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#1 Jhirschauer

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Posted 18 November 2018 - 07:54 PM

I have been trying to find a formula for determining the physical size ( not field of view ) of the image circle (at infinity focus) for any telescope of a given aperture and focal length.

It seems that this should be a pretty basic thing so consequently it must be that I am missing something pretty basic.

Can anyone help me out?

I've seen some other threads on this web site about this but there did not seem to be any conclusions.



#2 Alex McConahay

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Posted 18 November 2018 - 08:24 PM

I do not think you can calculate the image circle from just focal length and aperture. It would also depend on some mechanical features. Of course, there may be more to know, and somebody may have the real answer.

 

Alex



#3 ccs_hello

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Posted 18 November 2018 - 08:47 PM

Image circle is an OTA's design spec. and many (at least, inexpensive ones) do not publish it.

 

My method is to point the OTA at the sky in daylight (but not Sun) and place a piece of white paper at the focal plane.

Adjust focus knob to find the sharp focus (see a reverse image sharply in focus, as a far away tower.)

You'll find a bright circle on that piece of paper and guess what?  That's the answer you are looking for.


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#4 bobzeq25

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Posted 18 November 2018 - 10:08 PM

Image circle is not a hard and fast thing.  Any optical system has some vignetting (and processing exaggerates it).  How much is too much?   Star shapes get a bit distorted at the edges.  Again, how much is too much?

 

There are no real standards for those questions.  Most manufacturers don't want to get into disputes about "defective" scopes by publishing an image circle.  A few high end manufacturers have enough faith in their products to do so.  Or they may publish actual numbers for percent vignetting at various distances from the center, and charts of star size and shape at various distances.  Those things are much more objective standards.


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#5 Jerry Lodriguss

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Posted 19 November 2018 - 04:50 AM

 

I have been trying to find a formula for determining the physical size ( not field of view ) of the image circle (at infinity focus) for any telescope of a given aperture and focal length.

It seems that this should be a pretty basic thing so consequently it must be that I am missing something pretty basic.

Short answer: There are many image circles. One for each quality you want to consider like illumination, spot size, coma, etc.

 

TL;DR answer follows...

 

I agree with Bob. smile.gif Bob may find that amusing. lol.gif But I love Bob. We actually agree on like 99% of things. Then we get into it on the little bit left because we are both old and like to yell at each other because we are deaf and we can't hear each other.grin.gif

 

On really high end scopes where they are on the far end of the quality curve, they actually measure many different scope attributes and each has an image circle. So, it's actually "image circles" that you need to worry about. Each of these things can be calculated, and then compared to an ideal. Then you have to figure how each influences each other.  Ha. Pretty complicated stuff.

 

- Spot size

- Illumination

- Coma

- Astigmatism

- Spero-Chromaticism

- Surface flatness

- Surface curve accuracy

- Field flatness

 

You have to define the size of the aberration you are willing to put up with at a certain distance from the center of the sensor - this is the image circle for that aberration.

 

And all of this stuff gets worse as you move away from the center of the field, and the image circle gets bigger.

 

Then you also have to consider stuff like mechanical vignetting, not associated with the optics at all, but stuff like drawtube size at a given distance from the sensor.

 

Bigger sensors require larger corrected image circles and larger everything else associated with it.

 

So, really, the important thing is that you need to think about all of these things and the size of the sensor you want to cover.

 

Then you balance that with your checkbook. 

 

Then you decide how much you can put up with each parameter as you make the system faster, plus how much that costs in money in terms of materials, design, and actually manufacturing a system that can hold those tolerances when you carry it in the back of your truck bounding down the dusty roads to your dark sky observing site.cool.gif

 

In the end, you have to compromise somewhere. I put up with some aberrations in fast systems because I can't afford the expensive pieces of glass (meaning everything mentioned) that corrects them better at the apertures I want to shoot at.

 

But you would be surprised at what you can get away with if you don't flopping pixel peep.lol.gif

 

And if you are one of those people who just have to have the best because you can afford it, there usually is a pretty good correlation between quality and cost - as there is for most things.frown.gif

 

Jerry


Edited by Jerry Lodriguss, 19 November 2018 - 05:19 AM.

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#6 jdk

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Posted 19 November 2018 - 05:56 AM

I'm not sure why things like MTF charts are not available "as standard" for telescopes like they are for camera lenses. That would convey a lot of the information that Jerry describes.



#7 Goofi

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Posted 19 November 2018 - 12:57 PM

You're getting good responses, I'll add one more thing to consider ....

 

When  a company advertises the size on an image circle, that's kind of a minimum guaranteed image circle, not a maximum. Often, you can find a scope with a claimed 44mm image circle. It isn't unusual to get a little more, maybe 50mm, but all the company is claiming is if your sensor is <44mm, you'll be fully illuminated.

 

Also, don't forget reducers impact the size of the image circle.


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#8 Alex McConahay

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Posted 19 November 2018 - 01:44 PM

I know most of you don't care what an eyepiece is, but, c'mon.....I gotta talk about them sometimes. 

 

I have a 82 degree apparent field of view eyepiece that costs less than $100 at 30 mm. An equivalent Focal Length Nagler would cost three or four times that. How did I get one so inexpensively. The manufacturer thoughtfully neglected to put in a field stop that eliminated the aberrations that one normally finds at the edge of a field. This left me with a larger image circle than the lens should have had.

 

Nagler and company wasted all that time designing better lenses that actually were sharp out to the edge when they could have just put a larger field stop in a very much cheaper design. 

 

SO, I have a bigger image circle than I should have because they did not include this field stop. It enables me to see a wider field. Of course it is not as nice as being able to see sharper stars out at the edge. But it is an 82 degree field of view. 

 

This is one reason there is no simple definition of Image circle. 

 

Alex


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#9 ccs_hello

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Posted 19 November 2018 - 08:17 PM

I think people ought to see compromises are everyday life of astro.

People use simple inexpensive double lens ($40- $200) as focal reducer while a very high quality (multi-element, multi-group) F.R. would cost upward $500 -$700.

Aberration or not, from almost no illumination (sort of 10% partial illumination) to fully illumination, people won't complain and said flat "will fix it".

 

Even it's now a smaller 100% fully illuminated area, there is yet even smaller sweet-spot area that has less optical distortion. 

 

My point: a simple estimation using eyes provides a ballpark figure that one can use to estimate how far off...

 

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Edited by ccs_hello, 19 November 2018 - 08:26 PM.



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