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Astrophotography with the Pi astro-cam

Pi astrophotog


The Pi camera

The Pi camera uses the OmniVision OV5647 CMOS sensor, which is similar to the sensor used in the Microsoft LifeCam Cinema and Studio webcams (which are also used for some entry-level planetary imaging). However the Raspberry Pi camera is slightly different from the typical webcams.

First, the sensor used in the Pi camera module has very small pixels = 1.4 micrometer (v's 5.6um for the DMK/DFK/DBK21 or Philips SPC900NC).
The Pi camera Mk2.1 uses the 4yr. old Sony IMX219, which has even smaller pixels at 1.12m sq. however is claimed to be better in low light conditions (not that you have any choice, since you can't get the 5Mp version any more)
Small pixels means high resolution at 'fast' focal ratios (techo. speak for 'it needs a lot of light' = wide aperture) and thus it has a rather shallow depth of field (techo. speak for 'it has to be accurately aligned and is hard to focus').
While the typical 5.6 micrometer pixel based camera would operate at around f/20 (typical refractor telescopes), the Raspberry Pi camera requires f/5 or better (typical with Newtonian reflectors).
One way to increase the f/ number of your telescope is to fit a 'focal reducer' - for example the typical x0.5 reducer halves the focal length and reduces the f/ number to about 2/3rd (so f/20 becomes f/13, f/10 becomes f/6.5)

The Pi camera has one of the smallest pixel sizes of any camera you can find, which is a real advantage when it comes to auto-guiding and planetary imaging

In theory, when imaging the planets, you 'only' need one 'web cam' (video camera) since the video frames can be used by the auto-guider s/w as well.

One really useful site for CCD related formulae is Wilmslow Astro


Driving the Pi camera


Access to the Pi camera is via the closed source GPU code, which means 'you get what you get'. However the API is quite 'low level' and it's possible to control a whole range of parameters (even to the point of accessing the camera image data via multiple 'streams' - so, for example, it's possible to extract still images at the same time as a video stream - or thumbnails at the same time as a full resolution still).

Even so, some things can't easily be changed - for example, when you switch 'mode', there is a 'shutter delay' of some seconds as the camera 'auto-white-balances', and whilst 'binning' is supported it's done 'automatically' (if you drop the res. to 1/4, the data is 2x2 binned).
Further the max. amplification that can be applied is ISO 800.
Finally, the longest shutter speed (i.e. max. exposure time) is 6 seconds (which may keep thermal noise down to acceptable limits but is next to useless for deep sky imaging (even a consumer DSLR can manage 30 seconds .. although the Mk2 (8M pixel) Pi camera max. exposure time is reported to be 10s

Mounting the camera

Typically you will want to continue to sue your telescope visually, which means fitting the Pi Astro-cam with an 'eye piece'. An adapter made from a 35mm film canister will plug straight into most inch and a quarter focus assemblies. The main problem will be in achieving focus.

Typically, Dobsonian (and other Newtonian OTA's) have a problem 'focusing in' far enough, whilst on a Refractor it's often impossible to 'focus out' far enough for the camera.
To adjust the camera 'further out' for a Refractor is easy enough = for example, you could use a 90 degree mirror/prism to increase the optical path, or fit a short 'extender' tube.
To adjust a camera so it's 'further in' for a Newtonian is harder. If your focus assembly is theaded, one trick is to add a Barlow lens (which has the effect of moving the focus point further out up the tube) = or, if you have a 2" focuss assembly, mount the Pi camera on it's own inside (part way down) the 2" focuss tube (if it's 1.25" you are out of lick = the Pi camera is just slightly too big to fit within a 1.25" tube)

The next problem is that (unless you have a machined adapter) the tiny Pi camera CCD sensor will not be in the exact center of the field of view (although this can be an advantage as twisting the eyepiece will move what's in the cameras field of view).

The easiest way to focus is to 'aim at the Moon' = this guarantees that 'something' will be in the camera field of view.
A nice bright Moon means you can run the camera in 'movie mode' and manually adjust the focus whilst looking at the (local) display screen.
Once you have focus it's a 'good idea' to mark the position (for nights where there is no Moon :-) )

For more on drivng the Pi camera, the V4L2 driver 'usage examples' are a good place to start

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