Don Pensack

6 minutes ago, John said:

The Tele Vue hybrid barrels work well enough with "Hi-Hat" style 1.25 inch adapters where the adapter slips up inside the 2 inch barrel part which gives the 1.25 inch clamp a longer barrel to grip. The only ones I have experience with are the Ethos 13mm, 8mm and 6mm though. I have used those eyepieces in a standard 1.25 inch fitting but much prefer the security that a 2 inch fitting provides. The downside of the hybrid design is that I needed to fit barrel extensions to these 3 eyepieces so that I could use 2 inch filters with them. On the whole I prefer the approach used with the 4.7 and 3.7 Ethos SX eyepieces where you are provided with a 2 inch barrel section that screws over the 1.25 inch barrel. Again, for security reasons, I prefer to use these long and heavy eyepieces in 2 inch mode. Also the bottom end of the 2 inch eyepiece barrel contains an additional light baffle which I think improves the contrast a wee bit.

On the number of eyepieces used in sessions, I have had sessions where I've used just one and others where most of my collection have had a turn in the drawtube. It all depends ......

 

I don't worry about where to put filters.  I use a Paracorr and mount all filters on the Paracorr.

In my refractor, I put all the filters on the 2" adapter.

Hence, all 2" filters, even with 1.25" eyepieces.

With the threaded-on adapters, all the weight of the eyepiece is hanging on a few mm of 28.5mm thread.

I don't think it is more secure than using the eyepiece as a 1.25"--maybe less.

Though, in practice, it works fine.

15 hours ago, globular said:

The 70 degree versions claim 20mm/15mm eye relief and the 85s claim 20mm/12mm.

The 20mm is measured from the centre of the top face of the top lens.  And the 15mm & 12mm are from the top of the rubber eye guard.

The eye guards are the same for both eyepieces so they sit the same distance above the outside edge of the lens housings.

You can see from my measurements that the centre of the glass to the top of the eye guard differs by 7 - 4.5 = 2.5 mm between the two EPs.  

This is close to the 3mm difference in usable eye relief claimed - so I think their figures are pretty good / my measurements are fairly accurate - although my wife says they feel closer to each other in use to her bespectacled eyes.

How much wiggle room do you have when using the XW40 @Louis D?
You could try winding it out 2.5mm from the fully in position on your XW40 and that might give you an idea of how the 85 degree versions will behave. 

I found the difference just enough that the 85° were a NO-GO with glasses, while the 70° are very comfortable.  I'm very sorry about that, because the optics in the 16.5mm are excellent.

22 hours ago, Louis D said:

Thanks so much.  Yes, I was measuring at the edge.  That might explain the 20mm/12mm eye relief difference for the XW-85s if the 20mm refers to the design eye relief which will be ~8mm more than the usable eye relief (12mm).

I measured the depth and assumed the eye relief from the glass was 20mm.

The depth of the lens below the rubber eyecup was exactly 8mm.

18 minutes ago, Louis D said:

I like my Redline (AT AF70, etc.) eyepieces' take on 2"/1.25" barrels.  For the 17mm on down (in terms of focal length), they have a screw-on/off 2" skirt that is parfocal with the 1.25" barrel.  You can leave it on all the time (as I do) or off all the time (for 1.25"-only scopes), or swap it in and out if that's your thing.  I'd like to see more eyepiece adopt this very useful feature.

I haven't found any particular advantage to the Tele Vue style non-removeable skirt on my 12mm Nagler T4 or 14mm Meade 4000 UWA smoothie.  There's generally not enough 1.25" barrel sticking out to safely lock it into a 1.25" focuser, not to mention it vastly increases the in-focus requirement in this mode.  Also, you can't use 2" filters with the skirt unless you screw on a 20mm 2" extension first.  It seems like a solution in search of a problem.

The 2" skirt fits OVER the 1.25" focuser and a thumbscrew in the 2" skirt tightens down against the 1.25" focuser.

The TeleVue Hi-Hat adapter fits up inside the 2" skirt to grab a longer section of the 1.25" barrel and the slot in the 2" skirt allows the thumbscrew in the adapter to be accessed.

TeleVue's eyepieces and adapters were made to work with each other.

I do note, however, that the 2" skirt idea is now down to only 4 of TeleVue's eyepieces.

If you want to use the eyepiece as a 2", the Ethos 4.7 and 3.7 and all the APM XWAs and the TeleVue Apollo have a threaded-on 2" adapter that works fine.  It's a "better mousetrap" for a dual-size eyepiece.

And I agree, Louis, the Barsta 70° eyepieces have a nice adapter.

You might like this other site, which is comprehensive, but a little less technical, by Bruce MacEvoy:

https://www.handprint.com/ASTRO/ae1.html

https://www.handprint.com/ASTRO/ae2.html

https://www.handprint.com/ASTRO/ae3.html

https://www.handprint.com/ASTRO/ae4.html

https://www.handprint.com/ASTRO/ae5.html

https://www.handprint.com/ASTRO/ae6.html

Not all his data or illustrations are correct, but, by and large, the information is quite accessible by someone not educated in optics.

Judging by the lenses added in the lower barrel, it doesn't look like they could push the design much farther and keep it a 2" eyepiece.

But APM is unlikely to pay Mark Ackerman to design a 35mm when the 30mm is already quite good.

Here is an illustration of the UFF designs, with corrected field stops that correspond to the field diameters seen in the eyepieces.

Note: the 10mm is actually 10.5mm:

4 hours ago, Louis D said:

I'll have to look for EOFB sometime in it.  I once noticed my 12mm NT4 had brightening all the way to the center.  Only a small point appeared to be relatively dark in the center.  I swapped it with my 12mm ES-92, and the brightening went away.  Swapped them again, and the brightening was back.  I'm not sure what was going on that night, but it was highly disappointing to see in a TV product.  I even checked for fogging, but there was none.  I'll have to do a test sometime with those two plus the 13mm Redline, 12.5mm APM Hi-FW, and 14mm Morpheus.  It might even be scope specific.  I can't remember which scope I saw the EOFB in the NT4, but it was probably my Dob.

I noticed a slight bit of EOFB in the 17mm T4, but not in the 12mm.

It has been reported that the longer the f/ratio, the narrower the EOFB, but the brighter it is as well.

That points to it being related to inadequate internal suppression of scattered light from spacers, or lack of baffling, lens edges, or possibly angular magnification distortion in combination with those.

Many new 1.25" eyepieces come with a threaded-on 2" adapter, and that adapter has an additional baffle in the bottom of the adapter.  A brief look in several of them with and without that additional baffle showed me

that contrast in the outer field improved when the 2" adapters were added.

Accordingly, if you have one of those eyepieces (Ethos 3.7mm and 4.7mm, Apollo 11, APM/Astrotech/Stellarvue XWA), I recommend using the eyepiece as a 2" for maximum contrast unless you experiment and find that, in your scope,

there is no visible difference (some scopes have pretty aggressive baffling).

14 hours ago, Louis D said:

Literally every one of my eyepieces but those two I mentioned above make extended objects appear larger and larger as they approach the edge; thus higher magnification at the edge than in the center.  It's pretty obvious in my ruler images.

Put another way, the moon gets stretched into an egg shape with the large end toward the edge in practically all eyepieces to some extent.  By way of comparison, those two eyepieces I mentioned compress the moon's side facing the edge.  Perhaps we're describing edge magnification in different terms.  I'm describing in terms of area of the AFOV covered which is a rectilinear description, I believe.

Yes, rectilinear distortion is a radial distortion.  It stretches things at the edge along a radial, but not tangential, line.

Angular magnification distortion compresses the field in both radial and tangential directions, which is what happens when the entire field has a lower magnification.

Distortion is distortion, however.  Neither form, in the percentages found in ultrawide eyepieces, allows the moon to stay round at the edge.

In star patterns, you'd never know, however, unless the eyepiece is panned or the power is high enough the field moves across the eyepiece fairly quickly.

And you might never know if the scope tracks.

AMD usually is visible on straight lines as barrel distortion (negative RD), as the edge is compressed.  That leads to the visibility of "rolling ball" or "globe" distortion, as if looking at a globe rather than a flat field.

Zero AMD and high RD (positive, or pincushion) makes the field appear somewhat bowl shaped, but it is not different than how the eye sees the sky, as closer at the zenith and farther at the edges.

Since star patterns do not follow straight lines, this is usually the preferable type of distortion.

Pulling back a bit on the correction of AMD and allowing some to be in the field reduces the amount of RD seen and this is not uncommon in eyepieces (e.g. 14mm Morpheus).

But the Moon will still distort as it nears the edge.  Distortion is distortion.

One thing I learned is that you don't view the moon at low power near the edge of any ultrawide eyepiece because of distortion and because those eyepieces are rarely free of lateral chromatic aberration.

But having a fairly large amount of positive RD in the form of pincushion does not preclude having a flat field.  The 30mm UFF is an example--obvious pincushion, yet a map-flat field.

It's too bad the shorter focal lengths of UFF don't have the same internal prescription.  That would have been an impressive 70° line, albeit more expensive than they are.

P.S. the 13mm Hyperion shares this issue, just to a somewhat lesser degree.

11 hours ago, Louis D said:

Are you saying that even at f/10 those two Hyperions are not sharp to the edge?  If so, are they pretty close to that goal?  By way of comparison, my 13mm and 17mm Redlines (Astro Tech AF70 version) massively improve going from f/6 to f/12, but not quite perfect.  They are very similar to the Hyperions in design parameters, so I would expect similar improvement.

My problem with the Barsta 70° series (AF70/Redline/Olivon/et.al) 13mm is that it has the worst Edge of Field Brightening (EOFB) of any eyepiece I've ever seen, extending inward from the field edge 50% of the field radius and making the entire field look like you are looking into the center of a planetary nebula as large as the field.  It's even been photographed:

1 hour ago, Louis D said:

I've only got two eyepieces in my collection that have this property, at least in terms of radial magnification.  One is the APM Hi-FW 12.5mm and the other is a vintage Bausch & Lomb 15x WF (16.7mm) microscope eyepiece.  You can see this manifested as the millimeter tick marks growing closer together nearing the edge in my ruler images.

Below are the two group shots showing this.

APM Hi-FW 12.5mm which is 12.6mm on axis and 13.4mm at the edge:

 

Note that the APM squeezes in more true field in a narrower apparent field than does the 12mm ES-92 at just 0.6mm longer focal length on axis.  It's a pretty neat trick.

Bausch & Lomb 15x WF which is 17.1mm on axis and 17.5mm at the edge:

 

It makes sense that a microscope eyepiece would try to minimize magnification distortion across the field for research purposes.

There is some tangential magnification growth in both going from center to edge as can be seen in the bowing edges of the rulers.  It's just that it's a lot less in these two than in the other eyepieces.  I'm sure there's a way to work out angular versus rectangular distortion from these two types of magnification changes, but I haven't studied optical theory enough to know what it is.

Here is a chart showing AMD and RD.  Notice both increase as the field gets wider.  If AMD is corrected, the uncorrected RD is the span between the curves, or vice versa.

5 hours ago, Louis D said:

Not in my experience.  Only a very few eyepieces yield less magnification at the edge than at the center, at least in a linear manner rather than an angular manner.  I'll have to work out how to measure angular magnification someday.

Examples of lower edge magnification than center: Docter 12.5mm, APM Hi-FW 12.5mm, et.al.

I have seen many many eyepieces with some AMD, and they ALWAYS have had lower magnification at the edge than in the center.

Can you name one eyepiece where the edge has a higher magnification than in the center?

Yes, Morpheus all the way.

An alternative for the Mak is the 40mm 1.25" Plössl.

The exit pupil is larger (brighter image).

The magnification is lower (~39x), and the eye relief is a bit longer than a 32mm Plössl.

Apparent field will be 40-43°, about that of an Abbe orthoscopic.

**note: your "127mm Maksutov actually has a clear aperture of 121mm and a focal length with 1.25" diagonal of 1540mm, making the scope f/12.7.

That makes the exit pupil of a 40mm only 3.1mm, so a 40mm will be about the brightest image you can get.

True field will be 1.00°

If the visual back is shortened (the threaded portion on the visual back is removed), the focal length shortens to 1518mm and maximum true field increases.

The UFF 24mm actually gives you a bit more true field due to its larger field stop--1.03°, though 64x as a lowest power is a bit high for that aperture.

So I'll agree with Louis--a 32mm Plössl is a good compromise--48x and a 1° field.

9 hours ago, Ratlet said:

So the 17.5mm Morpheus turned up.  Nice bit of kit.  I've got clouds till the end of time so decided to do a completely unscientific comparison between it and the OVL 16mm and setup the 130PDS and used the moveshootmove camera holder.  I suspect I still need to dial everything in for using the camera holder, but it works well.  Getting a good view on the Morpheus was a bit of a challenge, probably due to the eyerelief and the fact that my Pixel 6 has protrouding cameras which removes some of the room for adjustment.  In both cases I tried to get the best possible focus on the flag pole.  Happy with the performance of the camera itself.

Here we can see the image from the Morpheus which looks pretty well in focus to the edge.

And here we can see the OVL.  To my eyes the OVL focus goes pretty wonky on the way out of the field.

Now the stupid question, is this what we mean when we talk about flatfield?

If the flagpole is a cylinder (likely), then there is some angular magnification distortion in your images.

The edge looks more magnified in both eyepieces.

I suspect this is in your camera as AMD in eyepieces is almost always the reverse--the edge has a lower magnification.

Your images also reveal the edge vignetting in the 16mm (often reported), and a slight brightening of the edge n the 17.5mm (not reported).

Also, the 17.5mm has the common edge of field CA--the blue ring, as in most complex negative/positive designs.

The 16mm appears to have none--it could simply be the vignetting that reduces its visibility.

48 minutes ago, bosun21 said:

Thanks for this. What f ratio is your scope? I’m just wondering whether the spacing will be different. Mine is f5. I will buy the GSO coma corrector as soon as I track one down. Thanks again 👍

The spacer length depends on what eyepiece you use that has the most in-focus, not the f/ratio of the scope.

The "working distance" for the lens is 75 +/- 5mm from the focal plane of the eyepiece.

But 2" eyepieces have their focal planes from more than an inch down into the lower barrel (below the "shoulder") all the way up to almost an inch above the shoulder.

To correctly choose the right spacer between the upper section of the GSO coma corrector and the lens, you first need to do a couple things:

1) determine where the focal plane of your scope is.  This can be done by putting translucent scotch tape across the 2" opening of the focuser, pointing the scope at the Moon, then focusing the moon on the tape.

That is where your scope's focal plane is.  Measure how far out the focuser is and write it down.

2) Insert the eyepiece you own that you know takes the most in-focus (or experiment to see which one it is).  Put it in the scope and focus the scope on the Moon.

Measure how far out the focuser is and write it down.  It will be a shorter distance than the focus on the tape.

The in-focus distance that eyepiece requires is the key figure.  Write it down.

3) measure the length of the top section of the GSO coma corrector and subtract that from 75mm.  Write down that dimension.

The spacer length you need will be 75mm minus the length of the top of the coma corrector MINUS the in focus distance for the eyepiece that needed the most in focus.

[example: 31mm Nagler needs about a 10mm in travel from the focal plane of the scope.  75-46-10=19mm spacer]

If a spacer of that length is inserted in the CC, the focal plane of the eyepiece will be the correct 75mm distance from the lens when inserted in the CC.

Then:

4) insert the eyepiece in the CC with the correct spacer added, and focus the scope.  Lock the focuser (or just don't move it).

For every other eyepiece you own, insert it in the CC and slide it out of the CC until it is in focus.  That will be the correct position for that eyepiece relative to the CC lens.

All your eyepieces will focus at the same place in the focuser travel when the CC is installed and the eyepieces correctly set.

Since that is not easy to do every time, I recommend parfocalizing rings be installed on your eyepieces so each one can be returned to exactly the same place each time it is inserted.

You can save time by parfocalizing all your eyepieces with the one that needs the most in-focus before you get the CC.  Then, after you use that in-most eyepiece to determine the spacer length in your CC,

all your eyepieces will be simple "drop-in" accessories that will already be in focus when inserted (+/- 0.5mm).

1 hour ago, Louis D said:

Same here in the US.  They won't ship to any country with an official Baader distributor different from themselves.

They're only $249 in the US, though.  You won't get them any cheaper from the EU.

4" f/8.6 = FL of 875mm, I would round off.

If it's a decent instrument, and it is, magnifications up to 240x are possible.

That is a 3.6mm eyepiece.

If you have the scope mounted on a tracking mount, the 3-6mm TeleVue Zoom is a strong possibility.  It's not just a single eyepiece, it's a whole set of planetary eyepieces in one because all the in-between magnifications can be used.

If you have the mount on a non-tracking mount, then I would not recommend a 50° eyepiece.  I would try to go wider--at least to the 62° of the TeleVue Delites (which come in 3mm, 4mm, 5mm, 7mm, etc.)

or to even wider apparent fields like 68-80°.  You may hear endless comments about how this eyepiece or that eyepiece is a better planetary eyepiece, but if you have to push the mount so often you only have a few seconds

when the scope has settled down to yield a sharp steady image before you have to push it again, those super-high magnifications will not be enjoyable to use.

It's a reason I use a 3.7mm 110° eyepiece a lot for planetary and lunar observing in my 4" refractor.  If the seeing supports such a high magnification, the eyepiece in the focuser will give a great view, even if it's not a Zeiss, or Takahashi.

All that presumes you don't need glasses at such small exit pupils, and few observers do.

What may be the case, however, is that the really small exit pupils might start showing you floaters in your own eye.

My 4", for instance, has great optics and can easily support a 3mm eyepiece, but my vision cannot.  Floaters interfere seriously when the exit pupil gets smaller than ~0.6mm (roughly a 4mm eyepiece).  I can only go smaller on double stars.

So that is the one caveat I would mention for using eyepieces shorter than, say, 4.5mm in your scope.  For double stars, the limit is only determined by seeing and telescope optical quality.

The Leica is 8.9-17.8mm, so not as applicable as the APM 7.7-15.4mm

It is also a LOT more expensive and needs a special adapter for telescope use.

The APM has a wider apparent field in the 15mm range as well.

I compared the 30mm APM UltraFlatField, 31mm Baader Hyperion, 30mm ES 82°, 31mm TeleVue Nagler and 30mm Pentax XW.

I kept the 30mm APM.

It's available now under several labels.

For purchasers in the UK, the StellaLyra UFF is a no-brainer.

Highly recommended.  It has superb contrast in its focal length range and a truly flat field.

It's usable with glasses, the field stop diameter is 36.3mm.

45 minutes ago, Highburymark said:

The problem is to achieve the very lowest magnification possible through a binoviewer, through this frankly ridiculous optical path:

ERF - 120mm refractor - 2x extender - solar filter incorporating 3x barlow - binoviewer - eyepieces.

Even 40mm plossls are too much. If some enterprising designer could make a pair of 1.25”, 100mm lightweight eyepieces, with 15mm eye relief, a decent AFOV, and minimum glass, that would be perfect…… It’s a question for another thread, or perhaps another universe.

There is a 100mm 1.25" eyepiece from Vixen.  I read it has an 11° apparent field, but I think that is overstated on that one.

The apparent field in 1.25" eyepieces is directly related to focal length:

Here are theoretical maxima in simple eyepiece designs:

24mm--68°

32mm--50°

40mm--43°

55mm--28°

70mm--22°

100mm--15°

So what you want is not physically possible in 1.25".

If you expand that to 2", 100mm maxes out at 26°

If you expand that to 3", 100mm maxes out with 40°.  That's about the same as 1.25" in 40mm, but, of course, 3" eyepieces and binoviewers don't match.

2 hours ago, cajen2 said:

Yes, as I said in my OP, I will need to wear glasses. 

Taking into account eye relief, FOV and cost, the XW does seem the best compromise.

Or I could Barlow my Morpheus 6.5 mil. I'm a bit hazy on the difference in effect of a Barlow v. a focal extender. I already have a 2x ES Focal Extender. Would that be better or worse for exit pupil than a standard Barlow?

A Barlow can be telecentric or telenegative.

As far as a 2X magnification is concerned, either type will do the job.

Your ES is a telecentric Barlow but will work fine to provide the magnification you need (Focal Extender is merely an advertising name).

Morpheus eyepieces work fine in Barlows.

Assuming that you have the seeing conditions to support a 3.25mm eyepiece (220x), it will be a good test of whether floaters are a problem for you at that high a magnification.

If they aren't, the 3.5mm XW would be a possibility, though there isn't anything wrong with using the Barlow, either.

20 hours ago, Highburymark said:

This is exactly what I need now - a pair of 1.25” 50/55mm plossls, though I have the excellent TV40s. Why? Because I have a solar Ha filter that needs as slow a light beam as possible to show surface detail well - so ideally I’d stick a 2x extender in front of the 3x barlow/extender built into the filter. Trouble is, that would still give me far too much magnification with my TV40s in a binoviewer. A pair of 55mms would be perfect - though not even I am demented enough to try two TV55s in a 2” binoviewer. 
Yet there may be a solution. I believe Russell Optics in Vermont does a 50mm 1.25” plossl. But I’m sure they would be extremely challenging with eye relief. Don’t know if Louis or Don have any experience with Russell, or even more unlikely, with such an unlikely eyepiece?

Apols for diverting the thread anyway….. I liked your story John

My advice is to achieve the longer f/ratio with a 25-26mm Plössl and a 2X Barlow instead.

You will still have a 45-50° apparent field instead of a 28° apparent field (55mm 1.25" Plössl)

29 minutes ago, cajen2 said:

I'm after a decent short f/l EP for my new-to-me Starfield 102 ED. Currently, my shortest f/l is my Pentax XW 5mm, giving me 143x and I'm sure the Starfield will handle more. It must have good eye relief and a medium to wide FOV so I can wear glasses with it and not be silly money!

I've considered an XW 3.5, which coincidentally would give me exactly 204x (twice the aperture) but has anyone got any other recommendations? 

When you say "good eye relief", do you mean compatible with glasses?

If so, there are some possibilities with medium to wide apparent fields:

TeleVue Delite 4mm (179x) or 3mm (238x)

TeleVue Delos 3.5mm (204x)

Pentax XW 3.5mm (204x)

Stellalyra 80° 4mm (179x)

If you mean just comfortable eye relief lone enough to avoid accidental eyelash contact with the eyepiece, I would add:

TeleVue Ethos 3.7mm (193x)

APM XWA 3.5mm (204x)

TeleVue Nagler Type 6 3.5mm (204x)

There are others, of course, at lower price points.

And, of course, adding a 2X Barlow lens to anything from 6-8mm would put you there as well.

Anything narrower that 60° of field will require a lot of pushing of the scope and smooth mount movements.  With a tracking mount, that doesn't matter.

I own a 102mm refractor of the same focal ratio/focal length.  I would caution you about small exit pupils and floaters.

You will experience no issues with floaters in the high power if it is for double stars.

But if it is for the Moon or planets, floaters may interfere strongly.

I cannot use a 3mm eyepiece on the Moon or planets not because the scope or seeing cannot support it but because my eye has too much junk in it to yield an image without visible floaters.

I max out at 193x (a 3.7mm eyepiece), with its 0.53mm exit pupil, and some of the time max out at 159x (0.64mm exit pupil) because of that.

I have a scope with >2.5x the focal length for when I want really high powers without floaters.  In that scope, 493x is a 0.64mm exit pupil.

You know your own eyes, but were I to advise an approach, I would try Barlowing a lower power eyepiece first to see if floaters interfere instead of getting an expensive wider field eyepiece with adequate eye relief.

Then you'll know if you can use such a small exit pupil without the issue of floaters.

On 21/04/2023 at 07:15, niallk said:

O-III for sure on DSOs: makes elusive detail visible on a wide range of objects:

https://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/

A wide variety of emission nebulae, you mean.

It serves no useful function for anything with stars, or dark nebulae, or reflection nebulae.