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simagic
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Reged: 09/01/13

Focal Extenders new
      #6080556 - 09/14/13 02:52 PM

Mr. Newbee/Novice here again. So, lets say I have a 2" 18mm eyepiece (link) http://store.explorescientific.com/82degreeseries_18mmeyepiece.aspx .Now lets say I wanted to double my magnification. As a newbee,(and after what I'm reading), I would have thought to purchase a 9mm eyepiece (or thereabouts) That would have been my "logical" thought. Now I see something called a focal extender. ANNND what I read on "explorer scientifics" website, would be "better" than a Barlow for the reasons they give in this link. (link) http://www.explorescientific.com/focalextenders/ . If they both were ""similar"" in price (although I think the extender is more), would one give me a "better/clearer/brighter" image over the other (both terrestrial and sky??). If the extender is 25% to???% more in cost but gives a much better/clearer/brighter image, then I'll go for it. I've read that the shorter eyepieces (for instance if I got the 9mm, would be "dimmer"(or whatever terminology used). So if I got the focal extender instead of the shorter mm eyepiece, would it be as bright as the 18mm is?? Short version of above. Which is better?? Thanks for any info.

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SkyGibbon
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Re: Focal Extenders new [Re: simagic]
      #6080627 - 09/14/13 03:45 PM

You would be better getting the eyepiece if you are concerned about brightness. Plus, you change eye relief and add glass when usuing an extender/barlow. But, if you have multiple eyepieces, you in a sense doubled your eyepiece collection, but adding certain things to the view. I have the ES 2x Focal extender and I must say it is fantastic. Much better than my Televue barlow that I also have and cheaper also. The one good thing about a shorty barlow or some other barlows, is you can pull the element off and screw them into eyepieces, adapters, etc. Makes for many different combos of magnifications. The focal extender works better for my AP. I know I am getting 2x no matter where I have to put the extender.

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desertlens
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Re: Focal Extenders new [Re: simagic]
      #6080644 - 09/14/13 03:51 PM

There will be some loss of brightness with either option. This is an expected result of increasing magnification (decreasing exit pupil). The function of a quality extender is to preserve the qualities of the eyepiece, not improve them. There may be some EPs that perform better in some respects but I wouldn't expect anything radical.

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Jon Isaacs
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Re: Focal Extenders new [Re: simagic]
      #6081046 - 09/14/13 08:35 PM

Quote:

So if I got the focal extender instead of the shorter mm eyepiece, would it be as bright as the 18mm is?? Short version of above. Which is better?? Thanks for any info.





A focal extender is really just another name for a Barlow, it serves the same basic purpose and works in basically the same way.

If you compared the image brightness of a 9mm eyepieces and an 18mm with a 2x Barlow/Image extender, the image brightness would be, for all practical purposes, the same. The magnification is doubled, that means the same amount of light is spread out over 4 times the area, the image is one quarter as bright. The actual insertion loss of light from the Barlow is small, below the ability of the eye to see.

There are some advantages to Barlows.. For Plossls, Kellners and Orthos, simple eyepieces, the eye relief is about 0.7x the focal length, a 18 mm eyepiece will have about 12mm of eye relief.. sufficient.. But a 9mm eyepiece will only have about 6mm of eye relief. It's tight. An 18mm eyepiece with a 2x Barlow(Telecentric/Powermate/Focal Extender) will maintain that 12mm of eye relief..

Another advantage of a Barlow is that it helps eyepieces deal with fast focal ratio telescopes. Most eyepieces show significant off-axis astigmatism viewing through an F/5 scope.. the center is sharp, the edges are pretty rough... Add that 2X Barlow, the scope is looking at an F/10 "light cone" and things are much better.. stars at the edge are sharper, cleaner.

So.. if Barlows are so great, one would have to wonder why manufacturers don't just build in a Barlow to an eyepiece???

The answer is, well, many/most modern eyepieces do incorporate a Barlow-like Smyth section that does take advantage of the Barlow effect. Such eyepieces are termed "negative-positive", Barlows/Smyths are telenegatives, a magnifying eyepiece is a positive.. So, when you see a 7mm eyepiece with 14mm of eye relief or a 82 degree eyepiece that is reasonably sharp across the field of view, it already has a Barlow/Telenegative/FocalExtender built in.

Jon


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SteveG
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Re: Focal Extenders new [Re: SkyGibbon]
      #6081066 - 09/14/13 08:48 PM

Quote:

Plus, you change eye relief and add glass when usuing an extender/barlow.




The ES focal extender is a telecentric design like the Televue Powermate. There should be no increase in eye relief with this barlow.

Jon's advise is about as good as it gets.


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T1R2
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Re: Focal Extenders new [Re: SteveG]
      #6081082 - 09/14/13 09:06 PM

Yep, my ES 1.25 FX is awesome, but some have said they like the barlow over the FX because if they want 3X they just put it infront of the diagonal, So if you had a 16mm and barlowed it to 8mm, when the bar is put under the ep, but if you put it in front of the diag. it will give you a 5.3mm, most imagers like the Focal Ex. while visualers that don't have a lot of ep's like a good barlow. Just thought I'd throw that in.

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Robert Cook
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Re: Focal Extenders new [Re: SkyGibbon]
      #6081102 - 09/14/13 09:18 PM

simagic wrote:
Quote:

Mr. Newbee/Novice here again. So, lets say I have a 2" 18mm eyepiece (link) http://store.explorescientific.com/82degreeseries_18mmeyepiece.aspx .Now lets say I wanted to double my magnification. As a newbee,(and after what I'm reading), I would have thought to purchase a 9mm eyepiece (or thereabouts) That would have been my "logical" thought.




Well, a 9mm eyepiece would give you double the magnification, so I don't see what's so illogical about that.

simagic wrote:
Quote:

Now I see something called a focal extender. ANNND what I read on "explorer scientifics" website, would be "better" than a Barlow for the reasons they give in this link. (link) http://www.explorescientific.com/focalextenders/ . If they both were ""similar"" in price (although I think the extender is more), would one give me a "better/clearer/brighter" image over the other (both terrestrial and sky??). If the extender is 25% to???% more in cost but gives a much better/clearer/brighter image, then I'll go for it.




If a particular Barlow and a particular telecentric focal extender are properly designed and of comparable quality, then neither should produce a better image, in terms of aberrations, than the other in practical use. As for brightness, a 2-element Barlow (typical) would lose less light than a 4-element focal extender (typical), but whether this is visually significant is debatable.

A focal extender does have certain advantages because instead of diverging the rays it fully (or very nearly so) changes the focal ratio of the incoming light cone (i.e. light that is focused by the objective lens or mirror), changing its effective focal length before it reaches the eyepiece. The benefits are: much less of a chance of vignetting (darkening or blackening the outer portion of the field of view, reducing the width of the apparent field of view--in such cases, it would provide a better view than a Barlow in this way) with long-focal-length eyepieces, far more predictable magnification that is not dependent on placement (e.g. always close to 2X instead of 2-2.5X depending on the particular eyepiece), and no change in eye relief so that using the eyepiece feels the same; by the way, I suspect that Barlows also tend to make spherical aberration of the exit pupil a bit worse, so that the eyepiece is more prone than usual to blackouts and "kidney-beaning" (partially blacking out), while focal extenders probably do not.

So basically telecentric focal extenders are generally better than simple Barlows in a number of ways, but they won't necessarily produce a better or brighter image.

simagic wrote:
Quote:

I've read that the shorter eyepieces (for instance if I got the 9mm, would be "dimmer"(or whatever terminology used). So if I got the focal extender instead of the shorter mm eyepiece, would it be as bright as the 18mm is?? Short version of above. Which is better?? Thanks for any info.




The reason the image gets dimmer at 2X the magnification is the magnification itself, and there is no magic to work around the laws of physics--this phenomenon affects both configurations equally. What is not equal is that the 2" 18mm ES82° + 2" 2X ES focal extender combination has a total of 10 lenses and weighs 37 oz (about 2 1/3 lb!), while the 1.25" 8.8mm ES82° would give you close to the same magnification and true field of view with only 7 lenses (brighter, if anything) and weighs only about 12 oz with the 1.25" adapter (and costs only $100 compared to $150 for the focal extender).

Now, sometimes it really is advantageous to use a Barlow/focal extender with a longer-focal-length eyepiece instead of an equivalent shorter-focal-length eyepiece, such as with Plössls, in which case any miniscule loss in brightness would be more than compensated for by more comfortable eye relief and better correction (less aberrations) when using "fast" telescopes. But in your case the 8.8mm ES82° is an extremely well-corrected eyepiece, even in fast telescopes, that actually has slightly better eye relief than the 18mm ES82°, in addition to being much smaller and lighter. Obviously these are not Plössls, so the rule doesn't necessarily apply; actually, the 8.8mm almost certainly has a Smyth lens group in its barrel that functions like a Barlow/focal extender (in addition to other corrections), so you could say that the same rule applies but in this case it has already been applied internally.

SkyGibbon wrote:
Quote:

Plus, you change eye relief and add glass when usuing an extender/barlow.




If eye relief is changed to any significant degree with a telecentric focal extender, then perhaps it is not doing a good job.

SkyGibbon wrote:
Quote:

The one good thing about a shorty barlow or some other barlows, is you can pull the element off and screw them into eyepieces, adapters, etc. Makes for many different combos of magnifications.




Lately I've been using my 6.7mm ES82° in this manner experimentally for approximately 1.5X its normal magnification, and have been surprised--shocked, really--at how well it works. I had expected noticeable spherical aberration and perhaps some coma and other aberrations near the edge of the field, but the image seems just as sharp as ever. The results have been so good that I've put off my planned purchase of the 4.7mm, at least for now (until I find something wrong). Interestingly, I've had absolutely awful results doing the same thing with my 24mm ES68°--even worse aberrations than I had expected.


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Jon Isaacs
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Re: Focal Extenders new [Re: Robert Cook]
      #6081132 - 09/14/13 09:35 PM

Quote:


Lately I've been using my 6.7mm ES82° in this manner experimentally for approximately 1.5X its normal magnification, and have been surprised--shocked, really--at how well it works. I had expected noticeable spherical aberration and perhaps some coma and other aberrations near the edge of the field, but the image seems just as sharp as ever. The results have been so good that I've put off my planned purchase of the 4.7mm, at least for now (until I find something wrong). Interestingly, I've had absolutely awful results doing the same thing with my 24mm ES68°--even worse aberrations than I had expected.




Typically Barlows work well with short focal length eyepieces, issues may arise with long focal length eyepieces, this where the 4 element designs like the Powermates, the TeleCentrics etc are valuable.

I am not sure why you would expect coma and spherical aberration with a Barlow used in the 1.5x mode, it's really no different.. With eyepieces that do have off-axis astigmatism in a fast scope, the 4mm TMB Planetary at F/5.4, it really cleans up the field of view. Doubles that were sharp in the center but soon faded away because of aberrations, I can watch them drift past the field stop.

Jon


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SkyGibbon
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Reged: 02/09/13

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Re: Focal Extenders new [Re: simagic]
      #6081301 - 09/14/13 11:43 PM

Quote:

The ES focal extender is a telecentric design like the Televue Powermate. There should be no increase in eye relief with this barlow.




Quote:

If eye relief is changed to any significant degree with a telecentric focal extender, then perhaps it is not doing a good job.




Yes, yes you are correct. Wrote out like that, since it seems most people consider them the same thing. Sorry about that. It is one of the two reasons I got my focal extender. Anyways.........they are always good to have in your kit. At least for me. Meaning both barlow and focal extender this time.


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Robert Cook
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Re: Focal Extenders new [Re: Jon Isaacs]
      #6081323 - 09/15/13 12:04 AM

Quote:

I am not sure why you would expect coma and spherical aberration with a Barlow used in the 1.5x mode, it's really no different..




Regarding spherical aberration, I figured that a basic Barlow (that has spherical surfaces), which is simply a negative element, would be corrected for a particular focal length (its own, that is) and magnification factor. I would not expect this to be an issue over a fairly wide range of magnifications, and my previous experience confirms this (same goes for chromatic aberration), but since this was a lower magnification factor, I wasn't sure what to expect, going in.

Regarding coma, I had no thoughts or expectations about it whatsoever, going in. Why would I? I don't know, either.

Then I tried screwing the Barlow lens assembly directly into the barrel of my 24mm ES68°, which, by the way, seems to work fine with this GSO Barlow in its normal 2X configuration, aside from some vignetting. Yikes! Most of the FOV was dominated by what looked like massive coma, and even the center of the FOV would not come to as sharp of a focus as this eyepiece normally does. It is well corrected for astigmatism so of course it normally shows the coma that we'd expect from an f/5 mirror, but the "coma" (was it? ) that I saw when in "1.5X mode" was way exaggerated.

Naturally, given the disastrous results I just had, including completely unexpected coma, I wondered what would happen with the 6.7mm, despite knowing that different eyepiece designs could respond quite differently and that Barlows tend to work better with short-focal-length eyepieces anyway--this experience gave me a new, if extreme, data point on how wrong things could go. I thought there was a decent chance that there would be less serious but noticeable aberrations, and was pleasantly surprised that the image in the 6.7mm in 1.5X mode was as sharp across the entire field as I could have hoped for (accounting for the increase in magnification) and better than I was expecting with my newly recalibrated expectations. Seeing was exceptionally good yesterday morning, and I had a nice, detailed view of Jupiter at 168X (thereabout--I haven't measured it empirically yet).

Quote:

With eyepieces that do have off-axis astigmatism in a fast scope, the 4mm TMB Planetary at F/5.4, it really cleans up the field of view. Doubles that were sharp in the center but soon faded away because of aberrations, I can watch them drift past the field stop.




All true, and I'm aware that some folks don't even notice coma when using many types of eyepiece due to astigmatism (of the eyepiece), and may erroneously blame the Barlow for the coma of the primary mirror that they can see only while using the Barlow, but the two eyepieces that I've tested in this manner so far are well corrected for off-axis astigmatism in fast scopes.

Now, perhaps what I saw technically wasn't coma, but it sure looked like coma and it wasn't pretty.


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Robert Cook
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Re: Focal Extenders new [Re: Robert Cook]
      #6081440 - 09/15/13 02:37 AM

I wrote:
Quote:

Now, perhaps what I saw technically wasn't coma, but it sure looked like coma and it wasn't pretty.




Perhaps it was coma from the f/5 primary mirror + tangential astigmatism from the eyepiece with the Barlow lens screwed directly in. In all honesty, I didn't spend much time studying it--one quick look was all I needed to know that this combination didn't work (I was really just experimenting out of curiosity anyway).


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simagic
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Reged: 09/01/13

Re: Focal Extenders new [Re: Robert Cook]
      #6081606 - 09/15/13 07:06 AM

Yeh, I'm the novice who stated this thread. Whats meant by a fast focal ratio telescopes.

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Jon Isaacs
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Re: Focal Extenders new [Re: Robert Cook]
      #6081613 - 09/15/13 07:12 AM

Quote:


Regarding coma, I had no thoughts or expectations about it whatsoever, going in. Why would I? I don't know, either.




A Barlow shouldn't affect coma one way or the other... It should improve off-axis astigmatism.. I have a 24mm Meade SWA, same optics as the ES, I will give it a look with the GSO Barlow at 1.5x, see what I see.

Jon


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Jon Isaacs
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Re: Focal Extenders new [Re: simagic]
      #6081623 - 09/15/13 07:26 AM

Quote:

Yeh, I'm the novice who stated this thread. Whats meant by a fast focal ratio telescopes.




Dennis:

Fast and slow are terms that come from photography to describe the focal ratio of a telescope. A "fast" focal ratio exposes film quickly, a "slow" focal ratio exposes film much more slowly.

What is a fast focal ratio? F/5 is a fast focal ratio, F/10-F/12 is a slow focal ratio. One way to understand it is to think of two telescopes with the same focal length, say 1500mm and then consider their aperture. A 1500mm scope with an F/5 focal ratio will have an aperture of 1500mm/5= 300mm, a 1500mm focal length scope with at F/15 will have a 100mm aperture. The scope with the 300mm mirror would collect more light and expose film more quickly.

In the telescope world, that is the world of eyepieces and telescopes, fast and slow are not so important, telescopes are "Afocal" devices. What is important is the magnification and the aperture (or the exit pupil). In theory at least, two 100mm telescopes at 100x will provide the same brightness, the same resolution regardless of their focal ratio.

Your 127mm Mak has a focal ratio of about F/12, it's relatively slow. That means eyepieces have an easy time of it and simpler widefield designs like Konigs and Erfle's do a reasonably good job.

Jon


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Robert Cook
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Re: Focal Extenders new [Re: Jon Isaacs]
      #6081717 - 09/15/13 09:16 AM

Quote:

Quote:

Regarding coma, I had no thoughts or expectations about it whatsoever, going in. Why would I? I don't know, either.




A Barlow shouldn't affect coma one way or the other...




That's what I had thought, but it doesn't explain what I saw.

Quote:

It should improve off-axis astigmatism..




This has always been my understanding, and with cheaper eyepieces it's been my experience as well, but what I actually saw in this case was an obvious and substantial degradation of the image quality of this eyepiece in comparison to how it performs alone and for that matter with the very same Barlow when used in the normal 2X fashion. Why? Currently, I am at a loss to explain it. This is intriguing enough of a mystery to explore it further, though.

Quote:

I have a 24mm Meade SWA, same optics as the ES, I will give it a look with the GSO Barlow at 1.5x, see what I see.




Sounds good. This is not a configuration that I need or even intended to actually use, but it would be nice to figure out what's going on in order to further our or perhaps only my understanding. Note, however, that the 24mm Meade SWA and 24mm ES68°, though related, are not identical optically, as indicated in the following review by Bill Paolini with regard to angular magnification, for one thing:
http://www.cloudynights.com/item.php?item_id=2729


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Robert Cook
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Re: Focal Extenders new [Re: Robert Cook]
      #6089356 - 09/19/13 04:11 AM

Quote:

Quote:

It should improve off-axis astigmatism..




This has always been my understanding, and with cheaper eyepieces it's been my experience as well, but what I actually saw in this case was an obvious and substantial degradation of the image quality of this eyepiece in comparison to how it performs alone and for that matter with the very same Barlow when used in the normal 2X fashion. Why? Currently, I am at a loss to explain it. This is intriguing enough of a mystery to explore it further, though.




OK, tonight I managed to spend more time on and make a more careful interpretation of what I'm seeing in my 24mm ES68° eyepiece with a GSO Barlow lens cell screwed directly in for ~1.5X (actually closer to 1.7X with this particular eyepiece). Admittedly, all I did previously was take a quick look, fiddle with the focus a bit, made a note of how poor the image looked, and moved on to other things.

After more closely examining the "coma" that I had thought I had seen, I realized that it was actually a combination of severe field curvature and equally severe astigmatism, both of which significantly affected most of the field (any actual coma was overwhelmed by these other aberrations). In contrast, the eyepiece by itself only shows off-axis coma from the f/5 paraboloidal mirror, which is the dominant aberration. With the Barlow used in the normal 2X manner (actually closer to 2.2X in this case), some field curvature near the edge of the field (the outer 10% at the most) is introduced, but with no obvious astigmatism. I still don't know why this is happening, but at least I have a much better idea now of what I am seeing.

By the way, for comparison I can see none of these strange aberrations when using the same Barlow with my 6.7mm ES82° eyepiece, for example--stars are always sharp across the field (aside from coma from the objective) in any configuration, including 1.5X (which actually is very close to 1.5X with this eyepiece). While I'd expect Barlows to work better with short-focal-length eyepieces, the degree of difference I'm seeing and what I'm seeing were still unexpected. In addition, I comparison-tested a Celestron Ultima Barlow (same as the Orion Shorty Plus), and interestingly at 2X with the 24mm ES68° (actually 2.2X as with the GSO) it apparently added a little astigmatism to the field curvature (the latter being similar to that of the GSO in this configuration). Its lens cell is not compatible with the filter threads on eyepieces, so I didn't try the 1.5X configuration with it, but another thing I noticed was that it requires inward focuser travel while the GSO Barlow requires outward focuser travel.


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Jon Isaacs
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Re: Focal Extenders new [Re: Robert Cook]
      #6089405 - 09/19/13 06:05 AM

Quote:



After more closely examining the "coma" that I had thought I had seen, I realized that it was actually a combination of severe field curvature and equally severe astigmatism, both of which significantly affected most of the field (any actual coma was overwhelmed by these other aberrations). In contrast, the eyepiece by itself only shows off-axis coma from the f/5 paraboloidal mirror, which is the dominant aberration. With the Barlow used in the normal 2X manner (actually closer to 2.2X in this case), some field curvature near the edge of the field (the outer 10% at the most) is introduced, but with no obvious astigmatism. I still don't know why this is happening, but at least I have a much better idea now of what I am seeing.

By the way, for comparison I can see none of these strange aberrations when using the same Barlow with my 6.7mm ES82° eyepiece, for example--stars are always sharp across the field (aside from coma from the objective) in any configuration, including 1.5X (which actually is very close to 1.5X with this eyepiece). While I'd expect Barlows to work better with short-focal-length eyepieces, the degree of difference I'm seeing and what I'm seeing were still unexpected. In addition, I comparison-tested a Celestron Ultima Barlow (same as the Orion Shorty Plus), and interestingly at 2X with the 24mm ES68° (actually 2.2X as with the GSO) it apparently added a little astigmatism to the field curvature (the latter being similar to that of the GSO in this configuration). Its lens cell is not compatible with the filter threads on eyepieces, so I didn't try the 1.5X configuration with it, but another thing I noticed was that it requires inward focuser travel while the GSO Barlow requires outward focuser travel.




Robert:

After reading your observations and since it was still clear, I decided to see what I could see for myself. I used a number of eyepieces with a couple of different Barlows. For a telescope, I used a TeleVue NP-101 which is a nearly ideal test for an eyepiece since it's F/5.4 and it is corrected for field curvature, any aberrations visible are in the eyepiece. The Pleiades were well situated for this test, a comfortable 50 degrees elevation, they are bright and make a good test of the wide field performance of an eyepiece.

Basically my observations confirmed yours, my Meade 24mm SWA has the same optics as the 24mm ES 68 degree. With the GSO shorty attached in the 1.5x mode, the field curvature and off-axis astigmatism were severe. Observing a star nearer the edge of the field, the star "focused" to a sharp lines at 90 degrees on either side of best focus. In the standard 2x mode, there aberrations were reduced by still visible.

I tried a number of other eyepieces including a 32mm Celestron Plossl, a 24mm TeleVue Widefield, a 20mm Celestron Erfle, a 16mm Nagler type 2, a 15mm TeleVue Widefield and a 9mm Nagler Type 6. What I found was a pattern. Eyepieces that are at least partially corrected for astigmatism, the Naglers, the Widefields plus the Meade SWA, they all exhibited astigmatism and field curvature. It seemed to lessen as the focal length decreased, it could be seen in the 9mm Nagler but it was minimal.

The two eyepieces that are not corrected for astigmatism, the 32mm Plossl and the 20mm Erfle, both showed improvement in their edge correction, particularly the 20mm Erfle which has a 60 degree AFoV and is poorly corrected for astigmatism at F/5.4. The Barlow in either configuration, 1.5x or 2x cleans up the view so it quite sharp near the edge.

I also tried the various eyepieces in my 2X TeleVue Barlow, I saw no problems.

Jon


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Robert Cook
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Re: Focal Extenders new [Re: Jon Isaacs]
      #6090855 - 09/19/13 11:26 PM

Quote:

Basically my observations confirmed yours, my Meade 24mm SWA has the same optics as the 24mm ES 68 degree. With the GSO shorty attached in the 1.5x mode, the field curvature and off-axis astigmatism were severe. Observing a star nearer the edge of the field, the star "focused" to a sharp lines at 90 degrees on either side of best focus. In the standard 2x mode, there aberrations were reduced by still visible.




Thanks for taking the time to verify and expand upon what I observed, Jon.

Quote:

I tried a number of other eyepieces including a 32mm Celestron Plossl, a 24mm TeleVue Widefield, a 20mm Celestron Erfle, a 16mm Nagler type 2, a 15mm TeleVue Widefield and a 9mm Nagler Type 6. What I found was a pattern. Eyepieces that are at least partially corrected for astigmatism, the Naglers, the Widefields plus the Meade SWA, they all exhibited astigmatism and field curvature. It seemed to lessen as the focal length decreased, it could be seen in the 9mm Nagler but it was minimal.

The two eyepieces that are not corrected for astigmatism, the 32mm Plossl and the 20mm Erfle, both showed improvement in their edge correction, particularly the 20mm Erfle which has a 60 degree AFoV and is poorly corrected for astigmatism at F/5.4. The Barlow in either configuration, 1.5x or 2x cleans up the view so it quite sharp near the edge.




This matches what I've seen with my more limited set of eyepieces. Another "classical" eyepiece design that visibly benefits is the Kellner, as well as "MA" variants of it. I wonder if the aberrations of the Barlow actually help correct those of such eyepieces. At first glace, the Barlow's aberrations seem too severe for this, but the Barlow has "tangential focus" while eyepieces usually have "sagittal focus," for what it's worth.

Quote:

I also tried the various eyepieces in my 2X TeleVue Barlow, I saw no problems.




Interesting.... Does the Tele Vue 2X Barlow seem to help clean up the view in classical eyepieces as much as generic Barlows do? If so, then the latter must not be a case of opposite aberrations canceling each other out.

I suppose that the following practical rules of thumb could be added to the principles laid forth in this discussion:

1) High-quality Barlows have certain disadvantages in comparison to high-quality telecentric focal extenders, but should not noticeably degrade image quality.

2) Ordinary, generic Barlows can improve the image quality of basic eyepieces (e.g. Plössls) and work well enough with virtually all short-focal-length eyepieces, but can introduce noticeable and sometimes rather severe aberrations, namely astigmatism and field curvature (closely related aberrations), when used with high-quality, well-corrected eyepieces of medium-to-long focal length.


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Jon Isaacs
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Re: Focal Extenders new [Re: Robert Cook]
      #6090898 - 09/20/13 12:05 AM

Quote:

Interesting.... Does the Tele Vue 2X Barlow seem to help clean up the view in classical eyepieces as much as generic Barlows do? If so, then the latter must not be a case of opposite aberrations canceling each other out.

I suppose that the following practical rules of thumb could be added to the principles laid forth in this discussion:

1) High-quality Barlows have certain disadvantages in comparison to high-quality telecentric focal extenders, but should not noticeably degrade image quality.

2) Ordinary, generic Barlows can improve the image quality of basic eyepieces (e.g. Plössls) and work well enough with virtually all short-focal-length eyepieces, but can introduce noticeable and sometimes rather severe aberrations, namely astigmatism and field curvature (closely related aberrations), when used with high-quality, well-corrected eyepieces of medium-to-long focal length.





Robert:

I think the issues here are probably specific to the GSO Shorty and not Barlows in general. I did find that the TeleVue 2x seemed to clean up the 32mm Plossl and the 20 mm Erfle and did not add unwanted aberrations to the other, better corrected eyepieces.

Jon


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Robert Cook
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Re: Focal Extenders [Re: Jon Isaacs]
      #6090937 - 09/20/13 12:46 AM

Quote:

I think the issues here are probably specific to the GSO Shorty and not Barlows in general.




Well, the Celestron Ultima Barlow (also known as the Orion Shorty Plus) appears to have the same issues despite being of a quite different design (three elements versus two), and having come out of a different factory located in a different country. That's at the normal 2X, but I'll try to find out whether the aberrations get to be as bad as those of the GSO Barlow (also sold under several brand names) when the lens is brought closer to the eyepiece. It's probably safer to say that some Barlows have such issues (to this degree).


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