Tom - Mars Reconnaissance Orbiter question

Great news jrich

Tom,

Do you know what the "depth of focus" is on the MRO's highest resolution camera? Or does anyone else?

Actually, I may be talking about "depth of field". What I would like to know is what is the vertical distance from the surface of Mars up, that will remain in sharp focus on an image taken by the MRO's highest resolution camera, when travelling at it's lowest orbit.

Thanks.

rd

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by rderosa</i>
<br />Do you know what the "depth of focus" is on the MRO's highest resolution camera?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Everything on Mars is "at infinity" for a camera in circular orbit several hundred kilometers up. -|Tom|-

Hello rderosa,

These terms are often confused. Depth of field refers to the distance between the lens and the object being imaged. Depth of focus refers to the distance between the lens and the imaging medium (film, CCD array, etc) within the camera. I suspect you are asking about depth of field.

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The really short version - it only matters when the light rays entering the lens are significantly non-parallel. If the object being imaged is much more than 10 meters from the lens it is effectively at "optical infinity". If anything at optical infinity is in focus, everything at optical infinity is in focus.

A strong telescopic lens will push the minimum distance for optical infinity further away, but not by much. It would probably be necessary to get the spec sheet for the camera's lens system to know what it is, but I'd be surprised to find that it was more than a few hundred meters.

Even for Hubble, optical infinity is probably not more than a few kilometers.

(NOTE - I'm not speaking from actual experience here [except for a few lab experiments 30 years ago], but from some general principals. I'm pretty sure I'm right but there may be some details where I have oversimplified.)

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It is a safe bet that everything within 10 kilometers of the surface will be in focus.

LB

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by tvanflandern</i>
Everything on Mars is "at infinity" for a camera in circular orbit several hundred kilometers up.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

If I understand you correctly that would mean that pretty much everything from the camera to the surface would be in focus at the same time. In other words if there was a small space craft hovering at an altitude of about 50 km, it would also be in sharp focus. Correct?

Ok, assuming that's correct, let me re-phrase my question. Suppose there was a feature on the planet surface that could only be seen with the naked eye at an altitude of 50 km. Just for the sake of argument assume we were hovering in a hot air balloon at 50 km, and we could see some feature perfectly clearly with the naked eye. However, if we took the hot air balloon to any altitude higher or lower, we could no longer see the feature clearly. As we went higher, the naked eye could no longer resolve the feature, and as we went lower, we get lost into a "forest for the trees" type of situation, sort of like trying to view the Mona Lisa under a microscope.

My question is this. Assuming this type of situation, would the feature be seen clearly from the MRO camera?

rd

<i>Originally posted by Larry Burford</i>

I suspect you are asking about depth of field.


Yes, that's correct, I'm speaking about depth of field, and I understand your and Toms answer. But please see my next post, I'd be interested in your response to that one also.



rd