Field flattener test for Takahashi FS60C

[willis]

It is reported that the edge performance of Tak 0.72X reducer/flattener is not very good when compared with the Tak zero power flattener for FS60C. http://www.hkastroforum.net/viewtopic.php?t=18638
As a matter of fact, the focal length of FS60C is just 355mm, there is no need to reduce it for me. So I choose the Tak zero power flattener for my FS60C.
This time I did the test in corridor. I used the same artificial star which I used in previous Equinox 80ED test to perform the test. viewtopic.php?f=5&t=243&p=1753#p1753
The procedure is the same.

The testing detail is follow:
Telescope: Takahashi FS60C
Distance between the telescope and the artificial star: about 36m.
Camera: Canon 5D II
ISO 1600
Shutter: 1/125 s

Before applying the flattener, the edge performance is bad. The imaging circle is very small. It shows strong coma. Without the flattener, the edge performance just ruins the photo. After applying the flattener, the edge performance improved. However the stars are not very round even at the middle of the top border. This means even for APS size DSLR coma is detectable. In addition, the image at position 3, upper left corner, shows distortion (See the "Test Result (100% Chop)" photo, locations) . However, there is a bonus of applying the flattener is that the contrast improved a lot. This is quite obvious from the photos. As a conclusion, without the Tak flattener, FS60C is a visual telescope only. If you really want to do image with it, a flattener is a must buy item.

[社長]

謝謝Willis很清晰地展示了平場鏡的作用。

我至今仍認為，業餘愛好者的最佳方案，是選一台頂級目視APO，拍攝時再配個平場鏡。

吹毛求疵的看，這個組合或許跟頂級攝星儀如Riccardi Honders等的解像度有點分別，但在正常的視寧度下，APO+平場鏡的照片質量，足夠讓絶大部份滿意了。