ICEINSPACE
Most Read Articles
Moon Phase
CURRENT MOON Waning Crescent
3%
The Sun Now
Time Zones
Sydney*
10:06 pm
Perth
7:06 pm
Auckland*
12:06 am
New York
6:06 am
Paris
12:06 pm
GMT
11:06 am




Go Back   IceInSpace > Equipment > Eyepieces, Barlows and Filters

Reply
 
Thread Tools Rate Thread
  #21  
Old 20-07-2017, 01:45 PM
electric
Registered User

electric is offline
 
Join Date: Jul 2017
Location: Sydney
Posts: 6
Of course the focal ratio counts for something. As you say, it affects the radius of curvature of the focal plane along with the focal length of the telescope. This is most significant with photographic applications. One example being the requirement of scope specific field flattner and focal reducers. If your scope is an ED80 f/7.5 refractor, you need flattner specific for that scope - you cannot use one made for a 100mm f/9 refractor. Astrophotographers are most aware of this.

As for catadioptric cassegrains, these produce a convex focal plane due to the, shall we say "hyperbolic" or negative secondary mirror. It is the shape of the secondary mirror that increases the native focal length of the primary to what the telescope is specified as being, and changes the shape of the focal plane. Despite the concave primary mirror.

This can be seen with the way the convex specific eyepieces also perform well in catadioptric cassegrains. Also those cheap and nasty 0.5X focal reducers, these work ok as general purpose focal reducers in refractors and catadioptric scopes, but not Newtonians. Even in slow Newtonians these reducers do not perform as well as with Cats.

Field curvature also be seen in convex specific eyepieces when used in fast refactors. These eyepieces are also cheaper. Even the older Masuyama eyepieces, SOME individual focal lengths can exhibit a little field curvature in very fast refractor. Remember, the original Masuyama eyepieces were designed when refractors were typcially slower than what is available today. Yet the degree of field curvature is nowhere as significant as in a Newtonian. Again, this is not a flaw in the eyepiece, but a factor of the design parameters of the time thirty years ago. Whether these new Masuyama eyepieces are designed for slower refractors as its predecessors or the more contemporary faster refractors, we have to see. I would suspect as yes to faster refractors due to the very large apparent field of view that they offer. But to dump all telescope designs as being the same with regards to shape of focal plane and only making the distinction as "faster" and "slower", that's disingenuous.

It comes to understanding the pedigree of the eyepieces you are using and coupling them to the most appropriate instrument. Unfortunately this is the hard part, and many reviews are written by people who do not understand this enough or have a vested interest.

Anyone have an f/5 APO to try out one of these? Oh, and most fast APOs come with built in field flattners, making things even more complicated, like the Skywatcher Esprit line.

Collin, did your refractor have a field flattner in it when you used the Masuyama?
Reply With Quote
  #22  
Old 20-07-2017, 01:53 PM
FlashDrive's Avatar
FlashDrive (Col)
Senior Citizen

FlashDrive is offline
 
Join Date: Apr 2008
Location: Queensland-The Promised Land
Posts: 3,185
Quote:
Originally Posted by electric View Post

Collin, did your refractor have a field flattner in it when you used the Masuyama?
@electric .... no, not at all .... stock standard TAK78 f/8

Col...
Reply With Quote
  #23  
Old 20-07-2017, 02:17 PM
casstony
Registered User

casstony is offline
 
Join Date: Feb 2006
Location: Warragul, Vic
Posts: 3,338
I expect edge performance of the Masuyama to be similar to that of other wide angle 5 element eyepieces (ie. challenged in many types of telescope). Televue and Explore Scientific don't use a bunch of lenses to correct aberrations just for the fun of it.

It might be good in the Celestron Edge scopes with their flat fields and slow f/ratio, as well as slow refractors.

Note to Electric: most fast APO's don't come with built in flatteners, they're usually an add on device for imaging (including the Esprits which are triplets with an accessory flattener). There have been a few designs released more recently which do include flatteners built into the design, in addition to the long standing FSQ and NP.
Reply With Quote
  #24  
Old 20-07-2017, 05:24 PM
Wavytone
Fringe Lunatic

Wavytone is offline
 
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 2,560
Quote:
Originally Posted by AstralTraveller View Post
So how do SCTs, Mak, and RCs fit into this picture? Are they curved similar to newts of fracs?
Tony is correct.

In short there is no such thing as the perfect eyepiece for all telescopes.

Unfortunately eyepiece manufacturers do not state what the field curvature is, which is unfortunate as a lot of observers waste money on eyepieces that aren't a particularly good match for their scope. Morton you're reading this far, right ?

The issue is that the curvature of the focal plane can be convex or concave towards the objective (and sky). For refractors, SCTs, maks and cassegrains it is concave towards the sky. For Newtonians however the focal plane is CONVEX.

The reason this matters is that the curvature inherent in the Masuyama (and for that matter a nice 38mm ProStar eyepiece I have) is that the curvature in the focal plane of the eyepiece is a fair match for that in small refractors and cassegrains.

Much like stacking two soup bowls on top of each other, a close fit.

But in a Newtonian the curvature is opposite to that of the eyepiece and the result is like stacking soup bowls back-to-back - they touch at one circle (in focus) but for the most part are far apart (out of focus). In a scope, you can't get the whole field to focus and have to rack the eyepiece in and out only to find there is no sweet spot where most of all of it is in focus.

While at f10 or even f13 it might not seem much of an issue, it is noticeable in sharp optics at f7. At f5 it is irritating to the extent you'll be looking for something that is a better match. And while field flatteners may help they don't really fix this especially for fast Newtonians.

Conversely the humble Plossl has field curvature that is a great match for Newtonians, but not refractors.

Televevues main market is the US where big dobsonians rule. So it is no surprise their eyepieces are a good match for these. But in Japan and Europe smaller refractors are the biggest seller and hence their eyepieces are intended primarily for these.

Last edited by Wavytone; 20-07-2017 at 09:07 PM.
Reply With Quote
  #25  
Old 27-07-2017, 04:49 PM
bytor666
Registered User

bytor666 is offline
 
Join Date: Dec 2004
Posts: 363
Apparently not good in fast scopes...
Reply With Quote
  #26  
Old 30-07-2017, 07:54 AM
Tropo-Bob (Bob)
Registered User

Tropo-Bob is offline
 
Join Date: Sep 2013
Location: Cairns
Posts: 451
Quote:
Originally Posted by Wavytone View Post
Tony is correct.

...

Unfortunately eyepiece manufacturers do not state what the field curvature is, which is unfortunate as a lot of observers waste money on eyepieces that aren't a particularly good match for their scope...

The issue is that the curvature of the focal plane can be convex or concave towards the objective (and sky). For refractors, SCTs, maks and cassegrains it is concave towards the sky. For Newtonians however the focal plane is CONVEX.

The reason this matters is that the curvature inherent in the Masuyama (and for that matter a nice 38mm ProStar eyepiece I have) is that the curvature in the focal plane of the eyepiece is a fair match for that in small refractors and cassegrains.

Much like stacking two soup bowls on top of each other, a close fit.

But in a Newtonian the curvature is opposite to that of the eyepiece and the result is like stacking soup bowls back-to-back - they touch at one circle (in focus) but for the most part are far apart (out of focus). In a scope, you can't get the whole field to focus and have to rack the eyepiece in and out only to find there is no sweet spot where most of all of it is in focus.

While at f10 or even f13 it might not seem much of an issue, it is noticeable in sharp optics at f7. At f5 it is irritating to the extent you'll be looking for something that is a better match. And while field flatteners may help they don't really fix this especially for fast Newtonians.

Conversely the humble Plossl has field curvature that is a great match for Newtonians, but not refractors.

Televevues main market is the US where big dobsonians rule. So it is no surprise their eyepieces are a good match for these. But in Japan and Europe smaller refractors are the biggest seller and hence their eyepieces are intended primarily for these.
Well, I am humbled. I have not heard this concept before, which is remarkable since I have been in the hobby for over 50 years. It shows that one continues to learn something every day!

I assume from the above quote that orthos are a better fit for refractors than reflectors? (It certainly seems valid from my experiences).
Reply With Quote
  #27  
Old 27-11-2017, 01:24 AM
Wilso (Darren)
Registered User

Wilso is offline
 
Join Date: Nov 2016
Location: Gawler
Posts: 65
If anyone's interested there is a review/article on CN about the 26mm masuyama eyepiece as well.
Cheers
Attached Thumbnails
Click for full-size image (image.jpg)
63.7 KB31 views
Reply With Quote
  #28  
Old 27-11-2017, 07:18 PM
LewisM's Avatar
LewisM
Xenomorph

LewisM is offline
 
Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 6,539
I LOVE my Masu 25mm (the original short version).

I have owned and tried a LOT of TV's, and have yet to find one I like. Just not MY flavour.
Reply With Quote
  #29  
Old 05-12-2017, 09:06 AM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 339
Reviews of 3 different Masuyamas:
https://www.cloudynights.com/article...26mm-85°-r3122
Reply With Quote
  #30  
Old 05-12-2017, 09:08 AM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 339
Quote:
Originally Posted by ausastronomer View Post
Hi Lewis,

The same eyepieces were sold as:-

Parks Gold Series
Antares Elite
Celestron Ultima
Orion Ultrascopic

They were essentially identical except for some slightly different coating specs. They all seemed to perform the same to me in the field. Optically they are excellent performers, outside of the 50~ degree fov and tighter eye relief in the shorter focal lengths. I like them a lot, really nice sharp contrasty images.

Meade also did a 5 element Series 4000 super plossl for a few years which was very similar and performed at the same level.

The Tak LE's use a similar optical design with a different housing. I am fairly sure they have fractionally longer eye relief (by about 1mm) for a given focal length than the others.

Cheers
John B
Righto.
Also:
Tuthill Plössls
Omcon Ultima
Astroplan "Super Plössls"
All were made in Japan.
The Meade 5-element Series 4000 "Super Pössl" was made by Kowa in Japan.
Of them all, the only one still in production is the Takahashi LE.
Reply With Quote
  #31  
Old 05-12-2017, 10:10 AM
Tropo-Bob (Bob)
Registered User

Tropo-Bob is offline
 
Join Date: Sep 2013
Location: Cairns
Posts: 451
Quote:
Originally Posted by Don Pensack View Post
Righto.
Also:
Tuthill Plössls
Omcon Ultima
Astroplan "Super Plössls"
All were made in Japan.
The Meade 5-element Series 4000 "Super Pössl" was made by Kowa in Japan.
Of them all, the only one still in production is the Takahashi LE.
I have some LEs, but much prefer the Tak Orthos. Then again, I mainly use refractors so maybe its horses for courses ...
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 09:06 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Advertisement
OzScopes Authorised Dealer
Advertisement
FLI Cameras and Imaging Accessories
Advertisement
NexDome Observatories
Advertisement
Lunatico Astronomical
Advertisement
Meade Instruments
Advertisement
Tasco Australia
Advertisement
Bintel
Advertisement
Atik 16200
Advertisement
SkyWatcher Star Discovery
Advertisement
Astronomy and Electronics Centre
Advertisement