Analysis of my Tele Vue Plossl 20mm

[Piero]

Back in 1998, I bought my first premium eyepiece, the Tele Vue Plossl 20mm, based on suggestion by a local vendor. At the time Tele Vue Plossls were quite expensive and, as a teenager, it took me quite a lot of saving to afford one.

 

Anyway, there are two reasons why I am not selling this eyepiece:
1. as mentioned earlier, it was my first premium eyepiece;
2. unfortunately, this is one of the very rare cases of a defective Tele Vue product. 

As far as I know, I only read of another case 'potentially' similar to mine, and it was about a Tele Vue Plossl 25mm. In that case, the person (a member here on SGL, actually) reported that his TV plossl 25mm was 'soft at the edge'.

Apart from these two cases, I only read very positive comments on Tele Vue products, and this should tell something about the overall quality process of this company, which to me is just outstanding.

 

I decided to write this thread to report my description about the aberration(s) detected on this eyepiece. So, let's see if I did my homework correctly. 

First of all, all this argument is done using the following equipment: 

- Tele Vue-60 F6 f.l. 360mm
- Tele Vue Everbrite diagonal 1.25"
- Celestron 114mm F8 f.l. 900mm 
- Tele Vue Plossl 20mm
- Tele Vue Panoptic 24mm  (control)
- Tele Vue Nagler 13mm T6 (control)
- Tele Vue Nagler 7mm T6 (control)
- Tele Vue Nagler 3.5mm T6 (control)
- Tele Vue Powermate 2.5x
- Tele Vue Bresser SA 2x
- Celestron SMA 25mm (control)
- Vixen SLV 5mm (control)
- Orion Sirius Plossl 10mm (control)
- Lunt Solar Wedge filter (necessary for observing the Sun with the TV60)

Targets for comparison: Moon, Sun, stars fields. 

None of the listed eyepieces apart from that Tele Vue Plossl 20mm showed the aberrations described below. Therefore, this can exclude defects on the two telescopes, diagonal, Panoptic, Naglers, Lunt, Vixen, and Orion equipment. The Tele Vue Plossl 20mm is in excellent condition.

- No colour distortion was detected on the selected targets. Therefore this is a monochromatic aberration. 

- If considering the Moon (but it is the same for the Sun), the body appears elongated when located at the edge. Assuming that 
the Moon is about 0.5 degree, and that a Tele Vue Plossl 20mm with the TV60 shows about 2.8 degrees of sky, this elongation is well noticeable when a target is about 36% from the edge (0.5 * 100 / 1.4). 

- Unfortunately the two telescopes are in different countries so I cannot compare them side by side. However, I can say that I never noticed this aberration with my Newton 114mm F8, whereas this is noticeable with my TV60 F6. Possibly I did not notice it because of my inexperience, but my feeling is that this aberration is inversely proportional with the telescope f-number like all the other aberrations.

- When the target is placed at the edge, focus is almost, if not completely, impossible.

- When tele-extenders or powermates (see list above for models) are used, this aberration is somehow reduced, but not removed. My feeling is that it is reduced by the factor of the tele-extender. Therefore, a powermate 2.5x makes this aberration noticeable at about 14% away from the edge (36% / 2.5). Not sure about this measure, but the eyepiece is just more usable due to reduced aberration when a powermate (or a bresser sa 2x) is used.

- The elongation is uniform for all edges. Considering the Moon, this is elongated horizontally when placed at the left or right edges, whereas it is elongated vertically when placed at the top or bottom edges. Same thing for top-right, top-left and so on. This reminds me of coma, but it does not happen with my Panoptic 24mm or Nagler 13mm T6, clearly excluding coma shown by the TV60.

- Outward focus vertically elongates the target located at the edge. If the Moon is placed, let's say, at the left edge, it appears horizontally elongated when focused. If outward focus is applied, this elongation becomes vertical. This vertical elongation is independent of the edge. 

- Inward focus does not seem to change the elongation instead. 

I am not an eyepiece expert, so for me it is a bit difficult to precisely say what is/are the aberration(s) for this eyepiece. 
I would be tempted to say that there is astigmatism. 
What is your opinion? 

 

REMARK NOTE:
Members, don't sell defective equipment in the second hand market. I know it is a shame that you bought an expensive good, but this should be rather returned to the vendor. There are many inexperienced people out there who are simply unaware of aberrations or defects and buy in the second hand market just because they cannot afford buying new equipment. An inexperienced person can really be put off by a defective eyepiece (or telescope), instead of enjoying this hobby as one should do. In addition, selling defective gear ruins the second hand market beside your credibility. So, if you have some defective equipment (e.g. an eyepiece) and you cannot return it for some reason, then keep it and rather use it to show other people how to recognise the problems that that piece of equipment has. This is useful for yourself and the other people. 

[Mak the Night]

Interesting, if slightly disconcerting about TV quality control. I have a 20mm TV Plossl, although since I acquired a 19mm Panoptic it doesn't get as much use as it once did. I think it was one of the first TV Plossls I bought as well. It seems fine, although for some strange reason, ever since I first got it, I seem to be perpetually leaving thumb or finger prints on the eye lens. A couple of weeks ago I fumbled about in my Orion case to find a 25mm Plossl when I absent mindedly put my fat greasy thumb right on the 20mm's eye lens due to the (often ill fitting) dust cap falling off. It must be jinxed! I think I've cleaned that 20mm Plossl more than any other eyepiece I've ever owned. 

[John]

Interesting report Piero.

Tele Vue claim to check and test each and every eyepiece before releasing it for sale.

I've recently sold a 20mm TV plossl so I checked it before sale and it did not exhibit the sort of issues you describe.

The distortion that you saw on the Moon is known as pin cushion distortion but AFAIK the T V plossls are not known for that. Their wide field ranges do show this though and it's the way that TV minimises or removes astigmatism.

The more I read of your test report the more I'm wondering if the lenses have been removed to clean them and then someone (not you !) has put one element back in the wrong way around ?

 

 

[Piero]

18 minutes ago, John said:

Interesting report Piero.

Tele Vue claim to check and test each and every eyepiece before releasing it for sale.

I've recently sold a 20mm TV plossl so I checked it before sale and it did not exhibit the sort of issues you describe.

The distortion that you saw on the Moon is known as pin cushion distortion but AFAIK the T V plossls are not known for that. Their wide field ranges do show this though and it's the way that TV minimises or removes astigmatism.

The more I read of your test report the more I'm wondering if a previous owner has removed the lenses to clean them and then put one element back in the wrong way around ?

 

Thanks John for your comment. I would be very interested in knowing what the problem is, just for curiosity. I am actually the first and only owner of this eyepiece. It was bought new from a shop that now does not even exist. So, this problem comes directly from Tele Vue, unless that vendor opened it, which I doubt. Unfortunately I didn't notice this problem because 1) I simply was not aware of eyepiece aberrations when I was 15; and 2) it wasn't so obvious in my previous F8 telescope. I noticed this problem on my TV60, but wanted to test this properly before reporting it as it is very uncommon in Tele Vue products.

Interesting that you say about one element could have been put in the wrong way. It does not seem to me there is an easy way to do this on my 20mm. Possibly the mounting is different in those produced in the last few years.. 

Edit: 

I have noticed that pin-cushion distortion on both my Panoptic and Nagler 13mm. In the latter it was more evident, although still very moderate generally. I apologise to doubt your comment, but when I see it on these two eyepiece, I can still reach focus. The image is sharp, just a tiny bit elongated. On my Plossl 20mm the image is not sharp. Detail is lost in that 36% from the edge. Just a doubt of course. I am totally inexperienced in this, so I might be likely wrong in my understanding of that aberration.

 

p.s. out of topic. The Terminagler in your photo is really HUGE!

[jkwhinfrey]

It might be worth contacting Televue about this. I've heard of them putting faulty items right without the vendor being involved, maintains their reputation. You never know until you ask.

James

 

[DRT]

23 minutes ago, John said:

I've recently sold a 20mm TV plossl so I checked it before sale and it did not exhibit the sort of issues you describe.

Good to know, John

Piero - I know from experience that many TV eyepieces have barrels that easily unscrew or come loose. I don't know if the 20mm Plossl is one of them but can you remember this happening to it at any time throughout your ownership? If so, perhaps when you tightened it back up one or more of the lenses might not have been properly seated?

Is the aberration consistent through 360°? For instance, if you were to put the Moon at the left hand side of the FOV and rotate the EP through 360° would the extent or shape of the elongation change?

[Piero]

2 minutes ago, DRT said:

Good to know, John

Piero - I know from experience that many TV eyepieces have barrels that easily unscrew or come loose. I don't know if the 20mm Plossl is one of them but can you remember this happening to it at any time throughout your ownership? If so, perhaps when you tightened it back up one or more of the lenses might not have been properly seated?

Is the aberration consistent through 360°? For instance, if you were to put the Moon at the left hand side of the FOV and rotate the EP through 360° would the extent or shape of the elongation change?

Thanks for your comment Derek. 

The barrel screws 'properly' and does not come loose. I am very careful with my things, so I can guarantee that it was never dropped accidentally by me. So, if there is a mis-allineament in the lens, I do certainly not know how this happened. 

Regarding to your second question, yes, if the eyepiece is rotated, the elongation follows the rotation, I would say consistently and perpendicularly to the field stop. Strange isn't it? 

[dweller25]

Hello Piero,

I had exactly the same problem with a second hand 10mm TV smoothside plossl - the lenses had been incorrectly reversed by a previous owner, probably after cleaning. Once corrected the eyepiece was excellent.

As suggested above, contact Televue for advice.....

 

[YKSE]

Piero,

I'm no expert in optics, nor have I look through a TV plossl, here's my wild guess anyway.

I agree with John, the elongated Moon is typical pincushion distortion, here's TV's patent about its plossl, here's quote from the patent general description:

" For astronomical viewing, pupil aberrations and geometric distortions are not as important as the correction of coma and astigmatism which control image sharpness at the edge of the field. "

To my ears, it sounds like TV has introduced extra aberrations at exit pupil and geometric distortions(pincushion in this case) for astigmatism correction, and you can easily check the pincushion in day time looking at straight lines, which should be arc shaped closer to the edge.

In my mind, for your f6 scope, 24Pan and 13Nagler should show the same elongation of the Moon too, while 7mm and 3.5mm Nagler should show much less to none-existent elongation of Moon, since our eyes are more diffraction limited when exit pupil smaller than ca 2mm, while aberration limited forexit pupil bigger than ca 2mm, i.e. with eyepieces produing exit pupil larger than 2mm, we'll see more aberrations (geometric distortion is a kind of aberration too).

[Piero]

6 minutes ago, YKSE said:

Piero,

I'm no expert in optics, nor have I look through a TV plossl, here's my wild guess anyway.

I agree with John, the elongated Moon is typical pincushion distortion, here's TV's patent about its plossl, here's quote from the patent general description:

" For astronomical viewing, pupil aberrations and geometric distortions are not as important as the correction of coma and astigmatism which control image sharpness at the edge of the field. "

To my ears, it sounds like TV has introduced extra aberrations at exit pupil and geometric distortions(pincushion in this case) for astigmatism correction, and you can easily check the pincushion in day time looking at straight lines, which should be arc shaped closer to the edge.

In my mind, for your f6 scope, 24Pan and 13Nagler should show the same elongation of the Moon too, while 7mm and 3.5mm Nagler should show much less to none-existent elongation of Moon, since our eyes are more diffraction limited when exit pupil smaller than ca 2mm, while aberration limited forexit pupil bigger than ca 2mm, i.e. with eyepieces produing exit pupil larger than 2mm, we'll see more aberrations (geometric distortion is a kind of aberration too).

Thanks for your comment and reference. 

Yes, you are right that both my 24Pan and 13Nagler also show elongation for the Moon, while my 7mm and 3.5mm Naglers don't. However, the elongation as shown in the former two eyepieces is far less than that one in my Plossl 20mm, and the image is anyway sharp, whereas in the plossl is not. 

Interesting to read that there might be as solution for this problem! 

 

[Piero]

14 minutes ago, dweller25 said:

Hello Piero,

I had exactly the same problem with a second hand 10mm TV smoothside plossl - the lenses had been incorrectly reversed by a previous owner, probably after cleaning. Once corrected the eyepiece was excellent.

As suggested above, contact Televue for advice.....

 

Hi dweller25, did you reverse the lenses yourself to correct the problem? If so, how did you do it?  

[YKSE]

Just a thougth, as to difficult to focus star near edge, have you tried to move/rotate your head slightly?

The increased spherical aberration in exit pupil results in image in exit pupil not in the same distance for center and edge, kidney bean effect in some T1 Naglers are the extreme cases, while milder effect might be (only my wild guess) can be that you need to move your head slightly for looking at the edge than in center.

[Piero]

3 minutes ago, YKSE said:

Just a thougth, as to difficult to focus star near edge, have you tried to move/rotate your head slightly?

The increased spherical aberration in exit pupil results in image in exit pupil not in the same distance for center and edge, kidney bean effect in some T1 Naglers are the extreme cases, while milder effect might be (only my wild guess) can be that you need to move your head slightly for looking at the edge than in center.

In my Naglers I have to move my head to see the field stop, whereas this is not required in my 24Pan and definitely not in the 20Plossl. In any case, if I move my head with all these eyepieces or rotate them, the image does not come to focus on the 20Plossl, whereas it does in the other eps. I don't think it is about the eyepiece focal length. The image shown at the edge in the 20Plossl is well different from that one shown in the other eyepieces. It is significantly blurred and focusing does not work really, whereas in the 24Pan or 13Nagler the image is just elongated, but well focused until the edge. The latter eyepiece might require some focusing adjustment, but this always work as expected, whereas for my Plossl 20, you can rotate the eyepiece, rotate the head, move the eye, whatever, it does not reach focus at the edge. When I slightly defocus, I can even see the outer border of the Moon becoming duplicating itself, as if there were two borders . This for the part of the moon pointing to the centre of the field of view, so perpendicular to the field stop. 

No matters how many times I try this, it is always visible, and of course it is well noticeable when observing the Moon or the Sun, due to their bright and large shapes, although it is also visible in star fields. 

Unless the lenses came out from production with some defect, I'd suspect that a reversed placement of the lenses might make sense actually.

[dweller25]

46 minutes ago, Piero said:

Hi dweller25, did you reverse the lenses yourself to correct the problem? If so, how did you do it?  

Hello Piero,

Yes I did it myself - I found a schematic on Google that showed me the correct orientation of the lenses.

Then I took the eyepiece carefully apart (nothing lost as it performed so poorly) to see if they matched the schematic - they did not !

So after adjusting the lens orientation and re-assembling and testing I had a very good eyepiece 

[Piero]

8 minutes ago, dweller25 said:

Hello Piero,

Yes I did it myself - I found a schematic on Google that showed me the correct orientation of the lenses.

Then I took the eyepiece carefully apart (nothing lost as it performed so poorly) to see if they matched the schematic - they did not !

So after adjusting the lens orientation and re-assembling and testing I had a very good eyepiece 

Thanks for the information. The patent document Yong attached above contains the optical scheme.

I wonder how to disassemble it though.. It doesn't seem straightforward to me.. Mm.. Could you give me some direction for this? Thanks!

[YKSE]

Piero,

You can have a visual check before disassembling.

According to the schematic in the patent, the cocave sides should be facing outwards...

[John]

In case the optical layout on the Patent is not clear, here is a link to a the optical layout of the TV plossl vs the Nagler which Al Nagler used in an article (he should know !):

http://cs.astronomy.com/cfs-file.ashx/__key/communityserver-blogs-components-weblogfiles/00-00-00-00-51-Telescopes/3365.Nagler04.jpg

With the 20mm plossl I think the chrome barrel can be unscrewed and removed without affecting the lens elements. The lens elements are held in place by the field stop unit which doubles as a lens retainer, as I recall. There will be a spacer keeping the two lens elements from touching each other.

 

[Piero]

Thanks everyone for your help!

The top lens seems flat to me. I cannot see whether the lens inside is flat too.

 

 

As John says the barrel can be removed without moving any lens. In the image above, the barrel screws on the external screw thread. I have no idea what the internal screw thread is for.. Maybe to attach smaller 0.96in barrels?

Not sure but there seems to be some glue on the internal thread possibly put to be sure the lens body doesn't come off?

Do you think the lenses are accessible by unscrewing this 'internal screw thread'?

Not sure how well I described this...

[DRT]

I suspect that is the case, Piero - the internal thread will be holding the lens groups in place, just like the retaining ring on the from end of a refractor holds the objective lens in its seat.

If you shake the eyepiece gently does it rattle or do the elements feel like they are firmly seated?

[Piero]

4 minutes ago, DRT said:

I suspect that is the case, Piero - the internal thread will be holding the lens groups in place, just like the retaining ring on the from end of a refractor holds the objective lens in its seat.

If you shake the eyepiece gently does it rattle or do the elements feel like they are firmly seated?

All firmly seated. It seems the internal thread is also glued. Not sure how to unscrew it without causing a damage. 

I suspect the internal lens group is the one which is reversed. 

[John]

Actually, if you can loosen the retaining ring, tapping the eyepiece slightly with the eyepiece pointing upwards can settle the lenses in their correct places which might do the trick. I've also come across plossls (not TV) where the spacer ring that separates the lens elements is missing. If the internal, convex, surfaces of the lens elements are touching it can cause problems. They should be slightly separated as the diagram shows.

 

[Piero]

I think I understood, but the retaining ring is basically blocked, likely due to the applied glue... uff! 

Whoever blocked it in that one, certainly didn't want someone to open it again! 

[John]

41 minutes ago, Piero said:

I think I understood, but the retaining ring is basically blocked, likely due to the applied glue... uff! 

Whoever blocked it in that one, certainly didn't want someone to open it again! 

That may have been the manufacturer. They put a little dab of cement across the threads. It can sometimes be broken or cut with a craft knife. It depends on how far you want to go to getting into the optics. I guess if the eyepiece is not working properly there isn't much to loose ?

[jetstream]

My TV plossls show pincushion distortion, but not the focus issue you mention. Sounds like you don't have much to lose Piero, just think the surgery out before grabbing the scalpel...

[Piero]

I'm currently in the operating room.. just giving anesthesia to the kiddo here.. 

Hope he's going to make it..!