Increase in Eyepiece diameters??

[Zubenel (Wes)]

Hi , May I ask a question?? With the rise of big Dobs and increase in diameter of primary mirrors has there been an increase in eye piece diameters?? As far as I can see ( pun intended) the answer is no. In last months Sky and Tel a 55mm eye piece was used in the 100'' Hooker Telescope on Mt Wilson giving 5.5arc minutes!! I know that apparent FOV's have increased dramatically in recent years but would a 100mm eyepiece need a 4" focuser as I love a wide FOV?

[Tropo-Bob (Bob)]

I have heard of 3inch EPs being used in observatories. Fact is though, such telescopes are intended for photography/imaging; not direct observing.

In my younger days, many Oz amateurs used Unitron .965inch EPs that only had a field of view of aprox 37 degrees. Imagine only having those with no Telrads or Goto.

[Wavytone]

Rough rule of thumb is the eyepiece barrel diameter needs to somewhat larger bigger than the eyepiece focal length otherwise the apparent angular field of view is going to be quite limited. In typical modern eyepeiece designs this would give a modest 40-50 degree field of view which isn't anything soecial.

You could build a 100mm eyepiece with a 1.25" barrel but it would be light looking through a toilet roll - with a field of view may be 10-15 degrees.

Big telescopes have big focal lengths. Even with big eyepieces these scope are high-power only. For example a 1 metre f/15 cassegrain has a 15,000 mm focal length; putting a 100mm eyepiece behind that gives 150X.

[AEAJR (Ed)]

Originally Posted by Zubenel

Hi , May I ask a question?? With the rise of big Dobs and increase in diameter of primary mirrors has there been an increase in eye piece diameters?? As far as I can see ( pun intended) the answer is no. In last months Sky and Tel a 55mm eye piece was used in the 100'' Hooker Telescope on Mt Wilson giving 5.5arc minutes!! I know that apparent FOV's have increased dramatically in recent years but would a 100mm eyepiece need a 4" focuser as I love a wide FOV?

I am not an optics expert so my comments may not be appropriate but I like to enter these kinds of discussions and see what comes of it.

The benefit of a larger barrel on the eyepiece is to enable a wider field of view at shorter focal lengths. This has little to do with the aperture of the scope as far as I know.

Most scopes today accept 1.25" eyepieces and there are variants that go to 82 degree AFOV and wider, but they are typically at the shorter focal length/higher mag end of the spectrum.

2" eyepieces allow wider FOV at longer focal lengths and are generally used for low power wide field use. Some have 100 or higher AFOV.

I don't think the use of bigger mirrors in hobby astronomy world really necessitates more than a 2" eyepiece unless you were going to very very long focal lengths.

As noted above there are larger eyepieces that are used at observatories but even a 25" mirror F5 scope would likely not need larger than a 2" focuser.

The 25" Obsessions, for example, use 2" focusers
http://www.obsessiontelescopes.com/a...sers/index.php

[OzEclipse (Joe Cali)]

The limiting factor becomes the exit pupil not the eyepiece diameter or field of view.

The eye pupil entrance diameter of a young person can only dilate to about 7mm. This reduces with age. Older people tend to be the ones who can afford the giant telescopes.

Exit pupil(mm) = Mirror diameter(mm) ÷ magnification

or

Exit pupil(mm) = Eyepiece focal length(mm) ÷ f number

Regardless how big the diameter of a long FL eyepiece is the exit pupil becomes too big.

The 55mm eyepiece used in an f4 dob (of any mirror diameter) will have an exit pupil of 14mm. Your eye will only be able to accept 1/4 of the light exiting the telescope. Alex has previously made the point in the past that he likes being able to scan his eye around in the eyepiece of a scope with oversized exit pupil. His comments related to short FL refractors. I'm sure that even Alex wouldn't suggest using a 20-30 inch scope to scan the heavens.

Let's make the example specific
A 25inch f4 dob with a 55mm eyepiece will have a 2500mm FL 14mm exit pupil 46 power and about a 1 deg field of view assuming 50 deg eyepiece FOV.

But your eye will only be able to accept 25% of the light which is coming from a 12 inch diameter part of the primary aperture and any one time.

A 12inch f4 dob with a 28mm 82 degree eyepiece will have a 7mm exit pupil, 45x magnification, a nearly 2 degree field of view and a fully dilated eye can accept all the light.

Large telescopes excel at many things. Big wide views are not one of them. Smaller scopes are best for this.


Joe

[mental4astro (Alexander)]

Eyepieces with barrels larger than 2" are certainly available, but there are only a couple of manufacturers. Ziess makes some, but finding info is like pulling hen's teeth. Siebert Optics is better known for large barrel eyepieces, all the way to 4.3" suckers. Explore Scientific makes one 3" eyepiece, but this is really just a one-of curio, and can't really say they are a dedicated large barrel EP maker.

As mentioned earlier, there are a few factors that affect the barrel size for scopes. The first one is cost. You won't see a 2" focuser in a department store cheapie. And then there is a difference in the quality of 2" focusers too. Then there is physical property of the cone of light that is typically produced is sufficient for 2" eyepieces, and to make longer focal length eyepieces with a larger barrel usually means an excessively large exit pupil with fast scopes, or introduces vignetting in most SCT's, Maks. With larger scopes (anything over 10"), it's a waste of time as these are not intended to be "sky scanners" as Joe mentioned. An 8" f/4 scope can give upto a 3° TFOV (I love mine! ), which is as large an aperture RFT sky scanner as practical, maybe include a 10" f/4 too. I certainly don't use my 17.5" for the same sky-scanner experience. There really is nothing significant to gain from a 4" 30mm eyepiece for a big dob.

But why not more larger barrel EPs, like 3", as there are now scopes being offered with 3" focusers? Mainly because there is not enough of an economic or visual gain to make 3" eyepieces for scopes that are primarily photographic instruments.

Large barrel eyepieces are intended for more specialised observatory instruments. Here again you are getting into the realm of primarily photographic instruments with a casual visual spin-off. You'll notice that all the instruments in the Siebert site that use their large eyepieces are firstly photographic instruments. And even with the longest fl EPs, the native fl of these scopes is several meters, so the TFOV will be very small.

The Mt Wilson telescopes have recently been decommissioned from professional work, and have been converted to dedicated visual instruments for the general public. You can hire the 60" and 100" instruments for your own exclusive use for a whole night!!! In the following link you'll find the tech specs of the 60" scope, f/ratio, focal length, eyepiece fl, and TFOV factors. You'll notice that the TFOV is tiny, 11 arc-minutes, despite the 100mm 4" eyepiece barrel. The blurb also says that these instruments are not good for very extended objects as these scopes' TFOV is too small for them! As visual instruments these are very specialised instruments.

* Visual tech specs of the 60" Mt Wilson scope

* Hiring the 60" Mt Wilson scope for 1/2night or full night

* Hiring the 100" Mt Wilson scope for 1/2 night or full night

[mental4astro (Alexander)]

There is one manufacturer who makes long focal length eyepieces in the 2" format, all the way to 85mm, Russell Optics - 2" eyepieces.

There is one thing you need to remember - the ultimate limiting factor to the maximum True Field Of View (TFOV) is the diameter of the barrel, regardless of it being 1.25", 2", 3", etc. You can get a 100° AFOV eyepiece, but the longest focal length obtainable by a given barrel size will be that which is allowed by the barrel. You won't see an 100° 30mm in 2" - it's just not possible. The longest focal length that will allow a 100° AFOV in 2" is around 25mm.

The above 25mm EP will exhibit the exact same TFOV as a 50mm 2" eyepiece - the 50mm being limited by the barrel/field stop.

Why would you want a 50mm eyepiece? A 100° eyepiece is not for everyone. A 2" 50mm Plossl eyepiece, such as the one from Russell Optics, has roughly the same AFOV as all other Plossls, approx 52°, which still makes for comfortable veiwing. This EP is actually really nice in a C8 and similar and larger Cats.

The longer focal length EPs all show the same TFOV, but as the fl gets longer, the AFOV gets smaller, like looking through a longer and longer pipe. But they serve a purpose that satisfies the requirements of the folks who purchase them.

I have one of these 50mm plossls from Russell Optics. It produces a lovely image in my big dob, though I am aware that when using this EP with this scope I am effectively stopping down the scope from 17.5" to something like 8". In my C8 out bush, it is also lovely. This EP is very much only for when one is observing out bush with any type of reflector as the shadow of the secondary will be all too evident under urban skies.

Alex.

[Zubenel (Wes)]

Wow , I'm overwhelmed by the time you all have given to this topic. The quest for a large FOV from a long Focal length ( Large diameter) scope is not at all an easy proposition when factoring limitations such as exit pupil, TFOV, large diameter eye piece/focuser combinations. As Alex has attested to, he loves his 8" with a 3° TFOV in a dark sky. (I'm with you Alex and thank you for your contribution....) and yes, a 25" F4 with a 55mm Plossl will see your retina only receive 1/4 of the primary's light but hey , I love looking around in there , trying to count numerous galaxies that an 8" would have no chance .
The 20" isn't the scope for sky scanning !With Big aperture comes fainter and brighter visual objects . Reducing the F ratio still evokes the tyranny of exit pupil. By the time we reach an age where a Large Dob is possible, if you are anything like me you have a shed filled with your previous favourite telescopes in descending aperture diameters. So for wide field I'm dusting off my 10" F5 ATM and doubling the FOV to 2°.
Observing through The Mt Wilson 100" Hooker Telescope was an interesting read which I thought would fuel Mega aperture fever. Well, it DIDN’T. The author referred to the poor seeing conditions described by Norm Vargas , when looking at Neptune as "We don't get excited unless the seeing is better than 1 arc second". Robert Naeye ( Author )put it this way." From a pure observing perspective, I've had better views of many of these objects through much smaller scopes operating under darker and steadier skies...".From personal experience when comparing views at this year’s QLD Astrofest between Zamar, my 20" F5 and Peter R's 30" F4.8 at similar magnifications I found the views through the 30" to be softer having been affected more by the seeing. (Larger may not be better except on rare occasions)Even the Mt Wilson 60" isn't the scope to view Galaxies!!
So rest assured that in Australia if you have a large Dob in a Dark Sky with sub arc seeing, your views may surpass that of the mighty Mt Wilson 100" Hooker telescope !! And forget about the quest for a Big Dob with an ultra-wide FOV It just ain't going to happen.