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Sterling 40mm Plossl and the Astro-Tech 40mm Titan Type II ED


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I have been looking for a nice wider field 40mm eyepieces recently to compliment my TSA-102. With the TSA’s f/8 focal ratio, and the resulting 5mm Exit Pupil a 40mm eyepiece gives, it would be a perfect match as I find this Exit Pupil produces for my eye the most consistent “diamond on velvet” effect. With my recent expedition to NEAF 2009, and the kind offer from fellow CNer “Gary” (Member # 10111) to borrow his 40mm, I found in my possession two very nice (and affordable) 40mm’s to test.



First up, the 2 inch barreled 40mm Sterling Plossl. This relatively newcomer to the field boasts a 55 deg Apparent Field of View (AFOV), a nice tapered barrel (I hate undercuts), 24mm Eye Relief, a listed weight of 13.6 ounces, and a street price new of approximately $100. Look and feel of the eyepiece is excellent with no part of it seeming like a shortcut or quality compromise was made – so a nice quality look and feel. While there are no rubberized grips on the eyepiece, its satin texture always allowed a firm grip in use. This eyepiece also has no rubber eye guard or other mechanical light control feature. The manufacturer boasts it has a 40 layer multicoat which I assume to be a 10 layer coating on each air-to-glass surface of the 4-element Plossl design.

Next, the Astro-Tech Titan Type II ED. This 2” barreled 40mm eyepiece is a more complex design, with 6-Elements, boasting a 68 deg AFOV, 14mm Eye Relief, rubberized grip panels, rubberized top housing to protect eyeglasses, a rotating adjustable height eye guard for stray light control, a listed weight of 18 ounces, and a street price new of approximately $160. The Titan, like the Sterling, is a well made eyepiece which gives an overall impression of quality.



Below is a table summarizing the manufacturer major points of each eyepiece, as well as some direct measures.


Attribute

Sterling Plossl

Titan Type II ED

Eye Guard

None

Rotating, Adjustable height, Rubberized top

Housing Grips

None

Large wide panel

encircling the housing

Barrel

2”, Tapered, Black Anodized

2”, Tapered, Black Anodized

Height (measured)

127mm (5in)

122mm/132mm (4.8”/5.2”)

Weight

13.6 oz

18.0 oz

Eye Lens Diameter (measured)

49mm

32mm

Eye Relief

24mm

14mm

AFOV

55º

68º

Design

4 Elements (2 Groups)

6 Elements


Look and feel impressions aside, now time for how they each performed. I used each eyepiece, side-by-side, for several weeks in both my TSA-102 f/8 APO, and just to see how the off-axis would be handled I also gave both eyepieces a run in my Orion XT10 f/4.7 Dobsonian. Given the very low magnifications these eyepieces afforded in either instrument, targets consisted of a variety of star fields, much time enjoying the M44 cluster, and of course the Moon.

A summary of the general observing results can be found in the table below. In reviewing this table, I have included results related to most of the typical aberrations. However, the indented items (i.e., #3 field curvature through #6 chromatic aberration) are listed for individual results, the impact of these many aberrations as a whole has been summarized in the Row #2, Off-Axis Sharpness. So if you are more interested in the impact to the view overall, rather than the specifics, you can just read the Off-Axis Sharpness results summarized in Row #2, and then find the detailed notes below the table which directly correspond to the numbered items within the table.


#

Performance Criteria

Sterling Plossl

Titan Type II ED

1

Sharpness (On-Axis)

Excellent

Excellent

2

Sharpness (Off-Axis)

Soft 25% from Edge @ f/4.7;

Soft 10% from Edge @ f/8

Soft 25% from Edge @ f/4.7;

Soft 15% from Edge @ f/8

3

Field Curvature

Yes

Yes

4

Astigmatism

Minimal

Moderate

5

Coma

Paracorr Not Corrected

Paracorr Corrected

6

Chromatic Aberration
- On Axis (Longitudinal)
- Off Axis (Lateral)


None Apparent on-axis

Minimal off-axis


None Apparent on-axis

Minimal off-axis

7

Contrast

Tie

Tie

8

Transmission

Tie

Tie

9

Distortion (Barrel or Pincushion)

Minimal off-axis

None Apparent off-axis

10

Eye Positioning

Easy

Easy

11

Eye Relief

Comfortable

Comfortable

12

Field Stop Definition

Sharp

Sharp


  1. On-axis sharpness was quite excellent through both eyepieces when used in either scope. Stars were nice pinpoints showing no readily apparent scatter and Lunar details were very crisp. I did however have the impression that lunar features were slightly crisper in the Sterling, but this was too close to say it was anything more than just an impression.

  2. Both eyepieces showed a less than sharp image in the last 25% of their FOV in the XT10 at f/4.7 -- stars appeared bloated off-axis in this region in the Sterling and in the Titan-II stars appeared both bloated and linear or V-shaped in this region. Similarly Lunar features softened and blurred in this same FOV region for both eyepieces. For the Sterling, the image softening and deformed stars was almost entirely due to Field Curvature, so an 1/8 of a turn of the coarse focus knob would clean the star points up to a sharp focus almost to the very edge of field. In the Titan II any refocus would shift the star image, which was more linear in shape, rotating it my 90 degrees which is a classic characteristic of astigmatism. Moving to the TSA-102 at f/8, the Sterling was sharp over 90% of its FOV with the outer 10% having field curvature that could be correct with a focus adjustment. The Titan-II did not perform as well as the Sterling in the TSA-102 with the off-axis area approximately 15% from the edge being moderately astigmatic and uncorrectable with any focus adjustment.

  3. Both eyepieces showed the same level of field curvature at the very edge of their AFOVs, taking about a 1/8 of a turn of the course focus knob to correct. However, given that the Titan II’s AFOV is much larger (68 degrees vs. 55 degrees), at the same True Field of View (TFOV) position from the center of the field, the Titan II showed a less out of focus star. It should also be noted however that both Newtonians and non-Petzval refractors like the TSA have main objective’s which generate some degree of field curvature, therefore exactly how much of this Field Curvature was from the eyepiece vs. the scope I could not determine.

  4. As summarized in #2 above, the Titan II showed moderate astigmatism in the outer 25% of its FOV through the XT10 at f/4.7 and in the outer 15% of its FOV through the TSA-102 at f/8. So when viewing the Beehive Cluster M44 as an example, as long as the cluster was centered the entire cluster was very pleasing. However, moving the densest part of the cluster to the edge of field did not produce a pleasing view as the stars elongated. The Sterling did exhibit some astigmatism, but it was quite mild and not obvious.

  5. No inherent coma generated from either eyepiece was detected, however the XT10’s fast mirror did show coma as expected. Given the very low magnification these eyepieces produced in either scope, the star deformation from the coma was not excessive visually and was primarily problematic in the outer 25% of the FOV. As a big plus for the Titan II, using the TV Paracorr in the XT10 eliminated this problem and stars were nicely tight across almost the entire field of view. As the days and weeks progressed, the Titan II became my preferred eyepiece in the XT10. Unfortunately, and somewhat unexpectedly, I discovered that using the Sterling with the XT10 that coma was not corrected by the Paracorr. Regardless of the tuning position I used on the Paracorr, the off-axis coma could not be corrected in the Sterling as it had with the Titan II.

  6. No Chromatic Aberration (CA) was noted on-axis for either eyepiece in either scope. In all the observing sessions the only time transverse color (i.e., lateral color) was noted was on the limb of the Moon. However, this only occurred when the limb was in the outer 25% or less of the FOV and only showed itself as a very fine line of color. At no time did other parts of the lunar surface inside the limb show any CA. So excellent performance by both eyepieces in both of my scopes.

  7. Scanning star fields, both eyepieces produced nice visuals with high contrast renditions – and the 4mm Exit Pupil provided numerous diamonds on velvet effects through the TSA. The Titan II in my opinion did show slightly better contrast however when viewing the Moon. This was particularly obvious when viewing the Lunar Maria as these regions showed darker darks relative to the brighter mountainous regions and also showed subtle shadings within the Maria more easily than through the Sterling. For Lunar observing, this perception of better contrast was easily observable. Conversely, on star fields the Sterling was the eyepiece that seemed to give an impression of more contrast between the stars and the background of space. I conjecture that this might have been a perceptual effect due to the smaller AFOV as I notice this occasionally when using medium to small AFOV eyepieces in general. Since each eyepiece gave an impression of more contrast in varying situations, both were judged as a tie overall.

  8. Transmission was judged equal for the two eyepieces. At no time were any dimmer stars noted or more easily observed using either eyepiece.

  9. Observing star fields, no apparent barrel distortion or pincushion distortion (both commonly grouped as rectilinear distortion) was noticed with either eyepiece. However, once the Moon was observed it became obvious that the Sterling had a slight level of this distortion in the outer 25% of the FOV. When the Moon was moved to this region you could detect a slight elongation and the Moon went “out of round” once it was positioned close to the Field Stop. Overall, I felt it was rather inconsequential and only worth mentioning from a technical standpoint. Practically I felt it had no impact on any observing session conducted and it never detracted from enjoying the eyepiece.

  10. Eye positioning was easy with either eyepiece. At no time did I feel finding and keeping proper eye position difficult, nor were any blackouts or kidney bean effects evident.

  11. Eye Relief felt comfortable for both eyepieces. Even though the Eye Relief was quite large in the Sterling, 24mm according to the manufacturer, it never felt excessive or uncomfortable. Likewise the Titan II was very easy to use and the adjustable eye cup was an added, and welcomed, convenience. For those observers who like to rest their forehead against the eyepiece housing while viewing, the Titan II excelled here as one could adjust the eye guard position to easily accomplish this.

  12. The Field Stop appeared sharp and distinct in each eyepiece. At no time was it difficult to view the entire FOV and peripherally see a distinct Field Stop.


CONCLUSIONS

Overall, both eyepieces performed very well. Even in the Orion XT10, where they produced an Exit Pupil in excess of 8mm, the views were still very pleasant and at no time was a shadow or darkening from the secondary mirror visible, even when viewing the Moon. Overall, at times I would enjoy the Titan-II more, then at other times I would prefer the Sterling. Basically, the one I preferred during any particular observing session depended more I believe on my mood – if I was preferring a sharper edge then the Sterling was preferred, but if I was wanting more context from a larger TFOV then the Titan-II was the one I reached for more often.

While either eyepiece was not judged perfect in the far outer portions of their AFOVs, both were still quite wonderful low power performers, each delivering much enjoyment and providing enchanting views of expansive targets like the M44 Beehive cluster. Given the two scopes I was using, I felt overall however that I had a slight preference for the Titan II as with Paracorr it really did provide excellent views almost to the very edge using the XT10, and although not as sharp to the edge in the TSA, the greater off-axis context was appreciated. The Sterling 40mm Plossl and the Astro-Tech 40mm Titan Type II ED are well constructed, easy to use, and in my opinion well worth their respective street prices.


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