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Longest useful eyepiece


David Levi

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Any chance of some of you more experienced astronomers clearing up a conundrum that has just occurred to me? I was reading the last thread in this eyepiece forum and there were several mentions of quite long focal length eyepieces i.e. 32mm and 40mm. According to the book Turn Left At Orion there is a 'Longest useful eyepiece' based on the focal ratio (f) of the telescope and the width of your eye's pupil (generally about 7mm). The formula being: longest useful eyepiece < 7 x f. This means that an f5 telescope has a longest useful eyepiece of 35mm. The book goes on to relate this formula to the minimum useful magnification as M =(approx) A/7 (aperture/7). For a telescope with a 150mm mirror this will then be 21.4x. They say that your eye's pupil gets smaller as you get older and so the situation is only going to get 'worse'.

To state this in another way: Is it worth having a 40mm focal length eyepiece with an f5 150mm telescope?

 

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Hello and welcome to the SGL.

I work using the same assumptions. I use the focal ratio of my telescope to provide me a practical high power eyepiece. f/6 gives me 6mm producing 200x power with an exit pupil of 1mm. 
I double this to give me an eyepiece which matches the so called sweetspot. An eyepiece that works otpitamally with the telescope.
I then multiply my focal ratio by the size of my eye pupil to get a ball park measurement for my longest / low powered eyepiece. This eauates to about a 30mm eyepiece, but looking at my signature, I have adopted the Sky-watcher Panaview 32mm.
This is a 2" EP with 70° afov.

For your f/5 a 5mm will hit the practical limit of 150x, but you can exceed this, and 5 times your pupil size, thereabouts ?

The conditions you mention don't get worse like the flick of a switch,  it takes time, but does change as we age. I average 4.9mm entry pupil, I'm under 60!! I could probably use a longer focal length or be happier with a shorter length, wider field

HTH.

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One of the issues is you start to see the secondary mirror in your view,  the increased brightness  of the larger exit pupil cant be utilized beyond your entry pupil and the  lower magnification,  the less detail  you`ll see,  just lots of space, like looking through binoculars through the objective end?
I understand there is no limit to a refractor ?

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Any eyepiece exit pupil larger than your own will lead to light being wasted, so unless you can guarantee you have 7mm it may be wise to stick to eyepieces of 32mm or less. If, for example, your eye is only capable of 6mm and the 150mm scope is giving 7mm, then the effective aperture of the scope will be reduced to around 125mm - the rest of the light is lost.

Unless you are in a dark sky location, a 7mm maximum exit pupil will be wasted anyway. Sky brightening will wash out any faint objects; this is because at higher magnifications light from the sky is spread out, making the background appear darker - stars, being points of light, remain the same.

I used to have a 250mm f4.7 Newt. I also have a 42mm LVW (72°) eyepiece. Not only did it show me less than the 22mm Nagler I had due to sky brightening, I could clearly see the secondary as a shadow which was very disruptive.

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In a word no.

If you have pristine skies it might be worth it but with any light pollution I'd stick to a 6mm exit pupil or less with an f5 scope. If you want wider field/lower magnification then buy and additional scope with a shorter focal length.

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