Is a high framerate better than resolution?

[Tippon]

I've got an action camera that I'm considering modding to use for imaging, but it's got several video modes. It can do 4K at 30fps, 1080 at 30/60fps, and 720 slow motion video at 90/120fps.

At least to start with, I'd be imaging the moon and planets. Would I be better off setting it for 4K or 120fps?

My telescope is a Celestron Astromaster 130EQ with the motor drive.

[happy-kat]

I think I would use 1080 at 30fps. Hoping the least demanding setting would yield the best data. But experiment to see what works best for your camera.

[jambouk]

Yes, you need to experiment. DSLR -type cameras compress the files and in doing so lots of data is lost. This will be less of an issue with the Moon than Jupiter as the Moon is so much bigger and brighter. I would start out in 1080 60fps mode and then try the 4K and then slow mo. But trial and error. To stack lunar images you want as many frames as possible, but you also want each of those frames to be as good as possible and as least compressed as possible, so it is a balancing act.

[vlaiv]

Primarily high fps.

Planets fit nicely on 640x480 as they are small. Moon is exception and it likes larger resolutions - but not too large with your scope which has coma. If you use too large field of view, the Moon will be blurred at the edges of the frame due to coma from Newtonian telescope.

 

[Tippon]

Thanks all. I'll have a play with it if the weather ever clears up

 

[symmetal]

It also depends as to whether you camera just downscales the full sensor resolution to the specified video format, or it enables you to use centre crop video instead where the video format just uses the specified resolution cropped from the centre pixels of the sensor, ie no downscaling so no loss of resolution.

Canon DSLRs can use the MagicLantern firmware to enable centre crop video, and also uncompressed video, if it doesn't have it normally.

If you can't use centre crop video then 4k will give the highest image resolution. A 5000 x 4000 sensor for example downscaled to 720 means a 5 x 5 pixel area of the sensor ends up as 1 pixel on the video. For planets in particular you need the highest resolution you can use.

If you can use centre crop video then use a resolution that best matches the target. Probably 720 for planets, and 1080 or 4k for the moon. 4k using more sensor may show some coma effects so 1080 for the moon may be preferable.

A higher frame rate just lets you capture your 1000 to 2000 frames or so quicker, it doesn't give you a better image. 🙂

Alan

[vlaiv]

7 hours ago, symmetal said:

A higher frame rate just lets you capture your 1000 to 2000 frames or so quicker, it doesn't give you a better image. 

I believe it is quite the opposite - high number of captured frames is better image.

There is limited amount of time you can spend on a target since planets rotate (even with derotation). Large number of captured frames means large number of good frames given equal probability of a good frame due to seeing at the time of the capture. Large number of frames stacked means better SNR and better SNR means that you can sharpen image more without bringing the noise to the surface.

[symmetal]

4 hours ago, vlaiv said:

I believe it is quite the opposite - high number of captured frames is better image.

There is limited amount of time you can spend on a target since planets rotate (even with derotation). Large number of captured frames means large number of good frames given equal probability of a good frame due to seeing at the time of the capture. Large number of frames stacked means better SNR and better SNR means that you can sharpen image more without bringing the noise to the surface.

Oh! I agree vlaiv that the more captured frames is better. If Tippon's camera can't do video centre cut-out then the 720p at 120fps will give a very small image of the planets, and Jupiter's rotation effect likely won't be resolved even with a long video. 2 mins at 4k vs 30s at 720p would give the same 900 frames to start off with for stacking, and the 4k will give a much higher resolution image, and the rotation effect, if visible, will be small.

My advice was to go for resolution if centre cut-out wasn't available and fps if it was, particularly for planets.

For the Moon you may as well go for resolution as video duration is not such an issue, though depending on the accuracy of Tippon's motor drive the target may drift off frame on a long video.

Alan

Edited August 14, 2020 by symmetal