I was speaking with Kevin some months ago about astrophotography, and he mentioned that his camera, an Orion StarShoot Solar System IV, did not seem to be working correctly. The two problems that he mentioned were that he could not see the entirety of the moon in a single frame, and that he could not see Jupiter at all. I surmised that there was nothing wrong with the camera, and I explained that video-type planetary imagers like this had notoriously small sensors that have a very narrow field of view. Precisely aligning the telescope to cause Jupiter’s image to fall on the small sensor would therefore be difficult, and this would explain why only a small portion of the moon would be visible. He loaned me his camera for testing, and I have concluded that I was correct about the nature of his problem.
I tested the camera on my C11, and obtained the results seen in the images below, which seem so confirm that the camera was working. As I began to write up the test, I learned that the camera’s sensor format was 1/3-inch. To put this fact into perspective, 6 of these sensors arrayed side by side would be required to span the entire width of the 2-inch rear aperture of the telescope. In other words, the sensor is only 16% as wide as the aperture, which explains why Kevin would not have seen Jupiter in the sensor frame unless its image was almost perfectly centered in the rear aperture.
The focal length of the C11 with a 2x PowerMate is 5600mm. This results in a .14 x .14 arcsecond field of view on each of the camera’s 3.6 x 3.6 micron pixels. I set the camera to a 640 x 480 mode to improve the frame rate, thinking that I was selecting a smaller region of interest, or a subset of the sensor’s pixels at the center of the sensor. What I did not realize is that this binned the output into 2×2 arrays of pixels that would be output as a single 7.2 x 7.2 micron pixel. This had the effect of halving the resolution that the camera was capable of producing. I would like to be able to repeat this test in the 1280 x 960 mode.
Before mounting the camera on the telescope, I wanted to make certain that Jupiter would be visible on the camera sensor by using eyepieces to center up Jupiter in the field of view. I started with a Panoptic 35mm, which I found very difficult to determine if Jupiter is centered. This is because this eyepiece’s exceptionally wide field of view made it impossible for me to see two opposite sides of the visible field at the same time, so centering at this stage was very much an estimation. Next, I replaced the 35mm with a 17mm Nagler, and repeated the process. It was better, but it was still somewhat difficult to determine if Jupiter was centered with this eyepiece. Next I put in a 12mm crosshair eyepiece, and was able to center up perfectly. When I replaced this eyepiece with the camera, and brought it up on the laptop screen, Jupiter was centered perfectly. Finally, I put the 2x PowerMate in the optical path, and only needed to adjust slightly.
On March 30, 2015, the night of the test, Jupiter’s apparent diameter was 41.6 arcseconds. I used PhotoShop to measure the diameter of the image produced by the combination of the optical configuration and the sensor. The measurement across the center of Jupiter’s white equatorial band was 142 pixels. This compared favorably to the computed prediction of 154 pixels. I attributed the difference between the computed and actual values to me being less than completely careful in taking the measurement and the limb darkening on the disk of Jupiter. Being able to predict the image size increased my confidence that Kevin’s camera is working just fine.
Just a few words about the clarity of the images as I wrap this up. As I previously mentioned, the resolution of the images could have been improved by using a 1280 x 960 setting instead of 640 x 480, thus doubling the resolution. The night was fairly still and dry, so the “seeing” probably did not affect the image quality very much. Focus seems to be very difficult to achieve while watching video stream off of the sensor, so that could have been a factor affecting image clarity. I made no attempt to rule out focus as a problem by capturing several frame sets while slightly changing the focus each time. And lastly, I believe that the telescope might have some issues, which I believe can be ruled in or out by eliminating focus as an issue.
For good measure, I did a couple of captures on the moon. Clarity, seems to be an issue here too. This did demonstrate that the entirety of the moon’s surface is not likely to be visible in Kevin’s telescope. Note that in the Plato photos, adding the 2x PowerMate magnified the blurriness.
All of the images that follow were lightly processed. I stacked the frames in Registax, and then I adjusted the curve and levels in PhotoShop.
© James R. Johnson, 2015.