Highest Practical Power(Potential)

[Pryce]

Hey lads and ladinas! 

 

I have a question, about magnification and barlows.

So, I've ordered a SW 150p which has x300 listed as it's highest practical power(potential) 

But what does this mean exactly? 

 

Is the limitation at x300 based on limitations in the scope/apeture/ep or is it based on atmospherical conditions? If(and thats a big if) seeing was absolutely optimal, would the potential max be higher? Say, a 2,5mm or a 4mm with a 2x barlow at x600 or x375 respectively? 

 

And that brings me to my second part.  Should I just throw the barlow away and get better EPs instead?  As far as I understand, the more low grade glass you're lookikg through the worse of an image you will see. 

[John]

It means nothing really.

Plan on 150x - 220x as the maximum useful magnification.

Telescopes for astronomy are more about light grasp than magnification. Most observing is done at 50x - 100x in reality.

Stepping up from the stock eyepieces is a good plan and not using the barlow lens is another one.

The BST Explorer eyepieces are a very good move away from the stock eyepieces:

https://www.firstlightoptics.com/bst-starguider-eyepieces.html

Is your scope the dobsonian or the equatorialy mounted 150mm newtonian ?

 

 

[Pryce]

Oh yeah, for sure, I'm aware that I shouldn't opt to press for higher magnifications.  I'm just trying to understand what the actual limiting factors(And why the Highest Practical Power(Potential) is set to x300) are.
In theory, given perfect seeing and no atmospheric disturbances or light polluting, would the potential practical power be higher or is it based on limitations of the equipment, is kind of what I mean.

I is the Newtonian on a EQ3-2 mount(Not motorized)

[Louis D]

Depends on the criteria used.  If you're splitting double stars and just looking for elongation rather than a clean split, then that allows for a claim of higher working power.  Realistically, most beginners need not concern themselves with such technical esoterica.  John has pretty much nailed what is typically the maximum working power for your scope and observing conditions.  If on the other hand you were observing from certain coastlines, deserts, or coastal mountain tops with excellent seeing conditions and are using a 16" to 30" Dob, it's quite possible to successfully use powers in the 400x to 1000x range.

[Louis D]

1 minute ago, Pryce said:

In theory, given perfect seeing and no atmospheric disturbances or light polluting, would the potential practical power be higher or is it based on limitations of the equipment, is kind of what I mean.

More specifically, it's based on the Rayleigh criterion.  Wikipedia has a good article on angular resolution.

[Pryce]

Oooooooooooooooooooooooooooooooooooooh that makes so much sense! Thanks Loius! That was exactly what I was looking for!  

Obviously I understand that there are other factors that will affect the maximum practical magnification as well though 😁

Yeah, thanks for your replies, both of you:D

Edited August 13, 2020 by Pryce

[Gfamily]

Barlows can be useful because they allow you to use longer focal length eyepieces and keep the benefits of a larger eye lens and very often a bit more eye relief.

A Barlow with a t-thread can also be useful when imaging planets using a dSLR or mirrorless camera.

[vlaiv]

9 hours ago, Pryce said:

Oh yeah, for sure, I'm aware that I shouldn't opt to press for higher magnifications.  I'm just trying to understand what the actual limiting factors(And why the Highest Practical Power(Potential) is set to x300) are.
In theory, given perfect seeing and no atmospheric disturbances or light polluting, would the potential practical power be higher or is it based on limitations of the equipment, is kind of what I mean.

For a bit more technical discussion on that topic, see here:

 

[Zermelo]

barlows ... yes, it's generally true that the more glass you put in the path of the light, and the more surfaces it has to navigate, then the more likely it is to degrade the image. A cheap barlow will almost certainly do that to some degree. Counter-intuitively though, a decent barlow combined with an average eyepiece *may* perform better than a single eyepiece with their equivalent focal length. This is because the barlow has optical characteristics that are "opposite" to those in eyepieces, so they can have the effect of reducing some of the aberrations present in the EPs.