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Does large aperture prohibit large true FOV? Why?

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#1 Huan

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Posted 04 August 2020 - 11:18 AM

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?



#2 drt3d

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Posted 04 August 2020 - 11:28 AM

The actual field of view depends on the magnification, so you cannot compare the field of view of the 8x42 pair with the field of view of the 100mm BT with 18mm eyepieces, which provides 31x magnification.

 

You can compare the apparent field of view, which is listed at 65 degrees for the 100mm BT and 8x (6-8.5 deg) = 48-68 degrees in your 8x42 example, so the the 100mm BT has the same apparent FOV as the 8 deg 8x42mm pair.

 

You can use even wider eyepieces in binoculars that take different eyepieces, for example the 76 degree Morpheus.

 

George



#3 PEterW

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Posted 04 August 2020 - 11:56 AM

Your need low power and very wide apparent field of view which are complex and large eyepieces. Your eye spacing is likely too small to enable you to use them... imaging two 3m type 5 Naglers. You could use a faster objective lens, but then you’d need to use expensive glasses to keep the aberrations that occur. Possible, but not cost effective. If we could get someone to design slim long focal length ultrawide eyepieces it would open up the possibilities.

Peter

#4 Jon Isaacs

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Posted 04 August 2020 - 11:59 AM

If you had a pair of 100 mm F/3 Binos and a pair of 21 mm Ethos eyepieces, you could achieve a 6.9° field at 14.3x with a 7 mm exit pupil.

 

F/3 Binos are not available, they could be made and would need a field flattener. You'd need large prism.

 

A bigger problem is that the eyepieces are 3 inches in diameter and less than 1% of the population could use them.

 

Field of view depends on the focal length of the objective and the field stop of the eyepiece.  Larger apertures have longer focal lengths which mean narrower fields of view. Binoculars are limited because the eyepieces must fit side by side.

 

Jon


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#5 TOMDEY

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Posted 04 August 2020 - 12:01 PM

No, can't do that for a somewhat subtle practical reason: Max apparent field is limited to around 120o for plain refractors, and about half that for binoculars (because of the prisms). So, as the aperture goes up, you would need to maintain the same magnification as the smaller binos. A 100mm bino operating at your referenced 8x would have a 12.5mm exit pupil --- way too big for your eyes. So, even though that would theoretically work, it would be performing at around 8x42 or possibly 8x50. No free lunch!    Tom


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#6 Huan

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Posted 04 August 2020 - 12:37 PM

 

F/3 Binos are not available, they could be made and would need a field flattener. You'd need large prism.

 

 

Does combining two reflectors together work? I'm under an impression that reflectors can achieve fast ratio more easily.



#7 Huan

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Posted 04 August 2020 - 12:47 PM

 

A bigger problem is that the eyepieces are 3 inches in diameter and less than 1% of the population could use them.

 

Do you mean exit pupil? I realized that this is impractical after reading replies from you and Tom. I think I'm looking for a design that can show the greatest number of stars in the eyepieces. That's the experience that I've always been dreaming of.



#8 Rich V.

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Posted 04 August 2020 - 12:54 PM

The main thing you appear to be missing is that generally it takes short focal length instruments to get wide fields; an 8x42 only has about a 180mm focal length.  A 100mm bino has a focal length of 500+mms, limiting FOV with available eyepieces.  Large aperture and short focal lengths are at odds with each other, particularly in binos. 

 

As you increase aperture, a proportional increase in focal length is the natural outcome.  Wide fields are harder to attain.  Many of us deal with this by having a range of different sized instruments to provide the different sized fields we want to examine.

 

Rich


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#9 Mr. Bill

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Posted 04 August 2020 - 02:36 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?

Physics...wink.gif


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#10 Grimnir

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Posted 05 August 2020 - 02:50 AM

I'm looking for a design that can show the greatest number of stars in the eyepieces. That's the experience that I've always been dreaming of.

 

Then read this article:

 

https://www.cloudyni...star-counts-r88

 

Graham


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#11 Ant1

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Posted 05 August 2020 - 04:04 AM

Thanks Graham for pointing it out, this article certainly is a good read!

 

Ant1



#12 PEterW

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Posted 05 August 2020 - 05:25 AM

Reflectors need added beam path, so larger secondaries which affect the view, especially at low power. So for the fastest mirrors it’s not really practical for <12inch diameter for Making into binoculars.

Peter

#13 PEterW

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Posted 05 August 2020 - 06:48 AM

Another recent thread about pushing the boundaries and a lower cost option to the WX. https://www.cloudyni...m45-binoculars/

Peter
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#14 TOMDEY

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Posted 05 August 2020 - 05:06 PM



Do you mean exit pupil? I realized that this is impractical after reading replies from you and Tom. I think I'm looking for a design that can show the greatest number of stars in the eyepieces. That's the experience that I've always been dreaming of.

Yes --- this topic naturally leads to Rich Field Telescopes ... which is where the magnification (eyepiece selection) is set so that exit pupil matches one's dark-adapted eye. And lots has been written on that, over the decades.

 

e.g. here's a table from the classic ATM Trilogy edited by Ingalls, 1937. Looks like people were researching this over a hundred years ago (1916 S. L. Walkden, London).

These books are still available used. Highly recommended.    Tom

 

[I notice that the table assumes a eye pupil of 7.5mm, and concludes that a 2.5-inch telescope operating at 8.5x is the Richest of Rich Field Telescopes. 9x63 binoculars would therefore excel for scanning the Milky Way and indeed be Richest Field.]

 

~click on~ >>>

Attached Thumbnails

  • 63 richest field telescope table ingalles.jpg

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#15 Mark9473

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Posted 05 August 2020 - 05:23 PM

Then read this article:

 

https://www.cloudyni...star-counts-r88

 

Graham

I think Barry's formula for limiting magnitude isn't accurate for the low magnifications typical in binoculars.

I'm not saying mine is perfect, but it's close enough to my experience:

binocular limiting magnitude = naked eye limiting magnitude + 2.5*LOG(magnification) + 2.5*LOG(objective size in cm)

 

I also used NASA's formula on limiting magnitude found here: https://spacemath.gs...rs/6Page103.pdf

 

If I then assume all binoculars have eyepieces with 60° AFOV (and all operate at full aperture, and are of same quality, etc.), it's easily shown that the average number of stars in the FOV is not all that different across common sizes:

 

rich field binoculars.JPG

 

My conclusion if you want the richest field:

 - pick good quality instruments

 - go towards larger exit pupil

 - select the largest AFOV you can find


Edited by Mark9473, 05 August 2020 - 05:31 PM.

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#16 TOMDEY

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Posted 05 August 2020 - 09:31 PM

Yeah, I think the ~solution~ is to make sure you have an eyepiece for each of your scopes that provides Rich Field (for your pupil). Most nights, when I get weary of ~serious observing~, I pop in that low power eyepiece, and just randomly sweep around... kinda like the Herschels, Dobson, Levy... I love watching the field stars cruising on by and enjoy thinking, "I'll bet no one else is looking right here, right now!" Do at least some of that, every observing session... and you will "discover" familiar and unfamiliar asterisms, double/multiple stars, clusters, galaxies, nebulae, Do a lot of that... you begin to sensitize to things that look somehow different than before...    Tom


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#17 Mr. Bill

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Posted 05 August 2020 - 11:12 PM

I enjoy the way that the mind makes patterns out of the stars in a starfield....linear and circular.

 

Sometimes I think there is more than randomness involved; maybe interstellar density winds during star formation.....blush.gif


Edited by Mr. Bill, 06 August 2020 - 12:14 AM.

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#18 PEterW

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Posted 06 August 2020 - 08:04 AM

To really get loads of stars in the eyepiece an image intensifier with suitable filtration can show the urban dweller just what they’re missing and make tracing the Milky Way easy. Otherwise follow the advice above, there’s no secret optic spec... if there was then someone would be selling it!

Peter

#19 Mr. Bill

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Posted 06 August 2020 - 10:08 AM

Seek out the darkest skies....the optics are secondary


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#20 Huan

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Posted 06 August 2020 - 12:17 PM

If I then assume all binoculars have eyepieces with 60° AFOV (and all operate at full aperture, and are of same quality, etc.), it's easily shown that the average number of stars in the FOV is not all that different across common sizes:

 

attachicon.gifrich field binoculars.JPG

 

 

LwlQwyr.jpg

 

Seek out the darkest skies....the optics are secondary

So true! I've been looking at the Sadr Region in the Cygnus in Bortle 8-9 area through my APM 70mm + Pan 24s which should provide me 16.67X and 3.88° TFOV, but I have never seen even remotely close to 200 stars. It's one of my favorite region so I'm pretty confident that I'm familiar with the stars around Sadr. Most of the time I see around 15 to 20 stars. Nevertheless, I'm always amazed after I look at that direction with naked eyes again because all I could see is one faint star.  

 

I don't have any 8x42 right now but both my Canon IS 10x42 and 10x32 show around 15 star to 20 stars. The APM does reveal more stars in more adjacent region to Sadr but the Canons have larger FOV.

I didn't actually count when I was viewing through them in upstate in Bortle class 4, I would say around 40 stars in the FOV. I will test it again next time when I'm under Bortle 4 sky. Will I be able to see around 200 stars in the FOV with my equipment in international Dark-Sky Parks? I want to believe it but it's really hard for me to imagine that.


Edited by Huan, 06 August 2020 - 12:35 PM.


#21 Mr. Bill

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Posted 06 August 2020 - 12:23 PM

I'm in a Bortle 3/4 location and enjoy the region surrounding Sadr for the several patches of nebulosity which on a good night show up well with my 70mm APMs and 17.5mm Morpheus eps (23x)



#22 Mark9473

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Posted 06 August 2020 - 12:50 PM

Huan, did you notice that my calculations were based on a naked eye limiting magnitude of 6? In a poor sky you would obviously see less.

That said, Cygnus is one of the richest regions in the sky, and seeing only 20 stars in the FOV sounds like you are looking on a hazy Full Moon night from a Bortle 9 sky.

Perhaps sketch what you see and then count?

#23 Huan

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Posted 06 August 2020 - 01:54 PM

Thank you for the suggestion, Mark! I will do it.

 

Having said that, I'm indeed living in a Bortle 9 area unfortunately. Orange street lights are everywhere around my house and the small playground where I observe most of the time. Big Dipper always show me six stars and I need my vixen 2.1 x 42 to see Megrez. So I concluded that the naked eye limit in my area is magnitude 2.5-3.0.

Considering this I feel quite proud that I was able to find Neowise in two nights lol.gif 

98P9KAc.jpg

July 21 night shot with iPhone behind APM 70 mm with the original 18 mm eyepiece ( there's even a touch of greenish hue!! )

 

prqTcu9.jpg
The star hopping route that I shared with my photographer friend as well as commemorating the first comet I've ever seen in my life.
 


Edited by Huan, 06 August 2020 - 03:31 PM.


#24 Mark9473

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Posted 06 August 2020 - 04:37 PM

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:

 

APM70.JPG

 


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#25 jprideaux

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Posted 07 August 2020 - 05:26 AM

People in the EAA forum use two eyes to look at monitor screens to see near-real-time views using greatly reduced large aperture telescopes like a 8” scope reduced to F2 (400 mm FL). These systems are complicated and are no way as convenient as binoculars. One can only “cheat” the laws of physics so far.

Edited by jprideaux, 07 August 2020 - 05:27 AM.



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