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highest USEFUL magnification

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#1 rnc39560

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Posted 19 August 2013 - 09:22 PM

What was the formula to figure a scopes highest useful mag.? I can't remember. Also, anyone with a 8" f6 what is the highest mag you have used, (with good seeing) and maintained :question:clarity?

#2 bandhunter

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Posted 19 August 2013 - 09:45 PM

I'm not a math guy, so I shy away from formulas. But when you really consider useful magnification there are many variables that change from night to night that I feel are not accounted for. So here is my formula. The highest useful magnification is the one that increases details from previous closest lower magnification eyepiece/telescope combination without significant image breakdown, and the next highest eyepiece magnification eyepiece combination you have shows an unusable image. I mean it is really that simple. Various objects are going allow different mags, seeing conditions, transparency, and wind are always going to be changing so there you go. There will be times that you can use less mag than what a formula says and there will be times that you can use more. The more experience you have the more likely you will be able to tell what you will be able to use on that occasion, then there will be those times you are surprised.

Daniel

#3 Ebyl

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Posted 19 August 2013 - 09:46 PM

The general rule of thumb is usually given as 50-60x per inch of aperture, or 2.5x per mm. I assume that's what you're referring to?

#4 rnc39560

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Posted 19 August 2013 - 10:27 PM

It was something like that. Thanks.

#5 faackanders2

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Posted 19 August 2013 - 10:34 PM

Normally they say you rarely use 300x+, but I do with my 17.5 f4.1. I only use my 3.5xmm Ethos on Globular clusters, and only use my 2.3mm on Uranus and Neptune to blow up their disk and confirm it's a planet.

#6 GOLGO13

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Posted 19 August 2013 - 10:44 PM

I'd say it's a somewhat generic term and it depends on a lot of things. Aperture, seeing conditions, is the scope in optimal conditions (cooled down, collimated), object being observed etc.

I've had my 10 inch dob at 480x and it was extremely sharp. That required proper seeing conditions which are not always the case in my area. The highest the 10 inch would support in general would be 600x...and on the moon you may be able to push that a little.

One thing I do think is people sometimes shy away from high magnifications too much. My personal opinion is 200x is the best magnification for most smaller DSO objects. 250x-300x is great for planets. And this assumes a scope that can handle that much magnification.

I always start at low magnification...say 15x-50x, then go to medium 100x-150x. Then try high. 200x-300x. In general if conditions do not support high magnifications, I drop back down to medium. Whatever magnification is still sharp is the way to go. And that can change nightly.

On topic more...I'm not exactly sure how they come up with that useful mag thing. It's sometimes different than the max mag. For instance, many people would say 300x is the max useful magnification no matter what size the scope because of seeing conditions. But, on good nights that can be blown away. Also, not having a tracking scope can make really high magnifications annoying since the earth is turning pretty fast. But, it's not impossible with wider field eyepieces.

#7 GlennLeDrew

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Posted 19 August 2013 - 11:03 PM

To understand how the highest useful magnification is derived, think in terms of exit pupil. The exit pupil is the interface between the objective and eye. Its diameter controls image surface brightness and the visibility if diffraction effects. Diffraction imposes the ultimate on resolution, and its effects generally become visible at about a 1mm exit pupil. For brighter targets like the Moon and planets, the smallest exit pupil which will not result in too bad blurring from diffraction is about 0.7mm. For objects in a more 'intermediate' brightness regime, such as brighter planetary nebulae and compact globulars, one can use 0.5mm, and even down to 0.3mm exit pupils due to the eye's poorer resolving power at the lower brightness levels.

All this presupposes decent optical quality and a steady atmosphere. Often times, the atmosphere is the final arbiter on the bright targets, resulting in such oft-stated maximum magnifications of 250-300X. For the not-so-bright stuff, as alluded to above, the eye's lower resolution tends to be worse than atmospheric seeing blurring, permitting those tinier exit pupils, at least on extended objects.

#8 SpooPoker

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Posted 19 August 2013 - 11:29 PM

This works out logically. My 4.5" f/8 image starts breaking down on planets past 165X when the exit pupil is less than .7mm. This is the approximate magnification from a 5.5mm eyepiece. Of course images are still useful at higher magnifications from shorter FL pieces, but I find 150X to be that sweet spot when seeing allows. Naturally I find the exit pupil consideration to be more a measure of allowable magnification than the 50X and 60X rule - particularly for planetary.

#9 CosmoSat

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Posted 20 August 2013 - 12:17 AM

...anyone with a 8" f6 what is the highest mag you have used, (with good seeing) and maintained clarity?

400x with a 6mm and 2x barlow lens giving a 0.5mm exit pupil, so thats 50x per inch of aperture.

Clear Skies!

#10 Koala117

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Posted 20 August 2013 - 01:23 AM

Normally they say you rarely use 300x+, but I do with my 17.5 f4.1. I only use my 3.5xmm Ethos on Globular clusters, and only use my 2.3mm on Uranus and Neptune to blow up their disk and confirm it's a planet.


Do you have any images of this? I think it would be awesome to see those two planets as seen from an amateur scope! Personally, I stick to Jupiter and Saturn for planets. Those seem to be my easiest ones to find. :o

#11 David Knisely

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Posted 20 August 2013 - 01:55 AM

Here is what Uranus looks like in my 14 inch f/4.6 Newtonian:

Attached Files



#12 Koala117

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Posted 20 August 2013 - 02:04 AM

David, what a nice image! :)

Thanks for sharing it. That's much brighter than I imagined it could be. Seeing the moons there is also quite nice.

#13 Lamb0

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Posted 20 August 2013 - 02:42 AM

:idea: Ah, found it!

:kitty: ... in all it's glory:

Useful Magnification Ranges for Visual Observimg by David Knisely 04/17/03

:gramps:

#14 Jon Isaacs

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Posted 20 August 2013 - 03:20 AM

To understand how the highest useful magnification is derived, think in terms of exit pupil. The exit pupil is the interface between the objective and eye. Its diameter controls image surface brightness and the visibility if diffraction effects. Diffraction imposes the ultimate on resolution, and its effects generally become visible at about a 1mm exit pupil



To elaborate on what Glenn has said:

Since this is the Beginners forum, it is probably useful to explain diffraction, diffraction effects and how it affects the resolution of a telescope system.

- In general, we like to think that a point at infinity, that is a star, is imaged at the focal plane as a point and when it is magnified by the eyepiece, we see it as a point of light. But this is not actually what happens.

Because of the wave nature of light, light from a point at infinity, a star, passing through a hole, (the aperture of your telescope) produces what is known as an Airy disc structure, a central disk surrounded by a series faint rings. The angular size of this disk/dot is inversely proportional to aperture of the telescope, the larger the aperture, the smaller the disk. This is the basic reason why larger telescopes can resolve smaller details.

- The size of the disk can be approximated by what is known as the Rayleigh criteria, it depends on the wavelength of the light but for green light the angular size of the disk is:

D = (5.45 inches/(Aperture)) arc-seconds.

This means that a 5.45 inch scope produces disks that are 1 arc-second in diameter and can resolve two stars that are separated by 1 arc-second. A 10.90 inch scope produces disks that 0.5 arc-second in diameter and can resolve two stars that are separated by 0.5 arc-seconds.

- For an extended object like a planet, I think it is easiest to think of the Airy disks with the faint rings as pixels on a computer screen, the telescope is painting the image of Jupiter at the focal plane with pixels that are the size of the Airy Disk. When the eye is able to resolve the pixels, increasing the magnification results in no more detail.

- The maximum magnification guidelines are determined by the ability of the eye to resolve the image. A better eye, capable of finer detail, sees everything at a lower magnification than a poorer eye. It is the relationship between the eye's resolution and the the telescopes resolution that determines the maximum useful magnification, typically this is stated to be 25x-50x/inch or a 0.5mm exit pupil.

One can consider resolving the double-double as a rough measure of the eye's resolution. The double-double pairs are separated by 2.3 arc-seconds. In a 60mm scope, the disks are approximately 2.3 arc-seconds in diameter so the pairs represent disks 2.3 arc-seconds in diameter that are just touching. What is the lowest magnification that one can resolve this? In general my experience suggests that at about 60x, it seems like a split is possible and at 120x, one might see they were both kissing.

This suggest that the eye is capable of resolving something like 2.3 arc-seconds x 60 = 2.3 arc-minutes but is more comfortable with 2.3 arc-seconds x 120 = 4.6 arc-minutes.

60mm/60x = 1mm exit pupil, 60mm/120x = 0.5 mm exit pupil. One can invert these, to get the commonly used 25x/inch - 50x/inch guide lines. 60mm = 2.4 inches, 60x/2.4 inches = 25x/inch, 120x/2.4 inches = 50x/inch.

- That's a lot of analysis, it's good to understand it but the most important thing to take away that derivation of these guidelines are not hard and fast but rather based on the eye's ability to resolve fine details and as we know, this varies greatly with the object. As an object becomes dimmer, the eye has greater trouble resolving the details. But increasing the magnification, it makes the object dimmer..

- Enough theory. Airy disks and diffraction rings... this is what we are looking at.

More practical considerations:

The steadiness of the atmosphere, the seeing. The telescope is looking through a long column of air that is the aperture of the telescope in diameter, it might only be 60 miles long at the zenith, several hundred miles long closer to the horizon. That air column better be steady, small thermal cells appear like mirages and the magnified image is degraded, this limits the resolution possible with any scope.

There are a number of ways to measure the seeing, one common way is in terms of arc-seconds resolvable. Typical decent seeing in much of the US is probably around 2 arc-second, one is quite good. In terms of useful magnification, this does translates into something like a 200x-300x limit for bright objects, some "oversampling" is useful. Of course this does not increase the useful limit of a 4 inch scope beyond about 200x, it reduces the practical limit of a 12 inch scope...

- I happen to be fortunate enough to live and observe in a place where the seeing is generally quite good, 1 arc-second is pretty normal, 1/2 arc-second at the zenith, is relatively common. On such a night, with a larger scope, 400x or more on the planets is definitely possible. To achieve such magnifications, it is important that the scope be well cooled down. For my 10 inch, this means 2 hours with a sealed back fan running..

- Some Planetary Nebulae respond to high magnifications, they are small but are quite bright. The Eskimo nebula is one that reveals itself at high magnifications, 500-600x or even higher under decent seeing conditions is quite reasonable. With my 25 inch, I often use 760x on planetary nebulae. These are not as bright as a planet so the eye's resolution is poorer and greater magnifications can be useful.

- Double stars: When splitting the closest possible double stars, very high magnifications can be useful. About 150 years ago, William Dawes empirically determined that a human eye could resolve equal magnitude pairs that were closer than the Rayleigh criteria. The limit became known as the Dawes limit, and represents the two Airy Disks overlapping and the eye seeing a minima in brightness of only 5% below the centers. This appears as a faint dark line. "Working at the Dawes limit" requires that the Airy disk be larger enough that this detail be seen. With my 10 inch, on a night when the seeing is 0.5 arc-seconds, I will use 821x on a Dawes limit double..

-------------------

I think the important thing to take away from this discussion is that there are guidelines, even limits, to what is possible but most of the time, you will be limited by the conditions and by the limits of your optics. In general, you let your eye tell you which magnifications provides the most detail. In generally, if in doubt, choose the lower magnification. To facilitate this, you want to have a selection of eyepieces that cover the range of reasonable choices..

One of those reasonable choices is that the seeing will be perfect, that your scope is perfectly cooled. Those conditions are quite rare for most observers but those are nights one remembers for a lifetime, best to be prepared. :jump:

Jon

#15 Koala117

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Posted 20 August 2013 - 03:38 AM

Nice post, Jon! Long and informative, just the way I like them! :D :D I certainly learned from it.

You point out something that I have noticed a good bit, about how, many times, viewing is limited(sometimes very much so). Is there a certain time of night when skies, in general, tend to be the calmest overall? I know that the different layers of the sky are crazy in that they are always changing and many times unpredictable.

I wish we had a better way to determine when the sky will be nice and calm! Sometimes, it can seem like the most lovely evening... and then you look through your scope and POOF! It's like looking down a very hot road; all blurry and jumpy is the view we get. =(

Sometimes, even at only 36x magnification(the lowest I can currently go :)), stars or saturn and jupiter just giggle at me as they dance around in my eyepiece. Having a steady ability to 'predict' calm skies would be wonderful.

Edit: Also, I have seen on various websites that sell telescopes that some will recommend 25x mag. per inch of aperture. Others recommend all the way up to 40x! Is this because of the quality of scopes that they sell or is it just that there is no 'generally accepted standard'?

#16 dennilfloss

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Posted 20 August 2013 - 05:23 AM

Is there a certain time of night when skies, in general, tend to be the calmest overall?


2. Surface turbulence extends from the ground up to a few hundred meters in the landscape around the telescope, which when viewing at a zenith angle above 60° is within half a kilometer of the observing site. Surface turbulence often represents up to half of all the observed optical distortion; it is largely due to convection currents rising from heat stored in the sunlit earth during the day. Particular concentrations of convection currents can arise from nearby residences, paved roads, surfaces of masonry or concrete, commercial buildings, and from turbulence between low lying layers that form temperature inversion boundaries. At many locations, surface turbulence follows a diurnal cycle from a minimum just after sunrise, steeply rising to a peak during early afternoon, declining to a secondary minimum shortly after sunset, increasing during the early evening to a secondary peak at around midnight, before returning to a minimum in the hour or two before morning.


http://www.handprint....html#structure

#17 Koala117

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Posted 20 August 2013 - 05:33 AM

Oh, very nice, dennilfloss! That's really good to know for me. :D

I actually tried my hand at googlefu'ing it myself in the past. My results ranged from sites that wanted to 'tell my future' (lol?) to ones very similar to that weird 'ancient aliens' show on TV! Apparently, I did not use the proper terms in my search string. hehe.

Thanks for the link :) This is also helpful for knowing a good idea of which eye piece to use and when it could have the best results, with wider vs. 'close up' viewing!

#18 dennilfloss

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Posted 20 August 2013 - 05:56 AM

You're welcome. :) There's a Part 2 also with a nice visual scale at the bottom of the page: http://www.handprint...RO/seeing2.html .

#19 kkokkolis

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Posted 20 August 2013 - 05:59 AM

+ 1 for all said. I'd like to also notice that the eye's qualities limit the highest useful magnification. The smallest the exit pupil, the most annoying floaters might be, especially those close to the center of the vitreous body. When you are young you might have almost crystal clear eyes, but usually they get worse with age. That happens to vitreous body (floaters) and later to eye's lens (cataract). Thankfully presbyopia +/- myopia are annoying only when you have to take off and on your glasses from eyepiece to map or optical finder. But floaters get in the way. My 8" f/6 can give more but rarely I enjoy greater than 250x (ES 4.7) because of floaters. That's enough for most objects. When I split doubles floaters aren't much annoying on the dark background, so I put a Barlow or a Powermate on.

#20 rnc39560

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Posted 20 August 2013 - 06:10 AM

Thank you ALL for info! These replies are what I was looking for!

#21 dennilfloss

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Posted 20 August 2013 - 06:13 AM

Best way to move floaters out of your field of view is to quickly look up, then down, not side to side. My ophthalmologist told me that a few years ago and it works.

#22 rnc39560

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Posted 20 August 2013 - 06:26 AM

Best way to move floaters out of your field of view is to quickly look up, then down, not side to side. My ophthalmologist told me that a few years ago and it works.


Makes sense too! As a kid I'm sure we all tried rolling them side to side, cause they floated in your fov longer!

#23 Joe Aguiar

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Posted 20 August 2013 - 07:46 AM

i just wanted to add that that rule is for good nights some nights you cant even go to 50x per inch power or 2x times the power per mm, so keep bumping up the power till it becomes fuzzy then back down abit then you will know per night per item what the max power you can do.

I log all my viewings in a book with every detail like how the nigh was which scope i use which ep filter was it cold hazy windy etc, its very helpful plus once and awhile i go back to look at it, so have all that details helps me remember.
joe

#24 Achernar

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Posted 20 August 2013 - 08:39 AM

I have gone as high as 300x with a 6-inch, and nearly 400X with my 10-inch. If the seeing is excellent, you can certainly exceed 400X with a good 8-inch if you are looking at the planets or trying to pry apart close double stars such as Eta Corona Borealis, which has a separation of about .7 seconds of arc.

Taras






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