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39 replies to this topic

### #1 pjmulka

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Posted 08 August 2022 - 12:21 AM

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

### #2 ParticularDynamics

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Posted 08 August 2022 - 12:56 AM

Focal ratio (f/) is determined by focal distance and aperture. If you change this ratio by changing focal distance only while aperture is the same, then what you are saying is correct.
If you change the ratio by only changing mirror size, then the image appears more or less bright but object size is unchanged.
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### #3 Dave Mitsky

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Posted 08 August 2022 - 01:26 AM

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### #4 pjmulka

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Posted 08 August 2022 - 02:10 AM

When the newt calculations list linear resolution. Does that have to do with the field of view or the actual clarity of the image?

### #5 TOMDEY

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Posted 08 August 2022 - 03:33 AM

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

If by f/ you mean the Newtonian Primary mirror's focal length... the answer is yes!    Tom

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### #6 Tony Flanders

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Posted 08 August 2022 - 05:14 AM

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

No, that is true only if you are constrained to use one particular eyepiece. If you are allowed to select different eyepieces depending on the primary mirror's focal ratio, then to a good first approximation that primary mirror's focal ratio has no effect on what you see through the eyepiece.

Let me give an example. One of the most common telescopes is the 8-inch f/10 SCT (Schmidt-Cassegrain Telescope). Possibly even more common is the 8-inch f/6 Dobsonian. As it happens, 8 inches = 203.2 mm, but for the purposes of this discussion we will pretend they both have 200-mm objectives. That means that the focal length of the SCT is 8 * 200 = 2000 mm and the focal length of the Dob is 6 * 200 = 1200 mm.

Let's further assume that both telescopes are being used with a 10-mm Plossl eyepiece. Then the magnification of the SCT is 2000 / 10 = 200X, and the magnification of the Dob is 1200 / 10 = 120X. So using that same eyepiece, your initial statement is correct: the SCT shows more detail, and the Dob has a bigger field of view.

But if the Dob owner wants to achieve the same magnification as the SCT, all they have to do is switch over to a 6-mm Plossl. At that point the Dob's magnification is 200X, just like the SCT with a 10-mm eyepiece. So to a good first approximation, the view through an f/10 scope with a 10-mm eyepiece is identical to the view through an f/6 scope with a 6-mm eyepiece.

Conversely, if the SCT owner wants to achieve the same field of view as the Dob owner with the 10-mm eyepiece, they have to switch to a 16.7-mm eyepiece, at which point both scopes will be operating at 120X, and will show the same field of view.

Now for the fine print. First of all, the eye relief on a 6-mm Plossl is uncomfortably short. So the Dob owner wishing to achieve 200X is more likely to need a somewhat more expensive eyepiece than the SCT owner to achieve an equally comfortable view.

More importantly, there's a limit to how long an eyepiece's focal length can be. The maximum field of view possible through a scope with a 1.25-inch focuser is achieved with a 32-mm Plossl. That yields a magnification of 1200 / 32 = 37.5X with the Dob. Most 32-mm Plossls have 50-degree apparent fields of view, so the maximum possible true field of view of an 8-inch f/6 Dob with a 1.25-inch focuser is 50/37.5 = 1.33 degrees.

For the SCT owner to achieve the same magnification and field of view, they would need an eyepiece with a 53.3-mm focal length. But a 50-mm or 55-mm Plossl is too fat to fit in a 1.25-inch focuser. So the maximum possible field of view of an f/10 scope with a 1.25-inch focuser is smaller than the maximum possible field of view of an f/6 scope with a 1.25-inch focuser.

The same is not true at the high-power end of the spectrum, because while there are physical constraints on how long an eyepiece focal length can be, there are no physical constraints on making eyepieces with short focal lengths. So the maximum useful magnification for both scopes depends entirely on the aperture, and not on the focal ratio.

In other words, short-focal-ratio scopes are more versatile than ones with long focal ratios. Short focal-ratio scopes can achieve the same high magnifications as long-focal-ratio scopes by using eyepieces with ever-shorter focal lengths. But they can also achieve lower magnifications -- and therefore wider fields of view -- than are possible through scopes with long focal ratios.

The primary mirror's focal ratio does have some second-order effects as far as aberrations are concerned; in particular, mirrors with short focal ratios have more coma. But that's a whole 'nother discussion.

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### #7 kathyastro

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Posted 08 August 2022 - 06:43 AM

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

By "f/", I presume you mean focal ratio.  The ratio is a consequence of focal length and aperture; it is not something that you change directly.  So when you talk about a larger focal ratio, did you change the focal length or the aperture?

Increasing the focal length (which will increase the focal ratio) will reduce your field of view.  Reducing the aperture (which will also increase the focal ratio) will do nothing to the field of view, but will reduce the resolution.

Your statement about focal ratio changing the field of view, although it is often repeated on this forum, is incorrect or at least incomplete.  Changing the numerator of the focal ratio (i.e. the focal length) does change the field of view.  Changing the denominator (i.e. the aperture) does not.

Edited by kathyastro, 08 August 2022 - 07:23 AM.

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### #8 csa/montana

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Posted 08 August 2022 - 09:07 AM

Moved to Equipment for better fit.

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### #9 pjmulka

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Posted 08 August 2022 - 09:47 AM

So If I’m understanding the responses to my first question correctly. Saturn will be larger in a 6mm eyepiece with a 10” f/8 newt dob than it would be using a 6mm eyepiece with a 10” f/6 newt dob?
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Posted 08 August 2022 - 10:19 AM

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

Given a single eyepiece comparison you are correct.  The apparent FOV of the eyepiece is irrelevant to discussing the effect of f/ ratios.
Let us take the example of an F/5 8" Newtonian and an F/10 8" Newtonian.

If we view M42, Orion Nebula, thru a 25mm 60 degree eyepiece, in the F/5 example we will get the following field of view.

If we swap the eyepiece over to a 8" F/10 this is the field of view. Note the size of the nebula has been magnified in the field of view.

If we overlay the two images to see the change in the overall actual field of view this is the result with the AFOV 60 degree eyepiece.

Optically what happens when the focal ratio moves from F/5 to F/10 is the light of diffuse targets spreads across more of the focal plane(point sources of distant stars don't typically alter to our eyes but double stars apparent separation will increase).

The 25mm eyepiece has an exit pupil of 5mm at F/5 but reduces to 2.5mm when moving to F/10.  The effect to our eye is the target looks less bright in the F/10 example in the same eyepiece.  This is due to how rods & cones in our eyes react to the light. When the focal plane increases in size and diffuse targets are reduced in size on the focal plane more light hits a smaller group of rods and cones, so the diffuse target looks brighter.  This same effect is seen when swapping eyepieces to change magnification, and why when we increase magnification some less bright targets lose apparent brightness.  And conversely as we decrease magnification some targets are easier to see, though we see less nuanced detail(depending on the aperture).

One final word, the AFOV or Apparent Field Of View of eyepieces affects how much of the surrounding area you see WITHOUT affecting the magnification.  In this case the image below shows the effect of changing from 60>82>100 AFOV 25mm eyepieces.  The magnification stays the same but the amount of surrounding area increases.  Much like changing the size of the window on your house... you see more/less of the outside through it.

### #11 kathyastro

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Posted 08 August 2022 - 10:20 AM

So If I’m understanding the responses to my first question correctly. Saturn will be larger in a 6mm eyepiece with a 10” f/8 newt dob than it would be using a 6mm eyepiece with a 10” f/6 newt dob?

Correct.  2000mm focal length vs. 1500mm focal length.

Edited by kathyastro, 08 August 2022 - 10:21 AM.

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### #12 Sketcher

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Posted 08 August 2022 - 10:26 AM

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

It's not correct to say that a larger focal ratio will result in a smaller field of view, nor is it correct to say that a larger focal ratio will make the observed object larger in the eyepiece.

The focal length determines the scale if the real image that is formed by a Newtonian's primary mirror.  The longer the focal length is, the larger the true image will be at the telescope's focal plane.  It's the scale of that real image, along with how much of that real image can be taken in by a given eyepiece, that determines one's true field of view.  The mirror's focal length divided by the eyepiece's focal length determines magnification -- how large the object will appear to be in the eyepiece.  Focal ratio, by itself, has no bearing on these matters.

If you really want to understand these things, I suggest that you conduct some experimentation.  You can start by looking at the real image that's formed by the primary mirror (or by the primary optics of any telescope for that matter).

So If I’m understanding the responses to my first question correctly. Saturn will be larger in a 6mm eyepiece with a 10” f/8 newt dob than it would be using a 6mm eyepiece with a 10” f/6 newt dob?

That's correct!  And the reason it's correct is because the 10-inch f/8 has an 80-inch focal length and the 10-inch f/6 has a 60-inch focal length.  When using the same eyepiece with each telescope, the telescope with the longer focal length will produce the more highly magnified view.

If you had been comparing the views in 10-inch f/8 with a 20-inch f/6 (with the same eyepiece) it would be the 20-inch f/6 that would provide you with the larger view of Saturn, since the 10-inch f/8 has an 80-inch focal length and the 20-inch f/6 has a longer, 120-inch focal length.  The focal ratio isn't the relevant factor.  It's the focal-length of the primary optics that matter.

As for the eyepiece, a 6mm eyepiece magnifies a telescope's focal plane (real) image by the same amount, regardless of which telescope it's used in.  So, if one is using the same eyepiece with two different telescopes, that eyepiece will provide a larger view to the eye (a greater magnification) when used with the telescope that has the larger real image at its focal plane -- which would be the telescope that has the greater focal length.

### #13 pjmulka

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Posted 08 August 2022 - 11:03 AM

Thanks so much everyone for your time and experience. I don’t know why I’m having such a hard time wrapping my head around this.

### #14 davidgmd

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Posted 08 August 2022 - 06:42 PM

Some formulae to play with:

f/ratio = Telescope focal length/Telescope aperture
Magnification = Telescope focal length/Eyepiece focal length

True field of view ~ Eyepiece field of view/Magnification (approximately)

True field of view = 57.3 x Eyepiece field stop/Telescope focal length

This website can give you some reasonable field of view approximations for various scope and eyepiece combinations: https://astronomy.to.../field_of_view/

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### #15 Chris Johnson

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Posted 08 August 2022 - 07:23 PM

So If I’m understanding the responses to my first question correctly. Saturn will be larger in a 6mm eyepiece with a 10” f/8 newt dob than it would be using a 6mm eyepiece with a 10” f/6 newt dob?

Yes, that is because the 10”f8 scope has a focal length of 80” while the 10”f6 scope has a shorter focal length of 60”. The size of any extended image is only dependent on the focal length of the telescope. The longer the focal length the larger the image produced. It has nothing to do with the focal ratio.

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Posted 08 August 2022 - 07:55 PM

Not sure why people are afraid to discuss F/ ratio effects in the manner of the OP's statement.

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

The OP wasn't asking is a 36" F/5 brighter & smaller image than an 20" F/10. Nor did the OP ask, if the focal length was varied.  The OP asked if the F/ ratio change had the effects he described, that being smaller FOV & more magnified.  That means understanding what F/ ratio designates; which to those in the know is changing the focal length to change the ratio.

In the specific question the OP's statement is accurate and truthful because the F/ ratio does have the effect he describes when the F/ ratio is increased.

Worrying about the math behind F/ ratio (focal length/aperture) is tangential to the higher level question asked.

### #17 kathyastro

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Posted 08 August 2022 - 08:02 PM

That means understanding what F/ ratio designates; which to those in the know is changing the focal length to change the ratio.

That statement is untrue.  Focal ratio can be changed either by changing the focal length or by changing the aperture.  One of those changes affects field of view and magnification, the other doesn't.

It is really important, when talking about focal ratio, to separate the two effects, focal length and aperture, and to be clear about which one causes which effect.

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Posted 08 August 2022 - 09:04 PM

That statement is untrue.  Focal ratio can be changed either by changing the focal length or by changing the aperture.  One of those changes affects field of view and magnification, the other doesn't.

It is really important, when talking about focal ratio, to separate the two effects, focal length and aperture, and to be clear about which one causes which effect.

While you can make that case... in the OP discussion they are NOT intimating a change of aperture.
You're introducing the lower level math into a higher level general discussion of merely discussing changing A mirror's F/ ratio.  If you've got an 8 mirror blank then considering F/ ratio is your first step.

In ATM terms when deciding how you'd like a mirror to perform, whether for DSO or for Planetary it isn't necessary to first discuss focal length nor even aperture, but simply to state an F/5 instrument is more suited to DSO while an F/10 is more suited to Planetary.  After that point discussing specific focal length can come into play, but until then it's a secondary discussion. And also at that point discussion can involve aperture and focal length pros and cons where varying either is important to the end result.

John Dobson wouldn't worry about specific focal length in grinding sessions, but would simply steer you toward your goal (DSO/wide or Planetary).  If you ground an F/6.5 he'd say that'll be good for wide views, or if it was F/8.2 he'd say that'll be better for planets.

Don't overly complicate the simple.  I got my Bachelor's degree in understanding systems and being able to explain to clients what engineer technical figures were all about.  The client didn't care about nitty gritty in an overview... he wants to know in non-engineering jargon what he's going to get.  And likewise, talking to engineers, being able to translate a client's non-engineering demand for speed or flexibility or reliability into ranges of specifications an Engineer can take to the drawing board.

Edited by carolinaskies, 08 August 2022 - 09:05 PM.

### #19 kathyastro

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Posted 09 August 2022 - 06:10 AM

Don't overly complicate the simple.  I got my Bachelor's degree in understanding systems and being able to explain to clients what engineer technical figures were all about.  The client didn't care about nitty gritty in an overview... he wants to know in non-engineering jargon what he's going to get.  And likewise, talking to engineers, being able to translate a client's non-engineering demand for speed or flexibility or reliability into ranges of specifications an Engineer can take to the drawing board.

I'll see your Bachelor's degree in understanding systems and raise you a teaching certificate and years of experience to go with it.

The OP didn't mention ATM.  For all we know, he may be trying to decide between two telescopes he is considering purchasing.  If the goal is making a telescope, by all means offer whatever shortcuts work.  When the goal is understanding, it is important to keep dependent and independent variables separate.  Otherwise confusion results

Much of the confusion we routinely see on the beginner pages is due to misunderstanding focal ratio as though it were an independent variable.

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Posted 09 August 2022 - 07:18 AM

My best teachers didn't approach a topic by boring students with detail first. That's a recipe for teaching only the detail oriented teachers pet and leaving the rest of the class wondering what the hell the crazy teacher is talking about.

Got a family full of good teachers. As a student I didn't have a problem understanding. But I didn't earn the nickname professor by my fellow students for not helping them understand the crazy teacher. But by translation. As a teacher you need to not only know the subject you teach but the students perspective.

The OP question is SIMPLE. Is their theory correct? Answer. YES.

Your perspective "No because F/ ratio is meaningless" But the F/ ratio is NOT meaningless to the OP because it is part of the question asked, and you prefer to alter the OPs question to suit your perspective in a negative fashion. F/ ratio isn't a made up term; it's a term defined in our understanding of optical systems. You treat it blithely, rather than accepting it as a reasonable variable.

Simple answers don't have to be unpacked at high levels. You imply the OP thinks F/ ratio is an independent variable. But there is no issue treating it as such if the question and answer are framed correctly. Which both were. That is why my response to it was framed without involving either change in aperture or change in eyepiece. Nor the underlying variable of focal length. If asked "how is the F/ ratio changed for a specific optic?" Then discuss change in focal length.

When we discuss telescopes we are constantly modifying focal ratios and focal lengths. We discuss reducers in terms of changing focal ratios. The effects of a .7x or a .63x or .8x resulting in a new focal ratio. And the effect the reducer has in widening the field of view and reducing magnification.

So you see, the OPs question isn't bad, incorrect, or flawed. It's simply examining optical effect from a perspective of the F/ ratio.

Sorry to disassemble things so much, but you've got a teachers certificate so surely you can understand multiple perspectives.

### #21 pjmulka

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Posted 09 August 2022 - 10:38 AM

I grinding a 10” mirror right now. I over shot the sag by a lot and I’m trying to figure out where to go from here. Currently I’m at f/ 8. Something I’m 5’11” and live in a very light polluted suburb of Detroit. I have 5 kids and don’t get a night to drive to dark skies very often so I’m looking to go with the best f/ I can for planetary viewing that I don’t need to stand on anything at the eyepiece.

I'll see your Bachelor's degree in understanding systems and raise you a teaching certificate and years of experience to go with it.

The OP didn't mention ATM. For all we know, he may be trying to decide between two telescopes he is considering purchasing. If the goal is making a telescope, by all means offer whatever shortcuts work. When the goal is understanding, it is important to keep dependent and independent variables separate. Otherwise confusion results

Much of the confusion we routinely see on the beginner pages is due to misunderstanding focal ratio as though it were an independent variable.

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Posted 09 August 2022 - 02:08 PM

I grinding a 10” mirror right now. I over shot the sag by a lot and I’m trying to figure out where to go from here. Currently I’m at f/ 8. Something I’m 5’11” and live in a very light polluted suburb of Detroit. I have 5 kids and don’t get a night to drive to dark skies very often so I’m looking to go with the best f/ I can for planetary viewing that I don’t need to stand on anything at the eyepiece.

F/8 for planetary in a 10" is great. If you're planning mostly planetary observing the ecliptic is much lower so maximum focuser height at zenith isn't a huge deal.  Have you decided your rocker box configuration?  If you're going for low profile truss then even at zenith only a small step ladder is necessary for overhead objects... I know not what you want to hear.

This is why there are a lot more F/6.5 & F/7 ATM 10" as a compromise to get the eyepiece height down.  Planetary views will be pushing the eyepiece focal length shorter to get magnification up.  Companies use to offer 10" F/6 & 6.5 for clock drive mounts where the bottom of the tube was just above ground level when vertical.  I have seen people make single arm  ATM projects to do this manually, though with a 10" it would require adding counter-balancing weight to the base so as not to tip over.

### #23 Keith Rivich

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Posted 09 August 2022 - 08:34 PM

I’m trying to understand how a mirror’s f/ effects what is seen in the eyepiece. Is it correct to say that a larger f/ will have a smaller field of view but will make the object observed larger in the eyepiece?

Lots of technical stuff here.

If you have two 10" scopes, one f/4 the other f/10, and you use the same eyepiece in both scopes, your comment is absolutely correct.

### #24 Jon Isaacs

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Posted 09 August 2022 - 08:57 PM

I grinding a 10” mirror right now. I over shot the sag by a lot and I’m trying to figure out where to go from here. Currently I’m at f/ 8. Something I’m 5’11” and live in a very light polluted suburb of Detroit. I have 5 kids and don’t get a night to drive to dark skies very often so I’m looking to go with the best f/ I can for planetary viewing that I don’t need to stand on anything at the eyepiece.

Hi:

This is the way I think about it:

Magnification is a choice, I have a selection of eyepieces, I know the aperture and focal length of the scope,  I choose the eyepiece to provide the view I want.  Don't worry about focal ratio/focal length, there are eyepieces, Barlows to achieve any magnification.

For a Dob, the eyepiece height will be very close to the focal length. I'm 6ft. For me, a 68 inch eyepiece height is about the maximum without a stool or ladder.

For planetary, mirror quality is more important than focal ratio. Slower mirrors are easier to make.

Personally, I think F/6 is a nice place to be. F/7 is too tall.

Jon

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### #25 pjmulka

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Posted 10 August 2022 - 12:24 AM

It’s going to be a truss.

F/8 for planetary in a 10" is great. If you're planning mostly planetary observing the ecliptic is much lower so maximum focuser height at zenith isn't a huge deal.  Have you decided your rocker box configuration?  If you're going for low profile truss then even at zenith only a small step ladder is necessary for overhead objects... I know not what you want to hear.

This is why there are a lot more F/6.5 & F/7 ATM 10" as a compromise to get the eyepiece height down.  Planetary views will be pushing the eyepiece focal length shorter to get magnification up.  Companies use to offer 10" F/6 & 6.5 for clock drive mounts where the bottom of the tube was just above ground level when vertical.  I have seen people make single arm  ATM projects to do this manually, though with a 10" it would require adding counter-balancing weight to the base so as not to tip over.

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