2" Eyepeices for a fast scope

[Mr Spock]

4 minutes ago, johnturley said:

Mr Spock

Do you have a 42mm Vixen LVW, I was wondering what the edge performance would be like in my f7 Esprit, I assume that it would be superior to my Baader 36mm Aspheric.

I already have a StellaLyra 30mm UFF, which I use as my lowest power through my 14in Newtonian, hence was thinking of using in my Esprit (which is piggybacked on the Newtonian) at the same time, and selling my 36mm Aspheric, and possibly also my 56mm Plossl.

Not many suppliers currently list the 42mm LVW, so wondered whether it had been discontinued, in which case if I want one, I need to order it fairly soon.

John 

The LVW range was discontinued a few years ago. You'll probably only be able to find one used. Generally it's a very sharp eyepiece, but, it does have some astigmatism round the edge; it also has uneven magnification - it's 42mm in the centre but 34mm at the edge. This is why there's confusion over the field of view - it is 72° but at an average of 38mm not 42mm.  Some are labelled as 65° but that isn't right either - at 42mm it's the equivalent of 65° but that isn't the right way to do it. I wish they had just labelled it as a 38mm 72° eyepiece. Sounds confusing - you'll see what I mean if you look through one. The fov is huge compared to a 'normal' 65° LVW. You can calculate it back from the 47mm field stop.

[Franklin]

18 hours ago, lawsio said:

is that extra view worth nearly £200 more?

No.

[bosun21]

7 hours ago, Mr Spock said:

The 42mm LVW has a field stop of 47mm  I had a WO diagonal and it vignetted noticeably - I can't recommend that diagonal or its clones to anyone.

Here's the Revelation:

None of my eyepieces have a field stop that large so I don’t get any vignetting and I have found that the 1/12th wave mirror gives me a slight increase in clarity over the other diagonals i have owned. I also like the fact that the sides can be removed for easy cleaning of the mirror. One mans food is another mans poison I suppose.

[John]

It's interesting to note that the diagonal is further up the "wobbly stack" of issues which impact the quality of the view than eyepieces. This is from a comparison of planetary eyepieces from Roger Vine's excellent "Scopeviews" website:

"The Wobbly Stack

What you see is a pretty subjective thing anyhow, but let’s look at some of the theory behind it.

In Star Testing Astronomical Telescopes, Richard Suiter describes the telescope system as like a wobbly stack of filters which at each level takes away a bit more contrast; tip it too far and the whole thing falls over. Items on the stack include:

1 Aperture

2 Seeing (not transparency, but the level of atmospheric disturbance which distorts the image moment to moment)

3 Quality of the primary optics

4 Central obstruction size

5 Alignment of the optics

6 The diagonal (mirrors scatter much more than lenses)

7 The ability of the focuser to deliver critical fine focus

8 The eyepiece

9 The skill and fatigue level of the observer and their eyes"

Edited September 27, 2023 by John

[bosun21]

4 minutes ago, John said:

It's interesting to note that the diagonal is further up the "wobbly stack" of issues which impact the quality of the view than eyepieces. This is from a comparison of planetary eyepieces from Roger Vine's excellent "Scopeviews" website:

"The Wobbly Stack

What you see is a pretty subjective thing anyhow, but let’s look at some of the theory behind it.

In Star Testing Astronomical Telescopes, Richard Suiter describes the telescope system as like a wobbly stack of filters which at each level takes away a bit more contrast; tip it too far and the whole thing falls over. Items on the stack include:

1 Aperture

2 Seeing (not transparency, but the level of atmospheric disturbance which distorts the image moment to moment)

3 Quality of the primary optics

4 Central obstruction size

5 Alignment of the optics

6 The diagonal (mirrors scatter much more than lenses)

7 The ability of the focuser to deliver critical fine focus

8 The eyepiece

9 The skill and fatigue level of the observer and their eyes"

Nothing to go wrong then?

[globular]

11 minutes ago, John said:

In Star Testing Astronomical Telescopes, Richard Suiter describes the telescope system as like a wobbly stack

In the copy of Suiter's book that I have the 'wobbly stack' items are listed without numbers or an order of importance - rather they are roughly stacked in the order that light from the object travels through the 'system'.  Read your list above with that in mind and you might agree that the same applies to Roger Vine's reproduction of the stack too.

[John]

30 minutes ago, globular said:

In the copy of Suiter's book that I have the 'wobbly stack' items are listed without numbers or an order of importance - rather they are roughly stacked in the order that light from the object travels through the 'system'.  Read your list above with that in mind and you might agree that the same applies to Roger Vine's reproduction of the stack too.

That makes sense apart from 1 and 2 which could arguably be reversed perhaps ?

 

 

 

 

[globular]

1 hour ago, John said:

That makes sense apart from 1 and 2 which could arguably be reversed perhaps ?

 

 

 

 

Agreed. If I remember correctly, that part of the book is talking about the importance of aperture - so I think he couldn't help but put it first.

[RobertI]

2 hours ago, John said:

2 Seeing (not transparency, but the level of atmospheric disturbance which distorts the image moment to moment)

Interesting that transparency is not included - I would have thought poor transparency would reduce contrast in diffuse objects such as galaxies and nebulae? We could probably have a whole thread devoted to discussing that list! 

[John]

7 minutes ago, RobertI said:

Interesting that transparency is not included - I would have thought poor transparency would reduce contrast in diffuse objects such as galaxies and nebulae? We could probably have a whole thread devoted to discussing that list! 

Transparency is more relevant to this thread I agree.

Edited September 27, 2023 by John

[johnturley]

18 hours ago, Mr Spock said:

The LVW range was discontinued a few years ago. You'll probably only be able to find one used. Generally it's a very sharp eyepiece, but, it does have some astigmatism round the edge; it also has uneven magnification - it's 42mm in the centre but 34mm at the edge. This is why there's confusion over the field of view - it is 72° but at an average of 38mm not 42mm.  Some are labelled as 65° but that isn't right either - at 42mm it's the equivalent of 65° but that isn't the right way to do it. I wish they had just labelled it as a 38mm 72° eyepiece. Sounds confusing - you'll see what I mean if you look through one. The fov is huge compared to a 'normal' 65° LVW. You can calculate it back from the 47mm field stop.

According to Harrison Telescope's website they still list the Vixen 42mm LVW with availability in 5-7 days, and tried doing a dummy purchase and it went into my basket, but not sure whether they still have one in stock. 

Even if they still have one available, I'm a bit undecided as not sure whether it would be a significant improvement over my 36mm Aspheric.

John 

[Mr Spock]

According to this http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p41976 the aspheric has very bad image quality. The 42mm LVW does ok and the other LVWs rate as excellent image quality.

[johnturley]

23 minutes ago, Mr Spock said:

According to this http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p41976 the aspheric has very bad image quality. The 42mm LVW does ok and the other LVWs rate as excellent image quality.

Thanks for the info.

I have now successfully placed an order with Harrison Telescopes, so remains to be seen whether they will actually be able to supply one.

Assuming it does arrive, I will compare it with my 36mm Aspheric, and 56mm Meade Plossl, and if it compares favourably, possibly sell both of these. My Meade 56mm Plossl is actually one of the original 5 element made in Japan eyepieces, so may fetch a bit of a premium price.

John 

[John]

1 hour ago, Mr Spock said:

According to this http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p41976 the aspheric has very bad image quality. The 42mm LVW does ok and the other LVWs rate as excellent image quality.

I did not think too highly of the Aspheric 36 that I had and that was in an F/12 refractor. I could still see some astigmatism towards the field edges.

[johnturley]

Just now, John said:

I did not think too highly of the Aspheric 36 that I had and that was in an F/12 refractor. I could still see some astigmatism towards the field edges.

I don't find the Aspheric 36 too bad in my f7 Esprit 150, but the edge quality is rather poor in my f5 Newtonian, hopefully (assuming that it arrives) the Vixen 42mm LVW will be better in both, besides giving a slightly wider field of view.

John 

[John]

1 minute ago, johnturley said:

I don't find the Aspheric 36 too bad in my f7 Esprit 150, but the edge quality is rather poor in my f5 Newtonian, hopefully (assuming that it arrives) the Vixen 42mm LVW will be better in both, besides giving a slightly wider field of view.

John 

That is a big exit pupil in the F/5 !

 

[lawsio]

Right then. 

Taking on board everyone's comments, I've ordered the scope but not the eyepieces yet. Was going to try and get them all together to save on postage but clearly this needs some thinking about and with the diagonal included it's going to be in the region of £300 extra I need to spend so want to get this right. 

In the meantime, I have a set of 1.25" Orion Sirius Plossl eyepieces, one of which is a 40mm and using the online calculator it looks like it gives me near as dammit the same FOV as the 2" SL UFF 30mm. I assume this will fall very much into the category of 'it's going to look hideous' on this scope but I'll persevere with it for a bit to ensure this is the right FOV im looking for and I'll appreciate the expensive one all the more for it down the line im sure!

Thanks for your advice all, I'll come back to this when it's time to buy!

 

[Ricochet]

20 minutes ago, lawsio said:

one of which is a 40mm and using the online calculator it looks like it gives me near as dammit the same FOV as the 2" SL UFF 30mm

I think there must be something wrong with the online tool you have used. You can't get a wider true field of view from a 1.25" eyepiece than with a 32mm/50° Plossl. As a result the 40mm Plossls have a more restricted apparent field of view, about 40°. The online tool probably had the 40mm listed at 50° like the rest of the line. 

Congratulations on the new scope purchase. 

[John]

6 minutes ago, Ricochet said:

I think there must be something wrong with the online tool you have used. You can't get a wider true field of view from a 1.25" eyepiece than with a 32mm/50° Plossl. As a result the 40mm Plossls have a more restricted apparent field of view, about 40°. The online tool probably had the 40mm listed at 50° like the rest of the line. 

Congratulations on the new scope purchase. 

Agreed. A 40mm plossl in the 1.25 inch fitting has a max apparent field of around 43 degrees. Some are less than that.

 

Edited September 28, 2023 by John

[lawsio]

This is what I've based the above comment on ^

 

 

Edited September 28, 2023 by lawsio

[johnturley]

55 minutes ago, lawsio said:

This is what I've based the above comment on ^

 

 

As some have mentioned previously, it is not possible to get a 52 degree field with a 1.25in 40mm eyepiece, unfortunately some manufacturers do overstate the apparent field of view of their eyepieces, or quote the average for the range, and the longer focal lengths have smaller APFOV's. 

John 

 

Edited September 28, 2023 by johnturley

[lawsio]

Fair enough - that's what the AFOV is listed as online. What's a more accurate number to us?

[John]

6 minutes ago, lawsio said:

Fair enough - that's what the AFOV is listed as online. What's a more accurate number to us?

42-43 degrees at best. As has been said, some are less.

Edited September 28, 2023 by John

[lawsio]

Thanks for that, makes sense! Still not a million miles out, will do me for now anyway whilst I decide what others to get!

 

[John]

16 minutes ago, lawsio said:

Thanks for that, makes sense! Still not a million miles out, will do me for now anyway whilst I decide what others to get!

 

If you have the 32mm plossl, that will show you the same amount of sky as the 40mm but with a more pleasing, and wider, apparent field. The exit pupil will be smaller as well so you should see a slightly darker background sky which helps pick out deep sky objects.