Delos Pincushion Distortion

[russ.will]

I will start by saying that I'm not one of those that is particularly fussed by this particular issue, such that i've suffered it to date. I will also state that in my one night time session with my 12mm Delos, it was a non-issue with the moon remaining absolutely pin sharp and perfectly round as it met and crossed the field stop. That was what I was looking for and the Delos excelled.

My 17.3mm Delos arrived today and not being one to wait, I plonked the Manfrotto tripod outside, bolted the ES ED80 to it and set to observing roof tops, aerials, birds and anything else that would stop still for long enough, spotter style. As with the 12mm, I found the sharpness of the field edge to edge to beyond reproach. I mean that not even the slightest shift of focus is required when you move a subject toward the edge of field to check this.

One of the subjects I use is a very long, very straight metal chimney from a wood burner on the roof of our friends behind us. Yes, it's a thermal nightmare mid winter, but that's not the point. As I moved it toward the edge of field of the 17.3mm, I noticed (a slight understatement) the chimney start to bow outward at the top and bottom in a way that I have never seen before in any eyepiece. And I do mean bow as in 'that's extremely curved'. If i had a camera lens that did this at wide angles, I'd bin it (read sell it on Ebay) but we're not dealing with anything like the wide angles approached by a camera lens. Indeed this is a far higher magnification than my Tamron would achieve at 300mm.

I had read in the Alpha Lyrae review that it is impossible to correct for field curvature and rectilinear distortion at the same time, but never having had an EP that majors so exclusively in the former at the expense of the latter, I wasn't sure how noticeable this effect is.

I can assure you, it is VERY noticeable and it's going to be interesting with the return of actual night, to drop this in the Dob, where one tends to pan about the sky with an eye at the EP. I can imagine a few people would find it quite objectionable and even I'm wondering. Wondering how massive the same effect must be in the even wider AFOV of an Ethos for instance?

I would observe that having noticed it in the 17.3mm, I could also see the same effect in the 12mm, but to a far, far lesser degree. Is this the reason Deloi stop at 17.3mm? I'm interested to hear other users experiences, because one does get the impression these things are beyond reproach, but clearly not without compromise in this regard.

Russell

[kalasinman]

Sounds like a poor EP. Return it?

[YKSE]

Sounds like a poor EP. Return it?

I don't think we can draw that conclusion this fast.

As OP has mentioned, ALL the wide field eyepiece (wider than 57 deg) will HAVE TO balance between AMD (Angular magnification Distortion) and RD (rectilinear distortion), because these two type of distortions can't be corrected at the same time. Eyepices with RD WILL show either Barrel or Pincushion distortion. That's why (wide field) eyepieces for daytime use will have better correction on RD, and left more AMD uncorrected. While for observing night sky, the RD is usually considerred less disturbing than AMD.

our eyes usually interpretate slight pichusion as straight lines, as demontrated here:

http://www.holgermerlitz.de/globe/distortion.html

[Adrian]

Correct me if I'm wrong, but it's my understanding that you can have one or the other: a round moon to the edge or straight lines across the field of view, but not both.

To go back to your camera lens analogy, if you have a rectilinear extreme wide angle lens then the straight lines will appear straight, but if you have someones head near the edge of frame then it will look stretched out of shape. If you want round heads to the edge of frame then you need a fisheye lens and your lines will have massive barrel distortion.

Same for the eyepiece - in order to keep the moon round to the edge then distortion is required, but this time it is pincushion, not barrel. Reduce the pincushion distortion and the moon will only be round in the middle of the FOV.

Televue have chosen to accept the rectlinear distortion because they are astronomy eyepieces, so round discs are more important, but a  manufacturer of a daylight-use (spotting scope) eyepiece may choose the opposite approach, making their eyepieces less useful for astronomy due to distortion.

Edit: Too long typing 

[russ.will]

That is my understanding too, albeit you have been more concise.

My issue as much as anything else (and believe me, these ARE lovely EPs) is that it is never mentioned when recommending EPs is TV are the sharpest going, but may make you throw up if you like to pan around a lot!'

Not having had dark or clear enough skies to have given mine a stern run-out as of yet, I'm simply interested to hear other users experience on this issue.

Russell

[YKSE]

Same for the eyepiece - in order to keep the moon round to the edge then distortion is required, but this time it is pincushion, not barrel. Reduce the pincushion distortion and the moon will only be round in the middle of the FOV.

Televue have chosen to accept the rectlinear distortion because they are astronomy eyepieces, so round discs are more important,

Edit: Too long typing 

I'd be cautious about drawing these general conclusions. Each and every eyepiece (same focal length) may very well have different corrections of AMD and RD, as russ pointed out in his first day-light.

Here's an review mentioning Nagler 26mm NOT showing round moon at the edge.

http://stargazerslounge.com/topic/195838-nagler-26mm-against-meade-24mm-uwa/

[andrew63]

 A lot of wide eyepiece have this issue and lots tele vue ones. Point some Naglers at jupiter and see what strange shape you get - eg the 13mm.   It's all well being sharp to the edge and correcting to F4 etc but flat they are certainly not.  If you want flatter try Vixen wides 

andrew

[John]

Having tried many eyepieces and most of the Tele Vues (not the Delos though) I reckon all eyepieces have their vices, some in more generous portions than others. 

I guess the thing is to find the formula that works best for you where the compromises that are made are ones which don't bother you.

This is why eyepiece preferences can often be very personal with one persons dream eyepiece not suiting someone else quite as well.

I believe the Delos range is not a "scaled" design so the optical layout and characteristics will vary across the range in the same way they do with the Pentax XW's. The focal point of the 17.3mm and I believe the 14mm Delos are quite different to the other Delos models too and TV have actually produced an adapter I understand to bring them into line. 

[russ.will]

I did notice that. They are significantly less parfocal than any other EP line I've had - nearly a full turn of the 1:1 between the 12 & 17.3.

[andrew63]

I noticed this quite strongly with the maxvision/meade 20mm 68 deg - the moon looks peculiar.  I assume as you have the max 20 also Russell the effect will be similar - might be worth comparing both as you have that nice luxury.

andrew

[russ.will]

I do and I will.

[michael.h.f.wilkinson]

This is a common trade off, as has been mentioned. I found the 14mm UWA from Meade had slightly less pincushion distortion than Naglers, but also slightly less sharpness. The perfect EP does not exist, everything is a trade off.

[YKSE]

I noticed this quite strongly with the maxvision/meade 20mm 68 deg - the moon looks peculiar.  I assume as you have the max 20 also Russell the effect will be similar - might be worth comparing both as you have that nice luxury.

andrew

I've just checked all my EP's edge distortion. the 20/24mm 88 deg do have the most obvious pincushion among all my EPs, the BCOs showed very slightly pincushion too, much to my surprise actually, while the wide field EPs like Baader 31mm and Nikon 7mm show straight lines across the FOV.

[russ.will]

Interesting. What do circular shapes look like at the edge in the Baader and Nikon?

Russell

[jetstream]

My BC0 18mm,Nagler 16mm ovals near any edge-some others do too,I forget at the moment.The best way to check this I've found is using my wedge with the green Soar Continuum filter in my short frac,very easy to spot this with it.The views provided with the wedge using these 2 eyepieces is fantastic regardless of the ovaling though.

[YKSE]

Interesting. What do circular shapes look like at the edge in the Baader and Nikon?

Russell

 I've not checked that yet, will report back when I have the chance to look at the Moon.

[John]

The Vixen SLV 6mm had mild pincushion distortion in the outer few % of the FoV. The 12mm and 20mm versions did not show this and neither did the older Vixen LV 6mm. The 6mm Baader CO becomes less sharp in out last 5% of it's FoV but I was not sure that was due to pincushion.

[YKSE]

Interesting. What do circular shapes look like at the edge in the Baader and Nikon?

Russell

The Moon will not be visible in a couple of days, so I did some indoor tests to start with:

A paper will circles taped on the far end of the wall, both C8 and 80ED were used, with similar results.

1. MV 20mm(EP with pincushion):  the circle neareast edge got egg-shaped, clearly in C8 (100x), also noticeable in 80ED(30x)

2. Leica in 17.8mm setting (EP without pincushion/barrel): no noticeable change of circles when approaching/passing edge in any scope(34X and 114X).

3. Nikon 7mm(EP without pincushion/barrel): invisible change in 80ED (86x); slightly flat nearest edge in C8(290x), in lesser amount than MV20's egg-shape in 80ED.

Ti my eyes, EPs with well-corrected RD are certainly more preferred in daytime use. Just need to find out how the round Moon looks like in these EPs.

[YKSE]

Had somr brief sessions among the clouds, here's my impressions:

1. Paning star fields: No globe effect in any of EPs I tested (with RD or AMD), stars followed a straight line from to edge in Leica and Nikon (with AMD), a slightly curved line in Maxvision(with RD), nothing distracting in any way.

2, Splitting doubles near the edge: The double-double was the target, clear split in the center of FOC with both MV20mm(100x) and Leica in 17.8mm (114x), moving to the very edge, both double couldn't be splitted because of the astigmatism in MV20, even though the distance between the stars were seemingly unchanged. In Leica, the pair with equal distance to the edge was still the same split, while the other pair with uneqaul distance to the edge couldn't be splitted. Moving the double-double to different edge just confirmed the observation.

3. The Moon (with 80ED): The shape of the Moon more or less confirmed the daytime viewing, it got egg-shaped in MV20(30x), while virtually unchanged in Nikon 7mm(86x).

[russ.will]

Cheers for that. I'm still waiting for a confluence of time and moon to make my own conclusions. But drawing on my ever evaporating short term memory, I'm pretty darn sure I'll find the same as you!

My Delos 8mm should turn up during this next week and as a bit of writing money should also land, I have a decision. Do I buy the 10 & 14mm, or a 10mm and a TAL 100RS OTA? The 14mm may be a bit redundant, based on the fact that I used to frequently jump from the MV16mm to the ES82 11mm, by-passing the ES82 14mm, even though I thought it a really nice EP. The TAL option based is purely on the hype one continually reads about them and not wanting to be caught out as OVL aren't going to import them any more. Fools. Are they THAT good, or just THAT good for the price, given that I have a decent F9.4 125mm frac anyway?

That said, my real hobby is tempting me with a pair of MiniDSP Plate Amps, which is something I've wanted for along while, but keep side-stepping every time I fancy something astro. The thing is, I like to cut my cloth during the closed season and then live with the decisions during the following prime viewing months, so at this particular point, EP choice is important. Hmm.

Russell

[Don Pensack]

I will start by saying that I'm not one of those that is particularly fussed by this particular issue, such that i've suffered it to date. I will also state that in my one night time session with my 12mm Delos, it was a non-issue with the moon remaining absolutely pin sharp and perfectly round as it met and crossed the field stop. That was what I was looking for and the Delos excelled.

My 17.3mm Delos arrived today and not being one to wait, I plonked the Manfrotto tripod outside, bolted the ES ED80 to it and set to observing roof tops, aerials, birds and anything else that would stop still for long enough, spotter style. As with the 12mm, I found the sharpness of the field edge to edge to beyond reproach. I mean that not even the slightest shift of focus is required when you move a subject toward the edge of field to check this.

One of the subjects I use is a very long, very straight metal chimney from a wood burner on the roof of our friends behind us. Yes, it's a thermal nightmare mid winter, but that's not the point. As I moved it toward the edge of field of the 17.3mm, I noticed (a slight understatement) the chimney start to bow outward at the top and bottom in a way that I have never seen before in any eyepiece. And I do mean bow as in 'that's extremely curved'. If i had a camera lens that did this at wide angles, I'd bin it (read sell it on Ebay) but we're not dealing with anything like the wide angles approached by a camera lens. Indeed this is a far higher magnification than my Tamron would achieve at 300mm.

I had read in the Alpha Lyrae review that it is impossible to correct for field curvature and rectilinear distortion at the same time, but never having had an EP that majors so exclusively in the former at the expense of the latter, I wasn't sure how noticeable this effect is.

I can assure you, it is VERY noticeable and it's going to be interesting with the return of actual night, to drop this in the Dob, where one tends to pan about the sky with an eye at the EP. I can imagine a few people would find it quite objectionable and even I'm wondering. Wondering how massive the same effect must be in the even wider AFOV of an Ethos for instance?

I would observe that having noticed it in the 17.3mm, I could also see the same effect in the 12mm, but to a far, far lesser degree. Is this the reason Deloi stop at 17.3mm? I'm interested to hear other users experiences, because one does get the impression these things are beyond reproach, but clearly not without compromise in this regard.

Russell

As eyepiece apparent fields get wider than about 40 degrees, the solution to eliminate rectilinear distortion (pincushion or barrel distortion, dependent on sign--either )|( 0r (|) is the result) results in increasing amounts of angular magnification distortion (wherein things either shrink or grow at the edge of the field).  RD and AMD cannot be simultaneously solved for, and the wider the apparent field, the more the two curves diverge.

Because you wouldn't want the separation between stars to change at the edge of the field, and you wouldn't want a planetary nebula to grow smaller, most designers of astronomical eyepieces solve for AMD and let RD fall wherever it may.  Having a line of stars slightly change shape as it nears the edge of the field isn't usually a problem.

AMD also results in what they call "rolling ball" or "globe" distortion when panning, and this is extremely annoying.

The way the human eye works, we can see up to 7% RD as undistorted, too, so solving for AMD seems a better choice for astronomy.

Now, if you were using the eyepiece for daytime views of  trees or buildings or any objects with straight lines, then RD becomes more of a problem, and leaving some AMD in the eyepiece is often done in binoculars (which are typically used in daylight).

Perhaps this is your first experience with widefield eyepieces.  Just know the problem gets worse as the apparent field expand.  100 degree eyepieces and wider have a serious choice to make about distortion correction.  Fortunately, all are used primarily for astronomy, so reducing AMD is the (correct) choice the designers make.

If you're curious, I can be a lot more technical about the subject, but the above explanation is it in a nutshell.

[russ.will]

That's basically what the Alpha Lyrae review I linked to said.

My point was that coming to the Delos from the even wider ES82s, it was even more noticeable than it had been in those UWAs - I do occasionally use my ED80 on a camera tripod as a spotting scope, although I'm not interested in buying EPs specifically for this purpose - It's too occasional and mostly because UK cloud has bored me into daytime use!

I was articulating that in an EP that is extremely well corrected for RMA, it's really noticeable on daytime subjects - far more notable than it was in my former ES14 & 18 (although I didn't like the latter for a number of reasons unconnected to the subject of this thread) despite their wider FOV, so their designer had presumably made a different set of choices.

To be clear, this was just a statement of what I saw, rather than a criticism of the Delos, two more of which have been added to my collection since and another may yet join them.

Russell

[Paul73]

Hmmm. I've seen this before. Once an eyepiece case has contracted green, the infection can spread quickly. Cutting of the supply of cash to the area is the only known treatment......

[Paul73]

Hmmm. I've seen this before. Once an eyepiece case has contracted green, the infection can spread quickly. Cutting of the supply of cash to the area is the only known treatment......

12mm Delos appears to be free of noticeable distortion when tested in the field last night. Daylight non astro objects might show errors which would be largely academic in the astro arena.

Back on thread!

Sorry Russell

Paul

[alan potts]

Russell,

I do not have the 17.3mm but I have a good few of the others. I have never noticed any pincushion in any of these. I think that viewing a straight pipe like you are would reveal faults in any eyepiece with a wide field. The same with camera lenses, I have some of the best you can buy and you can get the same effect when testing them the same way you are. I think that if the eyepiece offers a pleasing view with the full Moon then that is a test passed, I don't really think any of the wide eyepieces will ever better what you are seeing, maybe the only really true flat field is an orthoscopic. I must find something to look at and repeat the test with some of my eyepieces.

Slightly off topic but one reason behind me getting rid of the 26mm Nagler was the large pincushion where as the 24mm UWA was not really noticeable in comparison.

Alan