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Camera system

Ideal camera for Asteroid Photometry.

The concept of using a CCD camera for asteroid photometry is not as easy as it may seem. So many different types of CCD chips are available in the market used by different manufacturer. Without going to details of the different make of camera, I am intending to potray the reasoning behind the final selection of the best camera CCD chip for the asteroid photometry.
The problem lies in the movement of the asteroid during an exposure. A problem that limit the exposure time per image acquisition. if an exposure is too long, the asteroid will create a streak in the final image causing a reduction in Signal to Noise Ratio.
Also the price matters in the decision of which camera to purchase. The best CCD chip without doubt is the back illuminated version. This chip since it is thinned, the light will not need to go through the thick substrate used for the gate structure that causes loss of light. the QE invariably up around the 95% mark. The price however is 3-4 times deared that its Front illuminated counter part for the same dimension. The point though is that the Chip size is secondarty to its sensitivity when it comes to Asteroid photometry or stellar photometry.
So the objective is to find the best bang for bucks in asteroid photometry camera in the market.



Figure 1
U16M apogee camera.
Images were 5 minutes exposures at - 40C.
Clear filter
Bin 1x1 image
KAF16803 ME chip.
ABG

The image is measured in Canopus V10 software. The resultant SNR is shown on the bottom row. The mark show the value of SNR to be 26 at an instrumental magnitude of 16.56 compared with the sorrounding stars. No colour band magnitude was done yet but this will give an idea of comparison.

The test is done by taking the same exposure time consecutively with different Bin setting and similar atmospheric condition using the same telescope for that range of bin setting. The images are then measured using Canopus photometric software to obtain the Signal to Noise Ratio (SNR). This value is later listed and compared with accordingly. Finally any differences will be tabulated and concluded with different prices from different manufacturer.

To achieve that, we will need to review the different physical properties of a CCD.
A good reference is found here..
For simplification, the following is the list of properties of the ccd that is relevant.  These physical features are those that can effect the senstivity of the chip.
1. The larger the pixel size the deeper will be the well depth.
2. With anti blooming gate, the smaller will be the size of the pixel well depth because a small portion of the pixel area is being used to provide buffer for the charge to bleed. The chip should be a non anti blooming type for not allowing any sensitivity reduction.
3. If possible, always select the back-illuminated CCD chip rather than front-illuminated type. The Quantum Efficiency (QE) is much higher for the Back illuminated CCD.
4. Binning does not necessarily increase sensitivity as can be imagined since increasing the bin size means that the pixel size will be multiplied by the number of the bin. So the area will be quadrupled as the bin size is increased. However this does not quadrupled the well depth as can be envisaged if we follow the logic shown in point 1 above. In reality, the sensitivity does not change at all and it even reduce some what. this is applicable to the KAF 402ME, KAF 1603 ME , KAF 261 ME and the KAF 16803 ME I own.
5. Microlens chip will improve the sensitivity somewhat.

Apogee U16M
5 minutes exposure at - 40 C. Data reduced with Bias, Dark and flat frames.
Bin 2x2 image

The test is done by taking the same exposure time consecutively with different Bin setting and similar atmospheric condition using the same telescope for that range of bin setting. The images are then measured using Canopus photometric software to obtain the Signal to Noise Ratio (SNR). This value is later listed and compared with accordingly. Finally any differences will be tabulated and concluded with different prices from different manufacturer.

Bin 3x3 image.

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