Author: Michael Wong
Translator: Yilia Yu
The focal ratio that allows planetary imaging to achieve the optimal sampling rate is five times the camera pixel size in micrometers. In the past, camera pixel sizes were approximately around 3-5 micrometers, resulting in optimal focal ratios between 15-25. Even when using a native F/10 Schmidt-Cassegrain Telescope (SCT), a Barlow lens was necessary. However, now cameras with pixel sizes of 2 micrometers or lower have emerged in the market, leading to optimal sampling rate focal ratios of 10 or lower. This allows us to directly capture planetary images using an SCT or even an F/7 refractor telescope at their native focal points.
I have received these two cameras for quite some time, but the weather in Hong Kong has been unfavorable. Yesterday evening, we had a rare clear sky, and I conducted a preliminary test of the two cameras: the QHY5III678M (2-micrometer pixel size) and the QHY5III715C (1.45-micrometer pixel size)
The QHY5III678M performs exceptionally well in terms of signal-to-noise ratio in the visible spectrum, definitely surpassing the QHY5III290M and QHY5III462C. The initial impression of using it with an IR685 filter is also very good. I will not try it with a methane filter until the Jupiter rise higher. Its frame rate is not as high as the QHY5III290M or QHY5III462C, and is similar to other QHY cameras with a Type-C interface. It can reach approximately 100 FPS on my 6th generation i7 laptop with a 700 x 700 ROI. The overall impression of the QHY5III678M is no less than, or even better than, the QHY5III200M. Combined with the advantage of not requiring a Barlow lens when using an SCT, it is undoubtedly a winner. It will become my primary monochrome camera.
The QHY5III715C is a pleasant surprise. It is extremely affordable, yet its sensitivity is comparable to, or even better than, the QHY5III462C. The camera has an IR-blocking window, so I haven’t tried IR imaging yet. I will replace the window later and attempt IR imaging. If you are looking for a planetary imaging setup without the need for a Barlow lens or a filter wheel, this is definitely a good choice. Although I don’t have a QHY5III678C, I have a camera of the same model from another manufacturer. It seems that the color version of the 678 performs better in terms of signal-to-noise ratio, but considering the price and pixel size, the QHY5III715C is definitely the winner.
The New Era of Planetary Imageing without a Barlow Lens
After a long period of technological development, we have now entered a new era of planetary photography that no longer requires a Barlow lens. This will greatly enhance the convenience for astronomy enthusiasts in the field of planetary imaging. In the future, we have every reason to anticipate the emergence of more cameras with smaller pixel sizes and higher sensitivity, providing us with even more exciting astronomical photography experiences.