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Focal Length Calculation for ASI Air Polar Alignment


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I thought I knew how to calculate the focal length for my main camera image train ( Canon EOS 700D +0.85 Field Flattener+ Evostar 72ED). The formula I used was 1.6 for the crop sensor x 0.85 for the field flattener x 420mm for the scope = 571mm

However..... on trying out my new ASI air to polar align tonight I was unable to get it to plate solve. It found plenty of stars but would not plate solve. Eventually I decided to set the focal length in the app to zero and let it calculate it itself. It came up with 376mm and plate solving worked first time. 

I guess I used the wrong formula or I'm missing an element?

 

 

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I could be wrong but don't believe a camera sensor has anything to do with the optical train focal length, it's simply seeing/receiving what the scope and any glass in between is exposing it to. So your calculation should be scope FL x reducer, it's not always accurate as glass optics vary.

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2 minutes ago, Elp said:

I could be wrong but don't believe a camera sensor has anything to do with the optical train focal length, it's simply seeing/receiving what the scope and any glass in between is exposing it to. So your calculation should be scope FL x reducer, it's not always accurate as glass optics vary.

Thanks. That would make sense - gets me to 357mm so pretty close to its own calculation

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With the asiair I usually always set the main FL to zero (as I setup and break down every time, the guidescope doesn't change so I always input its value for that field) so leave it to work the main one out itself.

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34 minutes ago, Elp said:

I could be wrong but don't believe a camera sensor has anything to do with the optical train focal length, it's simply seeing/receiving what the scope and any glass in between is exposing it to. So your calculation should be scope FL x reducer, it's not always accurate as glass optics vary.

Exactly what he says 👆

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Yes it's probably likely that you need to include any spacer/adapter added for the camera.

So if your 420mm is extended by a 21mm spacer that would be 441 x 0.85 = 374.85. Near enough to the AsiValue (my term for the result it works out).🙂

Just a guess on my part based on the limited evidence in this thread. ZWO can send me some free gear if they want me to verify it properly.

 

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17 minutes ago, StevieDvd said:

Yes it's probably likely that you need to include any spacer/adapter added for the camera.

So if your 420mm is extended by a 21mm spacer that would be 441 x 0.85 = 374.85. Near enough to the AsiValue (my term for the result it works out).🙂

Just a guess on my part based on the limited evidence in this thread. ZWO can send me some free gear if they want me to verify it properly.

 

Thank you. Yes, I think you are right. I didn't take account of the adaptor ring!  I'll measure it tomorrow just to satisfy myself

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The whole business of linking sensor size to focal length in the term 'crop sensor' leads us right up a gum tree!  The idea that a crop sensor increases focal length is false. How can it? Focal length is a property of the glass in the lens. The term is useful in regular photography as a way of comparing the image produced by the same lens when used in full frame or APSc cameras. Clearly the APSc field of view is reduced but, more importantly, there are effects of perspective which also change. On a full frame camera, features in the foreground and background will have the same relative size as the naked eye view in a 50mm lens. The 1.6x formula allows a photographer to work out how to retain those proportions with a smaller sensor.

None of this applies in AP. The only effect your chip size has is to increase or reduce your FOV.

Also, pixel size, not the megapixel count often quoted in normal photography, defines your resolution.

Olly

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