18/2 = 9

[Richard N]

I have an 18mm eyepiece and a x2 barlow. I have just ordered a 9mm eyepiece. Im hoping that it will be better than the combo. Am I deluding myself?

[scotty38]

I guess it depends. If the 18mm and barlow are Televue and the new 9mm is out of a lucky bag then maybe not. Vice versa then maybe so 🙂

[Louis D]

I agree with @scotty38.  I'd take my 17mm ES-92 Barlowed to 8.5mm any day over my generic 9mm Kellner.

[Zermelo]

To compare oranges with oranges, assume that the two eyepieces are from the same range and have the same internal design. What effect might adding a barlow have (apart from the increase in magnification)?

Well, you're adding a further component into the optical train that typically has between 2 and 4 elements. You will therefore have the light losses (and hence image dimming) associated with absorption within the elements (dependent on the quality and thickness of the glass) and reflection at their surfaces (dependent on the quality of the coatings). You will be introducing aberrations due to the light passing through the extra surfaces (dependent on the accuracy of their figuring and the design decisions/trade-offs made by the manufacturer). You may also be introducing a new source of light scattering and ghosting (minimized by good design, edge blackening, etc.)

That's the case for preferring a shorter eyepiece over the (longer eyepiece + barlow) combination, if comparing like with like. But actually, a decent barlow will do a good job at minimizing those detrimental effects, and there may even be some advantages. In particular, a barlow usually preserves (and can even extend) the eye relief of the eyepiece being used, and for some designs (e.g. plossls) where the relief decreases with focal length, it can mean that the barlowed combination may be significantly more eye-friendly than the single-eyepiece alternative of the same focal length. It can also be true that the barlowed option actually reduces certain aberrations overall (such as field curvature), when compared with a single eyepiece, though this depends on the specifics of the eyepiece, barlow and telescope in which it is being used. There are so many possible permutations that it is difficult to predict in advance which eyepieces will "barlow well" with a particular barlow, though observing reports can be a useful guide.

So it's not clearcut, even when the eyepieces in question are of the same type. And as others have mentioned above, if the 18mm is a premium model, and the barlow is also of decent quality, then it's very possible that the combination might be preferable to a 'lesser' 9mm. Personally, I've experienced both scenarios while I've been experimenting with my kit.

Do post your own conclusions, when you've had chance to do a comparison.

[bomberbaz]

I use a televue powermate with my Nikons, I don't have high powered eyepieces for my taurus as I prefer the combination and utility of the nikons + powermate.

Without trying to sound braggy these are top quality chunks of glass so I expect top performance from them.

I do also use a ES focal extender which I use for my 1.25" EP's which I use with my frac and solar scope and I have been very pleased with results from this ES which is a much cheaper option that the TV PM.

If you have a good focal extender/powermate or similar and link this up with good eyepieces then personally I would be ok with it.

I do have a barlow which is a decent make but I much prefer the focal extender / powermate option due to the difference in the way they work. 

[Peter Drew]

Zooms and many wide field eyepieces are already Barlowed.       🙂

[bomberbaz]

1 minute ago, Peter Drew said:

Zooms and many wide field eyepieces are already Barlowed.       🙂

So the wide field EP's with the 100+ fov are likely barlowed already to help achieve this @Peter Drew

[Peter Drew]

Yes, they are used as a means to achieve wide apparent views with large lens elements despite being relatively short focal lengths.  Simply put, they are long focal length eyepieces incorporating a Barlow element to produce the shorter focal length.     🙂

[Richard N]

The results are in. The comparison was between a Starguider ED 18mm + Astro Essentials 2x Barlow and an Svbony 68 uw 9mm (red bands). And (drum roll...) the difference was... 

...nothing. I have spent a while swaping them on various stars and there is no dicernable difference. The faintest stars were just visible on both eyepieces.  However, I'm likely to keep the Svbony 9mm as it's a lot smaller, lighter and less cumberson than the combo. I did expect to see some difference but even with full imagination engaged I have admitted defeat. Maybe I'm looking at the wrong characteristics? I will try them on some doubles next.

[Zermelo]

25 minutes ago, Richard N said:

The results are in. The comparison was between a Starguider ED 18mm + Astro Essentials 2x Barlow and an Svbony 68 uw 9mm (red bands). And (drum roll...) the difference was... 

...nothing. I have spent a while swaping them on various stars and there is no dicernable difference. The faintest stars were just visible on both eyepieces.  However, I'm likely to keep the Svbony 9mm as it's a lot smaller, lighter and less cumberson than the combo. I did expect to see some difference but even with full imagination engaged I have admitted defeat. Maybe I'm looking at the wrong characteristics? I will try them on some doubles next.

It would be interesting to know the scope you tested these with.
Did you try them on a variety of targets?

[Richard N]

1 hour ago, Zermelo said:

It would be interesting to know the scope you tested these with.
Did you try them on a variety of targets?

ST80 (fast refractor).  I have just come in from a double star session. I rather like the new 9mm eyepiece. 9mm seems something of a sweetspot for this scope. I didn't try a comparison on DSOs. Might that show up more subtle differences?

[Zermelo]

6 minutes ago, Richard N said:

ST80 (fast refractor).  I have just come in from a double star session. I rather like the new 9mm eyepiece. 9mm seems something of a sweetspot for this scope. I didn't try a comparison on DSOs. Might that show up more subtle differences?

Different objects have different characteristics that might favour one or other eyepiece.

For example, contrast isn't very important with double stars, but more so with planets and some DSOs.

 

[bomberbaz]

5 minutes ago, Richard N said:

ST80 (fast refractor).  I have just come in from a double star session. I rather like the new 9mm eyepiece. 9mm seems something of a sweetspot for this scope. I didn't try a comparison on DSOs. Might that show up more subtle differences?

You could try it on faint fuzzies such as planetary nebula to see what the difference is.  Try the larger ones such as the Owl M97, Iris NGC 7023, Dumbell M27 and M81, 82 galaxy pair. They might show you a difference in contrast !

[Louis D]

On 14/11/2021 at 12:21, Richard N said:

The results are in. The comparison was between a Starguider ED 18mm + Astro Essentials 2x Barlow and an Svbony 68 uw 9mm (red bands). And (drum roll...) the difference was... 

...nothing. I have spent a while swaping them on various stars and there is no dicernable difference. The faintest stars were just visible on both eyepieces.  However, I'm likely to keep the Svbony 9mm as it's a lot smaller, lighter and less cumberson than the combo. I did expect to see some difference but even with full imagination engaged I have admitted defeat. Maybe I'm looking at the wrong characteristics? I will try them on some doubles next.

Try them during the daytime to see if SAEP (kidney-beaning) is more of an issue with one than the other.  I've repeatedly read that the 9mm UWA (at least the older gold band version) has strong SAEP that becomes obtrusive with small entrance (iris) pupils.  This can happen during white light solar observing, lunar observing near full moon, and daytime spotting scope usage.  It's the main reason I've never bought the 6mm and 9mm UWAs despite their low price, wide field, decent eye relief, and good field correction because I'm hyper sensitive to SAEP.

Alternatively, I've found that some shorty Barlows can induce SAEP in some longer focal length eyepieces, so the 18mm+Barlow combo might also exhibit SAEP.

[Louis D]

On 13/11/2021 at 14:02, bomberbaz said:

So the wide field EP's with the 100+ fov are likely barlowed already to help achieve this @Peter Drew

Notice in the following eyepiece raytrace diagrams that the field lens group (small diameter group on the right in each diagram) in pretty much all of the UWA and HWA eyepieces (diagrams 20 through 28) diverges the incoming rays just like a Barlow:

[bomberbaz]

@Louis D  27 and 28 both 100 FOV appear to be more like a tele extender arrangement rather than barlow but I get they are using a barlowing effect to improve the FOV. Very clever arrangements and no wonder they are so expensive. 

[Louis D]

24 minutes ago, bomberbaz said:

@Louis D  27 and 28 both 100 FOV appear to be more like a tele extender arrangement rather than barlow but I get they are using a barlowing effect to improve the FOV. Very clever arrangements and no wonder they are so expensive. 

Substitute negative focal length group for Barlow to make the statement more generic.  Even tele extender/magnifiers use an initial negative focal length group.

Notice that even in positive only designs like 19 (Panoptic-like), a negative power element is used for the field element to improve field correction.

[bosun21]

On 12/11/2021 at 14:51, scotty38 said:

and the new 9mm is out of a lucky bag then maybe not.

“ A lucky bag “

Now that does bring back memories