Choosing Eye Pieces handy formula.

[Sirius Starwatcher]

In my past research I came across a formula for choosing EP's Hope this will help in deciding what to buy.

Taking The focal length of your scope if you multiply by 3/4 this result will give you a High Powered EP.

Focal length multiplied by 1 1/4 will give a Medium High powered EP

Focal length multiplied by 2 will give a Medium Low powered EP

Focal length mutiplied by 3 will give a Low powered EP.

The chart below gives the example of a 120 mm F5.0 Startravel scope. 

 

[John]

Thats useful Garry

Knowing the true field of view that the eyepiece / scope combinations generate is also useful as is the exit pupil that the combination generates so you could add some further columns to your table.

The former can be found using the formula: True Field = Apparent Field of Eyepiece ÷ Magnification

and the latter: Exit Pupil = Focal Length of Eyepiece ÷ Focal Ratio of scope

[Charic]

2 hours ago, Sirius Starwatcher said:

Taking The focal length of your scope if you multiply by 3/4 this result will give you a High Powered EP.

 

I get 900 ?

Don't you mean ' Taking the focal ratio! ' 

 

[Sirius Starwatcher]

6 hours ago, Charic said:

I get 900 ?

Don't you mean ' Taking the focal ratio! ' 

 

Yep. Senior Moment !.

[Louis D]

To abstract this, what the equation is trying to do is achieve specific exit pupils by the following formula:

(Exit Pupil) * (Focal Ratio) = (Eyepiece Focal Length)

For exit pupil picks, I would probably go with the following:

Highest Power : 0.9

Mid-High Power : 1.6

Mid Power : 2.5

Mid-Low Power : 4.0

Lowest Power : 6.0

These would yield 4.5mm, 8mm, 12.5mm, 20mm, and 30mm for the aforementioned f/5 scope.  The list of eyepieces generated by the OP's original exit pupil choices is weighted far too heavily toward higher powers.  Seriously, a 15mm for lowest power on any scope?  It would have to be about an f/2.5 to achieve a 6mm exit pupil at 15mm.  I know there are a few f/3+ dobs out there, but they're very uncommon.

[ronin]

The only "rule" I use is for highish power pick one at the same value as the focal ratio, this also means that the Mag = the diameter.  Since it is not always possible then it really is just a case of as close as makes sense.

One other aspect, that is I know my preference, is if I like the eyepiece performance I tend to end up with the set, and the eyepieces in a set do not always follow our formulas. Planetary viewing will means more at shorter focal lengths, a 5mm may not deliver one night when a 6mm might.

[Ricochet]

1 hour ago, Louis D said:

To abstract this, what the equation is trying to do is achieve specific exit pupils by the following formula:

(Exit Pupil) * (Focal Ratio) = (Eyepiece Focal Length)

For exit pupil picks, I would probably go with the following:

Highest Power : 0.9

Mid-High Power : 1.6

Mid Power : 2.5

Mid-Low Power : 4.0

Lowest Power : 6.0

These would yield 4.5mm, 8mm, 12.5mm, 20mm, and 30mm for the aforementioned f/5 scope.  The list of eyepieces generated by the OP's original exit pupil choices is weighted far too heavily toward higher powers.  Seriously, a 15mm for lowest power on any scope?  It would have to be about an f/2.5 to achieve a 6mm exit pupil at 15mm.  I know there are a few f/3+ dobs out there, but they're very uncommon.

It looks to me like a copy of the advice in the sticky thread linked below with the low power switched from a 5mm exit pupil to a 3mm. I agree with you about it being weighted too much towards high powers, but I expect that is because I use a dob, while the guidelines were written with a smaller telescope in mind where you will be inclined to take a bit more magnification over a larger exit pupil. 

 

[Louis D]

2 hours ago, ronin said:

One other aspect, that is I know my preference, is if I like the eyepiece performance I tend to end up with the set, and the eyepieces in a set do not always follow our formulas. Planetary viewing will means more at shorter focal lengths, a 5mm may not deliver one night when a 6mm might.

That's why the TV Nagler zooms, SW 5-8mm zoom, and barlowed longer zooms (such as the Leica ASPH) are popular for planetary observing to eek out the maximum usable power in moments of good seeing.  Even run of the mill 8-24mm zooms do well in barlowed binoviewers on planets.

[Charic]

The data plate on a telescope is also very helpful, and you really don't need a calculator, the information is there for a reason?. Take my F-1200 A-200 f/6 Skywatcher Newtonian Reflector for example.

The '200' describes the aperture, the size of my telescope in mm, but this number also describes for me, a 'maximum'  power that this scope is  capable of producing (provided the conditions are right for visual observations under the night sky). For me therefore, the telescopes aperture in mm is a good marker of the telescopes power.

The '6' describes the focal ratio, the speed of the scope? derived from dividing the telescopes focal length by its aperture 1200/200=6 ( mine is middling to fast, whereas an f/4 is faster, f10 is slower?) Important for astrophotographers maybe, but advisers often say  "just ignore this number if your a visual only user".......Why?
This '6' describes  to me the focal length of the eyepiece required for this scope, in order to achieve the 200x power that the scope is capable of producing from its 200mm mirror (its actually plus side of 203mm). So from the outset, a 6mm can be regarded as my high power eyepiece, also providing me with a 1mm exit pupil, perfect for high magnification targets!

If I double the focal ratio, (guide lines again) I could use a 12mm eyepiece for medium power, with the scope now optically perfect for me? (sweetspot) 100x power, 2mm exit pupil, with whatever field of view I choose ( I favour 60° ) brand/type dependent.

I can now further extend the focal ratio either way by doubling or halving, in order to select further eyepieces, but I will reach the scopes visual limits (what it's capable of) pretty quickly. I have a 3.2mm eyepiece, almost purely by the fact that it completes the 'set' rather than being an essential eyepiece, although it does get used on the moon sometimes. There are also limits on a reflector scope when you use longer focal length eyepieces, to the extent that all you will see is the secondary mirror blocking your view? The limit I advise is eye-pupil size x focal ratio, or thereabouts! Therefore I should be using an eyepiece of about 30mm, I like my 32mm Panaview!

Like ronin ( and others ) I favour using  the  telescopes focal ratio as a guide to purchasing the first eyepiece for the scope in question, it might not produce the most used eyepiece in your collection, but one that is  most suitable without pushing the limits of magnification ( again, the seeing conditions will have more of a bearing on what you may see on a given night ). Also check that the eyepiece you buy is affordable in-case you become a collector? The TeleVue Delos for example  at +£330 new, becomes  quite an outlay, should you get a 'set'.

Whatever methods folk choose to select/advise and buy their own eyepieces,  even with good old 'experience' from trusted and meaningful folk, at the end of the day, any eyepiece, any brand, will always come down to the simple fact, its a personal thing, only you can tell, but you WILL find the eye-piece thats right for you..........eventually!

[JOC]

So for my F6 Dob.

Taking The focal ratio of your scope if you multiply by 3/4 this result will give you a High Powered EP. = 4.5mm (266x mag)

Focal ratio multiplied by 1 1/4 will give a Medium High powered EP = 7.5mm (160x mag)

Focal ratio multiplied by 2 will give a Medium Low powered EP = 12mm (100x mag)

Focal ratio mutiplied by 3 will give a Low powered EP. = 18mm (66.7x mag)

The other comments notwithstanding I think it is also a bit location dependant.  I seem to hit a 'sweet spot' at about 7-8mm (currently I'm searching for my 'Nirvana' EP in this size - must be wide angle (70" or wider), clear across the whole EP, 15mm+ eye relief - ideally 20mm, coated inside, no 'kidney beans', lets lot of light in and the blow to all this is cost effective for the small amount of time I get to use it).  I love my Baader Morpheus, but they don't come in a 7-8mm size and full price they are too costly.  So I'm in the second-hand market waiting for the right one to appear.  However, at 4.5mm I find things start to go blurry - I can just about get away with the moon, but I think 160-200x mag. is about the limit of what is useful from my garden.

[JOC]

My baby Vixen 80S has a different set of figures.  It is 80mm wide and 400mm focal length which puts it at F5.

Taking The focal ratio of your scope if you multiply by 3/4 this result will give you a High Powered EP. = 3.75mm (106x mag)

Focal ratio multiplied by 1 1/4 will give a Medium High powered EP = 6.25mm (64x mag)

Focal ratio multiplied by 2 will give a Medium Low powered EP = 10mm (40x mag)

Focal ratio mutiplied by 3 will give a Low powered EP. = 15mm (26.7x mag)

What I don't know is whether it is the magnification under UK skies that limits what is possible/useable, or if it is the telescope that limits what is possible/useable, i.e. I can get away with 100x magnification easily with the Dob, will I lose things at that point in the Vixen as that would be considered high magnification in the Vixen.  Or is losss of image down to atmospherics i.e. if I can see 100X magnification clearly in the Dob, will it also be clear with the Vxen in the UK - if so might a better quality 3-4mm EP be worth purchasing? 

What these two sets of figures do show is that I am unlikely to ever get the same magnification out of the Vixen as I do from my 8" Dob.  which is a shame.  Maybe this is why aperture fever sets in.  I wonder how many people ever do down-size and are happy with the results.

[Louis D]

57 minutes ago, JOC said:

What I don't know is whether it is the magnification under UK skies that limits what is possible/useable, or if it is the telescope that limits what is possible/useable, i.e. I can get away with 100x magnification easily with the Dob, will I lose things at that point in the Vixen as that would be considered high magnification in the Vixen.  Or is losss of image down to atmospherics i.e. if I can see 100X magnification clearly in the Dob, will it also be clear with the Vxen in the UK - if so might a better quality 3-4mm EP be worth purchasing? 

What these two sets of figures do show is that I am unlikely to ever get the same magnification out of the Vixen as I do from my 8" Dob.  which is a shame.  Maybe this is why aperture fever sets in.  I wonder how many people ever do down-size and are happy with the results.

The refractor (assuming it is at least ED glass) will take power better than an obstructed system.  If an achromat, not so much.  Still, about 130x is the useful maximum due to exit pupil issues on an 80mm objective.  An 8" dob is loafing at 100x.  Mine is easily capable of 200x on globular clusters to partially resolve them.  Texas has fairly steady skies most of the time, so that helps.

Where the 80mm ST refractor shines is at low powers for scanning star fields and observing large open clusters that the longer focal length dob just can't do.

[JOC]

31 minutes ago, Louis D said:

The refractor (assuming it is at least ED glass) will take power better than an obstructed system.  If an achromat, not so much.  Still, about 130x is the useful maximum due to exit pupil issues on an 80mm objective.  An 8" dob is loafing at 100x.  Mine is easily capable of 200x on globular clusters to partially resolve them.  Texas has fairly steady skies most of the time, so that helps.

Where the 80mm ST refractor shines is at low powers for scanning star fields and observing large open clusters that the longer focal length dob just can't do.

Hi Louis D - Yes it's an  Achromat - I've read that it is an additional lens that is needed to correct chromatic aberration - which I understand makes all refracted colour wavelengths line up and hit the focal plane at the same point so that colour slews aren't evident.   I have no idea if there is any disadvantage/advantage to a scope that needs such a correction. 

My 200P flex-tube seems ideal at the 160-200x mark - I just compared the 100x (ish) magnification to the Vixen as this seemed to be the overlap between one telescope's 'middle ground' and the others 'top end' and I found the comparison interesting - hence the question whether optics or atmosphere has the greatest impact on the maximum magnification that can be achieved.  I must admit I hadn't considered the advantage to a wider field of view with the smaller telescope (I only know that when I viewed Saturn in both that the 200P Dob had first place by miles) - maybe something like the veil becomes a better target in the smaller telescope?  However, the big telescope isn't so portable.  I am off to the New Forest for a few days with my son in a couple of weeks and the plan (if we have space) is to lob the small box with the Vixen and a few EP's into the car with my camera tripod.  We are camping so if it clears some night-time viewing might be on the cards.   I shall need to decide which EP's to take!

[Louis D]

1 hour ago, JOC said:

Hi Louis D - Yes it's an  Achromat - I've read that it is an additional lens that is needed to correct chromatic aberration - which I understand makes all refracted colour wavelengths line up and hit the focal plane at the same point so that colour slews aren't evident.   I have no idea if there is any disadvantage/advantage to a scope that needs such a correction. 

My 200P flex-tube seems ideal at the 160-200x mark - I just compared the 100x (ish) magnification to the Vixen as this seemed to be the overlap between one telescope's 'middle ground' and the others 'top end' and I found the comparison interesting - hence the question whether optics or atmosphere has the greatest impact on the maximum magnification that can be achieved.  I must admit I hadn't considered the advantage to a wider field of view with the smaller telescope (I only know that when I viewed Saturn in both that the 200P Dob had first place by miles) - maybe something like the veil becomes a better target in the smaller telescope?  However, the big telescope isn't so portable.  I am off to the New Forest for a few days with my son in a couple of weeks and the plan (if we have space) is to lob the small box with the Vixen and a few EP's into the car with my camera tripod.  We are camping so if it clears some night-time viewing might be on the cards.   I shall need to decide which EP's to take!

I have both an ST80 and a 72ED, and the difference is not subtle.  Everything in the ST80 has a contrast reducing haze around it while they are pinpoint sharp in the 72ED up to 100x.  The 72ED starts showing a little haziness beyond 100x, but it isn't bad.  It isn't an additional lens element that makes the difference, it's the use of low dispersion glass in the existing lens elements.  FPL-51 glass yields a very good ED scope while FPL-53 glass yields an APO nearly as good as using a flourite element.  Cost increases as you go from achromat -> FPL-51 -> FPL-53 -> Flourite, thus the increasing cost of ED and APO scopes.  Going from a doublet to a triplet can also improve color correction using less exotic glass or improve correction of more wavelengths using exotic glass.

If the reduced contrast and color fringing don't both you, then don't worry about upgrading to an ED or APO scope.  Personally, spending $2000 on a large dob rather than a 4" APO makes more sense to me if you want to see detail in planets and faint nebulae.  Just be aware that large dobs are back killers if you can't roll them everywhere or mount them in an observatory.

Definitely take the ST80 camping.  It's the only scope I plan to take to Nebraska for the eclipse this summer.  It will work perfectly fine for me with its solar filter to monitor the partial phases.  It will also work perfectly fine at a campsite without taking up too much room in the car for you.  I would recommend a 32mm plossl for scanning the Milky Way from a dark site at low power.  Once you see something interesting, drop in a 10mm to 16mm eyepiece to get a bit better image scale.  You could throw in a 5mm to 7mm for highest power viewing, but I can't guarantee the ST80 will yield a pleasing image at those powers.

[John]

3 hours ago, JOC said:

So for my F6 Dob.

Taking The focal ratio of your scope if you multiply by 3/4 this result will give you a High Powered EP. = 4.5mm (266x mag)

Focal ratio multiplied by 1 1/4 will give a Medium High powered EP = 7.5mm (160x mag)

Focal ratio multiplied by 2 will give a Medium Low powered EP = 12mm (100x mag)

Focal ratio mutiplied by 3 will give a Low powered EP. = 18mm (66.7x mag)

 

Personally I'd want more options between 160x and 266x - at least one more and preferrably two.

I've also always wanted something that gives the maximum true field while retaining a sensible exit pupil. With an F/6 a 32mm will be OK for this.

So just 3 more eyepieces / focal lengths then ?

[JOC]

4 hours ago, Louis D said:

I have both an ST80 and a 72ED, and the difference is not subtle.  Everything in the ST80 has a contrast reducing haze around it while they are pinpoint sharp in the 72ED up to 100x.  The 72ED starts showing a little haziness beyond 100x, but it isn't bad.  It isn't an additional lens element that makes the difference, it's the use of low dispersion glass in the existing lens elements.  FPL-51 glass yields a very good ED scope while FPL-53 glass yields an APO nearly as good as using a flourite element.  Cost increases as you go from achromat -> FPL-51 -> FPL-53 -> Flourite, thus the increasing cost of ED and APO scopes.  Going from a doublet to a triplet can also improve color correction using less exotic glass or improve correction of more wavelengths using exotic glass.

If the reduced contrast and color fringing don't both you, then don't worry about upgrading to an ED or APO scope.  Personally, spending $2000 on a large dob rather than a 4" APO makes more sense to me if you want to see detail in planets and faint nebulae.  Just be aware that large dobs are back killers if you can't roll them everywhere or mount them in an observatory.

Definitely take the ST80 camping.  It's the only scope I plan to take to Nebraska for the eclipse this summer.  It will work perfectly fine for me with its solar filter to monitor the partial phases.  It will also work perfectly fine at a campsite without taking up too much room in the car for you.  I would recommend a 32mm plossl for scanning the Milky Way from a dark site at low power.  Once you see something interesting, drop in a 10mm to 16mm eyepiece to get a bit better image scale.  You could throw in a 5mm to 7mm for highest power viewing, but I can't guarantee the ST80 will yield a pleasing image at those powers.

I have no idea what sort of glass this old 1970's Vixen 80S has in it.  However, I harbour a slight hope that it isn't too shabby.  The build quality of the entire telescope feels high so perhaps it contained the best glass available at that time (whatever that happened to be), but I don't suppose there is any way of knowing for certain.  I've seen adverts for these old telescopes which suggests that a Guide Book came with them, but I can't find one online which might have explained what they were made from.   However, your experience with these smaller telescopes is very interesting and the most I have been able to find out related to my one, and I shall take your advice about the EP's to take so thank you. 

[John]

The Vixen 80S is an achromatic refractor so the glass used with be crown and flint.

Here is a 10 year old thread on this scope:

 

[JOC]

Hi John, that thread does seem to be one all the search engines end up on.  There seems rather little info in internet land iutside that and the references therein.

Errrr....could you please excuse my ignorance and explain crown and flint glass to me in terms of what it is and, given its makeup, what properties that then confers on the telescope.  Sorry, the scientist in me likes the 'what's and why's' filled in.

[John]

An achromat objective lens uses two lenses of different glass types to bring the wave lengths of light to focus at as close to the same point as possible. Crown and Flint are types of glass that have slightly different refractive qualities. Used together they kind of compensate for each other. There is much more to this of course. This diagram might help:

 

[Stu]

I think the advice to not venture below 1mm exit pupil can be useful for larger dobs, particularly 250mm and above, but you will certainly be leaving a lot of mag on the table by doing this in a decent apo refractor.

This would mean sticking to x120 in an ED120 for example which makes no sense. You can easily use exit pupils down to 0.5mm in a decent apo refractor be it doublet or triplet. The main downside is the potential for floaters becoming overly visible but that is well worthwhile.

[Mark at Beaufort]

When I consider a new eyepiece I usually refer to this website - http://www.stargazing.net/naa/scopemath.htm

I then make a chart (see below) which is placed in my observing book. It shows the Scope, Eyepiece, Mag, FOV and Exit Pupil

I have not included my TeleVue 8-24mm zoom or the Baader 2.25X Barlow - the Zoom is mainly for the PST and when used both together they are for my Double Star work.

[Louis D]

15 hours ago, JOC said:

Hi John, that thread does seem to be one all the search engines end up on.  There seems rather little info in internet land iutside that and the references therein.

Errrr....could you please excuse my ignorance and explain crown and flint glass to me in terms of what it is and, given its makeup, what properties that then confers on the telescope.  Sorry, the scientist in me likes the 'what's and why's' filled in.

I found this Edmund webpage to be very informative about the various glass types.  As for using them to create achromat, ED, and APO objectives, here are several references that might help.

[JOC]

John and Louis D - quite fascinating thank you.  I hadn't even considered the qualities of the glass that is used to make lenses or its refractive qualities and how combining various lenses would affect it all.  There's certainly a lot more to making a functional telescope than I had ever considered.