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How Sensitive is the image sensor used in the Starfish camera? To answer this question, we setout to image a known star field with one of the prototype cameras. We imaged the bright star 'Caph' in the constellation Cassiopeia since it has some fairly dim stars nearby. Exposures of from 1sec to 30sec were taken. The telescope used was a Takahashi FSQ-106 refractor. Only an IR Block filter was used in between the starfish camera and the telescope optics. The images below were dark frame subtracted and then contrast stretched for display. Only a single frame was taken at each exposure setting. The images were scaled down to 683 x 512 pixels for the web page. Remember that the full size images are 2047 x 1536 pixels!

Click on each thumb nail image below to bring up the larger image. I have annotated some star magnitudes in the frame for your reference.

1 Second
2 Seconds
5 Seconds
10 Seconds
20 Seconds
30 Seconds
The image sensor used in the starfish camera also has an analog gain stage in between the pixel array and the analog to digital converter. All of the above pictures were taken with the gain set to a nominal value of '8'. Higher gain settings will result in greater intensity levels but at the expense of increased noise in the resulting image. To illustrate this increase in signal level we have taken a second image with an exposure setting of 2 seconds but with the gain set to '40'. This is an increase of 5x over that used for the 2 second exposure above.
2 Seconds, gain = 40
Starfish camera with Micron MT9M001 image sensor:
We were able to test a version of the starfish camera with the Micron MT9M001 image sensor. This sensor is a monochrome imager with 5.2u pixels. The quantum efficiency is 56%. This is more than twice as sensitive as the MT9T001 image sensor. The spatial resolution is 1280 X 1024 pixels but since the pixel size is 162% larger, the optical format is the same at 1/2" giving the same filed of view as the MT9T001 image sensor.
Starfish monochrome image sensor
Starfish monochrome image sensor
2 second exposure
30 second exposure

We used a modest telescope objective size of around 4" at f/5 for the sensitivity tests. This is because most guide scopes will not be much larger than this. Exposure settings for guiding will ideally range from 1 to 5 seconds in order to give the guider software frequent enough guide star position updates. In this exposure range, stars of magnitude 8 or less are readily visible and should provide a suitable star image for guiding. Increasing the image sensor's gain setting can provide additional signal level intensity. The resulting image noise would not interfere at all with guide star position calculations.

Tests with the more sensitive MT9M001 image sensor were very promising. The sensor has a maximum QE of 56% and would provide a broader selection of suitable guide stars in guiding applications. We are currently planning to offer this image sensor in a version of the starfish camera.

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