Quote:
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Originally Posted by circumpolar
I remember reading in the IIS Reviews(13mm Nagler T6 shootout) that the Pentax XW14mm suffers from edge coma.
Can other member confirm this?
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Yes, as RE pointed out this is due to some field curvature, which I was going to mention but I wanted to wait till I tested mine in the 4" Mak.
Quote:
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Originally Posted by circumpolar
Maybe it's only a prob with 10" dobs 
Thoughts... |
No it is not just fast dobs. The field curvature (FC) is a property of the eyepiece itself and it is a gradual shift in focus from centre to edge (so you have to refocus for sharp stars near the edge compared with those in the centre). It is quite obvious with the XW14 in both my Dobs f/5 and f/6, and in the f/13 Mak, as well as in a barlow in the f/5 Dob (effective f/11). I have also seen similar FC with Geoff's XL14 in an ED80. It was rather annoying when I was trying to hunt for planetaries the other night. I'd think I found one near the edge only to realise it's just a defocussed star.
The FC can be most dramatically observed on omega centauri. In the 12" it fills the FOV of the XW14 and then some. If you place the glob in the centre and focus and then shift it to the edge it goes from tack sharp to fuzzy. With the glob's centre on the edge of field, you can clearly observe the field curvature. It starts at about 50% of the way from centre to the edge, and becomes quite severe in the last 10 or so degrees. A large aperture that brings out lots of stars helps to see what is going on. To perform the same test with the small Mak I'd need much darker skies. But shifting a single bright star across the field confirms the FC is present with the Mak too.
The FC is a bit of a shame and I can understand why it made Mike prefer the 13mm T6 Nagler over the XW14 in his review. But the T6 has other shortcomings which I consider more of an issue.