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Delos Dilemma: 17.3mm Delos or 17mm Vixen LVW?

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#1 bluenote

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Posted 20 June 2014 - 07:23 PM

I recently asked if a 17.3 Delos would be a good choice for my Skywatcher 10" Dob...The general consensus was that it was. However, a good number of people
preferred the Vixen 17mm LVW. Is there a board member who has/had both who could give me some subjective feedback here? Eye relief is important to me as I wear glasses. Thanks

#2 tonyt

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Posted 20 June 2014 - 07:32 PM

I've owned both and the Delos is the obvious choice, even though the LVW's are very good eyepieces. The Delos has a wider AFOV and more comfortable (and easier to fold) eyeguard.

#3 Scanning4Comets

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Posted 20 June 2014 - 07:46 PM

Depends on your focal length. I own the 17mm Vixen LVW and it has better edge correction than the 17.3mm TV Delos in a fast reflector.

The 17.3mm Delos is in dire need of a coma corrector, a lot more than the 17mm Vixen LVW does, if your scope has a short focal length.

If coma does not bother you, the 17.3mm Delos has a bigger "immersion" factor and a much wider field of view.

The 17mm Vixen LVW is actually more comfortable to use than any of the Delos eyepieces if you do not wear glasses, but seeing as you do wear glasses, the 17.3mm Delos would be the better choice.

#4 karstenkoch

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Posted 23 June 2014 - 07:58 AM

I own the 17mm Vixen LVW and it has better edge correction than the 17.3mm TV Delos in a fast reflector.


I believe you, but that still surprises me. I thought Delos was THE STANDARD for edge to edge correction down to f/4. TV couldn't outdo an aging eyepiece design with their latest and greatest? Have I just swallowed the hype?

All I know for sure is that my 14mm Delos is effectively perfect (to my eye) at the edge in my 72mm f/6 refractor, but that's not exactly the toughest test imaginable...

#5 sixela

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Posted 23 June 2014 - 08:03 AM

Depends on your focal length. I own the 17mm Vixen LVW and it has better edge correction than the 17.3mm TV Delos in a fast reflector.


<raises eyebrows>

Not my experience.

The 17.3mm Delos is in dire need of a coma corrector, a lot more than the 17mm Vixen LVW does, if your scope has a short focal length.


<raises eyebrows even more>

Firstly, the need for a coma corrector depends on the focal ratio, not focal length.

Secondly, apart from the old Pretoria, I know of no coma correcting eyepieces. The only way to make coma less apparent is by blurring the comatic tails through field curvature, but that's hardly what you want in an eyepiece.

If you want no coma, then you need a coma corrector, period.


#6 eklf

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Posted 23 June 2014 - 10:19 AM

Depends on your focal length. I own the 17mm Vixen LVW and it has better edge correction than the 17.3mm TV Delos in a fast reflector.


<raises eyebrows>

Not my experience.

The 17.3mm Delos is in dire need of a coma corrector, a lot more than the 17mm Vixen LVW does, if your scope has a short focal length.


<raises eyebrows even more>

Firstly, the need for a coma corrector depends on the focal ratio, not focal length.

Secondly, apart from the old Pretoria, I know of no coma correcting eyepieces. The only way to make coma less apparent is by blurring the comatic tails through field curvature, but that's hardly what you want in an eyepiece.

If you want no coma, then you need a coma corrector, period.

:lol: You must be new here, Sixela

#7 csrlice12

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Posted 23 June 2014 - 10:25 AM

Yea, his near 15K posts show that.... :lol:

One must consider that Markus believes the paracorr to be an evil device robbing photons from the eyepiece..... :flame:

#8 Scanning4Comets

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Posted 23 June 2014 - 10:40 AM

I believe you, but that still surprises me. I thought Delos was THE STANDARD for edge to edge correction down to f/4. TV couldn't outdo an aging eyepiece design with their latest and greatest? Have I just swallowed the hype?

All I know for sure is that my 14mm Delos is effectively perfect (to my eye) at the edge in my 72mm f/6 refractor, but that's not exactly the toughest test imaginable...


True.

The 14mm Delos is excellent and almost the same as the 17mm LVW far as edge correction goes, but throw in the 17.3mm Delos and coma is very strong.

Yea, his near 15K posts show that....

One must consider that Markus believes the paracorr to be an evil device robbing photons from the eyepiece.....


Yea, your near 10,000 posts in less than 2 years and I had figured you'd learned something by now like the rest of the people going on & on & on about "coma correctors" when they are not really needed with certain eyepieces. Maybe you should get out more and observe with those 64+ eyepieces, instead of trying to cut me up. :p PS: It's near 13K posts, not 15K. :shocked: Where did you learn how to count? :roflmao:

You don't need it with the 14mm, 10mm or 6mm Delos, but throw in the 17.3mm Delos and it is a new ball game. With the shorter FL ones you don't see coma as much as the 17.3mm.

Did you get that? or should I spell it out for you? :tonofbricks:

Firstly, the need for a coma corrector depends on the focal ratio, not focal length.

Secondly, apart from the old Pretoria, I know of no coma correcting eyepieces. The only way to make coma less apparent is by blurring the comatic tails through field curvature, but that's hardly what you want in an eyepiece.

If you want no coma, then you need a coma corrector, period.


Ohh, sorry, my bad, Captain Coma.....please refer to my above statement. ;)

I recently asked if a 17.3 Delos would be a good choice for my Skywatcher 10" Dob...The general consensus was that it was. However, a good number of people
preferred the Vixen 17mm LVW. Is there a board member who has/had both who could give me some subjective feedback here? Eye relief is important to me as I wear glasses. Thanks


A person asked , and I gave my opinion. Sorry if I rustled some feathers. :bawling: :foreheadslap: :tonofbricks: :lol:


#9 ausastronomer

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Posted 23 June 2014 - 02:57 PM

I recently asked if a 17.3 Delos would be a good choice for my Skywatcher 10" Dob...The general consensus was that it was. However, a good number of people
preferred the Vixen 17mm LVW. Is there a board member who has/had both who could give me some subjective feedback here? Eye relief is important to me as I wear glasses. Thanks


I agree 100% with Markus on this. In a 10"/f4.7 reflector the EOF vperformance of the 17.3mm Delos is far from perfect, without a paracorr. The 14mm Delos is clearly superior in this regard IMO. I would consider going with the 14mm Delos as opposed to the 17.3mm Delos if you don't own a paracorr and don't plan on buying one in the near future.

Notwithstanding what I have said above, in a 10"/f5 ish scope I find a 14mm eyepiece to be more usefull than a 17mm eyepiece in any case. Although that could be influenced by the other eyepieces you have. In my 10"/f5.3 newt I usually go from a 27mm Panoptic to my 14mm or 10mm Pentax XW's, without Paracorr. In my 14" and 18"/f4.5 scopes I usually go from my 31mm Nagler to my 17mm Nagler with Paracorr. The 17mm Nagler (about 14mm with Paracorr) has a place in the larger scopes due to their longer focal lengths and higher magnification, in fact its my favourite eyepiece in both of those scopes, but I never use it in my 10" scope. I just find the slightly less than 3mm exit pupil a lot better for my needs than an over 3mm exit pupil, when I don't need the extra FOV. If for whatever reason you are set on the 17mm focal length, the 17mm Vixen will have better EOF performance than the 17.3mm Delos when both are used in a 10"/f4.7 reflector without a paracorr. In most other performance criteria the Delos will just edge out the Vixen. My first choice however without Paracorr would be a 14mm Delos

Cheers

#10 eklf

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Posted 23 June 2014 - 04:49 PM

Yea, his near 15K posts show that.... :lol:

Well, the number of posts don't mean much..just consider the current point of discussion. :foreheadslap:

Jocularity apart, I am well aware of Sixela's contributions (which I hold in high esteem); he belongs to a small group of CNers (Starman1, Jon Isaacs and Mike Hosea{btw, where is he?}) whose every post in eyepiece forum I follow.

#11 Scanning4Comets

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Posted 23 June 2014 - 05:03 PM

Yes, the current point of discussion was a reply to a question to which knowledge I have, and I wanted to help the OP. You don't need to respect my opinions or knowledge Kumar, and since you don't, you just had the need to post the above, because I have a bit of an attitude....and for a BIG reason.

Thank-You. :cool:

Drama. Oh my, it's everywhere!!! :lol:

#12 eklf

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Posted 23 June 2014 - 06:41 PM

Chill out Marcus.

Csrlice's statement "Yea, his near 15K posts show that...."was addressed to me and was a humorous rejoinder to my post about Sixela ("You must be new here, Sixela"). Note that Sixela has 14,000+ posts.

The point that Sixela is trying to make (one that Jon Isaacs and recently Starman1 have explained many times) is that coma is a function of a fast mirror. It is not possible for typical eyepiece (except aforementioned UO's pretoria) to correct for it. Therefore its not possible for 17mm LVW to show less coma than any other typical modern widefield in a fast reflector. It is right there, and present in bucketloads. Not all can identify it, and not all are sensitive to it. It may be true that it does not show other abberations.

Bottomline, 17mm LVW shows as much coma as any other modern widefield in a fast reflector, and will need a coma corrector just as much as any other 65* FOV eyepiece.

If your knowledge and experience suggests otherwise, than let the OP be aware that there are two schools of thoughts/experiences.

#13 bluenote

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Posted 23 June 2014 - 10:43 PM

Thanks to everyone for their input ..much grist for the astronomical mill here.
obviously, this is not a cut and dried issue and I appreciate the detailed responses.

#14 Scanning4Comets

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Posted 24 June 2014 - 12:27 AM

Bottom line here:

The 17mm Vixen LVW is much better in a fast scope, (correction-wise), than the 17.3mm Delos. I answered the OP with a simple answer that you or a few others just can't seem to swallow!

Sorry there isn't "bucketloads" of coma from the 17mm LVW , or I would have sent it packing ages ago and I would have mentioned it as well. What it does actually show is a very slight amount of field curvature. A Paracorr might help the 17mm LVW, but for MY EYES, I don't want, or need one.

The 22mm LVW is even better in a fast reflector w/o coma correction with no FC at all.

Oh, thanks for the pro-tip of where the coma comes from, I wasn't aware of that in my 30+ years in observing and reading about telescopes and mirrors bro. :waytogo: Really, I mean that.

#15 ausastronomer

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Posted 24 June 2014 - 02:48 AM

Chill out Marcus.

Bottomline, 17mm LVW shows as much coma as any other modern widefield in a fast reflector, and will need a coma corrector just as much as any other 65* FOV eyepiece.


Bottomline, there is more going on here than just coma and the 17mm Vixen LVW offers better EOF performance than the 17.3mm Delos in a fast Newtonian without a paracorr.

While the paracorr is really designed to correct for coma only, by virtue of some necessary design parameters to achieve the coma correction, it also helps correct for field curvature and to a lesser degree, off axis astigmatism with some eyepieces. A case in point of this is that the paracorr does an excellent job of correcting the field curvature of the 14mm and 20mm Pentax XW's. It also corrects a lot more than just coma in the 17mm Nagler T4, which without a paracorr is pretty average in a fast Newtonian and is absolutely superb with a paracorr.

Cheers

#16 youngamateur42

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Posted 24 June 2014 - 11:17 AM

Psssh! All of you guys and your short focal ratio scopes! Save the trouble and get a long scope :mrevil: :lol:

#17 sixela

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Posted 24 June 2014 - 11:47 AM

Bottom line here:
The 17mm Vixen LVW is much better in a fast scope, (correction-wise), than the 17.3mm Delos.


"Is" would mean that it's an objective truth.

Yet I happen to own three scopes, an f/5 and an f/4.8 which I use without a coma corrector and an f/4.5 which I use with one, and I find the edge of field correction (degree for degree, i.e. at the same apparent angle in the field) better in the Delos.

It does show coma in the scopes without a Paracorr, but an eyepiece is supposed to show coma unless it's a coma correcting eyepiece (and the LVW isn't one either). The comatic tails are a bit less easy to recognise in the LVW unless I refocus, but that's an additional aberration in the LVW (one which is fairly personal, since it depends on how well you accommodate for field curvature).

To me, coma is glaringly obvious in both the LVWs and the Delos at 17mm-ish in sub f/5 scopes (since the exit pupil is much too large for the diffraction to blur the comatic tails). But of course observing with a Paracorr has made me fairly intolerant of it.

In the scope with the Paracorr, the coma is simply gone, in both the LVW and the Delos. Then the edge of field astigmatism is lower in the Delos, and there's probably less field curvature as well (although the field curvature in an LVW is also very easy to accommodate for, so it's hard for me to tell precisely).

But then, I may have confirmation bias, since the Delos is mine and the LVW no longer is (I used to have one since I preferred it to the Pans in those focal length, but I sold it when I bought my 16T5 Nagler).

That's why I write "not my experience"; apparently _you_ need no such disclaimers.

#18 Scanning4Comets

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Posted 24 June 2014 - 11:49 AM

Bottomline, there is more going on here than just coma and the 17mm Vixen LVW offers better EOF performance than the 17.3mm Delos in a fast Newtonian without a paracorr.

While the paracorr is really designed to correct for coma only, by virtue of some necessary design parameters to achieve the coma correction, it also helps correct for field curvature and to a lesser degree, off axis astigmatism with some eyepieces. A case in point of this is that the paracorr does an excellent job of correcting the field curvature of the 14mm and 20mm Pentax XW's. It also corrects a lot more than just coma in the 17mm Nagler T4, which without a paracorr is pretty average in a fast Newtonian and is absolutely superb with a paracorr.

Cheers


Indeed John,

I've used a TV Paracorr on a few eyepieces in a friend's 16" F/4.5 and it cleaned the edges up nicely. I know it would make the 17mm Vixen LVW perfect and clean up the very MINIMAL FC it exhibits in my own 10" F/4.7, but the FC is so miniscule, that IMO, it doesn't need the Paracorr.

It seems every time I post about an eyepiece, people TRY DESPERATELY to lecture me on where coma comes from and that I have to have a Paracorr or some type of Coma Corrector in my scope.

People, I KNOW what is going on here, and I don't need to be lectured. Please read more carefully.

I've already made my point to the OP, there really is no use in going on & on about it.

Psssh! All of you guys and your short focal ratio scopes! Save the trouble and get a long scope


:roflmao:

My observing buddies joke around abut this all of the time with me.

"Well, if you had of bought a 10" F/8 reflector..." Yep.....would be awesome....but @ 80", I'd always be "ladder bound".

:lol:

Cheers,

#19 sixela

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Posted 24 June 2014 - 11:51 AM

t seems every time I post about an eyepiece, people TRY DESPERATELY to lecture me on where coma comes from


We don't lecture you, we say it for the benefit of the readers of the threads (it's also not a matter of opinion, but of fact).

And you don't have to be sorry to 'ruffle some feathers', but that doesn't mean you have to be adversorial if we disagree with you. Apparently the mere fact of disagreeing with you is also enough to ruffle some feathers in your corner.

#20 Scanning4Comets

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Posted 24 June 2014 - 11:59 AM

Excellent.

This gives me another opportunity to do another "shootout"

17.3mm Delos Vs. 17mm Vixen LVW in a fast reflector without a coma corrector / Paraccorr away from the city, with at least 6 or 7 of us!

Mike, Tony, Chris, Steve, Brett, Vince & Myself!

Hopefully we can all get out there this Thursday night. ;)

Let the games begin!

#21 sixela

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Posted 24 June 2014 - 01:55 PM

Couldn't you _also_ compare with a Paracorr?

I'm curious -- f/4.5 vs. 5.1 with Paracorr plus the way field curvature changes could well give different results with and without Paracorr. I don't expect it, but I don't actually exclude it.

#22 George Kiger

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Posted 24 June 2014 - 02:08 PM

The Delos has a field lens about 2-1/2 mm larger than the LVW - could this explain the perceived additional coma in a fast scope?

#23 Scott in NC

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Posted 24 June 2014 - 02:08 PM

:4

#24 Scanning4Comets

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Posted 24 June 2014 - 02:21 PM

The Delos has a field lens about 2-1/2 mm larger than the LVW - could this explain the perceived additional coma in a fast scope?


I don't think so. The 17.3mm Delos has a larger field stop, (I think?), so you're seeing more of the mirror. There are other factors involved as well, like Positive and Negative field curvature of the eyepiece.

#25 Starman1

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Posted 24 June 2014 - 02:33 PM

Well, though a 10" or 12" newtonian, at f/5 or so, has little field curvature, it does have some.
And we don't know, because we lack charts of design parameters, exactly what the field curvatures of the 17 LVW and 17.3 Delos are.
A few possibilities come to mind, and I am not at all arguing with what people see, merely trying to explain it:
1.the 17.3 Delos might have more field curvature than the LVW and, therefore, show stars defocused at the edge by a slight amount. This would inflate the size of the comatic star image and make it appear worse than the actual amount of coma in the scope
2.the 17.3 Delos is a little longer focal length and has a little wider field of view. This might make the comatic star images a tad worse at the edge, visually.
3.the Delos might have field curvature matching the field curvature of the scope, making the combination of scope plus eyepiece worse. Then, see point 1.
4.the nature of astigmatism is to make stars elongate radially on one side of focus and circumferentially on the other. If the field curvature curved the direction of circumferential elongation, a radial elongation in the star image (coma) could be slightly shortened in the radial direction and widened in the circumferential direction. This might slightly reduce the size of the blur image (rounding it slightly) and seem to reduce the effects of coma. A slight combination of astigmatism and field curvature could result in less visibility of coma. A lot of field curvature in that direction would show seagulls in combination with coma, but my experience with the LVWs shows that astigmatism and field curvature is very mild in the series.

If #4 is true, then it is ironic that an eyepiece with a flat field and no astigmatism could show a worse edge-of-field star image.

Whatever the case, Markus' and John's comments show that there are definite issues with one over the other in a scope without a coma corrector.

Armed with that information, what do you do? There is a choice, here.






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