20mm, 15mm, 9mm & 6mm

Click to Enlarge Generic Chinese Wide Angle Long Eye Relief Eyepieces

Introduction

I must commence this review by stating that eyepiece preferences and selection are very personal things.  Everybody’s eyes are different and while reviews such as this one are useful, I recommend that if all possible, you try before you buy.

The comments in this review are merely my personal observations.

This article has been peer reviewed by Darren Wong.

Background

Several years ago, at the start of the quest for wide-angle eyepieces that do not cost a fortune, I acquired a 20mm Generic Asian manufactured wide-angle eyepiece.  I believe this eyepiece is manufactured by Synta and looks remarkably similar to the Orion Expanse series of eyepieces.

I found I preferred the 20mm to my 32mm Celestron plossl due to easier eye placement and more immersive views.  I found it OK to use at F6 although once I had acquired some UO Konigs, they were my preferred option as they were sharper than the 20mm wide angle Synta eyepiece. 
Over the next couple of years I acquired, via the second hand market, the three other eyepieces to complete the set.  Upon reflection these eyepieces are in fact not too bad.  Of course they are not in the same league as the TeleVue or Pentax products.  However, at about $80 new or $50 per eyepiece on the secondhand market, for a new starter or casual observer, or those on a tight budget, they do offer a reasonable alternative to more expensive types.

With some regret, I have now teamed up this eyepiece set with a 114mm Tasco Newtonian I have refurbished for the local astronomical society which will be used as a hire scope for new members.

Availability

These eyepieces appear to be available from various sources under various names.  The Orion Expanse eyepiece set (from the USA) looks very similar and a search of other web sites indicates very similar products being advertised by various suppliers.  The giveaway is the 20/15/9/6mm focal lengths combined with the 66 degree AFoV.  In Australia I have only found one supplier advertising these eyepieces on the web; http://www.telescopeshed.com.au/.

Description

All units are of a consistent, reasonable quality construction.  They come with fold down soft eye guards, the eyepiece body has smooth sides devoid of any form of non-slip grip and there is a safety undercut on the barrel.  The barrel itself appears to be slightly undersize and can be a loose fit in a focuser.

I have not been able to determine the optical design of these eyepieces and I have not dismantled them to find out. However, given the AFoV and observed aberrations I think it is likely they are based on Erfle or Konig designs.  In addition, the 9 and 6mm units are a “Barlowed” design where a Barlow or Smyth lens is mounted in the eyepiece barrel. 

They all come with nice big eye lenses that, based on the color of the coating, appear to be multicoated.  However, it is not clear as to the coating type on the remaining lens groups and I suspect the lens edges are not blackened.

Performance

I have used all four eyepieces in 114mm F7.8 (also stopped down to 100mm/F9) and 200mm F6 Newtonians, as well as my Saxon 70mm f/5 refractor.   I have also tested them using a Celestron Ultima 2X Barlow.

My interest is predominately with deep sky objects, as a result the impressions I have gained with these eyepieces have been heavily influenced by their performance in this area.

On deep sky objects these eyepieces seem to perform quite well.  As well as offering sharpness and contrast comparable to mid quality Plossl types, they offer a much wider AFoV that I found quite helpful when star hopping and while not in the 80 degree “ultra wide” FoV class, they do provide a more “immersive” experience than a Plossl can offer.

Eye Relief

For several years, I have used 1.0 diopter “pharmacy” reading glasses for close work and reading, however, I do not normally use glasses for observing.  For this review, to assess useability for those who have to wear glasses while observing, I specifically used all eyepieces with and without reading classes.  Without glasses all these eyepieces have comfortable eye relief that subjectively feels a little less than the 20mm provided by a Radian eyepiece and appears to be around the 15mm mark. I found with the rubber eye guard rolled down, the 20mm, 9mm and 6mm were easily useable while wearing glasses and I could readily see the field stop with my glasses just clear of the rolled down eye guard.  I noted the 15mm had less eye relief than the others and even when I rested my glasses on the eyepiece I could not quite take in the full FoV without moving my head a little.  I estimate when wearing glasses, I could readily use 90% of the FoV of the 15mm eyepiece without moving my head and 100% for the others.

Field of View

As best as I could measure, all eyepieces appeared to have an apparent field of view as advertised at 66 degrees. The field stop was reasonably distinct in all eyepieces.

Focal Plane

These eyepieces were very close to being parfocal.  The 9mm and 6mm were effectively parfocal and the 20mm and 15mm are within a couple of mm.  I found the actual focal plane position to be in a very similar position to my 32mm Celestron plossl (c1996) eyepiece and as such these eyepieces should not have any trouble reaching focus with most commercial telescopes.

Abberations

Not surprisingly, given the purchase price, all four eyepieces exhibited some optical aberrations.

Astigmatism.

Both the 20mm and 15mm eyepieces demonstrated off axis astigmatism that became more apparent with reducing “F” ratio. At F5 I found the 20mm only marginally acceptable and the 15mm just acceptable, at F6 they were both OK with the 15mm performing a little better than the 20mm, and both were really quite good at F9.  As might be expected, when used with my Ultima Barlow lens, the astigmatism was markedly reduced in both eyepieces. The 9 and 6mm units did show a little image softening towards the edge of the field at F5 but this was minor and for casual observing they could be considered virtually sharp to the edge for F6 ratios and higher.

Field Curvature. 

I was not able to see any obvious field curvature with any of the eyepieces.

Ghosting. 

Both the 20mm and 15mm were “haunted”.  The brighter planets and stars generated ghost images at times.  For casual use I did not find the ghost images particularly intrusive and didn’t “see” them after a while.  I did not see any ghosting in the 9mm or 6mm.

Other Aberrations. 

Both the 9mm and 6 mm exhibited additional interesting aberrations:

9mm. 

After using this eyepiece a few times, I began to notice a faint haze or glow close to the field stop and completely circling the FoV.  When you looked carefully, it appeared to intrude over the outer 10% of the FoV. For casual use I did not find this significant.

6mm. 

When a planet or bright star was in the very outer FoV a series of bright reflections would appear across the FoV.  These reflections rapidly disappeared as the object moved to the inner 50% of the FoV.  In its unmodified form I found this eyepiece very difficult to use on the moon due general glare across the FoV that I initially assumed was this reflection characteristic repeated on a grand scale. 

Blackouts.

When testing these eyepieces I used the 70mm F5 refractor in spotting scope mode by day.  I have found a distant pine plantation to be an excellent field to assess eyepieces in this mode.   By day I found the 9 and 6 mm eyepieces not entirely satisfactory due to blackout and kidney beaning requiring very precise eye positioning which rapidly became tiring.  This characteristic was not noticeable at night at all in the 9mm and only slightly with the 6mm when used on the moon.

9mm and filters. 

On the 9 mm the Smyth/Barlow lens housing projects sufficiently to be level with the bottom edge of the barrel.  You need to be very careful when attaching filters to this eyepiece to ensure the filter glass does not contact the Smyth/Barlow lens housing.

Modifications to the 6mm

After a little use I felt the internal reflections/glare exhibited by this eyepiece were unacceptable and I applied flocking paper to some of the internal eyepiece structure in an attempt to reduce the reflections.  Interestingly, to my surprise, this modification allowed the eyepiece to be used comfortably on the moon although it did not completely resolve the internal reflection problem.

Summary

Liked

• Price
• Weight/Size
• Parfocal
• Wider AFoV compared to a Plossl
• Nicely matched set

Not Liked

• Optical Aberrations
• No non-slip grip.
• Loose fit in focuser
• 9mm potentially not filter friendly

Despite their aberrations, I liked having this eyepiece set in my observing kit and I sold them with some regret.  I particularly liked the 9mm and 6mm for DSO’s due to their wide flat field and good contrast, they were also quite nice for planetary use as well. I liked the 20mm and 15mm as easy to use, general purpose eyepieces.

Overall

While these eyepieces are not in the same category as the TeleVue or Pentax products they are quite useable and I feel they are very suitable and will provide very satisfying viewing for casual or inexperience users, in particular, for those who chose or need to wear glasses when observing.

This photo below shows the eyepiece set with their new partner ready to help new starters explore the Southern Skies.

Click to Enlarge Custom built scope