Skywatcher LER 2mm EP - Why?

[Stargazer33]

I was wondering if anyone can shed some light on a question I have been pondering since I bought my new scope?

When I bought my C8 the guy who sold it threw in a heck of a lot of stuff with it, including an anti-dew heater strip, 2 mirror diagonals and five eps (I was totally amazed when I opened the package, even though he had told me in a telcon that he was including them).

Anyway, the 32mm, 26mm, 25mm and 10mm I can understand and have been using - on the very odd occasion that we have had clear night skies here! What I cannot work out is when/where you would use a 2mm ep?!? In my scope with a 2000mm focal length that would give me 1000x magnification.

As most people on here seem to say that 200x - 300x is the max you could reasonably use in the UK, even with excellent seeing, I was wondering who would have a use for such an eyepiece? Do some scopes come with 400mm focal lengths where this eyepiece could be used?

It's a Skywatcher Long Eye Relief (LER) 2mm ep by the way.

It's not really that important, it was just that I set my scope up last night and found the ep in my ep box. Notice I said I set my scope up and not, I did some observing, as when I went out after half an hour ready to use my by now cooled down scope, guess what? Yep those damn clouds had rolled in again!

[Moonshane]

Televue do a 4-2mm zoom and many with short fracs do use these as I understand it. I have a 1840mm dob and use my 6-3mm Nagler zoom at 3mm infrequently but it's occasionally fun on the moon and useful on tight double stars in excellent seeing. 2mm would be pushing it a bit though!

[Stu]

Agreed. I have a 2.5 t6 nagler which I use for very high magnifications in shorter focal length refractors, 388mm and 690mm giving x155 and x276 respectively. It its fairly extreme and not useable very often, occasionally on mars for instance in very good seeing.

On my mak it gives x1600! Not much use there then, and same for the 2mm in your C8. It probably came as part of a set or something like that.

Stu

[Naemeth]

It depends on the FL of the scope, 2mm may be of some use in short focal lengths, it would give 200x in a 400mm focal length telescope, and gives 325x in my Heritage. Mind you, I'd only ever use that on the Moon! (I tried it, it wasn't bad but not great either, plus trying to keep the Moon in view was a nightmare on Manual tracking).

[Stargazer33]

Ah, okay, I didn't realise that scopes went down to those sorts of focal lengths! I may just keep it and have fun with it if I get a night with really good seeing. A night with any seeing at the moment would be nice!

At least my CG5 Advanced GT should be able to keep my object in the ep. I can't imagine manually tracking with 1000x mag!

Thanks guys, have some green.

[John]

Tele Vue do a 4mm-2mm zoom eyepiece so there is a market for these very short focal lengths.

[alan potts]

Thinking back years, but I haven't seen it for a while cheap scopes were sold on how magnification it could give. You used to see 60mm refractor of questionabe quality with a big label stating X400 magnification. I remember my first 3 inch came with a 4mm eyepiece and a barlow and yes I tried them together, I think it was X600 or something like. So one use for a 2mm eyepiece would be the people that don't know any better market.

The other is simply short F/L refractors when high magnification can be used. With my 115mm I can use at X266 which is a 3mm eyepiece, to be fair with my sky here it can be used a fair bit. However 2mm would push it up to just over 400, maybe only the moon and Venus and not a great deal of good. I will barlow to it next time out and see what it looks like.

Alan.

[John]

I had a 2.5mm T6 Nagler for a while (to complete the set !) but hardly ever used it, even with my shortest FL scope. I also find that eyepieces with an FL of 3mm or less show the floaters in my eye up much more - I think that's because the exit pupil I was getting was just too small for me.

[alan potts]

I had a 2.5mm T6 Nagler for a while (to complete the set !) but hardly ever used it, even with my shortest FL scope. I also find that eyepieces with an FL of 3mm or less show the floaters in my eye up much more - I think that's because the exit pupil I was getting was just too small for me.

,

John,

What does this mean, "show the floaters in my eye up much more".

I was thinking of getting a 3mm Radian to finish the set of Televue from 13mm-3mm, I don't think I would ever use 2.5mm and could always powermate the 5mm.

Alan.

[John]

They are tiny bits and bobs within the fluid in your eyeball. Some folk get them more than others. This Sky & Telescope article mentions them in the "High Power Effects" section:

http://www.skyandtel...html?page=3&c=y

They can be darned annoying when trying to spot fine Lunar or Planetary detail at high magnifications.

[Moonshane]

I think that floaters increase (or become more obvious anyway) with decreases in exit pupil too.

[Naemeth]

They are tiny bits and bobs within the fluid in your eyeball. Some folk get them more than others. This Sky & Telescope article mentions them in the "High Power Effects" section:

http://www.skyandtel...html?page=3&c=y

They can be darned annoying when trying to spot fine Lunar or Planetary detail at high magnifications.

Come to think of it, I've seen these floaters on the Moon at 325x with effectively a 2mm eyepiece, which is a 0.4mm exit pupil... Hmm, 2.5mm may be my absolute limit in this scope.

[Stu]

I struggle with them at higher power with my 106mm. x200 is roughly 0.5°. I must do a comparison at similar mag with the 200mm which gives an exit pupil around 1mm which in theory won't show them as much

Stu

[Stu]

I of course mean mm, not °. Doh!

Stu

[KaStern_Former_Member]

Hello Brian,

it may be that the SC owner did own a fast Telescope too.

So if you combine for example an f/4 telescope with a 2mm eyepiece

you get 0.5mm exit pupil. That is o.k. for double star splitting.

With an f/10 scope exit pupil gats way too small.

It would be only 0.2mm and so you get annoying floater effects.

And since stars do not appear as points but as a tiny spot,

surrounded by rings, in a telescope scope these diffraction effects

will be very apparent at extremely high magnification.

The airy disc will appear very large and extended objects

will appear softened and unsharp.

You can sell the 2mm eyepiece to someone with an f/4 or f/3.5 scope.

Cheers, Karsten

[great_bear]

Good eyepiece for star-testing your telescopes though...

[Naplesnut]

2mm very useful for daytime terrestrial viewing as far for mag possible with a scope than at nite. I have used the 2mm in my f5 80mm refractor giving 200x and even tried it with a barlow giving x400 but not so good at distant targets.

Also the floaters that u see in your eyes are cells left over from foetal development. Cant do anything about them except blink!!

[Stargazer33]

You have foetuses growing in your eyes! Sorry, couldn't resist!

BigMak, KaStern: The Skywatcher 2mm LER ep has an exit lens that is quite big (I would say at least 15mm - I'm at work and don't have it to hand). Does this make any difference to the exit pupil or would you still get a 0.5mm - 1mm image in a 15mm wide lens?

Good eyepiece for star-testing your telescopes though...

That is a really good idea Bear!

[KaStern_Former_Member]

Hello Brian,

for the exit pupil size it makes no difference if the lenses are big or small,

as long as they are big enough.

The lenses of the 2mm LER are big because the exit pupil is situated about 20mm away from the lens.

I do not think that the 2mm eyepiece is good for star-testing your f/10 telescope.

The resulting exit pupil is too small and the magnification too big.

Suiter proposes a magnification that is about aperture diameter in mm multiplied by 2.

So 400x will serve well for your C8.

Cheers, Karsten

[michael.h.f.wilkinson]

My XW 7mm (290x) is the shortest EP I tend to use in my SCT. On Mars I have used the 10mm with 2x barlow, but that is the furthest you can sensibly go. The 2mm would even be a stretch in my kids' mini-Dob. Having both a 25 and 26mm is also a bit odd. I wonder if the 26mm is a Celestron Plossl (black top barrel with orange letters) that came standard with the C8 at the time I bought mine. Those were made by Vixen and are very nice indeed. I only got rid of it when I got my Nagler 22T4.

[Naplesnut]

You have foetuses growing in your eyes! Sorry, couldn't resist!

BigMak, KaStern: The Skywatcher 2mm LER ep has an exit lens that is quite big (I would say at least 15mm - I'm at work and don't have it to hand). Does this make any difference to the exit pupil or would you still get a 0.5mm - 1mm image in a 15mm wide lens?

That is a really good idea Bear!

Good one there Diver. More like placentas though now I am older since that post 

[rwilkey]

Hi Bryan, the 2mm LER (LET is the correct original name) came with a set of four sold in a silver case, it was a smart set apart from the 2mm.  There are actually six in the complete set, supplied by Sky Watcher with nice twist-up eyecups, Sky Watcher also do one 2" version at 28mm, which they often supply with new telescopes.  I have to say, the 1.25" set is pretty good and very comfortable to use.  They retail at around £29 from Phil Burton Photography, and are a good set to choose if your budget does not extend to the BST Explorers/StarGuiders.  However, the 2mm is much too much for most telescopes.  My wife uses them on her 90mm frac and is very happy with them.

[Stargazer33]

Ahhhh! Zombie thread! 

Robin, I doubt if I will ever use this 2mm ep. If your wife would like a spare then she is welcome to it. I'm sure it is in my ep box somewhere!

[rwilkey]

Ahhhh! Zombie thread! 

Robin, I doubt if I will ever use this 2mm ep. If your wife would like a spare then she is welcome to it. I'm sure it is in my ep box somewhere!

Hi Bryan, no, the 2mm is absolutely no use at all, not sure why they made this one.

[Stargazer33]

Ah! I misunderstood, sorry. Perhaps I can use the barrel for something.