what eyepiece?

[Chippy3476]

Hi all,
Yet another question for you all, I am trying to decide what lower power eyepiece to get for my 8" dob, at the moment my lowest is a 25mm which gives me 48 zoom. Do I really need lower power? I am tossing up between a 30mm or 40mm, 40 zoom or 30 zoom,
So in your opinions what would be better?
Thanks all
Dan

[SkyWatch]

A 40mm eyepiece will give you an exit pupil of 5mm, which is about as much light as you can get into your eye, unless you are under 50! The 30mm wont give much different to your existing 25mm, so if I were you I would look at a 40mm.
The other thing you might want to think about is the field of view of the eyepiece: plossl eyepieces generally have around 50 degree apparent field: so you would about 1.6 degrees true field with a 40mm plossl (about 3x the width of the moon). Some of the more expensive eyepieces have 68, 80, even 100 degrees apparent field of view, so you can get up to 2x the true field for the same magnification, depending on the eyepiece.
It all depends on what you want to look at, and how deep is your pocket!

- Dean

[Renato1]

It depends what size eyepiece you are talking about. If you are talking 1.25" eyepieces, the field of view from a 32mm standard plossl is around as big as you are going to get (which won't be much bigger than in a 25mm wide angle eyepiece). If you use a 40mm eyepiece you see the same field as you would have seen in the 32mm, you have a brighter image with a bigger exit pupil, but it feels like you are looking through a tunnel to see the image.

If you are talking 2" eyepieces, then a wide angle 40mm eyepiece will show you the biggest field, which will be noticeably bigger than that of a 32mm wide angle eyepiece, and around the same as from a standard angle 50 or 55mm eyepiece.
Regards,
Renato

[julianh72]

Yes, if you are shopping for "affordable" 1.25" eyepieces, 32 mm Plossl is about as long as it is worth going. E.g. take a look at BinTel's range of their own-brand Plossls:

http://www.bintel.com.au/Eyepieces-and-Barlows/Bintel-/Plossl/116/catmenu.aspx

All of them have an Apparent Field of View of 50 degrees, EXCEPT for the 40 mm, which has an apparent Field of View of only 43 degrees, which gives it pretty much the same ACTUAL Field of View as the 32 mm, but you get the impression of looking through a cardboard tube because of the narrower Apparent Field of View.

I presume this is a fundamental limitation of the Plossl design, because you seem to see a similar reduction in Apparent Field of View from other manufactures of 1.25" Plossl eyepieces as well.

You you can get a wider Apparent Field of View if you go up to 2" eyepieces, and you don't have to spend a lot more money; e.g. the BinTel "SuperView" range:

http://www.bintel.com.au/Eyepieces-and-Barlows/Bintel-/Superview/117/catmenu.aspx

[SkyWatch]

Yeah, the limiting factor for true field is the diameter of the field stop (ie: the diameter of the maximum "cone" of light that can make it through the end of the eyepiece without being cut off by the field stop). For a 32mm diameter eyepiece, this is around 27mm.
This is why a 32mm FL plossl gives the biggest possible true field for a 32mm diameter eyepiece, and why a 40mm plossl (or longer) at 32mm diameter has a reduced apparent field. If it had a full 50 degree field there would be vignetting, or light cut-off from the field stop.
Al Nagler has written an interesting article about all this. See: http://www.skyandtelescope.com/astronomy-equipment/an-eyepiece-primer/

- Dean

[Chippy3476]

So a 40mm would be pretty useless with an 8" dob by the sounds of it?
So how would I go about getting a smaller magnification? I assume I will need less than 48* for lunar observing? Apart from that would there be any need to get a smaller magnification at all?
Thanks
Dan

[ausastronomer]

Hi Danial,

A 40mm focal length eyepiece with a 2" barrell will work fine. It will give you an exit pupil of 6.67mm in your 8"/f6 dob which is ideal for your lowest power eyepiece under your rural skies.

A 40mm eyepiece with a 1.25" barrel IMO is completely useless and manufacturers should not even be allowed to make them. Because of the restriction on the field stop aperture, they have a maximum Apparent Field of View (AFOV) limited to about 42 degrees. By comparison, a 40mm eyepiece having a 2" barrel can have an AFOV up to 70 degrees.

Cheers
John B

[ausastronomer]

Danial,

A couple of posters have confused you a little.

A 40mm eyepiece with a 2" barrel is ideal for your needs in an 8"/f6 newtonian under rural skies.

This is one to consider

http://www.ozscopes.com.au/celestron-40mm-e-lux-series-2-plossl-eyepiece.html

If you are happy to spend a bit more money then the quality improves significantly. However, that eyepiece will be good enough to keep you happy for a while and its cheap

Cheers
John B

[SkyWatch]

Woops: sorry about the 5mm exit pupil I said earlier: I divided 200 by 40 instead of 30... :screwy:

But I agree if you have a 2" focuser it is worth looking at a 40mm eyepiece for wide-field views.

The GSO Superview 2" 42mm is a nice eyepiece for the money too: as is their SP 65 series 40mm. (Check out Andrews Communications)

[ausastronomer]

Dean,

I am not quite sure why you're dividing 200 by anything to work out the exit pupil. The easiest way to calculate the exit pupil is to divide the focal length of the eyepiece by the F-ratio of the telescope. In this case we have a 40mm eyepiece and an 8"/f6 newtonian.

Exit pupil = 40/6 = 6.67mm exit pupil. No 200's anywhere in there.

Cheers
John B

[N1]

Geez, and I thought the view through my 40mm Plossl was actually pretty good, especially when using filters on extended DSOs under dark skies. Oh well, better chuck it I suppose, along with all the Orthos. :lol::screwy:

[SkyWatch]

Yeah you are right, I know the aperture cancels out, but for some reason I was thinking magnification in my head. Aperture/magnification in this case is 200/30=6.67...
Put it down to a long day. ;)

[Renato1]

Not useless, but not as satisfying as a 32mm eyepiece would be for general usage - that is giving the widest field of view to observe deep sky objects that just fit inside that field.

But if you were using a H-beta filter to try see the Horsehead nebula in your telescope, you might see it with the 40mm, but won't with the 32mm.

I'm not sure why you want such low power for lunar observing. Even a 15 mm standard plossl eyepiece will have all of the moon sitting totally inside the field of view. Typically one would then use higher power to look at lunar features closer.
Regards,
Renato

[ausastronomer]

It does absolutely nothing that cannot be done at least as well and in most cases a whole lot better by a 40mm eyepiece in a 2" barrel.

One thing I can tell you is this. On the basis that the only telescope Danial owns is an 8"/f6 dob on an undriven mount, without setting circles of any kind and that he doesn't have any other eyepieces having a focal length > 25mm; the last eyepiece he needs is a 40mm 1.25" plossl. It does very little he can't do with a 32mm plossl. On the other hand he will find a 2" 40mm eyepiece quite useful as a low power finder eyepiece and for widefield views of those extended objects you are getting a restricted field view of.

I own a full set of high grade orthos btw but I don't use them for low power views of extended objects. I do use them for high power views of galaxies and other extended objects.

Cheers
John B

[N1]

John, that is the least anyone might expect. Otherwise why have 2" eyepieces in the first place, eh? Incidentally, a 1.25 will cost less, weigh less, won't require adaptor fiddling when changing eyepieces and use the same filter size as the other EPs.

I fail to see how something so non-essential as the AFOV at low power (yes looks pretty but that's not the same) can determine whether a given focal length is or

I got my 1.25" 40mm specifically to get a large exit pupil and low power when it matters.



I don't disagree with you there. I just think the extra AFOV is the only advantage of the 2" over the 1.25". In every other aspect, it's less convenient. So that's what the OP must decide I suppose. I maintain that the 1.25" has its place, even on an undriven f/6 dob. It's not that hard to track an object by hand at 30x, even at 43° AFOV.

I also got the impression from your earlier post that a wide AFOV was essential for enjoyable observing. To me, it's not. Same as telescope aperture. Nice to have (yes, I have some wide angle EPs too) but that's about it. It's all down to target selection and personal taste.

Finally, I agree that the 32 Plossl might actually be a good alternative :thumbsup:

Cheers

[ZeroID]

My GSO Super View 25mm (1.25") has the same feild of view as my 25mm Plossl. And for the price is a good eyepiece.
I also have a 32mm Nagler in 2" and a 30mm 2" plossl. They are great for big widefield immersive views, almost 'goldfish bowl' like but only work in the 10" f5 newt with any real effect.