What is really BLACK?
|Image taken with MX7C of three different
visually "black" materials under a standard UK mains lamp - tungsten
filament approx' 2950C colour temp. (all 3 appear black to the naked
eye!). Image is out of focus but that's not critical to the test.
Left: 220GSM storebought "Black card"
Middle: Black velvet.
Right: Black felt.
|I decided to build a custom "Dew
cap" to reduce the effect of light pollution (see
Site). I was going to use a simple lightweight card tube fitted to
heavyweight card reinforced support rings at each end. The mounting end
reinforcement acting as a fixture to the 'scope.
However after some experiments I was getting no improvement in CCD contrast so I started testing the materials used:
The visual appearance of materials is not a real guide to how they are viewed by the MX7C camera. All CCD imaging systems are sensitive to near infrared radiation (thus the need for an infrared filter to achieve "real" colour balance). The human eye is not at all sensitive to this near infrared radiation so materials appear very different when viewed with the two systems.
The black card (left) and black felt (right) both reflect a significant amount of near infrared. (the card almost 70%) So although they would appear to improve contrast for visual observation the effect on CCD imaging would be to potentially increase the near infrared light pollution! The only material natural or painted (with several brands of so called black paint) that I found to be truly black in both visible and near infrared was the velvet.
I lined my dew cap with the truly black velvet. This not only reduces the light pollution problem and also reduces the dewing effect on the corrector plate. (The moisture is absorbed by the velvet.) I have also fitted a heating system in the base of the dew cap (a ring of low value 0.5 watt resistors connected to an adjustable 12 volt source.) for those really damp nights.
The final "Dew cap" is a simple 150gsm card tube with velvet fully covering the inside down to the level of the 'scope fixture. To avoid separation of the card and velvet when made into a tube the two were bonded together whilst wrapped around a cylinder matching the final required diameter. The reinforcing ends were made in a similar way, except the first layer of the end that fixed to the 'scope was wrapped around the corrector plate mount, to provide a good fit.
I made an end cap to protect the system and for dark frame exposures, body and support ring are made from heavy duty card (approx' 1.5mm thick). The end cap incorporates a Hartmann mask (the three 40mm holes) and an internal blanking plate to cover the holes. This is held in with velcro so can readily be removed and replaced.
The final "Dew cap" is visible on the Site page.