Widest FOV eyepiece for a C8?

[ryandsimmons]

Hi all,

I have my C8 on a CG5 goto which I am playing around with doing a mixture of observing and AP and was just wondering what eyepiece people would suggest to round out my kit?

I have a 20mm that I got with the c8/CG5 collection. a 9mm X-Cel which is fantastic and a X-Cel 2x barlow.

Just wondering what kit I need to add to that for the visual side of things?  I have LP and moon filters, but apart from that am not sure where my next upgrade should be?  I was thinking something for wide field views?  Maybe a 40mm?

But then reading around too large an eyepiece won't work well as the scope itself will limit the FOV?

Any suggestions what the largest usuable EP is?

[PeterCPC]

I used a Celestron 40mm with my CPC800 and the views were very good.

Peter

[YKSE]

I have two 2" wide field EPs,  Baader 31mm aspheric is my most used wide field EP because of its light weight (380gram), its 72 deg FOV shows 1.1 deg TFOV, when I need the max FOV in C8, the 40mm SW Aero can give me 1.36 deg TFOV, which is close to the max TFOV you can get from C8, 0.02 deg shy of a 42mm Vixen LVW. if I remember somewhat correct.

Both 31mm asperic and 40mm Aero work very in C8 and f7.5 80ED if it's of interest to know.

[John]

If you don't mind moving to 2" accessories, a 2" diagonal plus a Vixen NLVW 42mm will give about as wide a field of view as can be obtained. It's an expensive approach though (~£400 for both) and it's probably somwhat less expensive to buy an 8" F/5 newtonian for wide angle viewing. 

A 40mm eyepiece in the 1.25" fitting shows only a really tiny bit more sky than a 32mm in the same size due to the field stop restrictions of the 1.25" barrel diameter. I prefer the view through a 32mm as it's less constrained.

[LukeSkywatcher]

I have an 8se and all of my EP's are 1.25 format. I do have the 2 inch adapter which came with the scope (or did it come with my Hyperion Mk III 8-24mm zoom?). It would be pretty expensive to start buying 2 inch EP's to replace all of my EP's. 

I'm used to/enjoy the 50 degree FOV (apparent or otherwise) that all of my scopes and EP's offer. 

"A 40mm eyepiece in the 1.25" fitting shows only a really tiny bit more sky than a 32mm in the same size due to the field stop restrictions of the 1.25" barrel diameter. I prefer the view through a 32mm as it's less constrained".

I did once buy a second hand 40mm EP (Celestron E-Lux.........i think) in the 1.25 format, and used it in my other scopes (not the 8se) and i totally hated it. It was like looking down a very narrow cardboard tube. It didnt take me long to sell it on.

As John has said, the views with a 32mm in the 1.25" format are far more pleasing than the views of a 40mm EP in the 1.25" format.

My favourite EP ever is the 1.25" 32mm GSO. 

[acey]

Any suggestions what the largest usuable EP is?

The true field of view of any telescope depends on its focal length (FL) and the field stop diameter of the eyepiece (FS). The field stop in a plossl is the ring inside the bottom end that holds the lower lens in place - in wide-field eyepieces it's inside the lens and not directly accessible. The diameter of the field stop is limited by the diameter of the telescope focuser (and the eyepiece), i.e. any 2-inch eyepiece must have a field stop of less than 2 inches. The formula for FOV in degrees is

FOV = FS/FL x 57.3

(in case you're wondering, the 57.3 is to change from radians to degrees). I believe a C8 has focal length 2032mm, in which case the field of view is FS x 57.3/2032 for FS in mm. Any 2-inch eyepiece must have a field stop smaller than 50.8mm (i.e. 2 inches) so the maximum possible field of view is 50.8 x 57.3/2032 = 1.43 degrees. In practice I believe the largest possible field stop is actually 46mm, giving FOV = 1.02 degrees. For a 1.25" focuser the widest field stop is 27mm, giving widest possible FOV = 0.76 degrees.

The other limiting factor is the exit pupil EP, for which a formula is EP = FE / f where FE is the focal length of the eyepiece and f is the f-ratio of the telescope. You generally want the exit pupil to be no larger than the eye pupil diameter p, hence you want the eyepiece focal length FE to be no bigger than f x p. The C8 s f10 I believe, so the eyepiece focal length should be no bigger than 10 times the eye pupil diameter. For a 7mm eye pupil this would be a 70mm eyepiece, which I don't think anyone makes, so any focal length should be OK.

In that case, if you want the widest possible field, then all you're looking for is a 2" eyepiece with a 46mm field stop or a 1.25mm eyepiece with a 27mm field stop. The TeleVue 32mm plossl has a 27mm field stop, so it gives the widest possible true field for a 1.25mm eyepiece. That's one reason why it's so popular. I've had mine for years and still love it. The TV 40mm plossl also has field stop 27mm so it gives the same true field, but at a slightly higher magnification. I've never used one because the exit pupil would be too large for my dob, but it would be fine in your f10.

I don't know what the field stop size is in the GSO 32mm plossl mentioned in another post - maybe it comes out of the same factory as the TV and is the same size. Or maybe someone who owns one could get a ruler and measure it.

[ryandsimmons]

Many thanks for taking the time to write that up and explain!