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Old 24-10-2021, 05:28 PM
Rod-AR127 (Rod)
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What size EP's do most people use, 1.25" or 2"?

So l now have a 2" star diagonal, what do l do with it?
That's a rhetorical question.
Now after looking around at 2" EP's I'm left wondering what most people use.
Clearly on modest telescopes like mine you wouldn't want a dozen 2" EP's, you be better off with a better OTA.

I guess what I'm asking is do those who use 2" EP's have one or two, ( or three ) for their favorites viewing and run 1.25" the rest of the time?

Is a 2" SD worth it if your only using 1.25" EP's?

Is a 1.25" EP in a 2" SD limiting the EP or no more than a 1.25" SD is limiting the OTA?
So many questions.
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Old 24-10-2021, 05:50 PM
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The Mekon (John Briggs)
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You can see from my signature what telescopes I have, So how many 2" eyepieces do I have and how often do I use them? Answer - one only a 26mm Nagler which is used about 10% of the time. Most use is 1 1/4" eyepieces like my 17.3mm Delos, 13mm, 9mm, and 5mm Naglers.
Only reason to use 2" is for wide field.
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Old 24-10-2021, 06:37 PM
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Stonius (Markus)
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As John said, they're good for widefield. I use my 30mm ES a fair bit. It's a good 'finder' eyepiece when looking for objects. Plus I kinda like widefield views because it gives context to the object, the stars are tighter, everything just feels kinda 'cleaner'. And I never really get sick of looking at swirling dust clouds. If I had to get by with only 3 EPs I'd have one that gives the widest views possible with my telescope before the exit pupil becomes too big for my dark adapted pupil, another that corresponds to the best seeing conditions that I'm *likely to have (so it can be used *most nights) and something somewhere in between with an exit pupil of around 1 or 2mm. But yeah, I don't think you need 50 widefield 2" eyepieces.


Cheers


Markus
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Old 24-10-2021, 06:42 PM
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mura_gadi (Steve)
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I only use the one and haven't seen a need to replace it or add to it. That's my wide field at 30mm 80degree and a 5mm exit pupil. The rest are all 1.25", though I generally use a 1.25" zoom. I think I could happily get away with the wide field, 8-24mm zoom and a powermate 2.5 as a full kit tbh.

I would suggest you try astronomy tools and check to see what ep you need to fit in something like M42(ie a large visible target) well framed, a consideration for the high powered ep would be does the field allow the moons of Jupiter and Saturn to be seen etc.

The exit pupil, eye relief are well worth considering and getting a little reading in on.

Last edited by mura_gadi; 24-10-2021 at 06:54 PM.
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Old 24-10-2021, 07:26 PM
Rod-AR127 (Rod)
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Interesting stuff and thankfully about what l was hoping for.
I've done some reading on apparent and true FOV, exit pupil and eye relief.
I'll make up a spread sheet with my 2 GSO 50° and my planetary EP and work out all the numbers for my 127mm aperture and 1200mm FL and over 50 yr old eyes.
I won't be rushing out to buy a 2" EP just yet, l need to plan because l need to got to Japan as soon as covidly possible. (I can see a small apo in my future.)
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Old 24-10-2021, 08:58 PM
astro744
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The field stop diameter is what determines the true field of view of an eyepiece in any given telescope. Eyepieces with 1.25” barrels are limited to 27mm maximum field stop diameters. An eyepiece with 27mm field stop diameter could be a 40mm Plossl with 43 deg apparent field, 32mm Plossl with 50 deg apparent field or 24mm Tele Vue Panoptic with 68 deg apparent field. Each will show the same true field but at different magnification for a given telescope.

Eyepieces with 2” barrels allow for 46mm maximum field stop diameters giving much larger maximum true fields for a given telescope compared to 1.25” eyepieces. Examples include 50mm Plossl and 41mm Tele Vue Panoptic.

There is no reason one would want to only own 2” eyepieces since focal lengths and apparent fields with field stop diameters under 27mm only require 1.25” barrels and almost all are manufactured such. Tele Vue has produced some dual barrel eyepieces but the 2” skirt is simply a convenience as the eyepiece is really only 1.25” (13, 11, 9mm original Nagler, 16, 12mm Type 2 Nagler, 22mm Panoptic, 12mm Type 4 Nagler).

Your favourite eyepiece need not be 2” but one that frames the object you are viewing nicely and this could very well be a 1.25” eyepiece.

If you only have 1.25” eyepieces then a quality 1.25” star diagonal which allows for full illumination of 27mm field stop eyepieces is all you need. Note though with a 1.25” diagonal you would be using the edge of the reflective surface which may not be to the same accuracy as the centre but if you buy a quality one the difference may not be noticeable as the mirror may be oversized to avoid any edge defects being perceived. A 2” diagonal would mean the edge would not come into play if using only 1.25” eyepieces.

One 2” low power and one 2” med power wide apparent field eyepiece is all you really need in 2”, something approx 30-40mm and approx 17-22mm.
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Old 24-10-2021, 10:28 PM
Rod-AR127 (Rod)
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Wow, thankyou, greatly appreciated, not fully understood but give me a day or two and a few more reads on doing the maths on random EP's and l should pick it up.
Thanks heaps everyone for taking the time to reply.
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Old 24-10-2021, 11:25 PM
Renato1 (Renato)
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Most people would use 1.25" eyepieces.

The 2" diagonal is handy for the occasion when one wants very wide fields of view, like with a 2" 40mm wide angle eyepiece or 2" 55mm Plossl, or a wider field of view with a 2" 30 or 32mm eyepiece than in their 1.25" counterparts.

For me, the downside of 2" diagonals is that they are a bit more unwieldy than a 1.25" diagonal - they weigh more, and one has to be a bit more extra careful when tightening them up. And most of the time I'm putting 1.25" eyepieces in them.

I have 40mm and 50mm 1.25" eyepieces which I haven't used much at all because of their narrow fields of view. Their 2" counterparts I've used only a bit more. I do now keep a 2" 40mm in my 5" Mak because I like that combination, though I wasn't fond of it in my 6" Mak or 8"SCT.

Judging from what I have, the field of view in my 1.25" 25mm wide angle eyepiece is only very slightly smaller than the field of view in my 2" 25mm wide angle eyepiece.

Anyhow - you have the flexibility with a 2" diagonal to get a few 2" eyepieces. But there is no reason to rush.
Regards,
Renato
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Old 25-10-2021, 12:31 AM
Rod-AR127 (Rod)
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Geez l am missing something here.
How do you know the field stop of each EP? Or is it set at 27 or 46 respectively.

A 15mm EP of 68°AFOV, in a 1200 FL scope.
TFOV= Field stop 27mm / 1200 X 57.3 = 1.289°?
So it does not matter the 15mm Or the 68° AFOV.
All the 68° EP's from 3mm to 33mm with a 27mm field stop will all have the same 1.289 TFOV?
ARRGGGHHHH.

Last edited by Rod-AR127; 25-10-2021 at 12:51 AM.
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Old 25-10-2021, 01:29 AM
Renato1 (Renato)
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Quote:
Originally Posted by Rod-AR127 View Post
Geez l am missing something here.
How do you know the field stop of each EP? Or is it set at 27 or 46 respectively.

A 15mm EP of 68°AFOV, in a 1200 FL scope.
TFOV= Field stop 27mm / 1200 X 57.3 = 1.289°?
So it does not matter the 15mm Or the 68° AFOV.
All the 68° EP's from 3mm to 33mm with a 27mm field stop will all have the same 1.289 TFOV?
ARRGGGHHHH.
True field of view = Apparent field of view / magnification
So,
1200mm/15mm= 80X magnification.

The true field of view you see in the eyepiece is 68degrees/80X= 0.85 degrees.

Regards,
Renato
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Old 25-10-2021, 06:06 AM
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mura_gadi (Steve)
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Hello,

Try the link below, it has some really good resources for FOV etc, though you can do similar with Stellarium etc.

There is also an extensive list of eyepieces via an excel spreadsheet on CN which you can download and use the excel filter function to fine tune a possible selection of EP's based on ER/EP/Price etc.

"Russian talks" also has a fairly good write up of ep's based on performance at different ratio's, I would suggest having a look through what Ernest says. A great list to find performance without the price tag imo.


Steve
https://astronomy.tools/calculators/field_of_view/
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Old 25-10-2021, 08:11 AM
astro744
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Quote:
Originally Posted by Rod-AR127 View Post
Geez l am missing something here.
How do you know the field stop of each EP? Or is it set at 27 or 46 respectively.

A 15mm EP of 68°AFOV, in a 1200 FL scope.
TFOV= Field stop 27mm / 1200 X 57.3 = 1.289°?
So it does not matter the 15mm Or the 68° AFOV.
All the 68° EP's from 3mm to 33mm with a 27mm field stop will all have the same 1.289 TFOV?
ARRGGGHHHH.
Field stop diameter for Tele Vue eyepieces can be found at https://www.televue.com/engine/TV3b_page.asp?id=214

Lots of other advice here https://www.televue.com/engine/TV3b_page.asp?id=154

Note from the table eyepieces with the same field stop diameter give the same true field in a given telescope. Magnification and apparent field of view is what will vary with different eyepiece designs.
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Old 25-10-2021, 09:21 AM
Rod-AR127 (Rod)
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Thankyou, l was on track earlier doing what Renato laid out for me.
Thanks for the links and advice.

Damn cloudy over here for another week, watching my bid on a cheap tripod (61 inches high) for my 10x 50 bino's, 4 hrs to go.
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Old 25-10-2021, 11:31 AM
astro744
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Quote:
Originally Posted by Rod-AR127 View Post
Thankyou, l was on track earlier doing what Renato laid out for me.
Thanks for the links and advice.

Damn cloudy over here for another week, watching my bid on a cheap tripod (61 inches high) for my 10x 50 bino's, 4 hrs to go.
The true field = apparent field/magnification is a good approximation but doesn't take into account angular magnification distortion. Good enough for most purposes but there is a slight difference if you use the field stop formula. I use the simpler formula all the time myself but its good to understand both methods. There is another method involving arctan but not one I've used much except for working out binocular true fields when only xxx feet at 1000 yards is stated.

Note if your 15mm eyepiece had a 100 deg. apparent field then your true field would be about 1.25 deg. The field stop would be approaching 27mm. Note if you look at the Tele Vue tables you can see that the 100 deg. 13mm Ethos has a field stop diameter of 22.3mm and the 17mm Ethos has 29.6mm. The latter is over 27mm thereby requiring a 2" barrel.

The field stop diameter is a linear representation of the true field at the focal plane. Think of cameras where a full frame 35mm (36x24mm) has a larger area than an APS-C (24x16mm approx. for 1.5x crop) sensor. The full frame gives a larger true field than the APS-C for the same focal length lens.
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Old 25-10-2021, 01:02 PM
Rod-AR127 (Rod)
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Originally Posted by mura_gadi View Post
What a great tool. Nothing like visual learning.
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Old 25-10-2021, 03:48 PM
Renato1 (Renato)
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There is another way to measure True Field Of View which is pretty accurate and doesn't rely on the manufacturer's stated Apparent Field of View or any other esoteric calculation. It relies on having a big detailed Atlas or a print-out of an atlas, which has the scale of degrees to millimeters on the page.

Aim at a star near celestial equator, look for faint stars near it, then put that star at the edge of the field of view, and find a little star at the opposite side of the edge - so that the two stars mark out the full diameter of the eyepiece.

Then go to the Atlas or printout, use a ruler to measure the millimeters between those two stars, then go to the scale and see how many degrees of True Field of View that translates to.

Sometimes one finds that the True Field of View is smaller than what one would have expected had the manufacturer's stated Apparent Field of View been accurate.
Regards,
Renato
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Old 26-10-2021, 05:50 AM
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mura_gadi (Steve)
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"What a great tool. Nothing like visual learning."

With "Visual learning" you have nailed a very important aspect. From my book/web reading they say for visuals its:
1) Seeing conditions
2) Experience
3) Telescope and eyepieces.

An experienced visual user will pick up objects quicker and see fainter objects etc than a new observer using the same telescope(conditions of the eyes excluded). Much the same way you can train your eye at being good at jigsaw puzzles, visual work improves with time and experience.
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Old 27-10-2021, 07:32 PM
Startrek (Martin)
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After a year into the hobby ( 2017 ) I bought a book called “Choosing and Using Astronomical Eye Pieces” by William Paolini ( circa 2013 )
I must say it’s the A to Z book for Eye Pieces , very comprehensive book , easy to read and understand. It should answer the majority of your queries
Thoroughly recommend it ( Retails for around $85 ?)
I believe William is an IIS member as well ( he lives in the US )

I started with basic Plössl eye pieces both 1.25” and 2” ( 9mm to 32mm ) and 4 years later ended up exclusively using Televue eye pieces in my 12” Goto dob at my Bortle 3 dark site getaway on the South Coast. I use a complement of 1.25mm and 2” TV eye pieces and they all have a specific viewing purpose

Cheers
Martin
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Old 17-11-2021, 10:32 AM
donavan.jones
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Yes I have found this very confusing, have some old ones and new ones, is there really that bigger difference in the old ones V's the new glass, I have some new televues and old Unitron but struggle with which are best, mind you just the price alone would make you think that new one are, but could just be in my head.
Cheers
D
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Old 17-11-2021, 10:33 AM
donavan.jones
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Quote:
Originally Posted by mura_gadi View Post
"What a great tool. Nothing like visual learning."

With "Visual learning" you have nailed a very important aspect. From my book/web reading they say for visuals its:
1) Seeing conditions
2) Experience
3) Telescope and eyepieces.

An experienced visual user will pick up objects quicker and see fainter objects etc than a new observer using the same telescope(conditions of the eyes excluded). Much the same way you can train your eye at being good at jigsaw puzzles, visual work improves with time and experience.
Yea i think this is great advice for newbies, i need more time on the glass
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