Using dithering with LVI II

[willis]

I bought the LVI II for some time, but there is not much time to explore its functions. Usually, the weather and my works are the killers. They just ruin my sky time. Anyway, I tried to experiment the dithering last night. I went to my school's roof and did the test for 45 mins and rushed back home. I just mounted the guide scope to the HEQ5Pro and collect the 40D to LVI II via the LVI shutter cable. I didn't do real imaging, but just see if it works. At 6:15pm, the sky was not very dark. The only two stars I could see were Vega & Altair. What a lovely pair! I pointed the guide scope to Vega & LVI stared to guide with dithering.

If dithering is turned off, the guide star will stay at the same point on the sensor. Under ideal condition, the guide star will stay at the same point on the sensor through out the guiding, but so as the hot pixels, bright pixels, dead pixels etc. This is very hard to remove these defects in post-processing. When the dithering is enabled, LVI II will search the guide star after each exposure again. This will result in a shift of the guide star on the sensor. Those defects, hot pixels, bright pixels & dead pixels etc will move around and the corresponding defect image can be easily removed in post-processing.

Before the guiding with dithering, I need to set the exposure time for each frame, delay time, interval between subsequent exposures etc. When everything set, off we go!

There is one drawback of dithering is that it takes about 2 more mins for LVI II to find (to displace) the guide star and load the calibration again. So the profit is higher if one images longer exposure for each shot. Since this is the first time, I tried the dithering. I don't know how fast other guiding systems are. Maybe 2 mins is already a reasonable or fast dithering.

[社長]

Dithering 對於降噪有明顯的功效，月前在Astronomy雜誌中，Tony Hallas的文章也說得很清楚，你看我在西藏天文台的照片，做與不做也有明顯分別。

而且做了dithering，疊加時再會drizzle，便可提升影像解像度。

LVI II的就是因為提供了單反操控，才能實現這步，SG4就不可能了。

LVI II做dithering的最低成本要每幅照片2分鐘也不算太長，一來沒有其他不用電腦的自動導星器能實現單反的dithering，根本沒法比較，另外以冷凍CCD拍攝時做dithering，例如以Paramount赤道儀及STi配合CCDSoft運作導星，軟件也建議每幅照片之間預留1分鐘時緩讓導星器把導星推至另一位置，有電腦操控的尚且如此，獨立運作的LVI實在很成功了。

[funoooo]

Dithering 對於降噪有明顯的功效，月前在Astronomy雜誌中，Tony Hallas的文章也說得很清楚，你看我在西藏天文台的照片，做與不做也有明顯分別。

而且做了dithering，疊加時再會drizzle，便可提升影像解像度。

LVI II的就是因為提供了單反操控，才能實現這步，SG4就不可能了。

LVI II做dithering的最低成本要每幅照片2分鐘也不算太長，一來沒有其他不用電腦的自動導星器能實現單反的dithering，根本沒法比較，另外以冷凍CCD拍攝時做dithering，例如以Paramount赤道儀及STi配合CCDSoft運作導星，軟件也建議每幅照片之間預留1分鐘時緩讓導星器把導星推至另一位置，有電腦操控的尚且如此，獨立運作的LVI實在很成功了。

非常之好！ Willis, 你要教我如何運作！
另社長, shutter 線要多少錢？ 50D 及 450D 那種。
這樣看來 LVI II 比 LVI 強得多， LVI 就........... 重新考慮！

另外

疊加時再會drizzle，便可提升影像解像度。

不明白，可否詳細點。

題外話，我剛用 LVI II 做 Guiding。發現 600sec 時有部份相拉線，900sec 時就全部拉線。
會不會 guiding scope's focual length 太短而導致，或是 LVI guiding 時間上限只有 10min。
謝謝解答我這個蠢問題！

[社長]

價格在此： viewtopic.php?f=9&t=110

拉線我也看不出跟LVI有什麼關係，在Astrocafe我已寫過不下一萬字。

以我理解沒有多少人用單反拍星，會用上dithering功能，而用電子快門線控制單反也很方便，在那情況下LVI I其實已滿足大部份的要求，因為運算簡單些，操作起來比LVI II暢順。

[willis]

非常之好！ Willis, 你要教我如何運作！
另社長, shutter 線要多少錢？ 50D 及 450D 那種。
這樣看來 LVI II 比 LVI 強得多， LVI 就........... 重新考慮！

The dithering in LVI II is simple. Basically plug & play. One just needs to set a few parameters, say exposure time, no. of shots, interval between exposure etc..
As u have LVI II already, if u want to buy one more for both HK & mainland use, go for LVI I. It is because LVI I is more responsive & cheaper.