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Question about maximum magnification

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#1 Agreegator

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Posted 25 November 2020 - 02:03 PM

I have a Meade 127mm MAK. Now based on the usual "Max Magnification" rule, my instrument should give max useful magnification of ~ 250X. However, even at a magnification of ~ 250X, I feel that I am severely underutilizing the scope. For example - Jupiter is a very sharp - but glowing, overexposed disk at 250X with all moons visible at this "maximum magnification" and I have to set my polarizing filter at full darkness + reduce camera exposure to get any kind of useful image  - Maybe its to do with the UHTC coating, that Meade advertises so much ?

 

In fact, I have seen some stacking tutorials for 127mm MAKs where people are working with far, far darker and magnified images of planets, than I am limiting myself to - and they seem to get great results after stacking. So my question is this : Of course I am not going to image DSOs with a MAK - So what if I go to a magnification of 300X - 350X or more ? What more harm will it do ? 

 

Focusing works great on the scope and I can feel I have the room to focus more precisely. I have no issues with tracking at higher mags - as I am practicing my hands on manual tracking and its working great. And I can of course live with reduced brightness - that would be a blessing. So is there something else that need to look for at higher than 250X mags ? I am thinking of ordering a 2X barlow to see how it goes. I'll really appreciate if someone can tell me the technical reason for the 2X rule .. so I can understand if its relevant for me or not.

 

Thank you 



#2 rhetfield

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Posted 25 November 2020 - 02:30 PM

The magnification limitation is more about what your eye can see than what the scope can see.  What your eye can see is based on exit pupil - how big the image at the eyepiece is.  A young adult may have a maximum pupil of 7mm in the dark.  An older person might only get 5mm.  If the scope presents an exit pupil too large, the person can't see it all.

 

Once exit pupil drops below 1mm, it starts to get dimmer and grainier due to how little of the eye gets used to see it.  By the time one gets to 0.5mm exit pupil,  most people find that the image is unacceptably dim and grainy.  Less bright targets degrade quicker than bright ones like the moon.

 

On your scope, 127x is an exit pupil of 1mm due to the aperture.  254x is 0.5mm.

 

The camera is able to take longer exposures than your eye, is able to see better in the dark as a result, and often has higher pixel resolution per mm^2 than your eye.  As a result, it can see good at a lower exit pupil than you can.  That is why people can take good pictures with a small scope.  For astrophotography, the real limitation is how well can a scope track a target.  On the planets, the rotation of the planet can be an issue as well - too long of an exposure will cause the planet to motion blur. 


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#3 MellonLake

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Posted 25 November 2020 - 02:32 PM

The issue is exit pupil. Basically at about an exit pupil of 0.5mm optical aberrations in our eyes become an issue and the image degrades. Bright objects tend to compensate for this a bit so you may get a bit more magnification on the moon and Jupiter.

The exit pupil is calculated as the focal length of the eyepiece divided by the focal ratio of the telescope. For a 0.5 mm exit pupil this will be equal to the diameter of the telescope in mm times 2. Beyond this magnification the exit pupil is smaller than 0.5mm.

So the issue is not that the optics of the telescope can't do more. It is that our eyes can't do more.

The advantage of bigger telescopes is that the exit pupil for a given magnification is larger. However in reality we are often limited more by atmospheric seeing than even exit pupil considerations.

I would also add the at some point adding magnification does not add detail. For the conditions use the magnification that produces the best view. That might be 120X or 250x. Increasing magnification does not necessarily result in more detail unless the conditions allow for it. I have had many nights where the view at 140X is far superior to 240X.
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#4 TOMDEY

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Posted 25 November 2020 - 02:34 PM

You can always cut down brightness with a "moon filter". Some people prefer more mag, some prefer less. I get the impression that one's visual acuity also enters into it. If your eyes are 20/20 the 50x/in rule-of-thumb limit should apply... if your vision is worse, you will prefer/need to go higher; if your vision is better, you will probably prefer lower.    Tom


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#5 Redbetter

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Posted 25 November 2020 - 04:21 PM

This seems to be more of an imaging question.  I have no idea about the imaging aspects.  

 

With regard to visual, I have the ES 127 Mak (f/15) which is the ES clone of the same OTA, but with a better back plate arrangement than on most of the Meades--allows access to the collimation screws.  For my sample and my eye, I have found that in very good seeing that a 9mm eyepiece for ~211x is about the limit for maximum planetary detail.  Most of the time seeing limits me to a 13 or 11 for 146x or 173x.  

 

When it comes to theoretical limits used in specs, they are generally provided using a 50x/inch rule of thumb.  This is typically about right for me with a well corrected ED/apo refractor on planets, but obstructed scopes like cats and newts are a bit different.  The combination of mirrors and sometimes large central obstruction (e.g. ~40% in 127mm Maks) reduce the contrast.  The result is that I find the optimal detail is somewhat lower/discounted with large obstructions.  With my SCT and Mak the result is an optimum magnification for planets in stable seeing that is about 40x/inch, give-or-take a little.  

 

Different folks have different eyes and preferences, but much of this comes down to what level of magnification begins to reveal the diffraction effects.  On a suitably bright star I can begin to see the first diffraction ring by about 1.5mm exit pupil (~17x/inch.)  Some with sharp eyes might be able to see it at 2mm exit pupil (~12.5x/inch).  Most will be able to detect it by 1mm (25x/inch).  The 0.5mm exit pupil (50x/inch) is at about the point that diffraction effects begin to noticeably blur extended objects.  One can continue to increase magnification but it is unlikely to reveal anything more, and it might even make it harder to discern detail as the image becomes dimmer.  [Note, this resolution loss is somewhat different than the contrast loss from reduced transmission and from the obstruction of reflectors and catadioptrics which result in some additional discounting as described earlier.]  Exit pupils somewhere below 1mm tend to show more of the structure in the eye as well (floaters, some texture, etc.)

 

My advice with respect to finding optimal planetary magnification for a scope is to approach it incrementally, beginning with a magnification that should be useful on most decent nights.  ~150x is useful even on mediocre nights when stability is not good.  ~175x or so is usually there on decent nights with the 127 Mak. ~200x is for the better nights.  If a person starts with an eyepiece that is nearly always useful for planetary, then adding the next increment will likely reveal more on some/many nights.  If that one still seems to have some room left, then add the next increment.  Eventually one will find that the final eyepiece never seems to be providing more detail, even on the best nights.      

 

Starting at 50x/inch is unlikely to prove useful in that regard even with perfect optics and perfect collimation.  It will almost certainly overshoot for planets.  At 50x/inch one is not accounting for obstruction effects, or more importantly seeing and thermal effects.  Most of the time seeing will be limiting.  

 

So where will 50x/inch or even higher prove handy?  Very close double stars.  With tight double stars one wants to see and inspect the diffraction patterns associated with the spurious disk and first diffraction ring.  This is a range that eyepiece and Barlow combinations cover well.


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#6 Eddgie

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Posted 26 November 2020 - 08:27 AM

It will do no harm at all.  Use as much magnification as you think the subject will bear.  Keep in mind though that as you magnify the image, lower contrast detail may become harder to see but high contrast detail like shadow transits of the Jovian Moons, or double stars can take considerable power. If you are happy with the views, it does not hurt anything to use as much as you like. Higher powers often don't give as good a result as lower ones, but that is what you will learn by experimentation.  


Edited by Eddgie, 26 November 2020 - 08:28 AM.

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#7 Jon Isaacs

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Posted 26 November 2020 - 08:42 AM

I have a Meade 127mm MAK. Now based on the usual "Max Magnification" rule, my instrument should give max useful magnification of ~ 250X. However, even at a magnification of ~ 250X, I feel that I am severely underutilizing the scope. For example - Jupiter is a very sharp - but glowing, overexposed disk at 250X with all moons visible at this "maximum magnification" and I have to set my polarizing filter at full darkness + reduce camera exposure to get any kind of useful image  - Maybe its to do with the UHTC coating, that Meade advertises so much ?

 

 

From your description, it seems you are using camera and not your eye.  

 

The 50x / inch rule of thumb applies to visual observation, it is about matching the telescope to the heye. it doesn't apply to astrophotography.  For astrophotography, matching the camera to the telescope is done differently and it depends on the camera as well as the telescope.

 

These days, planetary photography is done with high speed video cameras.  Hundreds even thousands of uncompressed (or losslessly compressed) images are taken and then processed via stacking.  Maybe you are doing this.

 

By the way, reduced camera exposure is a good thing, it negates the effects of atmospheric turbulence, you should be shooting at the fastest possible shutter speeds with the lows ISO settings.  

 

Jon




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