TV 55mm Plossl questions

[Doc]

What are your views on the Televue 55mm Plossl.

In my scope at 400mm aperture and FL 1829mm

It gives the following:

Mag = x33

FOV = 1.5 degrees

Limiting mag = 16.5

Raliegh Limit = 0.33

Exit pupil Diameter = 12.01mm

I'm concerned about the Exit pupil of 12.01mm. Normal pupils have a

diameter of about 6mm is that correct so will the extra light be wasted.

The reason I'm asking this is I'm after a widefield eyepiece for my birthday in September, a long way off but I'm bored so making plans

[Ben Ritchie]

I think the exit pupil's going to be a problem Mick - 12mm is huge.

[John]

I'de be concerned about the exit pupil as well - I won't go above 7mm which means something in the 30-35mm range for my F/5. Al Nagler was not daft when he designed the 31mm Nagler T5 !.

John

[Ben Ritchie]

I try and keep to 6mm in my moderately light-polluted location, so something like the 28mm UWAN is ideal for my f/5 - more expensive than the TV Plossl, but not absurd used.

I've always rather assumed that the long focal length Plossls (the TV or Meade 50mm+ ones) are effectively SCT-only eyepieces due to the exit pupil limitations.

[Doc]

Thanks guy's thats what I was thinking as well but just wanted to double check.

John if I could afford a 31mm Nagler It would be the first thing I would buy.

[Ben Ritchie]

Mick, the 28UWAN is 90%+ of the performance of the 31T5, less than 50% of the price... i've seen them for £140 used and mine wasn't much more than that, a lot of eyepiece for the money...

[Doc]

Loooks good Ben

28mm UWAN

Results are:

Mag = x65.3

FOV = 1.26 degrees

Limiting mag = 16.5

Raliegh Limit = 0.33

Exit pupil Diameter = 6.13mm

Half the exit pupil value of the TV55 and only 0.24 of a degree less.

[Doc]

As my other eyepieces are Baader hyperions and I'm planning getting almost all of them how about this:

Hyperion Aspheric 31mm

Results are:

Mag = x59

FOV = 1.22 degrees

Limiting mag = 16.5

Raliegh Limit = 0.33

Exit pupil Diameter = 6.78 mm

[acey]

Even an exit pupil of 6 or 7mm is risky. If the exit pupil is too big you can have trouble holding the image - you look in and see nothing, then have to tilt your head until you get the right position, but still maybe see black areas. This has been my own experience using a 32mm plossl on an 80mm F5 scope (exit pupil 6.4mm) when my eyes haven't been fully dilated because the site wasn't dark enough. At a dark site I'm OK. So do take into account the degree of light pollution you normally encounter, as this reduces your pupil size below the normally quoted figure of 7mm. So does age. My father (in his late sixties) looked through the 32mm at a dark site and couldn't hold the image.

Best thing would be to try before you buy, or you could try measuring your pupil. Put yourself in a dark room with a mirror in front of you and a ruler beside your eye and take a flash photo.

Otherwise maybe better play safe and go for an exit pupil no larger than 6mm.

Andrew

[acey]

BTW for your scope that would mean looking at up to 28 or maybe 29mm, so the UWAN would seem a good choice.

[acey]

I just looked at the weight of it - a little over 2lbs. So if you go for it, make sure you've got counterbalance worked out.

[Doc]

Will do Andrew.

It won't be till September but just thinking ahead.

Thanks for your help.

[John]

...John if I could afford a 31mm Nagler It would be the first thing I would buy.

The 28mm UWAN would be a very good alternative - I owned a 16mm UWAN for a while and it was pretty much just as good as the Nagler equivilent.

John

[ollypenrice]

A lot of folks at the UK club I used to belong to preferred the 35 Panoptic to the 31 Nagler. I'm one of them. The big Nagler is incredible but it has one issue for me - blackout zones that move about as you shift your eye (as you have to) in roaming that fabulous field. They are not where you are looking but you are aware of them.

Of course others might disagree or not even get the effect.

Olly

[John]

A lot of folks at the UK club I used to belong to preferred the 35 Panoptic to the 31 Nagler. I'm one of them. The big Nagler is incredible but it has one issue for me - blackout zones that move about as you shift your eye (as you have to) in roaming that fabulous field. They are not where you are looking but you are aware of them.

Of course others might disagree or not even get the effect.

Olly

I've not seen that effect in my Nagler 31mm with either my F/6.5 refractor or my F/5 dobsonian. Some Naglers, particularly the earlier models were very prone to it though.

John