Eyepiece maths (but not that sort)

[keybaud]

I've been trying to work out what eyepieces to get and I have read a number of guides, but I'd be grateful if someone could explain why my idea is wrong. I have looked at 4 eyepieces that, with a 2x Barlow, give a range where each larger value in value is an increase of approximately 40%. The eyepiece range is then as follows:

7mm, 10mm, 28mm and 40mm. This gives an equivalent range of 3.5, 5, 7, 10, 14, 20, 28 and 40mm, which is an increase of 1.5, 2, 3, 4, 6, 8 and 12.

Yes, I did spend some time with an Excel spread sheet to arrive at these numbers...

[Langy]

I guess someone will ask what scope you have to know the magnification. You have some very low numbers there which gives the high magnification but have you started wide enough? After all the widest EP will usually go in the scope first before anything else.

[keybaud]

Other ranges with a similar effect are:

5.5, 8, 22 and 32

or

6.5, 9, 26 and 36

The recommended values from various guides for my MN190 were calculated as:

6, 10, 18 and 32

[keybaud]

I guess someone will ask what scope you have to know the magnification. You have some very low numbers there which gives the high magnification but have you started wide enough? After all the widest EP will usually go in the scope first before anything else.

I'd be surprised if 40mm wasn't wide enough. I thought it might even be too wide, as it gives a 7.6mm exit pupil on a MN190, hence my follow-on calculations.

[YKSE]

There're many ways to calculate the "optimal" eyepiece set, one of the most important factor is your scope's focal ratio, e.g. if your scope is a f15, the 7mm will most likely be useless, or if your scope is f4.5, the 40mm is not that useful.

[Naemeth]

I think the 6,10, 18 and 32mm are best.

Just so you know, a 40mm plossl won't show anymore sky than a 32mm plossl unless it's 2"

HTH

[Langy]

Apologies, I missed the 40mm 

[Reeny]

I assume you will be using a 2-inch Barlow to accommodate the 28mm and 40mm eyepieces.

The suitability will depend on the final magnification from your scope.

e.g. 3.5mm will give 200x on a 700mm focal length, and be just about perfect if you have enough aperture to get there (over 4 or 5 inch diameter)

Not sure what the spec is for an MN190

[keybaud]

MN190 is a 190mm 1000mm focal length Mak Newtonian. As the magnification is a function of the focal length of the eyepiece and of the telescope, it is irrelevant, as the ratio is constant for all telescopes.

As for the 6,10, 18 and 32mm, is there a reason why, with a 2x Barlow, I would want eyepieces that are 10 and 9, as well as 18 and 16, as these are only a 10% difference, instead of a set that gives me a 40% change for all values between 6 and 32?

[Langy]

I wouldn't buy the extras just for the sake of numbers. I'm still new to all this but if I was looking at this in a photographic perspective of buying prime lenses then I would want a wide angle, normal view and a reasonable telephoto for a minimum. Photography is like astronomy in that you would want more EP dependent on the use you have. For example in photography a macro lens or very long telephoto like a 500mm mirror.

So in my option as long as the widest and narrowest both work well then a range of 3 or 4 EP should most needs with the Barlows. With limiting the amount of EP to a small amount you can then spend more money on quality EP to suit your needs. Having 3 good quality EP will far outweigh 5 cheap EP.

[Naemeth]

MN190 is a 190mm 1000mm focal length Mak Newtonian. As the magnification is a function of the focal length of the eyepiece and of the telescope, it is irrelevant, as the ratio is constant for all telescopes.

As for the 6,10, 18 and 32mm, is there a reason why, with a 2x Barlow, I would want eyepieces that are 10 and 9, as well as 18 and 16, as these are only a 10% difference, instead of a set that gives me a 40% change for all values between 6 and 32?

It's unlikely you'll find it satisfying barlowing a 32mm/40mm eyepiece, because you'll end up with about 30mm of eye relief and thus have to move your eye significantly backwards in order to see the whole field.

Don't get too worried about the numbers

[keybaud]

Don't get too worried about the numbers

But I'm an engineer, I can't help it!

[keybaud]

It's unlikely you'll find it satisfying barlowing a 32mm/40mm eyepiece, because you'll end up with about 30mm of eye relief and thus have to move your eye significantly backwards in order to see the whole field.

I thought the Barlow doesn't change the eye relief, which is why you are better off using a Barlow to get a higher magnification, as it overcomes the issue of shoving your eyeball into the eyepiece glass.

I am new, so I am more than willing to be shoved in the right direction, so apologies if this comes across as me being awkward, I'm just an inquisitive git that likes to understand 'why', not just 'what'.

[ronin]

I find the arguement slightly irrelevant.

To get what you have determined you need fairly specific eyepiece focal lengths: 7mm, 10mm, 28mm and 40mm.

Looking round I think the only common 40mm is a plossl, a 7mm plossl is likely to be uncomfortable, actually a 40mm plossl could be as well as your eye would have to be a long way back from the eyepiece - about 25-28mm away.

If you get eyepieces from different sets then when you change eyepieces you will spend time altering the focus, which can be a problem when you go to a higher magnification.

To change both eyepiece and barlow takes time, half expect things to drift out if no motors to track. You would/could be making your life difficult.

[Naemeth]

I thought the Barlow doesn't change the eye relief, which is why you are better off using a Barlow to get a higher magnification, as it overcomes the issue of shoving your eyeball into the eyepiece glass.

I am new, so I am more than willing to be shoved in the right direction, so apologies if this comes across as me being awkward, I'm just an inquisitive git that likes to understand 'why', not just 'what'.

A Powermate / Tele-Centric doesn't extend eye relief, but a barlow does.

[Avocette]

But I'm an engineer, I can't help it!

If you like maths I trust you have read the posting by the Warthog in http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/ ? And then the boundary conditions need to be played with in your spreadsheet, notably the 32mm limiting case for a 1.25" focusing tube mentioned above and conversely the maximum useful magnification in reality due to typical 'seeing'.

[keybaud]

If you like maths I trust you have read the posting by the Warthog in http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/ ? And then the boundary conditions need to be played with in your spreadsheet, notably the 32mm limiting case for a 1.25" focusing tube mentioned above and conversely the maximum useful magnification in reality due to typical 'seeing'.

That article is where I got the 6,10, 18 and 32mm values from, but I still don't see why I'd want combinations that are close together and far apart in other places, such as 9 and 10mm, and 16 and 18mm.

The ratios I came up are based on the upper and lower limits suggested within guides and include calculations for minimum exit pupil diameter and seeing conditions.

[keybaud]

That article is where I got the 6,10, 18 and 32mm values from, but I still don't see why I'd want combinations that are close together and far apart in other places, such as 9 and 10mm, and 16 and 18mm.

The ratios I came up are based on the upper and lower limits suggested within guides and include calculations for minimum exit pupil diameter and seeing conditions.

I've just reread the article and the suggested eyepieces are without a Barlow, so do follow my logic; however, but they are approx. a 70% increase as you go up.

[Crebles]

Eyepiece magnifications in our 8inch and 12inch scopes.xlsx

I graphed eyepiece FL vs magnification once (I hope it'll attach here) and realised its exponential.

I found it quite helpful when considering what eyepieces to buy and the 'diminishing returns' of differences in eyepieces of Focal Lengths greater than about 20mm. Similarly the steep change in mag between say 6 and 7mm was interesting. I'd never seen this anywhere else, and wasn't 'obvious' to my little brain without the graph - so I'll share it here in case its of some use.

[keybaud]

I graphed eyepiece FL vs magnification once (I hope it'll attach here) and realised its exponential.

I found it quite helpful when considering what eyepieces to buy and the 'diminishing returns' of differences in eyepieces of Focal Lengths greater than about 20mm. Similarly the steep change in mag between say 6 and 7mm was interesting. I'd never seen this anywhere else, and wasn't 'obvious' to my little brain without the graph - so I'll share it here in case its of some use.

Thanks for the link. My calculations provided me with a list of eyepieces that had a linear increase in magnification, because of the exponential effect your graph clearly demonstrates. As I said, it turns out that the original suggestion was actually a linear progression, but adding a Barlow changes it. I need to look at which eyepieces work well with a Barlow and which don't and then come up with a revised list. In the mean time, I've bought a second hand Meade 8-24mm from Astrobuysell until I figure out what individual eyepieces I ought to get.

[Avocette]

I think the one thing that doesn't quite compute is the eye/brain's ability to zoom in on what you want to see within the field of view. So where a particular magnification might be theoretically optimum to just contain, say, a globular cluster, your eye wouldn't mind having a 40% wider FOV and do its own 'zooming' around this. I also have a reluctance to use a 2x Barlow, with its many extra lenses even if all multicoated, except where I want the highest magnification. So I would stick with the 32 and 18 used alone, and the 10 and the 6 used alone and also occasionally with the Barlow.

[Crebles]

Ahh, forgive me, now I see what you're aiming for. Good thinking. Love to hear how your plan goes - keep us posted.

[Crebles]

ps/ just one more thought before bed. I've been very disappointed with the quality of the view whenever I've used a Barlow - though to be fair I've only ever used relatively cheap and nasty ones - a stock Skywatcher one and a £30 Meade one.

So - if one of your 'key' or 'likely-to-be-used-most-often' magnifications comes from a 'Barlowed' EP - I suggest you invest in a really good Barlow, or you might find you achieve the mag range you're after, but the actual visual quality alternates = weakening your otherwise excellent plan.

Keep us posted.

[astronymonkey]

With eyepieces there is more to the thought process than just magnification as the field of view is probably the most important thing.

With wide field eyepieces you get the benefit of seeing a bigger chunk of sky fir a given magnification whereas with a narrow field eyepiece you would need two eyepieces to do the same..... one for the magnification and another for the wide field.

so rerun the calculations, but this time include the fov, and do two options 82 degree and 50degree, youll be suprised at the results and probably half the number of eps you need.

cheers

[Reeny]

I worked out my ideal eyepiece sizes on some scrap paper.

50x / 100x / 150x / 200x

That was the easy part.