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Old 06-09-2019, 02:23 PM
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MortonH
Deprived of starlight

MortonH is offline
 
Join Date: Nov 2006
Location: Sydney
Posts: 3,762
Hi Alex,

There some slight negatives about off axis performance in a Newt without Paracorr in Bill Paolini's review (excerpt below) but there are more comments on CN. Overall however I may have exaggerated the issue!


3.e. OFF-AXIS PERFORMANCE (Dob without Paracorr)

As many observers choose to not use a coma correcting device in their fast Dobs, I also experimented with the behavior of the Baader Morpheus compared to the Pentax XWs in this regard. Without a coma corrector, such as the Tele Vue Paracorr, you can expect coma from a fast telescope's mirror to begin impacting star points at about 50% of the way from center in the AFOV, especially for longer focal length eyepieces. But for shorter focal length eyepieces, sometimes you can get away with fairly good performance to the edge if your Dob is not much faster than f/4.5. Without the coma correcting Paracorr in the XT10, all the Morpheus eyepieces behaved the same as far as star points. When brighter stars were moved further into the coma field of the Dob's mirror they were overwhelmed with coma as the primary aberration. Neither noticeable field curvature or astigmatism could be seen or separated from the coma. For these tests of star point behavior without a coma corrector I used bright magnitude zero Arcturus.


Moving from the clinical star point test to some real observations, I started with the Mizar complex of bright stars. Using the Morpheus 14mm I felt that the resulting true field of view (TFOV) was large enough using the longer 14mm eyepiece that the far off-axis of the eyepiece was intruding into a rather severe portion of the XT10's coma field. As a result I did not like the appearance of these bright stars in the off-axis of either the 14mm or 12.5mm Morpheus on this Mizar grouping. However, in the shorter 9mm, 6.5mm, and 4.5mm Morpheus focal lengths I felt observing the bright stars in the Mizar complex was entirely acceptable without a coma corrector. Turning to M39, which has fairly bright components, again I did not like the 14mm but this time I felt that the 12.5mm Morpheus was providing a fairly good view without Paracorr. And of course, without Paracorr all the shorter Morpheus focal lengths were fine as well on the field of view of bright stars around Mizar. When turning to the fainter stars like in the open cluster M11, I found that even the 14mm Morpheus provided a great view for this celestial object without Paracorr as long as I did not position the cluster it right at the field stop. Overall, without a coma corrector in a fast Dob, I felt the 12.5mm and shorter Morpheus focal lengths demonstrated that they could provide very nice views.

Last edited by MortonH; 06-09-2019 at 02:35 PM.
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