Author Topic: Mars Helicopter Shadow  (Read 739 times)

Mars Helicopter Shadow
« on: 22 April, 2021, 04:45:18 pm »
When Ingenuity takes pictures of its own shadow the shadow of the blades is not as dark as the shadow of the body and legs, anyone know why? Are the blades thin enough to be translucent?
https://mars.nasa.gov/resources/25839/in-flight-image-from-ingenuitys-second-flight/

Jaded

  • The Codfather
  • Formerly known as Jaded
Re: Mars Helicopter Shadow
« Reply #1 on: 22 April, 2021, 04:54:26 pm »
Probably in a warehouse in Area 52 🤣

I think the rotors are carbon fibre, and they rotate about 4 times faster than they would have to here. So quite possibly they are thin.
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Re: Mars Helicopter Shadow
« Reply #2 on: 22 April, 2021, 10:16:21 pm »
Answer here:
https://twitter.com/sdamico/status/1384205372668350465

Light continues to leak into the sensor after the shutter is closed, so you're seeing a long exposure image of the body overlaid with a snapshot of the blades.

Re: Mars Helicopter Shadow
« Reply #3 on: 22 April, 2021, 11:06:44 pm »
If this was film (or had a conventional shutter), then wouldn't this just be the exposure time being longer than the time it takes the blades to traverse their own width?

According to the NASA info sheet, they're 1.2m across and do 2,400rpm - I reckon that means the tip is doing a touch over 542km/h, or a fraction under 151m/s.

Even if the exposure is 1/1000 of a second, the blade tip will have travelled 15cm: if the blade is 20cm wide, then its image would appear to cover 35cm, with only the central 5cm looking solid, and the 15cm on either side appearing translucent - but the centre of the blade, with a lower rotational velocity, would appear more solid on the film.

But it ain't film, and the whole blade appears evenly, so clearly it's a different effect ...

Re: Mars Helicopter Shadow
« Reply #4 on: 22 April, 2021, 11:16:07 pm »
But the blades have sharp edges in the picture. If it were a simple long exposure you'd either not see them at all or see a blurred edge, with the centre the same shade as the body.

Digital camera sensors* work by collecting photons in one set of buckets that faces the lens. To end the exposure the buckets are decanted into a second layer of buckets behind the first and the amount of photons in each counted.

The tweet indicates that this sensor design allows some photons to sneak past the first set and land in the second even after the exposure has finished.

If this were a film camera you could achieve the same effect by taking a very fast picture (freezing the blades in position) then taking a second much slower exposure (where the blades would be invisible) through a very dark filter to avoid obliterating the first.

(* or at least this one. There are other approaches)

Re: Mars Helicopter Shadow
« Reply #5 on: 24 April, 2021, 09:51:02 am »
Are the blades thin enough to be translucent?

My money would be on that explanation.

Re: Mars Helicopter Shadow
« Reply #6 on: 25 April, 2021, 07:58:59 pm »
They're carbon fiber and at least 2 layers, so I doubt they would be translucent.
Also this still image from the rover shows a solid shadows


Re: Mars Helicopter Shadow
« Reply #7 on: 27 April, 2021, 07:39:12 am »
marshelicoptercattest(1) by rogerzilla, on Flickr

(Credit to Pedrohin on b3ta)
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