Louis D

Since Ningbo Sunny is probably the same as Ningbo Barride (or at least closely related), and Ningbo owns Meade since 2013, it can probably be said to be a genuine Meade product.

Try the barlow without the diagonal. I have a feeling the focuser can't be racked in far enough to reach focus with it in the diagonal. Alternatively, put the diagonal in the barlow (which is inserted in the focuser) for a 3x or more magnification factor. That combination should reach focus. If the lens elements unscrew from the end of the barlow tube, see if they can be threaded onto the bottom of the eyepieces where filters would normally be threaded. That should yield about 1.6x magnification. One more experiment, if the above worked, try threading the barlow lens element onto the end of the diagonal that goes in the focuser. That should yield about 2x or a bit more.

Of course, an even older Zeiss line, their professional observatory monocentrics, are thought to be even better than either their commercial monocentrics or their much later ZAOs by at least one dedicated observer/collector.

But, if the same imaging sensor was used in both scopes, would there be any way for the f/10 to rule the f/5? I guess throughout my thought process, I assumed that the same imaging sensor would be used to eliminate that variable from the equation. After all, what happens if you stuck that larger sensor from the f/10 example on the f/5 system? I would assume it would create like-density images more quickly but at a different magnification.

So, about the photographic equivalent of moving to a larger format sensor and increasing the ISO at the same time while maintaining similar SNR. For this to actually work, though, the f/10 would need to have a significantly larger image circle than the f/5 if the f/5 were already maxing out the fully illuminated image circle. I'm thinking of a 4" refractor versus a 6" SCT where this probably wouldn't hold true due to the the rear baffles.

Please explain one example where an f/10 scope produces an image of equal density in less time than an f/5 scope at equivalent magnification, noise level, and level of detail. My curiosity has been piqued.

For a given ISO, at exactly the same rate as any other f/2.8 camera lens or telescope. The only difference would be the magnification based on the focal length.

Fast relates to photography. Smaller focal or "f" ratios (the focal length divided by the aperture in matching units) require shorter exposures to reach a given image density so images can be taken "fast"er. That's all fast means in this context. The light isn't being drawn in faster, it's just creating a brighter, per unit area, image. This is unrelated to magnification which is strictly a function of the focal length. Thus, a 1000mm focal length f/4 telescope will be able to achieve a denser/brighter (more photons collected per unit area) image in a given amount of time than a 1000mm focal length f/8 telescope. However, the magnification would remain the same. The magic here is that the objective must be twice as large in diameter to collect four times as much light for the f/4 telescope as compared to the f/8 telescope. Thus, the f/4 scope will create the image "faster" by collecting more photons per unit of time. The downside is increased optical tube bulk, weight, and cost. For a given aperture, the smaller the f-ratio, the shorter the focal length and generally the shorter the physical tube. Visually, fast scopes are nice because, for a given aperture, you have a shorter focal length and thus can achieve wider true fields of view. They're also more compact because the physical tube is shorter which makes them easier to mount and transport. It is possible to use a focal reducer near the focal plane and have a smaller f-ratio than the physical tube length would imply. However, the image circle is also reduced by doing so. You're basically squeezing the photons down to a smaller image circle. That's why you can't use 2" eyepieces behind an SCT focal reducer. The image circle has been compressed to much less than the initial 46mm diameter. TANSTAAFL Fast scopes are much more difficult to make to the same optical level of excellence as slower scopes. For Newtonians, the figure of the mirror is more difficult to polish and the increased coma is more difficult to control at the eyepiece. However, you can more easily observe with a 20" f/3 Dob than with a 20" f/5 Dob because the eyepiece is about 60" off the ground compared to 100" for the f/5. For refractors, color dispersion is extremely difficult to control in shorter focal ratios. In fact, as aperture goes up, the focal ratio must go up as well in order to maintain the same level of color control without changing glass types or number of objective lenses. That's why APO telescope prices increase very non-linearly with linearly increasing aperture at a constant f-ratio. Eyepieces need to have a more sophisticated design to deal with shorter focal length scopes because the light cone is converging fairly steeply near the edge. That's why TV Ethos eyepieces cost so much. It requires a sophisticated design along with special glass types to successfully bring together all of the various ray bundles along with their various associated wavelengths of light into their proper positions at the exit pupil across a 100 degree apparent field of view. Older eyepiece designs generally have narrower AFOVs and were intended for slower focal ratio scopes common at the time of their design. Older, wider eyepiece designs sacrificed good edge correction for their wider AFOVs. Wider AFOVs were important in military applications to avoid tunnel vision. Motion and contrast could still be perceived in the blurrier outer field, allowing the user to swing the optic around to center the object of interest as in binoculars and periscopes. Why are steeply converging light cones difficult to control? That's actually a fairly technical subject that I have only a rudimentary understanding of. Someone like @Peter Drew could probably explain it better.

Doesn't this also lead to a soft field stop since you have moved the focal plane away from the physical field stop location?

I'm good with my 30mm APM UFF at 30mm. I've found at least one comparison that liked it better than the XW except in comfort. Since I find the UFF comfortable enough, I'll stick with it. I'm also good with my decloaked 40mm Meade SWA 5000 at 40mm. I picked it up for $125 back during the big Meade blowout sale when they were bought by Sunny. I can't imagine that the XW will be three times better to justify the expense for an eyepiece I use just to get my scopes roughly on target and to occasionally scan star fields at low power.

This is interesting from the Zeiss webpage: Extremely Broad Wide-Angle Field of View Subjective angle of view of 72° throughout the entire zoom range for a noticeably greater overview of the terrain. Which made me read more about this scope. Note the following: A notable innovation of the Victory® Harpia is the relocation of the magnification ring from the eyepiece to the spotting scope body, alongside the focus wheel. This tells me that the zooming action is most likely internal to the scope like a zooming telephoto lens, not the eyepiece. That would explain the relatively small form factor of the eyepiece. It's might actually be a fixed focal length eyepiece rather than a zoom and that grip ring on the eyepiece is just that, a grip ring and nothing more.

Abbe orthos are closer to 80% of focal length for eye relief, so slightly better than Plossls.

Check out how well this throwaway Meade "Silvertop" Plossl that came with my daughter's 127mm Mak did against much more costly offerings. Sure it's somewhat narrow in AFOV and short on eye relief, and I doubt it's worth even $10 on the used market, but it seems pretty sharp across the field. Not Vixen LV sharp, but not blurry by any means.

Maybe the OP could try @John's trick with the barlowed zoom to judge maximum usable power.

Alright, back to basics then. Remove the eyepiece, point it at the moon, and look in the focuser hole from a distance of about a foot. You should be able to see an aerial image of the moon in there. Now, take a piece of white printer or notebook paper and move it outward from the focuser (which should be racked all the way in/down) until a sharp image of the moon is formed on it. That distance is roughly where the shoulder of your eyepiece needs to be to reach focus at infinity. If it's above the maximum outward position of the focuser, you'll need an extension tube or possibly both adapters stacked together. If it just keeps getting more and more defocused as you move the paper outward, the telescope is reaching focus below it's most inward position. That would mean your primary mirror needs moved upward in the scope's tube. However, this shouldn't be the case for a new scope, and it should be returned for replacement for a properly spaced primary/focuser combination.

If you want to stick with Meade, their recently discontinued 6.5mm HD-60 5000 is quite nice. It works about as well as my 7mm Pentax XW, but with less edge chromatism, a softer field stop, 1mm less usable eye relief, and a measured 5 degree narrower apparent field of view as seen below. All in all, a very good eyepiece for the money.

Back to the OP's original question, I find I can't use very high relative magnifications with my 127mm Mak because of cool down issues and lack of perfection in the figure of the optics. I can push my hand figured Newtonian mirrors much further than my little mass produced Mak relative to the their respective theoretical limits within the same cool down period and beyond.

Since it hasn't been brought up before, I have to ask, do you have significant astigmatism in your observing eye? By significant, I generally mean a cylinder (CYL) correction for your eyeglasses of 1.0 diopters or more. If so, you'll probably want to either observe with eyeglasses, toric contacts, or use TV Dioptrx correctors on each eyepiece. I have 2.0 diopters of astigmatism in my observing eye, so even at best focus, stars are a spiky mess for me at all but the highest powers where a small exit pupil masks the astigmatism in my eye. As a result, I almost always have to observe with astigmatism correction. I prefer to wear distance corrected eyeglasses so I can look back and forth at the sky and eyepiece without having to futz with taking them on and off. This means I need eyepieces with at least 16mm of usable eye relief, which generally equates to 20mm+ of claimed eye relief, to fit my glasses in between my face and the eyepiece. This brings me around to the ES-92 versus Ethos choice. If you don't need eyeglasses, the Ethos is probably the right choice because they're wider in AFOV, smaller, and lighter. If you need wear glasses, the ES-92 is pretty much you're only hyper-wide choice. Luckily, they're very good. Some folks actually prefer the presentation of the ES-92 over the Ethos despite not wearing eyeglasses because you don't have to bury your eye in the eye cup to take in the entire view, so it's more relaxing. My other long eye relief choices include Pentax XL and XW, Delos, Morpheus, APM UFF, Meade HD-60, Vixen LV, Nagler T4, AstroTech AF70, 40mm Meade 5000 SWA, and 30mm ES-82 mushroom-top eyepieces.

Except at the highest powers, sufficient eye relief to use while wearing eyeglasses is the primary deciding factor for eyepiece choice. Secondarily, I like wide apparent fields of view. Not necessarily ultra-wide (~80 to 90 degrees) or hyper-wide (~90 to 120 degrees), just "super" wide (~65 to 80 degrees) is usually good enough for my tastes in observing. Third, I demand low levels of edge astigmatism and field curvature since I use undriven mounts which means I have to observe objects while they drift from edge to edge.

So, you might not be opposed to being banished to live in the shed. 🤔

To carry your car analogy even further, would you put high mileage touring tires on your BMW such that you are constantly spinning your wheels and sliding through corners? No, you'd put high performance tires on it to maximize your enjoyment of it. Likewise, putting 100 degree eyepieces into an f/4.7 Newt without a coma corrector of some sort won't allow them perform to their full potential. You'll be wondering why the stars bloat in the outer parts of the field in such a well corrected eyepiece. If you've got the budget, the TV Paracorr II can't be beat. If you get lucky, you might be able to find a used Paracorr I with tunable top for much less. It's not quite as good, but it's way better than nothing. If your budget is tight after picking up so many expensive eyepieces, I would recommend the Revelation/GSO coma corrector (on sale for a really great price). It performs about the same as the original Paracorr I without the tunable top. It flattens the field as well as only magnifying by 10% instead of 15% for the Paracorrs. It also requires only 12mm of in-focus (backfocus). If you parfocalize your eyepieces and tune the optical element to eyepiece holder distance with spacer rings, it's a great solution.

I guess it depends on how nice the shed is. If it's a fully outfitted observatory with heated/cooled living quarters off to the side, it might not be too bad at all.

Or even more perfect with the living conditions swapped?

Then you should also consider the 12.5mm Nikon NAV-HW which is generally rated as slightly better than the 13mm Ethos. That, and it comes with their EiC-10 element to convert it to a just as good 10mm.

Since you're probably on a tight budget, I'd recommend some decent plossls. 32mm, 20mm, and 12mm to start with. Even the inexpensive ones are reportedly fairly decent. I'd also probably get a decent 2x shorty barlow as well. My 80mm short tube came with 20mm RK, 12.5mm K, and 9mm K eyepieces, where K means Kellner and RK means Reverse Kellner. The Kellners were surprisingly good, the RK was less good, but not bad, so give the included eyepieces a chance before tossing them. Be aware that due to extreme false color thanks to the scope's fast achromat, you'll never get super sharp images, especially at higher powers. A minus-violet or light yellow filter can help tame some of this false color at the expense of giving everything a yellow cast.