想问下电跟的一个基本问题

[Wah!!]

應該是"恆星速"吧?

上面2.3的理論, 對於電動的情況仍有效. 所以才會在長焦拍攝時對齒輪的工藝, 對極的準確度, 或者導星有所要求.

[豆豸苗]

Ooops. 係"恒"星速. 打錯字添.

[Wah!!]

回到主題: 窮人的輕便赤道儀
長焦等高要求的情況便不必考慮了, 所以手動的精度已足夠.

[鄧登凳]

這裡提到的有兩個概念:"速度"和"頻率"

平均速度上, 無論電動或手動都跟行星速.
但頻率上, 電動應是固定的(步進?), 手動則會有偏差, 阿華說的是人手扭時, 不同焦距可容納的偏差.

Sidereal speed = 恆星速 (雖然恆星速也可在數小時內追蹤行星)

MANDII: 在乎你的鏡是什麼焦距, 視場角度(FOV)有多濶, 如FOV為10度的相機鏡, 那五分鐘差0.01度, 根本不是問題; 但長焦望遠鏡的FOV, 可能只是0.25度, 差0.01度, 星點已經畫線

[MANDII]

這裡提到的有兩個概念:"速度"和"頻率"

平均速度上, 無論電動或手動都跟行星速.
但頻率上, 電動應是固定的(步進?), 手動則會有偏差, 阿華說的是人手扭時, 不同焦距可容納的偏差.

Sidereal speed = 恆星速 (雖然恆星速也可在數小時內追蹤行星)

MANDII: 在乎你的鏡是什麼焦距, 視場角度(FOV)有多濶, 如FOV為10度的相機鏡, 那五分鐘差0.01度, 根本不是問題; 但長焦望遠鏡的FOV, 可能只是0.25度, 差0.01度, 星點已經畫線

May be I am wrong for the previous conclution .
It should be :
Although the all electonic mount is set fix to the Sidereal speed and it didn't need to know the scope or camera for setting it's speed , however,different scope can produce different tracking effect of the image . A bad scope ( e.g a long focal length scope , little FOV) can easily produce the tracking error image .

The above saying apply for both elctronic or manual tracking .

[Wah!!]

May be I am wrong for the previous conclution .
It should be :
Although the all electonic mount is set fix to the Sidereal speed and it didn't need to know the scope or camera for setting it's speed , however,different scope can produce different tracking effect of the image . A bad scope ( e.g a long focal length scope , little FOV) can easily produce the tracking error image .

The above saying apply for both elctronic or manual tracking .

那是錯誤的理解.
望遠鏡焦距長短不會引入追踪誤差.
追踪誤差一直已經在赤道儀上存在, 只是攝影器材和條件能否容許這誤差的存在!

[PTS]

wha 兄,我又5明多口问一下了。
跟踪还要考虑扭螺絲的频率的吗？
你说的是手动还是电动的呢？
如果是手动扭螺絲跟踪，那么是肉眼边看镜头边扭螺絲的吧，只要眼看到星点有移位便要扭制的吧，那样不存在固定的每分或每秒的动制频率吧。
如果是电动，那么市面上卖的电跟又如何知道你的镜子的性质来定它的扭动速度或者扭动频率呢？如果市面的电跟在不用得知镜子性质的情况底下能够實现跟踪，那么也说明与镜子本身无任何关系吧，并且都是同一个速度和地球同步的吧，短焦长焦都无关吧 。

Hi,
Wah is right.
Depending on the focal length of the lens and the pixel size of your ccd, you can actually calculate the time required for the star image to move across 2 pixels. This time is the interval for you to move the drive by the same amount to the west, thus keeping the star image to stay within the 2 pixels, equivalent to a good tracking.

PTS

Hello PTS ,
I just have a simple question to ask you .
How do the mount know your scope focal length or CCD to set it's speed to track the star correctly ? You know there is many different scope and CCD and mount , Or should I each time need to manually set the speed of my mount to fit my scope or CCD ?

Hi,
It is not your mount that knows the focal length (or indeed anything), but you know this.
I am not saying you should change the setting with different optics/ccd combination. Rather, you should design your mount/tracking based on the optics/ccd combination.
If you have several telescopes and ccds, then you should choose a mounting/tracking system that match the requirement of the longest FL/smallest pixel combination.
There is no need to "lower" the accuracy to suit a short focus/large pixel system because if the tracking is good for a long focus telescope, it will certainly do better for the short focus ones.

PTS

[MANDII]

呵呵,可能英文表达的不够准确吧。
我意思应该是: 现今赤道仪不用也无法按镜子的焦距,镜头的FOV等等因素而设定其合当的运转速度以作准确的跟踪,而短焦距，大FOV的镜子或相機更加能 减底跟踪的误差。

[Wah!!]

一定要分辨清楚追踪 "速度(SPEED)" 和 "精度(ACCURACY)" 的意義有什么不同, 不要混淆了.

任何情況下, 赤道儀的平均追踪速度都一定要是相同的(恆星或太陽速).
至於不同赤道儀的精度便有所不同.
對於短焦距拍攝, 可以使用容易制作或平宜的較低精度赤道儀, 因為赤道儀的誤差不影響成像.

[MANDII]

wha 兄,我又5明多口问一下了。
跟踪还要考虑扭螺絲的频率的吗？
你说的是手动还是电动的呢？
如果是手动扭螺絲跟踪，那么是肉眼边看镜头边扭螺絲的吧，只要眼看到星点有移位便要扭制的吧，那样不存在固定的每分或每秒的动制频率吧。
如果是电动，那么市面上卖的电跟又如何知道你的镜子的性质来定它的扭动速度或者扭动频率呢？如果市面的电跟在不用得知镜子性质的情况底下能够實现跟踪，那么也说明与镜子本身无任何关系吧，并且都是同一个速度和地球同步的吧，短焦长焦都无关吧 。

Hi,
Wah is right.
Depending on the focal length of the lens and the pixel size of your ccd, you can actually calculate the time required for the star image to move across 2 pixels. This time is the interval for you to move the drive by the same amount to the west, thus keeping the star image to stay within the 2 pixels, equivalent to a good tracking.

PTS

Hello PTS ,
I just have a simple question to ask you .
How do the mount know your scope focal length or CCD to set it's speed to track the star correctly ? You know there is many different scope and CCD and mount , Or should I each time need to manually set the speed of my mount to fit my scope or CCD ?

Hi,
It is not your mount that knows the focal length (or indeed anything), but you know this.
I am not saying you should change the setting with different optics/ccd combination. Rather, you should design your mount/tracking based on the optics/ccd combination.
If you have several telescopes and ccds, then you should choose a mounting/tracking system that match the requirement of the longest FL/smallest pixel combination.
There is no need to "lower" the accuracy to suit a short focus/large pixel system because if the tracking is good for a long focus telescope, it will certainly do better for the short focus ones.

PTS

Ohh, my originally question to ask you mean that :
If the nowadays mount can do tracking CORRECTLY , and if the mount do not need to know the focal length or CCD information to set it's speed , then it means the mount's accurate tracking speed do not have relationship with to the focal length or CCD or other equipments .

[PTS]

平均速度上, 無論電動或手動都跟行星速.
頻率上, 電動應是固定的(步進?),

那么,宗合以上结论会是 : 所有的電動跟踪赤道仪是以固定的行星速(about 1/4degree/min) 来跟踪的 ，因此赤道仪并不用知道是怎么样的镜子或CCD。 而手动跟踪时，影像的跟踪效果会与器才有关。应该没错了吧 。

Yes, normally the tracking system is for 恆星速, unless it is a dedicated solar telescope.
However, almost all tracking systems allow adjustments so that we can track the Sun or planets. In the old days, these adjustments were done by mechanical means (like changing gears). In the 1970's, this was mainly done electronically. During that period, most tracking drives made use of "sychronous motors" whose speed is a function of the frequency of the AC electricity. We made circuits that alter this frequency by +/-10% for adjusting the tracking speed.
When stepper motors are used later, the tracking speed can simply be implemented by varying the frequency of the pulses that drives the stepper.
Of course all these old tricks have no control on the actual tracking accuracy. We still have to "guide" (either with a guiding scope or an off-axis guider) during long exposures.
The modern tracking mostly uses "servo-motors" with positive feedback on the actual position of each axis. This system improves on the tracking a lot and guiding is only needed for long exposures.

PTS

[kurt]

wha 兄,我又5明多口问一下了。
跟踪还要考虑扭螺絲的频率的吗？
你说的是手动还是电动的呢？
如果是手动扭螺絲跟踪，那么是肉眼边看镜头边扭螺絲的吧，只要眼看到星点有移位便要扭制的吧，那样不存在固定的每分或每秒的动制频率吧。
如果是电动，那么市面上卖的电跟又如何知道你的镜子的性质来定它的扭动速度或者扭动频率呢？如果市面的电跟在不用得知镜子性质的情况底下能够實现跟踪，那么也说明与镜子本身无任何关系吧，并且都是同一个速度和地球同步的吧，短焦长焦都无关吧 。

Hi,
Wah is right.
Depending on the focal length of the lens and the pixel size of your ccd, you can actually calculate the time required for the star image to move across 2 pixels. This time is the interval for you to move the drive by the same amount to the west, thus keeping the star image to stay within the 2 pixels, equivalent to a good tracking.

PTS

Hello PTS ,
I just have a simple question to ask you .
How do the mount know your scope focal length or CCD to set it's speed to track the star correctly ? You know there is many different scope and CCD and mount , Or should I each time need to manually set the speed of my mount to fit my scope or CCD ?

Hi,
It is not your mount that knows the focal length (or indeed anything), but you know this.
I am not saying you should change the setting with different optics/ccd combination. Rather, you should design your mount/tracking based on the optics/ccd combination.
If you have several telescopes and ccds, then you should choose a mounting/tracking system that match the requirement of the longest FL/smallest pixel combination.
There is no need to "lower" the accuracy to suit a short focus/large pixel system because if the tracking is good for a long focus telescope, it will certainly do better for the short focus ones.

PTS

Ohh, my originally question to ask you mean that :
If the nowadays mount can do tracking CORRECTLY , and if the mount do not need to know the focal length or CCD information to set it's speed , then it means the mount's accurate tracking speed do not have relationship with to the focal length or CCD or other equipments .

理論上如果個mount係１００﹪完全同步地球自轉的話，無論用幾長焦距同幾細粒ccd都無問題，都會得到完美的星點

[MANDII]

So now raise up another issue by PTC !
The automatic adjustment of the speed/position of the mount during tracking to enhence accurate tracking . Somehow it need to know how much it need to adjust for the motors .

Therefore ：
现今赤道仪不用也无法按镜子的焦距,镜头的FOV因素而设定其合当的运转速度以作准确的跟踪,但必需透过有关的器材(e.g servo motor,guide scope or off-axiso scope)作出跟踪的调节(速度/位置)。 而短焦距，大FOV的镜子或相機更加能减底跟踪的误差。

那也即是说要做个电动的赤道仪，得有 1)固有的恒星速 2)变速 3)位置变更 的 3 个功能了.. .

[Skyobs]

而短焦距，大FOV的镜子或相機更加能减底跟踪的误差。

Not reduce the error, but can tolerate a bigger tracking error.

Nothing is perfect. If you have a good auto-guide system, you may neglect most of the common imperfections. However, astrophoto is still possible with F=1500mm, small CCD, unguided, cheap mount.

[PTS]

wha 兄,我又5明多口问一下了。
跟踪还要考虑扭螺絲的频率的吗？
你说的是手动还是电动的呢？
如果是手动扭螺絲跟踪，那么是肉眼边看镜头边扭螺絲的吧，只要眼看到星点有移位便要扭制的吧，那样不存在固定的每分或每秒的动制频率吧。
如果是电动，那么市面上卖的电跟又如何知道你的镜子的性质来定它的扭动速度或者扭动频率呢？如果市面的电跟在不用得知镜子性质的情况底下能够實现跟踪，那么也说明与镜子本身无任何关系吧，并且都是同一个速度和地球同步的吧，短焦长焦都无关吧 。

Hi,
Wah is right.
Depending on the focal length of the lens and the pixel size of your ccd, you can actually calculate the time required for the star image to move across 2 pixels. This time is the interval for you to move the drive by the same amount to the west, thus keeping the star image to stay within the 2 pixels, equivalent to a good tracking.

PTS

Hello PTS ,
I just have a simple question to ask you .
How do the mount know your scope focal length or CCD to set it's speed to track the star correctly ? You know there is many different scope and CCD and mount , Or should I each time need to manually set the speed of my mount to fit my scope or CCD ?

Hi,
It is not your mount that knows the focal length (or indeed anything), but you know this.
I am not saying you should change the setting with different optics/ccd combination. Rather, you should design your mount/tracking based on the optics/ccd combination.
If you have several telescopes and ccds, then you should choose a mounting/tracking system that match the requirement of the longest FL/smallest pixel combination.
There is no need to "lower" the accuracy to suit a short focus/large pixel system because if the tracking is good for a long focus telescope, it will certainly do better for the short focus ones.

PTS

Ohh, my originally question to ask you mean that :
If the nowadays mount can do tracking CORRECTLY , and if the mount do not need to know the focal length or CCD information to set it's speed , then it means the mount's accurate tracking speed do not have relationship with to the focal length or CCD or other equipments .

Yes, as Wah said, the tracking speed accuracy is FIXED for a mount. You have to determine yourself if this accuracy is good enough for your optics/ccd combination.

PTS