Is my 2" Explore Scientific 9mm wasted in my scope?

[osbourne one-nil]

Playing around with some eyepiece formulae this evening, as you do, I noticed that the field stop of my 9mm 100degree 2"eyepiece was coming out as "N/A" put simply, would I be just as we'll off with a 1.25"eyepiece?

[John]

What scope do you have ?

My guess is that it would work very well with any scope.

[osbourne one-nil]

Sorry, it's a Televue 85.i just remember reading somewhere about wasted light in certain situations...but that it wasn't a big deal. Just trying to get my head around it!

[John]

I think the issue that you are referring to is when you use an eyepiece / scope combination that delivers a really large exit pupil. Something like a 40mm 2" eyepiece used with an F/5 scope for example. No danger of that with the TV 85 as it's an F/7 scope - you can use even a 40mm eyepiece without a problem.

I would have thought the views through your scope with the ES 9mm / 100 degree eyepiece would be rather lovely

[RT65CB-SWL]

+1 for John's second comment.

I think would be overkill in my TeleVue Ranger. (Mind you, if you want to donate it...

...damn! the Ranger is 1.25").

[John]

+1 for John's second comment.

I think would be overkill in my TeleVue Ranger. (Mind you, if you want to donate it...

...damn! the Ranger is 1.25").

The 13mm and shorter focal length eyepieces are actually 1.25" eyepieces with a 2" barrel fitted as the weight / mass of the eyepiece is such that using a 2" fitting focuser / diagonal is much more secure.

The 13mm Tele Vue Ethos would actually fit into the Ranger using the 1.25" drawtube. The effect can be slightly comical though as the photo below of my 13mm Ethos in the 1.25" diagonal and focuser of a Celestron 5" SCT shows. The views were pretty good though .....

[alan potts]

I am only going to agree with what has been said. The 9mm should be a fine eyepiece in almost any scope. The only thing is in one of mine it may be a bit short and give too much power in the 3048mm F/L of the LX. but in the others would be superb. These ExSc 100's are up there with the best by all accounts.

Alan

[osbourne one-nil]

Oh good - saves me selling it then! The views are fantastic, which is all that counts...I was just getting hung-up on my spreadsheet!

[RikM]

I think spreadsheets and calculators are useful, but what really counts are how the view looks to your own eyes. If you like it, then it's good.

[osbourne one-nil]

Absolutely, but I was being a bit mercenary too. You see, I have a Panoptic 35mm, 24mm, 15mm and a Delos 6mm. That could be considered in making the ES 9mm a luxury (although they each give approximately the same step-up in magnification), and knowing its resale value, I wondered....well, you know. I'd much prefer my clients to pay me rather than thinking it's optional!

[great_bear]

The 9mm sits in a perfect space in your line-up - there would be too much of a gap without it.

[RikM]

Personally, I would would swap the ES 9mm for a Delos 10mm. That would keep a very similar aFOV across your range of focal lengths. But then I prefer 70° to 100° eyepieces and like to have matching image circles when changing eyepices or if anything, larger aFOV at lower magnification. That's just me though!

[osbourne one-nil]

I have wondered that myself, as whenever I've used it, I've had to shift my eye around a lot to get the full field of view, which seems a bit daft to me. It doesn't matter....it just seems a bit of a gimmick, although I know many claim to be able to take it all in at once.

[John]

I find that I need to fold the rubber eyecup down when I use my ES 20 / 100 to see the full FoV. Not that looking at the edges of the field stop is what it's about with hyper-wide eyepieces

[Damo636]

I have wondered that myself, as whenever I've used it, I've had to shift my eye around a lot to get the full field of view, which seems a bit daft to me. It doesn't matter....it just seems a bit of a gimmick, although I know many claim to be able to take it all in at once.

I personally wouldn't regard 100° eyepieces as a gimmick. I think it all comes down to how we as individuals like to observe. I don't even attempt to take in the entire fov, as its a surefire recipe for eye fatigue! I'm aware of the field stop in my peripheral vision, but there's no way I can view it all at once. To me, the whole point of a 100° eyepiece is to create the illusion of a frameless window into space, and in that regard 100° afov eyepieces deliver and then some :-)

[Stu]

Totally agree with that Damo636

Stu

[michael.h.f.wilkinson]

some EP formulae bork at 100 deg, because they are wrong:

The theoretical field stop diameter D of an EP (assuming no distortion)

D = 2 x F x sin (AFOV/2),

with F the focal length, and AFOV the apparent FOV in degrees. plugging in the numbers gets 13.79mm

[Bart]

I personally wouldn't regard 100° eyepieces as a gimmick. I think it all comes down to how we as individuals like to observe. I don't even attempt to take in the entire fov, as its a surefire recipe for eye fatigue! I'm aware of the field stop in my peripheral vision, but there's no way I can view it all at once. To me, the whole point of a 100° eyepiece is to create the illusion of a frameless window into space, and in that regard 100° afov eyepieces deliver and then some :-)

What he said! Bang on Damo, that's it in a nutshell,......a big nutshell, of course!

Barry

[Stargazer_00]

The views are fantastic, which is all that counts

This is the most important thing. If you like it, regardless of all else, then that is all that is important. It's you that's looking at it, what else matters?

[great_bear]

some EP formulae bork at 100 deg, because they are wrong:

The theoretical field stop diameter D of an EP (assuming no distortion)

D = 2 x F x sin (AFOV/2),

with F the focal length, and AFOV the apparent FOV in degrees. plugging in the numbers gets 13.79mm

tan surely? Not sin

[michael.h.f.wilkinson]

tan surely? Not sin

That was what I thought, but plugging tan in gets results far removed from real figures. Take the Paragon 40mm with a 69 deg AFOV. 80 x tan 34.5 = 54.98 mm which is impossible in a 2" (50.8mm outer diameter) barrel, plugging in the sin function yields 45.3mm, very close to the measured 45.6mm. The same holds for a load of other EPs. The field stop for a Panoptic 24mm would be 32.4mm with the tan, but 26.8 according to the equation with the sin

[great_bear]

The tan equation assumes there's no lens in the eyepiece... D'oh!