Don Pensack

On 16/05/2021 at 16:53, Mr Spock said:

I'll be using it as an all-rounder. The only thing missing will be fuzzies - too much LP here. Planets are too low at the moment too, so it'll be moon, doubles, clusters.

The orthos I favour for planets/doubles with Barlow would give x202, x280 and x360. So, a nice spread. Without the Barlow I'll be using LVWs as they are better corrected for a fast Newt.

And:

--carbon stars

--bright planetary nebulae

Just make sure to match the magnification with the object.

Here in my Bortle 10 environment in Los Angeles (you can read a newspaper at night), I get great views of clusters and planetaries at 150-240x in my 4" refractor.

Using 100x is questionable and anything lower is a waste of time.

At f/5, the exit pupil of the eyepiece is 2.5mm

At f/6, the exit pupil of the eyepiece is 2.08mm.

That calculates to a 44.5% brighter image in the f/5 scope.

Now, that's the brightness of the field in unit area measurements.

In terms of how faint a star could be seen, the higher magnification and smaller true field in the longer scope should lead to fainter stars being seen at the limit.

So, in answer to Rob's question, yes the field will appear brighter in the 150mm.

But the dimmest star visible should be dimmer in the 200mm if/when you do that kind of observation.

rl is right.

A difference of 10% transmission in and eyepiece is a difference of 0.1 magnitude.

The average reflector loses 40% of its light at the edge of the field and very few can see it.

So transmission differences in eyepieces are not worth worrying about.

That doesn't mean that some eyepieces can't see deeper than others, but it doesn't seem to be related to transmission.

It does appear to be related to two main issues: spot side and lens polish.

Spot size is the focus of the eyepiece to bring a star image to a point.  If the spot size is less than the Airy disc size of the scope,

then the limit to resolution and sharpness will not be in the eyepiece, but in the scope..  This is the average situation, though many of us have found that the smaller the spot size the sharper the eyepiece,

so spot size becomes important in the smallness of details you can see.  Seeing conditions completely swamp this parameter, so we are talking about superb seeing conditions.

Lens polish determines things like light scatter in the field and around stars and basic contrast in the eyepiece, assuming coatings are excellent.  Superb polish and better contrast makes light transmission SEEM higher,

even if it isn't really.  And,  combination of the two qualities may allow you to see deeper.

[ Aside: Kellner eyepieces are notorious for internal reflections and ghosting, but I haven't seen a commercial Kellner with the high end coatings found in TeleVue eyepieces, which might let us know if it's the design or the execution.

Since they also aren't free of Chromatic aberration or lateral astigmatism, we're unlikely to see a high-end line of Kellners, though.  State of the art in 1849 is not the state of the art today. So coating technology plays a role in determining

light scatter, too.]

But, to answer your questions more directly, the Ethos, Delos, and Delite eyepieces were designed to be as color-neutral as possible.  The Plössl, Panoptic, and Nagler designs were not.

Note that that has nothing to do with spot size or lens polish, though the Delites are exemplary in both cases.  In my opinion, the Delites are TeleVue's "orthoscopic" eyepieces, in a sense.  Lunar/planetary observers have been asking

TeleVue for years to make a line of orthoscopic eyepieces.  They kind of did--it's just that they have long eye relief and 62° fields.  They aren't of course, actually orthoscopic, but then neither are most Orthoscopics.

People have their favorite focal lengths in every series, but keep in mind that telescope size, and type, and exit pupil, and seeing conditions, and personal preferences all enter into the picture.

One person may find a 27mm Panoptic perfect, yet another not like the eyepiece at all.

So there can be no "universal" answer to your question.  I've owned and used nearly every TeleVue eyepiece over the years, but my "favorite" has changed like the weather.  I could give you a list of my favorites since 1980,

but there have been almost as many as the number of years that have passed since then.  I have always favored the ones that yielded the tightest star images and the best contrast, though, which gets back to my point above.

One "NOT" reason for zooms can be found here:

http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p41976

Look at the Baader results at f/4.  The results will be better at f/10, but this maps to my experience too.

Look at the results for the Pentax XF zoom for comparison.

Zooms have narrow fields and poor star images and reduced contrast compared to fixed focal length eyepieces, IMO.

Well, some fixed focal length eyepieces.  Look at the Meade QX results and GSO Superview results.  Equally as bad.

So a conclusion can be made.  In long focal length (>f/8) scopes, zooms are perhaps passable.

Below that, not as good.  At f/5 and below, to be avoided.  The original poster is getting an f/5 scope.

I'm another vote for avoiding zooms.

6 hours ago, badhex said:

Thanks. I might look into sub-4mm as an option, as 3mm gives me 140x which is really the max. mags I think this scope will be comfortable with. I'm actually not sure what's on the market at that focal length which has decent eye relief at F6 and doesn't break the bank! I don't want much do I?  

The 62° TeleVue Delite comes in a 3mm.  I recommend it, and use one myself.

It is less expensive than the Takahashi TOE 3.3mm and 2.5mm

The BST Starguider ED 60° eyepiece comes in a 3.2mm.  Available under many labels.

The Celestron X-Cel LX 60° eyepiece comes in a 2.3mm.

The Altair Lightwave LER 55° comes in a 3mm. Inexpensive.

All have fairly long eye reliefs except the Takahashis.

23 hours ago, Louis D said:

Due to edge distortion compressing the outer part, they provide the same TFOV at a given focal length as competitors' 100 degree AFOV eyepieces which stretch the outer part to get to 100 degrees.  Effective AFOV (eAFOV) is about 90 degrees for both variations on 100 degrees.

That would be profound angular magnification distortion if the percentage were that large.  I haven't read anyone whose review mentioned a high degree of AMD in this eyepiece.

The people measuring the eyepieces used tests that revealed the narrower apparent fields than claimed.

A simple daylight test would reveal if the eyepieces have pincushion distortion (which I suspect) or barrel distortion.  If the latter, a simple billboard test would reveal the AMD.

3 hours ago, Piero said:

 

Yeah, the 24 Pan is another great eyepieces (currently my only TeleVue!), not ideal for observing the Moon, but lovely for other things (like star fields).

You made a good point regarding on/off axis. Both AMD and RD are off-axis. Not everyone observes planets or double stars off-axis.. In a dob at high power, one could end up observing the target at the edge to avoid constant nudging. 

 

I am aware of that link, but I am not sure whether it is fully correct regarding AMD / RD. In fact, the text calls RD as positive distortion, whereas AMD as negative distortion, letting one feel that they are kind of opposite. As far as I know, both can be positive or negative. For example, regarding AMD, if positive the image is "magnified" on-axis, and "minimised" off-axis, whereas negative AMD is the other way around. Not sure how common negative AMD is. Regarding RD, it is easier to think as if the image is projected on a sphere. If positive, you are looking at this image from the inside of the sphere, whereas if negative, you are looking from the outside. Not sure how common negative RD is.

What we see near the edge with common eyepieces having these distortions: 

- AMD: the moon pretty much remains round, distance between stars shrinks

- RD: the moon becomes an egg, distance between stars pretty much remains unchanged

 

Anyway, this is a bit off topic. Personally, I would not bother too much about the presence of some AMD in this eyepiece. To me, there are more important factors (e.g. how much back focus needs, how much the view remains sharp at the high-end of the magnification range, comfort, colour tone, etc). 

Very pleased to hear that this new zoom will be available in the astro market. If it has the optical quality of a Pentax XW / Delos, ~ £400 does not sound much to me. The Zeiss zoom I have is out of production, the Leica zoom costs nearly two times. Zooms are great not just for slimming down, but for the added practicality of getting your favourite magnification for a certain target without having to swap eyepieces.

With AMD the moon diameter closer to the edge shrinks and the moon becomes oval

With RD, the moon diameter stretches radially as the moon nears the edge and the moon becomes oblate.

Distortion is distortion, as the opticians say.

I suspect the best lunar eyepiece might only partially solve for either distortion if a wide field, but the best lunar eyepiece with the least distortion would be a narrower field eyepiece.

On the other hand, if sharpness at field edge is critical, and you always pull the lunar feature to the center to examine it, an excellent widefield can be a revelation.

My first view of the Moon through a 6mm Ethos was shocking--2/3 of the moon in the field in sharp focus to the edge.  At 304x.  Moving the edge to the center showed some shape changes, but didn't reveal any more details.

At low power the distortion of the shape of the Moon seems far more annoying than at higher power.

We need to wait to see what people think when using the eyepiece at 7.7-10mm.

11 minutes ago, Barry-W-Fenner said:

Enlightening Read Don. Thanks.

 I have read that you view through a 12.5" scope on a few occasions now. Out of interest what scope do you have?

Cheers

It's in my signature, but in case you block the signatures, it's a 12.5" Teeter TT Classic dob at f/5 (f/5.75 with Paracorr II), mirror by Carl Zambuto.

My observing site is reasonably dark, averaging 21.45 mpsas over the last 16 years, though it has been brighter than that the last 2 times out (21.35 and 21.40 respectively), and high (2550m altitude).

Seeing is usually good to excellent.  Darkness is usually OK, though not superb.  And transparency ranges from awful to incredibly excellent on a random basis, just like every place I've ever been.

If you are a beginner, you don't know what you like in eyepieces yet.

If you wear glasses, you are obviously looking for longer eye relief eyepieces (say, 18.5mm or longer eye relief), but you don't know yet whether you prefer 60° fields or 80° fields or 100° fields.

So here is a rational way to approach it:

But some inexpensive 60° apparent field eyepieces that yield multiples of 60x (OK in a 10"), so 60x, 120x, 180x, 240x, 300x

[divide the focal length of the scope by the desired magnification to discover what focal length eyepiece yields that power]

That should cost you £300-350 total if you shop.

Now you have something for every object size and brightness, from a large faint nebula to a small planet.

Then, as you observing experience grows, replace one of the eyepieces with, for example, an 82° apparent field eyepiece (say, the 120x one).  See if you like it.  If you do, then maybe try the same apparent field for the next higher magnification

or even go larger to 100° and see if you like that.  If you really like the wider fields, then you know where to go and you can ask advice here about brands and models.

But at least start with something you can use.  You may never go to larger fields or more expensive eyepieces.  Or, you might try a wide eyepiece and decide you need to change them all.

You don't know that yet.  Only experience in the field will tell.

But first, you have to become familiar with the scope and how different magnifications work on different objects.  So, eyepieces first, then upgrades later.

On the other hand, if your budget is unlimited, then your "starter set" might be the aforementioned Morpheus eyepieces or even something more expensive.

On 09/05/2021 at 12:16, Xilman said:

I don't know about you guys but I find magnifications > x200 or so essentially unusable with a Dob. The speed at which objects traverse the field of view (not to mention having to focus during that short interval) and the difficulty finding them again afterwards makes calculations based on exit pupil rather academic.

Perhaps it's just old age and incompetence but I restrict myself to x150 or lower.

Only if you have a decent drive should you start worrying about diffraction-limited magnification or excessively small exit pupils. Again, IMO.

 

My 3.7mm eyepiece yields 495x in my dob.  It's a 110° eyepiece.  If I push a small object out to 3/4 of the way from center to edge on one side of center, and let it drift until it is 3/4 of the way from center to edge on the other side,

I get 40 seconds of viewing at that magnification before I have to move the scope again.

Try holding your breath for 40 seconds to see how long that is.

At only 230x, I get over 2 minutes between pushes.

So it is obvious the apparent fields of your high power eyepieces are too small.  In an undriven dob, high powers should be really wide field eyepieces to give you more observing time in between pushes.

As your magnification goes down, you can tolerate narrower fields because the movement through the field is slower.  I personally go from 70° at low power to 110° at high power in a smooth manner so the field size doesn't drop rapidly.

Here is what I mean: I've added every eyepiece I use (One great flaw to this presentation is that the object in the center doesn't have its size changed with magnification.  At the lowest power, the galaxy would be a point.  And it would not be as large as in the image until the magnification got to about much higher powers.:

The Morpheus don't disappoint.

They perform far beyond their price points and equal many much more expensive eyepieces.

In addition, my field test reveal they have superior light transmission as well.

A 40% spread runs 4.5mm, 6.5mm, 9mm, 12.5mm, 17.5mm, so the 14mm is the odd man out, so to speak.

In my 12.5" f/5 scope (f/5.75 with Paracorr II coma corrector), ALL the focal lengths perform superbly.  Even the 14mm.

The 1825mm equivalent focal length is so flat that I don't even see FC in the 14mm, which many people do on shorter focal length scopes.

Ernest's lab tests at f/4 show the field edges have star images of:

4.5mm 14'  dominant edge aberration chromatic aberration 78°

6.5mm,  16' dominant edge aberration chromatic aberration  79°

9mm 14' dominant edge aberration astigmatism 78°

12.5mm 16' dominant edge aberration field curvature 78°

14mm 24' dominant edge aberration field curvature 78°

a 10' figure is essentially indistinguishable from a perfect point, so the 3 shortest focal lengths are truly superb, in that this performance is at f/4 and longer f/ratios yield better results.

Distortion figures were average for the apparent fields.

At f/5.75 (my scope with Paracorr), the performance is better than this.

Hmm.  Exit pupils will be smaller in longer f/ratio scopes.  One would think that the smaller exit pupils would make SAEP less noticeable, as astigmatism is.

I'll have to think about that.

But, presbyopia doesn't influence spherical correction much, if at all, though if it is strong enough to prevent accommodation at distance as well as close up,

it will make a difference in where the scope focuses.  I haven't noticed much difference, as my vision has drifted toward fixed focus, to my sensitivity to SAEP.

The eyepieces that were bothersome are still bothersome.

Have you?

On 07/05/2021 at 11:29, Stardaze said:

 

Well, I'm pretty committed now as the 8mm and 6.7mm have sold..

So my idea of a lineup would be:

30        41x.        6mm ep

20        64x.        4mm 

13         98x.       2.6mm

10        127x.       2mm

7          181x.       1.4mm

5.         254x.      1mm

3.5.      363x.      0.7mm

Looks great from an exit pupil perspective but there are some large jumps from 7mm down. I agree that a 3.5 wouldn't be used much and so bottom of the shopping list for sure. It's a massive jump as Don highlights from the 5mm but it does provide a 32x magnification? Even 5mm to 7mm is a large jump.

@Louis DI've never thought of a 40mm as being of use really as it gives an 8mm exit pupil but magnification of 32x? Obviously some of that light will be wasted but is the FOV with a filter possibly still worth it? 

 

Look at the magnification jumps you have:

23x (how do you even see the difference?), 34x, 29x, 54x, 73x, 109x.

If you observe in a place with excellent seeing, I might be able to see the high magnification jumps.  After all, most high mag. objects are quite small.

But having the low power eyepieces be that close together means it is unlikely you will use them in sequence.

I'd bet you either run from 30 to 13 to 7 much of the time or from 20 to 10 to 7.  Having two ranges isn't such a bad idea if your seeing is highly variable.

But your range could have been different, like a 25-26mm, 12.5-13mm, 8-9mm, 6-6.5mm, 5mm, 4mm and have been more evenly spaced.

There is never a wrong answer if you are happy with what you have, but in my experience you never feel the need for an eyepiece in between if the jumps are not too large,

and never feel you have too many eyepieces if the jumps are not too small, and that is up to your own determination.

SAEP is more of a problem when the eyepiece has a large exit pupil and less of a problem with a smaller exit pupil, though I don't think there is a hard cutoff on that scale.

I wonder, but have never read anything about it, whether spherical correction (myopia or hyperopia) in the eye can influence one's sensitivity to it.

Choosing eyepieces based on exit pupil has some problems.

1) It results in having the low power magnifications be too close together.

Take the 8" f/6 scope.  A 24mm eyepiece yields 50x and a 4mm exit pupil.

A 1.414 increase (50% difference in brightness) is an exit pupil of 2.8mm, but is only 70x.  That's too close to 50x to see any appreciable difference in magnification.

From experience, I can tell you it takes 35-40x spacing to even notice the difference in magnification.

2) it results in high power eyepieces being too far apart.

Take the same 8" f/6 scope. A 1mm exit pupil is a 6mm eyepiece at 200x.

A 1.414 increase in exit pupil is an exit pupil of 0.7mm, with is an eyepiece of 4.2mm.  

But 4.2mm is a magnification of 285x.  That is too much of a jump at the high end where you might be bumping into the ceiling allowed by your Seeing conditions.  Smaller jumps,

like 40-50x are more practical as you edge up on the limit, specially if you are doing lunar/planetary observing.

So I advocate ignoring exit pupil and picking eyepieces based on a rational pattern of magnification increases.

In that 8" scope (I owned one that size for 11 years), I found jumps of 50x just about ideal to have enough change from one to the other but not make the high power jumps too large.

And a 50x/100x/150x/200x/250x sequence is exit pupils of 4mm, 2mm, 1.3mm, 1.0mm, 0.8mm

The 27mm Panoptic is impressively sharp at the edge, but it does have strong rectilinear distortion in comparison.

8 hours ago, Stardaze said:

Only just stumbled across this thread. Interesting link above but my own experiences of ultra wide follow similar. I have found some EP's take me a while to acclimatise to and funnily enough the least widest I own, XW 5, is the most difficult for eye placement where I do get black outs depending. The eye cup seems to affect where I want to be (noting that the Pentax eye relief is a much more solid type than all of my others) and I find myself hovering, or lightly resting, with that EP instead of nestling it into the socket as per my 100 degree EP's, even though the cup is as far raised as possible. I seem to have acclimatised best to my two APM 100 degree EP's but you do have to roll your head around to take them all in. I use the 13 and 20 for so much of my viewing that when I use something else, it can take a little time to readjust, but that's just my own time served that I think that's due to. 

Just a note: to adjust to a new experience, or, in this case, eyepiece, one acclimates to the eyepiece, with the spoken accent on the first syllable.

When one ascends a mountain and has to get used to the altitude, or one steps out of an air-conditioned house to a hot outdoors, one acclimatizes (or acclimatises in British English) to the new conditions, with the accent on the second syllable.

I only mention this because you used it incorrectly twice in your post.

Not that this is an egregious error, of course.

It's not a perfect eyepiece, because the edge deteriorates a tad near the edge at f/6 and faster, but the long eye relief makes it easy to use (effective eye relief of about 18-19mm because the eye lens is recessed),

and the light transmission, when directly compared to a 24mm Panoptic, is excellent.

The star images are very tight out to about the 90% point but don't really get wonky until very near the edge, so it is an excellent full-field 1.25" eyepiece.

In my f/7 apo, it is sharp to just about the very edge, and in anything longer, it should be perfect.

Ackermann designed it for f/5, so it is only a questionable choice under f/5.

It has an effective field diameter of 27.3-27.5mm (different people have measured different diameters in the field), so a little wider than most of the competition.

The 65° field is consistent with 27.3mm

This review should be read by someone contemplating the 8.5mm Pentax XF:

https://www.cloudynights.com/articles/cat/user-reviews/review-of-at-paradigm-8mm-and-pentax-xf-85-mm-r2737

It's a fine eyepiece for everything except the Moon and planets, where internal reflections are an issue.

20 hours ago, Mr Spock said:

Thought of that. But, the thread on the Meade tube isn't the same as the diagonal.

You misunderstood.  I meant use the entire Barlow in front of the diagonal.  That won't work of course, if your diagonal threads on, but you talked about threading the lens to the diagonal, so I assumed you had a refractor-style diagonal

that has a forward tube section.  In other words: scope >> 1.25" visual back >> barlow complete >> diagonal >> eyepiece.

What about using the #140 in front of the diagonal in its own tube?  Should be 3x there.

Have you tried all of these:

19 hours ago, Barry-W-Fenner said:

I have put the extension ring in the 6.5 & 4.5 now. The 4.5 has never been an issue for comfort to be honest. However I now find the 6.5 to be an improvement from an eye relief perspective. If I put my eye well pressed on the eye cup I have had some great views of the moon this week with my Bresser AR102s   it feels like I kind of need to view  somewhere in between the extension spacing  as I really press my eye into the eyecup to get the best views.

Cheers

Baz

You do realize you can use the extension ring and fold the rubber eye cup down?

13 hours ago, johnturley said:

Don

Apologies for my ignorance, but what do OCA and GPC stand for.

John 

They are essentially the same thing.  In the US, the term OCA is used (Optical Corrector Assembly), while in Europe GPC is used (Glass Path Corrector).

They move the focal plane backwards to compensate for the additional inches of light travel through the binoviewer prisms that would normally require inches of in-focus.

I see Louis already answered this.

4 hours ago, Nikodemuzz said:

Hi all!

I recently had a chance to have a go with binoviewers. For me, the experience was similar to the first time I tried clipless pedals on a bicycle. Transformative might be an appropriate word. So comfortable, and looking at the moon for example, it was enthralling, more real (with the lack of a better description). Some of the more memorable views of the moon, to be honest, and that was with humble 32mm Plossls.

Now, this got me thinking. Going the binoviewer route is yet again one of those choices that is bound to get expensive for obvious reasons. However, a few strategically chosen eyepiece pairs and a barlow for example could go a very long way. What I wouldn't like is the prospect of changing back and forth between mono and binoviewing. In my case that would require either a new diagonal, or screwing adapters in the dark, which personally is a big turn off. I like keeping things simple so that I can focus on doing what I enjoy rather than faffing about with adapters and a countless possible combinations of eyepieces, extenders etc. 

In order to keep things under control I would have to decide between mono and binoviewing. Not both. Going full stereo means goodbyes to 2" eyepieces, which could actually cover some of the cost. The most concrete sacrifice would be the low power views. With my scope the maximum 1,25" FOV with a Panoptic 24mm or equivalent would yield 42X and a 1,55 degrees. Not a panoramic vista, I suppose, but still considerably wider than I used to get with my C11 and ES82 30mm (93X, 0,88 degrees). By comparison, the 30mm would get me 33X and 2,46 degrees in cyclops mode. 

Granted, only I can judge whether the low power views are, in my personal case, worth sacrificing for ease of use, lower investment and overall astronomical feng shui. But I would like to ask you, have any of you gone down the same road? What did you decide and why?

 

Don't neglect to consider the effect of the OCA (GPC) on the magnification.  In many binoviewers, a 24mm behaves as a 12mm eyepiece.