Monthly Archives: June 2012

Solargraphy 2012

 

Solagraphy is the art of photographically capturing the path of the sun across the sky. As the sun takes a day to cross the sky, the exposure time for a solargraph is at least a day. To really capture the path of the sun as the seasons change (the sun climbing higher as we head from winter to summer, for example) we need exposure times tending towards the length of a season or two – three to six months. You’re not going take these photographs with your DSLR or point and shoot. You need the low sensitivity and long exposure times that come with a pinhole camera.

This is one of the results of my second attempt at six month solargraphy. It is not excellent, but does show the arc of the sun as it crosses the sky. It also show just how few uninterrupted sunny days we have had so far this year.

Dagenham Solargraphy

Dagenham Solargraphy

The photograph isn’t very sharp due to a combination of reasons:

  • Movement of the camera – keeping something fixed in position outside in the weather for six months isn’t easy.
  • Movement of the photo paper in the camera – humidity and heat and rain all cause the paper to expand and contract and move.
  • The size of the pinhole really should be about 0.3mm for optimum sharpness according to optical theory. I used a hole about 0.2mm larger and didn’t thin the material as well as I could first.

I think I shall start having a play with shorter exposure pinhole photography, to develop the skills and materials needed before I try another solargraph.

 

Playing with ImageMagick

For some Slow Scan TV (SSTV) experiments I want to be able to overlay text on images from the command line or within a script. The Swiss-army-knife of image manipulation is ImageMagick, so I’m using that.

Starting with a a photograph I took a few years ago, I resized it to 320×256 pixels – that’s the Martin-1 SSTV mode frame size.

An SSTV sized image I want to overlay text on

I can then use the following ImageMagick command to add some text in a shaded box.

convert  -fill white -background '#00000080' -gravity North -size 320x40 caption:'G7UVW SSTV' small-electronics.png +swap -gravity north -composite small-electronics-captioned.png

For now it just adds simple static text, but in production it’ll add dynamic data as well as static text, for example, computer load, camera pointing and RF subsystem data.

Captioned SSTV Frame