Don Pensack

That is true of the 2" to 1.25" Click-Lock adapter as well.

Though this would change the magnification factor of the Barlow by pulling the eyepiece back from the lens. By how much, I don't know, but it would likely increase the Barlow's magnification by 1.15-1.25x.

Easy enough to replace brass or steel screws with nylon ones, or teflon-tipped steel ones. You only need two screws with large heavy eyepieces. Otherwise, one screw is enough. As an aside, though, if it leaves a mark on the eyepiece, that will affect its optics exactly zero. It might net you a trace less in re-sale, but do you buy every eyepiece with the presumption it will be resold? As for tilt, you can perform this experiment--tip the eyepiece ever-so-slightly as you tighten it down. Can you see a difference in focusability? Unlikely. And a brass split ring shoves the eyepiece against one side of the focuser or accessory no differently than a thumbscrew directly on the eyepiece. A brass split ring is not a collet, and it does not tighten down around the eyepiece uniformly, nor hold the eyepiece in the center. It is merely a way of putting a brass surface between screw and eyepiece. If you want to center the inserted accessory, then you need a Twist-Lock system.

Generally, additional back focus means the same as additional IN focus. When an adapter is used with an eyepiece that pulls it out more from the focuser, additional in-travel of the focuser is necessary. 0.965" (24.5mm) eyepieces are typically so short, though, that only a trace of in-travel is necessary, if any.

That calculator uses the inaccurate TF=AF/M formula to compute true fields. The actual difference is a 36.3mm field stop versus a 34.8mm field stop, so the true field of the 30mm UFF is 4.3% wider than the 20mm, a bit more than shown in the illustration. With a 1200mm focal length, the true fields should be ~1.73° versus ~1.66°, so your point is valid--the 4.2' difference isn't significant. For the Pleiades, though, the lower magnification makes the cluster appear more "cluster-like". For the Pleiades, the view in an 80mm refractor at 15-20x is really the best way to view the Pleiades. It looks a lot more impressive with 2.5° and more in true field. M44, though, would be very nice in either eyepiece, though it, too, looks better at a low power.

An eyepiece can have any apparent field of view you want, from 20° up to 100-120°. The True field on the sky that you see in the eyepiece is dependent on an inside focal plane diameter called the Field Stop. Field stops get smaller with shorter focal length. Once the field stop easily fits inside a 1.25" barrel, it is no longer necessary to make the eyepiece in a 2" diameter. And as the apparent field gets larger, at any given focal length, the field stop grows as well. With currently-produced eyepieces, the transition point between 2" and 1.25" is: 41-40mm at 40° apparent field. 41mm and longer will be 2" eyepieces because the field stop will be too large to fit in a 1.25" barrel. 33-32mm at 50° 27-26mm at 62° 25-24mm at 68° 17-16mm at 82° 14-13mm at 100° So you don't need 2" eyepieces to get a large apparent field and large true field with shorter focal length eyepieces.

Step 1 is to learn the sky, i.e. learn the constellations and the names of the 30 or so brightest stars. To help out with that: skymaps.com monthly maps (which show the planets) a David Chandler two-sided planisphere and, use the "nightly Tour" in your telescope's hand controller.

Bortle 9+. You need to understand that once you get to a 10mm eyepiece, anything shorter will heavily depend on Seeing (atmospheric turbulence). The less air movement there is, the higher the power you can use. Just stay away from nebulae and galaxies for a while, as they are the hardest things to see in the night sky. Try star clusters, because no two look alike, and there are thousands of them visible in a 6" scope. Urban astronomy can be: Moon Planets double stars (many are quite lovely, and do display striking colors) carbon stars (large red giants near the ends of their lives) Variable stars Bright open star clusters Bright globular star clusters The cores of the brightest galaxies The brightest emission nebulae (using a nebula filter) Then, when you get to darker skies: Galaxies, Emission nebulae, dark nebulae, reflection nebulae, supernova remnants, all star clusters, telescopic comets, and everything seen in the city. Your term "smudges and blurs" is a good one to describe how nebulae and galaxies appear in a scope. The words "faint fuzzies" are often used. We do not, by and large, see colors, or see the details in photographs, so put that out of your mind. It can be exciting to see a small faint fuzzy when you know it is a galaxy of billions of stars 250 million light years away and that the light you are looking at was emitted back before the dinosaurs walked the Earth.

A note: the field stops of 100-110° eyepieces (and anything narrower in apparent field) fit inside a 1.25" barrel at any focal length shorter than 14mm. So you don't get any wider a field with a 2" barrel below that point. 2" may be convenient, or support a large heavy eyepiece more safely (e.g. 9mm 120°), but to all intents and purposes, 1.25" eyepieces can be as wide as they get when the focal length is shorter than 14mm. If the goal is nice "framing of DSOs at shorter focal lengths", one needn't use a 2" eyepiece to attain the necessary field.

Maximum magnification depends on Seeing (atmospheric turbulence), which varies from night to night and even from hour to hour. For a 6": 21-60x is considered low power 60-120x is medium power 120-180x is high power 180-300x is ultrahigh power. At a useful minimum, you want to have eyepieces that yield one low power, two medium powers, and one high power, maybe 40x/80x/120x/160x, or eyepieces of 38mm, 19mm, 12.5mm, 9mm Your environment is heavily light-polluted, so the 40x is likely only going to be used on large open star clusters on the very clear nights. That does not mean you wouldn't want more eyepieces eventually, since 200x ( 7.5mm) is often a useful magnification for a 6" scope. You will likely have to observe often to get skies steady enough for 200x, or even 160x for that matter. Pay attention to collimation (SCTs don't go out of collimation easily, but excellent collimation is critical for high power), Cooling of the optics (the scope will not yield sharp high power images when first taken outdoors, but images will get better over the couple hours the scope will take to cool down to the ambient temperature, and the conditions of the night sky. I refer to this as the three "C"s. Seeing at high power will be better when the jet stream is not overhead, 2-3 days after the passage of a front, when the wind has died and the air seems thicker and harder to breathe. You want non-moving, stable air. To see deeper in magnitude, you want the front to have just passed, when the air is very transparent and clear and light scatter of the atmosphere is at a minimum. I live in L.A., and right after a front has passed, and there is no moon in the sky, I have seen magnitude 5 stars at the zenith. On the days of really good seeing, I can barely make out Polaris and magnitude 2, and the air is thick.

Then a compromise: the 30mm 70° UltraFlatField from United Optics. It's available for £179 from FLO as a Stellalyra Ultra Flat Field: https://www.firstlightoptics.com/stellalyra-eyepieces/stellalyra-30mm-ultra-flat-field-2-eyepiece.html Magnification won't be too low, so the field won't be completely washed out. The step to the 17.3mm is a good jump up in magnification. It's one of the finest 30mm eyepieces out there today. I find it sharper than every other 30-31mm eyepiece except the 31mm Nagler. The true field is 34.4% wider than the 24mm Panoptic. In a typical 8" dobsonian, 1.73°.

See: https://www.cloudynights.com/topic/862337-2023-eyepiece-buyers-guide/?p=12473522 Here is what is available in 6mm (you can scroll the list): [It is plenty of choices!]

The black section containing the lens is smaller in O.D. than the 1.25" barrel above it. That does not augur well for the threads on the lens portion to be the 28.5x0.6 filter tread. Highly unlikely, I'd say.

Simply use a 2" barrel extender threaded to the skirt and you have a 2" eyepiece. If the 2" skirt is not threaded to accept barrel extenders, then an attached adapter works. The eyepiece is usable as 2" in base form, but it takes 1.25" filters.

This is the case in places with poor to average seeing. But, most of the time, the highest power in a scope will be more like 50-60x/inch of aperture, which makes the highest magnification eyepiece 1/2 of the f/ratio or even shorter. This only applies in certain circumstances of excellent to superb seeing, and often only on double stars or the Moon, where seeing interferes a little less than it does on planets (primarily because planets are small and have a lot of low contrast details). I'll presume you have a 10" scope. 200x is hardly a limiting magnification in a scope that large. I have friends who regularly use 300x in their 10" scopes, but they do have good seeing. Given good optics and good seeing, the limit of high power in a scope is: --when and where floaters in the eye start interfering with the image. --when and where the image gets too dim. For me, the first comes before the second, and I draw the line, with my eyes, at about a 0.65mm exit pupil (about 40x/inch). But I have seen nights where, on certain objects, I have used 62x/inch and seen a clean sharp image. It does require superb seeing, though. Your favorite eyepieces will tend to fall in the 1mm to 4mm exit pupil range on most DSOs, but in neither case is a limit. I read all the time about people observing objects with exit pupils as large as 10mm (in refractors) and as small as 0.3mm. You should try observing some planetary nebulae at 300-400x in your 10". You'll see a LOT more details.

After well over a decade with Ethos eyepieces and now needing to wear glasses at the eyepiece, I have been using Morpheus eyepieces for much of my observing, though I do use an 11mm TeleVue Apollo 11 (85°) a lot. But, below 8mm, I can simply take my glasses off and so every eyepiece I tend to use from 8mm down is 100-110°. The Morpheus are my "narrow field" eyepieces, and I do have the 6.5mm and 4.5mm in my eyepiece case. The 4.5mm Morpheus, in particular, is a super-sharp eyepiece, and I use it for double stars and the Moon in my 4" apo.

1) the 85° eyepieces are not compatible with glasses unless the distance from your eye to the lens is very very short. I tried it, but I had to jam the glasses lens into my eye socket to the point my eyelashes brushed the eyeglasses every time I blinked and it made my glasses hurt. So I don't regard them as eyeglasses compatible. And I can use eyepieces with as little as 14.5mm from the rubber up at 85°. 2) They are not compatible with the DioptRx unless the DioptRx is somehow attached to the rubber eyecup. I regard these eyepieces as incompatible with the DioptRx. Since the eye relief is only 12mm from the rubber up and the DioptRx eats up about 8mm of eye relief, even if you could attached the DioptRx, you'd have almost no eye relief left. 3) the 16.5mm has a flat field, no vignetting, and sharp stars. The 23mm had noticeable vignetting at the edge of the field, but I note that I used the eyepieces with a Paracorr so I would not evaluate the edges of the field with coma present, and it is possible some of the vignetting is from the Paracorr. I did not look to see if the vignetting disappeared without the Paracorr because then nearly the entire field would have visible coma. Even with vignetting, the edge of field was nice and sharp--not perfect on the 23mm, but very good. So, I think these are very good eyepieces for an observer who does not wear glasses. It would be a rare glasses-wearing observer who could use them with glasses.

What generation? Pre-2017, or after, when the bandwidth was made narrower?

https://www.eyepiecesetc.com/Choosing_a_Nebula_Filter_p/nebula-filters-explained.htm

On the Russian test site: At f/5 Svbony 3-8mm star size center/mid field/edge of field (10 is approximately perfect to the eye) 3mm 8/8/30 4mm 6/7/14 5mm 4.5/7/14 6mm 4/6/10 7mm 3.3/5/12 8mm 3/6/24 Judging by how I've seen other eyepieces with similar figures perform, I think the eyepiece measures great from 4 to 7mm and has only edge problems at 3mm and 8mm. On the test bench, it looks like a winner, price-considered.

R$290 = US$15.37

Well, one objection I have to the Baader Zoom Barlow is that when it's attached to the zoom, the entire mass of the zoom is outside the focuser or adapter. I never felt comfortable hanging all that weight on just a few mm of threads.

One is a conventional Barlow in its own tube. The other is a threaded-on version with adapters for 1.25" and T-Thread use. If this is to be used visually in your scopes, get the Q-Barlow unless it is to be used for imaging too. Also, the Q-Barlow cannot be used at the lower power advertised unless the bottom barrel of the eyepiece is completely empty and the bottom lens of the eyepiece is up inside the upper barrel of the eyepiece. If used as a 1.3x Barlow, it inserts quite a ways into the eyepiece's barrel. Generally, it's best to think of both Barlows as 2.25x.

Ernest Maratovich got 8.9mm: http://astro-talks.ru/forum/viewtopic.php?f=32&t=2962#p57122 However, I can believe a difference in measurement techniques can easily yield the different result. He also measured the 9mm Morpheus apparent field at 78.3°.

P.S. some measured focal lengths on the Morpheus eyepieces: 17.5mm--17.2mm 14mm--13.9mm 12.5mm--12.4mm 9mm--8.9mm 6.5mm--6.7mm 4.5mm--4.8mm