Otto Piechowski's Review of the Orion Sirius Plossl and UO MK-70 40mm Eyepieces

Eyepieces Facts and Features
Rationale for These Particular Eyepieces
Observational Comparison:
Decision and Conclusion
Addendum: Methods of Calculating the TFOV

Eyepieces Facts and Features

ORION 40MM SIRIUS PLOSSL:

UNIVERSITY OPTICS 40MM MK-70 KONIG:

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Rationale for These Particular Eyepieces

The telescope for which these eyepiece were obtained and with which these are used is the 150mm MK-67 maksutov-cassegrain system. Though the optical quality and performance of this scope is uniformly very good (see MK-67 scope review) and well respected within the stargazing and scoping community, one of its limitations is that it has a significantly smaller actual field of view (TFOV) compared to short focal length Newtonians or refractors. To partially ameliorate this limitation for visual use, one option is to obtain a long focal length eyepiece with as large an apparent field of view (AFOV) as possible. In my particular case, the threshold I was trying to obtain was a TFOV of 1 degree or more. My first attempt to reach this threshold was to obtain Orion’s Sirius 40mm Plossl from Eric’s Telescopes of Lexington, Kentucky. At the time of the purchase of the Sirius 40mm Plossl, I was operating under the assumption that my telescope had a focal ratio of F12 or 1,800mm. Under this assumption, the actual field of view (TFOV) should have been .96 degrees for this eyepiece listed as having a 43 degree AFOV. (See: Addendum: Method #1). However, later observations and measurements led to the conclusion that this eyepiece was only providing .88 degrees TFOV (See: Addendum: Method #2 and Method #4). Comparisons with other eyepieces confirmed that the 43 degree AFOV listed for the Sirius Plossl was correct. This led to the conclusion that the telescope had a focal ratio of F13 or 1960mm of focal length. This accounted for the diminished TFOV. The enlarged focal ratio of the telescope also meant that the 40mm eyepiece gave a magnification of 49X rather than 45X.

My second attempt to reach this observational threshold began with the purchase of a 2 inch diagonal. I selected an inexpensive diagonal sold by Surplus Shed of Pennsylvania that sold for $59 plus $5 for shipping and handling. I have been very satisfied with the quality and performance of this diagonal. I was attracted to Surplus Shed’s diagonal because Surplus Shed is also a distributor of Paul Rini eyepieces that I have long enjoyed. At the same time I purchased the diagonal, I also purchased a 2 inch, 80mm focal length Rini plossl being sold for less than thirty dollars. With its reported 38 degree AFOV, according to the calculations, the eyepiece was able to provide around a 1.4 degree field of view. Though the images were good and a large swath of the sky was visible, I soon grew dissatisfied with the 80mm. The eye relief was uncomfortably long, the apparent field of view felt narrower than even the stated 38 degrees and it suffered from severe problems with black-out. However, I have subsequently been informed that Paul Rini has recommended that these eyepieces be used only in unobstructed telescopes such as refractors.

Out of dissatisfaction with the sub-one-degree TFOV of the Sirius and the problems with the 80mm Rini, I then sought the advice of Bill Burnett at Internet Telescope Exchange. I asked him if a focal reducer would provide me a 1degree plus TFOV for visual use. After consulting with Mike Palermiti, Mr. Burnett returned my email and told me that the use of a focal reducer with this telescope was not optimal for visual use. They both recommended that I obtain the 2 inch MK-70 40mm Konig. Finding the price of $200 prohibitive, I spent fruitless weeks searching Astromart, Astronomy Mall and Ebay for used MK-70 40mm eyepieces with no luck. During this time I was fortunate to make the acquaintance of J.W. Seyfried, the president of University Optics. Through his assistance I was able to obtain a very slightly used and, to my eyes, optically and cosmetically perfect MK-70 40mm for less than $200. In the same telescope, and using the same equations listed below, it was determined that this eyepiece provided a TFOV of 1.43 degrees.

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Observational Comparison:

My initial reaction to the Sirius Plossl was a "this is a good eyepiece." My initial reaction to the MK-70 40mm Konig was "Wow!"

Individual Merits:

The Sirius gives a flat field with pinpoint stars all the way across. Contrast and image brightness are good. It has a comfortable amount of eye relief. It provides an adequate, even enjoyable, wide view. The MK-70 provides a very wide field of view. The fourteen or so brighter stars of the Pleiades are all visible in the field of view. Image brightness is excellent and contrast seems to be good. Stars appear as pinpoints. The eyepiece provides comfortable eye relief.

Individual Limitations:

The Sirius does not provide as bright of images as the MK-70. This was noticed when observing Messier galaxies in Leo in severely light polluted city skies. Whereas two galaxies were detectable with difficulty in the former, in the latter they were obvious. Because the background brightness in both eyepieces seemed equivalent, the increase in object brightness without any noticeable increase in contrast seemed to be due to the superior coatings of the MK-70. Another limitation of the Sirius is that the field of view of the Sirius is much less than that of the MK-70; just over 60% less. Finally, though the Sirius can be used with a Barlow and does provide good images with the Barlow, the eye-relief becomes uncomfortably long when it is used with the Barlow. Further, when used with a Barlow, the Sirius has a noticeable amount of black-out. The MK-70 also has limitations. The field of view of the MK-70, unlike the Sirius, is not completely flat. Star images elongate at the edges of the field of view. The center "degree" of the 1.43 TFOV appears perfectly flat. Stars appear horizontally elongated in the remaining .4 of the 1.43 TFOV, becoming more apparent as the star approaches the edge of the field of view. It should be noted that this elongation occurs in that area of the MK-70 that is not even available in the Sirius. Also, it should be noted that one does not notice this "bending" when one is gazing into the middle of the eyepiece. "Kidney beaning" is not a problem with the MK-70, but it is present. One does not see it when looking directly into the eyepiece or even when moving the eye around a bit. Under normal observing techniques, one does not notice kidney beaning. Another comparative deficit of the MK-70 is that, because of its 2 inch aspect, the MK-70 cannot be used with the 1.25 inch Barlow and 1.25 inch filers employed with the 1.25 inch eyepieces in my collection. The use of 2 inch accessories in the MK-67 when used with the cg4 mount provides better declination balance than without it. On the other hand, the weight and the heft of the 2 inch diagonal and ½ kilogram MK-70 places some strain on the restriction ring within the Crayford focuser. Unless the end of the diagonal which inserts into the Crayford focuser is roughened up a bit (i.e. sanded), it tends to slip out no matter how firmly the finger screw is tightened. And finally, though the MK-70 gives a very nice view of the full moon, the image in the Sirius appears more pleasing. This may be due to the fact that the brightness of the full moon, in causing the pupil to contract, causes one to lose a sense of the edges of the field of view in the MK-70; whereas, in the Sirius, the lunar disk is viewed within a discernible field of view. It may be that this edge provides a visual frame of reference that the eye needs, to feel comfortable with viewing an image. This effect can also be caused by intrusions/narrowings in the optical system that vignette (narrow down) the AFOV allowed by the field stop of the eyepiece. Though possible, this does not seem to be the case with the MK-70 when used in the MK-67. The field-stop of the MK-70 is equivalent to the interior wall diameter of the eyepiece; around 47mm. According to calculation (see Addendum: Method #3) the full potential field of view is being maximized. This means that there is nothing in the optical train limiting the potential field of view more than the field stop itself.

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Decision and Conclusion:

I've kept both of these eyepieces. The MK-70 provides superior views of extended heavily populated star fields and for its utility in sweeping for galaxies. I like the Sirius in that, though it provides good but inferior views, it is the eyepiece I use for providing low powered views at public viewings. The MK-70 has a more exposed lens face and is too expensive to hazard the additional risk of damage by observers unfamiliar with the gentle treatment optical glass requires. Further, I like the Sirius in that it is much less expensive than the MK-70 and thus is more affordable. Finally, another attraction of the Sirius, because of its 1.25 inch aspect, is that it is more conveniently useable as one of a group of eyepieces. It provides a low powered and relatively wide field of view for tracking down an object that one knows ahead of time is going to be observed at higher magnifications later. Though the merits and limitations of these two eyepieces differ, both are good and worthy of consideration for inclusion in one’s eyepiece collection.

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Addendum: Methods of Calculating the TFOV

Note: TFOV = True Field of View, AFOV = Apparent Field of View

1: The Apparent FOV Method

2: The Stellar Drift Method

3: The Field Stop Method

4: The Star Chart Method

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