Louis D

I've yet to notice the EOFB in the APM. Of course, I'm generally not under dark skies like Mt. Pinos where I might be able to detect it.

I recomposited my 12mm to 14m UWA eyepiece ruler images together so it's easier to see the differences in presentation. It's also obvious which have objectionable SAEP. All are sharp to the edge at f/6.

I have the Morpheus 14mn rather than the 12.5mm. I haven't done that much head to head competition between them. They have very different distortion characteristics, so are a bit hard to compare. The Morpheus stretches objects close to the edge like 98% of well corrected astro eyepieces while the APM squashes them. As a result, the APM actually has the wider true field of view of the two. It's even a little wider than my ES-92 12mm due to it also stretching objects.

It's unlikely to have much impact on a 14mm 80 degree eyepiece since the 13mm Ethos is a 1.25" eyepiece (with 2" skirt), the 14mm Morpheus (measured 78 degrees) is a 1.25" eyepiece, and the 16mm Nagler T5 is a 1.25" eyepiece. I think they put it in a 2" housing to ease the design slightly. It really depends on the diameter of its field lens. Even then, I doubt it much exceeds 27mm in diameter.

Try sliding your glasses down your nose slightly to increase the distance until resting them on the flipped down eye cup is at the proper standoff distance.

The Arcturus ones are more or less self-centering. The ones with thumbscrews can push eyepieces off-center making merging more difficult. Baader Click-locks are not self-centering. They actually push the eyepiece to one side.

You could fit a 2" to 1.25" step ring into the eyepiece to allow for 1.25" filter use:

I have the APM Hi-FW 12.5mm and really enjoy using it. It's better in every respect than my Nagler T4 12mm (eye relief, lack of SAEP, etc.). Personally, I have never noticed EOFB in the APM. It's really bad in the 12mm NT4 by way of comparison. You might also check into the Founder Optics Marvel / StellaLyra LER UWA 14mm 80 degree. It is 2" only, but it gets great reviews.

Among affordable eyepiece pairs with near max true field of view for affordable binoviewers, I've used the 23mm Aspherics with my Arcturus binoviewer (made by Norin Optech), but much prefer the Svbony 20mm 68 degree UWAs for their much better clarity, contrast, and sharpness. They're best at f/12 and slower. I use the optical nosepiece of a vintage Meade 140 2x Barlow to reach focus. I just screw it into the insertion barrel of the BV.

Well, it's like having a minimum of 6 eyepieces in one, perhaps more if you dabble in half-mm focal lengths. So, that makes just over 2 pages per covered focal length! That doesn't seem so excessive then.😁

How long did you allow for acclimation? I see spiking with my 90mm triplet APO that looks like pinched optics for up to about 30 minutes. It eventually disappears after acclimation. Give your scope an hour to acclimate and check again.

I kind of wondered if/suspected based on the 50 degree AFOV.

Has anyone discussed theories on the optical and mechanical organization of the Televue Nagler Zooms and the Svbony 3-8mm zoom? My theory, they have a 4 element image forming positive group up top and a 2 element negative, Smyth, group down in the insertion tube. To increase magnification, they simply move the two groups apart like a varifocal eyepiece such as my Speers-Waler 5-8mm "zoom". This also maintains the size of the AFOV as in the S-W. The mechanical trick to maintain near parfocality is to move the negative lens group downward during zooming in (higher power). I have to do this with the focuser knob to maintain focus while zooming in with the S-W.

On axis, off axis, edge, all? At 3mm, it is very good across 70% of its field. It's only in the outer portion it starts to fall apart in faster scopes. I have no idea how well corrected the 3.3mm TOE is edge to edge. Most folks only discuss on, or nearly on, axis performance of it and the Vixen HRs since few use these eyepieces for anything but planetary viewing or double star splitting.

That 38% linear secondary obstruction doesn't sound very conducive to good planetary contrast.

If it sharpens up refocused, it's entirely field curvature. If it is still mushy, just less so, then there's most likely astigmatism involved as well. It's best to use a star to examine these effects. Astigmatism turns stars in lines. They'll be tangential on one side of focus and radial on the other side. You'll also be able to see chromatic aberrations which turn stars in to radial rainbows. Coma is rare in well corrected eyepieces in my experience. As a rule of thumb, if an eyepiece exhibits chromatic issues, it will generally also exhibit coma. This manifests itself as a rainbow that fans out center to edge. It gets more diffuse the closer the star is moved to the edge. A coma-free eyepiece shows a nice linear rainbow pointing to the center.

I've found my 90mm Triplet takes about 30 minutes or longer to acclimate, or I get what looks like pinched optics. That might just be due to a poorly engineered objective cell with a coefficient of expansion greatly mismatching that of the glass. What I'm seeing is definitely not tube currents during acclimation. I forget where I read it, but eyepiece acclimation is much less of an issue due to the smaller total thermal mass, and the fact that it is at the back end of the optical train where variance in the optical figure has less effect than at the objective. As for dew heaters, the trick is to use as little as possible. You want to warm the optical device just above the dew point. That generally won't noticeably affect the figure of the optical element.

I buy flat field eyepieces to work toward with non-flat field scopes. Once I added a coma corrector to my Newts, it reduced their low curvature even more. I then added TSFLAT2 flatteners to my 2" diagonals to flatten my refractor fields. I was even able to compensate for the field curvature of my 14mm Pentax XL by over-correcting the field flattening with the TSFLAT2 by increasing the working distance another inch or so, IIRC. It was amazing to see the XL sharp edge to edge without refocusing.

No, you don't need to ever justify a purchase to anyone else (except perhaps your family CFO, if you know what I mean). I'm just trying to figure out what I've been missing with this scope. I'll try some more widefield viewing with it.

My 152 Achro is fine for sweeping star fields, but so is a 6" f/5 Newtonian which is much lighter, cheaper, and color free. The 152 does very poorly on planets and double stars at moderate to high power thanks to excessive CA. DSOs appear about the same as in the Newt. @Stu1smartcookie What exactly do you find the 152 Achro to do particularly well or better than other comparable scopes? The only thing I've found it good for is extreme testing of violet-cut and red-cut filters to find out which (combination) yields the best image of various objects. Because I love tinkering with optical combinations, I find it fun. I'm trying to justify why I'm holding onto it.

I tried them both in the 72ED, 90 APO (both with TSFLAT2 field flatteners), 8" Dob with GSO CC, and 127 Mak. I don't think I had the 6" f/5 Newt or 152 Achro at the time. Both performed pretty consistently across all of them except for the slow Mak which closed the gap between them. I will say the background seems darker in the NT4 than the Redline. It might be for the same reason as with the Vixen LV line appearing darker: the use of rare earth glass types that contributes to their higher prices relative to their competitors.

I won't lie, the 22mm NT4 is nice and sharp edge to edge compared to the 22mm Redline 70. It was enough different to persuade me to retire the Redline to the B-Team case in favor of the NT4 despite the tighter eye relief and slight SAEP in the NT4.

If you look down the tube from the open end, you should see a magnified image of your face like in a shaving mirror when you have your head at the right distance above the end. It's generally around 6 to 10 inches above the end, IIRC. This is just a quick test to see if the mirror can produce any sort of image.

Will probably need a photo down the tube to see if the spider and optics are still there.

APM claims it weighs 331g. I'll have to get it and my gram scale out again to double-check.