#1  
Old 07-10-2021, 11:47 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,298
Higher powered Bino EP's?

Hi all,


I've just jumped into the world of binoviewing with a Baader Mark V paired with TV 24mm panoptics for widest FoV, but now I'm trying to decide whether I should get a couple of higher powered EP's for viewing of planets.


One option would be to just use powermates to get me there, but given a similar final magnification, is there much difference between using a 4x powermate with 24mm panoptics, or simply using the Baader 1.7x glasspath corrector/paracorr with an 11mm Delite or Plossl?


I notice the field stop on the 4x PM is kinda small -maybe that's a factor?


And if a second pair of shorter focal length EP's is better, Would I be better to just buy a second XW10, or a new pair of Delite 11mm, or Plossl 11's?


Cheers


Markus
Reply With Quote
  #2  
Old 08-10-2021, 04:56 AM
astro744
Registered User

astro744 is offline
 
Join Date: Apr 2007
Posts: 1,074
I’m not familiar with the Baadar but with my Tele Vue Bino Vue, I follow their guidelines. see https://www.televue.com/engine/TV3b_...d=63&Tab=_ause

I use the 2” 4x Powermate fitted with the Tele Vue T- ring adaptor screwed to the bottom of the Bino Vue replacing the 1.25” 2x amplifier. I get an amplification factor of 4.3x this way.

There is no issue with the small diameter field lens of the 4x Powermate since the high magnification factor only requires a smaller field stop. In mono mode I have used a 35mm Panoptic with 4x Paracorr with superb results although it did cause me balance issues on my Dob at lower altitudes.

With a combination of a Powermates (and Tele Vue T-ring adaptors) I can get a range of powers using pairs 24 and 19mm Panoptics and 16mm T5 Naglers.
Reply With Quote
  #3  
Old 08-10-2021, 09:27 AM
Rainmaker's Avatar
Rainmaker (Matt)
Strictly Visual......

Rainmaker is offline
 
Join Date: Jun 2019
Location: Canberra
Posts: 422
I tried pairs of DeLites but found then too bulky with their eyecup set to correct height, I preferred the Nagler T6 pairs but in the end went with Tak LE pairs in 5, 7.5, 18 and 30mm focal lengths.

They’re nice and compact, very sharp and, as I only use them on a tracking mount, their 52 degree field is sufficient.

I sold the Pano 24s, preferring the LE30 …
Reply With Quote
  #4  
Old 08-10-2021, 12:08 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,298
Quote:
Originally Posted by Rainmaker View Post
I tried pairs of DeLites but found then too bulky with their eyecup set to correct height, I preferred the Nagler T6 pairs

I'd discounted the Naglers - I thought their field would be too wide for binoviewing. You didn't find that an issue?
Reply With Quote
  #5  
Old 08-10-2021, 12:28 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,298
"There is no issue with the small diameter field lens of the 4x Powermate since the high magnification factor only requires a smaller field stop."


<quote button not working for some reason>


I have to admit, I'm foggy on field stops and entrance pupils. It seems criminal to throw away a large part of the light cone. I'd always presumed it was because the optics design couldn't correct for aberrations any wider than that, but thinking about it, maybe it's simply that, being closer to the focal plane the outer part of the image is simply not required at higher focal lengths.


Makes me wonder if using the 2.5x PM will be a good idea, since it's only 1 1/4". The entrance pupil on that is a similar size to the 4x, which doesn't seem right. I wonder if I'm discarding precious photons?


Markus
Reply With Quote
  #6  
Old 10-10-2021, 09:55 AM
astro744
Registered User

astro744 is offline
 
Join Date: Apr 2007
Posts: 1,074
I also have the 5x Powermate and it has an even smaller field lens. You are not losing any light from the primary as all rays focus at the field stop. Not any different to a short focal length eyepiece with small field stop diameter. You’re still getting all the light that is available from the primary.

Think of the Powermate as part of the eyepiece. A 41mm Panoptic (68deg) has a 46mm field stop diameter. The 2” 4x Powermate makes it a 10.25mm (68 deg) Panoptic with an effective 46/4=11.5mm field stop diameter. The clear aperture of the 4x Powermate field lens is about 17mm (rough measurement) and this is well over 11.5mm needed for 10.25mm/68 deg eyepiece.

The 5x has an even smaller opening because it is higher amplification but too is only 1.25” so only needs to accommodate eyepieces with max. 27mm field stop diameters. 27/5=5.4mm. It’s clear aperture looks about 10mm (didn’t measure but well above 5.4mm).

You are not losing any light and the clear aperture of the Powermates is well above that needed for any eyepiece with maximum field stop diameter for a given barrel size. It is no different to using a shorter focal length eyepiece with smaller field stop diameter. Note the Powermates are all Parfocal with only the 2x Powermate requiring a slight focus shift. See https://www.televue.com/engine/TV3b_...id=53&Tab=_app
Reply With Quote
  #7  
Old 10-10-2021, 12:52 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,298
Thanks astro744 for attempting to explain it to me. It kind of makes sense, but I still struggle to understand some aspects of this whole field stop malarkey. For example;


Why does the 2.5x powermate have a 1 1/4" barrel, while the 4x is a 2", (even though the field stop on the 4x could easily be accomodated by a 1 1/4" barrel)?


Also, I understand that the field stop will likely get smaller as magnification increases, but it doesn't seem to be a linear relationship. As mentioned, the 2.5x and the 4x are about the same at 10mm or so. I also have an ES telecentric 3x that has a field stop even smaller than the TV 4x (it's around 5mm).


Clearly, there must be something I don't quite understand yet. But I guess life would be boring if you understood everything all at once.


Cheers,
Markus

Last edited by Stonius; 10-10-2021 at 03:38 PM.
Reply With Quote
  #8  
Old 10-10-2021, 01:43 PM
astro744
Registered User

astro744 is offline
 
Join Date: Apr 2007
Posts: 1,074
The decision to have some Powermates 1.25” and others 2” is a Tele Vue call. They could have easily all been 2” or all 1.25”. If the 5x Powermate was in a 2” barrel it would have had a larger clear aperture that it does now so it could accommodate the larger field stop diameter possible in a 2” eyepiece. Similarity a 4x Powermate in a 1.25” barrel would not need the clear aperture it currently has since it only needs to accommodate a maximum field stop in an eyepiece of 27mm (max for 1.25” barrel) and not 46mm (max for 2” barrel).

See https://www.televue.com/engine/TV3b_page.asp?id=214 for field stop diameters of Tele Vue eyepieces. The eyepieces with the same field stop diameter give the same true field of view on the same telescope. Magnification will vary with eyepiece focal length.

The field stop diameter is simply the linear true field at the focal plane. The wider it is the more sky you see (for a given telescope). To get the true field in degrees you multiply the field stop diameter by 57.3 and divide by the focal length of the telescope.
Reply With Quote
  #9  
Old 10-10-2021, 04:34 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,298
Quote:
Originally Posted by astro744 View Post
Similarity a 4x Powermate in a 1.25” barrel would not need the clear aperture it currently has since it only needs to accommodate a maximum field stop in an eyepiece of 27mm (max for 1.25” barrel) and not 46mm (max for 2” barrel).
DING! Lightglobe goes on - thanks! So I guess the size of the 2.5x powermate's field stop is the size it needs to be in relation to the maximum possible field stop of the eyepiece that can be inserted into it. Makes sense.

Quote:
Originally Posted by astro744 View Post
The field stop diameter is simply the linear true field at the focal plane. The wider it is the more sky you see (for a given telescope). To get the true field in degrees you multiply the field stop diameter by 57.3 and divide by the focal length of the telescope.
<Lightglobe goes *pop*>

Okay, so if I were to remove the field stop from an eyepiece or a barlow, would the result be the same? ie, that I see more sky? (Just a thought experiment, BTW - I'm not *actually considering doing this!).

In my head I'm thinking I *would see more sky, but that extra bit would be outside of the design spec and be uncorrected / severely aberrated. Maybe I'd also get lots of internal reflections/scattering where light now reflects off things it was never meant to touch (lens edges, internal threads, whatever)? So, in theory could you enlarge the TFoV of a given Plossl by simply removing the field stop, even though the new bits of the image would be 'bluergh'?

Apologies for all the questions, I had never thought too hard about field stops until I was looking at getting a binoviewer.

Markus
Reply With Quote
  #10  
Old 10-10-2021, 07:14 PM
astro744
Registered User

astro744 is offline
 
Join Date: Apr 2007
Posts: 1,074
Yes you could remove the field stop in your eyepiece but the results wouldn’t be pretty. Firstly the field stop if positioned correctly at the focal plane defines the sharp black circular edge the you see in the field. Secondly you would see significant edge of field aberrations that the designer of the eyepiece wanted to control with proper placement of the field stop and selection of glass types, number of lenses and curvature of all surfaces. The wider the apparent field required, the more complex the lens groups and larger some of the lenses need to be.

The field stop of simpler eyepieces is normally a black sharp edged ring that you can see if you turn your eyepiece over and look into the barrel. Often it unscrews but shouldn’t be tampered with or you lose the sharp edge as any new position may not be at the focal plane as intended. On complex eyepiece designs the field stop is internal and you cannot remove it without removing some lenses first. NOT recommended.

Note Powermates and Barlows are different in that the Powermate lets the eyepiece behave exactly as designed with or without the Powermate in place and the only difference being the amplification factor. With a Barlow things are more complex and many factors contribute to a successful combination.

I remember a very long time ago I used a Tele Vue 24mm Widefield (65 deg.) eyepiece in a 2.8x Klee Barlow and noticed some obvious vignetting. The same eyepiece in a 2x Clave Barlow showed no obvious vignetting. I always thought it was the small clear aperture of the Klee compared with the very large, almost full aperture of the Clave but there was one other difference and it was that the Klee was very short and the Clave very long and the eyepiece exit pupil behaviour and eye relief was considerably different between the two. A Barlow extends the eye relief whereas a Powermate does not. If the eye relief is extended too far then vignetting of the exit pupil is noticeable.

I got kind of off topic a bit. I don’t have the binoviewer you have but I thought I’d share how I achieve higher powers with a set of Powermates (and Tele Vue T-ring adaptors) and a set of three focal lengths. I chose the 24mm, 19mm Panoptics and 16mm Naglers because of their form factor (volcano top shape makes binoviewing easier at shorter eye relief). I also have a pair of Tele Vue 8-24mm click-stop zooms which also work well. These are not volcano tops but apparent field of view is less and the eye relief is ample to see the entire view when binoviewing. (Haven’t used this combination for a while so going off memory).
Reply With Quote
  #11  
Old 10-10-2021, 11:19 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,298
Quote:
Originally Posted by astro744 View Post
I got kind of off topic a bit.
Not at all, it's interesting - or at least it is to me. I have a feeling I used to know some of what you said, but as with many things these days, if I'm not actively using that knowledge, it disappears.

Quote:
Originally Posted by astro744 View Post
I chose the 24mm, 19mm Panoptics and 16mm Naglers because of their form factor (volcano top shape makes binoviewing easier at shorter eye relief).
Yes, I've got the Pano 24's as my standard BV EP's. I was also considering the 19mm Panos too. They look like they will interleave nicely with the 24's to get a range of focal lengths without doubling up too much.

Cheers,

Markus
Reply With Quote
  #12  
Old 11-10-2021, 04:52 AM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 482
Field diameters in eyepieces determine how much of the telescope's field you see. The image scale is on the scope's field and the eyepiece acts like a magnifier for the portion of the field of the scope seen.

Barlows expand the image scale of the primary so the eyepiece sees less field. That's why the field seen is smaller and at a higher magnification. The eyepiece is seeing a telescope with a longer focal length.

All eyepieces perform better when the rays entering the bottom are closer to parallel than converging. PowerMates and other telecentric multipliers make the converging rays enter the eyepiece closer to parallel and improve the performance of many eyepieces while providing an expansion of the image scale at the same time.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 12:20 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Limpet Controller
Advertisement
NexDome Observatories
Advertisement
Bintel
Advertisement
Astronomy and Electronics Centre
Advertisement