45 replies to this topic

[Tony Cifani]

You really think it's wise for a new amateur astronomer to roll out a 6mm ortho with a non trackable dob with so many good and inexpensive 60-82º widefield options? Sounds a bit harsh lol.

I agree with this in general, although my first two eyepiece purchases were Plossls. My second two were old volcano top orthos purchased here on the CN classifieds, a 7mm and 18mm, which I still use. The 7mm is excellent for planets and doubles, and both orthos are very sharp to the edge (in an F/8 scope and not bad in an F/5), but yes, narrow AFOV. More recently, I started buying Astro-Tech/Starguider Dual EDs. In regards to the limited eye relief of the 7mm ortho, I don't wear glasses.

[Procyon]

Having said that, everyone should try Orthos at least once, even newcomers. I just thought they'd enjoy them more later on....with a bit more experience.

Edited by Procyon, 22 May 2025 - 01:19 PM.

[Oldfracguy]

Having said that, everyone should try Orthos at least once, even newcomers. I just thought they'd enjoy them more later on....with more experience.

I use KK Fujiyama HD-OR Orthoscopics in my f/6 Newtonian and f/6 refractors when I want the sharpest image possible at higher magnifications, at the expense of a wider overall field of view.  I'll use them to look at double stars, the planets and the Moon, even "Barlowed" in an ES 3x Focal Extender, if the seeing permits.  I mainly use the 4, 5 and 6mm focal length models with these f/6 scopes, and reserve the 7, 9 and 12.5mm Orthoscopics for scopes with longer focal lengths and higher focal ratios.  A couple years ago I did try the 18 and 25mm Fujiyamas, but found that they exhibit that "blackout" effect similar to a 1.25" 40mm Plossl.  Above an Exit Pupil of about 1 mm (25x per inch, or 1x per millimeter of aperture), I switch to other types of eyepieces with more forgiving Eye Relief and wider AFOV at the medium to lower magnifications.  

Edited by Oldfracguy, 22 May 2025 - 11:25 AM.

[PKDfan]

I recently got an 8mm TV Plössl that is so crisp it seemingly cuts the object into tiny pieces of excaliber level sharpness. Eye relief is Just enough and i'd suggest that focal length as a first step into minimal glass eyepieces rather than a shorter focal length ortho or Plössl.

It does 150X in the OP's scope. A very versatile power for a new observer with a decent 1.333mm exit pupil.

I'd never suggest a shorter ortho or Plössl than 8mm to a beginner as 8mm will be rather difficult to track by hand as it is, but its not impossible.

With enough power to easily see fine details a 8Pløssl has been a revelation for me, but its only 50X in my small F/6.45 apo, nevertheless A11 trio of craters was an easy peasy catch. Eye relief is Just sufficient. I can't imagine a 6mm Pløssl.

7mm ortho has slightly better ergonomics as its design has slightly better relief than a Plössl.

I suggest saving up for one very carefully selected eyepiece of high quality and for an 8inch F/6 scope a 12mm is 100X. An all purpose power. 2mm exit pupil maximizes the eyes acuity. 24mm a perfect 50X.

Read all the reviews you can and then dive into your choice du jour confidently.

If many others like it then you probably will to.

Good Luck !!


Lance
CSS

[Procyon]

6mm ER huh. Brave lol. Under 10mm ER I need to do this:

 

 

Tom

Edited by Procyon, 22 May 2025 - 06:30 PM.

[davidgmd]

I manage to successfully use a 6 mm Plossl (4.2 mm ER) without suffering a corneal abrasion or gumming up the eye lens with eyelash oil. But I wouldn’t recommend it to a beginner.

[T1R2]

tasco made men outa kids early in life...they supplied a SR5mm (160x whoah!) and a 12.5mm with their 60x 800mm scopes, I learned at an early age of 11 how to use them, now I'm not a whimp anymore and can punch a big kid in the eye on the playground and take his lunch money as bodyguard payment...lol

[joeser]

Whatever eyepiece(s) you choose, especially ones that will give you the medium-to-lower magnifications, be sure that they will work well at your scope's f/6 focal ratio. The performance of certain types of eyepieces starts to degrade at the lower ("faster") focal ratios compared with how they perform at the higher ("slower") focal ratios.

I struggle with understanding what type(s) of eyepiece(s) work well or do not work well with an f/6 focal ratio. Can someone explain that for me? I am new to all of this as well.

[Jay_Reynolds_Freeman]

I struggle with understanding what type(s) of eyepiece(s) work well or do not work well with an f/6 focal ratio. Can someone explain that for me? I am new to all of this as well.

It is a matter of eyepiece design, and is connected with the history of astronomical telescopes.

 

Note that a telescope eyepiece is really not much more than a glorified magnifying glass, whose purpose is to magnify the focal-plane image so that you can view it.

 

Now consider the image of a star in the focal plane of your telescope. The light from that star approaches the focal plane in a converging cone, with the wide end of the cone being the objective itself and the point of the cone being the image of the star. For long focal-ratio telescopes, the cone is narrower -- more pointy -- than for short focal-ratio units. As the light continues past the image, it forms a diverging cone, whose narrowness or pointyness is exactly the same as for the inbound cone.

 

Thus when the diverging light cone reaches the eyepiece, which is at a certain distance from the focal plane, the width of the cone there depends on the focal ratio of the telescope: In a short focal-ratio telescopes, the bundle of light from a star that reaches the eyepiece will be wider than for long focal-ratio telescopes. For the eyepiece to work well, it must be capable of dealing with the full width of that bundle of light, and I suspect it will make sense to you that any given eyepiece has a harder time dealing with the wide bundle of light, from a short focal-ratio telescope, than with the narrow bundle of light from a long focal-ratio telescope: A wide bundle of light goes through a wider chunk of the glass of the eyepiece than does the corresponding narrow bundle of light, and the optical designer must work harder to provide good correction across the wider portion of the glass. (Note also, that there must be reasonable correction not only for just one bundle of light -- one stellar image -- but also all across the entire usable field of view of the eyepiece.)

 

Providing better eyepiece correction generally requires more complexity of design, typically featuring some combination of more lenses, fancier glass types, or both, which makes such eyepieces more expensive. If the eyepiece in question is supposed to have a wide angular field of view, it will already be more complicated -- and more expensive -- than a narrow-field-of-view unit, and making it work at fast focal ratios as well only makes the designer's task worse.

 

Early astronomical telescopes generally had focal ratios that varied from fairly long to outrageously long, so eyepieces could be of relatively simple designs and still work properly. That was just as well, because fancy glass types and clever techniques for fabricating complex lens assemblies were not available in the old days. Some of the early eyepiece types are still available today -- they include Huygens, Ramsden and perhaps Kellner eyepieces (Kellners are a bit more recent, but still a simple design). They have relatively narrow apparent fields of view, and still work well with long focal-ratio telescopes. More recent telescopes have tended toward shorter focal ratios, and visual observers have become more interested in wide-field views of many kinds of objects. It follows that more recent eyepiece designs have tended toward wider fields of view, or good quality images at fast focal ratios, or both, and thus have become increasingly complicated and increasingly expensive.

 

I hope that helps.

 

 

Clear sky ...
 

[luxo II]

More specifically the issue is the curvature of the focal plane (field curvature) of the eyepiece, vs that of the scope - particularly with

- small refractors with focal lengths under 1000mm,

- newtonians with focal lengths under 1200mm,

- SCT's 5" - 8" aperture.

 

For refractors (doublets and triplets) and SCTs the focal plane is concave towards the sky.

For newtonians it is convex towards the sky.

Flat-field scopes designed specifically for imaging have a (fairly) flat-field and fir these, flat-field eyepieces are the best match.

 

Eyepieces also have field curvature and if you choose one that has similar curvature to that of the scope, NIRVANA !

 

But if you choose an eyepiece with field curvature opposite to that of the scope you will be unhappy.

 

The possible combinations are:

 

1. (( = nirvana 

2. )) = nirvana

3. || = nirvana

4. () = horrible

5. )( = horrible

6. |( = OK but not perfect

7. |) = OK but not perfect

8. )| = OK but not perfect

9. (| = OK but not perfect

 

Legend:

Left is the eyepiece, right is the scope, sky to the right

( = concave to sky, ) = convex to sky, | = flat field

 

Notes:

 

1. Erfles and derivative designs like SWA match short refractors (see 1 above). But they are horrid with Newtonians.

2. Plossls play nicely with small Newtonians, not so well with short refractors (see 2 above).

3. APM Ultra-flats are perfect with flat-field refractors and Rumaks (see 3 above).Good (OK but not perfect) with any other scope (see 7, 8, 9 above)

4. Televues are designed to suit their primary market in the US - which is big fast Newtonians (see 2 above).

5. Japanese eyepieces (Nikon, Vixen, Pentax) are mainly designed to suit their primary market in Japan, which is small refractors (see 1 above).

 

Lastly, while the field-curvature of various telescope designs are well known (see https://www.telescope-optics.net), with very few exceptions eyepiece manufacturers do not disclose  the characteristics of eyepieces, so it is a guessing-game to extract more money from your pocket. The exception being the APM ultra-flats which work extremely well with flat-field scopes designed for imaging (drum roll, boom-tish).

Edited by luxo II, 29 May 2025 - 10:09 PM.

[rdjamieson]

Plenty of good advice here, and you should pay particular attention to eyepiece mavens like Starman1 and JonIsaacs, both of whom I believe are Dob owners.  You will find that most (but certainly not all) CNers hold the following eyepiece lines in high regard: Televue Ethos and Nagler, Baader Morpheus, Pentax XW, and Nikon NAV-SW/HW.  The problem is that these EPs can cost more than a car payment, and I'm not talking about a Nissan Versa.  IMHO, Astro-Tech (house brand of CN's host, Astronomics) is peerless in terms of value.  The XWA line equals the sharpness of most of the royalty brands (OK, not the Ethos), has the same amazing 100-degree AFOV as the Ethos, and every eyepiece in the line comes in at less than $300.  If ~$280 is too much of a reach given your $200 budget, then consider the Astro-Tech UWA line.  It has the same 82-degree AFOV as the Televue Naglers (which for many years were the widest field eypieces you could buy), CN members routinely rave about them, and they can be had new for $125 for the 1.25" sizes, and $225 for the 2".  

 

If you are looking for something that will excite your nascent passion for the hobby, grabbing an eyepiece with at least an 82-degree AFOV is the way to go.  The difference between a plossl and a widefield will astonish you--it's the difference between reading about it and being there. 

Edited by rdjamieson, 29 May 2025 - 09:56 PM.

[Oldfracguy]

I struggle with understanding what type(s) of eyepiece(s) work well or do not work well with an f/6 focal ratio. Can someone explain that for me? I am new to all of this as well.

I saved this post from years ago by Don Pensack, aka Starman1 here on CN.  He operated an eyepiece and other visual accessories business for many years, and retired last year.  He still posts a lot on CN, and provides his outstanding Eyepiece Excel Spreadsheet that is available in the Eyepiece Forum in the Equipment section of CN. It explains a lot of what you are wondering about:

 

--------------------------------

Eyepiece Tutorial from Don Pensack, “Starman1” on CN

Vendors
Posts: 55,075
Joined: 24 Jun 2003
Posted 17 September 2013 - 11:27 PM

 

It is because all eyepieces perform best when the light rays entering the bottom are parallel. To correct the eyepiece for all aberrations and yet have the rays enter the bottom at greater and greater oblique angles requires additional lenses or a different design than if the eyepiece is not designed for those oblique rays.

 

The light rays from the edges of an f/10 scope enter the eyepiece closer to parallel than those from the edges of an f/5 scope. So, many older designs which were designed back in the eras when f/12 was considered a "short f/ratio" scope do poorly on today's shorter f/ratio scopes.

 

The most common problems induced by using an eyepiece below its "critical f/ratio" are astigmatism and later chromatic aberration [there are other potential problems, but these are most easily noticed]. That would be the case at high powers as well as low.

If the eyepiece is designed to handle the f/4 light cone without added aberrations, it will also do well at f/12. The reverse is not necessarily the case.

 

Many simpler designs (Huygens, Ramsden, Kellner, Plossl, Abbe orthoscopic, Erfle, Brandon, Konig) were not designed to handle the short f/ratio light cone. They certainly can be modified to do so, and some modern derivations have been so designed. But, in general, if you want a widefield eyepiece design, and you want it well corrected to f/4 (or faster), it will have more internal lenses or be a more complex design and that will add to the cost.

 

If the apparent field is kept narrow, the corrections to the simpler designs may not add much money, if any. It is when the apparent field gets wide and you want the ability to handle the shorter f/ratios that the eyepieces get expensive.

 

One of the paradoxes of modern astronomy is that a high percentage of low-priced scopes are short f/ratio dobsonians, yet most of the inexpensive eyepieces are not well corrected to work with short f/ratios.

 

I would like to see us return to the time when manufacturers stipulated the "Critical F/ratio" of their eyepieces (below which the light cone might induce additional edge aberrations and sometimes unpredictably).

 

Some examples:

Huygens--f/12 (better at f/15)
Ramsden--f/10
Kellner--f/6-8
Abbe Ortho--f/6
Plossl--f/4
Konig I--f/4
Erfle--f/6
Erfle II--f/4
RKE--f/6
Mod.Erfle--f/4
Konig II--f/4
Zeiss Astroplan/Masuyama--f/4
All current TeleVue--f/4

 

Some of us who take for granted the superior correction of more modern types might take issue with the manufacturer's stated minimum f/ratio figures (like a Konig at f/4), but we also have to remember that most of these eyepieces were designed to have narrower apparent fields than modern production pushes them to. The Konig I, for instance, was supposed to be a 55 degree field eyepiece, not the 65 degree fields found on some of the modern production. And Plossls were supposed to be 45-50 degrees, not the 55-60 degree fields found in some recent incarnations. When eyepieces are pushed beyond their designs, it is not surprising to find the edges of the fields aren't as good as they should be.

Don Pensack
http://www.EyepiecesEtc.com

-----------------------------------------------------------

Edited by Oldfracguy, 29 May 2025 - 09:47 PM.

[gwlee]

Found myself with around $200 to spend on upgrades to my beginner setup. I currently have an 8" Dobsonian (f/6) and a UHC nebula filter, but I'm still using the default eyepieces that came with it (Meade 25mm and 10mm Plossl). Since I've heard eyepieces can make a big difference, I think this is where I have the most room to upgrade. 

 

Are there any good recs within my price range? I'm interested in both deep sky and solar system objects, so really any recommendation is welcome. I also realize it's fairly low budget compared to a lot of other folks on here, so if it isn't worth upgrading without spending more, I'd appreciate y'all letting me know. I can maybe spend a little more if there are some strong recommendations.

If cost effective is your goal, add a 2x Barlow to your 25mm and 10mm EPs. 

[Apnee44]

My vote would be 12, 8 and 5 Astrotech Paradigm.

Upgrade your low power when you have more to spend.

Agreed from above, I have used a Dobson 8inch f6 many years, the focal 12, 8, 6/5 will be used every night. For brand/model, I never tried the above one mentioned, but I can read a lot of good reviews about them. As low power, no hurry, to start a simple plossl 32 will be great.

[Elroy]

A long term approach would be browsing through the classifieds here on CN once or twice a day, then learn how to buy and sell. You can land really good deals on eyepieces and then sell if that particular purchase didn't work for you. And keep reading here on CN, the amount of information on astronomy is staggering. A 19mm or 24mm TV Panoptic would be a great get out of the classifieds. IMMHO.

 

good luck to you!

[CowTipton]

My vote would be 12, 8 and 5 Astrotech Paradigm.

Upgrade your low power when you have more to spend.

 

Quoting just to get this out there again.

 

100% my vote as well.

Very good eyepieces, great value too.

[splashout]

But a different path, and probably the best one, is to go ahead and get the AstroTech 7 & 10mm UWAs. These two don't cover as much ground, magnification-wise, because the 7mm is really an 8mm eyepiece, so you're getting 8mm and 10mm eyepieces, but with the huge true field of view they'll put into your eye, you will be quite happy with the result in your 8" dob.

CollinofAlabama, do you know if the AT 7mm XWA is also an 8mm? It seems like a Televue Ethos 8 clone. And is the 20 actually a 21, the 3.8 a 3.7 and so on?

[fjjoachim]

I really liked my Paradigm 25mm, 15mm, and 8mm. I also had a Celestron Utima 2.2X Barlow. These eyepieces gave my f5.9 8 inch Dob 48X, 80X, 106X, 150X, 176X, 330X.  

[Jon Isaacs]

 

 

More specifically the issue is the curvature of the focal plane (field curvature) of the eyepiece, vs that of the scope - particularly with

- small refractors with focal lengths under 1000mm,

- newtonians with focal lengths under 1200mm,

- SCT's 5" - 8" aperture.

 

The curvature of a Newtonian is equal to its focal length. 

 

The curvature of a refractor is about F/3.  

 

The curvature of an 8 inch SCT is about 220 mm. 

 

Field curvature is significant in the refractor and SCT, this thread is about a 8 inch Dob with a focal length of 1200 mm. Field curvature is a non issue for the Dob.

 

Field curvature decreases quickly as the the field of view narrows as magnification is increased.

 

I have a set of Astro-Tech Paradigms to go along with my Naglers, Ethos's, Panoptics and such.

 

The 12 mm, 8 mm and 5 mm Paradigms are good performers and have a reasonable amount of eye relief for non-eye glass wearers.  I enjoy them in my 10 inch F/5 Dob.

 

Jon

[Dave Mitsky]

A 32mm Plössl, a 7-21mm Svbony zoom eyepiece, and a 2x Barlow will give you a variety of magnifications in concert with your present eyepieces for less than $200.

[vahe]

 

 

Are there any good recs within my price range? I'm interested in both deep sky and solar system objects, so really any recommendation is welcome. I also realize it's fairly low budget compared to a lot of other folks on here, so if it isn't worth upgrading without spending more, I'd appreciate y'all letting me know. I can maybe spend a little more if there are some strong recommendations.

 

My two cents;

A pair of DIRT CHEAP Orthoscopics (Tak Starbase Ortho's) along with equally CHEAP binoviewer will outperform most expensive eyepieces used in mono mode on planets.

Try it , you will be surprised.

.

Vahe

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