Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

The controversy of highest pratical magnification!?

  • Please log in to reply
52 replies to this topic

#1 Supernova74

Supernova74

    Messenger

  • -----
  • topic starter
  • Posts: 417
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 26 May 2020 - 10:36 AM

Hi guys I’m new to cloudy nights and live in the uk and this will be my first topic of interest and will be interested in hearing your views and opinions on the listed topic and I apologise in advance if I’ve posted in wrong section.

 

from time to time we all can be creatures of habit and perhaps set in our own ways in trying something new.and the knowledge and advice we receive on social media sites and various other sources of information Around Astronomy can be easily accessible aslo through our favourite dealers etc.and to my knowledge similer information and advice is pretty much the same on the other side of the pond to you guys in the United States.and myself including jump on the band wagon in what monkey do what monkey does kind of scenero this is based on the knowledge and advice we can receive a general rule of thumb.for exsample regarding the telescopes we own there is a guideline we follow in what’s the lowest usefull magnification and what is highest which to most principals seem to be around 40-50x per inch of aperture of your scope and a definite yes dark,clear seeing conditions is key and low Atmospheric turbulence makes or breaks useing your favourite low focal length eyepiece. Quote al nagler once said the god father of Televue EP his best views was through an 3” ED,APO telescope looking at Jupiter at 400x.which is well and beyond the freshold that the telescope in theory should be capable of and the best ever views he ever had.so that 40-50x highest pratical magnification has gone out the window then.however we are not comparing an department store telescope here that has pretty pictures on the box of the planets,moon and galaxies etc which claims to have potential of reaching 525x magnification those are just ridiculous claims and the purchaser who brought it is often left feeling very disappointed.myself personally I truly feel tho in reasonable conditions not even 100% full capacity which my sky bortale 5-6 sky’s can produce we can achieve well above the recommended 40-50x per inch of aperture to a certain degree of course and for the guys who own high end apo,s you know what I mean there.so anyway here in the uk and were I’m located the recommended maximum usefull magnification is around 300x in general good seeing conditions I just don,t believe it as last year even tho Jupiter and Saturn where low in the meridian I was observing both the planets at over 300x thease were very average conditions so am I mad don,t think I am as views were still sharp the atmospheric turbulence was reasonably low and I could easily gone up a notch or two with a lower focal length EP and still maintain half decent views with contrast and detail intact.another exsample two months ago I observed the moon started off low then worked my way up and the moon was near full phase I used the televue 10mm Delos accompanied by an Exsplore scientific x2 Barlow at first was,nt go to exspect to much and to my amazement the view were which I consider above average this was a shade over 600x it was almost like I could reach out and touch it kind of moment unbelievable.so my point is really just because someone says you can only walk sometimes we can run type of analogy!? And certain sources of information is a rough guide line only doesn’t mean it’s not achievable and another important factor is to consider is the quality of the optics of today are so much better of years gone by.



#2 Shorty Barlow

Shorty Barlow

    Mercury-Atlas

  • *****
  • Posts: 2,514
  • Joined: 15 Sep 2015
  • Loc: Lloegyr

Posted 26 May 2020 - 10:45 AM

It's all to do with conditions.


  • Jon Isaacs and happycamperjohn like this

#3 emilslomi

emilslomi

    Messenger

  • -----
  • Posts: 484
  • Joined: 12 Nov 2015
  • Loc: Alps

Posted 26 May 2020 - 10:50 AM

There are no fines for staying above or below certain numbers. I have used an 8+mm exit pupil on poorly dark adapted eyes, and I was not put into jail - just wasting a lot of light. A barlowed 6mm in my 102/F11 achro giving me 420x did teach me how fast planets are moving! It did not have any legal consequences either. Do what seems to be fun doing and enjoy what seems to be enjoyable to you. A lot of things that are fun are not very useful.

 

Emil


Edited by emilslomi, 26 May 2020 - 10:51 AM.

  • Astrojensen, russell23, happycamperjohn and 1 other like this

#4 epee

epee

    Soyuz

  • *****
  • Posts: 3,840
  • Joined: 30 Nov 2006
  • Loc: Suh-van-nuh, Jaw-juh

Posted 26 May 2020 - 10:56 AM

Sky quality, optical quality and tracking quality all play a part. Even when seeing is simply average there will be moments of steadiness to reward the patient. These moments come more often and with longer duration as conditions improve. Then it is a matter of your optics and how well you can track your target.


  • Dougeo likes this

#5 junomike

junomike

    Hubble

  • *****
  • Moderators
  • Posts: 18,834
  • Joined: 07 Sep 2009
  • Loc: Ontario

Posted 26 May 2020 - 10:59 AM

IMO It's depends on location/seeing, target/object, telescope/optics, Aperture and lastly personal preference.

 

For me It's usually It's the seeing that's number one followed closely by personal preference.

I can't recall how many times people I'm viewing with raved about (insert Planet) being amazing at 400X+, only to have a glance and find

that although It's large and bright, It's a mess (mushy/soft views).

The same holds true for them, as they comment to me  on how "you can go up in mag ya know", when I've already went up in mag, found it soft and came back down

to where I find it sharp at the max. mag (for me).


  • havasman and oldmanrick like this

#6 mikeDnight

mikeDnight

    Apollo

  • ***--
  • Posts: 1,151
  • Joined: 19 Apr 2015
  • Loc: Wild Wild West - North West England

Posted 26 May 2020 - 11:09 AM

  Go as high as your optics and seeing conditions will allow you to go, or until you run out of exit pupil. Although you can't exceed the resolution limit, a truly good lens or mirror will still present a well defined image well beyond the generally accepted 50X per inch threshold, allowing for great lunar, stellar, and some planetary views.


  • russell23 likes this

#7 John Carlini

John Carlini

    Vostok 1

  • *****
  • Posts: 159
  • Joined: 10 Sep 2018
  • Loc: Northern Wyoming

Posted 26 May 2020 - 11:11 AM

I usually follow the 50x per inch rule but it's seeing, environment/location, mount stability and optical quality that can push the limits.


Edited by John Carlini, 26 May 2020 - 11:17 AM.


#8 TOMDEY

TOMDEY

    Fly Me to the Moon

  • *****
  • Posts: 6,769
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 26 May 2020 - 11:34 AM

Hi, SN74! Most beginners go way too high on magnification, and also very quickly grow out of it. The one and only approach is empirical, for your scope, eyes, and varying conditions. Just provide eyepieces that allow you to experiment, covering the range of 4x to 40x per inch. Start at ~too low~ magnification and work up, until you find what is most pleasing for you. Each target is different and each individual is different, and that will vary from night to night, depending on the atmosphere.

 

People with very good eyesight generally prefer lower magnifications. Three decent eyepieces and a Barlow are all that are needed!

 

Most beginners tend to over-think and under-try alternatives. Most experienced observers tend to over-coach and pontificate.    Tom


  • Stelios, Mark Strollo, DHEB and 1 other like this

#9 Berny

Berny

    Ranger 4

  • *****
  • Posts: 390
  • Joined: 27 Jan 2015

Posted 26 May 2020 - 11:51 AM

Interesting Post, but the almost total lack of punctuation marks, sentence spacing and capitalisation of the first letter in a sentence makes it very difficult to read.


  • Stelios, John Huntley, plyscope and 3 others like this

#10 t.r.

t.r.

    Fly Me to the Moon

  • ****-
  • Posts: 6,167
  • Joined: 14 Feb 2008
  • Loc: 1123,6536,5321

Posted 26 May 2020 - 12:00 PM

Go as high as your optics and seeing conditions will allow you to go, or until you run out of exit pupil. Although you can't exceed the resolution limit, a truly good lens or mirror will still present a well defined image well beyond the generally accepted 50X per inch threshold, allowing for great lunar, stellar, and some planetary views.


Actually you can exceed the resolution limit, quite easily in fact. A telescope has reached its resolution capability at approximately 1x per 1mm of aperture. So a 100mm refractor has resolved all it can at 100x magnification. However, it is easier for the eye to see the resolved detail at a higher magnification for most. While magnifying above 1x/1mm is called “Empty Magnification”, for most people going higher, up to a point (usually at .5 to .8mm exit pupil) they will better be able to make out the detail presented by using a larger image scale to stimulate more of their eye. No hard fast rules...experiment for what works for you. And yes seeing, optical quality and eye quality have an impact!

Edited by t.r., 26 May 2020 - 12:02 PM.

  • Eddgie, markb and mikeDnight like this

#11 sg6

sg6

    Cosmos

  • *****
  • Posts: 7,997
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 26 May 2020 - 12:49 PM

There are likely too many factors involved: Scope itself, quality of the optics, shape of the optics, eyepiece quality, sky quality, your eyeball, your ego. Peoples ego comes higher then many think.

 

The "simple" and rather unfortunate rule of 2x aperture in mm is not based on optics. It is based on where the eye starts to produce problems. This occurs at a 0.5mm exit pupil size and that equates to 2x aperture in mm. Whether the scope can actually produce a usable image at that magnification is kind of ignored, forgotten, brushed under the table.

 

What does 400x mean on a 3" scope. Would suggest that you could if lucky recognise it as Jupiter, a big fuzzy somewhat indistinct Jupiter. But that recognition also means that you know what Jupiter looks like. How many 4 year olds can draw Saturn and have never seen Saturn? Quite a large percentage. Knowing what you are looking at makes a difference. That roundish blob thing must be the GRS and suddenly it is the GRS and equally suddenly you have seen the GRS at 400x and soon after it was so sharp it was obviously the GRS. Many will be convinced they could read the sign above it saying "GRS Here"

 

We need to stop quoting magnifications, or at least add further information.

 

Will a Skywatcher ST 80 match my WO 81 triplet for magnification? Oddly yes, the result will be garbage in the ST at say 150x, the WO should be acceptable. But stick the right eyepiece in and mathmatically both have the same magnification. Just one is usable the other is not.

 

Quality of optics comes into it fairly significantly. GSO mass produced equalling a Zambuto? You know which will outperform the other. Another way to realise quality of optics - remember the slight problem that Hubble had with a slightly low quality mirror?

 

Will a 130 spherical mirror match a 130 parabolic mirror? Again we all know the answer. Then somehow people get told exactly the same for either one = 260x is your maximum by us who know a) it isn't true and b) it isn't going to happen.

 

I bought a Skywatcher 72ED, last year SW said 144x max, now SW say 216x max. Has the glass evolved over a year into greater perfection? I suspect not, I also suspect it will never make 144x. Maybe 100x if the gods of astronomy smile on me. Maybe a large burning pyre of sacrifical dobsonians will help.

 

I recall SPM said 1.5x aperture was a sensible maximum, usually now quoted is 2x and at times 3x appears.

Is it also not a bit strange that no-one points ot that the greater the magnification then the less you see? Someone found out a few weeks ago that M45 and an 8SE do not go together.

 

Sales and marketting making excessive claims I half get. But a lot of people here almost blindly repeating the same I do not get.



#12 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 17,100
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 26 May 2020 - 12:54 PM

Magnification also has a lot to do with image scale on the eye and the resulting image surface brightness. That differs for object type: faint fuzzies, point sources, and lunar and planetary. The image on the focal plane is unchanged with magnification, however the afocal image on our eye is often significantly affected. Some objects cannot take high magnification, others can.

It has as much to do with object type and our own physiology as it does quality and seeing conditions. Remember, the actual sensor actually seeing the telescopic image at different magnifications is our own eyes. That afocal image should be tuned to our acuity, and that's different for each class of object and observer. However, that's not to say some rules of thumb based in resolution and average acuity are not without merit.

Some low contrast images of faint fuzzies require a certain critical size for detection without becoming too dim to see. Magnification governs image scale and the exit pupil relates to image surface brightness. The best magnification is usually a good balance between the two.

#13 Shorty Barlow

Shorty Barlow

    Mercury-Atlas

  • *****
  • Posts: 2,514
  • Joined: 15 Sep 2015
  • Loc: Lloegyr

Posted 26 May 2020 - 01:24 PM

 

 

I bought a Skywatcher 72ED, last year SW said 144x max, now SW say 216x max. Has the glass evolved over a year into greater perfection? I suspect not, I also suspect it will never make 144x. Maybe 100x if the gods of astronomy smile on me. 

 

 

A 72mm aperture should have a 144x maximum magnification if you aim for a 0.5mm exit pupil.

 

At 50x per inch of aperture it will give 140x. I can regularly get 60x per inch with it on the Moon which is 168x.

 

The 216x is basically 77x per inch. I've had 210x on the Moon in really good conditions, although it's rare.

 

I'm not sure where the Sky-Watcher marketing department are getting the 216x figure from, although they do emphasise it is a potential magnification. I don't think the glass needs to evolve, I just think it could be capable of 216x with the right conditions and target. I took mine to 210x on the Moon once and thought I could have pushed a bit more to be honest.

 

So, I'm guessing it has the potential at least. That and it's basically a claim by the Sky-Watcher marketing department, possibly trained by the Sirius Cybernetics Corporation



#14 Supernova74

Supernova74

    Messenger

  • -----
  • topic starter
  • Posts: 417
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 26 May 2020 - 01:45 PM

from above 

 

The ED,APO,s always seem to handle powers better then your conventional type telescopes reflectors,sct cassagrains etc as a perfectly sculpted lens will always outshine a mirror based design unfortunately the down side much more exspensive to produce.so your 60x per inch in good steady seeing conditions will be easily achievable then some 



#15 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 26,328
  • Joined: 01 Feb 2006

Posted 26 May 2020 - 02:23 PM

It is a very complex subject and the maximum magnification an observer can use can vary a great deal by observer.

 

The important thing to know is the eye and how it responds to illumination and visual noise, and what the individual's contrast sensitivity threshold is.

 

See, our daytime eye has very high contrast sensitivity.  Rather than write everything I know about contrast sensitivity here, I am going to give you a link that explains it, but this link primarily focuses on the photopic eye (daytime or night time indoor illumination levels). 

 

https://www.aoa.org/...Sensitivity.pdf

 

At night, the eye moves from photopic mode to scotopic mode and in between, it passes through a mode called mesopic.

 

As it goes though these changes, the contrast sensitivity threshold (the amount of contrast a detail has to have to be seen against the background rises meaning that someone that can see contrast of 2" or 3" might need to have 10% contrast to be seen at night (and again, refer to the link above).  

 

Now the point here is that not everyone has the same contrast sensitivity threshold and a great deal depends on the angular size of the detail and it's starting contrast.  

 

For example, if you looked for a faint festoon 2 arc seconds wide on Jupiter you could struggle to see it, or not see it at all. This is a low contrast detail and if it falls below your contrast sensitivity threshold, you can't see it.  But what about the shadow of a Jovian moon crossing the planet?  This is also only maybe 2 arc seconds, but the difference is that it starts at very high contrast (probably 80% or more) so even though it is the same angular size, you can see it easily.  

 

Now next is the luminance of the subject. The brighter a subject is, the better you will see low contrast detail on on it.  If you make the exit pupil larger, the detail becomes brighter, but if you make the power too low, it becomes to small to resolve.  If you use a smaller exit pupil, it becomes larger, but if you make the exit pupil too small, it becomes to dim to see. 

 

So, the point is that it is very complicated and different people will have different contrast sensitivity thresholds, so a power that works well for one person may not work well for another person. A person with better contrast sensitivity might see something at a higher power than someone with a lower contrast sensitivity threashold.

 

Subjects with very high contrast sensitivity (double stars, shadow transits, the Cassini Division) can take far more power than a faint festoon on Jupiter's surface or subtle belt shading on Saturn.

 

The bottom line is that no one can tell you what the highest power you can use would be.  It depends on the starting contrast of the subject, the illumination level, the angular size of the detail, and the contrast sensitivity and visual acuity of the observer.

 

What is not enough power for one person might be too much for the other.

 

Rules are made to be broken and the rule of 50X per inch is a silly one because the maximum power you can use depends on a lot more than the aperture.  


Edited by Eddgie, 26 May 2020 - 02:29 PM.

  • doctordub, elwaine, Asbytec and 1 other like this

#16 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 26,328
  • Joined: 01 Feb 2006

Posted 26 May 2020 - 02:27 PM

And this is why you can image detail that is invisible to the eye. The camera handles the luminance issue by using a very long exposure (or multiple exposures), and the camera has a fixed contrast sensitivity threshold (less than 2% which is far better than the mesopic or scotopic eye.)

 

Here is a great page on the telescopic eye. If you really really really want to understand why the 50x per inch rule is total BS, read these pages.

 

https://www.aoa.org/...Sensitivity.pdf

 

What it is going to say is what I said at the beginning of my previous post which is that it is complicated. 



#17 TOMDEY

TOMDEY

    Fly Me to the Moon

  • *****
  • Posts: 6,769
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 26 May 2020 - 02:53 PM

Hi, SN74! PS: I also enjoy your ~stream of consciousness~ writing style! When I was a kid, the nuns would gleefully slap me around for violating their school-marmish Rules of Order. I would, of course, therefore violate conventions all the more. Years later, word got back to them, that I scored 1600 on the SATs, at the tender age of sixteen... and that they took the news by being... even more incensed!

 

Keep up the good work, astronomy is an expansive hobby!    Tom, "Last Laugh Tom"



#18 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 26,328
  • Joined: 01 Feb 2006

Posted 26 May 2020 - 06:00 PM

And if anyone bothered to read the article I linked on contrast sensitivity, they talk about a sine wave grating.  This grating is not only used in contrast sensitivity testing, it is also the way to express the modulation transfer function (MTF) of telescopes. 

 

The sine wave is used for the eye in the same way.

 

The MTF plot for a telescope shows how much contrast it will loose at a given angular size of the lines in the same way the it is used for patients. After all, both are optical systems, and both have the function of contrast transfer, and both can have reduced contrast transfer for a variety of reasons.

 

This is what the chart is telling us.  The lines shown are 100% contrast and the x axis is showing what happens with the frequency (angular size of the detail)  As the frequency drops (detail gets smaller) the scope looses more and more contrast.

 

Now for a 100% contrast, this example shows that the detail would be visible until it got to the size shown in the bottom grating at what would be about the .7 of the maximum frequency.

 

As the detail gets smaller, it becomes harder to resolve the detail but more than that, the eye itself starts to cause contrast loss.

 

 

 

 

If the detail starts with very low contrast (indicated by the dark blue line) even though it is a larger detail, once it falls below the contrast sensitivity line, the observer would not be able to see it.  It would blend into it's background.  

 

Contrast sensitivity.jpg

 

(Note.  The numbers used here are not intended to be exact representations.  They are simply chosen to explain how contrast sensitivity threshold works. The human eye cannot see most of the detail the scope can show.)

This would be like the example I gave earlier. A moon transit of Jupiter is easy to see because even though it is small, it starts with very high contrast.  A festoon on the other hand, can be quite large but starts with very low contrast.  In very small scopes, even the Great Red Spot can be difficult to see because all other things being equal, the smaller the aperture, the more contrast it would loose but the GRS is well within the ability of a very small telescope to resolve for the camera with quite good detail.) 

 

So, this is why there can be no hard and fast rule for an observer, but a camera can capture far higher resolution of very low contrast detail.  A camera would capture even 2% resolution all the way to near full resolution of the telescope. The eye, by comparison, even for a sharp eyed observer, will not approach the performance of a camera and the contrast sensitivity threshold is quit variable form one observer to the next.  Someone with better contrast sensitivity will see a bit smaller or a bit lower contrast detail, some with poorer contrast sensitivity would not see all of the detail that is even shown above the blue line here.   

So, magnification is greatly influenced by the observer's contrast sensitivity threshold and the amount of magnification that can be used will depend on a variety of different factors.

 

And this is why we have different power eyepieces. 

 

I am a big advocate of zoom eyepeices for planets because it makes it very easy to try a large variety of magnifications. This is important for both the ability to find the best magnfication for a given detail, or to adjust the magnification to match seeing conditions.   


  • Jon Isaacs and Berny like this

#19 havasman

havasman

    Voyager 1

  • ****-
  • Posts: 11,010
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 26 May 2020 - 06:25 PM

My accomplished ophthalmologist tells me I have very good eyesight. I believe him and have reason to do so.

 

I typically observe at high and often very high magnifications. The site I use, my gear, my preferences and objects I typically observe allow and benefit from this practice.

 

There is no controversy.

 

I do not care for high magnification observation of planets.

 

 

And yes, the OP might want to learn to format and edit if he expects to be read.

 

Hi guys I’m new to cloudy nights and live in the uk and this will be my first topic of interest and will be interested in hearing your views and opinions on the listed topic and I apologise in advance if I’ve posted in wrong section.

 

from time to time we all can be creatures of habit and perhaps set in our own ways in trying something new.and the knowledge and advice we receive on social media sit.......

That's about as far as I bothered with that mess.



#20 skybsd

skybsd

    Aurora

  • *****
  • Posts: 4,821
  • Joined: 01 Feb 2008

Posted 26 May 2020 - 06:31 PM

Hi guys I’m new to cloudy nights and live in the uk and this will be my first topic of interest and will be interested in hearing your views and opinions on the listed topic and I apologise in advance if I’ve posted in wrong section.

 

from time to time we all can be creatures of habit and perhaps set in our own ways in trying something new.and the knowledge and advice we receive on social media sites and various other sources of information Around Astronomy can be easily accessible aslo through our favourite dealers etc.and to my knowledge similer information and advice is pretty much the same on the other side of the pond to you guys in the United States.and myself including jump on the band wagon in what monkey do what monkey does kind of scenero this is based on the knowledge and advice we can receive a general rule of thumb.for exsample regarding the telescopes we own there is a guideline we follow in what’s the lowest usefull magnification and what is highest which to most principals seem to be around 40-50x per inch of aperture of your scope and a definite yes dark,clear seeing conditions is key and low Atmospheric turbulence makes or breaks useing your favourite low focal length eyepiece. Quote al nagler once said the god father of Televue EP his best views was through an 3” ED,APO telescope looking at Jupiter at 400x.which is well and beyond the freshold that the telescope in theory should be capable of and the best ever views he ever had.so that 40-50x highest pratical magnification has gone out the window then.however we are not comparing an department store telescope here that has pretty pictures on the box of the planets,moon and galaxies etc which claims to have potential of reaching 525x magnification those are just ridiculous claims and the purchaser who brought it is often left feeling very disappointed.myself personally I truly feel tho in reasonable conditions not even 100% full capacity which my sky bortale 5-6 sky’s can produce we can achieve well above the recommended 40-50x per inch of aperture to a certain degree of course and for the guys who own high end apo,s you know what I mean there.so anyway here in the uk and were I’m located the recommended maximum usefull magnification is around 300x in general good seeing conditions I just don,t believe it as last year even tho Jupiter and Saturn where low in the meridian I was observing both the planets at over 300x thease were very average conditions so am I mad don,t think I am as views were still sharp the atmospheric turbulence was reasonably low and I could easily gone up a notch or two with a lower focal length EP and still maintain half decent views with contrast and detail intact.another exsample two months ago I observed the moon started off low then worked my way up and the moon was near full phase I used the televue 10mm Delos accompanied by an Exsplore scientific x2 Barlow at first was,nt go to exspect to much and to my amazement the view were which I consider above average this was a shade over 600x it was almost like I could reach out and touch it kind of moment unbelievable.so my point is really just because someone says you can only walk sometimes we can run type of analogy!? And certain sources of information is a rough guide line only doesn’t mean it’s not achievable and another important factor is to consider is the quality of the optics of today are so much better of years gone by.

This is weird and awkward to read.., 

 

skybsd 


  • elwaine and Berny like this

#21 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 85,034
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 26 May 2020 - 06:42 PM

Actually you can exceed the resolution limit, quite easily in fact. A telescope has reached its resolution capability at approximately 1x per 1mm of aperture. So a 100mm refractor has resolved all it can at 100x magnification. However, it is easier for the eye to see the resolved detail at a higher magnification for most. While magnifying above 1x/1mm is called “Empty Magnification”, for most people going higher, up to a point (usually at .5 to .8mm exit pupil) they will better be able to make out the detail presented by using a larger image scale to stimulate more of their eye. No hard fast rules...experiment for what works for you. And yes seeing, optical quality and eye quality have an impact!

 

The telescope's resolution limit has nothing to do with the magnification. The telescope provides an image at the focal plane.. magnification, as you suggest, is all about the eye.

 

The eyepiece magnified the image so our eye can see the detail. Estimates of magnification per inch are based on estimates of the resolution of the eye, which is a strong function of the brightness of the object.. 

 

The smallest Galaxy in Stephan's quintet is about the size of Jupiter.. to see it might take 200 x but there's really nothing to resolve, it's too faint..

 

Listen to your eye, it'll tell you the optimal magnification. I use 80/inch on very close doubles, a 7.8 mm exit pupil for Barnard's loop.

 

Jon


  • doctordub and Asbytec like this

#22 Supernova74

Supernova74

    Messenger

  • -----
  • topic starter
  • Posts: 417
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 27 May 2020 - 01:37 AM

Oh dear never mind you kind of get a few negative responses from time to time.

however why do some folk comment in first place if thay did,nt like the content and the way it was written??


  • doctordub, TOMDEY, SonnyE and 1 other like this

#23 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 17,100
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 27 May 2020 - 03:38 AM

Oh dear never mind you kind of get a few negative responses from time to time.

however why do some folk comment in first place if thay did,nt like the content and the way it was written??

We read it because we want to reply to an interesting question you want an answer to. However, paragraphs do make it much easier to read. But, we're drifting from topic. I said my piece above. 

 

I'd only add it's hard to separate magnification from the observer. The telescope provides an image on the focal plane according to the laws of physics. However, that afocal image is projected onto our eye for us to perceive it in terms of our own acuity. Rules of thumb based on average acuity can be guidelines, especially for point source resolution which many deal with. They do not apply to other types of extended objects, like dim galaxies. For that there are critical size of detection theories and guidelines.

 

The best advice has been mentioned a few times above, try to bracket your magnification - start a little low, go higher, then back down a notch if needed - to find the most productive image that works for you under your observing conditions and depending on the type of object. Generally, our eye likes a large object that is still bright enough to be seen well. Magnification, in my experience, is all about striking that balance between size and surface brightness.

 

The fully resolved telescopic image still has to be resolved on our eye so we can perceive what the telescope can deliver using our eye/brain system. The trick, IMO, is to see as much of that telescopic image as is humanly possible or simply framing a celestial scene that pleases us (wide colorful double stars or large nebulae). Humanly possible often means experience, patience, and technique, as well as any other factor including the telescope, the type of object, and our environment. 

 

To answer your question more directly, I might argue the highest practical magnification is found just before the object becomes too dim to see well by the increasingly smaller exit pupil. For example, I can still see Jupiter at 0.3mm exit pupil (~ 675x with a 200mm aperture). It's still resolved in all it's glory on the focal plane, but the very large afocal image is too dim for my eye to see well. I am usually better off at 0.5mm exit pupil (400x with a 200mm aperture) because the additional brightness stimulates my high resolution photopic vision.

 

Various bright NGC galaxies are seen well between 2mm exit pupil and less than 1mm exit pupil, but we are observing a different class of object using scotopic dark adapted vision on the low resolution part of our retina. Some small bright planetary nebulae are best seen at very high, almost ludicrous magnification (at small exit pupils) because their image retains a lot of its surface brightness allowing us to put a larger bright image on our eye to resolve more detail. (The exit pupil arbitrates the afocal image brightness and results from the magnification. Exit Pupil in mm = Aperture in mm/Magnification). 

 

The Rosetta, Orion nebula, the Andromeda galaxy, and other large objects, however, may benefit more by low magnification, wide field framing of the subject. That would probably be the highest practical magnification, though you could certainly crank it up on any of those objects, too. But, you are not likely to see the annular structure of a small bright planetary nebula at that low magnification and probably won't split a tight double star, either. Double stars can really take magnification, in fact I often prefer upwards of 80x per inch for bright high contrast objects. Mars, too, as much as "practical". Productive. 

 

Well, I added too much already...


Edited by Asbytec, 27 May 2020 - 03:41 AM.

  • Stelios, Jon Isaacs and Dougeo like this

#24 SonnyE

SonnyE

    Surveyor 1

  • -----
  • Posts: 1,915
  • Joined: 01 Nov 2015
  • Loc: Cali for ni a

Posted 27 May 2020 - 03:55 AM

Oh dear never mind you kind of get a few negative responses from time to time.

however why do some folk comment in first place if thay did,nt like the content and the way it was written??

Well, and then there is me..

I was photographing the night sky, and became interested in the Orion Nebula, and spent two nights finding it in my spotting scope.

It was 20 x 60 power magnification. I was so excited to finally see a Nebula!

I spent a month trying to decide if I wanted to photograph the night skies. I was hooked on Nebula, still am.

So I began researching how to image my interest. Funny thing, over the next 4+ months as I refined how to do what I wanted, the telescopes got smaller.

I found my nice simple 80mm was sufficient for DSO. And what I needed was actually a camera lens in the form of a telescope. And what I needed was not my DSLR, but an Astro Camera.

After reading, studying, and a few asking's, I was ready to "pull the trigger" on one of 3 wish lists. I finally decided on my ED80T CF because it was the best I could do for what I wanted to pursue.

 

I expected a learning curve. It was going to be a climb learning all the new junk and figuring out how to run it. But it took several years to overcome the boulders the false advertising rolled down the learning curve at me.

But I have the tenacity of an octopus. I won! I always win! Over 5 years I persisted, and I persevered. My first camera was utter junk, replaced 3 times in the first year, 5 friends tried to process it's files and couldn't. A dear friend took me under his wing and sold me my Atik Infinity OSC camera from his collection of gear. It was a complete success from the first image the very first night.

My first mount failed the first week I had it. The vendor, and Amazon Prime, both screwed me over that piece of doo-doo. Celestron worked on it twice in the first year, and it finally died the Error 16 and Error 17 death last November. 4 years and dead. I now own a Losmandy GM811G HD, with a 12" extension to make it a portable pier design of my own doing. So I'm learning it, and having huge successes with my equipment now.

 

So, this is a different sport. And everybody seems to wear a different Jersey to the field. I don't think any two of us use the exact same stuff. And I know I use some pretty unorthodox equipment myself. Like a Tasco Red Dot firearm sight as my spotting scope, that I made my own custom mounting for. But it works great and has 11 settings of brightness ( I usually use the 1, dimmest, setting.)

I made my own adjustment paddles to level up my tripod before erecting the column, head, Dec, and telescope imaging rig on it. And I'm a stickler for accuracy so I use a digital level to get things 0.0 degrees of level on 3 directions. People have argued it isn't necessary. I argue that less than your best effort is lazy and slothful.

 

But the bottom line is that almost everybody bangs around until they are happy with their kit. I made some bad choices because I trusted names I had known most of my life. I suppose I should be glad I don't have a garage full of mistakes from buying willy-nilly and being scatter brained.

I knew what I wanted to do, set out on a path, and 5 + years later I'm living my dream. I have the Pelican Nebula cooking on my side monitor, being WiFi'd in from my mount computer, and writing this while operating my Mount and imaging telescope remotely.

 

So.... what do you want to see? It's a lot like the old racers adage, How fast do you want to go, and how much money do you have to spend? wink.gif lol.gif

(And don't worry about the grammar cops. They pick on your lack of Paragraphing to hide their reading comprehension disorders. grin.gif  )

 

Welcome to Cloudy Nights!


  • Supernova74 likes this

#25 Supernova74

Supernova74

    Messenger

  • -----
  • topic starter
  • Posts: 417
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 27 May 2020 - 04:29 AM

Thankyou Guys for your positive responses yes I will work on my literacy skills and take that on board.but on the over side of the coin it shows I’m passionate about amateur Astronomy!?call it speed writing ✍️ 


  • Jon Isaacs, Asbytec, eros312 and 2 others like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics