Max magnification - can it be exceeded?

[vlaiv]

6 minutes ago, John said:

I think often we see the Encke Minima rather than the Encke Division.

 

Not sure about that. Minima is very low contrast feature and is quite extended and soft:

[John]

I'm not sure about it either come to think about it. It's probably a bit beyond the scope of the question asked by the original poster anyway

[vlaiv]

5 minutes ago, John said:

I'm not sure about it either come to think about it. It's probably a bit beyond the scope of the question asked by the original poster anyway

Yes - it occurred to me that I should have checked if initial post was in beginner section before I started whole theoretical discussion.

[ollypenrice]

Planetary images in telescopes are very small so it is naturally tempting to want to make them bigger. Personally, I think this temptation is best resisted.* I find it more productive to concentrate on the image I have than to hanker after a bigger one. 

Olly

* I hope my wife isn't reading this since my ability to resist temptation is tenuous at best...

[Peter Drew]

The problem with theory is that it's relatively fixed whereas practice is generally not.  Some of us are getting on a bit now and despite experience the visual acuity starts to drop. I now find I need more aperture and magnification than I used to use to be able to see the same as I did in the long past. It also depends a lot on the nature of the object, a line can easily be seen although a section length of the line equal to its width would be invisible. This is how the Cassini Division can be resolved with telescopes whose apertures are theoretically not large enough.   🙂

[PEMS]

I suggest that most "maximum magnifications" are almost irrelevant and certainly rarely are what the manufacturers state. They know that people will see 200x maximum on one scope and 300x on another and buy the 300x, oddly even if they are obviously the same scope.

I have a small 72ED, Skywatcher. Here is the fun bit: The scope had a "maximum" of 144x, your standard (2x diameter). As thescope is for wide views and at the time the Mercury transit, the maximum does not interest me greatly.

Now Skywatcher updated their information and surprise the maximum is now quoted as (3xdiameter), so has become 216x. Same scope, same glass, same paintwork, same alu case. Nothing has changed on the scope but now 50% increase in magnification apparently overnight.

So DK what does that mean? I suspect it is marketing, not physics or optics.

Many quotes of a maximum are apparently based on the exit pupil and how the eye can function. Some numbers say the eye has an exit pupil limit of 0.5mm and that is a magnification of (2x diameter). But that does not say the scope is capable of that magnification. That just says the eye may be capable of handling it.

Have read a statement with the maths that an exit pupil of 1mm is "best", that means a magnification equal to the diameter (mm). Have read another that says the best is a 2mm exit pupil so (diameter/2) - close to Vlaiv's.

Many find it good to go with the 1mm idea, it is in the middle and in a way easy. To get 1mm exit pupil you buy an eyepiece equal to the focal ratio, then the magnification delivered is equal to the aperture.

I would expect a young eye - childs - to handle a smaller exit pupil better. How old is your eye? What works now could well not work in 8 to 10 years, maybe not so well in 2 years.

You can push the magnification "higher", put a 5mm eyepiece in and a 5x barlow and if you do the numbers the magnification is "higher". Very likely utterly useless, but it is higher.

[John]

Telescopes with very high optical quality seem to support high magnifications while maintaining good image quality. Although the maths still apply these scopes with premium optics do seem to be able to "bend the rules" surprisingly often.

 

[Louis D]

Back to the OP's original question, I find I can't use very high relative magnifications with my 127mm Mak because of cool down issues and lack of perfection in the figure of the optics.  I can push my hand figured Newtonian mirrors much further than my little mass produced Mak relative to the their respective theoretical limits within the same cool down period and beyond.

[jetstream]

@vlaiv thanks for the explanations, I always like the theory.

I can absolutely see both the Enke minima and Enke gap, back when Saturn was high using my 10" dob (&15"). I cannot see that thin line dividing the gap. My eyes test 20/15 and 20/10 last time checked. I totally agree that there is a difference between resolving and detecting.

@davekelley I do not focus my attention on theory eventhough I enjoy it. What I do when observing is to maximize things in my control ie cooling, collimation and thermals. Once done I then try everything I have on an object-ie magnification etc. Doing this allows you to get to know your own scope and its limits as well as your seeing conditions.

Some objects seem to defy magnification limits ie Saturn and the moon and we can throw Mars in there to.IMHO.

[Rob Sellent]

To the question 'what is maximum useful/optimal magnification?' There seems to be a whole range of ideas....

• 0.45mm x aperture in mm.
• 2mm x aperture in mm/50x per inch of aperture.
• 1mm x aperture in mm/25x per inch of aperture.
• focal ratio of scope x2 / focal length of scope (2mm exit pupil).
• focal ratio of scope x1 / focal length of scope (1mm exit pupil).
• Half focal ratio of scope / focal length of scope (0.5mm exit pupil).
• TeleVue recommends 2.5x mm of telescope aperture.
• Experienced observers going way beyond even these limits etc

and so on and so fourth....

Depending on who you ask, a 100mm f/10, for example, will either have a maximum useful/optimal magnification of around 45-50x, 100x, 200x, 250x, or more. Plug in other factors like optical quality, collimation, seeing and atmospheric conditions, visual acuity, the type of object being viewed and these numbers may start to vary once again. In other words, all the detail through your scope will be seen at about (a, b, c, d, e) x per mm of aperture and that any magnification above will fail to reveal any more detail, although the detail might be easier to see. 

Clearly, as this thread is showing, such statements cannot be right. What magnification is useful is evidently variable. As such, all options should be explored when observing, or at least those which maximise the observer's experience.

[vlaiv]

4 minutes ago, Rob Sellent said:

As such, all options should be explored when observing, or at least those which maximise the observer's experience

This is much in line with what I've gained from this thread. Up until now I was under impression that x2 aperture in mm is based on theory (and it is but with a bit flawed assumptions) and that was one of my primary drivers in eyepiece selection - you need one eyepiece that will show what scope can deliver under optimum viewing conditions.

It turns out that chasing short FL comfortable eyepieces is not going to provide that because x2 aperture is not that magical figure. It sort of lifted that burden of aiming for particular magnification based on physics of things and set me free to experiment with both lower and higher magnifications depending on target (primarily without fear that I'll miss out on some finest detail if I use lower power eyepiece that is sharper due to less mag and also due to atmosphere).

[jetstream]

26 minutes ago, Rob Sellent said:

As such, all options should be explored when observing, or at least those which maximise the observer's experience.

There is a hillbilly saying "run whatcha brung"! (and bring lots lol!)

[johninderby]

2 minutes ago, jetstream said:

There is a hillbilly saying "run whatcha brung"! (and bring lots lol!)

And bring a zoom so you can try different mags to see what works at the time. 

[davekelley]

2 hours ago, Louis D said:

Back to the OP's original question, I find I can't use very high relative magnifications with my 127mm Mak because of cool down issues and lack of perfection in the figure of the optics.  I can push my hand figured Newtonian mirrors much further than my little mass produced Mak relative to the their respective theoretical limits within the same cool down period and beyond.

I don't find cool down a major issue in the middle of a British winter.  I can take the scope out to the shed for an hour before dark and bring it out to setup, half an hour and I think it's all good!  I was asking this question because I do desire a little more mag on the planets although it is true that often images are far better at lower powers.  In fact it's hard to beat the sharpness of a good 32mm super plossl on a lot of objects.  I just really like the Nirvanah ep's and looking at the moon (an easy target I know)  the scope was handling the 7mm with great ease.  I thought This thing can definitely handle a 5mm ep.  Sadly the Nirvana only do a 3.2.......I suppose I know that is just was too much and looking at planets is a different thing from looking at the moon.  In fact I've ordered an ND filter for the purposes of lunar observing!

regards

Dave

 

 

[jetstream]

1 minute ago, johninderby said:

And bring a zoom so you can try different mags to see what works at the time. 

Absolutely, a zoom with "catch" the seeing very well. I own 3 nice zooms one of which gives ortho like views in the centre of the field. I;m playing around with it in the ParacorrII for f4.

Also for the OP - there is also the idea of matching eyepiece tone to the target- I like an icy tone on Saturn but a warmer tone on Jupiter for example.

[jetstream]

On 17/12/2019 at 09:44, davekelley said:

Now I have a tracking scope I wonder if fov is not so critical on planets (no need to keep nudging it along to keep the object in the fov)

I use orthos on a tracked refractor with ease as well as the Vixen Hr's. Long Perng made some really nice 5mm planetary eyepieces with 20mm eye relief. Zumell, Stellarvue, Orion (Edge ON?) and Williams optics offer variants of these or did. The 5mm I have is VG.

[John]

46 minutes ago, davekelley said:

I don't find cool down a major issue in the middle of a British winter.  I can take the scope out to the shed for an hour before dark and bring it out to setup, half an hour and I think it's all good!  I was asking this question because I do desire a little more mag on the planets although it is true that often images are far better at lower powers.  In fact it's hard to beat the sharpness of a good 32mm super plossl on a lot of objects.  I just really like the Nirvanah ep's and looking at the moon (an easy target I know)  the scope was handling the 7mm with great ease.  I thought This thing can definitely handle a 5mm ep.  Sadly the Nirvana only do a 3.2.......I suppose I know that is just was too much and looking at planets is a different thing from looking at the moon.  In fact I've ordered an ND filter for the purposes of lunar observing!

regards

Dave

 

 

There is a 4mm Nivana eyepiece but it is probably still too much power to be useful:

https://www.harrisontelescopes.co.uk/acatalog/ovl4nirvana.html

The Skywatcher UWA Planetary eyepieces come in a good variety of focal lengths and are not too expensive:

https://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-uwa-planetary-eyepieces.html

 

[Louis D]

Maybe the OP could try @John's trick with the barlowed zoom to judge maximum usable power.