Don Pensack

Cameras can capture things invisible to the eye, but the view through the eyepiece can encompass a greater dynamic range. You can see the Trapezium in M42 at the same time you can see wisps of nebulosity in the Fish Mouth. You can see details in the Regio Centralis at the same time you can see the very faint nebulosity almost a degree away that closes the oval. Here is a sketch that captures what can be seen visually, which would be an impossible image: https://www.cloudynights.com/topic/399146-sketch-of-m42-m43-dobsonian-22/?p=5112451

If you were a visual observer, you'd appreciate that: There is a lowest power that works in the scope. There is a highest power that works in the scope. The range of usable magnifications is likely a 15:1 range or larger. Seeing conditions often limit the maximum to a lower power than the highest. You want the magnification changes to represent a noticeable difference when you move up in power. Objects in the night sky come in a huge variety of sizes, from, perhaps, an arc second up to over 14,000 seconds. needless to say, the magnification for one is not the right magnification for the other. So, to yield a reasonable range of magnifications, you will need around 6-8 eyepieces. And if planetary observing is a featured use, due to the seeing conditions, many like to have several high power eyepieces close together to accommodate the fact each night may have a different highest power usable. I think people build up much larger collections of eyepieces because they have multiple telescopes, or because they simply don't get rid of older, unused ones.

It looks like there is evidence the lower retaining ring was loosened and retightened with a tool that had blades wider than the slots in the retaining ring. And that caused the scuffing on the paint. However, I'm a bit concerned about the stuff on the lens right inside the retaining ring. Either the lens is filthy or, maybe, the same tool that chipped the paint scratched the lens as the ring was tightened. I hope not, and that it cleans right off with a cotton swab and some alcohol.

This process, called "blinking", doesn't work well in light polluted environments because the ambient light reflecting off the nebula filter is brighter than what can be seen through it. It works OK in really dark skies, but, as a technique, its true value is in planetary use, where you can quickly see what filter you want to use to yield the detail you want to concentrate on.

You don't focus on the nebula, you focus on the stars in the field (and there is always a star to focus on). And unless the filters are perfectly parfocal, which they realistically are not unless extremely expensive, like Astrodon filters, having the clear filter is a waste of time. Because you'll have to refocus on the next filter anyway. The H-ß filter will be the darkest of all, so having it follow another filter with a wider bandwidth might be a good idea.

The 30mm Ultima Edge focuses only 5.1mm above the focal plane of the scope. That says your scope's focal plane is very far out in the focuser travel if you had to use an extension tube. I wonder if it's possible to lower the primary mirror in your scope by a few mm so you don't have to use an extension tube.

I've discovered a lot of eyepieces have internal vignetting, which can make the field stop somewhat vague in the flashlight test. It's usually pretty easy to see where light goes to zero, though. The more difficult part is determining where the exit pupil is because if it's large and there is SAEP, its position becomes quite vague. Though, this doesn't make a large difference in the calculation of apparent field if the circle measured is at least a meter from the scope. I discovered that this measurement can be improved by stopping the scope down, making the f/ratio longer and the exit pupil much smaller. It is pretty easy to determine the exit pupil position when the exit pupil is small. Some day when I have time, I'll do this with a bunch of eyepieces to determine the accuracy of their eye relief figures. At this point, I don't have enough data to determine how accurate that figure is for most eyepieces. I propose an index card with mm markings on it that can be held up to the eyepiece to measure the effective eye relief, added to a measurement of the lens depth. The advantage would be a determination of both design and effective eye reliefs at the same time.

http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p41976 Go down the page to the second chart, which has hyperlinks to each individual review. Personally, I use the flashlight test to measure apparent fields, but the best I can do is +/- 0.2°. Ernest is a designer, so likely has more sophisticated instruments to measure the apparent field, like: http://www.southastrodel.com/Page209.htm#:~:text=A dynameter is an astronomical,particular eyepiece through a telescope https://adsabs.harvard.edu/full/1970JBAA...81...24C https://trioptics.com/us/products/optitest-visual-measurement-instruments/ https://www.cloudynights.com/topic/574401-an-easy-way-to-measure-apparent-field-of-view/ S&T uses a pivoting magnifier similar to a theodolite that can measure apparent field to <0.1°

Many brands Astromania BST (Barsta) Lunt Omegon Orion OVL (First Light Optics) Skywatcher Telescope Service

Well, not quite dead on: XW: 40 68° 30 68° 20 70° 14 69.4° 10 68.4° 7 69.3° 5 69.3° 3.5 68.5° XL 40 63.3° 21 61° The point is that without some serious pincushion distortion (not Pentax' general rule), a 28mm is more likely to be ~52° in a 1.25" barrel.

That's true. Distortion can make a difference. But are you sure the XL28 was really 55°?

My field analysis: https://www.cloudynights.com/topic/849945-new-85°-pentax-eyepieces/?p=12369694 and the following post as well. If you just touch the eyecup with glasses on, but do not press, I think you will see maybe a 75° field? I didn't analyze that, but the field didn't get that much larger when pressing hard to see the field stop. It also seemed a little easier in the 16.5mm than in the 23mm.

Sorry to burst your bubble, but that 27mm focal length has a 53° apparent field in all 8 labels (maybe more) it's sold under. The whole line is also sold in colors if you look up "AngelEyes". https://www.aliexpress.us/item/2251832657095693.html?spm=a2g0o.productlist.main.3.74a32ed8hivC3Y&algo_pvid=9c214683-03aa-4896-a23e-04b5f0916fb0&algo_exp_id=9c214683-03aa-4896-a23e-04b5f0916fb0-1&pdp_ext_f={"sku_id"%3A"65093639638"}&pdp_npi=2%40dis!USD!69.9!48.93!!!!!%4021021d7b16716629313185371d0793!65093639638!sea&curPageLogUid=RhVSeCCVfg2F 8, 12, and 16 are 60°. The 19mm is 65°. And the 27mm is 53°. When dealing directly with China, beware false advertising. There's a lot of it. Likewise, a 36mm 1.25" Plössl would have at most a 47° field, and likely smaller.

Low price: BST Starguider ED or the Barsta 58° eyepieces that are very common. Both are available under many different labels. FLO has the Starguider EDs. A little more: Celestron X-Cel LX or Meade S5000 HD60. The Meades would only be available used. FLO has the Celestron X-Cel LX. High-end in the same apparent field: TeleVue Delite. Looks like FLO has those too.

You are assuming the engineering diagrams are wrong, and they are not. The problem is that the virtual field stops, the figures relevant to our figuring out the true fields in our scopes, have not been quoted. Other companies have not published the engineering diagrams and so we do not know the physical field stop figures for them. So they are not discrepancies, merely not figures relevant to amateur astronomers. I agree, they should be corrected for amateurs to have a better idea of what true fields they can see in their scopes.

21.7mm was measured in a lab. It could be in error a couple tenths, I suppose, but unlikely in error by much. A distortionless 18mm 65° would have a field stop of 20.4mm. Of course, it isn't distortionless. 23.4 is the diameter of the diaphragm in the eyepiece, the mfr's quote, a figure irrelevant for our purposes. You can unscrew the barrel and measure it. 27mm is simply a misprint, as it is impossible in a 1.25" 18mm eyepiece since that translates to an apparent field that cannot fit into a 1.25" barrel at that focal length.

FLO, like APM and Altair, merely copied the numbers from the mfr, which are the actual FS diameters, not the virtual field stops that matter to us. Effective field stops ARE usually bigger than the internal physical stop but not always. Sometimes it depends on the aperture of the next lens up or the type of distortion in the field.

Eyepieces with negative field lenses have a physical field stop in the form of an iris between the negative lens and the positive set higher in the eyepiece. One thing that happens is that the negative lens moves the focal point up in the eyepiece, away from the physical field stop to a "virtual" field stop in between the lenses. Since the image is expanded by the negative lens, this virtual field stop will be a different size than the physical stop itself. The confusion comes from the listing from the manufacturer of the actual physical field stops, which no other manufacturer quotes in negative/positive designs. All the other manufacturers quote the "virtual" field stop figures, because THEY are what determines the actual true field you see, not the physical stop in the eyepiece. I find it useful to think of the virtual field stop as merely a magnified view of the physical stop that is below it. The measured virtual field stops are (lab, not field measurements): 30mm--36.3mm (though a couple people have seen 36.4mm, an inconsequential difference) 24mm--27.6mm (though the edge is a bit vague, and some have measured 27.3-27.5mm) 18mm--21.7mm 15mm--18.2mm 10mm--(actually a 10.5mm FL)--11.2mm

Only that in your two shorter f/ratio scopes the outer 20% of the field won't be very sharp. If the eyepiece is used for planets, planetary nebulae, small globular clusters, and double star observing in a scope that tracks the turning of the Earth, that will not matter at all. If the eyepiece is used to watch objects drift across the field in a non-tracking scope, or for Moon observing, the lack of sharpness in the outer field may bother you (or may not). However, in your two longer f/ratio scopes, there won't be a problem and the edge will clean up significantly. Get the Fine Tuning Rings for the Hyperion, and you also have other focal lengths with the same eyepiece: 4mm (14mm FT ring), 3.2mm (28mm FT ring), and 2.6mm (14+28mm FT rings)

It's more likely the Zeiss Astroplan configuration: https://stargazerslounge.com/topic/307483-plossl/#comment-3362020 or https://www.cloudynights.com/topic/52439-schematichelp-needed-for-meade-5000-55mm-plossel/?p=682332

1. True. 2.True 3.True, though in my experience the rubber eyecups don't scratch the glasses. 4.True, though you have to adjust it every time the scope is moved unless the scope is a refractor or SCT. 5.And that is easy to accomplish with the DioptRx, though they are eliminating the quarter diopter steps in favor of half diopter steps. 6.Like many presbyopic observers, if I use the telescope without glasses, neither the sky nor my notes are in focus. So I need to put glasses on to look at the sky or my notes or my DSC screen. I might as well wear the glasses at the scope, too, because that way I can share the view with others, whether they wear glasses or not, and I don't need any extra optics. 7.You're lucky. 8.That depends what you use a low power eyepiece for. If it's for large star clusters, then astigmatism is contraindicated. 9.Exactly. 10.I have compared as well, but I don't see a difference, though I do see a difference between eyepieces.

No eyeguards are in the way when a DioptRx is used. Any/all eyeguards are removed prior to mounting a DioptRx. And it is true that, when used on some eyepieces, the turnability of the DioptRx is compromised and turning of the eyepiece is required. That has no effect on the eyepiece, though, since the only eyeguard that can be used is the round one on the DioptRx, which doesn't have a direction. It's a simple flip up/flip down type.

I often start with that one, and simply bypass the longer focal lengths. I like the 9mm a tad better, though.

Jeremy, The DioptRx also fits: --most ES eyepieces --Baader Hyperions --Baader Hyperion Zoom --Baader Morpheus --Pentax XW --APM UFF 24mm --APM Hi-FW 12.5mm --APM Super Zoom And I'm sure a lot of others. Basically, any eyepiece with at least a 2mm long top section 41-43.9mm in diameter. It can even be made to fit other eyepieces in various creative ways.

A little lacking in details, and the 4.8mm and 3.5mm were not reviewed, but, overall a fair comparison. I was shocked he could use the 20mm with glasses. He must have shallow eye sockets and glasses lens very close to the eye. In doing a lot of comparisons, I find the XWAs differ across the line in both correction and contrast. The 7mm and 4.8mm are both excellent in that regard--the best in the series in a coma-corrected dob. They're all worth trying at the very low price they currently sell for.