Field flattener comparison for Equinox 80ED
發表於 : 2010-04-29, 17:06Picking the right flattener is not easy especially when there is no stock flattener provided by the same manufacturer of the telescope. One needs to match the telescope and flattener. It is because flattener A works well on telescope X, it doesn’t guarantee the performance on telescope Y.
I tested the Skywatcher 0.85X reducer/flattener and the TS zero power flattener on Equinox 80ED. I did the test in physics laboratory. I used the RGB and white artificial star to perform the test. viewtopic.php?f=20&t=181
(Note: The green LED is not well aligning with the hole, so the brightness is weaker. I will fix it later.)
The testing detail is follow:
Telescope: Equinox 80ED
Distance between the telescope and the artificial star: about 15m.
Camera: Canon 5D II
ISO 1600
Shutter: 1/250 s
The focusing is critical in this test. I found that a bit off the focus, the image of the artificial star will be enlarged considerably. I used a DIY large focusing dial to manifest the focus. For each flattener I take about 20 sets of shots of different focuses. And select the best set for final comparison. The artificial star was placed at three locations in the photos:
1: Centre
2: Middle of upper border
3: Upper left corner
The Upper left corner image reflects the performance in full frame DSLR, while the middle of upper border also reflects partially the performance in the case of APS size DSLR. Note the distance between corner and the centre in APS size DSLR is about 13.4mm and the distance between the middle of upper border and the centre is 12mm.
The result shows that both flatteners improve the edge performance a lot. The stars are very round at both centre and the middle of upper border. However, both flattener over correct the coma at the extreme edge. The stars seem a bit smaller in Skywatcher 0.85X. I guess part of the reason is that the focal length is shorter in the case of Skywatcher 0.85X flattener. One interesting observation is that both flatteners reduced the chromatic aberration at the centre. Look the white star at the centre. This is unexpected.
The distance between the telescope and the artificial star is about 15m and this is practically not far enough. But I dare not to performance the test in the corridor as students may hit my setup. If I do the test in corridor, I probably need to do it after 7:00pm. Anyway, the test result could be useful for Equinox 80ED users.
I tested the Skywatcher 0.85X reducer/flattener and the TS zero power flattener on Equinox 80ED. I did the test in physics laboratory. I used the RGB and white artificial star to perform the test. viewtopic.php?f=20&t=181
(Note: The green LED is not well aligning with the hole, so the brightness is weaker. I will fix it later.)
The testing detail is follow:
Telescope: Equinox 80ED
Distance between the telescope and the artificial star: about 15m.
Camera: Canon 5D II
ISO 1600
Shutter: 1/250 s
The focusing is critical in this test. I found that a bit off the focus, the image of the artificial star will be enlarged considerably. I used a DIY large focusing dial to manifest the focus. For each flattener I take about 20 sets of shots of different focuses. And select the best set for final comparison. The artificial star was placed at three locations in the photos:
1: Centre
2: Middle of upper border
3: Upper left corner
The Upper left corner image reflects the performance in full frame DSLR, while the middle of upper border also reflects partially the performance in the case of APS size DSLR. Note the distance between corner and the centre in APS size DSLR is about 13.4mm and the distance between the middle of upper border and the centre is 12mm.
The result shows that both flatteners improve the edge performance a lot. The stars are very round at both centre and the middle of upper border. However, both flattener over correct the coma at the extreme edge. The stars seem a bit smaller in Skywatcher 0.85X. I guess part of the reason is that the focal length is shorter in the case of Skywatcher 0.85X flattener. One interesting observation is that both flatteners reduced the chromatic aberration at the centre. Look the white star at the centre. This is unexpected.
The distance between the telescope and the artificial star is about 15m and this is practically not far enough. But I dare not to performance the test in the corridor as students may hit my setup. If I do the test in corridor, I probably need to do it after 7:00pm. Anyway, the test result could be useful for Equinox 80ED users.
