highest USEFUL magnification
Posted 19 August 2013 - 09:22 PM
Posted 19 August 2013 - 09:45 PM
Posted 19 August 2013 - 09:46 PM
Posted 19 August 2013 - 10:34 PM
Posted 19 August 2013 - 10:44 PM
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.
Posted 19 August 2013 - 11:03 PM
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.
Posted 19 August 2013 - 11:29 PM
Posted 20 August 2013 - 12:17 AM
400x with a 6mm and 2x barlow lens giving a 0.5mm exit pupil, so thats 50x per inch of aperture.
...anyone with a 8" f6 what is the highest mag you have used, (with good seeing) and maintained clarity?
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
Posted 20 August 2013 - 01:55 AM
Posted 20 August 2013 - 02:04 AM
Thanks for sharing it. That's much brighter than I imagined it could be. Seeing the moons there is also quite nice.
Posted 20 August 2013 - 02:42 AM
... in all it's glory:
Useful Magnification Ranges for Visual Observimg by David Knisely 04/17/03
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.
- 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.
Posted 20 August 2013 - 03:38 AM
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'?
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.
Posted 20 August 2013 - 05:33 AM
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!
Posted 20 August 2013 - 05:59 AM
Posted 20 August 2013 - 06:10 AM
Posted 20 August 2013 - 06:13 AM
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!
Posted 20 August 2013 - 07:46 AM
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.
Posted 20 August 2013 - 08:39 AM