Ursa Major FMC FF 25 mm

[Ags]

Will I be disappointed? Thinking about getting this EP for max 1.25" FOV in an F9 scope, although I might use it at F6 too.

https://www.firstlightoptics.com/ursa-major-eyepieces/ursa-major-fmc-flatfield-eyepiece-25mm.html

 

[Zermelo]

1 hour ago, Ags said:

Will I be disappointed? Thinking about getting this EP for max 1.25" FOV in an F9 scope, although I might use it at F6 too.

https://www.firstlightoptics.com/ursa-major-eyepieces/ursa-major-fmc-flatfield-eyepiece-25mm.html

 

Did you see this thread:

 

[Mr Spock]

We'll let you be the guinea pig  Let us know what you find.

[Ags]

1 hour ago, Mr Spock said:

We'll let you be the guinea pig  Let us know what you find.

At this price I may very well include it with my next purchase… A few of these might make a nice outreach set.

[Ricochet]

As above, these are so new that I don't know of any reviews so we need someone to be a guinea pig and find out. Having said that we can look at the specifications and make some guesses based on that. 

The 15.5/19/25 look like a scaled design so their performance is likely to be similar to each other. I would guess that the 5 elements/3 groups are all in the body of the eyepiece but given the high eye relief/focal length ratio perhaps the first element is a negative lens. The eye relief of the 25mm doesn't quite match for a scaled design though. Elsewhere it is listed as 4/3 so perhaps it is an even more basic design than the other two. Given these are billed as "Flat field" when the same manufacturer also has an "Ultra Flat Field" we can assume that there is a bit of field curvature in the eyepiece, but as these are made by UO I think it is likely that this curvature matches (and so minimises) the direction of curvature in a refractor. Field curvature and astigmatism are opposite sides of the same coin and the design appears too basic to correct both so I think we can assume that that astigmatism will be present at the edges.

The 10.5mm is an outlier in that it appears to just be the 10mm UFF rebranded to fill a gap in this line. This is a known good eyepiece and reviews can be found elsewhere. Whether the lower cost has been made possible by some cost cutting somewhere (coatings?) or the manufacturer are simply accepting lower margins remains to be seen.

The 3.5/5.5/7.5 look like a "modern" design with a standard set of lenses in the body and a negative group in the nose. Specifically it looks like these are based on the 15.5mm. See how the outer body design of the 15.5 and 7.5 is identical in size. As the number of elements and groups increase by 1 this is a simple negative lens so the 7.5mm is probably quite nicely corrected but maybe the 3.5mm is pushing the design a little bit too far so it might be a bit soft in comparison. Also note how the AFoV has been trimmed from 65° to 60°. Perhaps this is a marketing trick to make the 10.5mm eyepiece not stand out so much, but also consider that if we double the magnification moving from the 15.5 to the 7.5 then we also double the size of the aberrations at the edge, which is indicative that perhaps the edge performance is not so good and needed to be hidden.

If your only desire is for use in an F9 scope the 25mm might perform well enough, but if you also wish to use it in the F6 scope I would suggest spending the additional money on the 24mm UFF instead (or ES68, Panoptic).

[Don Pensack]

These are not new in the world.

Most resellers sell them as "Premium Flat Field" eyepieces.

Look up reviews on the Astrotech PF eyepieces to see some comments about them.

They are also sold by Artesky, Astromania, Astrotech, Auriga, Lacerta, Omegon, Sky Rover, Tecnosky, and Telescope Service.

FLO has a good price.  

Edited April 20 by Don Pensack