Why are high powered plossl eyepieces best suited to long focal length telescopes?

[bendiddley]

I'm wanting to know why high powered plossl eyepieces are best suited to long focal length telescopes. Also if they are used in shorter focal length telescopes what limitations are there with this? Cheers.

[wulfrun]

I'm open to being corrected if my understanding is wrong or incomplete but I'd say it's largely down to eye relief.

The shorter the focal length of a Plossl, the shorter the eye relief. Say you have two scopes, a 1000mm and a 500mm F/L. If you want 100x magnification, you'll need a 10mm EP with the former but a 5mm with the latter. You'll need your eyeball on the lens of a 5mm Plossl in the 500mm scope.

If you use a 10mm EP in both scopes, you'll get only half the magnification with the 500mm one versus 1000mm but the eye relief would be the same.

[vlaiv]

I'm not sure first part is true.

Here are some things that you should consider and see if you meant any of that:

1. Simple eyepieces with few elements and narrower field of view work better in slower scopes (not necessarily longer focal length) - meaning slow F/ratio - like F/8 and above. This is because of angles involved. Fast optics produces shallow angles - so does wide field eyepiece designs. Only best eyepiece designs with many elements can deliver sharp to the edge wide field views in fast scope.

One of the reason to prefer simpler eyepieces like kellner, plossl, ortho and similar is because they are cheap and they work well in "long focal length" scopes (but not because of focal length - but because of slow F/ratio - it is easier to have slow long focal length scope - because short focal length scope that is slow - would just not appeal to anyone because of tiny aperture. Say you have 400mm F/10 telescope - it is only 40mm of aperture).

2. It is easier to use plossl design at longer focal lengths because of eye relief. Eye relief with designs like plossl and ortho is function of their focal lengths - shorter the focal length, shorter the eye relief. At around 10mm and below - these eyepieces start to be uncomfortable for use.

Say you want to observe planets with 1500mm FL scope - you can comfortably use 10-12mm eyepiece for ~ x120 magnification. However, if you try to use 600mm focal length scope to observe planets at x120 - you would need 5mm eyepiece. Plossl would be very hard to look thru at that focal length - you would almost need to press your eyeball against eye lens (something like 3mm of eye relief or something like that).

3. Some telescope designs extend eye relief. I must admin that I don't fully understand what happens here, but if telescope design has negative element in optical path - it extends eye relief of eyepiece used. This happens when you use barlow (it is negative optical element that diverges rays), but also with scopes like Maksutovs and SCTs - because their secondary mirror diverges rays (acts as amplifying mirror - similar to barlow).

This makes short focal length plossls more comfortable to use as it improves their short eye relief.

[wulfrun]

Hmm, I got half the story then @vlaiv. I chose to ignore the distortions aspect since I thought it wouldn't be the major factor.

[vlaiv]

Just now, wulfrun said:

Hmm, I got half the story then @vlaiv. I chose to ignore the distortions aspect since I thought it wouldn't be the major factor.

That really depends on observer. I did not pay attention to edge performance of my 32mm GSO plossl in F/5 scope until I read that plossl eyepieces are not quite good for that F/ratio.

After that, I payed attention, and sure - edge of the field shows little seagulls - but that really does not bother me when observing as I concentrate on the central section of the FOV. Some other people are probably much more bothered by this.

[Louis D]

6 hours ago, bendiddley said:

I'm wanting to know why high powered plossl eyepieces are best suited to long focal length telescopes. Also if they are used in shorter focal length telescopes what limitations are there with this? Cheers.

I'd like to know where you got that pearl of wisdom from since I've never heard it before.

[mikeDnight]

I really like a good plossl in a shorter focal length scope. For my kind of observing its on axis where it really counts, and the humble plossl can offer very sharp on axis performance, performing better than many more expensive complex designs.  

[Highburymark]

I also love Plossls. Though anything below 10mm I find difficult to use because of inadequate eye relief. The TeleVue plossl range is corrected to work well with fast scopes, even down to F/4-5. Contrast, sharpness and transmission up there with the very best eyepieces. But they have a slightly different design to more traditional Plossls.