Low power EP for fast refractor?

[mental4astro]

Hi all,

I'm looking for a good EP match for an f/5 achro. 2" format, focal length between 34 & 40mm, AFOV preferably between 68* & 82*. Edge performance doesn't need to be 100% (not overly important to me. But astigmatism, pincushion & as far as possible field curvature controlled pretty well.

Any suggestions for a ripper rich field EP?

Mental.

[Wavytone]

:confused2: is there a type you haven't tried ? How about that nice Masuyama ?

Ive culled my surplus ones, though I still have a Skywatcher 2" 28mm LET lying around... That would do pretty well.

[anj026]

I'm happy with a 21mm Ethos in my Jaegers 6" f5 achro. The next best choice (especially away from the city) is the 31mm Nagler for the slightly wider field and 6mm exit pupil. Haven't tried a 26mm Nagler but would expect it to be a good compromise.

If I couldn't have those I would look at a 35mm Panoptic or 27mm if viewing mostly from the suburbs.

The f5 ratio is demanding on eyepieces and focusing. When your focal length is 760mm or less then field curvature is hard to avoid for visual.

[MattT]

This is a hard one Alex.
When I had my 4" f5 achro I couldn't find any eyepiece to do what you want and ended up using a 30mm plossl that gave 3º FOV from memory. It was the only one that had a fairly flat field. Tried ES 28mm 68º and was pretty bad half way out and was only 3.8º... RKE 28??? looks bigger than it is, also not 2" as you know. :shrug:

[omegacrux]

Hi Alex
I just have a 2in 30mm gso
Does me fine but when you move it you can notice its not flat !
There cheap as though

David

[mental4astro]

Thanks for the replies, gentlemen.

Wavy, I think there's just a couple of EPs I'm still to try, :rolleyes:

David, the Superviews are Erfle designs, which is the most common EP design in binos. Certainly worth a try as I too have the 30mm Superview. I do know some bino EPs have small field stops put into them to control abberations, so the view with the 30mm should be interesting. I tend to forget my old workhorse these days. I'm glad I still have it. I'll report what I see with it in the next few days.

Matt, I too am not too keen on my 28mm Meade SWA in the 4" achro. BUT the 34mm is pretty good I have to say. I'm just getting greedy with wanting even more than the 4.5deg TFOV it gives me. Not that I can get too much more, but a 5deg TFOV sounds mighty goooood! :rolleyes: ;) :D

Haven't tried a 31mm Nagler yet. I'll see what I can do. See Wavy, one I haven't tried yet...

[AG Hybrid]

Well in a 4" my 30mm ES 82 gives 3.5 degrees. In yours it will give you 4.5 degrees TF. Reviews indicate the view is nearly identical to a nagler 31. You can have a loan of it next time we meet.
Isn't 4.5 degrees about 9 Moon diameters? Do you really need the 10th?

[brian nordstrom]

:) Hi Alex , H grabbed a SW 100mm f5 , it was cheep and I wanted to see how it performs , anyway , I have found my AT Titan 40mm 2 inch is a great match for this scope , about 13x and its good out to about 75-80 % to the edge , great for deep sky scanning .
Brian.

[Satchmo]

You're probably enjoying about 70mm of aperture there but they can't take the wide field away from you ...

[mental4astro]

In the words of the Great Austin Powers "YEAH! Baby!"

Can never have enough wide field, :lol:

Mark, there in lies a dilemah- stick to exit pupil & lose TFOV. Grab as wide a field as barrel size allows & reduce effective aperture. For my "thing", I'm happy to go a bit big in exit pupil.

[brian nordstrom]

Right , but if the last 1/3rd is un-usable is it worth it ?

Brian.

[Camelopardalis]

Have you tried a Paragon? Weren't they supposed to be orthoscopic?

[Satchmo]

Why would the outer third of your aperture be unusable .

[brian nordstrom]

;) Coma , my friend , Horrendous COMA ..:eyepop: .
Brian.

[Satchmo]

I think you are confusing the term `aperture' with `field of view' - when I say aperture I 'm talking about the usable diameter of the telescope whose light you are funneling in to your eye, not the the quality of the last 30% of your field of view.

[Wavytone]

Maybe, maybe not... With all lenses there comes a point off-axis where image quality deteriorates really badly, very suddenly. On a small refractor it's quite possible for that to occur within the field of a 2" eyepiece. Winged seagull shapes will be likely.

Alex, there's an LVW42 waiting...

[brian nordstrom]

:shrug: WHAT ? , tell me one thing , what use is a full say , 4 degrees of sky if only 2 degrees is usable ? as Wavytone says , seagulls don't do it for me either ;) .
That's where my 40mm AT Titan is OK , seagull city only the last 10-15 % , which is OK ,,, for me .
And every one is different .
Brian.

[Fox]

Alex, I guess it boils down to field stop. The largest possible field stop for a 2 inch EP is 46mm. IMHO my first choice would be Televue, because Al Nagler obviously has his f/5 scopes in mind. My second choice would be Explore Scientific. If you stick as close as possible to the 46mm maximum field stop for greatest TFOV, a 100 deg vs 82 deg vs 68 deg FOV eyepiece will give you progressively smaller and smaller images for the same TFOV.

I agree with everyone above; I don't see much point in buying somewhat less expensive brands and designs if the outer edges of the field results in stars which look like seagulls and comets. And I would also add that the smaller the focal length of the eyepiece will give smaller images and less contrast - so the sky may look too washed out.

Another alternative - looks as though some manufactures are beginning to make 3-inch eyepieces. Awfully pricey I bet, and perhaps the size of your focuser will start becoming an issue.

Fox

[ausastronomer]

Hi Mark,

I think you're wasting your time, he hasn't got a clue what you're talking about :)

Cheers,
John B

[ausastronomer]

In fact running the numbers and making an assumption of 50 yrs of age and a 6mm maximum pupil dilation, the scope effectively becomes a 75mm/F6.67 scope. Plenty of wasted light going west there. Even with a 6.5mm maximum pupil dilation the scope becomes an 81mm/F6.1 scope

Cheers,
John B

[mental4astro]

Which is why I always take a bucket with me to catch all that "wasted" light. I then donate it to other fellows with smaller scopes.

I have no problem with an over large exit pupil. If the FOV the EP gives satisfies, then what's the problem? No wasted light. Not the most effective, but certainly not wasted. Sometimes the carry on that happens about exit pupil, pin-sharp edge to edge, etc, etc, really, just takes the fun out of this. To carry on like this is just nasty. Relax, boys.

Wavy, thanks mate, I certainly will take you up on that 42mm EP in my f/5 refractor.

[Satchmo]

If the light from the objective does not make it into your eye then I think wasted is an appropriate word. You might call it `unwanted' then we are getting into philosophical territory :)

[mental4astro]

:rolleyes:

Yeah, philosophical... :) Would be good to sit with you for a while and pick your brains a bit, Mark, :)

Folks, please can we stick to the question I posed in the original post. Ok, exit pupil has been mentioned, and it will be big, and I knew that. Fine, finished. Not my question though.

If you don't have anything more constructive to add, then don't add anything else. I'm not interested in CAN'T. I'm after pushing the limits of what the gear can do. If it spins you out using a 40mm EP in an f/5 scope, then look the other way. But, if you can add to the sorts of aberrations or lack of one can see, please type away. THAT'S what I'm after, :)

[anj026]

These three quotes are from articles written by Al Nagler.

"If the exit pupil of the telescope is larger than the eye's pupil, then the full aperture of the telescope is not being utilized. However, neither image brightness nor resolution is reduced at that low magnification."

"For reflector telescopes, it's best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes."

"For best low-power viewing, use the highest power that properly frames the subject. You will see the most detail and best contrast with the least contribution of eye defects."

[Satchmo]

This is a rather sinister piece of semantics ! ..... Do you really expect a company that wants to encourage the sale of as many different eyepieces as possible to give a consice answer ?

If you observe at a lower magnification that your aperture can support for full light cone acceptance - then in relation to the actual scope capability you have in your hands - you are definitely loosing potential resolution.

The larger you can make a DSO object for the light available the more detail you will see as at lower light levels the resolution of the eye drops to a whopping 1 degree or more ! So the higher magnification you can have with full aperture for a given apperture - the more detail you will see. So to say that in relation to the aperture you have that overfilling your pupil with a too low magnification is not compromising resolution is nonsense..


It is true that once your pupil is full - you will not see a brighter image at that magnification by increasing the aperture - but if you actually have a larger aperture available that is not being utilised - then you are definitely loosing resolution on fainter objects ...

[anj026]

Well, I have no doubt that Al Nagler likes to sell eyepieces and telescopes but I have difficulty seeing any sinister implications here.

I found those quotes on the Tele Vue website in the advice section;
http://www.televue.com/engine/TV3b_page.asp?id=154

My impression is that there is no harm in using a refractor with exit pupils larger then 7mm if you choose to do so.

If you only have one telescope and it is a refractor then it has that extra level of functionality in being able to achieve wider fields by lowering the magnification.

If I want wider fields than the 3 degrees available in the 6" f5 Jaegers with 31mm Nagler (25x, 6mm exit pupil) then I go to a TV60 with 24mm Panoptic (4.3 degrees, 15x, 4mm exit pupil) or binoculars.

[Satchmo]

You will not come to physical harm if you choose to do so :) but it is absolutely false to make a claim that you will not lose resolution by using a too large exit pupil , particularly for dim DSO objects as outlined below in my last post . To claim that it is not so in order to encourage people to buy more long focal length eyepieces that are not matched to their scope and eyes is an exercise in selling more eyepieces ? ...

If anyone doesn't understand the physiology of what I am pointing out then please ask for further clarification .

[mental4astro]

Thanks Andy. One person seems to understand my question.

Resolution may be lost (reducing aperture will do that), but again that's not a concern for me. It's not like I'm looking through a brick either. If I'm reducing my effective aperture from say 100mm down to 70mm, I'm still getting the wider field I'm after, and a 70mm aperture is still mighty fine for the application I'm after.

I hope I'm making myself clear here now.

Reducing effective aperture is no sin, and thankfully no physical harm either, :). If you can't understand that, then that's a shame and I'll leave you to your sub 7mm exit pupils. The effect I'm after isn't predicated by adhering to a pucker tight definition of exit pupil. It IS dictated by pushing the FOV to as wide as possible. If there is a trade off in exit pupil, I'm happy to live with that. Now, can we get back to these blasted eyepieces...

[Satchmo]

Mental - I don't think anyone has been unclear about what you were doing/seeking to do from what I have seen on the thread. The posting of mis-information from Televue website needed correcting as it was posting a conflicting message.

You would be interested in a little set of home made binoculars I have - they take the principle of sacrificing aperture to get field of view to the extreme!
The objectives are 55mm camera lenses of 25mm aperture wide open . With the 25mm EP's giving 2 X magnification they give a massive 25 degree field of view. With a probable true used aperture of 12mm in a dark sky the views of the milky way are not that much improved over naked eye. But the 2 X magnification helps a little in resolving large clusters. The original owner/ builder was very disappointed in their performance as he didn't understand the relationship of magnification and exit pupil.

[mental4astro]

Mark, I certainly am interested! 25* FOV! Caawww! Should be interesting the image they produce from a dark site.

[Camelopardalis]

But surely you're just wasting light not being able to see the WHOLE sky all at once :D :D

Sorry, couldn't help myself ;)

[Wavytone]

Alex it would make more sense to take a really fast lens around 75-100 mm, and mate that to an ES 9mm 120 degree eyepiece, to produce a magnification around 10X and a field of view over 10 degrees wide. The main thing is to find a really fast lens a couple of stops faster than anything in binoculars.

Candidates include the Olympus 75mm f/1.8 lens (for m4/3), Canons 100mm EF f/2, there's also the Panasonic/Leica Nocticron 45mm f/1.2, phenomenaly fast but it's a bit short, you'd really need to mate that with a 4.7mm Nagler.


Taking the 75mm f/1.8 as being the only one remotely affordable, with the 9mm 120 degree eyepiece it will give a magnification a tad over 8X and an exit pupil around 8mm. Even allowing fir a smaller exit pupil effectively limiting the aperture it's still going to be a whole 2stops faster than any binoculars - 4X brighter. The 15.7 mm field stop implies a FoV around 12 degrees.

A monster monocular. Whether it would actually be any good is another matter, as the lens will be designed to give a flat field, whereas I'll bet the ES eyepieces don't have a flat field and have negative coma. At f/1.2 any focal plane mismatch due to find curvature will have truly savage consequences (defocus), to the extent I'd expect the result to be pretty awful off-axis.

[Satchmo]

My point about the binoculars was that they were a disappointment to the builder who had gone to some trouble - despite the 1" clear aperture - at 2X the eye- pupil was actually stopping the camera lens down to 10 or 12mm.

The builder did not understand about matching magnification to the eye pupil. Despite the 28 degree true field , and 50 degree apparent field, the overall view was hardly any different from the naked eye - in fact the naked eye view was far better at some 140 degree true field !

Wavytone - I'm not sure what you mean about finding a combination that wil be `two stops faster than any binocular '? .

(Edit : perhaps you were wanting to use a fast lens with a very short FL eyepiece to take advantage of the large apparent field of some of those shorter FL eyepieces ?)

Surface brightness of an extended object is a function of exit pupil. The eye does not care what optical system delivers it. Once you fill your pupil at certain magnification you cant increase brightness any more.
In fact the apparent brightness of an extended object nebulous object is fixed by the pupil diameter not by the aperture of the telescope.

Magnification change while keeping the pupil constant is affected by changing the aperture , but the surface brightness remains the same as the naked eye view at the same pupil setting. A simple example would be if you view the Moon one naked eye and in the other with large telescope fitted with a diaphram fitted at focal plane / field stop that allowed you to mask the view of the moon down to 1/2 degree apparent field , you would find that the images were of identical brightness .

This can be a revelation to people in understanding telescopes and their intimate relationship with the physiology of the human eye - the surface brightness of an extended object like M42 per arc minute is the same in a 6 meter as a 6" telescope at the same pupil setting- it is just a lot larger and more detailed ! As we don't have CCD sensors at the back of our eyes we are always limited by the constraints of getting an image in via a 7mm hole at the front :)

[Wavytone]

Mark I appreciate that - it's immediately obvious...

Yes it's possible to use camera lenses but the results are usually pretty ordinary compared to binoculars. Personal experiments suggest you won't get much joy from lenses less than 135mm focal length; if designed for APS-C or full frame cameras they should work well enough as finders with a 20mm eyepiece. A lens designed for full frame can cover a 40mm circular field, which suggests an ultra wide eyepiece around 20mm in a 2" barrel.

Big, heavy and expensive for something that ultimately isn't any better than a pair of 7x50 binocs.