What is the point in a 2.5mm eyepiece?

[tomo1515]

As a beginner I'm trying to get to grips with all these eyepiece specs. I've read that the lower the 'mm' number the more powerful the eyepiece is. The usual sort is around 10mm-30mm, then what is the point of a 2.5mm. The 2.5mm gives a power of 480X on a 1200mm focal length scope and with a 2X barlow a power of 960X. If it becomes unusable around 350X what are the uses for it. I just can't see any.

http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-uwa-planetary-eyepieces.html

[adarshajoisa]

The name says it all. Some planets like Saturn and Mars need high magnifications. And to get a decent magnification on a fast scope, you need low fl EPs. My scope, for example is 130mm dia and 650 mm fl. So a 2.5 mm ep would give me 260x, which is below the theoretical max for my scope. Under really good seeing, I can use 260x.

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[John]

Some scopes have a short focal length but are still capable of high magnifications so you need a short focal length eyepiece to get there. Using a barlow lens with a longer focal length eyepiece is another way.

I've found that floaters in my eye become very distracting when I'm using an eyepiece with less than 3mm focal length. That varies person to person though.

[ghostdance]

Apart from any question of pushing scope maximum mag, the trick as adarshadoisa mentioned, is the seeing...

So for me under London skies, my 2.5 is pretty near useless. I can see stuff at lovely high mag, but about as sharp and detailed as a soggy Jacob's cracker. Still hoping it will snap into focus with the Moon tho, on an exceptional night. Optimist eh?

Edit\ And John is so right about floaters. Pesky things: my eyes need a flush-through

[Scosmico]

High mAgnification for double stars. Doubles take considerably more mg because they are a point of light. you can do more magnification per inch then normal.

[Ruud]

I have a 500mm f/5 refractor. On it a 2.5 mm eyepiece gives a 200x magnification, which is also the 'theoretical maximum' for the 4" diameter of the aperture.

[Charic]

.....having  the higher magnification does have its uses for me,  only if the seeing is good and the target is large enough, ie the Moon.

I  have only looked at Jupiter in detail, not having the chance to view the other Planets due to restrictions / conditions? but on the Moon my  3.2mm is  good. I haven't quite found any  foot prints  yet    but it does get you close,  just depends on where your looking? the detail in the terminator is really impressive.

Sticking  within  the parameters of your telescope makes sense here in the UK for magnification choices, depending on the specifications of your telescope.   My 200P - f/6   easily offers 200x  with a 6mm ( suggested limit 400X ) Going  shorter  with  the  focal length  is down to personal choice,  just  pushing  the limits, rather than being essential.  The   BST 3.2mm copes well  within the limitations of my Skyliner. 

[alan potts]

Some time ago I did a review using a variation on a TMB 2.5mm planetary eyepiece, whether it was a genuine one or not I do not know. I recall using it in a couple of my scopes and it was much better than i though on my 115mm F7 APO where if we were talking the Moon, you could use it many nights and as already stated double stars can call for plenty of power to split them sometimes.

Televue make a zoom which is highly regarded that goes to 2mm. There are a a good few short very high quality scopse that handle these powers, it's more often the case that our eyes can't for floater reasons.

[adarshajoisa]

Kinda off topic here, but many people mentioned floaters. I have the same problem with with floaters when I look at Jupiter with high mag. Are there any tricks to reduce the effect of floaters?

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[John]

I think the "trick" is to know at what focal length eyepiece you start to get floaters and to avoid it.

For me 3mm is the minimum. I see floaters at 4mm too but they are not distracting. At 3mm they are much more noticable, especially on an extended bright object. 2.5mm was "no go" for me so I did not hang on to my 2.5mm Nagler for very long

[Stu]

The important thing to understand is not the focal length of the scope or the magnification but the exit pupil that the eyepiece gives in your scope.

Floaters generally start to become an issue at around 0.5mm exit pupil. Below this I find them too intrusive. That said, in my smaller refractor scopes I generally find a 3.5mm to be a minimum although 3mm is occasionally useful.

Like every rule there are exceptions though, I used to have a Televue Genesis which is a 4" scope with a focal ratio of 500mm (see how I mixed my units there?!). A 2.5mm eyepiece in that scope is very useable at x200 and 0.5mm exit pupil. In my 60mm Tak it would give x142 and a 0.42mm exit pupil, max mag and useable for doubles but too much for most planets except Mars really.