Louis D

At f/6, the WO SWAN is going to start showing astigmatism at around 60% out and be a complete blur in the last 15%.

The APM UFF is going to be perfect edge to edge.

I guess it depends on your tolerance for edge imperfections.  If you nudge your Dob often enough, you could keep the object in the inner 50% of the WO where it will be sharp.  The problem I found with these types of eyepieces is that they fail as finder eyepieces because stars look like nebula in the outer regions, so it makes it difficult to locate globular clusters and planetary nebula and center them for higher power viewing.

Compare the lens design of the WO SWAN (5 elements in 4 groups) with the APM UFF (9 elements in 5 groups):

The APM UFF is a technological tour de force.  It employs a novel telecompressor stage before the field stop such that the field lens (40mm) is larger than the physical field stop (30.4mm), while the effective field stop is larger (36.4mm) than the physical field stop.  This keeps the eyepiece very narrow for it's focal length and AFOV while allowing for near perfect correction.

The 30mm APM UFF is so good, I swapped out my venerable 27mm Panoptic for it in my A-Team eyepiece case.

Compare that to the 35mm Baader Scopos Extreme which is a giant beast with a typical telenegative stage ahead of the physical field stop, which is larger than both the field lens and the effective field stop diameter:

Since the 30mm APM UFF is so much better in the same price range, I don't know why anyone would look at the 31mm Hyperion.

2 hours ago, Ratlet said:

The guess it's a bit like normal binoculars in a sense, only rather than focusing each eye, you are matching image scale?  That's really cool.

Exactly, the image scale has to match to merge.  However, it's really easy to judge because objects not only have different sizes between images, they are also displaced radially from the center by different amounts.  It's actually super cool to see the merge happen and the brain locking the two images together suddenly with much greater fidelity than either image alone.

9 minutes ago, Franklin said:

Handy for binoviewing though.

Not really.  I just grab both zooms and twist them in sync until the power roughly frames what I want to view.  I then slowly adjust the zoom at my nondominant eye back and forth a bit until the two images snap into merger.  It's pretty quick and obvious when it happens, much more so than when best focus has been achieved.  I originally had concerns it would be an issue merging non-click-stop zooms in a BV, but it hasn't been an issue at all.

Besides, with click-stops, you're relying on tight manufacturing tolerances for the two eyepieces to be perfectly matched at every click-stop.  If one eyepiece's click-stop magnification is slightly off from the other eyepiece's, it would be hard to move the zoom mechanism of one of them off by a small amount because the click mechanism would want to force you onto the nearby detent.  Thus, you're having to force your brain to merge two images of slightly different magnifications in that situation.  It's doable, but not comfortable long term.

To avoid a headache or eyestrain during power changes since my brain is trying to merge unmergable images, I concentrate on the view through my dominant eye since I can't twist the two eyepieces at exactly the same rate.  This would be an issue with click-stops as well unless you looked away entirely and just counted clicks.

22 minutes ago, Don Pensack said:

It is known that JOC sells these eyepieces with seals (Explore Scientific) and without (many direct-from-China providers).

They toss in a seal with each eyepiece?  Cool!!!

9 minutes ago, Don Pensack said:

Not to mention the large amount of spherical aberration caused by moving the mirror.  The SCT is only "diffraction limited" in a very narrow range of focus.

Do Maks also get a bunch of SA when moving the mirror far away from the design point?

28 minutes ago, Don Pensack said:

I don't think a reduction in aperture will be as important as a big increase in spherical aberration.

Does the R/C induce SA?

I compared the BST Starguiders (AT Paradigms for the set I picked up) to the Meade 5000 HD-60s in the following thread some years back:

I preferred the HD-60s due to much better eyeglass friendliness and somewhat sharper views.  The 12mm and lower focal lengths in both lines are the standouts.  Unfortunately, the HD-60 line has been discontinued.  The Celestron X-Cel LX line is the closest in performance to them from what I've read that are still being sold new.

13 hours ago, Millenium said:

What is the difference between the two above vs. this one Teleskop-Express: TS-Optics Ultra Wide Angle Eyepiece 20 mm 1,25" - 66° field of view , I'm thinking specifically for the top part, this one doesn't have adjustable part for eye relief, is that mainly for people using glasses? For non glass users, should I rather get non eye relief EP?

I compared the SVBONY 68° Ultra Wide Angle 20mm, which is optically the same as the TS-Optics UWA 20mm, to other eyepieces in the 20mm range in the following thread:

You can look at the AFOV images to get an idea of how it performs at f/6.

It's fine in slow scopes (f12 and up) in my experience, but I wouldn't recommend it for your scope (f/6.5).  They're also terrific in binoviewers when used at f/18 in my experience.

Rini eyepieces are generally made from uncoated or single coated surplus lenses, and as such, have loads of internal and surface reflections.  They also don't generally have defined field stops, so they have blurry field edges.  They're fine for beginners on a budget starting out as I was 20+ years ago, but just about any commercial offering gives contrastier and sharper views.  I wouldn't pay over $40 or $50 for one.  I have the 1.25" 35mm MPL, 2" 38mm MPL, 2" 29mm MPL, and 2" 42mm Erfle.  You can check out my images of each of these Rini's AFOVs in the following thread and compare them to commercial offerings in the same comparison images:

You could repeat the experiment at a narrower field of view with the eyepiece alone to see if the astigmatism was R/C induced.  If it goes away or greatly reduces, it was the likely culprit.  If there is no change, it's likely due to the eyepiece.  That's all I was suggesting to help narrow down the source of the astigmatism you were seeing.

I got a sharp view of Venus last year through an ST80 using a green filter.  I don't remember which one I was using (I have several), but it got rid of the red/blue smearing of both the achromatic lens and the atmosphere.  Mainly its phase became obvious.

The mirror's backside probably got a bit of rough handling during manufacturing, that's all.  I would shine a bright light through the glass from the side to look for deep imperfections like stress fractures.  Surface scratches on the backside of the mirror are purely cosmetic.
Thoroughly check the rest of the scope for real damage like bent spider vanes or tube crinkles.  Those can't be fixed by the end consumer.

Online eyepiece specs have been known to be wrong by wide margins, so take those numbers with a grain of salt.

I've read that the ES equivalents sold by Ali Express can be factory seconds based on user experiences.  Returns via AE can be a pain in that case.

I would hope that Opticstar sells first quality optics.

On 05/07/2022 at 11:02, bomberbaz said:

I have a ZWO DUO Band filter which had a finger print smear on it, so I decided to use baader fluid to clean it.

I usually just blow off dust and then huff some breath moisture onto the surface and lightly wipe the fingerprint off with a clean microfiber cloth.  Don't scrub.  Repeat huffing and gently wiping with clean sections of the cloth in various patterns until it comes off.  I've been using the technique on camera lenses, eyepieces, and filters for decades without noticeable degradation of the coatings.  The quicker you get the fingerprint off, the easier it is to remove in my experience.  When I'm in the field in the dark, I wouldn't trust myself using cleaning liquids around optics other than breath moisture.

I'm loath to put chemicals on optical surfaces unless it's to remove hardened/sticky gunk, which most fingerprints are not.  The exception would be fingerprints at outreach events.  But, I wouldn't have my nice optics available for touching in that situation.

You were both probably seeing diffraction spikes from the secondary mirror spider vanes if it was a cross shaped artifact.  Try creating an off-axis aperture mask from cardboard or foamboard to leave only an unobstructed circle between spider vanes.  That should get rid of the spike artifacts and decrease the dazzling effect of Venus in particular by masking down your aperture a bit.  Adjust the measurements below for your 8"/14" scopes.

12 hours ago, bosun21 said:

I’m a little puzzled 🤔 When you say move the mirror forward in a SCT or Mak, are you simply referring to the mirror movement by means of the focuser? When you say “forward” does it mean towards the EP or the corrector? Surely you will have to position the mirror to the place that provides the sharpest focus. I apologize for the incessant questions.

 

Yes, focusing an SCT or Mak generally moves the primary mirror unless you've opted to add a Crayford style focuser to the rear port.  To move the focus position further from the back plate, the mirror must move forward toward the corrector.  This extends more of the light cone behind the back plate.  I would think this would also cause some vignetting because a wider portion of the light cone is hitting the secondary and passing through the rear port.

Yes, you have to position the mirror to provide best focus, and if you move the position of the eyepiece back 100mm from the rear port by adding a BV, that means best focus has also moved back 100mm from the rear port.

Try the experiment again without the reducer in place.  See if the level of astigmatism gets better or worse at the edge.  You may also notice increased field curvature or possibly coma (IIRC) without the reducer/corrector.

23 hours ago, bosun21 said:

Louis does the binoviewer actually have a magnification factor on their own without an OCA or barlow? If so, then to what degree? Thanks  

     Ian 

None beyond what effect they have on SCTs and Maks in native focus mode without an OCA/Barlow.  That is, to reach focus with a BV in a catadioptric scope without an OCA/Barlow element requires about 100mm or more additional back focus.  To do this, the primary mirror must be shifted forward which increases the scope's focal length due to the magnification factor of the secondary mirror.  There are formulas for both types to calculate the added focal length based on added back focus.

You could try using a 0.63x focal reducer/corrector.  That way, you get to roughly the same widest field by using a widest field 1.25" eyepiece with it without having to invest in a 2" diagonal, 2" visual back, and 2" eyepieces.

That comparison only speaks to the quality of Long Perng made eyepieces in general since the OP asked about the 68 degree line, not the 80 degree line.

According to Ernest's measurements, the 16mm Levenhuk Ra UWA 82° branding of the 16mm Nirvana 82° has a 25mm diameter field lens and a 21.2mm field stop.  Technically, it wouldn't vignette based on the field stop alone, but since the field lens is slightly larger than the BV's clear aperture (~22mm), there might be a bit of vignetting at the edge.  Since you have to look in the center to hold both images, you'd be unlikely to see this level of vignetting in your peripheral vision, though.

The Altair Lightwave Premium LER 68 is the same as FLO's StellaLyra 68º LER / WA.  Here's a recent thread on it and the StellaLyra 80° LER / UWA:

Ernest in Russia tested the entire line 68º here under the Levenhuk Ra ER20 WA branding.

I forgot to mention I need to wear eyeglasses when observing with exit pupils larger than 1mm, so that limits my choices.  Microscope eyepieces intended for eyeglass wearers are also a good choice if you can get the proper barrel adapters.  They are generally compact and designed to be very sharp at f/18.  If you don't need to wear eyeglasses, you have many more options.

I've recommended it many times before, but here it goes again.  Look for a used, vintage Meade 140 2x Barlow to use as an OCS/OCA/GPC.  They regularly go for between $40 and $65 over here on our astro classifieds.

The nosepiece is a three element, filter threaded unit that can be screwed onto the front of a WO/Arcturus/etc. BV.  It operates right at 3x in that configuration and is insanely sharp.  I have no trouble reaching focus in my Dob that has a low profile focuser and 20mm of in-focus left with eyepieces that focus at the shoulder.  It also works great in refractors to avoid having to rack the focus in from its normal focus position.  Since I only view planetary sized objects with BVs, the 3x magnification is a plus.

If you are going to be using a 2x Barlow nosepiece operating at that or even a higher power, you can get away with using lighter, cheaper eyepieces.  I've found Svbony 20mm UWAs work really well at 3x (f/18 in my scopes).  I can't get my nose between fat eyepieces like the Morpheus.  That, and two of them plus the BV weigh a lot.