Jump to content


Photo

Help me decide on long eyepiece

  • Please log in to reply
24 replies to this topic

#1 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 29 January 2013 - 09:07 PM

I used a 6" newt for several years and gave it to my son. I miss observing, so I think I want to pick up an Orion XT12g. Mostly for the simplicity of setup, and the tracking capability.
I would like to also buy a long eyepiece to take advantage of the wide field at lowest useful mag, which I think comes out to a 35mm eyepiece. Should I buy the widest field I can afford?
I like the price of the Celestron Ultima LX 32mm, which is pretty close to 35.
Or, spend a little more for the ES68?
Luminos?
Or, cough up the dough to go Panoptic?
Any advice is very happily accepted.
Thank you

#2 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 23428
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 30 January 2013 - 02:05 AM

Does your eye's pupil open to 7mm? Unlikely if you're over 40.
Your new scope is about f/5. You might see coma with a very wide field eyepiece. But that can be fixed if you do.
Nonetheless, my comment is that, for that scope, a 30-31mm ultrawidefield would be a superior low power eyepiece and have a field of view a little LARGER than the 35 Panoptic. An eyepiece like, say, the 30mm 82 degree ES or something similar.
That would have a wider field than any you mentioned.
And certain large DSOs would just be perfect in that large FOV.

#3 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 30 January 2013 - 11:33 AM

Thank you, that makes good sense, I hadn't been thinking about the exit pupil (so much to learn...).
I probably mis-stated my purpose. My real goal is FOV. Low mag is just helpful to that.
I see the ES 30/82, and the Olivon 30/80 would both give great FOV and exit pupil. At $250 and $150.
I wonder if folks here think the ES is worth the extra $100?
Lastly, the Orion Q70 32/70 comes close in FOV, but, $80? I've seen some positive reviews here on CN, so I'm intrigued, but, How much would I be giving up to save all that dough?

#4 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 30 January 2013 - 11:48 AM

Your first step is to measure your exit pupil. Don't worry about standing in a dark room for 30 minutes, almost all of the dilation happens in 30 seconds. And if the room is not completely dark, that just gives you a conservative figure (a little margin for error). You don't need six decimal accuracy on this.

Once you have that answer, then it's just a "simple" matter of cost/weight/performance. Talk to ten different people and you'll get twelve different opinions.

#5 csrlice12

csrlice12

    Voyager 1

  • *****
  • Posts: 11494
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 30 January 2013 - 12:00 PM

Using cars as an analogy, in the 12g, Televiews/Pentax are Porshes, ES are vette's, and Q70s are Chevys....you don't even wanna consider Yugos.....

Not familiar with the Olivon, but that ES82* 30mm (and the 24mm) are absolutely fantastic in my 10XTi. I will admit the 30mm FOV is so large I can't take it all in, I have to move my head around. Its not like looking out a porthole, it's like getting thrown out the airlock door....but beautiful sharp to the edge views. A Paracorr will probably be a must with the 12g

#6 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 30 January 2013 - 01:07 PM

I notice that your 10" is a bit faster than the 12". (4.7 vs 4.9)
Do you need the paracorr for your scope with the 30/82?

#7 bleep

bleep

    Vostok 1

  • *****
  • Posts: 148
  • Joined: 06 Aug 2012

Posted 30 January 2013 - 02:22 PM

How do you measure your pupils?

#8 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 30 January 2013 - 05:55 PM

With the help of a friend, or just ask the eye doc to do it on your next visit.

#9 BillP

BillP

    Voyager 1

  • *****
  • Posts: 12048
  • Joined: 26 Nov 2006
  • Loc: Vienna, VA

Posted 31 January 2013 - 12:23 PM

Besides just the exit pupil considerations, I think you should consider the aperture/focal ratio as well. If widest TFOV is a "major" concern, then the 12" scope is a limiter unless you go for a more exotic shorter focal ratio. To illustate using the standard orion offerings, here's what you get when you keep the exit pupil near the same between them...

XT12 - 27 Pan - 5.5mm Exit Pupil - 1.2 deg TFOV
XT10 - 27 Pan - 5.7mm Exit Pupil - 1.5 deg TFOV
XT8 - 35 Pan - 5.9mm Exit Pupil - 1.8 deg TFOV

So the 8" will get you 50% more TFOV! Don't know what other considerations you have in going for a 12" scope...but just FYI the 8" will get you a lot more sky to view.

#10 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 31 January 2013 - 03:11 PM

Thanks, that's a great point.
My reason for the 12" aperture choice is light gathering ability for deep sky objects (aperture fever?).
For me, the XT12g seems like a good compromise between portability/aperture/ease of setup.
No messing with struts and daily collimation, fits in my truck, and within my weight lifting abilities.

I like the tracking ability that comes with the goto system,
but I think I'll be manually finding my objects, just for the fun and challenge of it.

With that in mind, my next step is to choose a good, long eyepiece for star-hopping and wide views of larger objects.

I'm thinking that the 1.3 - 1.5 degree FOV that it appears this scope can obtain (with reasonable exit pupil),
would be enough for most everything except perhaps Andromeda.

But, I'm not that experienced, so I welcome anyone who would disabuse me of that notion.

#11 taylornate

taylornate

    Sputnik

  • -----
  • Posts: 45
  • Joined: 04 Jan 2013

Posted 31 January 2013 - 03:19 PM

Besides just the exit pupil considerations, I think you should consider the aperture/focal ratio as well. If widest TFOV is a "major" concern, then the 12" scope is a limiter unless you go for a more exotic shorter focal ratio. To illustate using the standard orion offerings, here's what you get when you keep the exit pupil near the same between them...

XT12 - 27 Pan - 5.5mm Exit Pupil - 1.2 deg TFOV
XT10 - 27 Pan - 5.7mm Exit Pupil - 1.5 deg TFOV
XT8 - 35 Pan - 5.9mm Exit Pupil - 1.8 deg TFOV

So the 8" will get you 50% more TFOV! Don't know what other considerations you have in going for a 12" scope...but just FYI the 8" will get you a lot more sky to view.


What is stopping you from getting a 1.8 TFOV in a 12"? Some light may be unused in long focal lengths with the excessive exit pupil, but how is that worse than having less light to use in all situations?

#12 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 23428
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 31 January 2013 - 04:03 PM

A 12" f/5 scope has a focal length of 1524mm. The Orion is nominally 1500mm.
A "largest possible field" 2" eyepiece would yield, at that focal length, a true field of 46/1500*57.3=1.76 degrees.
It's not recommended to use a 40mm eyepiece (or longer) with an f/5 scope because:
--exit pupil is too large and the eye is stopping down the scope
--the shadow of the secondary may become noticeable
--the natural astigmatism of your eye will be at its maximum (do you see the stars as tiny little pinpoints without flares with your naked eye? I certainly don't.)
--the secondary size chosen is not designed to illuminate such a large field, so there will be vignetting.
--the magnification will be very low, and that will make the background sky in the eyepiece quite bright and possibly be less esthetically pleasing than a shorter focal length eyepiece.
--even if YOU don't notice any of the above, if you share your views with others, they might.

So what would I recommend?
Well, a 30-31mm ultrawide eyepiece might very well satisfy the field size need (yielding around 1.6 degrees), and avoid the above issues. Magnification will still be low enough (48-50X) to keep star clusters looking like star clusters instead of a random assortment of field stars, yet high enough to make the background a little more esthetically pleasingly dark.
AND, it will still have a MORE than large enough field to use as a "finder field" eyepiece. [when star-hopping, I prefer a field closer to 1 degree to make it easier to hop the right distances, so this is a "mega-field", really].
Last, the wider apparent field will be more "immersive", and it is at low powers this sensation is most apparent.

I'm just sayin'......

#13 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 31 January 2013 - 04:04 PM

I'm thinking that the 1.3 - 1.5 degree FOV that it appears this scope can obtain (with reasonable exit pupil),
would be enough for most everything except perhaps Andromeda.


Excellent observation. I've long thought that most of the large ones (2+ degrees) are photographic targets with perhaps a dozen or so showpiece visual objects that exceed this. Whether it is 12 or 20, the majority of targets are small.

Sometime this winter I will sit down with Astroplanner and do an analysis of the size distribution of visual DSO's. Say, everything above magnitude 12 in the Northern Hemisphere.

Point being, one has to judge whether the best approach is get the large expensive and heavy widefield (like a 31 Nagler) to squeeze a few more DSO's into view of the large scope. Or, get something easier on the wallet and telescope balance like the 26 Nagler, then use that 26 in a smaller scope (like the ubiquitous 80mm ED refractor) for the super-wide views.

Tough call.

#14 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 31 January 2013 - 04:05 PM

Besides just the exit pupil considerations, I think you should consider the aperture/focal ratio as well. If widest TFOV is a "major" concern, then the 12" scope is a limiter unless you go for a more exotic shorter focal ratio. To illustate using the standard orion offerings, here's what you get when you keep the exit pupil near the same between them...

XT12 - 27 Pan - 5.5mm Exit Pupil - 1.2 deg TFOV
XT10 - 27 Pan - 5.7mm Exit Pupil - 1.5 deg TFOV
XT8 - 35 Pan - 5.9mm Exit Pupil - 1.8 deg TFOV

So the 8" will get you 50% more TFOV! Don't know what other considerations you have in going for a 12" scope...but just FYI the 8" will get you a lot more sky to view.


Yep, stepping down in telescope focal length can buy you much more field than an increase in field stop.

#15 taylornate

taylornate

    Sputnik

  • -----
  • Posts: 45
  • Joined: 04 Jan 2013

Posted 31 January 2013 - 04:19 PM

A 12" f/5 scope has a focal length of 1524mm. The Orion is nominally 1500mm.
A "largest possible field" 2" eyepiece would yield, at that focal length, a true field of 46/1500*57.3=1.76 degrees.
It's not recommended to use a 40mm eyepiece (or longer) with an f/5 scope because:
--exit pupil is too large and the eye is stopping down the scope
--the shadow of the secondary may become noticeable
--the natural astigmatism of your eye will be at its maximum (do you see the stars as tiny little pinpoints without flares with your naked eye? I certainly don't.)
--the secondary size chosen is not designed to illuminate such a large field, so there will be vignetting.
--the magnification will be very low, and that will make the background sky in the eyepiece quite bright and possibly be less esthetically pleasing than a shorter focal length eyepiece.
--even if YOU don't notice any of the above, if you share your views with others, they might.


This was enlightening--thanks!

#16 BillP

BillP

    Voyager 1

  • *****
  • Posts: 12048
  • Joined: 26 Nov 2006
  • Loc: Vienna, VA

Posted 31 January 2013 - 04:36 PM

What is stopping you from getting a 1.8 TFOV in a 12"? Some light may be unused in long focal lengths with the excessive exit pupil, but how is that worse than having less light to use in all situations?


This is an excellent point. I usually make it as well but sometimes forget it appears :shocked: :lol: I very much enjoy viewing with a 40mm XW in my XT10 (8.5mm Exit Pupil). I get no secondary shadow and the views look just fine with and without Paracorr (better with of course). This larger exit pupil means my 10" is operating as an 8" and that is actually just fine since the extra TFOV is really nice(assuming I am dialated to 7mm).

If the OP used a 40mm XW or 41mm Pan in their scope, the exit pupil would be 8.16mm. Assuming their eye only dialated to 6mm max then their view would be as bright as a 9" scope, but they would get that 1.76 deg TFOV!!

#17 aa5te

aa5te

    Genial Procrastinator

  • *****
  • Posts: 1497
  • Joined: 30 Aug 2008
  • Loc: Clinton, TN

Posted 31 January 2013 - 04:45 PM

I get 2.07 degrees TFOV (measured with Stellarium) with my Meade 40mm SWA in my 10" f/5. The view is very, very nice.

#18 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 31 January 2013 - 08:36 PM

A 12" f/5 scope has a focal length of 1524mm. The Orion is nominally 1500mm.
A "largest possible field" 2" eyepiece would yield, at that focal length, a true field of 46/1500*57.3=1.76 degrees.
It's not recommended to use a 40mm eyepiece (or longer) with an f/5 scope because:

---
---
---


ZOW. Your post has really interested me.
I see in your "true FOV" calculation, you are using the field stop diameter of a TV eyepiece.

According to Explore Scientific (http://www.exploresc...ee_series1.html)
Their 40mm 68Deg EP has a Field Stop Dia of 52.2mm.

Using your calculation: 52.2/1500*57.3=1.99 degrees.
Since 52.2mm is MORE than 2", I am intrigued.

HOW do they get more than 2" of field stop in a 2" eyepiece?

Would I get 1.99 degree FOV with that eyepiece?
This exceeds the theoretical limit of the scope, since it's based on 2".

I understand the problems with viewing that can crop up with an oversized exit pupil.
Would this one (8.16mm) be likely to have big problems?

I am eagerly anticipating some lively technical discussion.

#19 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 23428
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 31 January 2013 - 08:57 PM

A 12" f/5 scope has a focal length of 1524mm. The Orion is nominally 1500mm.
A "largest possible field" 2" eyepiece would yield, at that focal length, a true field of 46/1500*57.3=1.76 degrees.
It's not recommended to use a 40mm eyepiece (or longer) with an f/5 scope because:

---
---
---


ZOW. Your post has really interested me.
I see in your "true FOV" calculation, you are using the field stop diameter of a TV eyepiece.

According to Explore Scientific (http://www.exploresc...ee_series1.html)
Their 40mm 68Deg EP has a Field Stop Dia of 52.2mm.

Using your calculation: 52.2/1500*57.3=1.99 degrees.
Since 52.2mm is MORE than 2", I am intrigued.

HOW do they get more than 2" of field stop in a 2" eyepiece?

Would I get 1.99 degree FOV with that eyepiece?
This exceeds the theoretical limit of the scope, since it's based on 2".

I understand the problems with viewing that can crop up with an oversized exit pupil.
Would this one (8.16mm) be likely to have big problems?

I am eagerly anticipating some lively technical discussion.

1) I used the typically largest possible field stop in a 2" eyepiece--a hypothetical largest field, like a 41 Panoptic or a 55 Plossl.
2) the 40mm 68 degree ES has a field stop of 45.3mm. That 52.2 is not possible in a 2" eyepiece. 52.2 sounds more like their 3" eyepiece coming out, but even it is smaller than 52.2 because of its 30mm focal length. It's a typo.
3)As to whether an exit pupil of 40/4.9=8.16mm is going to show all those issues, I can only say "Maybe." It depends how large your pupils get. It depends how sensitive you are to vignetting. It depends how much astigmatism your eye has. And it depends how bright your skies are.
But, as I've discovered, pursuing that last possible few minutes of field isn't really important if the field you do get is large enough. I went from a 1.5 degree field with the 31 Nagler to a 1.3 degree field when I added a Paracorr, but it wasn't ever noticeable since so much more of the field was "usable" when I added the Paracorr.

EDIT: 52.2mm is the field stop of the 30mm 100 degree 3" eyepiece they're bringing out. The 40mm has a field stop of 46mm.

#20 Sean Puett

Sean Puett

    Vanguard

  • *****
  • Posts: 2413
  • Joined: 06 Sep 2010
  • Loc: always cloudy, washington

Posted 31 January 2013 - 11:41 PM

Not trying to monkey wrench your plan but, another size to consider is 10". You get the same fov as an 8" with better light grasp. That being said, my 12" scope is part of the family. It is a vast improvement over an 8" in light gathering but, it is also twice as heavy (most of the time).

I own the 30mm es82 and it is a wonderful eyepiece that my scopes have to fight over when u have them both set up. It works great with a paracorr at f5 and is very usable at f5 without one if you aren't spoiled yet. I think it is a steal at $250.

#21 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • Posts: 914
  • Joined: 22 Dec 2006
  • Loc: Plymouth, Minnesota, USA

Posted 01 February 2013 - 10:11 AM

How do you measure your pupils?

I took a set of drill bits into a room that was almost totally dark. I held the shafts close in front of my eye until I found the smallest one that completely blocked a crack of light. Turned out to be 13/64 or about 5 mm.

Sky & Telescope used to sell a pupil gauge (I have one). This gave the same result. Telescope and eyepiece manufacturers should give away pupil gauges as promotional items.

#22 Thomas Karpf

Thomas Karpf

    Surveyor 1

  • -----
  • Posts: 1781
  • Joined: 09 Feb 2009
  • Loc: Newington, CT

Posted 01 February 2013 - 11:17 AM

How do you measure your pupils?

I took a set of drill bits into a room that was almost totally dark. I held the shafts close in front of my eye until I found the smallest one that completely blocked a crack of light. Turned out to be 13/64 or about 5 mm.

Sky & Telescope used to sell a pupil gauge (I have one). This gave the same result. Telescope and eyepiece manufacturers should give away pupil gauges as promotional items.


Very slick. Thanks for the information.

#23 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 07 February 2013 - 05:13 PM

Not familiar with the Olivon, but that ES82* 30mm (and the 24mm) are absolutely fantastic in my 10XTi. I will admit the 30mm FOV is so large I can't take it all in, I have to move my head around. Its not like looking out a porthole, it's like getting thrown out the airlock door....but beautiful sharp to the edge views. A Paracorr will probably be a must with the 12g


So this "moving the head around", is that due to an oversized exit pupil, or is it something else?

#24 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 07 February 2013 - 05:23 PM

Does your eye's pupil open to 7mm? Unlikely if you're over 40.
Your new scope is about f/5. You might see coma with a very wide field eyepiece. But that can be fixed if you do.
Nonetheless, my comment is that, for that scope, a 30-31mm ultrawidefield would be a superior low power eyepiece and have a field of view a little LARGER than the 35 Panoptic. An eyepiece like, say, the 30mm 82 degree ES or something similar.
That would have a wider field than any you mentioned.
And certain large DSOs would just be perfect in that large FOV.


The ES82 30mm IS a very attractive EP.

On the XX12, that would give FOV 1.6, exit pupil of 6.1mm.
The ES82 24mm would be FOV 1.3, exit pupil 4.9mm

Do you think I'd be safe with the 6.1mm exit pupil, or would I be taking a chance of being unhappy?

#25 HenryV1598

HenryV1598

    Sputnik

  • -----
  • Posts: 40
  • Joined: 01 Sep 2010
  • Loc: Houston

Posted 07 February 2013 - 05:24 PM

How do you measure your pupils?


If you're a teacher, you stand them against a wall and get a tape measure....

Ok, stupid joke over.

I read this nifty trick a year or two ago: Get a metric set of allen wrenches, if you don't have them in your kit already (and don't go to home despot to buy them, because it seems no one there knows what an allen wrench is... sigh). Go out to your favorite dark location and hold them up close to your eye, one at a time in increasing order, looking at the flat-edge. Pick a star (probably best with one of moderate brightness, but not too bright) and block out the light with the wrench. With those smaller than your pupil, you should be able to see around the wrench. Once you can't see the star without moving your head around, you should have reached roughly the size of your exit pupil.

I'm sure there's better and more accurate ways, but this is probably your best quick and easy method.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics