2" sub 40mm?

[gooseholla]

Hi

I was wondering whether it is worth getting two inch eyepieces 40mm and below e.g. 42mm, 50mm. I am not concerned about the field of view, but the low power of them. My scope has 20, 26, 32 mm 2" eyepieces, but these give 45x - 75x magnification, and I hear for some things, you need much lower powers to see clearly. Any thoughts?

Thanks

John

[RichM63]

I would have thought that the exit pupil on a 50mm or even a 40mm in a 12" Dob would be way too large to be of any benefit at all.

Except to the EP vendor.

[haitch]

It's the wide field of view that lets you fit the object in the field so you look at it and don't find yourself looking through it. Too low a magnification can adversely affect contrast as you lighten the sky background. That's one reason (together with exit pupil) why wide angle EPs sub 30mm are more popular than longer f/l EPs for most scopes.

[Moonshane]

I thought this until I tried a 40mm Televue Widefield in my 12"f4 dob. Ok not like my 26 nagler but very nice.

[alan potts]

If your 12 inch is the 1600mm model a 40mm 68 degree FOV eyepiece like say the ExSc 40mm SWA is as low as I would go as the exit pupil is getting rather large. If your scope is this one at F 5.3 it will sort out cheaper eyepieces and not give clear and sharp edges where as the ExSc of Meade 40mm will. If however the scope is F4 it will render even these a bit on the soft side and the ExPupil will bigger than is really useable but it could still be done.

Myself i like lowish magnifcation wide views but i have a very dark site to view from, in some of my scopes the long 40mm type of eyepiece is a must. I have used it (41mm Panoptic) a few times in my 1meter M/N 190mm and yes you do get a very wide FOV but in real terms the near 8mm ExPupil is rather wasting the size of the scope as some of the light will not enter my eye, but I do it anyway purely because I have the eyepiece.

If you site is dark and you really want the widest FOV try one, do not go to 50mm Etc as these show the same amount of sky at lower magnification in a smaller FOV.

Alan.

[John]

I've used 40mm and even a 50mm eyepiece with an F/4.8 scope and they are good fun but when I switched to view the same object, say M31 or M42, though a 32mm I realised that the overlarge exit pupil that the longer focal length eyepieces create does mean that the full aperture of the scope is underutilised which reduces the contrast and extent of the deep sky object. You tend to get orange or grey background sky too unless you are observing under the darkest skies.

Having tried these longer FL eyepieces I'm now sticking to around 32mm even with my F/5.3 12" OO dob.

As with many of these debates though (ie: moon filters, ultra wide eyepieces, etc, etc,) personal preferences and experiences will vary so I'd suggest trying it for yourself and see how you find it

[gooseholla]

Thanks for the replies. Interesting stuff.

I believe it is f/5.3 but I am never sure! All I know is the mirrors are Orion Optics, whether the tube is or not I often debate! I am reasonably happy from a quick tape measure that it is around f/5.

40 mm will give me around 35 x magnification. I think it might be worth trying one out to see. I know my 32mm gives quite nice views, although nothing seems to match my 26mm Panaview in terms of view in my scope. I think I'll see if I can borrow a 40mm or so for a couple of sessions to see how well it works.

[RichM63]

But wouldn't a pair of 20x80 binoculars give an even wider TFOV, coupled to a more sensible Exit Pupil and magnification down to 20x than even a 50mm EP could deliver in the 12" Newt?

OP wanted very low mag + big skies, hence the thought on Binoculars, and the reason I bought mine, Star Fields are superb. (dark site).

[Reeny]

But wouldn't a pair of 20x80 binoculars give an even wider TFOV, coupled to a more sensible Exit Pupil and magnification down to 20x than even a 50mm EP could deliver in the 12" Newt?

OP wanted very low mag + big skies, hence the thought on Binoculars, and the reason I bought mine, Star Fields are superb. (dark site).

Agreed,

I have a 42mm 65 degree in the 8" dob at 28x mag, and the 15x 70 binoculars give a wider view.

The 42mm is good for the Orion Nebula, Andromeda, and as a finder.

But it won't fit all of the Pleiades in, and the exit pupil is 7mm.

A 40x 30mm would have been better for me.

[John]

The widest true field you can get with an 8" F/6 dob is around 2.2 degrees, unless you can convert it to use 3" eyepieces !

An average pair of 15x70 binoculars gives you around 4.4 degrees, 20x80's around 3.7 degrees.

[russ.will]

I bought the ES Maxvison 40mm and 34mm SWAs, as they just seemed too cheap to pass up for a ridiculous amount of glass.

I've tried both in my F4.9 Dob and by all accounts, the results were a lot better than I expected - No central obstruction black out and coma wasn't that bad, but the sky is washed out. The 28mm and the 24mm see a lot more use. The latter particularly because I'm lazy and can't be bothered faffing about with adaptors.

That said, drop one of these in the F6 80mm Apo and it makes for one extremely nice monocular for sweeping the Milky Way with. Horses for courses.

Russell

[Ruud]

Hi John,

If your scope is an f/5.3 and your pupil is 6.5 mm big, the longest focal length eyepiece you should use is 5.3 x 6.5 mm, which is about 34 mm.

The point is that the shadow of the secondary forms a dark circle at the centre of the exit pupil. When the exit pupil gets bigger, that dark circle gets bigger as well.

The brightness of the view reaches a maximum when the exit pupil matches your own pupil.  When the exit pupil gets larger still, light gets lost because  the outer rays get blocked by your iris, while at the same time, the shadow of the secondary begins to fill your pupil. 

The 34mm Maxvision is a great eyepiece, but at 804 g it is heavy. I got mine here for €119. 

[gooseholla]

Thanks for all the suggestions. Last night I pulled out my 32mm and found it hard to position my eye to avoid blacking out. I think I may leave a 40mm for now. I do like the suggestion of large binoculars, but then I'd need a large tripod etc. Probably easier to get my smaller scope out as 32mm gives 30x in that.

The suggestions of eyepieces though has been noted and will probably think in the future about trying some of them out.

John