S/W Planetary, S/W Nirvana, & Celestron XCel LX EP Comparison

[cloudsweeper]

I thought it might be interesting to compare these EPs in the ST120 Frac since they all give about the same TFOV in it (0.5-0.6 degrees).  

 

S/W UWA Planetary 6/58 – the least expensive:   (mag x100)    Functional appearance.

Celestron XCel LX 5/60:  (mag x120)    Nicer to behold – matt finish, gold rings, twist-up eyeguard (which can come off with the lens cap!), chunky rubber grip-ring.

S/W UWA Nirvana 4/82 – the most expensive:  (mag x150)   Also pleasing in appearance – gloss finish, twist-up eyeguard.

 

I chose the Trapezium as my target.  All three EPs gave good, sharp views of it.  If I had to differentiate, I'd say the S/W Planetary was sharpest by a tiny margin, but that is probably because the mag was the least.

After focusing, I then moved the target to the edge of the field.  The S/W Planetary produced a noticeably distorted image - the de-focus effect was uneven (i.e. elongated) which I presume to be astigmatism.  I further presume there would also be some FC present too.  The XCel performed better, showing only a slight deterioration in sharpness (some FC), but the Nirvana stood out by remaining sharp at the field edge.

So yes, there were clear differences in performance.  But do I really need to upgrade the S/W Planetary?  I tend to concentrate on centre-field, and look to the sides just to recognise star patterns.  Personal preference, I suppose.

Any helpful comments about my procedure and conclusions would be very welcome!

Doug.

 

 

[John]

Good stuff Doug

Taking a challenging target that you know well is a good way to see the differences in eyepiece performance. F/5 pushes eyepiece performance off axis as well so that was a good choice.

It is challenging to compare eyepieces of differing focal lengths because you can't always be sure that differences you see are not, at least to some extent, a result of the varying magnifications produced.

The "do I need to upgrade" question is one only you can answer I reckon 

 

 

 

[Louis D]

Choose a moderately bright star, about magnitude 2 or so for the following tests.  Make sure you have a flat field instrument before you try this.  This could be either a long focal length, coma corrected Newtonian or a field flattened refractor.  Field curvature of the objective can completely overwhelm any field curvature present in the eyepiece.

To detect field curvature, refocus for the edge and then recenter and refocus again.  After a few iterations, you'll get a feel for how much FC is present, especially in comparison to other eyepieces you perform this same procedure with.  You can even note how much you have to turn the focuser knob to refocus each eyepiece if you want to quantify it.  About the flattest field eyepiece I own is a 10mm Delos which has none that I can detect in any scope.  If I put this eyepiece in an unknown scope, I can be assured that the curvature I'm seeing is strictly due to the objective.

To detect astigmatism, rack the focuser in and out through best focus at the center and at the edge.  If the out of focus image is ellipsoidal radially one side of focus and tangentially on the other side, you've seeing astigmatism.  There's a slim chance the objective could contribute astigmatism, but it's highly unlikely.  Coma won't change directions on either side of focus.

[cloudsweeper]

10 hours ago, John said:

Good stuff Doug

Taking a challenging target that you know well is a good way to see the differences in eyepiece performance. F/5 pushes eyepiece performance off axis as well so that was a good choice.

It is challenging to compare eyepieces of differing focal lengths because you can't always be sure that differences you see are not, at least to some extent, a result of the varying magnifications produced.

The "do I need to upgrade" question is one only you can answer I reckon 

 

 

 

Thanks, John!  Yes, I did think that comparing along the lines of same TFOV was a bit irregular as opposed to having the same FLs.  (Unfortunately I don't have any duplicated FLs, but next time I upgrade, I'll hang on to the older one to do a comparison.)

Doug.

[cloudsweeper]

4 hours ago, Louis D said:

Choose a moderately bright star, about magnitude 2 or so for the following tests.  Make sure you have a flat field instrument before you try this.  This could be either a long focal length, coma corrected Newtonian or a field flattened refractor.  Field curvature of the objective can completely overwhelm any field curvature present in the eyepiece.

To detect field curvature, refocus for the edge and then recenter and refocus again.  After a few iterations, you'll get a feel for how much FC is present, especially in comparison to other eyepieces you perform this same procedure with.  You can even note how much you have to turn the focuser knob to refocus each eyepiece if you want to quantify it.  About the flattest field eyepiece I own is a 10mm Delos which has none that I can detect in any scope.  If I put this eyepiece in an unknown scope, I can be assured that the curvature I'm seeing is strictly due to the objective.

To detect astigmatism, rack the focuser in and out through best focus at the center and at the edge.  If the out of focus image is ellipsoidal radially one side of focus and tangentially on the other side, you've seeing astigmatism.  There's a slim chance the objective could contribute astigmatism, but it's highly unlikely.  Coma won't change directions on either side of focus.

Thanks Louis - very informative!  

Doug.

[N3ptune]

On 2/27/2017 at 15:20, cloudsweeper said:

Celestron XCel LX 5/60:  (mag x120)    Nicer to behold – matt finish, gold rings, twist-up eyeguard (which can come off with the lens cap!), chunky rubber grip-ring.

I push the lens cap at 1/3 of their total depth only, they are much more easier to remove like that and they hold tightly.