TeleVue - a VERY brief first light

[Davey-T]

11 minutes ago, John said:

I'm very lucky that I have both. I thought that I might not need the 31mm Nagler once I got the Ethos 21 but I've found that I use it enough to hang onto it.

Snap 😂

Dave

[Raph-in-the-sky]

22 minutes ago, John said:

No. The ES that I've used and owned were the 24mm 68 degree and the 20mm 100 degree. They were both very nice eyepieces.

I've read enough reports on the 82 degree ES eyepieces to realise that you get 90% or even 95% of the performance of a Nagler in scopes down to F/5. Below that I rekcon the Naglers will be better corrected at the edges of the field of view but the ES 82's will still be doing a good job.

 

How big is the difference between f5 and f4.7?.... numbers seem to be close but I have no reference point

[John]

15 minutes ago, Raph-in-the-sky said:

How big is the difference between f5 and f4.7?.... numbers seem to be close but I have no reference point

.3 !

Really though, I don't have a reference point either I'm afraid, I just tend to regard newtonians with focal ratios of F/5 and lower as pretty fast scopes that place more demands on eyepiece correction.

[jetstream]

37 minutes ago, Raph-in-the-sky said:

How big is the difference between f5 and f4.7?.... numbers seem to be close but I have no reference point

Mel comes through again... punch the numbers in and it will tell you the coma in rms waves, very interesting. Just go to "eyepieces" and the calculator will come up.

http://www.bbastrodesigns.com/NewtDesigner.html#is aperture king

Edited August 29, 2019 by jetstream

[jetstream]

F5 10"

Manufacturer

Type

FocalLength

FieldStop

Field

ExitPupil

TrueFOV

MagX

PerInch

Res

MagLimit

Etendue

ComaAtEdge

TeleVue

Nagler 5

31mm

42mm

82deg

6.2mm

1.90deg

41x

4x/inch

5.9"

14.5

2317 cm^2deg^2

2.5 WavesRMS

Nikon

NAV

17mm

30.1mm

102deg

3.4mm

1.36deg

75x

7x/inch

3.2"

15.4

1190 cm^2deg^2

1.8 WavesRMS

Clave

Plossl

6mm

5.3mm

48deg

1.2mm

0.24deg

212x

21x/inch

1.1"

15.8

37 cm^2deg^2

0 .3 WavesRMS

"

 

F4.7 10"

"Manufacturer

Type

FocalLength

FieldStop

Field

ExitPupil

TrueFOV

MagX

PerInch

Res

MagLimit

Etendue

ComaAtEdge

TeleVue

Nagler 5

31mm

42mm

82deg

6.60mm

2.02deg

39x

4x/inch

6.2"

14.4

2622 cm^2deg^2

2.9 WavesRMS

Nikon

NAV

17mm

30.1mm

102deg

3.62mm

1.44deg

70x

7x/inch

3.4"

15.3

1347 cm^2deg^2

2 WavesRMS

Clave

Plossl

6mm

5.3mm

48deg

1.28mm

0.25deg

199x

20x/inch

1.2"

15.8

42 cm^2deg^2

0.4 WavesRMS

"

 

[bingevader]

And for the statistically challenged amongst us!

Could we have a lay mans interpretation of what difference 0.4 of a Wave means, please? 👋

[jetstream]

1 hour ago, bingevader said:

And for the statistically challenged amongst us!

Could we have a lay mans interpretation of what difference 0.4 of a Wave means, please? 👋

Lets ask an experienced observer- @John- I think you have used f4.7-f4.8 dobs and you own an f5.3. What is the difference with the 21E (or others) coma wise in these? The f4.8 comes in at 2.4 waves and the f5.3 at 1.9 waves for a difference of .5 waves.

[Ruud]

Below you see three light cones from f/3.5,  f/5 and f/10 objectives.
 

Between these three, it is easier to design an eyepiece for the f/10 light cone because such an eyepiece only has to handle rays that stay within three degrees of the optical axis.

The other two are said to have steep light cones. Faster objectives with their steeper light cones pose a  more demanding task for the  eyepiece designer to deal with them. Delos eyepieces are said to work well in telescopes as fast as f/3.5, showing sharp stars across the entire field of view.

Coma matters in a different way. It is more an objective issue than an eyepiece issue. Newtonians have coma and standard SCTs too. A good eyepiece faithfully shows the focal plane of the telescope. If coma is present in the focal plane, your good eyepiece will show you that coma. If that same eyepiece is used in a coma free instrument, it will show a coma free image. 

A coma corrector can be used to correct the coma present in a telescope's focal plane. Fast Newtonians benefit greatly from them. In Newtonians without a coma corrector, the wider the apparent view of your eyepiece, the more coma you'll see.

Coma correcting eyepieces also exist, the Pretoria is an example. You seldom see them though because such eyepieces are designed with a specific amount of coma in mind and they will not work well in a coma-free telescope.

 

 

 

Edited August 30, 2019 by Ruud

[Alan White]

Welcome to the world of Green & Black Jon.

A road you may regret sometimes financially, but boy I doubt you will ever doubt it when observing.
I have almost gone all Green & Black these days.
Don't regret it at all, just when I want another EP as they are at a premium price.

Naglers, love them, Panoptic love them too.
 

[parallaxerr]

15 minutes ago, Alan White said:

A road you may regret sometimes financially

This emotion I have already felt...

16 minutes ago, Alan White said:

but boy I doubt you will ever doubt it when observing

This one, I look forward to!

[bingevader]

Thanks Ruud, but I get the coma bit;

In order to avoid step ladders, short, large newts need big curvy mirrors.

Crass oversimplification, I know!

It's the wavey business I'd like someone to explain and what a difference of 0.4 WavesRMS means to the observer, please.

[Ruud]

Oops

The effect of wave front errors is difficult to predict, but it can be measured as the Strehl ratio. The Strehl ratio is 0.8 when 80% of the light that should end up in the Airy disk actually ends up there. Telescopes with a Srehl of 0.8 or higher are said to be diffraction limited, which simply means that the largest part of the blur in the image is caused by diffraction rather than poorly made optics.

Even higher Strehl ratios may matter for separating close double stars and discerning fine planetary detail, but most big Dobs aren't used for that and with their large aperture they pick up an awful lot of atmospheric seeing which is will probably obscure many shortcomings in the mirror shape.

Better optics tend to come with a test report. I suppose you'll have to rely on that.

Here's some info on the matter: https://www.telescope-optics.net/Strehl.htm#frequency

Sorry for misinterpreting your question. I somehow took it as one about the influence of fast optics on eyepiece performance.

[bingevader]

No need to apologise, my question was about the influence of fast optics on eyepiece performance.

I was hoping to understand what difference 0.4 WavesRMS made to the observer.

I think maybe I'm not supposed to (understand, that is)!

I read the info too!

I liked the pictures.

[Helen]

This might help a bit more https://www.telescope-optics.net/aberrations.htm  (rms = root mean square is a measure of the average error/uncertainty and expresses statistical deviation frpom perfect reference)

Helen

[bingevader]

Helen, that's the same one!

I especially liked the bit where it said, "In more general terminology..."!

The pictures are still good.

[John]

At F/4.8 the 21mm Ethos is very, very good. At F/5.3 it is very, very, very good.

The exchange rate is therefore: .5 wave = one very.

Hope that helps

I have referenced the telescope-optics website many times but rarely understand more than a few % of what I read on there.

I'm a bear of little brain when it comes to maths !

 

[jetstream]

15 minutes ago, John said:

At F/4.8 the 21mm Ethos is very, very good. At F/5.3 it is very, very, very good.

The exchange rate is therefore: .5 wave = one very.

Hope that helps

I have referenced the telescope-optics website many times but rarely understand more than a few % of what I read on there.

I'm a bear of little brain when it comes to maths !

 

Does f4.8 show more coma than f5.3 using the 21E John?

[Helen]

35 minutes ago, bingevader said:

Helen, that's the same one!

I especially liked the bit where it said, "In more general terminology..."!

The pictures are still good.

It's a different bit of the same website 😉 but I thought it explained wavefront and RMS a bit 🤣🤣.

We like graphs 😁

[bingevader]

1 hour ago, John said:

At F/4.8 the 21mm Ethos is very, very good. At F/5.3 it is very, very, very good.

The exchange rate is therefore: .5 wave = one very.

Hope that helps

I have referenced the telescope-optics website many times but rarely understand more than a few % of what I read on there.

I'm a bear of little brain when it comes to maths !

 

Thanks John!

I think that'll do.

I've hijacked the thread enough as it is!

Back to the Green and Blacks.

Mmmm, chocolate!

[RT65CB-SWL]

Copied from the SGL topic I posted earlier this week... [Prices with no box ?]...

This means I have one to many TeleVue 13mm e/p's ...

left: 13mm Plossl - centre: 13mm Nagler / Type 1 - right:  13mm Nagler / Type 6.

Edited September 1, 2019 by Philip R

[Piero]

On 29/08/2019 at 09:16, parallaxerr said:

After a few very satisfying sessions hunting DSO's recently, I've been questioning if I can improve my views with better eyepieces that may have better transmission. I am currently only using a MkIV Baader Zoom and Celestron Omni 32mm Plossl in my one and only 4" F7 refractor. 

In my opinion if you want to improve the views of DSOs, you should consider a telescope of larger aperture. Something like a 10in dobson will also complement your refractor.

Secondly, I would try to understand what you don't like about the views you get with your current plossl and zoom, and if you can or not live with it.

 

Said this, you mentioned "transmission". The highest transmission is on-axis. The eyepieces with the highest transmission I found are Vixen HR, Docter UWA, and Zeiss zoom diascope 20-75x, particularly when combined with Baader VIP Barlow. None of the above is cheap.

Televue eyepieces are particularly famous for the edge correction in fast telescope. A lot of Televue fans enjoy observing from edge to edge. Will a Televue eyepiece combined with a fast telescope with good optics be sufficient for providing one with pin point stars across the edge? No. Televue eyepieces will not introduce astigmatism (off axis aberration), angular magnification distortion (off axis distortion), nor chromatic aberration. However, to correct coma, one needs a coma corrector and Televue sells a good one. To correct rectilinear distortion which is the opposite of angular magnification distortion, you need eyepieces often used for terrestrial observation (there is biblical belief in the astro community that eyepieces for astronomy must correct AMD..). Field curvature is more complex as it is a bit of a combination between telescope and eyepiece. I have found this to be larger in short tube refractors where the focal length is quite short. All these aberrations and distortions mentioned in this paragraph are off-axis. How do Televue eyepieces work on-axis? They do work very well, but in my opinion the eyepieces I mentioned in the beginning of this post are better on-axis, in terms of contrast and sharpness under very good seeing and using a telescope with very good optics.

My two pennies.

 

P.s.

+1 for enjoy using a zoom. Zoom eyepieces have a terrific potential in my opinion.

Edited August 31, 2019 by Piero

[parallaxerr]

Hi Piero,

Thanks for the post. I agree that a larger scope would improve my views but it simply isn't an option for many reasons I won't go into in this thread. So, I'm sticking with the frac on the AZ4 and actually I'm happy with this combo, accepting it's limitations. Wide field is also therefore important due to manual tracking of planets at high mag and location of DSOs at lower power.

I guess on axis transmission is going to be close run between a lot of decent EPs but poor edge correction was bugging me in the Baader when locating DSOs which involves inspecting the whole FoV.

I've always fancied the Vixen HRs but the focal lengths are too short for my scope and seeing conditions. Actually, seeing conditions are most likely my limiting factor from home, I must get to some dark sites this winter to really test the scope and new EPs.

Oh...and the zoom - yes, I'll be keeping that!

P.S. just checked out the Docters and I thought TV were expensive 😱

Edited August 31, 2019 by parallaxerr

[Piero]

1 hour ago, parallaxerr said:

Hi Piero,

Thanks for the post. I agree that a larger scope would improve my views but it simply isn't an option for many reasons I won't go into in this thread. So, I'm sticking with the frac on the AZ4 and actually I'm happy with this combo, accepting it's limitations. Wide field is also therefore important due to manual tracking of planets at high mag and location of DSOs at lower power.

I guess on axis transmission is going to be close run between a lot of decent EPs but poor edge correction was bugging me in the Baader when locating DSOs which involves inspecting the whole FoV.

I've always fancied the Vixen HRs but the focal lengths are too short for my scope and seeing conditions. Actually, seeing conditions are most likely my limiting factor from home, I must get to some dark sites this winter to really test the scope and new EPs.

Oh...and the zoom - yes, I'll be keeping that!

P.S. just checked out the Docters and I thought TV were expensive 😱

 

As you like the "low power + zoom" combo (me too by the way  !) and want a wider FOV, what about improving this?

Here could be a long term solution:

• invest in a good 2" diagonal (this allows 2" filters)
• current 32mm plossl => 35mm TV Panoptic or 30mm APM UFF
• current Baader zoom => Leica ASPH zoom
• Baader VIP barlow to combine with the Leica zoom (you need an extra £30 Baader adapter to prevent the VIP from touching the mirror diagonal) 

You might need some altitude counterweight, but this can easily (and cheaply) be solved with a wrist counterweight attached to a bolt on the dovetail if needed. A combo like that can be reused in many telescopes. You get a larger field of view too and you might find them in the second hand market. It is not cheap, but a TV set is not cheap either.

 

For manual tracking at high power I would suggest to invest in a good Giro mount head (e.g. Giro Hercules or AOK Ayo2). Those are noticeably better than the AZ4.

Edited August 31, 2019 by Piero

[Timebandit]

 

6 hours ago, parallaxerr said:

I must get to some dark sites this winter to really test the scope and new EPs.

 

 

 

Hello. You can have the most fancy and highest cost eyepiece's out there. But for the faint DSO , to get the best out of your scope , you really really need to go to a dark site. The highest cost eyepiece is not going to alter the fact or cure the problems of seeing faint DSO in a light polluted area.  My 8" newt at a true dark site with a decent eyepiece in will best  my 14"  with a TV nagler in a light polluted area. So get to a dark site as you will really see the difference on the fainter DSO , compared to light polluted areas.

 

 

 

[John]

13 hours ago, parallaxerr said:

P.S. just checked out the Docters and I thought TV were expensive 😱

Thats an interesting point. There are a few eyepieces out there that offer slightly better performance than the Tele Vue equivalents in some aspects but they tend to cost more than the Tele Vues.

I spent quite a lot a while back on the Leica ASPH zoom and a Baader VIP barlow plus some extenders and other adapters to get a flexible setup. I thought that these investments would mean that I would soon be selling off my Tele Vue and Pentax eyepieces and contentedly living with a superlative zoom based set but, for me at least, the Leica ASPH zoom + VIP barlow combination, while very good indeed, was not really a revelation. While very sharp centre field, I found it's edge correction not quite up to Tele Vue standards with my F/5.3 dobsonian and that matters a lot to me. I also saw some ghosting and scatter from brighter targets that I felt should not be there in such a high end optical system.

After a few months of trying to get to love this expensive zoom, I got a bit frustrated with it and decided to sell it on. My Tele Vue and Pentax eyepieces are still with me.

Your mileage may vary though - I may be one of the only amateur astronomers who has not been bowled over by the Leica ASPH zoom

I do understand your situation regarding the scope. For ages I used a relatively modest couple of scopes but stuck with building a top quality set of Tele Vue and Pentax eyepieces to use with them. I could have bought a much larger aperture scope and used less expensive eyepieces with that but at that time I was constrained on scopes but not so much on eyepieces.