Don Pensack

Just a note: the UltraFlatField eyepiece he tested is a 24mm, not a 25mm.

König actually designed 38 different eyepieces during his career, including a design later used by Clavé in their "Plössl" line. It's unclear whether Clavé changed some glass types to yield better chromatic correction. The 3-element one is well-known, but the ones produced under his name in Japan and sold in the '80s and '90s had 4 and 5 elements, and were not the 3 element design for which he is best known. The first ones sold by University Optics were the König design in the top chart, not the bottom chart.

Current Lumicon UHC and O-III filters pass no red. Here are some other filter tests using prisms and spectrophotometers: NOTE: many tests are older and do not represent the current filters. Astronomik narrowed their filter bandwidths in the 2016-2017 range TeleVue did so in 2018 Lumicon changed their bandwidths and transmission spectra in 2018. So reports from earlier may not represent current filters. This site is quite current: https://searchlight.semrock.com/?sid=a08a1af9-84ee-49d2-959d-153d7e7c0eb8# As is this: https://www.cloudynights.com/topic/817105-2022-nebula-filters-buyers-guide/#entry12318142 Here is the list of test sites: http://www.loptics.com/articles/spectra/spectra.html http://astrosurf.com/viladrich/astro/instrument/filters/Filters.html http://karmalimbo.com/aro/reports/Astronomy Filter Research Paper_by Jim Thompson.pdf http://www.reinervogel.net/index.html?/Filter/Filter_e.html http://www.cloudynights.com/item.php?item_id=1520 http://karmalimbo.com/aro/reports/Band Pass Filters For Visual & Video Astronomy_Part1.pdf http://www.reinervogel.net/ http://www.astronomy.ru/forum/index.php/topic,68257.0.html https://www.cloudynights.com/topic/734419-radian-triad-triad-ultra-performance-vs-f-ratio/#entry10581011 http://www.reinervogel.net/index.html?/Filter/Filter_Hb_e.html http://www.astronomy.com/~/media/import/files/pdf/8/c/7/0805_nebula_filters.pdf http://karmalimbo.com/aro/reports/Test Report - LP Filters Under Dark Skies.pdf http://www.astrosurf.com/buil/filters/curves.htm#TeleVue NebuStar http://www.karmalimbo.com/aro/pics/filters/narrow.jpg http://www.karmalimbo.com/aro/pics/filters/o3b.jpg http://www.karmalimbo.com/aro/pics/filters/o3a.jpg http://home.freeuk.com/m.gavin/grism2.htm http://sweiller.free.fr/spectro/filters/Wratten/Filter 26-29-30.jpg http://www.astrosurf.org/buil/filters/curves.htm http://www.astrosurf.org/buil/filters/curves.htm#Lumicon UHC http://www.carlostapia.es/curvas_filtros/revisiones.html http://www.astroamateur.de/filter/ http://www.carlostapia.es/curvas_filtros/Comparison_OIII_Visual.html http://www.astrograph.de/artikel/Spectral Transmission.pdf

The Optolong, like many "UHC" filters, has a bandwidth almost twice as wide as the better narrowband "UHC-Type" filters. As such, it also yields much less contrast on nebulae. Yes, it will work some, but the better narrowband filters have 22-28nm bandwidths, whereas the Optolong is 47-49nm. Compare with David Knisely's comments about the Deep Sky broadband filter.

The Rosette Nebula has substantial H emission as well as O-III. A narrowband UHC-type filter will help you see the Rosette a lot better than an O-III. Something like an Astronomik UHC, or TeleVue Nebustar or DGM NPB or Lumicon UHC or Orion UltraBlock.

Louis forgot you don't pay the punitive 25% tariff for ES that US dealers have to pay. Still, a 25% add-on to the wholesale price should only add 20% to the retail price, so that doesn't explain a 65% higher price in the US than in the UK. Explore Scientific is well-known to jack the prices up and then put them on sale 20% off for extended periods of time, here in the US. So that may be what is going on.

Contrast and stars: The higher the magnification, the fainter the star is that can be seen. That's because the sky in the eyepiece dims with magnification, but stars don't dim until very very high powers. So the faintest stars are seen at high power. Contrast and extended objects: Just like the sky in the eyepiece dims with increased magnification, so does the extended object. But, it is also larger. So it becomes more visible and features within it become more visible with increased magnification. But only up to the point where you start losing the ability to see those features or the object itself. So in the two curves--decreased brightness with magnification, and increased size with magnification--there will be a magic point where the viewing of the object is optimized for you--your eye, and your scope. Where that is requires experimentation. For a large faint nebula it could be at the lowest power. For a small bright planetary nebula, it could be your highest power. You don't know until you look, but after a few years of doing this, you'll get a feel for it and pretty much know where to go, magnification-wise, on each type, size, and brightness of object.

Bear in mind that 2 things will happen: 1) you will need about 2" more in travel of the focuser to focus the eyepiece. 2) you will have horrible and significant vignetting, turning the 36mm Hyperion into a 53° eyepiece. I strongly urge you to invest in some full-field 1.25" eyepieces like APM 24mm UFF, ES 24x68°, TeleVue Panoptic, 24mm Hyperion, 32mm Plössl, ES 26x62° etc. and skip the idea of using a 2" eyepiece in a 1.25" focuser.

Since focal plane positions in eyepieces vary a lot, there is a simple way to determine the magnification of any barlow for any eyepiece and automatically take into account the focal plane position in the eyepiece: Point scope at star near the celestial equator (it can be anywhere, but the process is faster near the equator). Time the passage of any star from edge to edge in the eyepiece. Insert eyepiece in Barlow and time the passage of the same star across the field. Time 1 / Time 2 = Barlow magnification factor. You can duplicate the process with any eyepiece.

It sort of goes without saying that eyepieces should be capped if you step away from the scope.

Dew doesn't form in windy environments. The simple act of blowing air evaporates any condensation. I've accidentally fogged up an eyepiece on a night when my scope was sopping wet, and the small hand fan cleared the eyepiece in seconds. Now, I do have to say that if dew is a problem on the secondary, it might be necessary to use a heater of some sort. You can buy small 12V hair dryers (camping item, I'd guess), and they work fine to evaporate condensation on secondary mirrors and eyepieces. Your choice. A mildly cold environment where you accidentally exhale on the eyepiece can be handled with a fan or unheated blower. A seriously dewy environment like the dew shower I experienced in Australia will need heating.

So spring for a couple Morpheus eyepieces. Maybe buy one at a time. I have the entire set and the 4.5mm has the least eye relief, but it's still usable with glasses on with a bit of pressure of the glasses against the rubber. This is one example of spend more/get more. I'm an ocularholic, but I could be happy with them for the rest of my life. Since the OP is OK with 50° eyepieces, Vixen SLVs could be OK, and there are a LOT of focal lengths available.

It has multiple uses: https://www.opolar.com/products/2019-new-opolar-battery-operated-air-duster?currency=USD&variant=31955082051664&gclid=Cj0KCQiAveebBhD_ARIsAFaAvrGhr0QiwkIBOYZxmlCijifU_WqVr4SOqP0Urjx6JavhKqEwIdTSJPgaAsV8EALw_wcB Something similar might be available in the UK. Mine came from China directly. As for inadvertent fogging of the eyepiece with your breath, I keep a folding Japanese fan in my pocket and it works great to evaporate the moisture. You can find them easily on many websites. It's called a sensu. You can wave it vigorously and blow a lot of air at the eyepiece.

I always thought the sv was like the sv in svelte, as cajen2 suggests. Unless it's in Latin, where the v is the same as our short u. Yes, of course it is a made-up name for westerners. And, like the crazy drug names, it's almost like someone threw some alphabet blocks in a hat and pulled them out and arranged them. I can tell you it is very hard to trademark any name these days because so many good names are already trademarked. Eyepieces from "Svbony" originally had the name "Vite" on the side, but probably interfered with western trademarks and so it changed. As far as I know, the company is Henan Yisair E-Business Co., Ltd in Shenzen, Fujian, China. And I believe they only sell direct to consumers, though they have warehouses here and there.

Though all the Ethos can be used as 1.25" except the 17mm and 21mm.

Close. I prefer to have Morpheus size fields and wider, so my 30mm and 22mm eyepieces are 2", and the 17.5mm has a permanent 2" adapter attached, but from 14mm down to 3.7mm, I use all the eyepieces as 1.25" except the Apollo 11, which does not work as a 1.25" in the Paracorr. If an eyepiece is usable as 2" and 1.25", why use it as a 1.25"? Couple reasons: --many dual size eyepieces focus farther in as 1.25" eyepieces (e.g. Morpheus), so the focuser has less leverage out at the end of the eyepiece to cause sag. Some are lighter as 1.25" and heavier as 2". --I can leave the adapter in the Paracorr and merely switch eyepieces (applies to my 14mm, 12.5mm, 9mm, 8mm, 7mm, 6.5mm, 6mm, 4.7mm, 4.5mm, and 3.7mm) --used as 1.25", all the eyepieces use Paracorr settings clustered together (A-D), which is very useful, and on the closer-to-focuser end of the tunable top's adjustment range, also yielding less sag. --most eyepieces sit closer to the focuser, making the focuser knobs almost ideally placed. So 4 of 14 eyepieces are 2" and 10 are 1.25", or 71% 1.25". With an upcoming scope (2023), one of each size will leave, so it will be 3 to 9, or 75% 1.25". I'll never get rid of 2" entirely though.

I suspect 23mm will be the longest focal length because keeping up with Pentax's other set, the next logical focal length is 32-33mm, and that would be quite difficult to do in a 2" diameter eyepiece and keep it 85°. Plus, it's difficult to see what they could bring to the table in terms of performance that the 30mm and 40mm XWs don't already offer. If they end up being excellent eyepieces, a set of 23mm, 16.5mm, 12mm, 8.5mm, 6mm and 4.5mm would be a great progression.

In the flashlight test, remember the distance to the wall is measured from the exit pupil of the eyepiece, not the actual eyepiece itself. I found it helps to use an aperture stop on the scope when doing the test, as this significantly reduces the size of the exit pupil and makes the exit pupil to wall measurement more accurate. It also, by the way, makes measuring the eye relief of the eyepiece easier.

More to the point, however, is that apparent fields in eyepieces, when tested, rarely deviate by more than 2° from the advertised value. And truly, is 2° important? Not really, even if comparing 40° to 42°. So I think it is fair, unless the mfr specs have been tested and everyone knows the actual AFOV, just to assume the mfr's AFOV quote is correct. Especially with the better grades of eyepieces discussed here on SGL.

For a narrow field eyepiece, like an ortho, yes. Then globular's formula works. But it doesn't work for wide fields or ultra wide fields because distortion increases with apparent field (see chart). And to calculate apparent field from field stop and focal length when distortion exceeds 10% (as it does on all wider fields) requires using distortion in the formula. And unless we know the exact distortion %, the AFOV will not be accurate. The best way to test the AFOV is the flashlight test: https://www.cloudynights.com/topic/574401-an-easy-way-to-measure-apparent-field-of-view/?p=7958975 https://www.cloudynights.com/topic/574401-an-easy-way-to-measure-apparent-field-of-view/?p=7959408 Here is a chart showing how Angular Magnification distortion and rectilinear distortion increase with apparent field:

Let's do the math. A 20mm eye relief from the glass at 85° means the eye lens must be 36.7mm wide (exposed part). I'll presume a 39mm lens diameter to the edge and a 1.5mm wall thickness of aluminum around the edge = 39mm + 3mm = 42mm. So it is indeed possible there could be a 43mm thread under the eyecup, like the other XWs. If so, a Baader Morpheus eyecup could be threaded on and, folded down, likely increase the effective eye relief enough to use glasses. 15mm of eye relief = smaller lenses. (100° and 15mm eye relief = 35.7mm exposed eye lens) all lenses below the eye lens can also be smaller. 20mm of eye relief = larger lenses (92° and 20mm eye relief = 41.4mm exposed eye lens) To wit, the 25mm 100° weighs 1176g The 17mm 92° weighs 1159g A 22mm with the same characteristics could be 1450g (3.2 lbs). I can see why they might hesitate. Especially if it has to sell for ~$1000 in the US. Eye relief is measured from the glass, so whatever is above it, if machined off, will not increase the eye relief of the eyepiece. It will, however, increase the effective eye relief for glasses wearers. A Morpheus eyecup increases the effective eye relief on the 70° XW. I'm hoping it will do the same for the 85° versions.

At f/5, I recommend only the 22mm and 17mm in that line. The other focal lengths will all have significant problems in the scope. The 32mm will be particularly bad.

Ricoh told me they have no intention to discontinue the 14mm and 20mm 70° XW. The XW85 will be a new series. If successful, and I think they will be, it is likely these will only be the first focal lengths in the series. More of concern is that though they give a 20mm eye relief from the glass, they are stating only a 12mm eye relief from the rubber eyecup, ruling out glasses use. If there is a way to modify the top and change the eyecup to yield more eye relief, then they would be sold to glasses wearers. As soon as I get my hands on them, I will investigate this.

ES told me several years ago that 22mm was the maximum possible focal length for the 92° series, but that they were a bit scared to make it because it would be well over 3 lbs and physically larger than the 17mm. They thought that 2 shorter (than 12mm) focal lengths would come first. Then came slowdowns in production at JOC, then CoVid and then supply chain shortages. It seems to me that the 92° eyepieces have priced themselves out of the market in the US, though prices are more reasonable in the UK and EU.

In an earlier post, I commented about the difference between Design Eye relief (measured from the glass) and the Effective eye relief (measured from the rubber eyecup--the important measurement for glasses wearers). Here are some measurements I and others made: Eyepiece............eye relief/depth of lens/effective eye relief APM UFF 30.0 22.0 -4.90 17.10 NAGLER 22.0 19.0 -3.00 16.00 MORPHEUS 17.5 23.0 -2.30 20.70 MORPHEUS 14.0 18.5 -2.30 16.20 MORPHEUS 12.5 20.0 -2.30 17.70 APOLLO 11.0 18.0 -3.70 14.30 MORPHEUS 9.0 21.0 -1.90 19.10 ETHOS 8.0 15.0 -3.20 11.80 APM XWA 7.0 13.0 -4.00 9.00 MORPHEUS 6.5 18.5 -1.70 16.80 ETHOS 6.0 15.0 -3.10 11.90 ETHOS SX 4.7 15.0 -5.25 9.75 MORPHEUS 4.5 17.5 -2.10 15.40 ETHOS SX 3.7 15.0 -5.20 9.80 APM UFF 24.0 29.0 -12.4 16.6 APM UFF 18.0 20.0 -4.3 15.7 x-Cel 25.0 16.0 -2.2 13.8 x-Cel 12.0 16.0 -2.0 14.0 x-Cel 9.0 16.0 -2.0 14.0 APM Super Zoom 19.0 -2.5 16.5 8mm Delos is 1.9mm eyelens depth Effective eye relief 18.1mm 9mm Delite is 3.1mm eyelens depth Effective eye relief is 16.9mm 10mm Pentax XW is 4.9mm eyelens depth Effective eye relief 15.1mm