Webcams for Astronomy - brief summary of modifications SC1 and SC3
For the last few years webcams have provided a very cost effective way of starting in astrophotography. Gone is the need for expensive equipment.
The mainstream use of webcams in astronomy is for planetary and lunar imaging. However, there is a hardcore of users who make hardware modifications
to their webcams to allow longer exposures - enabling them to capture some of the brighter deep sky objects. Some people even get the fainter ones too. ;-)
I often get asked about the various types of modifications. The notes below summerise the various stages.
There are several stages of modification.
- The best webcam to use is the most expensive offering from the philips toucam range with a CCD, NOT CMOS!!! The CMOS types are useless for anything but the Moon so avoid them. A philips toucam will be superior to a Meade LPI.
- Basic modifiction to control shutter with parallel cable. (SC1). This allows software like k3ccdtools to make exposures of any duration you like with the webcam. Doing this modification requires a soldering iron and a steady hand. The limiting factors are as follows:
- After 30 seconds or so the glow from the on chip amplifier will start to ruin your image.
- The chip is colour and less sensitive than a black and white.
- The pixels are very small and unsuited to DSO work on many telescopes.
- Mod to control the on-ccd-chip amplifier to reduce (or remove) amp glow.
There are a lot of different ways of doing that. (SC1.5). I think my direct bias techinque is the best.
- Replace the colour ICX098 ccd chip on the webcam with a black and white
ICX098 chip. Straight swap. Artemis is the best place to get the CCDs. A long exposure toucam with a black and white 098 chip makes a great little guidecamera.
- Replace the ccd with a ICX424 chip, which has the same number of pixels, but
they are bigger (7.4um intead of 5.6um) hence bigger FOV. The icx424 has more
legs than the 098, so the mod is a bit more fiddly. 424 chip available in
colour or b+w. Using a colour one is a bit pointless.
- Stick the whole thing in a new box and make a fan blow into
- Use a peltier to either cool a cold chamber or a cold finger that
communicates with the ccd chip. This is tricky as the cold parts have to be
airtight to avoid condensation/ice. Make a nice anodised case.
- Modify the webcam to read interlaced frames to allow guiding with the same
chip (like SX do). Nobody uses this mod these days however.
- Modify the webcam to allow 2x1 binning. Again, more hassle than its worth,
nobody does this.
- Swapping the ccd for an ICX414 - which has even bigger pixels.
- Modify the webcam firmware into RAW mode.
The "homepage" of the webcam mods is http://www.pmdo.com/wintro.htm
These mods are designated by their SC number. SC = Steve Chambers who invented
the whole idea.
A table with the different designations is found here...
My camera is an toucam 840 with a black and white ICX424 sensor, long exposure
modded, Tom-modified martin burri amp off system supplemented by the
Tom-invented direct substrate bias injection modification. Its also peltier
cooled sometimes, when i have all the leaks plugged and enough dry dessicant to
hand. The peltier cools the ccd direct with a copper cold finger, which
keeps the lead length very short. As a result its one of the lowest noise SC3s
out there. Running in RAW mode.
The guide camera is a long exposure modified toucam 840 with a black and white
ICX098 sensor (for better guiding image scale) and RAW mode.
The finderscope camera is a totally untouched toucam 840. Although I might have
smashed the IR filter off. Can't remember.
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