Field flattener comparison for Equinox 80ED

[willis]

Picking 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.

[社長]

Willis, 謝謝分享，這個測試甚具參考性。

你的測試和我之前的相反(TS2優於SW0.85)，但沒問題的，稍後我們一起深入理解一下兩個結果異同原因。
viewtopic.php?f=5&t=123

想知道TS2的圖像如何?

另外那個Focusing Dial很有用，可說是廉價版的量化調焦系統了，配合魚骨板便可以系統性地確保調焦至10微米精度了，很接近昂貴的羽毛調焦的精度了。若有多餘一片的話，我很想要......

[willis]

Willis, 謝謝分享，這個測試甚具參考性。

你的測試和我之前的相反(TS2優於SW0.85)，但沒問題的，稍後我們一起深入理解一下兩個結果異同原因。
viewtopic.php?f=5&t=123

想知道TS2的圖像如何?

另外那個Focusing Dial很有用，可說是廉價版的量化調焦系統了，配合魚骨板便可以系統性地確保調焦至10微米精度了，很接近昂貴的羽毛調焦的精度了。若有多餘一片的話，我很想要......

What TS2? I only have SW 0.85X & TS on hand...
PS* If I have time, I can make u one focusing dial.

[社長]

The one you have on hand is TS2 (2" version), it peform differently from TS2.5 (2.5" version, for 2.5" Feather Touch Focuser).

any spot result for the TS2?

FYI, TS2 perform very good on my APM-LOMO 80/600, but not the TS2.5

[willis]

any spot result for the TS2?

Click the "Test Result" photo & u will see the TS2 result.

[Henry Wu]

Hello Willis 謝謝提供寶貴測試資料
看起來　Skywatcher 0.85X reducer效果也不差
社長:　那個Focusing Dial 是否可於香港買到
若可以購買到請先幫我買五個
先謝謝 帳再一起結

[funoooo]

Good test.
What's the price for Skywatcher field flaatener?
One more problem, It is only for Skywatcher telescope, not for the other brand scope. Is it right?
What's the price?

[社長]

Good test.
What's the price for Skywatcher field flaatener?
One more problem, It is only for Skywatcher telescope, not for the other brand scope. Is it right?
What's the price?

it's an art of mix and match.....

~HK$2,000

[社長]

Hello Willis 謝謝提供寶貴測試資料
看起來　Skywatcher 0.85X reducer效果也不差
社長:　那個Focusing Dial 是否可於香港買到
若可以購買到請先幫我買五個
先謝謝 帳再一起結

那片是Willis自己造的，是非賣品了....

Equinox 的慢速調焦鈕每一轉是1.2mm左右，以120格刻度來說，最細可以讀至1/2格，即可分別解調焦精度至5微米，是Feather Touch的水平的。

配合魚骨板，以Live View人眼判別調焦後，再在正負5個單位 (每單位半格即5微米) 拍共11張照片，對準亮星每張曝光5秒就可以，然後從記憶體內以最大放大率播放影像，只要視寧度不太差，你便可以找到最佳焦點位，若每次拍星可以花這一點功夫在調焦上，你的照片解像度會更上一層樓的。

當然若望遠鏡並非最高端攝星儀，自由組合的平場鏡後仍有少許場曲，那或許用軸外一點距離去判別焦點會合理些 - 理由是增加可用視場。

[willis]

Hello Willis 謝謝提供寶貴測試資料
看起來　Skywatcher 0.85X reducer效果也不差
社長:　那個Focusing Dial 是否可於香港買到
若可以購買到請先幫我買五個
先謝謝 帳再一起結

My testing result is different from 社長. May be 社長 & I need to find out what is going on. But what I can say is that for APS size DLSR, the difference between Skywatcher 0.85X & TS is small.

[willis]

Good test.
What's the price for Skywatcher field flaatener?
One more problem, It is only for Skywatcher telescope, not for the other brand scope. Is it right?
What's the price?

One can use different flatteners on different telescopes but adaptors are needed. As 社長 mentioned it is an art of mix & match. After I go through the recent tests, I found that the original flattener provided by the same manufacturer only guarantee “good performance”, but not necessarily “excellent performance”. See my Tak FS60C flattener test. viewtopic.php?f=5&t=244 Look at the performance of upper middle border, the star is obviously elliptical. I cannot find the spot diagram of the Tak zero power flattener in the Internet. But one can find easily the spot diagram of Tak F4.2 reducer/flattener. This is one of the example: http://xmcvs.free.fr/tests/red_taka.htm From the spot diagram, the coma at the 21mm is strong and the spot size at 21mm is much bigger than that at 0mm. (Note: 21.6mm is the distance between the corner and the centre of a full frame size DSLR) . The relative size of the spot diagram agrees with the relative size of the stars in my test. Well, even flattener is applied, the extreme edge performance is ok. This has been pointed out by universe24. See this link: http://www.hkastroforum.net/viewtopic.php?t=18638

PS* I found that there is distortion at the extreme corner too. Look at the shape of the box of my artificial star in the photo.

[社長]

Hello Willis 謝謝提供寶貴測試資料
看起來　Skywatcher 0.85X reducer效果也不差
社長:　那個Focusing Dial 是否可於香港買到
若可以購買到請先幫我買五個
先謝謝 帳再一起結

My testing result is different from 社長. May be 社長 & I need to find out what is going on. But what I can say is that for APS size DLSR, the difference between Skywatcher 0.85X & TS is small.

我會這樣看，兩個是針對不同要求的，TS2是等焦的，即以Equinox 80維持500mm拍攝。

而Sky Watcher 0.85x則把Equinox 80縮至425mm拍攝，光圈大了，但受制於調焦筒內徑及平場減焦鏡，全照明像場也縮小了15%，週邊減光也明顯了。

至於兩個組合像質如何，某程度上我看是在同一檔次的。