Astrophotography using DSLR video

[Moondogg]

Here's a question for you experts,

This has been bugging me for a while. With the arrival of large sensor DSLRs (Canon's 5d2 has a massive full frame) that can take 1080p video, why are we still using 640x480 webcams?

Bigger sensor = more photons, surely.

I am lucky enough to have a few Canons, and even the 550d can spew out 720p at 60fps, surely this must be preferable to the trusty Toucam and variations?

I know they do use a lossy H264 codec, so that might be the reason. Or perhaps Registax can't separate the individual frames, but I must say I am surprised at the lack of innovation in AP sensors over the past 10 years.

BTW, I am principally talking about planetary AP here, I have seem some great long exposure DSOs with DSLRs. Magic lantern must be a lifesaver here.

I'm sure I'm missing something obvious, but can find very little from uncle Google.

Thoughts?

[Cornelius Varley]

One reason, Canon EOS 550D body only @ £420 and weighs 476g, webcam, cheap & light. DSLRs are great for DSO imaging but a dedicated camera such as a D*K or QHY still beats a DSLR on planets and the moon.

Peter

[Moondogg]

But size aside, Peter, why are webcams preferable? Even high-end ones like the Flea etc still have risably small sensors and low resolutions.

The APS-C sensor in the 550d is 25.1 × 16.7 mm, compared to the 1/3 inch QHY sensor at 8.4mm wide. That is many times the light collecting power, for a very small premium in cost. We all aspire to higher apertures for our scopes, wouldn't it be better with bigger sensors?

I'm sure your right, buy the way, I just need to find the answer as to why APs don't use them for planets. There is very little that has been written about it.

[Digz]

I think, and I stress I may be completely wrong with this, it has something to do with the FOV generated by a webcam compared to a DSLR. That is to say a webcam will only effectively record part of the entire view generated by a telescope hence they are preferred for planetary imaging where the subject can still be relatively small even in the biggest of scopes.

Having said that I have photographed Jupiter with my DSLR and cropped the image, but this was a single shot rather than stacked video.

Hope that makes some sort of sense.

[paulk]

Here's one with a 550D Re: converting Canon 550D mov to avi for use with Registax5: Canon EOS 1100D - 300D Forum: Digital Photography Review

[Cornelius Varley]

Another reason why webcams and similar cameras are prefered is the codec. DSLRs use a lossy codec for video imaging so the video output is degraded to start with. The big sensor is also wasted for planetary imaging because to make use of it you would need a big telescope (I mean really big, think Hubble) to fill the sensor. The size of telescope used by amatuers means that big sensors are not practical for planetary imaging whereas a camera with a 640x480 resolution or similar gives a good sized disc on the sensor. Normally planetary/lunar imagers try to image at between f20 and f30 ish depending on the camera/telescope to get the right sort of pixel/arc second ratio needed for planetary imaging. A big sensor like those in DSLRs would still only have a planet disc of the same diameter as captured by 640x480 camera at the same focal length.

Peter

[paulk]

That makes sense, I'm going to get me a webcam soon

[Moondogg]

This makes sense. I guess if the scope was going to be designed for that size sensor, the the secondary mirror would need to change to cover the image (assuming a Newtonian). Still possible though, a Canon 5dMk2 in prime focus would (theoretically) collect 50 times more light than a 1/3 inch sensor.

Think of all that lovely Canon L glass, they go all the way up to 800mm f5/6, and they all cover a full frame camera sensor. They are Refractors in all but name.

OK, so that might explain the sensor size, but what about the resolution? 640x480 is rather meagre for 2012; small sensors can have much higher resolutions now (even the iPhone is theoretically capable of 4K).

Even if you were stuck with an optimal 1/3" size, wouldn't a higher rez sensor be better?

[Crimson Sunset]

If i recall correctly, the newer gen of canon DSLRs have a "crop" video mode that records 640x480, at least my 60D does. I remember seeing a topic about this on a different forum and somebody managed decent shots of jupiter using that mode.

[Moondogg]

Yes, the 550d has that. It means you are just using the very centre of the sensor.

Advantages:

1. It has a has a higher crop factor (remember this is a 18Mpix sensor), so you'll gain more apparent magnification.
   
2. There is no pixel binning/line skipping from the processor to get from 18Mpx down to 1080p or 720p, so there should be very little rolling shutter (jello effect), aliasing (jagged lines) and moiré (rainbow patterns on fine lines)
   

Disadvantages:

1. It's 640x480 only
   
2. So, we're back to the Toucam!