Noticed Saturn appears to move way faster than Jupiter.

[Chris]

I took some AVI footage of Jupiter and Saturn back to back and noticed that Saturn appears to move way way more quickly across the EP/sensor than Jupiter. I've never really noticed the big difference before, and giving it some thought I think it's mainly due to the rotation of the Earth raking out more distance out at the orbit of Saturn compared to Jupiter in an equal amount of time. 

Am I right? Is this the main reason, or am I missing any major reason?  

I know that actual movement of the planets will be negligible compared to the rotation of the Earth, but still I've got the feeling I'm missing something? 

Thanks and any thoughts? 

[Peter Drew]

I think the clue to the answer is "appears". I can't see in reality how there can be any true difference as observed from here. If anything Saturn should be slower, having a much greater orbit than Jupiter. Interesting observation though.   😀

[vlaiv]

No, that can't be the reason.

Any motion of the planet across the sensor will be due to Earth's rotation. Rate of relative motion of both planets to Earth is so slow that you need like an hour to notice something on the sensor (it's like couple of arc seconds an hour).

We need to ask a simple question - why did each planet move across the sensor in the first place? Follow up question would be - were both of them shot at same focal length (same setup).

If you were using equatorial mount and tracked both planets - then motion of the planets on the sensor can be explained by few factors:

1. Improper tracking speed (like Solar / Lunar instead of sidereal)

2. Poor polar alignment causing drift

3. Periodic error of the mount

Different focal length will cause different "appearance" of the same drift rate on the video simply because for same drift speed in arc second / second will translate into different px/second if sampling resolution (arcsecond/pixel) is different - regardless of the drift cause. At eyepiece - this is equivalent to magnification - in low mag EP drift seems slower than at high mag.

From above 1) would give same drift rate, so it is hardly the cause. 2) will give different drift rates in RA/DEC depending on the part of the sky and direction of PA error - this can give a feel of different drift rates on sensor if one is horizontal and other is vertical or at an angle.

Above point 3) will depend where on period cycle you are currently at - mount can be tracking slower then sidereal, faster than sidereal and about the same as sidereal. It might even happen that one planet is drifting one way and other - the other way if depending on whether the mount is leading or trailing compared to sidereal.

If you are using Alt/Az mount there could be different reasons for drift - like mount not being precise in "knowing" where it is pointing - that will cause small drift.

If using non tracked mount - it can be illusion because of already described - one planet could be drifting horizontally, and other at an angle. Diagonal of sensor is longer than it's side and planet will be visible for longer - that can give impression that it is moving slower, when in fact it is moving at the same rate - but longer distance.

[Chris]

24 minutes ago, Peter Drew said:

I think the clue to the answer is "appears". I can't see in reality how there can be any true difference as observed from here. If anything Saturn should be slower, having a much greater orbit than Jupiter. Interesting observation though.   😀

That's the thing, Saturn does orbit slower because the Sun has less pull on it and it's settled at a point where the gravity is balanced with centripetal force. Like you say it's how it appeared which is contrary, and the difference was massive!

[Chris]

7 minutes ago, vlaiv said:

No, that can't be the reason.

Any motion of the planet across the sensor will be due to Earth's rotation. Rate of relative motion of both planets to Earth is so slow that you need like an hour to notice something on the sensor (it's like couple of arc seconds an hour).

We need to ask a simple question - why did each planet move across the sensor in the first place? Follow up question would be - were both of them shot at same focal length (same setup).

If you were using equatorial mount and tracked both planets - then motion of the planets on the sensor can be explained by few factors:

1. Improper tracking speed (like Solar / Lunar instead of sidereal)

2. Poor polar alignment causing drift

3. Periodic error of the mount

Different focal length will cause different "appearance" of the same drift rate on the video simply because for same drift speed in arc second / second will translate into different px/second if sampling resolution (arcsecond/pixel) is different - regardless of the drift cause. At eyepiece - this is equivalent to magnification - in low mag EP drift seems slower than at high mag.

From above 1) would give same drift rate, so it is hardly the cause. 2) will give different drift rates in RA/DEC depending on the part of the sky and direction of PA error - this can give a feel of different drift rates on sensor if one is horizontal and other is vertical or at an angle.

Above point 3) will depend where on period cycle you are currently at - mount can be tracking slower then sidereal, faster than sidereal and about the same as sidereal. It might even happen that one planet is drifting one way and other - the other way if depending on whether the mount is leading or trailing compared to sidereal.

If you are using Alt/Az mount there could be different reasons for drift - like mount not being precise in "knowing" where it is pointing - that will cause small drift.

If using non tracked mount - it can be illusion because of already described - one planet could be drifting horizontally, and other at an angle. Diagonal of sensor is longer than it's side and planet will be visible for longer - that can give impression that it is moving slower, when in fact it is moving at the same rate - but longer distance.

It's a non tracking mount with the same scope, barlow lens, and modified webcam. 

I'll create a private video on youtube and link it here so you can see the difference in apparent velocity. The orientation is a bit different so could have some bearing as the chip is slightly rectangular, but the speed is at least twice if not three times as quick to my eyes.

Anyway I'll sort out the footage and you can see what you guys think : ) 

[vlaiv]

1 minute ago, Lockie said:

It's a non tracking mount with the same scope, barlow lens, and modified webcam. 

I'll create a private video on youtube and link it here so you can see the difference in apparent velocity. The orientation is a bit different so could have some bearing as the chip is slightly rectangular, but the speed is at least twice if not three times as quick to my eyes.

Anyway I'll sort out the footage and you can see what you guys think : ) 

Ah, forgot one very important thing - you say you did use modified web camera.

Saturn is fairly dimmer than Jupiter - this generally means that if you want to create video and have both planets properly exposed on that video - video of Saturn is going to use longer exposure.

Let's say you did 33ms exposure on Jupiter and something like 100ms exposure on Saturn.

You now created regular video out of both recordings - one that runs at 30fps.

Jupiter will move at normal speed because each frame is 1/30 of a second. Saturn will move three times as fast because one frame is now 1/10 of a second, but movie is displayed not at 10fps but at 30fps - three times faster speed than it was recorded at.

(don't know actual exposures that you used - just showing that because different exposure lengths you can get different playback speed and therefore drift rate that appears different when in fact it is playback rate that is different).

[Chris]

2 minutes ago, vlaiv said:

Ah, forgot one very important thing - you say you did use modified web camera.

Saturn is fairly dimmer than Jupiter - this generally means that if you want to create video and have both planets properly exposed on that video - video of Saturn is going to use longer exposure.

Let's say you did 33ms exposure on Jupiter and something like 100ms exposure on Saturn.

You now created regular video out of both recordings - one that runs at 30fps.

Jupiter will move at normal speed because each frame is 1/30 of a second. Saturn will move three times as fast because one frame is now 1/10 of a second, but movie is displayed not at 10fps but at 30fps - three times faster speed than it was recorded at.

(don't know actual exposures that you used - just showing that because different exposure lengths you can get different playback speed and therefore drift rate that appears different when in fact it is playback rate that is different).

Ah that could well be it! I completely forgot about this aspect! I'll check the source files and see what the rates for each AVI are. 

here are the AVI's anyway 

https://www.youtube.com/watch?v=DRDjDCvcdos&feature=youtu.be

[vlaiv]

6 minutes ago, Lockie said:

Ah that could well be it! I completely forgot about this aspect! I'll check the source files and see what the rates for each AVI are. 

here are the AVI's anyway 

https://www.youtube.com/watch?v=DRDjDCvcdos&feature=youtu.be

Video is private, so I can't see it. But you can check visually if different recording/playback speeds have been used - don't look at planet drift speed - look at speed of seeing - if it is "dancing" faster on Saturn video - you have it on "fast forward"

 

[Chris]

Camera setting for Jupiter:

[Logitech HD Webcam C270]
Resolution=1280x960
FPS=30.00
Colour Space=RGB24
Output Format=AVI files (*.avi)
Frame Rate Limit=Maximum
Exposure=15.6ms
Timestamp Frames=Off
Brightness=128
Contrast=32
Saturation=32
Sharpness=24
White Balance=3690(Auto)
Backlight Compensation=0
Gain=50
 

Camera settings for Saturn:

[Logitech HD Webcam C270]
Resolution=1280x960
FPS=30.00
Colour Space=RGB24
Output Format=AVI files (*.avi)
Frame Rate Limit=Maximum
Exposure=3.9ms
Timestamp Frames=Off
Brightness=128
Contrast=32
Saturation=32
Sharpness=24
White Balance=3690(Auto)

So the frame rate was around 4 times faster for Jupiter. That's the answer, cheers!  

 

 

[Chris]

8 minutes ago, vlaiv said:

Video is private, so I can't see it. But you can check visually if different recording/playback speeds have been used - don't look at planet drift speed - look at speed of seeing - if it is "dancing" faster on Saturn video - you have it on "fast forward"

 

Sorry, changed to public  

[LukeSkywatcher]

I'd say the scope alignment was off a bit when observing Saturn. First time I saw it, it raced across the FOV. Jupiter didn't.

The Moon used to race across too, until I tweaked alignment. Then it crept across.