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Does large aperture prohibit large true FOV? Why?

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31 replies to this topic

#26 Huan

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Posted 07 August 2020 - 11:40 AM

Just mag 3 visually, that is tough. Takes some real perseverance to go out out for viewing.

I plugged some data for your APM BT70 into my spreadsheet, confirming your observation:

 

attachicon.gifAPM70.JPG

Would you please share your spreadsheet? I would love to play around with different sets of parameters on it, mainly simulating various scopes with different NELM to have a general idea of what to expect with any specific equipment in a specific location.  



#27 Mark9473

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Posted 07 August 2020 - 12:11 PM

I've sent you a PM with the file attached.


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#28 MartinPond

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Posted 09 August 2020 - 10:18 PM

What prevents a 100 mm BT from offering a relatively larger true FOV, say 6° to 8.5° that a pair of 8 x 42 mm binoculars provides?

 

Or it could provide larger true FOV but would suffer from some other issues that I’m missing?

 

It is possible, but even well-funded  astronomers don't reach that high. 

Film producers, sometimes.

 

Aside from eyepiece issues, the abberation of the short barrel becomes severe,

  incurable by just 2-3 ED elements..

Looking at cinematic lenses, you could reach down to ~F2.8, but the 100mm telescope would

  require 5-7 elements and the price (for the same transmission and quality) would

  rise to maybe  30-60K$.   Your eyepiece issues would be partly alleviated, though.

 

It's doable, but for a very high price.    And you may need 

 a fine sensor do do what your eye cannot.

Given the same amount of money, astronomers would be

reaching for power and distance instead. 



#29 Swedpat

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Posted 10 August 2020 - 11:30 AM

This is an interesting topic! While the FOV of a prism optics system does not depends on the aperture like with a galilean system, it's still limited by the field stop. And the field stop increases with the aperture. 

So there is SOME relation between the FOV and aperture IF WE STRETCHES THIS TO THE EXTREME. Right?



#30 Rich V.

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Posted 10 August 2020 - 11:47 AM

This is an interesting topic! While the FOV of a prism optics system does not depends on the aperture like with a galilean system, it's still limited by the field stop. And the field stop increases with the aperture. 

So there is SOME relation between the FOV and aperture IF WE STRETCHES THIS TO THE EXTREME. Right?

Yes, as in the original question, with a 100mm f5.5 BT you could get an 8° FOV at 8x with a 70mm 64° AFOV eyepiece but the exit pupil would be 12.5mm, limiting the effective aperture for human eyes.  The field stop would be approx. 78mms so the eyepieces would be HUGE and the prisms would have to be sized likewise.  I doubt most people could look through the thing even if it could be made.     undecided.gif

 

Not likely, IMO.

 

Rich


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#31 MartinPond

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Posted 11 August 2020 - 12:50 AM

This is an interesting topic! While the FOV of a prism optics system does not depends on the aperture like with a galilean system, it's still limited by the field stop. And the field stop increases with the aperture. 

So there is SOME relation between the FOV and aperture IF WE STRETCHES THIS TO THE EXTREME. Right?

If you lower the f-ratio you gain field, whatever the aperture.

Of course, the bigger the aperture, the more aberration for the same f-ratio.

To tame that, the element count and price go crazy.



#32 stargazer193857

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Posted 12 August 2020 - 07:03 PM

Large aperture does not have to prohibit field of view if you use big eyepieces too big for most to view through simultaneously, and accept an exit pupil larger than your own. Lots of light would be wasted, but you might get higher contrast.


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