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If a slow scope is for planets...

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#26 dakinemaui

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Posted 25 March 2019 - 11:07 AM

0.5 mm is a practical exit pupil to call the magnification ceiling of any scope.

This is equivalent to the oft-cited "2x aperture in mm" rule of thumb, but I'm curious if you've rationalized 0.5 mm pupil as a limit on its own merit?



#27 SeattleScott

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Posted 25 March 2019 - 11:45 AM

I have often heard that the human eye cannot resolve additional detail under .5mm exit pupil. If the object is bright enough, you can crank up the power to .3mm or whatever. This will give you greater image scale, but not additional detail. An extreme example was when I briefly hooked up a VHS player to my new 50” plasma. I could see Will Smith bigger than ever but the image was very grainy. VHS was not made at high enough resolution for a 50” screen, and given the media is not digital, the TV could not autoscale to improve resolution. Similarly I have tried using 420x magnification with my 6” Mak. The image was big, but soft. A lot of empty magnification. The view was very nice at 265x which is within the useful range for the scope, at around 6mm exit pupil.

Scott
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#28 astro744

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Posted 25 March 2019 - 11:59 AM

I want to thank all of you for your input.

 

I guess I am trying to prove that my very first (and only, for now) telescope is not the wrong telescope, as subjective as this may be. I currently own a 2" 150/750 scope that is apparently optimized for astrophotography (secondary mirror is 63mm) and I have three 2" 70° eyepieces, 22mm, 13mm and 5mm. I do not do any astrophotography. Visual and DSOs is all I care about.

 

Living in a class 4/5 Bortle zone and a alt-az mount with a 9kg load capacity, I was wondering if I can somehow upgrade/optimize my rig. The obvious upgrade would be a dobson , though I hate the fact that it needs more storage space than my current reflector. Plus, I am fairly sure it is not as comfortable as my reflector on the mount (I do not use a chair).

 

A 6" reflector is too heavy for my mount and I have no intention switching to an EQ mount. A 4-5" reflector would probably be a downgrade for DSOs and (having tried a Mak in the past) I will probably find looking through it to be a very uncomfortable experience (neck-wise). 

 

Would a 150/1200 reflector be better for DSOs than my current scope or is a dobson the only way to go?

A 150/750 and 150/1200 will give you a similar visual experience with the latter offering better contrast due to the smaller central obstruction.  Both are Newtonian reflectors.  Both can be mounted on an EQ mount or on a Dobson mount.  The 150/750 is described as suitable for astrophotography not so much because of its focal ratio but because it has a large secondary mirror to give more off axis illumination, it probably has a focuser optimised to take a camera thereby requiring extension tubes for visual and it is generally always sold on an EQ mount or OTA only for use on an EQ mount as this is an essential component in long exposure astrophotography.

 

As for field of view this is purely a function of focal length of the telescope and field stop diameter of the eyepiece or linear dimensions of photographic sensor.  For any given eyepiece the telescope with the shortest focal length will give the greatest true field of view.  Likewise for any given camera.  Focal ratio does not come into it at all nor does magnification since you can have an eyepiece of the same field stop diameter but with different focal length.

 

True field of view (deg) = Field stop diameter (mm)  x 57.3 / Focal length of telescope (mm)

 

Exit pupil too is not going to determine how much sky you see.  If you say had a 90mm refractor with 750mm focal length giving approx f8, you will see the same amount of sky as your 150/750 telescope.

 

In general for DSO viewing you want a larger a aperture.  I started with a 6" f5.5 (152/840) and still have this telescope and it will give a lifetime of DSO viewing.  However a larger aperture telescope is capable of showing you the same objects but brighter and also previously undetectable objects become detectable (faint galaxies).  My largest aperture telescope is a 10.1" f6.4 (256/1640) Newtonian.  I have looked through larger and many DSOs take on a whole new appearance at larger apertures (20" - 50cm/any focal length or ratio), particularly globular clusters.

 

Most large aperture telescopes are on Dobsonian mounts and most used to be around F5 and many still are but as aperture goes up, the trend now has been for focal ratio to go down to give a more manageable setup so f3.5 is becoming common.  Focal length can still be quite long (say 2100mm for a 60cm telescope) so field of view is governed by the 2100mm focal length not the f3.5 focal ratio.  Whether you use this telescope for DSOs or planets is up to you as you'll get excellent performance on both (provided a first class mirror).

 

Note if you are considering another telescope then as you already have a 150/750 then perhaps consider either 200/1200 or 250/1200 in lieu if 150/1200 to give you greater aperture required for many DSOs.  Note they all have the same focal length and are therefore capable of giving the same true field of view.  Enjoy!



#29 dakinemaui

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Posted 25 March 2019 - 12:50 PM

I have often heard that the human eye cannot resolve additional detail under .5mm exit pupil.

The resolution limit of the eye is typically quoted at 1 arcminute. If you don't want to lose resolution, this constrains the minimum magnification at right around Mmin = D [mm] when you work through the math (visible light).

 

I've rationalized the maximum magnification similar to you as a desire to avoid "empty" magnification. Doing this, the idea is to avoid excessive "oversampling", and 2x is an arbitrary but perhaps reasonable criterion. (The "samples" have an angular apparent extent 1 arcminute.)  This yields to Mmax = 2D [mm], assuming seeing doesn't limit your resolution. This, in turn, gives the 0.5 mm pupil size as a result. (As an aside, this ignores the fact that resolution is typically constrained by seeing rather than aperture size, so likely a better "rule" would be independent of the aperture.)

 

At any rate, I've wondered if there's a more direct consideration to constrain pupil size but haven't ever come up with anything. I appreciate your thoughts!



#30 Sketcher

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Posted 25 March 2019 - 01:11 PM

1) a 150/750 (f/5 - fast) or 2) a 150/1200 (f/8 - slower) 

 

So, which of the two scopes is suitable for DSOs (visual only) and why?

It's all a matter of opinion.  In my opinion, both are suitable for visual, DSO use.

 

My preference would be for the 6-inch f/8 -- smaller obstruction, less serious coma, less sensitive collimation, less expensive eyepieces will perform well with it.

 

But since you have the f/5, use it and enjoy it.  It's a very capable visual, DSO telescope in it's own right.  With the right  eyepiece, you can achieve a wider true FOV than you could with the f/8 -- potentially more pleasing views of the Pleiades, M31-32-110, and other large objects.

 

No telescope is ideal for everything!


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#31 Tony Flanders

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Posted 25 March 2019 - 02:13 PM

I guess I am trying to prove that my very first (and only, for now) telescope is not the wrong telescope, as subjective as this may be.
 
I currently own a 2" 150/750 scope that is apparently optimized for astrophotography (secondary mirror is 63mm) ...


Your first telescope is never the wrong scope! Truth be told, all decent telescopes can be used for any kind of visual observing. Some may be tweaked a smidge toward one goal or another, but those differences are pretty small. In any case, the telescope's optical and mechanical quality are far more important than the f/ratio or the size of the secondary mirror.

 

Personally, I find f/5 to be the ideal focal ratio for deep-sky observing with a Newtonian, all other things being equal. Coma is visible but very modest, and it's easy to achieve a 7-mm exit pupil with widely available eyepieces.

 

In fact, I think that a 150-mm f/5 Newtonian with a 2-inch focuser hits a very particular sweet spot, achieving the best deep-sky views possible within its portability class. I am considering purchasing one to replace my current 130-mm f/5 Newtonian.

 

I would prefer a 150-mm f/5 to a 150-mm f/8 on optical grounds alone, and the fact that the tube is much shorter is a significant side-benefit. But mind you, if somebody gave me a high-quality 150-mm f/8 Newtonian, I would be thrilled, and I would use it very happily for deep-sky observing as well as planetary observing.

 

I would prefer a somewhat smaller secondary mirror. But the difference between a 63-mm secondary mirror and, say, a 45-mm secondary mirror is pretty subtle.

 

In very large apertures, Newtonian owners almost always prefer faster focal ratios simply to keep the instrument more compact, and to allow them to look through the eyepiece without using a ladder.


Edited by Tony Flanders, 25 March 2019 - 02:19 PM.

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#32 sg6

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Posted 26 March 2019 - 03:48 AM

..and a fast scope for astrophotography, what would be the ideal focal ratio for DSO (visual)?

 

Playing around with Stellarium, I can see that the Andromeda galaxy (one of the largest celestial objects, if I am not mistaken) requires a 2.1° FoV in order to fit in the eyepiece. I can achieve that using 1) a 150/750 (f/5 - fast) scope with a 23mm 70° eyepiece or 2) a 150/1200 (f/8 - slower)  scope with a 36mm 70° eyepiece.

 

So, which of the two scopes is suitable for DSOs (visual only) and why?

 

Clear skies!

 

A scope is not really "for" one or another. Aspects of a scope suit one or another a bit better but even then not exactly correct.

 

Fast scope for astrophotography.

Is it ?

Smaller image (brighter) so short exposure or shorter exposures is what you are told.

But, short focal length and so greater image curvature so poor image on sensor. Is what you are not told.

 

To image you are almost certain to need a coma corrector. So is the f/5 "optimised" for AP?

I suggest it is not.

 

The bigger secondary may or may not help, it is where the prime image falls that is relevant. Usually that means the primary mirror is higher up the tube and so the light cone at the secondary is wider. To capture the wider cone they have to put a bit bigger secondary in. Have they said the mirror is moved or located higher up.

 

All scopes for anything need to produce a good clear sharp high contrast image.

Getting that good quality image is the problem. Astronomers want the cheapest cost but expect the highest performance, and it does not happen. Really it is that simple. You want good, you pay for it.

 

Your scopes are pretty good, nothing wrong in them. But you need to understand their strong and weak points, and use the scope best suited.

 

At a recent outreach I was asked to point my ETX-70  at Mars for people, his scope had died. I just laughed and refused. The ETX was very good on the Pleiades - low power and wide - a 12mm or 8mm eyepiece. Mars was not an option, nobody would see anything and as the aim was for people to experience something I stayed on M45. Basically I know the scope and what it can do well, and more relevant I know what it cannot do.

 

If you want a number then try f/6 to f/8, reason is nothing to do with the target. At f/6 to f/8 they are easier to maintain collimation, require less work, mirrors are easier to produce a decent one and eyepieces are not as critical. Nothing to do with the target but a simpler life.

 

If you want good buy a Zambuto, but expect higher cost.



#33 Cirus

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Posted 28 March 2019 - 03:10 AM

I think quite often people over estimate the cons of over magnification.

It wasn't until I tried .33mm exit pupil and found that not only was the image larger, but I could see the finer detail more easily, it was then I realised that I have been mislead.

Think about the smallest details that you can only just see, but it's a hard squint, if you double it's size, it's no longer a hard squint.

Imagine opening a word document, and you make the font size to a setting where you have to squint to read, then try enlarging that size it's the same when viewing the tiniest detail (if it is bright enough)

On the moon, Jupiter and Mars I have benefited greatly from .33mm exit pupil over .5mm exit pupil. This is not always the case, if the seeing is poor than .5mm , .75mm and 1mm exit pupil is better. But don't always assume that .33mm exit pupil won't sometimes have its benefits.

.33mm exit pupil is not fool proof, if it's a dim object or if the seeing conditions are poor then it's only going to make things worse. But there are also a lot of nights where it can make observations better. It's a good option to have in the eyepiece arsenal!

This has been my experience with 100ED f/9, other people's experience may differ.

Edited by Cirus, 28 March 2019 - 03:11 AM.

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#34 Jond105

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Posted 28 March 2019 - 03:18 AM

I think quite often people over estimate the cons of over magnification.

It wasn't until I tried .33mm exit pupil and found that not only was the image larger, but I could see the finer detail more easily, it was then I realised that I have been mislead.

Think about the smallest details that you can only just see, but it's a hard squint, if you double it's size, it's no longer a hard squint.

Imagine opening a word document, and you make the font size to a setting where you have to squint to read, then try enlarging that size it's the same when viewing the tiniest detail (if it is bright enough)

On the moon, Jupiter and Mars I have benefited greatly from .33mm exit pupil over .5mm exit pupil. This is not always the case, if the seeing is poor than .5mm , .75mm and 1mm exit pupil is better. But don't always assume that .33mm exit pupil won't sometimes have its benefits.

.33mm exit pupil is not fool proof, if it's a dim object or if the seeing conditions are poor then it's only going to make things worse. But there are also a lot of nights where it can make observations better. It's a good option to have in the eyepiece arsenal!

This has been my experience with 100ED f/9, other people's experience may differ.


It’s been my experience with the 100ED as well at 300x. Especially in this time of year.
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#35 gnowellsct

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Posted 28 March 2019 - 01:31 PM

This is equivalent to the oft-cited "2x aperture in mm" rule of thumb, but I'm curious if you've rationalized 0.5 mm pupil as a limit on its own merit?

People can and do go higher.  I note that Suiter disputes that any additional details are yielded beyond 2x per mm.  I pretty much agree with him.  In addition to the narrow exit pupils' highlighting the defects of the eye, there are additional factors that as a practical matter intervene:  seeing conditions, for one, and the tendency of objects to dim out as magnification increases.  

 

That said there's nothing wrong planning for going beyond that.  With a 4x Barlow and a 2.5mm ocular I can go to .0625 exit pupil.  But aside from doing some silly stuff on a branch with a new scope it's not something I expect to use.  I note that the view at 730x in my 92 mm was inferior to the view at 184x.  Even in broad daylight an image can dim out.  But yeah you can go over 2x per mm.  2.2 to 2.5x.  In steady seeing I've had good experiences at 2.5x per mm which is 63x per inch.  One certainly hears about magnificent nights in Florida where people push to 800x etc.  But 800x, as improbable as it sounds, is around 2x per mm in 14 to 15 inch scopes, and less on even bigger scopes.

 

In my 102mm even on nights of good seeing I have found the views at about 180x to be all that I am interested in, and even so generally prefer it at 160x.  You lose color when the image is over-magnified.  But in greneral I think 2x per mm is a good ceiling cap around which to plan.  It's not against the law to go higher!

 

 

Greg N



#36 Conaxian

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Posted 28 March 2019 - 02:29 PM

Speaking of eye defects, I tried an eyepiece that produced a .47mm EP in my small mak, and the result was pure aggravation. I have a gozillion floaters that were showcased very well, but I couldn't seem to focus at all.

I'm getting old and so are my eyes, so I have reluctantly set my high power limit to .75mm EP from now on.

You younger folks (and those eagle eyed oldsters out there) are lucky. Enjoy the view while you can because there comes a time...


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#37 Redbetter

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Posted 28 March 2019 - 02:47 PM

I think quite often people over estimate the cons of over magnification.

It wasn't until I tried .33mm exit pupil and found that not only was the image larger, but I could see the finer detail more easily, it was then I realised that I have been mislead.

Think about the smallest details that you can only just see, but it's a hard squint, if you double it's size, it's no longer a hard squint.

Imagine opening a word document, and you make the font size to a setting where you have to squint to read, then try enlarging that size it's the same when viewing the tiniest detail (if it is bright enough)

On the moon, Jupiter and Mars I have benefited greatly from .33mm exit pupil over .5mm exit pupil. This is not always the case, if the seeing is poor than .5mm , .75mm and 1mm exit pupil is better. But don't always assume that .33mm exit pupil won't sometimes have its benefits.

.33mm exit pupil is not fool proof, if it's a dim object or if the seeing conditions are poor then it's only going to make things worse. But there are also a lot of nights where it can make observations better. It's a good option to have in the eyepiece arsenal!

This has been my experience with 100ED f/9, other people's experience may differ.

One needs to define "over magnification" first.  I would define over magnification as any magnification above the point that makes it more difficult to see detail or where details are lost.  There are several aspects that result in over magnification, any one of which can be the over riding factor for an individual/scope/conditions/target:

  • Observer's visual acuity.  Some of us see things at lower magnification/scale than others.  I am getting all of the information that my eye can detect at ~50x/inch if conditions and aperture support it.  Above that is like being too close to the TV or an impressionist painting and seeing individual pixels or brush strokes rather than the image.
  • Diffraction blurring.  As Scotty said, "I cannot change the laws of physics."  The light coming through the aperture creates diffraction blurring that is visible to they eye--the airy disk pattern seen for point sources.  I begin to detect the spurious disk separate from the 1st diffraction ring at about 1.5mm exit pupil, roughly 16 to 17x/inch.  That means that some level of diffraction blurring is already present in extended objects even at such low magnification, even though one will likely be unaware of it and further magnification to improve scale will still be helpful.
  • Defects of the eye.  By about 35-40x/inch I become more aware of small floaters/texture/etc. that is present in my eye, as well as any particulates on mirror/prism/eye lens surfaces.  By 50x/inch on bright objects I notice tiny patterns that are exactly the size and shape of white ovals on Jupiter. 
  • Aberrations of the scope as well as central obstruction, etc.  The quality of the optical figure/correction, collimation, central obstruction, baffling, etc. play critical roles in how much magnification is effective.  Poorly made achro refractors are probably the worst in this regard and can fail to be sharp at even low mag/inch.  A well made ED/Apo will be on the high end of the scale.  A quality Newtonian will typically be better than an SCT.
  • Seeing is still dominant.  Even with a very small aperture (such as a 60mm), seeing is still a determining factor in what is visible.  In good seeing with a 60ED I have been able to watch moons fully transit the polar regions of Jupiter (where the color/brightness contrast is better vs. the brighter central portions of the planet.)  In the same conditions, incrementally larger apertures show it better with more detail.  The 60ED typically tops out around 120x although I sometimes use 144x for scale to verify.  On a poor night I will still be in the 150x range with an 80ED and similarly limited with larger apertures.  As the seeing reaches mediocre I will still be in the 200x range with larger apertures for planets.  As the seeing cross over to good I will be at 250-275x--perhaps 300x, and when it reaches very good I will be at 350x-400x or so in larger apertures.  In my largest scope in excellent seeing I used 500 to 700x for planets...but I haven't ever had that level of seeing in the region I am now (four years running.)
  • Targets are important to determining the effective limit.  Double stars and limiting stellar magnitude can benefit from greater than 50x/inch for my eye.  Double stars can take the highest because I am trying to see the diffraction pattern itself, not a representative image of the actual star.  Limiting stellar magnitude is achieved when the spurious disk is blown up to a scale that just allows the central peak to stand out vs. the background without becoming so large that its surface brightness declines below the eye's threshold for detection.  For me the cut off with small aperture (refractor) in good seeing is about 60x/inch, roughly .4mm exit pupil.   High brightness planets tend to top out in detail for me at 50x/inch in the same scopes, but are limited to much lower values in larger scopes because of seeing. 

The combination of effects become interesting for DSO's.  While people often say that DSO's don't require good seeing, those folks probably aren't using large aperture scopes and pushing their apertures to the limit.  The level of detail seen in 13, 14 or 15th mag galaxies, and detection of cores, supernovae, dust lanes, etc. is highly dependent on the seeing with a large scope (20" in my case.)  In typical poor winter seeing I am limited to 227x or 278x and the detection threshold is limited as well.  If the seeing rises to mediocre/average I can use about 357x to detect galaxies down to ~17.0 visual mag (18 B mag.)  In good seeing I will be able to use 500x and more detail is seen along with 18 mag field stars.


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#38 Earthbound1

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Posted 28 March 2019 - 08:33 PM

Thanks for the question and thanks for the responses! Following with interest...

#39 gnowellsct

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Posted 28 March 2019 - 08:55 PM

Assuming the same aperture and different focal lengths, the maximum achievable magnification should be the same. If one can get the same FoV by playing around with eyepieces, why is one marketed as a planet scope and the other for astrophotography? What would be a DSO scope then?
.


Exposure times decrease radically with shorter focal lengths. That in turn makes it easier to get pictures with short subs of 1 to 5 minutes. An airplane or satellite has less chance of ruining your whole night. The demands for extreme accuracy in the mount diminish because fast exposures mean rounder stars.

Long focal lengths are challenging because magnification exaggerates small errors in tracking. A 6 inch apo for deep sky is not for beginners.

Fast is good for deep sky. Fast and small, think f/5 80mm apo.

Planet imaging places a premium on thousands of ultra short exposures, 30 a second, on fairly bright targets. Critical to success is a large image projected on to the Web cam. A large bright image requires bigger apertures and longer focal lengths.

Think c11 and c14 for planets.

It is true that there is some crossover into visual observing, slow scopes make it easier to reach peak magnifications. But you get focal LENGTH with an increase in APERTURE as well as from an increase in RATIO. I think advances in eyepieces and optics, better refractors, bigger Newts, make an f/15 4 inch refractor more of a curiosity than a contemporary engineering solution to the problem of observing planets. But a fast apo with full illumination of wide fields is king of a certain type of deep sky. Large apertures win for faint deep sky by making it bright. But not, typically, by offering a 4 to 6 degree field.
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#40 Jon Isaacs

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Posted 29 March 2019 - 08:13 AM

The resolution limit of the eye is typically quoted at 1 arcminute. If you don't want to lose resolution, this constrains the minimum magnification at right around Mmin = D [mm] when you work through the math (visible light).

 

I've rationalized the maximum magnification similar to you as a desire to avoid "empty" magnification. Doing this, the idea is to avoid excessive "oversampling", and 2x is an arbitrary but perhaps reasonable criterion. (The "samples" have an angular apparent extent 1 arcminute.)  This yields to Mmax = 2D [mm], assuming seeing doesn't limit your resolution. This, in turn, gives the 0.5 mm pupil size as a result. (As an aside, this ignores the fact that resolution is typically constrained by seeing rather than aperture size, so likely a better "rule" would be independent of the aperture.)

 

At any rate, I've wondered if there's a more direct consideration to constrain pupil size but haven't ever come up with anything. I appreciate your thoughts!

 

1 arc-minute applies to the eye during the day with optimal illumination.

Consider Albireo at 34 arc-seconds. Were the eye able to resolve 1.0 arc-second, it could be split at 2x.  Likewise, the double double would be resolved at about 25x.

Sidgwick's analysis is similar what you have done, you try to match the resolution of the eye to the resolving power of the telescope.  The difficulty is that the resolving power of the eye is not fixed.  For faint objects, it can be over a degree.  The smallest galaxy in Stephan's quintet is about the size of Jupiter but reveals very little detail at 200x-300x.  A photo will show detail but the eye cannot resolve it.

 

Own thinking is that I use exit pupil as a guide line but out under the night sky, I let my eye determine the optimal magnification for a particular object. There are just too many variables to consider to try to calculate what the optimal exit pupil might be and all the while my eye and brain are right there providing me with the answer.

 

It's like climbing a hill on a mountain bike.  One could try to calculate the right gear but that would be just a guess, your legs tell you the right gear.  

 

Jon

 


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#41 Araguaia

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Posted 29 March 2019 - 08:59 AM

Sometimes, on Mars, I can see more detail at 435x even when the seeing "won't support it" - I see a blurry swimming dark region instead of a sharper image without it. 

 

Sometimes, on Saturn, kids at outreach have an easier time seeing the Cassini division at 435x, even if to me it is all blurry and the view looks much better at 277x, with no detail lost.

 

With Jupiter, OTOH, seeing rules.  It quickly gets blurry if you overdo magnification.

 

Once I was able to observe Mars and Saturn with a barlowed 3.5mm T5 for 870x, and they were still sharp.  It lasted for only a few minutes at dusk, but was by far my best view ever of either planet.  Of any planet.


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#42 Starkid2u

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Posted 29 March 2019 - 09:19 AM

If you work the numbers backwards, 0.5 mm is a practical exit pupil to call the magnification ceiling of any scope. And based on my personal tastes 3.5 mm is the shortest fl eyepiece you want to use.

These two numbers alone give you f/7 as a ratio. The magnification with a 3.5 mm ocular will be 2x the aperture of the scope in mm. A 7 mm will give you the scopes aperture as magnification.

So the ceiling of each instrument is 2x the aperture. Go beyond this and you will see stuff but views will dim.

So an 80mm peaks at 160x, a 250 mm at 500x etc.

Now consider your sky conditions. If 300x is the usual ceiling a 250 mm aperture can afford to "spend down focal ratio" (relative to f/7) and still reach 300x. A 250mm f/5 will hit 250x with a 5mm ocular and 300x with 4mm. You are in the reasonable viewing zone. You pay for it at the other end, where large exit pupils mean you cannot physically use the aperture you have paid for. Other issues arise including coma etc.

Now let's say you want a 300x view in a 100 mm telescope. That's 3x per mm of aperture. An f/5 100 mm would require a 1.3 mm eyepiece. (5mm=100x, 300x means 5/3=1.3. Constraining the aperture forces the focal ratio out, but you will never ever get to 300x and stay above a 0.5 mm exit pupil in a 100 mm scope, because the 0.5 exit pupil is an ocular one half the focal ratio, always. Your long focal ratio scopes are thus designed to exceed the organic limits of the eye with easy to use oculars, an f/15 100 mm scope will reach 300x with a 5 mm eyepiece.

The field of view is strictly a function of magnification. At 100x all scopes have the same field of view.

So we conclude your question is wrong. Field of view is determined by magnification. Magnification is determined as a ratio to aperture. If the question is what will give you 2.1 degrees and 300x at 0.5 exit pupil, you fiddle with aperture till you get where you want. A 100 mm can't do 300x at 0.5 mm exit pupil. For that you need 150mm or larger aperture. And as it happens the increased aperture is optimal for DSOs.

To get portability and wider fields reduced apertures and shorter focal ratios do the trick. But it's absurd to call a four inch f/15 instrument optimized for planets. It is optimized to offer dim views at ultra restricted exit pupils.

The notion of long focal ratios as optimized for planets is rooted in the need to control CA at high power and the difficulty in using tiny glass oculars. It has some, but less critical, basis in small Newtonian design under 150 mm where a long focal ratio can have a small secondary and more accurate mirror than say a small f/4.5. GN

Hey, that's why there's different strokes for different folks, pardner. No definitive last word here...

 

STARKID2U



#43 Wouter1981

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Posted 29 March 2019 - 09:19 AM

There is also 1 aspect of a slow refractor that has not been mentioned. One of the reason I prefer them over any other telescope... long slow refractors are beautiful....


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#44 Araguaia

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Posted 29 March 2019 - 09:47 AM

There is also 1 aspect of a slow refractor that has not been mentioned. One of the reason I prefer them over any other telescope... long slow refractors are beautiful....

 

You can always put a long hood on a fast refractor...

 

Scope  - 1 (1).jpg


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#45 Tony Flanders

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Posted 29 March 2019 - 11:10 AM

There is also 1 aspect of a slow refractor that has not been mentioned. One of the reason I prefer them over any other telescope... long slow refractors are beautiful....

That they are. Beautiful and profoundly impractical. But who can resist looking through a 6-inch f/15 when one shows up at a star party? I mean -- everybody knows that's what a telescope is supposed to look like!


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#46 Araguaia

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Posted 29 March 2019 - 04:24 PM

I sometimes dream of getting an 8" doublet lens and making an F/15+ refractor on a pier to nail planets and small clusters... then I remember that it will probably require tracking to be usable.  Deal breaker.  In another lifetime I was notorious for taking half an hour to dock a schooner under sail in the weakest zephyr, rather than turning on the engine.



#47 gnowellsct

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Posted 31 March 2019 - 11:21 AM

That they are. Beautiful and profoundly impractical. But who can resist looking through a 6-inch f/15 when one shows up at a star party? I mean -- everybody knows that's what a telescope is supposed to look like!

If it involves crouching in the mud to get a view at zenith, I would find it easy to resist


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#48 Araguaia

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Posted 31 March 2019 - 11:57 AM

If it involves crouching in the mud to get a view at zenith, I would find it easy to resist

Easily avoided.  But then you will need heels higher than Louis XIV's to get a view lower down.


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#49 Hesiod

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Posted 31 March 2019 - 12:13 PM

If it involves crouching in the mud to get a view at zenith, I would find it easy to resist

Mud and glory...




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