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A note on Raspberry Pi camera and astrophotography

Raspberry Pi camera have been released and some already tried to use it for astrophotography. The camera uses OmniVision OV5647 CMOS sensor. Similar sensors are in Microsoft LifeCam Cinema and Studio webcams. Those are used for some entry-level planetary imaging too. However the Raspberry camera is slightly different from the webcams.

The sensor used in the camera module has very small pixels - 1.4 micrometer (5.6 in DMK/DFK/DBK21 or Philipcs SPC900NC). That will result in high resolution at fast focal ratios. While a 5,6 micrometer pixel based camera would use around f/20 the Raspberry camera requires only f/5. If you have a fast Newtonian you don't need a Barlow. If you have an f/10-12 Maksutov or SCT - the image will be over-scaled (when camera has no lens). A focal reducer would be required. Second option would be binning (the sensor supports bin2), but I don't know if and how it's supported in the software (and is it real color bin2 or pixel bin2 which looses color). Third would be eyepiece projection imaging where long focal length eyepiece would reduce the resolution (the camera module has a lens that rather isn't removable).

As for imaging - it's a color sensor with an IR/UV built in. No extra filters needed. It also has a lens, which makes some problems. Ideal configuration would be without that lens - for prime focus imaging. When the lens is used (like for the published results) then you get a projection imaging and it's harder to predict the resolution you would get (but if you have an SCT or Mak it's one of ways to reduce the resolution on the sensor).

For Deep Space imaging (on long exposures) this sensor would be even harder to use. First there are the small pixels (only bright lenses or some very low resolution eyepiece projection would be good), next is the sensor performance. Such CMOS sensors are noisy so they would have to be cooled. Also doing flat and dark frames would be a must. Bias frames maybe too. In general very hard/annoying. Typical Canon would be probably a much better solution for this (and Raspberry camera used as a guide camera)...

You always need guiding. There is a chance that open-PHD could run on ARM and guide with the Raspberry camera via RS232 connection with the mount. Canon does the imaging, Raspberry does the guiding and no bigger computer would be needed.

Compared to dedicated planetary cameras (good machine vision cameras) the Raspberry + camera board is cheaper, but also less friendly to use. You can't use FireCapture or SharpCap for AVI capture. You will also need good storage for the captured clips or frames - either HDD on USB or bigger USB stick. Depending in which format/codec you save the movie clip the stacking applications (Registax or Castrator/Autostakkert on Windows) may have problems reading it. A set of BMP (or eventually TIFF) files would be a safe choice in this case (or an AVI in RGB24 or similar supported format/codec).

Note that some camera vendors like Point Grey provide Linux ARM support (don't know if ARMv6 is supported - needed for Raspberry, or only ARMv7 for newer processors). So in theory some USB or Ethernet (GigE) machine vision cameras could work on Raspberry and similar ARM-based mini computers.

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Astronomy and Astrophotography, 4 June 2013, Piotr Maliński

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