Celestron Ultima Duo, Baader Hyperion, & Field Curvature

[cloudsweeper]

Can anyone with experience of these kindly tell me which (if any) is likely to be better corrected for use in a fast 'scope?

Also - would it be reasonable to surmise that any uncorrected Field Curvature effects would matter less in a shorter Focal Length EP, where the RFOV is quite small?  Or would it be noticeable across the range of Focal Lengths?

Many thanks!

Doug.

 

[John]

Hi Doug,

I've used all the Hyperions and they are not well corrected in fast scopes (ie: from around F/7-F8 and faster). The work fine at these faster focal ratios in the central are of the field of course (most eyepieces do) but the stars in the outer 50% of the field of view will not be pin points, the effect worsens towards the edge of the field. The extent to which this matters varies person to person I think but it's worth being aware of. This seemed consistent across the focal lengths of the range.

I believe this is caused by an abberration known as astigmatism primarily. The eyepieces may have some field curvature as well but the dominant one for me was astigmatism.

When the Celestron Ultima Duo's were lauched there was a lot of speculation that they are basically a re-branded Hyperion (based on the focal lengths, specs etc) but there is no way to prove this other than to compare the two types "back to back" and probably to dismantle them !

 

 

[cloudsweeper]

Thanks again, John!  Having used all the Hyperions, would you say these field curvature issues are more pronounced in the 24mm, say, rather than the 5mm (with smaller field of view)?

Doug.

[cloudsweeper]

..........thinking about it, I don't suppose it matters how much sky you are viewing, since the actual image in the EP is the same size, so will be just as prone to edge effects regardless of focal length.

And yet, fatter lenses (i.e. short focal length) might be expected to produce more edge distortion.

I think.

Doug.

[YKSE]

I've not used any of these EPs, only read some reports and optical tests about EPs, my understanding is that there are variations in ANY EP lines, even if the best line has variations among the different focal length, thatis about EPs' astigmatism correction, FC, CA etc. So cheery picking is, IMHO, better way than just buy a whole line.

As to FC in your short FL refractor, your scope has a lot of FC, an EP without FC will show the FC of the scope. Especially in low mag, since our eyes in larger exit pupil are just not as sharp in smaller exit pupil. Getting an EP with opposive FC as the scope will help you, but then this EP will show its own FC in a flat scope.

What are  the XCel LX you have? I would think that 7mm and 9mm are among the best of Xcel LX, if the edge sharpness is the main concern for you.

[cloudsweeper]

2 hours ago, YKSE said:

I've not used any of these EPs, only read some reports and optical tests about EPs, my understanding is that there are variations in ANY EP lines, even if the best line has variations among the different focal length, thatis about EPs' astigmatism correction, FC, CA etc. So cheery picking is, IMHO, better way than just buy a whole line.

As to FC in your short FL refractor, your scope has a lot of FC, an EP without FC will show the FC of the scope. Especially in low mag, since our eyes in larger exit pupil are just not as sharp in smaller exit pupil. Getting an EP with opposive FC as the scope will help you, but then this EP will show its own FC in a flat scope.

What are  the XCel LX you have? I would think that 7mm and 9mm are among the best of Xcel LX, if the edge sharpness is the main concern for you.

Thanks, very helpful.  I just have the one Hyperion (10mm), and 12 and 5mm in the XCel LX.  

My concern is really more academic than anything, having spent years in science and engineering.

I tried the Hyp as a 10mm and (without the nosepiece) as a 22.4mm in the 8SE, and didn't notice an intolerable deterioration off-axis.  I realise that this will be different in the ST120, which I am about to try this evening.  I am expecting the 22.4mm to show a wider FOV again, but to be sharp only in the central region.

Doug.

[YKSE]

3 hours ago, cloudsweeper said:

Thanks, very helpful.  I just have the one Hyperion (10mm), and 12 and 5mm in the XCel LX.  

My concern is really more academic than anything, having spent years in science and engineering.

I tried the Hyp as a 10mm and (without the nosepiece) as a 22.4mm in the 8SE, and didn't notice an intolerable deterioration off-axis.  I realise that this will be different in the ST120, which I am about to try this evening.  I am expecting the 22.4mm to show a wider FOV again, but to be sharp only in the central region.

Doug.

Try your EPs first is always the best starting point Just place a bright star like Vega close to the edge, with best focus, you might even see difference between 5mm and 12mm XCel LX.