measuring perioidic error of GP-DX + SS2k

[gary125]

Hi experts,

I have calculated the periodic error of my GP-DX mount / ss2k system ( PEC function is off) with the M36 image taken at TKO site on 13 Dec 03.

As I don't know if my method is correct or not, would you please comment my assumptions / deriving methods. all comments are welcomed . (remark: I have not conduct any precise polar alighment at all, I only point the scope north-wise and set the inclination to approx 22degree)


Number of image Pixel of the drifted star trails : 5 pixel
Total number of pixel of the image = 1325 pixel

= drifted image size at film = 5/1325 *24mm ( note 1)
= 0.0906 mm

applying the prime focus image size formula at the link http://www.philipstripling.com/astro.html , the image size of the drifted star image = 10.36 arc second ( note 2 )

== periodic error = +/- 5.2 arc seconds ??!! (note 3)


Note 1 : the scanned image has 1325 pixels at the 24mm side of the regular 24x 35mm film)

Note 2 : the image was taken on a 5 min ISO asa400 film.
the image size at film of prime focus photography should be somewhat like 2 pi f cos(theta) * xxx / yyy. However, I can』t remember the exact formula and therefore use the web page instead

Note 3: the image looks like a 「~」 and the peak-to-peak periodic error is 10.36 arc second. Therefore, if it has to be measured in peak-to-zero notation, then error should be denoted as +/- 5 arc seconds only!!

[Subaru]

假如上面幅圖上下是東西方向, 而個極特登擺唔準使粒星南北drift, 5 pixel是measure上下方向的peak to peak movement的話, 條數應該啱.

+/- 5.2弧秒是很高的精度! 你要好好珍惜這台mount啦.

[Wah!]

我之前以 200mm 焦距影 M1 失敗的 avi, 唔 align 直接 stack, 得出以下相片.波浪線的右下為開始,左上為結束.
發現個 AR mount 追得唔準!!!!!!!!!!!!!!!!!!!!!!!!!
個 mount 係追快左架!!!

每一粒像素係 5.6um*5.6um, 焦距 200mm, 即是像素間的視角為 5.8".

一恆星日有 86164.09054 秒, 個 mount 係 144 齒的, 即是每一個波浪有 598 秒.取其中 5 個浪則有 2992 秒.
五個浪的垂直高度有 46 像素. 即 266角秒.
追快左的速度係: 0.089度/小時= 0.089角分/分=0.089角秒/秒 (呢三個數一樣,真有趣!)

每恆星日追多左 2.13度, 誤差係 0.59%...

每個波浪 p-v 有 10 像素, 算出個週期誤差係 p-v 58角秒.

唉......原來個mount都唔係想像中咁好.......

[gary125]

thank you for your comment. I just return from China yestersday and the most valuable prize for me is the starry night and naked eye horse head.