Higher resolution or higher FPS?

[lorcog5]

I'm currently imaging with an entry level camera (Canon EOS 2000d) that has the option of 1080p at 25 fps or 720p at 50fps, the object I'm currently attempting to shoot is Jupiter and Saturn with a Skywatcher Classic 200P. What would be the best approach as I am unsure whether I should lose out on some resolution to get the higher FPS shooting.

 

Thanks.

 

 

 

[vlaiv]

Hi and welcome to SGL.

One has nothing to do with other.

Resolution - or sampling rate is dictated by pixel size and focal length. You can use barlow to modify focal length of the telescope. There is something called critical sampling rate and you should not go over it as there is no point - telescope aperture can't resolve more detail than that.

As a rough guideline - your F/ratio should be x4 pixel size in µm.

Exposure length should be as low as 5ms regardless of everything else. This is to freeze the seeing. Longer exposure lengths allow for seeing to change during single frame exposure and additional "motion blur" forms because of that. At about 5-6ms you "freeze" that motion and only distortion is recorded and not motion blur due to change in that distortion.

This means that fps that you want to achieve is over 150fps.

If your question is only about DSLR usage and two different modes, then I would choose higher FPS and if possible - raw video format. DSLRs tend to shoot compressed video. Avoid that as it introduces artifacts in your image (like poor quality jpeg - image becomes blocky). Find out about pixel size of your camera and then use barlow to get good F/ratio.

In most cases even with proper F/ratio - planets are small and even 640x480 format is enough to capture them, so don't worry about that part.

[lorcog5]

13 minutes ago, vlaiv said:

Hi and welcome to SGL.

One has nothing to do with other.

Resolution - or sampling rate is dictated by pixel size and focal length. You can use barlow to modify focal length of the telescope. There is something called critical sampling rate and you should not go over it as there is no point - telescope aperture can't resolve more detail than that.

As a rough guideline - your F/ratio should be x4 pixel size in µm.

Exposure length should be as low as 5ms regardless of everything else. This is to freeze the seeing. Longer exposure lengths allow for seeing to change during single frame exposure and additional "motion blur" forms because of that. At about 5-6ms you "freeze" that motion and only distortion is recorded and not motion blur due to change in that distortion.

This means that fps that you want to achieve is over 150fps.

If your question is only about DSLR usage and two different modes, then I would choose higher FPS and if possible - raw video format. DSLRs tend to shoot compressed video. Avoid that as it introduces artifacts in your image (like poor quality jpeg - image becomes blocky). Find out about pixel size of your camera and then use barlow to get good F/ratio.

In most cases even with proper F/ratio - planets are small and even 640x480 format is enough to capture them, so don't worry about that part.

Thanks for the response, it was very helpful!

I am a bit confused on what you meant by exposure length being in the milliseconds though, any chance you could expand on that?

[AstroMuni]

11 minutes ago, lorcog5 said:

Thanks for the response, it was very helpful!

I am a bit confused on what you meant by exposure length being in the milliseconds though, any chance you could expand on that?

I am sure vlaiv can provide a detailed explanation but In a nutshell the faster the exposure the less chances of capturing any disturbances contributed by atmosphere etc.

So each frame is as crisp as possible and anyway we stack several hundreds of frames to get final image so the detail is not lost.

Edited August 23, 2021 by AstroMuni

[vlaiv]

8 minutes ago, lorcog5 said:

Thanks for the response, it was very helpful!

I am a bit confused on what you meant by exposure length being in the milliseconds though, any chance you could expand on that?

When you take still with your DSLR - you set exposure to some value - say 1/60 or 1/30 - that is 1/60th of a second or 1/30th of a second.

I'm fairly confident that when you are taking a video with DSLR you can also adjust exposure length. It is easy to calculate how much it needs to be in fraction - if you just notice that one second has 1000 milliseconds.

1/60th of a second is then 1000 ms / 60 = 16.67ms, and similarly 1/30th is 1000ms / 30 = 33.33ms.

If you want to know "fraction" value of 5ms or 6ms - it goes the same 1000ms / 5ms = 200, so 1/200th is equal to 5ms. Similarly - 1/166 is equal to 6ms.

You may notice that this is equal to max FPS that you can theoretically produce, so if you set exposure length to 1/60th - you can produce at most 60fps and for 1/30th - 30fps.

Proper lucky type planetary imaging done with dedicated planetary cameras uses 5-6ms exposures and hundred of more FPS. With DSLR you'll probably be limited to said 50FPS, but never the less - go with short exposure like 1/200th (which is 5ms).

[lorcog5]

15 minutes ago, vlaiv said:

When you take still with your DSLR - you set exposure to some value - say 1/60 or 1/30 - that is 1/60th of a second or 1/30th of a second.

I'm fairly confident that when you are taking a video with DSLR you can also adjust exposure length. It is easy to calculate how much it needs to be in fraction - if you just notice that one second has 1000 milliseconds.

1/60th of a second is then 1000 ms / 60 = 16.67ms, and similarly 1/30th is 1000ms / 30 = 33.33ms.

If you want to know "fraction" value of 5ms or 6ms - it goes the same 1000ms / 5ms = 200, so 1/200th is equal to 5ms. Similarly - 1/166 is equal to 6ms.

You may notice that this is equal to max FPS that you can theoretically produce, so if you set exposure length to 1/60th - you can produce at most 60fps and for 1/30th - 30fps.

Proper lucky type planetary imaging done with dedicated planetary cameras uses 5-6ms exposures and hundred of more FPS. With DSLR you'll probably be limited to said 50FPS, but never the less - go with short exposure like 1/200th (which is 5ms).

Oh I understand what you mean now, thankyou!

[lorcog5]

21 minutes ago, AstroMuni said:

I am sure vlaiv can provide a detailed explanation but In a nutshell the faster the exposure the less chances of capturing any disturbances contributed by atmosphere etc.

So each frame is as crisp as possible and anyway we stack several hundreds of frames to get final image so the detail is not lost.

Thanks for the help!