Firecapture focal length data

[jambouk]

In the text document which accompanies the avi files firecapture saves, it gives a value for "focal length", which seems ball park correct.

How does it generate this data as it doesn't ask what scope you are using. Does it know how the apparent diameter of each planet (and the moon) for any given point in time and compare this to the size of the image on the sensor?

Jd

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[james_screech]

It can calculate the angular size of the planet, then by some simple trig. and the outline of the planet's image calculate the focal length. This is very much dependent on the size/position of the planet outline you use when measuring the planet. I've found that the given focal length can vary between 5060mm and 5410mm in my setup depending on how accurately I set the size of the planets outline.

James 

[jambouk]

Thank you.

Where / how do you "set the size of the planets outline"? I've never seen this option.

Does firecapture know when mars is 14 arcminutes and when it's 25 arcminutes? Or does it just estimate that it is always somewhere between the two?

James

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[james_screech]

Sorry I was thinking of some other software that I've been using (WinJupos). But FireCapture probably uses the same blob detection that it uses for autoguiding and uses the size of the detected blob to calculate the focal length. This is not always very accurate though as blob detection algorithms can be fooled if the seeing is bad. 

James

[jambouk]

I looked at the file and it suggested my focal length was 8790mm; i was using a c11 (2800mm), and a focuser and flip mirror which added 240mm, and a 2.5x powermate, so 7600mm by my rough calculations, but i have no idea where the mirror was and how different the actual focal length was to the native 2800mm.

So not bang on, but within 15% or so which seems pretty clever!

Jd

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[Zakalwe]

I never kew this. Just checked mine. Its reporting 6.86metres. Thats with a focal length of 2.8m (C11) PowerMate 2.5, Moonlite focuser and filterwheel.

Pretty damn clever! I guess that it knows the pixel size of the sensor and then works out the diameter

From the txt file:

Diameter=44.00"
Magnitude=-2.52
CMI=331.9° CMII=182.7° CMIII=359.2°
FocalLength=6860mm
Resolution=0.17"

[jambouk]

But does it have the data on the diameter of all the planets at any given moment in time?

Jd

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[james_screech]

With the correct information on the orbit of a planet it can work out it's distance from the Earth at any given time then knowing the actual diameter of the planet its relatively simple to work out it's apparent (angular) diameter. 

James

[JamesF]

Yup.  My recollection is that FC has ephemeris data tucked away somewhere, so it shouldn't be too hard to calculate the apparent angular diameter of Jupiter given it's position relative to the Earth at any point.  Using that and the "blob detection" algorithm to find the number of pixels across the planet and knowing the camera pixel size allows it to calculate the number of arcseconds per mm on the camera sensor and the effective focal length falls out of that quite simply.

James

[jambouk]

Fascinating. Thanks.

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[JamesF]

Just to add to this, I've been looking at the file that FireCapture writes (from my capture data last night).  At the top of the file is:

FireCapture v2.3  Settings------------------------------------Camera=ZWO ASI120MMFilter=RProfile=JupiterDiameter=37.49"Magnitude=-2.15CMI=348.7° CMII=162.0° CMIII=352.4°  (during mid of capture)FocalLength=5490mmResolution=0.14"

The "Diameter" line suggests that it does indeed have some way to find the current angular diameter of the planet.

James

[jambouk]

Yeah, skysafari suggests 38". Impressive.

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