Wider FOV 68 Vs 82 Degree for a fast scope

[Louis D]

19 hours ago, Louis D said:

They're $185 Euros in Germany (no VAT to US).  This equates to $203.  Factor in no sales tax or import duties and $30 shipping, and you're slightly ahead, especially if you can't get free shipping and a seller that doesn't charge your state's sales tax here in the US.  I've bought all of my Morpheus eyepieces from the UK in the past for these reasons.

With FLO's sale on these, that works out to £152.50 or $191.50.  Add in shipping, and the total comes to $216.78.  Not quite as cheap as from continental Europe, but close; and I'm pretty sure FLO will ship them to the US.  It's certainly cheaper than from US retailers, especially when you factor in they have to charge US sales tax and international sellers don't.  Being under $800, import duties are not an issue, either.  I'll have to think about picking up another Morpheus now. 🤔

[bosun21]

27 minutes ago, Don Pensack said:

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).

 

 

Thanks Don for your detailed guide of how to setup the GSO coma corrector properly. I have bookmarked your reply and will be ordering a bunch of parfocalizing rings along with the coma corrector. Thanks again 👍

[Ratlet]

Just now, bosun21 said:

Thanks Don for your detailed guide of how to setup the GSO coma corrector properly. I have bookmarked your reply and will be ordering a bunch of parfocalizing rings along with the coma corrector. Thanks again 👍

Welp.  My store credits gonna be short lived.  A coma corrector would be a nice thing to experiment with for visual, and my 130pds is now primarily for imaging soooo....

[Ratlet]

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?

[Louis D]

2 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?

Well, you'd have to refocus for the edge of field to see if the edge sharpness returns to be able to definitely assign the blurriness to field curvature.

My Pentax XL 14mm has near perfect edge correction, but it also has significant field curvature.  When I bought it in my early 30s, it looked sharp edge to edge.  Now in my late 50s, I have to refocus it to see that sharp edge again.

If when refocused for the edge it is still unsharp out there, you're probably dealing with astigmatism issues.  This assumes you've got a coma corrector in your 130PDS.  Without one, you'd be seeing quite a bit of coma as well.

Setup a pinhole light with some foil and a flashlight (torch) in a darkened room for more advanced testing.  Getting the pinhole small and round enough is the problem in my experience with this approach.  Alternatively at night, put a fairly bright star in the center and slew it to the edge.  See if it goes blurry.  Refocus it to see if gets sharp.  Look at the shape of the star on either side of best focus.  If it alternates between radial and tangential lines on either side of best focus, that's classic astigmatism.  If it simply looks comet like, pointing to the center, that's classic coma.  It can look like a combination of those two.  Commonly, chromatic aberration is also thrown in for good measure making pretty rainbows out of the star.

Edited May 4, 2023 by Louis D

[Don Pensack]

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.

Edited May 4, 2023 by Don Pensack

[bosun21]

Just wait until you look through it and you will see a noticeable difference especially near and at the Edge. You would have to spend a lot of money to better the Morpheus for clarity and edge correction.

[Ratlet]

48 minutes ago, Don Pensack said:

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 you 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.

I think the edge brightening might be an artifact of the image rather than the lens.  It was a struggle to get the camera far enough from the eyepiece to not get blackouts.  The think there is a slight amount of tilt as the phone holder was hanging on the edge of the eyepiece and it was threatening to black out.  You can kind of see the crescent shape of darkness.

Oddly the flagpole might not be a cylinder.  About 1/3 of the way in from the edge you can see a join from the larger lower section.  I didn't notice that till you mentioned it.  You've got a good eye!

Really appreciate @Don Pensack and @Louis D feedback and comments.  Really helping me to understand eyepieces.

[Ratlet]

1 hour ago, Louis D said:

Well, you'd have to refocus for the edge of field to see if the edge sharpness returns to be able to definitely assign the blurriness to field curvature.

My Pentax XL 14mm has near perfect edge correction, but it also has significant field curvature.  When I bought it in my early 30s, it looked sharp edge to edge.  Now in my late 50s, I have to refocus it to see that sharp edge again.

If when refocused for the edge it is still unsharp out there, you're probably dealing with astigmatism issues.  This assumes you've got a coma corrector in your 130PDS.  Without one, you'd be seeing quite a bit of coma as well.

Setup a pinhole light with some foil and a flashlight (torch) in a darkened room for more advanced testing.  Getting the pinhole small and round enough is the problem in my experience with this approach.  Alternatively at night, put a fairly bright star in the center and slew it to the edge.  See if it goes blurry.  Refocus it to see if gets sharp.  Look at the shape of the star on either side of best focus.  If it alternates between radial and tangential lines on either side of best focus, that's classic astigmatism.  If it simply looks comet like, pointing to the center, that's classic coma.  It can look like a combination of those two.  Commonly, chromatic aberration is also thrown in for good measure making pretty rainbows out of the star.

Might have test this out with an artificial star.  I'm quite good at making them now as I had to do a bit of trouble shooting on a couple Tair 3 lenses and eventually identified the issues as being astigmatism in one and being cleaned and assembled by an idiot (I put the front element in backwards) in the other.  Did an awful lot of testing with the hole in tinfoil.

[Louis D]

1 hour ago, Don Pensack said:

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 you camera as AMD in eyepieces is almost always the reverse--the edge has a lower magnification.

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.

[Mr Spock]

Same here. The 42mm LVW for example is only 42mm in the centre, at the edge it's more like 35mm, hence the confusion over its 72° fov. It actually averages 38mm which fits the actual observed fov nicely (I've covered all this before). It also has pincushion distortion too which complicates things.

I've never seen an eyepiece magnify less at the edge.

The 16mm Nirvana suffers from strong field curvature. It's the black sheep of the family and the 13mm, 10mm, 7mm and 4mm are all fine.

Eyepieces are quite complicated optical systems. Every eyepiece is going to fall down in one way or another. That's why some of us have so many 

[Ratlet]

This has been really educational.  I didn't know eyepieces could have different magnifications across the field.

Going to have to do some reading (cause with the weather for the foreseeable, I'm not going to get much observing done).

Cheers guys 

[Louis D]

2 hours ago, Mr Spock said:

I've never seen an eyepiece magnify less at the edge.

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.

[Don Pensack]

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?

[Don Pensack]

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.

 

[Louis D]

1 hour ago, Don Pensack said:

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?

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.

[Don Pensack]

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.

[Louis D]

1 hour ago, Don Pensack said:

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.

I'm sure it would have been overkill and expensive as you say to go with 9 elements and an exotic telecompressor design when a more traditional Smyth group negative/positive design would suffice.  I'm sure it would have created a niche following, though.

I would like to see the 30mm design pushed until the telecompressor field lens fills the 2" barrel.  It's only a bit over 40mm wide in the 30mm UFF.  I looks like it could be pushed to having a 46mm clear aperture.  The upper barrel elements might have to grow wider as well if the design is simply scaled.  The internal field stop would grow to 35mm from 30.4mm.  If all remains linear, the effective field stop would grow from 36.4mm to 41.9mm, or roughly the same as the 31mm NT5.  The focal length would probably grow from 30.3mm to 34.8mm, or roughly the same as the 35mm Panoptic.  It would have a 3mm larger field stop diameter than the Pan and probably be somewhat lighter and slimmer.  Is KUO listening?

[Don Pensack]

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:

 

Edited May 5, 2023 by Don Pensack

[Ratlet]

Do you guys have any suggestions for good reading materials on telescope optics?  This is all fascinating.

[John]

20 minutes ago, Ratlet said:

Do you guys have any suggestions for good reading materials on telescope optics?  This is all fascinating.

This source is very often quoted or linked to on forums:

Amateur Telescope Optics (telescope-optics.net)

Most of it is way over my head though 🙄

 

[Louis D]

7 hours ago, Ratlet said:

Do you guys have any suggestions for good reading materials on telescope optics?  This is all fascinating.

I often refer back to my copy of Telescope Optics: Evaluation and Design by Harrie G. J. Rutten and Martin A. M. van Venrooij.  Sky & Telescope took over Wilmann-Bell's catalog, so it might show up again in a new printing.  It mostly covers telescope optics and spends only a single chapter on eyepiece optics.  Still, it's an educational read that is less dense than telescope-optics.net.  It's still dense, but not overwhelmingly so.

Edited May 6, 2023 by Louis D

[Louis D]

7 hours ago, Don Pensack said:

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

No, I wouldn't either.  But someone in China paid an optical designer somewhere to interpolate/extrapolate Thomas M. Back's 30mm and 40mm Paragon designs to add a 35mm design.  It would not be absurd to think they'd do it again with the 30mm UFF design to adapt/scale it up or down.

[Ratlet]

Cheers guys,

I'll keep an eye out for the book, hopefully they will reprint it or one will come up second hand.

telescope-optics.net could do with coming in a print format.  I struggle with meaty reading when it is through a screen for some reason.  I suspect that there is probably a minimum understanding required that will make it more intuitive.

[bosun21]

53 minutes ago, Ratlet said:

I suspect that there is probably a minimum understanding required that will make it more intuitive.

Most definitely. I tried and fried my brain 🤯.