Telescope people - What is the advantage of a 2" lens over a 1-1/4?

My new 10" Dobsonian uses 2" lenses, but comes with a 1-1/4 adapter to use the “cheaper” lenses. The scope itself came with Plossl 25MM and 9MM lenses, in the 1-1/4" flavor. They seem to be of good quality.

Since I am probably going to want to buy a 6MM and a barlow (2 or 3x) is there an advantage of ditching the 1-1/4" and going in favor of the 2"? There is an obvious price difference between the two classes.

Help an amateur!


The advantage of the 2" instead on the 1-1/4" is that the 2" will gather more light, allowing you to see better (I’m assuming this is for star gazing). Back of the envelope gives me the 2" collecting about 2.5 times the light of the smaller one.

I don’t have any experience with these, so I can’t give recommendations.


Are you talking about eyepieces or the main lens? I assume you mean eyepiece, correct?

The two inchers are more useful in the longer focal length, wide field eyepieces (25-55 mm and beyond). They let you capture more of the light cone when the eyepiece is further from the focus point of the mirror. I can’t think of a good reason to go with the larger 6mm eyepiece. Your already practically on top of the primary focus, so the light cone hasn’t diverged much.

I should have explained myself better, yes, the eyepiece. It’s also the only lens on a reflector.

No real advantage over the larger one?

Lens diameter is the sole variable governing how (angularly) close two points can be before the telescope can no longer resolve them, hence the bigger the better.

A minor nit - your telescope uses eyepieces, not “lenses”. The term “lens” by itself is confusing, as it can refer to both the lenses found in eyepieces and the objective lens of a refractor or a finderscope (which perform two entirely different functions). Call them eyepieces, so people don’t get confused.

The advantage of the larger diameter eyepieces is that in long focal lengths, the barrel diameter of the 1 1/4 inch eyepiece limits the size of the field of view. A 2 inch diameter eyepiece allows larger true fields of view in very-low-magnification eyepieces.

If I were you’ I’d go with the 1 1/4 inch Barlow (since your 25 mm eyepiece is a 1 1/4 inch design, and you’re happy with it, and since you don’t typically use Barlows on ultra-long focal length eyepieces anyway), and a 1 1/4 inch 6 mm Plossl. There are fancier eyepiece designs that offer huge apparent fields of view (Naglers and their kin), but they’re very pricey - for my money, a good Plossl is hard to beat in terms of quality of image per dollar spent.

Hope this helps!

SM, this is the eyepiece we’re talking about not the primary mirror or lens. Eyepieces are made larger so that they can capture more of the light focused by the primary. An eyepiece forms a focused image when it is located about one (eyepiece) focal length beyond the focus of the primary mirror. The longer the F.L. of the eyepiece, the further it must be from the mirrors focus. Since the primary’s light cone diverges beyond its focal length, the longer focal length eyepieces benefit from being a larger diameter. Unless a telescope has a really crappy optical path, short F.L. eyepieces, i.e, 6mm will not.

OK. Thanks for the input, people. I am indeed very aware that I should have called them eyepieces from the get-go, but had a brain-fart last night when I posted!

I am quite happy with my field of view with the 25MM Plossl, but who knows… in the future I may want a larger one. I do know though, I want some more magnification. I think a 6MM should fit the bill, then I can exercise the option of using a barlow or not.

The next question I have is this though:

Would I be better off buying a 2" Barlow and 1-1/4" 6MM eyepiece? Then, can I use the 2" to 1-1/4" adapter on the Barlow for the other 1-1/4" lenses? I am thinking that way, in the future if I purchase any wider field of view eyepieces that are 2", I can also use the Barlow on those?

Or, is using the adapter on back of the Barlow just a bad idea?

OK. Just want to verify this for one last time, as I am going to buy this within the next day or two… Would I be better off buying a 2" Barlow and 1-1/4" 6MM eyepiece? Then, can I use the 2" to 1-1/4" adapter on the Barlow for the other 1-1/4" lenses? I am thinking that way, in the future if I purchase any wider field of view eyepieces that are 2", I can also use the Barlow on those?

That is what I plan on doing unless I hear otherwise from folks.


Sorry for the late reply - I was away from my computer for most of the weekend.

Buying a 2" barlow certainly offers you the maximum posssible flexibility; just don’t skimp on the quality and buy a CHEAP 2" barlow rather than a good 1 1/4" one in an effort to save some cash. You want the best quality barlow you can afford.

As for buying the 6 mm eyepiece - you’re probably not going to use the 6mm plus the barlow very often (in addition to atmospheric limitations that usually prevent use of such high powers, you’ll find it’s really hard to keep your dob tracking the object at extremely high magnifications unless you use an equitorial platform). You might want to consider getting a lower-powered 1 1/4" eyepiece that when used with the barlow will give you an effective focal length of 6 mm - the lower-power eyepiece will have better eye relief than the 6 mm one will, and you’ll be effectively getting two eyepieces for the price of one. And if you buy a 3x barlow, using it with the 9 mm eyepiece you already have will give you almost the same magnification as the 6 mm eyepiece would provide with a 2x barlow. So you might want to skip the 6 mm altogether. Just a thought.

Amen on artemis’s point about tracking speed. 6mm with a Barlow will, IIRC, give you about 400x magnification on that model, which is terrific in really good seeing for planetary viewing, but your targets are going to be zooming across the field of view at quite a high rate of speed. It’s remarkable how apparent the rotation of the earth is at high magnification.

If I have my 15mm eyepeice with a Barlow in - 160x power - and step away from the telescope for one minute, I have to reaim the scope. And on a Dob, that’s a hassle if you’re trying to following something around every ten seconds.

Thanks for the replies.

I ended up ordering a good quality 2x Barlow (Antares 3 element, reviews seem positive) and a ultra-wide 6mm Plossl (I think it is a Super-Plossl)

I think a 4mm might be too much… we’ll see how the 6MM works out.


Oh, they’re all 1-1/4"

I’m dropping the 2" eyepiece idea until I need them.