Definitions of Low, Medium and High power eyepieces and does it really matter?

[bomberbaz]

I got to thinking about this after the recent, "how many EP's do you have" thread.  In this someone referenced the minimum you might need and @Don Pensack posted up with the following rule of thumb type working out which is magnification per inch of telescope.

Low power: 3.5-10x/inch

Medium power: 10-20x/inch

High Power: 20-30x/inch

UltraHigh power: 30-50x/inch

I am not saying this is wrong or right although it probably lends itself better to the USA where skies are usually more settled than the UK.

I think those powers are over optimistic for my UK skies where we have the joys of the polar jet stream to contend with which often sits on top of us. 

Often mentioned on this forum as the maximum power one should work on over here is a simple x200, where anything more than this should be seen as a bonus. This simple rule does away for the x power/inch calculation but does it actually make things simpler for anyone, especially the beginner. 

Most of us have read or are at least familiar with this thread.  

It seems simple to seasoned observers but to a complete novice maybe not so, although it does align itself with the x200 rule of thumb which makes it all simpler but what does anyone else think? Is the x200 rule realistic for the UK skies or are there any of us who are regularly pumping it up to far higher levels?

Personally I have kind of disregarded the x200 RoT, although not entirely as it does always stick in the back of my mind when observing. Instead, I do kind of use Don's formula although not in the same scale and also in that I have a range I consider lower, medium and high power as well as glass I consider for those moments when seeing is excelling normal conditions and one can ramp it up.

If I am talking about my dob I have glass at 2 sizes for low power, 3 size med, 2 size high and a TV Nagler zoom for ultra-high power moments.  Incidentally these work out at roughly 7x per inch aperture so I think there definitely is something in this way of calculating your eyepiece range.

However as I travelled further down this rabbit hole I am reminded of the 2x aperture in millimetres.  For my scope that's x700 as a maximum. 🙀 EEK! Imagine trying to track your object manually at that level, the highest magnification where I manually tracked and coped with it ok were times 420 mag, I doubt I will go higher than that often if indeed at all.

At the end of the day though, does it all matter. You can have an eyepiece case stuffed full of glass but I can guarantee most of you, me included always reach for a particular eyepiece when you know what the task in hand is. For me the Nikon12.5mm eyepiece is my galaxy hunting EP of choice. (X140 = X10 per inch aperture) without fail and my line filter eyepiece the APM 24. Those two alone cover probably around 70% of what I observe.  That said, every one has been used at least once in my last two sessions.

So for all the formula of what you need, filling gaps in the eyepiece armoury as EG "I have a space between the 8 and 12mm eyepieces" and so on, do we buy what we want, or what we really need?

Oh and I have said this before, I have no intention of selling any of my perceived excess eyepieces because as soon as I do, you can bet your life I would be reaching for it when on a session. 😂😂

 

[vlaiv]

Here is another rule to try out

Maximum magnification (except for double star observing, people seem to like seeing these disks and not just detecting separation) is one that equates angular resolution of your vision to airy disk radius.

Here is nice table:

https://en.wikipedia.org/wiki/Visual_acuity#Expression

If you have 20/20 vision - your resolution is 1 minute of arc (MAR column - minimum angle of resolution)

For 200mm telescope, airy disk radius is 0.64"

60 seconds of arc / 0.64" = ~x94

We can round that to x100. If you have 20/20 vision - max power that you need is x100 (maybe you'll want more magnified image, but you won't need it to see all that can be seen).

If you have vision that is 20/10 - then you only need x50 power, but if you have vision that is 20/40 then you need x200

How much power you need depends on how good your eyes are. People with worse visual acuity need more magnification (and often don't as much mind higher magnifications - to them x500 does not look as blurred as to someone with sharp vision).

 

[vlaiv]

Just to add - above is high power that is needed. Then there is low power that is usable - it is related to size of exit pupil and how much one's pupil dilates in dark.

Medium power is smack right between those two

 

[cloudsweeper]

No Steve, I don't think it really matters.  We could all say different things.  Low power is to me up to say x50; medium to x150; high to x240; very high is above that, which can be up in the x300s or more.

I have about 30 EPs, because I like to change in small steps.  Differences in sharpness and field are better seen that way.  So Yes, I feel I need this amount.  The ones I sell on are generally either very bulky or have very low eye relief.

As for max mag, I work on getting down to an exit pupil of about 0.4mm, which is more that 2 x aperture in mm.

Doug.

Edited September 28, 2022 by cloudsweeper

[NGC 1502]

Difficult call……

My main scope is a 10” Dob.  I’m a visual observer of solar system and deep sky.  With deep sky it seems to me that they are not as seeing dependent (steadiness of atmosphere) as the main planets.  For me 150x is often ok with Jupiter and Saturn, that needs an 8mm eyepiece. Globular clusters, smaller planetary nebulae and tight double stars are usually good at 240x, that needs a 5mm eyepiece.  For manually finding objects and viewing large ones I use a 27mm eyepiece for 44x and not too large an exit pupil.  44x to 150x is too large a jump, so I fill the gap with a 14mm for 86x.

So that’s 44x, 86x, 150x, 240x.  That’s what works for me, perhaps not for you!

[scarp15]

Selectively there are the connotations, the in-betweens, that are probably more prevalent in user terms; such as medium-low. As for example my 21mm ultra wide field, operates at 29x, 57x and 88x respectively in each of my varied focal length scopes.

Equally medium high such as using a 10mm ep at  60x, 120x and 184x.

It's good to have a broad range to hand though pushing the boundary at both high and low margins and as Doug has commented, I to like to incrementally chop and change in small steps.  

[Guest]

Here in the US we do have some places with night skies that are a dream to the rest of us.  In my part of the US..... Ohio,  well our skies suck.  On our best nights 300x is about it for most DSOs.  The moon can take tons of magnification but thats about it.  I am usually running around 200x most of the time. 

[Louis D]

Around here in Texas, we're usually well south of the jet stream, so our skies are often very stable (it also leads to prolonged droughts).  If you have the aperture, 100x to 200x is cruising and 350x to 400x is quite usable.  It's part of the reason big Dobs are popular in these parts because aperture is really what limits resolution, not the atmosphere.  Of course, these same dead calm conditions are perfect for mosquitoes for a few weeks during our long summers after the infrequent rainstorms (I was batting them away last night despite our last rain being 2+ weeks ago).  200 miles west of here, and they are no longer much of an issue because of how arid it gets that direction (away from the Gulf).

Having lived in New York state for 6 years near NYC, I totally sympathize with your weather predicament.  I couldn't realistically take up astronomy there at all because of it.  I simply looked at lunar eclipses and planetary conjunctions with binoculars while living there.  My hat's off to anyone in the UK trying to stargaze.

[Mr Spock]

They are just labels. I tend to think of (approximately) x20 - x50 as low power, x75 - x150 as medium power and x200+ as high power, all regardless of the scope I'm using. I base it on the target and target size rather than the scope.

As for maximum useable in excellent seeing conditions, from experience I'd say -
x2 per mm for apo refractors
x1.5 per mm for Newts / Maks
x1 per mm for SCTs
Most of this is to do with the scope's spacial contrast (MTF) and how it renders image definition.

As always some will have different opinions - I can only go on what I have seen 

[Alan White]

Hard to define here in the UK as it changes every night and day with the atmosphere, more than many others enjoy.

Probably why I struggle to settle on scope and eyepiece choices, well that’s my argument with the Finance Officer anyway.

My local condition often limit me to 150x or often less.

 

[bomberbaz]

10 minutes ago, Alan White said:

Hard to define here in the UK as it changes every night and day with the atmosphere, more than many others enjoy.

Probably why I struggle to settle on scope and eyepiece choices, well that’s my argument with the Finance Officer anyway.

My local condition often limit me to 150x or often less.

 

finance officer, 😂

my brother in law refers to his other half as the entertainments manager when I ask if they are coming out

[Alan White]

54 minutes ago, bomberbaz said:

finance officer, 😂

my brother in law refers to his other half as the entertainments manager when I ask if they are coming out

It has a double meaning in our household

Herself fills that role well, but it’s actually her real job as well

 

[Mr Spock]

Oddly, in my last job (college) I was a Finance Officer for 8 years

[Louis D]

3 hours ago, bomberbaz said:

my brother in law refers to his other half as the entertainments manager when I ask if they are coming out

When asked if I'm free on a certain day or evening, I always say I first need to check with my social events coordinator.  I just go where and when I'm told to go. 😊

[F15Rules]

Looking back at the OP's question "..and does it really matter?", my response would be an emphatic " No, it doesn't!"

Why do I say that?

Because I believe there are SO many variables affecting each and every one of us, that a simple bench type formula simply can't cope with all those variables. Consider a few of them:

- Atmospheric conditions, moisture content, Jetstream, humidity etc

- Local geography and micro climates, ie hilly, flat, prevailing winds, grass/vegetarian/soil conditions

- light pollution (nearby eg street lights and further away eg city/town/industrial light domes)

- scope..size/type/quality of optics, cooling time, quality of focuser etc

- mount ..size, type, quality, eq or altaz/push to (the highest theoretical magnification of any scope will only ever be reachable in a meaningful way with a driven mount)..at 400-500x mag on those rarest of nights, "nudging" a scope every few seconds won't allow prolonged observing for several minutes, during which you might just get those few moments of perfect seeing.

- eyepieces..optical quality, focal length, whether used singly or in pairs ie binoviewing, coatings, fov etc

- critically, you/me, the Observer.. our eyesight, our age (tolerance of cold or heat), our motivation on a given night, our alertness or fatigue levels, our dark adaptation, our pre-preparation (observing lists or casual "quick peep"), observing alone or with friend(s)..

The list is almost endless.

In my own case, I know for a fact my right eye (historically my observing eye) has deteriorated in the past 5 years, so I've had to train my left eye to be my cyclops eye. I also binoview more as it definitely helps using two eyes on some objects (not all).

My present site is Bortle 4, so quite good, but we have a microclimate which often means seeing doesn't settle til midnight or later, whereas in my previous Bortle 5 area the light dome was more obvious, but the flat landscape meant that I could get out earlier on clear nights and have decent seeing. And, since I was working then, that was important to me, as I had to often rise early to travel for work. Now, being retired, it's easier for me to have a late night from a commitment perspective, but I'm now 5 years older and struggle more with motivation for late sessions (especially in cold winter nights!)

So, no formulas for me. But I do still tend to think of low, medium and high powers, and, with my 5" Tak, that is usually Low, c 30-50x (31mm UWA), Medium 70 - 130x (9-14mm, eg Morpheus 9mm, Pentax 10.5mm, Zoom c12-15 mm setting) and High (7-3.5mm, eg Barlowed Pentax/Morpheus/Zoom, for c (140-300x).

Note: My Tak manual says try up to x500 on the best nights! - but in the UK c 300x or so would be the very best I could use where I live, even though I don't doubt the scope's optics are good enough to go higher- if I lived in Arizona, perhaps.

So, these days I just enjoy whatever I can get and am grateful for that😊😊

Dave

Edited September 29, 2022 by F15Rules
Additional text info

[Louis D]

3 hours ago, F15Rules said:

And, since I was working then, that was important to me, as I had to often rise early to travel for work. Now, being retired, it's easier for me to have a late night from a commitment perspective, but I'm now 5 years older and struggle more with motivation for late sessions (especially in cold winter nights!)

Right there with you even though I'm a few years from retirement.  Youth is wasted on the young.  They (whoever they are) keep telling us this or that will extend our lives X amount of years.  Do I get to choose which X number of years to tack on, or do I just get X more elderly years?  I think back to 30+ years ago and marvel at what I used to be able to do in a single day.  It makes me tired just thinking about it.  I used to think, "When I'm retired, I'll be able to do all these things I've never had time to do".  I just didn't count on being old and decrepit at that time.  Getting old sucks (pardon my language).

Edited September 29, 2022 by Louis D

[F15Rules]

I feel your pain, Louis!

We can grow into GOGs (Grumpy Old Guys) together!😂

Dave

[Stu]

I find it best to have a general idea but not too many hard and fast rules.

Exit pupil is one way of looking at things which can be useful. At the low end, max exit pupil should match your fully dilated pupil eg 6 to 7mm.

At the high end, a lot of people say 1mm as a max which is basically the same power as aperture of your scope eg x200 with a 200mm scope etc.

I see two exceptions to this rule. Even though a large scope may go to x400 or more whilst giving a 1mm exit pupil, the seeing conditions will potentially limit this.

For small apo scopes, there is benefit on going down to 0.5mm or even smaller eg x200 or even x300 with a 100mm scope for doubles, lunar and some planetary observing. If you stick to a 1mm exit pupil in these scopes you would miss out on a lot of the potential performance.

DSO observing often benefits from an approximately 2mm exit pupil, whilst filtered views of nebulae using UHC or OIII filters are optimal above say 4mm.

In terms of not having hard and fast rules, I just judge the sky conditions as I see them, and use whatever power they can cope with. There used to be some respected members on here who would scoff at anyone using more than x250, saying it was a waste of time. It was interesting to see their views change over the years to more of a ‘use as much as the sky will take’ and using powers of x300 or more.

On excellent nights I’ve seen benefit from using x300 on the Moon or Mars with a 4” apo scope, or x360 or even x400 with my 8” f8. On many poorer nights there is no benefit to use more than x150.

My other comment would be that many people comment on particular eyepiece focal lengths. I’ve not seen that on this thread so this may be an unnecessary comment but in my mind you can’t talk about the benefits of an eyepiece without knowing the scope type, aperture and focal ratio it’s going to be used in. Extreme case in point, I used to have an 8” f20 Mak and a 4” f5 refractor. One had a 4000mm focal length, the other a 500mm focal length. A 31mm Nagler in the refractor gave a 5 degree field at x16, in the Mak it gave a 0.63 degree field at x129, bordering on planetary potential. That’s why exit pupil or mag per inch are more useful to consider than just focal length of eyepiece.

I’ll shut up now 😉

[Stephenstargazer]

43 minutes ago, Louis D said:

I just didn't count on being old and decrepit at that time.  Getting old sucks (pardon my language).

Well you should consider the alternative of not getting old! That's no fun at all 😄

[Don Pensack]

I posted the rules as explained to me by reading back in the '60s above.

I don't usually pay attention to those rules, though they make sense from the standpoint of magnification and exit pupil.

 

What I typically use is:

<100x  low power

100-200x Medium power

200-300x High power

>300x Good luck with seeing.  Ultra high power.

In my scope, that is

low <8x/inch (exit pupil 5mm down to 3mm)

medium 8-16x/inch (exit pupil 3mm to 1.5mm)

high 16-24x/inch (exit pupil 1.5mm to 1mm)

uber high>24x/inch (I max out about 40x/inch) Exit pupil 1mm down to 0.6mm

What do I look at?:

low--large star clusters, large nebulae, multi-object fields.  Less than10% of use.

medium--almost everything except planets and planetaries and Moon,  Use about 70% of the time

high--moon, planets, planetaries, small star clusters, most globulars, double stars.  Use about 20% of the time

ultra high--small planetaries, Neptune, Uranus.  Use about 1% of the time.

 

It's obvious to me that I could be happy with magnifications from 100-300x and exit pupils of 3mm to 1mm in the 12.5".

 

I cannot apply the rules above to my 4", where I typically use exit pupils from 1mm down to 0.5mm.

Why so small?  I don't really like magnifications <100x on most objects.  The only problem is that makes most objects dim in the 4".

If I want really wide fields and super low powers, I'll use binoculars.