TeleVue 55mm Advice

I am looking at the TV 55mm 2" Plossl eyepiece and I am wondering how that would match up with my 10" dob f/5 1250mm. I did some initial research and I have heard some people talk about the exit pupil being an issue and then some people say that EP works great. I really enjoy the TV Plossls. Just looking for some advice / comments on that EP. thanks

Justin

The exit pupil will be very large. This will mean that all the light will not fit into the pupil with resultant light loss. The big Plossl does perform well at night though. But daytime use is not good.

For a little bit more you could get a 27mm Panoptic, which is a wonderful ep and great match for your scope.

I tried the TV 55 Plossl in my F4.3 Starmaster. You could indeed see the shadow of the secondary with the monster exit pupil that ep gives a fast newtonian. Also, the TV 55 Plossl requires a good bit of back focus and I had to back the ep out of the focuser about an inch and could barely get it to focus at that. One last negative, even without seeing the secondary's shadow, I find exit pupils over about 6mm mean eye placement is very critical (blackout).

However, I think the TV55 Plossl is a great eyepiece. I love them with F10 SCTs and longer FR maks, etc. I had a C11 and found the 55 plossl to be a great DSO ep at F10 (I found it unusable with a F6.3 F/R due to exit pupil but that is a whole nother story).

Randy

Once you get used to a well corrected wide-field eyepiece a 50 degree AFOV is a bit disappointing particularly so with the additional drawback of an overly large exit pupil.

Dave Mitsky

Some basics to consider here.

1. F5 scope, 55mm eyepiece Exit pupil =11mm This is huge and essentially means that at best (7mm eye dilation) you will be cutting the effective aperture of your scope down to around 6 inches. If your eye dilates to something like 5mm then your effective aperture will be down around 4.5 inches.

2. 55mm eyepiece, F5 scope with a Central obstruction of 25% (typical for an Asian DOB.): The central obstruction has an Focal ratio of F20.

So the diameter of the hole in the center of the image will be 55mm/20= 2.75mm

This is large, definitely a problem during the day or viewing the moon but it might even be a problem in normal night time viewing. During the day your eye will only be open about 2mm meaning that the dark hole in the center will dominate the view. At night, if your pupil dilates to 5mm, then the effective central obstruction will be 55%

3. Large exit pupils show up the aberrations in the optics of your eye. Just like any lens, the human eye is best when only the center is used, poorest when the edge is included. A maximum exit pupil will bring out otherwise unnoticed (by your or your eye doc) astigmatism and make it seem that this eyepiece is a piece of junk...



4. These eyepieces are designed for use in F10 and longer scopes where the exit pupil will be within reason.
With an F5 scope you 30-35mm is probably about as long a focal length eyepiece as you should go. Fast scopes are nice because they are compact but they do require better eyepices to get the best views.

The trend towards fast scopes has been a big boone for TV because their eyepieces are some of the best corrected for use with F4 and F5 scopes. Uncle Al probably makes more money selling Naglers and Panoptics to DOB owners than anybody else makes selling the DOBs themselves...

jon isaacs