40mm eyepiece with 12" Flextube?

[pqloapq]

Hi guys

I am wondering whether a 40mm focal length eyepiece would work with my 12" flextube dob.

Or will i get the kidney bean effect?

Has anyone any opinion as to the longest focal length eyepiece that could be used with this scope.

I currently have a 32mm Panaview eyepiece.

[Moonshane]

hi there

kidney beaning is caused generally by incorrect eye placement; some eyepieces are more fussy than others about eye placement. the scope would not create it as such. in the case of longer focal length eyepieces, the problem is often than the eye relief is so long that you have to hold your eye away from the rubber guard to reach the good spot. too close or too far away and this may cause kidney beaning.

if your scope is an f5 scope then a 40mm eyepiece would create a large exit pupil of 8mm (40mm / f5 = 8mm). the average adult male over 40 has an exit pupil of 5mm or so and therefore some of the light gathered by the scope will be 'wasted' as it will hit the iris and not the retina. this effectively reduces aperture or at least light gathering capacity.

also, such a large exit pupil will mean that the sky will not be much darker than with the naked eye and may seem washed out compared with say your 32mm Panaview. This is even more the case when there's an element of light pollution at your observing site.

For me, I prefer to keep exit pupils to around 5mm maximum and therefore I'd consider even your Panaview too little magnification with an f5 scope; or at least it's at about the minimum I'd consider anway. I sold my 35mm 68 degree eyepiece to buy a 26mm 82 degree eyepiece for this reason.

The 40mm will certainly work in your scope but I'd consider other options than such a long focal length eyepiece unless you are looking to buy a slower scope in the future and the 40mm is a good deal. Others will disagree with this but that's my view

[NGC 1502]

Shane is spot on in post #2

Regards, Ed.

[acey]

As said, if the exit pupil is larger than the eye pupil then some light is "wasted" and the scope is effectively of smaller aperture: the 40mm EP with the 305mm flextube gives magnification 37.5 and exit pupil 8.1mm. If your own pupil were 5mm then the effective aperture would be 188mm, i.e. equivalent to a 7.4" f/8 scope (though with an unusually large central obstruction due to the secondary).

It could nevertheless be useful as a low-power finding eyepiece. Nineteenth-century visual astronomers routinely used such eyepieces on large observatory instruments: "wasted light" was not a problem in that situation; what they cared more about was having low power and wide field while finding.

I currently use a 32mm TeleVue plossl as finding eyepiece with my 305mm flextube: I haven't measured my eye pupil but given my age I suspect it will be less than the 6.5mm exit pupil. This eyepiece has the same true field as a 40mm plossl, which is the maximum possible for a 1.25" eyepiece (the limitation is the field stop). To get a wider true field I'd need to use a 2" eyepiece: a 55mm 2" plossl (largest possible true field) would yield magnification x27, exit pupil 11.2mm, and for a person with eye pupil 5mm would make a 12" f/4.9 flextube scope effectively a 136mm (5.4") f/11, again with a very much over-sized secondary. Not good for general observing but possibly useful for low-power finding.

[DOBBY]

Have you tried the 38mm panaview ?

It's big & boy is it heavy...but, it does show a wide field of view if thats what you're looking for

[acey]

Have you tried the 38mm panaview ?

38mm panaview in 12" flextube:

Magnification 39.5

Exit pupil 7.7mm

Effective aperture for 5mm eye pupil 198mm (7.8")

Effective aperture for 6mm eye pupil 238mm (9.4")

Effective aperture for 7mm eye pupil 277mm (10.9")

True field 1.8 degrees

32mm panaview in 12" flextube:

Magnification 46.9

Exit pupil 6.5mm

Effective aperture for 5mm eye pupil 234.6mm (9.24")

Effective aperture for 6mm eye pupil 281.5mm (11.1")

Effective aperture for 7mm eye pupil 305mm (12")

True field 1.5 degrees

You'd get a third of a degree more field size for a possible 16% loss of effective aperture. Hence I agree with Moonshane's advice, don't go for any longer focal length than you've already got - unless it's for use as a finding eyepiece.

[astroeddie]

My ten bobs worth.

I have a 12" dob.

The eye piece I use as my "finder", is the 30mm 2" Moonfish, giving me x50 mag and a true F.O.V.of 1.6*.

Working on figures above, exit pupil works out at @6mm. There is slight star distortion in the outer 15% or so of the f.o.v. My intension is to get a 24mm Hyperion for this job.

In my new frac the fov is a whopping 2*,affording a @3.75mm exit pupil. If my sums are correct.

[John]

I would agree at sticking at 32mm as the max focal length eyepiece for the reasons others have explained above. If you want to see more sky you could upgrade your Panaview 32mm to a UWAN / Nirvana 28mm or even a Nagler 31mm both of which deliver a very well corrected 82 degrees field.

[acey]

The Nagler 31mm would give an exit pupil of 6.3mm and true field of 1.7 degrees, almost the maximum possible with the 12" flextube, and about 28% more true field area than the 32mm panaview. But it costs about 500% more than the panaview.

A 28mm UWAN/Nirvana would have a true field of 1.53 degrees, compared to 1.49 degrees with the 32mm panaview, a field area increase of just 5% (and a price increase of 210%).

[SiriusB]

Once i did own a 12" reflector, lowest power gave an exit pupil about 5.5mm, the sky was pretty bright even in a dark rural village in the 1980's.

Sometimes i use a 7mm exit pupil in my small scopes, but to be honest 4mm & smaller seems to work best on dso most of the time.

My advice, stick with the 32mm.

[cs1cjc]

Hi,

I would differ from the posts here. My 38mm SWA has really helped me to locate DSOs. Not all the aperture of the scope is being used, the large exit pupil means that the image is as bright as possible.

[michael.h.f.wilkinson]

I use my 40mm TMB Paragon in my 80mm F/6 as finder, but I really want a 30 mm 82 deg FOV EP (Explore Scientific or Nagler 31T5) for better exit pupil at almost the same FOV.

What we clearly need is an Ethos 33mm and 2.5" focusers

:D