www.astrocafe.hk

Hi all,

Photo for sharing, took on 3 Aug 2010 at Yuen Long.

10 inch F4,7 newtonian + Paracorr, V4 filter, ST2000XM.
L 120s X 28, R 120s X 2 ,G 120s X 2 , B 120 X 3 , RGB 2x2 binning.

Guide by Vixen 80s + 2x with PHD.
Forgot to take flat ...

Ben

Hello Ben, 您的孩子什麼時候光臨?

我在這裡的所有照片都沒有Dark沒有Flat的.....

With post processing, flat field and dark frame etc are just like a lady wearing make-up. Otherwise, it is a lady wears no make-up. Anyway, it seems that there is CA. Is it comes from the corrector?

從各星有不同方向的色散看，那些應不是光學系統色差，或許做一下RGB Align會好些，我猜可能包括以下原因：

1. 大氣色散，因為M8的仰角不高，去年拍木星也會有這現象，而混入L通道後是拆不開的

2. 可能導星在視場外，或極軸未完全對準，有少許場旋，所以拍RGB的期間星點已有偏移

這只是我猜想，Ben會清楚一點，但肯定是後期處理可以大幅度解決的，這幅照片也展示了1200mm焦距應有的解像度。

willis 寫:With post processing, flat field and dark frame etc are just like a lady wearing make-up. Otherwise, it is a lady wears no make-up. Anyway, it seems that there is CA. Is it comes from the corrector?

Flat field is a must in CCD photography , it can eliminate the shadows of dust on the image and correct the vignetting of the setup .
There should be no CA for the Televue comacorr (Badder coma corrector has tiny CA and also spherical aberration and I am not using it in CCD photography). The alignment of the frames is not that great but I don't know why everytime I use PHD in guiding , the star in each frame is round but the field shifts a bit between frames . So after 1 hour L frame captured, the field shifted quite a lot..... There is no such problem if I use the self- guider or ST4 in Maxim DL.

Ben, you think anything related to field rotation? is your guide star inside the field?

I have no experience in PHD guide or ST4, I only pick up deep sky imaging recently because standalone and low power consumption auto-guider available, in additions to modified full frame DSLR with liveview.

社長 寫:Ben, you think anything related to field rotation? is your guide star inside the field?

I have no experience in PHD guide or ST4, I only pick up deep sky imaging recently because standalone and low power consumption auto-guider available, in additions to modified full frame DSLR with liveview.

No, the guide star is 2-3 degree away . Let 's see if you can hold a LVI workshop and demonstrate to me later

benben88 寫:

willis 寫:With post processing, flat field and dark frame etc are just like a lady wearing make-up. Otherwise, it is a lady wears no make-up. Anyway, it seems that there is CA. Is it comes from the corrector?

Flat field is a must in CCD photography , it can eliminate the shadows of dust on the image and correct the vignetting of the setup .
There should be no CA for the Televue comacorr (Badder coma corrector has tiny CA and also spherical aberration and I am not using it in CCD photography). The alignment of the frames is not that great but I don't know why everytime I use PHD in guiding , the star in each frame is round but the field shifts a bit between frames . So after 1 hour L frame captured, the field shifted quite a lot..... There is no such problem if I use the self- guider or ST4 in Maxim DL.

Hi Ben! I made an over-do statement. It should be post processing is just like a lady wearing make-up. However for the flat field, if the telescope & the flattener are optically excellent, there is no need to apply flat field in the processing. As 社長 mentioned "我在這裡的所有照片都沒有Dark沒有Flat的....." No kidding and I tried APM APOs with proper flattener, there is no need to apply flat field. There is no obvious vignetting in full frame DSLR. If u are talking about APS size DSLR, flat field becomes a waste of time. I know some well-known astrographs, they really need flat field. The word "Astrograph" becomes a comercial name. I suppose the field of an Astrograph should be reasonably flat. But the reality is not. I reckon the term astrograph should be defined by performance, not just a name. So flat field or not, depends on what telescope u are using.
Clear Skies!
Willis

Willis，平場除了處理漸暈外，也有很多功能的，例如Ben說到的麈埃問題，以及一些不平均的感光問題等。

我在台灣拍的照片全部都沒有後期處理平場暗場，一來是我還沒準備好座枱電腦，目前的手提電腦根本應付不了很複雜的處理，這兩星期收拾好辦公室後便會進台i7 core的模擬8核心電腦，另一個原因是我不介意把沒平場暗場的照片分享，讓大家看到我所用的APM-TMB折射鏡及Modified 5D Mark II的原始成像水平如何，但這一切不代表平場不重要，我稍後還是會一一處理的，所以你看到我在武嶺每一天都有拍暗場，也每一個光學組合都拍了平場，稍後便會重新處理照片，而M31那張也會補回一些Ha, OIII等通道。

benben88 寫:

社長 寫:Ben, you think anything related to field rotation? is your guide star inside the field?

I have no experience in PHD guide or ST4, I only pick up deep sky imaging recently because standalone and low power consumption auto-guider available, in additions to modified full frame DSLR with liveview.

No, the guide star is 2-3 degree away . Let 's see if you can hold a LVI workshop and demonstrate to me later

I don't think 2-3 degree will have any effect on field rotation.....

yes, you can come and join us for next deep sky imaging workshop, or you can join Willis', I just talked to David and Willis that I will focus on imaging Jupiter these 2 months, due to some mistakes, what I didn't last year is not good to my standard, will do much better this year.

viewtopic.php?f=16&t=43