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Equipment Discussions >> Refractors

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faackanders2
Carpal Tunnel


Reged: 03/28/11

widest TFOV refractor
      #5591959 - 12/27/12 11:14 PM

What is the widest TFOV refractor, and how would it compare to binoculars of same aperture?

What is the main benefit of a refractor over relectors. Is it no central blockage?

Note all my telescopes are dobsonian newtonian reflectors. I do have many binoculars also for wide FOVs.


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dlpville
member


Reged: 07/09/07

Re: widest TFOV refractor new [Re: faackanders2]
      #5591983 - 12/27/12 11:42 PM

I can't be sure, but I would say off-hand that the Celestron (Skywatcher)150mm f/5 would have the widest TFOV. Here's a sample, using an Explore Scientific 14mm 82* EP:
Focal Length (mm): 14
Apparent Field (deg.): 82
Magnification: 54
True Field: 1.53
Exit Pupil: 2.8 mm

I can't say what the comparison with a binocular of the same aperture would be, but who would use a 6" Bino?

The major point in using a refractor is the fact there's no central obstruction. Therefore, a great deal of energy is found in the central Airy disk, so that contrast is significantly increased over a Newtonian reflector.

I do personally observe with one, the views are wonderful for open and globular clusters, close galaxies, and none-too-subtle nebulae. If you're looking for planetary detail or high magnification, f/5 is not the way to go. Star party attendees seem to love the looks, too.
davel


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Hermie
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Reged: 04/20/05

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Re: widest TFOV refractor new [Re: faackanders2]
      #5591991 - 12/27/12 11:52 PM

The widest TFOV refractor is that which has the shortest focal length. The TV 101 comes to mind as a standout in the 4" range, but a 60 or 80mm refractor can give wider views. Binoculars offer wider views than comparable refractors, but are limited to lower powers.

The main advantages of refractors are:

flexibility - low, wide views to high power planetary.
hassle free - rarely need collimation, minimal thermal issues
high contrast, "pure" view

The disadvantages of refractors are:

cost - which is why they are usually relatively small aperture
color - if you don't choose an apochromatic "apo"

Compared to your dobs, the "hassle-free" advantage is most significant.

Hermie


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FirstSightModerator
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Re: widest TFOV refractor new [Re: Hermie]
      #5592017 - 12/28/12 12:13 AM

A Televue NP-101 (f#5.4) has a TFOV with either a 41mm Panoptic or 55mm Plossl of 4.9 degrees, and a TFOV of 4.5 degrees with a 31T5 Nagler. The other advantage of this particular scope/these eyepieces is the "flat" rather than fishlens-curved appearance across the field of view.

HOWEVER, my Stellarvue SV-60 finderscope, which is #f#3.7, produces a TFOV of 7 degrees with a 24mm Panoptic. The SV-60's focuser can only accept 1.25" eyepieces and the 24 Pan has the maximum-sized potential field stop for a 1.25" eyepiece, so it's doubtful any wider TFOV is possible with this particular scope with any other eyepiece. Even though the SV60 intended mainly for use as a finderscope, it is a true full-fledged compact refractor capable of independent grab n'go use with an appropriate mount & tripod, capable of accepting any 1.25" format eyepiece. While it might in principle be possible to construct a full-fledged refractor with an even more extreme short focal-ratio and wider TFOV, the SV60 is the most extreme functional example I know of.

Edited by FirstSight (12/28/12 12:19 AM)


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beatlejuice
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Reged: 04/05/11

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Re: widest TFOV refractor new [Re: faackanders2]
      #5592044 - 12/28/12 12:48 AM

How about an AT72 with a 41mm Pan for a FOV of about 6.5 degrees.

Eric


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: widest TFOV refractor new [Re: Hermie]
      #5592217 - 12/28/12 07:33 AM

The NP-101 with a 55mm eyepiece is at best a 70mm scope. Better to use the 41 mm Panoptic which provides the same 4.9 degree TFoV but with a more reasonable exit pupil.

As far the widest TFoV, this is focal length and focuser dependent which in turn becomes a function of aperture. One also has to consider the quality of the view. At f/4 the field curvature combined with the astigmatism from the eyepiece will get pretty messy. I consider F/5 to be as fast as is reasonable. The NP101 is an exceptional 4 inch but my vote for the widest practical field of view goes to a 80mm f/5 fit with a 2 inch focuser. With the 31mm Nagler its a true 6.0 degree field of view and with the right eyepiece it can be about 6.6 degrees.

Currently my 80mm f/5 is an iOptron but I am working on a visual f/5 focal reducer for my Williams Optics 80 mm f/6.9 FD. So far the results look promising and the current design seems to flatten the field as well. This particular scope is well suited for a focal reducer because it uses a 2 inch extension which can be removed to make up for the inward focuser travel a focal reducer requires.

Jon


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Jobryant
sage


Reged: 01/13/09

Re: widest TFOV refractor new [Re: Jon Isaacs]
      #5592334 - 12/28/12 09:23 AM

I'm pretty sure that the 60mm SV finderscope that FirstSight mentioned can be had with a 2" focuser so this might be the widest.

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Jon Isaacs
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Re: widest TFOV refractor new [Re: Jobryant]
      #5592368 - 12/28/12 09:58 AM

Quote:

I'm pretty sure that the 60mm SV finderscope that FirstSight mentioned can be had with a 2" focuser so this might be the widest.




There is no one answer to the question of the widest possible field of view. Shorter and shorter focal lengths provide wider and wider fields of view. I have kludged together a 35 mm binocular objective and cardboard tube and an eyepiece. It has a focal length of about 130mm and with a 31mm Nagler it theoretically provides a 17 degree TFOV at 4.2x with a somewhat oversized 8.2 mm exit pupil. With a 1.25 inch eyepiece, 12 degrees is possible.

But the view is not pretty and I disregard f/3.7 finders and binocular objectives as being acceptable telescopes. My 80mm UO finder has a focal length of 300mm and could provide an 8 degree Tfov but its messy at 5 degrees with well corrected eyepieces.

Note that a 60mm f/3.7 objective has a focal length of about 8inches and the optical path length of a 2 inch diagonal is 4 inches so there is not room for a real focuser but it's not hard to focus just slipping the eyepiece.

Jon


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Jobryant
sage


Reged: 01/13/09

Re: widest TFOV refractor new [Re: Jon Isaacs]
      #5592433 - 12/28/12 10:47 AM

The widest or I should say fastest objective I've ever seen is the 155mm (6.1") diameter air spaced achromatic triplet objective with a focal length of 195mm (f/1.25) that is sold on surplus shed. One could make a extremely fast refractor with it.

The Tak FS60 in conjunction with the Tak 2" adapter is also a good wide TFOV refractor. Borg also has some pretty wide refractors as well.


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FirstSightModerator
Duke of Deneb
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Re: widest TFOV refractor new [Re: Jon Isaacs]
      #5592561 - 12/28/12 11:57 AM

Quote:


But the view is not pretty and I disregard f/3.7 finders and binocular objectives as being acceptable telescopes. My 80mm UO finder has a focal length of 300mm and could provide an 8 degree Tfov but its messy at 5 degrees with well corrected eyepieces.




The key qualification for the SV-60's usefulness as a full-fledged achro refractor is NOT that it is primarily marketed for use as a "finderscope", but rather that it's f/3.7 objective demands very well-corrected (premium) eyepieces to produce a decent off-axis image, especially to go beyond 50 deg AFOV. However, with a 24mm Panoptic at a dark-sky site, an SV-60 provides an astonishingly worthwhile panoramic widefield view in its own right, and I've also looked at the view through the SV-60 with a 16T5 Nagler which is also extremely impressive.

SOMEONE ABOVE commented that the SV-60 is capable of being configured with a 2" focuser, which unfortunately I doubt is true. Even with the 1.25" focuser, a 24 Pan @27mm field stop will vignette very slightly to just below a true 68 deg AFOV, the tube connection for the diagonal on the SV60 is also only large enough for 1.25" diagonals, and the included focuser is helical (built into the included diagonal), so I'm not sure exactly how you'd attach any replacement 2" focuser even if it would fit.

THAT SAID, there's a huge difference between declaring the SV60 as too fast f# + too diminutive to suit one's tastes as a general-purpose refractor rather than just a finderscope, and validly declaring it as generally unsuitable for anything but finderscope use. I agree that I'm not inclined or tempted myself to detach it from my 12" dob and mount it as an independent scope, but others apparantly have done exactly that with the SV60 precisely because it is so conveniently compact and portable.


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John Rhodes
Vendor (Televue Rep)
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Reged: 02/21/06

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Re: widest TFOV refractor new [Re: Jobryant]
      #5592908 - 12/28/12 04:03 PM

Quote:

I'm pretty sure that the 60mm SV finderscope that FirstSight mentioned can be had with a 2" focuser so this might be the widest.




The TV 60"is" is a 360mm F/6 with a 2 in. dual speed focuser 8:1 and comes with a field flattener for imaging.
A true 7.32 FOV with the 41 Pan.


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Traveler
Pooh-Bah


Reged: 08/19/07

Loc: The Netherlands
Re: widest TFOV refractor new [Re: John Rhodes]
      #5593030 - 12/28/12 05:50 PM

Wide and picture perfect views: Takahashi FS-60CB.

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Mark9473
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Reged: 07/21/05

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Re: widest TFOV refractor new [Re: faackanders2]
      #5593086 - 12/28/12 06:27 PM

Quote:

What is the widest TFOV refractor, and how would it compare to binoculars of same aperture?




If they have the same aperture, the binocular's view will be noticeably brighter because you're using both eyes. It will also be brighter than using a binoviewer on the refractor.

If you use a high quality binocular, the weakest link in your viewing system will be your eyeballs, in particular the optical correction of your eye lenses at their wide open setting. This also affects the view through the refractor if it operates at the same exit pupil, largely eliminating the benefit offered by its sharper optics.

My personal view on the question is that a high quality binocular will have the benefit of a brighter image with better contrast (speaking of the same low magnification and equally large exit pupil in both instruments). The refractor will have the advantage of being able to use highly corrected large-AFOV eyepieces so you can reach a wide TFOV view at a smaller exit pupil, reducing the influence of eye aberrations (i.e. a sharper view), but at the expense of brightness. But the increased magnification would also show more detail, which is an important advantage.

So it would seem for a given TFOV the choice is between image brightness (the binocular) and sharpness + detail (the refractor). On this basis alone, I'd pick the refractor. In practice however, the handling and ergonomics of the two instruments are so completely different that this cannot be ignored.


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oldtimer
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Reged: 11/13/08

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Re: widest TFOV refractor new [Re: faackanders2]
      #5593129 - 12/28/12 06:51 PM

Another very important aspect of an RFT is the size of the exit pupil it will produce with varing eyepieces. If you live in a light polluted area where your eyes can never truely get 'dark sky adapted' you may have to set the upper limit at 5mmn. Also if your on the high side of say 50 years old, chances are your eyes are not going to open to 7mm or even 6mm anymore.

One more thing, Short focal length refractors (like F-5)require the very good (can you say Nagler type) eyepieces to get good edge definttion.

Since I view from a red zone and am 67 years old my current favorite RFT/eyepiece combination is: a 4" F5 achro with a 23mm Celestron Luminos yielding 21,74X, a 3.78 TFOV with a 4.6mm exit pupil.


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faackanders2
Carpal Tunnel


Reged: 03/28/11

Re: widest TFOV refractor new [Re: Jobryant]
      #5593217 - 12/28/12 07:42 PM

Quote:

The widest or I should say fastest objective I've ever seen is the 155mm (6.1") diameter air spaced achromatic triplet objective with a focal length of 195mm (f/1.25) that is sold on surplus shed. One could make a extremely fast refractor with it.

The Tak FS60 in conjunction with the Tak 2" adapter is also a good wide TFOV refractor. Borg also has some pretty wide refractors as well.




f1.25??? What type of coma corrector does it use?


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faackanders2
Carpal Tunnel


Reged: 03/28/11

Re: widest TFOV refractor new [Re: Mark9473]
      #5593236 - 12/28/12 07:55 PM

Quote:

Quote:

What is the widest TFOV refractor, and how would it compare to binoculars of same aperture?




If they have the same aperture, the binocular's view will be noticeably brighter because you're using both eyes. It will also be brighter than using a binoviewer on the refractor.

If you use a high quality binocular, the weakest link in your viewing system will be your eyeballs, in particular the optical correction of your eye lenses at their wide open setting. This also affects the view through the refractor if it operates at the same exit pupil, largely eliminating the benefit offered by its sharper optics.

My personal view on the question is that a high quality binocular will have the benefit of a brighter image with better contrast (speaking of the same low magnification and equally large exit pupil in both instruments). The refractor will have the advantage of being able to use highly corrected large-AFOV eyepieces so you can reach a wide TFOV view at a smaller exit pupil, reducing the influence of eye aberrations (i.e. a sharper view), but at the expense of brightness. But the increased magnification would also show more detail, which is an important advantage.

So it would seem for a given TFOV the choice is between image brightness (the binocular) and sharpness + detail (the refractor). On this basis alone, I'd pick the refractor. In practice however, the handling and ergonomics of the two instruments are so completely different that this cannot be ignored.




Good comparison. I have contemplated 150mm 45deg or 90deg binos but these are expensive. I have 25x100 binos that are straight through and hence can only observe between horizon and 70 deg above. From the responses so far, it appears that more binos are made for wider views than refractor telescopes (which may be optimized for higher power and detail), excluding finder scopes and possibly RFT imaging scopes.


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BarrySimon615
Pa Bear
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Reged: 03/01/04

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Re: widest TFOV refractor new [Re: faackanders2]
      #5593340 - 12/28/12 08:59 PM Attachment (23 downloads)

How low (and wide) can you go? Brings to mind the old "Limbo" song.

As others have said, you can get very low powers and wide fields of view if you reduce focal length and have the capability of using a 2" (or larger) focuser that does not obstruct the light path either with or without a diagonal.

I have a nice Zeiss (West Germany) copy lens scope that I have fitted with a 2" focuser and I can use it with a 2" diagonal. One of my monster eyepieces is a Jaegers 38mm in a specially machined adapter. The clear aperture of the field lens is 48 mm and the focal length of the objective (54mm aperture) is 300 mm. So I have a 54 mm f/5.6 telescope giving me 7.89x (6.84 mm exit pupil) with a 9.17 degree field. Edge correction is about equivalent to what I see in my Fujinon FMT-SX 7x50 binoculars (and that is pretty good). The true advantage of this setup is the eye lens of this eyepiece is huge! You literally fall into "space". Plus the scope was cheap to make.

Barry Simon


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Jon Isaacs
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Re: widest TFOV refractor new [Re: BarrySimon615]
      #5593780 - 12/29/12 04:03 AM

After reading this thread, last night I realized I had all the parts to put together a 42mm f/3.3 achromat with a 2 inch focuser. It's a 42 mm Meade binocular objective that just fit the front of a 2 inch Everbright diagonal. With the 20 mm Nagler it provides an 11 degree TFOV at 7x with a 6mm exit pupil.

It worked but the field curvature resulted in very messy views. With the 31 mm Nagler it would have been about 17 degrees with a 8+mm exit pupil at 4.5 x but it would not come to focus, luckily... The radius of the field curvature would have equalled the diameter of the field stop.


It occurred to me that for such a short focal length, one should start with a camera lens because it will have a built in field flattener. It also was apparent that binoculars are much better suited for these low power views. Both eyes makes a big difference in what one sees and the eyepieces can be matched to try to address field curvature.

Jon


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StarStuff1
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Re: widest TFOV refractor new [Re: Jon Isaacs]
      #5593794 - 12/29/12 04:47 AM

If a binocular type instrument is simply two "refractors" strapped together then my 2.4x40 Russian gallieans with a 27 afov would not be too shabby.

But, more along the line of the OP, I have a 50mm f/4 decent quality achro (bino objective) that can do 5X with a 2-in 40mm eyepiece resulting in a 14 afov. Obviously the exit pupil is 10mm but what did Uncle Al say...

Incidently, this objective also can provide decent images above 40X.


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faackanders2
Carpal Tunnel


Reged: 03/28/11

Re: widest TFOV refractor new [Re: StarStuff1]
      #5594081 - 12/29/12 10:24 AM

Quote:

If a binocular type instrument is simply two "refractors" strapped together then my 2.4x40 Russian gallieans with a 27 afov would not be too shabby.

But, more along the line of the OP, I have a 50mm f/4 decent quality achro (bino objective) that can do 5X with a 2-in 40mm eyepiece resulting in a 14 afov. Obviously the exit pupil is 10mm but what did Uncle Al say...

Incidently, this objective also can provide decent images above 40X.




I also have 2.3x40mm "blue planet optics" opera glasses, and other UW binos; but I was asking about refractor scopes which I know little about.


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