Usable Magnification...

[Rob]

Hi All,

Thought I'd start a thread to discuss magnification. So as we know a telescope has a usable magnification. Now this changes with focal length & aperture, but then we also factor in where you maybe in the World and seeing conditions.

So this got me thinking about producing this thread to provide all with guidance to achieve the best out of your scope on various targets!.

So lets categorize these targets. It would be great to have a number users to provide detail on where they are, scope aperture & focal length and max magnification they tend to find provides the best result.

• The Moon
• Planets
• Deep Sky
• Double Stars

I feel this may bring up interesting results, and I hope will provide help with observing.

Rob

[Neil H]

Hi Rob I am in Luton  I have the Orion. Optics VX8L 200/1200 for planets I have a planetary eyepiece 6mm and I use my 15mm with and without a x2 Barlow the 15mm gives me a small but detailed view . For the moon if it's a full moon 40mm or 20mm if looking at a creator then 10mm or 12mm

Not done deep sky yet to much light pollution

For double stars 20 mm to line it up then  9 mm or 6mm some times I Barlow the 9mm 

Edited June 8, 2020 by Neil H

[John]

This is a great idea but with 5 scopes I'm going to need to devise some sort of matrix to present the information in a useful form I think. I would also want to represent variables such as the seeing conditions and the varying requirements within the target categories that you have listed.

As an example, take the category "planets" and, with my 12 inch dobsonian I would say that I have found anything from 80x to 400x useful and usable depending on the planet, the conditions and my objectives at that time.

Its going to be complicated I think !

I'll give it some more thought though - not much else to do currently !

 

 

 

[vlaiv]

Some sort of statistical analysis should be considered here.

Besides already mentioned variables - observer plays a role as well. Some people tolerate slightly soft but larger image as it allows them to see detail easier. Others prefer detail to be small at edge of resolving but overall image to be sharp.

I see this in imaging also. There is not much difference in x2 sampling resolution in terms of what can be seen in the image and perceived sharpness. That is x2 in "magnification".

As for myself, I think I went thru different phases. Before - it was about magnification / image size - it allowed me to see better, but as time went on, I found that I prefer lower magnification and perceptually sharper image.

Btw, actual magnification that allows you to see all there is to be seen is quite low. For 8" aperture it is less than x100. Everything above that is just magnifying image to make it easier to see without additional detail being revealed.

[John]

This is not mine but it was posted on another forum by the experienced amateur David W Knisely. It might prompt some discussion anyway:

USEFUL MAGNIFICATION RANGES FOR VISUAL OBSERVING
IN ASTRONOMICAL TELESCOPES

LOW POWER (3.6 to 9.9x per inch of aperture)(7mm to 2.6mm exit pupil):  Useful for finding objects and for observing ones of large angular size like open clusters, large faint nebulae, or some larger galaxies. For lunar work in modest apertures, it is generally somewhat on the low side, but can show the crescent moon with background starfields well. This is also the range where Nebula filters tend to perform the best. Some of the wider double stars can also be best appreciated in this power range.

MEDIUM POWER (10x to 18.9x per inch of aperture)(2.5mm to 1.3mm exit pupil):  Useful for observing somewhat smaller deep-sky objects such as galaxies, some diffuse nebulae, smaller open clusters, and moderate to large planetary nebulae. Also useful in apertures 6 inches and larger for getting at least partial resolution on the brightest globular star clusters. Often used in moderate to large apertures for detecting very small galaxies which may be invisible at low powers and for revealing details in some galaxies like dark lanes, mottling, and star-like nucleii. Very useful for wide area views of the moon, or for showing the moon systems and some of the larger features of the planets.

HIGH POWER (19x to 31.9x per inch of aperture)(1.3mm to 0.8mm exit pupil):  A very useful power range for observing fine planetary and lunar detail. This is the range where the full theoretical resolving power of the telescope is becoming visible. Also useful in moderate to large apertures for getting better star resolution in tight globular clusters or for viewing detail in the smaller planetary nebulae, as well as resolving tight double stars. This power range is sometimes compromised in apertures larger than 5 inches by seeing effects (ie: disturbances in the Earth's atmosphere which can blur fine detail).

VERY HIGH POWER (32x to 46.9x per inch of aperture)(0.8mm to 0.5mm exit pupil):  Useful for study of certain specific planetary details, and resolving very tight double stars near or just above the resolution limit of the instrument. Also useful in larger telescopes for resolving the cores of some very tight globular clusters or for detecting the finer detail and faint central stars in the smaller planetary nebulae. Quite useful for telescope collimation tests or rough star-testing. This power range is not as frequently usable with larger apertures due to seeing disturbances. For planetary viewing, eye defects like motes and floaters (along with the somewhat lower overall light level), begin to become visible and slightly annoying in the upper half of this range.

EXTREME POWER (47x to 75x per inch)(0.5mm to 0.3mm exit pupil):  Mainly used for resolution of double stars at the resolution limit of the instrument, or for detecting elongation of unresolved doubles. Powers up to 60x per inch are sometimes usable in rather small instruments for making gross planetary detail easier for beginners to see (ie: Jupiter's main belts or the Cassini Division in Saturn's rings). This power range is not often used in apertures above 6 inches due to seeing limitations, and requires very good optical quality in the instrument. Even when conditions are good, lunar and planetary views using this power range can sometimes seem less pleasing overall than at somewhat lower powers due to the lower light intensity and increasing interference from eye defects like floaters. However, this range can be somewhat useful for certain *specific* targets or details which require extreme scale. Examples include (for large apertures) seeing Encke's Division in Saturn's rings, the central star in M57, detail in some brighter planetary nebulae, or for resolving a few small specific lunar details. Powers from 75x to 90x per inch are occasionally used for very close double star elongation, micrometer measurements, or for optical testing, but otherwise, powers well beyond 75x per inch can often be nearly useless, especially in inexpensive "department store" telescopes.
 

[Rob]

Great replies all, thank you.

I just see so much in the forum where people question why the images are not great etc. I feel the understand of correct magnification along with expectation is important.

[Neil H]

Rob this is a great idea , I think  John needs to sell some of his telescopes so it would be easier for him to answer this lol 

[Stu]

I’ve seen attitudes to magnification change over the years on SGL I think. It used to be that people thought if you used more than x200 or x250 max then you were crazy, and likely to be seeing mushy images.

Whether it is as kit quality has generally improved I’m not sure, but I think we are now at a sensible position which is more along the lines of ‘use whichever magnification works for the object and your kit and situation’

If the seeing is very stable, then you can jack the mag right up and still see very good images on the Moon, planets or doubles. With the planets down low currently then less is likely to be more.

I have recently used x280 on my little Zeiss Telementor using a nag zoom. On doubles this works very well as the airy disks show up beautifully and you get additional separation. It is good on the Moon too.

Generally on smaller apo scopes up to four or five inches I use exit pupils down to 0.5mm when observing the Moon and Doubles, sometimes on planets too. On larger scopes it tends to be more about what the sky can take. The guidance to use a 1mm exit pupil does not apply to smaller apps; if I only ever used x100 on my Tak I would miss much of the capability of the scope.

Jupiter tends to look best at around x180 ish, Saturn can take a bit more due to its higher contrast, x220 say and Mars a bit more to get the image scale, x250 ish.

At the low end it is more about framing of objects by choosing the best field of view and also having a large exit pupil if using narrowband filters. The Veil in my Genesis with over 5 degrees of sky and an OIII is lovely.

Play around with mags, exit pupils and fields of view and find what works for your situation. Read the theory and people’s opinions but don’t be limited by it would be my general comment.

[John]

The concept of "empty magnification" seems to be less relevant today perhaps ?

 

[vlaiv]

More affordable scopes mean more people can use larger aperture scopes and hence higher magnification?

Knowledge on optimization of high power viewing is now readily available online?

[Stu]

16 minutes ago, John said:

The concept of "empty magnification" seems to be less relevant today perhaps ?

 

Not sure John. I get that optically you only get more detail up to around 1mm exit pupil, but in the same way that being able to increase image scale on DSOs makes them easier to see, I think increasing the mag on high power objects like planets and doubles can also make detail easier to see. Obvious caveats about not losing contrast and working within the seeing conditions still apply of course.

With my little Telementor, the airy disks are big, and it is obvious that you are not adding any more detail by increasing mag, but it does increase the separation on doubles and can make it a little easier on the Moon to see the smaller detail provided you don’t go too far. Often x140 is plenty. Have yet to try it on planets....

[Rob]

Excellent responses here guys. Appreciate the time taken to add important input.

Best Rob

[John]

1 hour ago, Stu said:

Not sure John. I get that optically you only get more detail up to around 1mm exit pupil, but in the same way that being able to increase image scale on DSOs makes them easier to see, I think increasing the mag on high power objects like planets and doubles can also make detail easier to see. Obvious caveats about not losing contrast and working within the seeing conditions still apply of course.

With my little Telementor, the airy disks are big, and it is obvious that you are not adding any more detail by increasing mag, but it does increase the separation on doubles and can make it a little easier on the Moon to see the smaller detail provided you don’t go too far. Often x140 is plenty. Have yet to try it on planets....

I'm not sure either Stu.

Generally I prefer a smaller but sharper and more contrasty image but sometimes you need to increase the scale, as you say, to enable the eye to see intricate stuff.

I do find very high magnifications useful to pick out faint point source targets such as faint planetary moons.

 

[mikeDnight]

I think its a mistake to limit a scopes magnification based on resolution limit, which itself is not technically accurate. 

My personal thoughts on the matter are as follows:

Jupiter seems happiest around 180X,  Saturn 200X. While Mercury, Venus and Mars, which show mainly albedo features, can benefit from 300X +.  Uranus and Neptune too will take high powers well, but there's little in the way of detail other than subtle albedo differences across the globe. (I've only ever seen subtle detail on Uranus and none so far on Neptune, and the magnification used was around 400X in a 5" refractor).

Deep sky loses contrast when the sky background is too bright, so it's a balancing act to get the best contrast/magnification match. This will vary dependant on the type of DSO, the aperture of the scope, the stability and transparency of the atmosphere, and the eyesight and experience of the observer. M42 in a 4" scope looks spectacular at around 30X to 40X, with the dark nebulosity adding an almost 3D impression to the view. M13 in the same instrument needs a black sky background to really bring it to life, so 100X. M27 & 57 80X, while galaxies need good dark adaption and exceptional transparency to be seen well between 20X and 80X. 

The Moon is a magnification free for all, so 20X to 400X, tailored to suit the seeing. And stellar objects can  take even higher powers on occasion.

 

[paulastro]

Some great points are made here, and quite a few 'rules' suggested.  I would say though, that if someone reads this who hasn't a lot of experience and is looking for practical guidance they may very well find it rather overwhelming.

Personally, I have a more relaxed and less prescriptive attitude toward magnification.  When observing any object, I just try different eyepiece and barlows   (if necessary) combinations  to find the one that gives me the best possible view at the time.  That's it.

In fact I have to admit I'm often not

aware of the mag I'm using.  In my observing notes I tend to record what I was using rather than the resulting magnification.  I often use a x3 or x2 barlow with both single eyepiece, my Baader zoom and with different sets of Orthos in the binoviewer.  I will just use what is best at the time.

If pressed (with my telescopes and eyepieces)  my rule would be to use the mag between x18 and x775 that give the best view of the object I'm observing 😄.

(This is the total range of magnifications I have available - I think)

[PEMS]

I generally go by the maybe simplistic idea that a magnification equal to the aperture very likely defines a good point.

Many will go higher, however what I have seen usually means that the image quality begins to drop off at that point, although likely not on the good apo triplets. You are however then getting into the rather specialist areas.

The rather easy way to get Mag=Aperture is just to drop in an eyepiece equal to the focal ratio, that combination also delivers a 1mm exit pupil. In a way you do not need to think a great deal, a somewhat useful situation at times.

There are always exceptions, so the above is based on a middle of the road, average type scope. A good triplet apo will go beyond it easily, whaereas a fast ST80 achro will struggle. As I expect one of the Bird-Jones type reflectors and also the small but again fast reflectors. Some beginners ones are f/4 and that is I would say too fast.

If the scope is average and reasonable then you may well get 20% to 25% more, an 80mm f/7 or f/8 I would say will deliver 100x. However I doubt more without dropping detail. Then it becomes how much detail sacrificed to a bigger image.

[John]

It took me a long time to realise that there are some occasions when using very high power is a useful "tool" for certain tasks. For example, I had observed Uranus and Neptune in the past at 200x or so, enough to see them as disks. I read somewhere that it was possible to see some of their moons with my scope but very high magnifications would help tease these tiny points of light out of the background sky. Using 300x - 400x has helped me to see Neptune's moon Triton and Titania and Oberon. I would still like to see Umbriel and Ariel and also Phobos and Deimos at Mars 

The views of the planets themselves are not (usually) augmented by such high magnifications of course.

While they may not be the the most used tools in the tool kit, very high power eyepieces do have their uses.

 

 

 

 

[russ]

For me a useable magnification is what ever works on the night. Start low and work up until the image looks worse than the previous view. Also depends on the subject and height in the sky. Jupiter looked rubbish the other morning at 100x but the moon, slightly higher in the sky, looked epic at 300x.  

I never bother with all the 50x per inch nonsense. Some scopes won't reach that, others surpass it with ease.

[russ]

On 09/06/2020 at 13:09, Stu said:

I think we are now at a sensible position which is more along the lines of ‘use whichever magnification works for the object and your kit and situation’

 

That's it in a nutshell.  

[andrew s]

I have no problem with do as you find best but just beaware that you can get to the point where what you are seeing are defects or structure in your eye.

There is a famous example of this in respect of Mars.

Regards Andrew 

Edited June 11, 2020 by andrew s

[JeremyS]

1 hour ago, andrew s said:

I have no problem with do as you find best but just beaware that you can get to the point where what you are seeing are defects or structure in your eye.

There is a famous example of this in respect of Mars.

Regards Andrew 

Yeah, but Percival Lowell wasn't using a Tak, Andrew!

 🤣

[mikeDnight]

I've never seen any of Lowell's canal's but I have seen and sketched a few of Schiaparelli's, which are nothing more than linear albedo markings. So far I've seen three within Solis Lacus which are named Eos, Nectaris, and one called Ambrosia ( easy to remember because I like rice pudding). Then there's a really easy one called the Indus, extending from Margaretifer Sinus to Acidalia, and Nilosyrtis extending from Syrtis Major northwards. Its a bit unnerving when you first realise you're actually seeing these features, but if you rotate the diagonal they retain their relative positions on the globe and so are not imaginary, or artefacts within the eye. 

Of course with this next Mars apperition likely to be spectacular, and with my new HR eyepieces and Zeiss prism, I'm expecting to see this.....

 

[JeremyS]

11 minutes ago, mikeDnight said:

f course with this next Mars apperition likely to be spectacular, and with my new HR eyepieces and Zeiss prism, I'm expecting to see this.....

Make sure you use that lovely Vixen HR 1.6 with a Barlow, Mike!

🙂

 

[John]

When I visited the Herschel Museum in Bath (highly reccommended by the way) I was struck at the really short focal length eyepieces that the Herschel's used when observing. Apparently over 6,000x magnification on some occasions

I came across this old paper, held by the RAS, on this topic by W H Steavenson for any that might be interested:

http://articles.adsabs.harvard.edu//full/1924MNRAS..84..607S/0000607.000.html

 

 

 

[Second Time Around]

+1 for the Hershel Museum.