3.5mm Nagler

[JamesF]

Saw one of these for sale recently and wondered when it might ever be useful, so I did some maths...

If we accept "twice the aperture in mm" as the maximum feasible magnification for a telescope then with this eyepiece the focal length divided by 3.5 must be no more than twice the aperture. In other words, the scope should be no faster than f/7.

If we accept that 250x is a realistic limit for magnification in the UK regardless of aperture then the 3.5mm eyepiece limits the focal length to at most 875mm (250 x 3.5).

So, to use this eyepiece one might expect to have an f/7 or slower scope of no more than 875mm focal length. There don't seem to be that many candidates. Sub 4" ED scopes such as the Synta-manufactured 80ED models and the Vixen 81 ED would fit the bill as would the FLT110, but you'd hardly be spoilt for choice.

Does this seem a fair line of reasoning, or am I making assumptions that aren't valid?

James

[Naemeth]

Saw one of these for sale recently and wondered when it might ever be useful, so I did some maths...

If we accept "twice the aperture in mm" as the maximum feasible magnification for a telescope then with this eyepiece the focal length divided by 3.5 must be no more than twice the aperture. In other words, the scope should be no faster than f/7.

If we accept that 250x is a realistic limit for magnification in the UK regardless of aperture then the 3.5mm eyepiece limits the focal length to at most 875mm (250 x 3.5).

So, to use this eyepiece one might expect to have an f/7 or slower scope of no more than 875mm focal length. There don't seem to be that many candidates. Sub 4" ED scopes such as the Synta-manufactured 80ED models and the Vixen 81 ED would fit the bill as would the FLT110, but you'd hardly be spoilt for choice.

Does this seem a fair line of reasoning, or am I making assumptions that aren't valid?

James

Sorry James, just have to correct you..

The scope should be no slower than F/7. It would be fine in my Heritage, with only 650mm it gives me 185.7x

[JamesF]

Sorry James, just have to correct you..

The scope should be no slower than F/7. It would be fine in my Heritage, with only 650mm it gives me 185.7x

Corrections are encouraged No slower than f/7 would open the field up a bit, certainly.

James

[Naemeth]

Corrections are encouraged No slower than f/7 would open the field up a bit, certainly.

James

I agree though, the selection of good quality scopes is quite small, considering a focal length of less than 850mm... I often feel that something like a 3-6mm and 2-4 Nagler Zoom is the best thing for scopes with small focal lengths, and they are light (which is a consideration for my scope) and able to get the best out of the conditions at any particular moment.

[RT65CB-SWL]

I agree with Brown Dwarf's comment about the 3-6mm Nagler zoom on small scopes. I have one and use it a lot with my TeleVue Ranger, (obj. 70mm f/l. 480mm - f6.8). Lovely eyepiece. Lovely views.

[Sirius Bizness]

I had this EP for my SW ED120 and it was amazing (x257). Gave some awesome views on particularly good seeing nights of Saturn, double stars, Orion trapezium stars and the moon. The whole double double fitted easily into the FOV which was nice. Only swaped it for the Nagler 3-6 zoom as the FOV wasn't as important to me with a tracking mount. I reccomend the zoom over this as its as good optically to my eyes but it's brilliant to be able to click through mag without faffing about with changing EPs to tease out the best detail.

[alan potts]

James,

I hope to get a 3.5mm while in England soon. This is mainly for my 805mm F/L F6.95 115mm refractor. I will also use it in the M/N 190mm which is 1000mm F/L but it's mainly for the former.

Alan

[Stu]

I think it is a fair line of reasoning James, but the are quite a few relevant scopes out there. I need to lose one of these but I currently have three which would be suitable.

Astrotech 106mm f6.5 = x197

Stellarvue 80mm f7 = x160

WO SD66mm f5.9 = x110

Put simply it gives a good, high power view at near the max for each of these scopes. Exit pupil is small so floaters are a bit of an issue but still very useful. I do have a 3-6 Nagler Zoom and often max out at around 3.5mm in each of the above scopes so have hankerings after a 3.5 and maybe 4.5mm Delos when funds allow.

Cheers

Stu

[Moonshane]

personally, I'd not buy such a short focal length unless I was a short frac fan. BUT I did buy a 6-3mm nagler zoom despite my scope having focal lengths of 1200mm (200-400x), 1600mm (267-533x) and 1840mm (306-612x) and I genuinely use it almost every session on any of my scopes that I happen to be using that night. mainly for lunar, doubles and sometimes planets depending on seeing. I got amazing views of Mars with it in my big dob last time round.

[John]

I've owned the 3.5mm and the 2.5mm Naglers. The 2.5mm showed too many floaters and was too much power for even my shortest FL scope so that was sold on quite quickly. The 3.5mm got a lot of use though - it produced an entirely practical 189x in my F/6.5 102mm ED Vixen and a top end but still useful 257x in my ED120 F/7.5.

I've since moved to a Pentax XW 3.5mm and that gets a fair amount of use too

[Tiki]

I currently own the 2.5 and 3.5mm Naglers. The 3.5mm gets tons of use as it gives a very useful 30' FOV at 150x in my TV101. The 2.5mm gets used much less; 216x with a 0.5mm exit pupil. I wouldn't like to part with either though.

[Stu]

Well I confess I had been watching that 3.5mm and did buy it in the end. I have had a 2.5mm in the past but found it a little too much in my scopes. As I mentioned, often 3.5mm with my nag zoom is a good maximum power so I thought I would try a dedicated ep.

If I get on with it, I will think about a Delos in future.

Stu

[John]

It's a very nice eyepiece Stu - congratulations

[Stu]

Thanks John, should be great in my 106mm in particular. I thought the price would go higher so quite happy with what I got it for - apparently new and unused for £164.

Stu

[John]

Thats a £100 saving over the new price Stu - can't be bad