EP advice for evostar 72ED

[W0nderste]

Hi, i just got a nice Evostar 72 ED today, thanks @Trevor N

what would be good EP choices for this scope?

I currently have an F10 SCT with 1.25" EPs although I did acquire a Baader Hyperon 8mm recently, so haven't got any experience with a faster scopes.

Assuming I can make use of Hyperon 8mm, i was looking for mid and low power EP.

i've just been reading that some EP's are poor for faster scopes but finding it hard to work out what good options are with making expensive mistakes

thanks

[Louis D]

Do you have a 2" diagonal for it?  If so, a 30mm APM UFF or equivalent (Altair Ultraflat, Meade 5000 UHD, Celestron Ultima Edge) would be a very good choice for wide field views.  It is basically flawless in my f/6 Astro Tech 72ED.  You may notice field curvature, though, due to the very small radius of curvature of your new scope (420/3=140mm).  I know I sure did with my 72ED.  I ended up springing for a TSFLAT2 mounted 15mm in front of the body of my 2" GSO dielectric diagonal to flatten the field.  I do remove it for high power work because it contributes some spherical aberration at high powers.

[W0nderste]

Thanks @Louis D

good place to start.

looking at the 2" EP's is much more confusing than the 1.25s as it appears some 2" EPs are poor with faster scopes

[Louis D]

This is in large part true because many 1.25" eyepieces these days use a negative/positive design, much like having a Barlow ahead of the image forming eyepiece.  This allows for better correction and longer eye relief.  That positive-only image forming section has a fairly long native focal length.

To accomplish the same thing with two inch eyepieces would require really large positive sections having a very long native focal length.  My 35mm Baader Scopos Extreme is an example of this approach.  Below is the optical design schematic (eye lens at left, negative field lens at right) and an image of mine side by side with other similar focal length eyepieces.  It's also 38 ounces, so right in between my two ES-92s in weight.  And yes, it has very good correction as shown in the last image.

[W0nderste]

15 hours ago, Louis D said:

This is in large part true because many 1.25" eyepieces these days use a negative/positive design, much like having a Barlow ahead of the image forming eyepiece.  This allows for better correction and longer eye relief.  That positive-only image forming section has a fairly long native focal length.

To accomplish the same thing with two inch eyepieces would require really large positive sections having a very long native focal length.  My 35mm Baader Scopos Extreme is an example of this approach.  Below is the optical design schematic (eye lens at left, negative field lens at right) and an image of mine side by side with other similar focal length eyepieces.  It's also 38 ounces, so right in between my two ES-92s in weight.  And yes, it has very good correction as shown in the last image.

Thanks Louis

[W0nderste]

So what 2" EPs is everyone else using on small 70ish refractors?