will be scattered into the
eyepiece. Better to have glossy black there, so that the light moves down
into the tube where you can kill it. Of course, this is only comparing paint
to paint, I still like velvet better.

> the problems of the upper tube assembly, i.e. the light that may
> enter the EP from areas outside the secondary. I would still need
> to blacken the area around the primary visible via the secondary,

I look at it this way. The most important part of making the field dark is
light entering from around the secondary. The second most important thing is
light reflected from the secondary. This can include light from the observed
object which is scattered by the mirrors and becomes veiling glare as it is
no longer focused correctly. Yes, by all means blacken around the primary,
but down there one would use matte (as you said before).

> but before I do that I would follow Galileo's example of actually
> finding out how things work

Wonderful!

John






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