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Equipment Discussions >> Eyepieces

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XTlegend
member


Reged: 01/20/07
Posts: 51
Loc: U.K 51 25 40N , 0 34 16W
No field stop?
      #3426925 - 11/03/09 07:23 AM

Sorry if this has been mentioned before but in the Meade (revelation) eyepiece kit I just bought the 15mm eyepiece does not have a defined field stop. I noticed this on a Celestron kit I bought a couple of years ago too - is this right? Surely they arnt designed this way and something is left out at the factory? To me this particular eyepiece is useless.

--------------------
Celestron ED80 AS-GT.
Celestron CR4 Refractor.
Skywatcher 114mm f5 reflector.
Celestron Nexstar 60 SLT.
DSI Colour
Various modified webcams.


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Luigi
Post Laureate
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Reged: 07/03/07
Posts: 4940
Loc: MA
Re: No field stop? new [Re: XTlegend]
      #3426935 - 11/03/09 07:31 AM

Sounds strange, as the field stop is usually integrated into one of the necessary spacers or retaining rings, but I suppose it's possible.

I don't know if this pertains in your case, but the field stop will appear sharp only if you have good distance vision. If you are myopic and view without glasses, it will be fuzzy.

--------------------
17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins


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XTlegend
member


Reged: 01/20/07
Posts: 51
Loc: U.K 51 25 40N , 0 34 16W
Re: No field stop? new [Re: Luigi]
      #3426956 - 11/03/09 07:46 AM

Well, in all the other eyepieces (9,12,20,32mm) the field stop is crisp and clear. The dealer I bought the kit off says that no one else has complained in the 2 years they have been selling them. Very odd if you ask me!

Quote:

Sounds strange, as the field stop is usually integrated into one of the necessary spacers or retaining rings, but I suppose it's possible.

I don't know if this pertains in your case, but the field stop will appear sharp only if you have good distance vision. If you are myopic and view without glasses, it will be fuzzy.




--------------------
Celestron ED80 AS-GT.
Celestron CR4 Refractor.
Skywatcher 114mm f5 reflector.
Celestron Nexstar 60 SLT.
DSI Colour
Various modified webcams.


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XTlegend
member


Reged: 01/20/07
Posts: 51
Loc: U.K 51 25 40N , 0 34 16W
Re: No field stop? new [Re: XTlegend]
      #3426959 - 11/03/09 07:49 AM

Looking into the bottom of the eyepiece concerned it looks like there is one but maybe its too big? That is, the wrong one fitted?

--------------------
Celestron ED80 AS-GT.
Celestron CR4 Refractor.
Skywatcher 114mm f5 reflector.
Celestron Nexstar 60 SLT.
DSI Colour
Various modified webcams.

Edited by XTlegend (11/03/09 07:51 AM)


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EdZ
Professor EdZ
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Reged: 02/15/02
Posts: 14731
Loc: Cumberland, R I , USA42N71.4W
Re: No field stop? new [Re: XTlegend]
      #3427057 - 11/03/09 09:40 AM

If a 1.25" eyepiece of 15mm focal length had no field stop, the Afov would then be determined by the inside of the barrel and it would end up being Afov>100°. That's proably not the case.

There probably is a field stop. it could be the retaining ring that holds the field lens in place, but it may not be positioned exactly at the focal plane, and therefore appears out of focus, or fuzzy.

edz

--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21


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deSitter
Carpal Tunnel


Reged: 12/09/04
Posts: 2926
Re: No field stop? new [Re: XTlegend]
      #3427066 - 11/03/09 09:42 AM

Well I've never seen any eyepiece of any make that omitted the field stop. Can you take a picture through the eyepiece (hold it next to your camera lens) and also down the barrel? Thanks.

-drl


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Mike Hosea
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Reged: 09/24/03
Posts: 3945
Loc: "Metrowest" Boston
Re: No field stop? new [Re: deSitter]
      #3427120 - 11/03/09 10:21 AM

I have. Had a couple of Rinis and an early-model 42mm GSO Superview without field stops. The 1.25" 32mm UO Konig II has a field stop, supposedly, but it's about 32mm in diameter and completely hidden behind the 1.25" barrel attachment. IIRC, its placement was such that even the fastest scopes would not be able to throw any light on it. The GSO was particularly disappointing because its lenses were undersized, so the true field was not as large as I expected, even though the AFoV appeared quite generous when it was not plugged into a scope (just due to ambient light tracking through the optics at all possible angles, some of which were not also supplied by a telescope).

--------------------
Mike

Stuff that I use:
  • 7" f/6.7 home-built Newt, eq platform, Pentax 40XW and 5XO, Tele Vue 13E and 2x Barlow, ZAO-II 6mm
  • 120mm f/8.3 home-built grab-n-go Newt with 7-21mm Nikon Zoom
  • Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars



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starrancher
professor emeritus


Reged: 06/09/09
Posts: 613
Loc: Northern Arizona
Re: No field stop? new [Re: XTlegend]
      #3427220 - 11/03/09 11:19 AM

Quote:

Sorry if this has been mentioned before but in the Meade (revelation) eyepiece kit I just bought the 15mm eyepiece does not have a defined field stop. I noticed this on a Celestron kit I bought a couple of years ago too - is this right? Surely they arnt designed this way and something is left out at the factory? To me this particular eyepiece is useless.




What is the (revelation) kit ?

--------------------
LXD75 AR5
LXD75 SN8
Series 4000 Plossls
Misc. other stuff


Fort Rock , Az .


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John Huntley
sage


Reged: 07/16/06
Posts: 249
Loc: South West England
Re: No field stop? new [Re: starrancher]
      #3427994 - 11/03/09 06:25 PM

Quote:

Quote:

Sorry if this has been mentioned before but in the Meade (revelation) eyepiece kit I just bought the 15mm eyepiece does not have a defined field stop. I noticed this on a Celestron kit I bought a couple of years ago too - is this right? Surely they arnt designed this way and something is left out at the factory? To me this particular eyepiece is useless.




What is the (revelation) kit ?




It's a set of re-branded GSO plossls, filter, case etc. GSO kit is often sold under the Revelation name over here.

John
SW England

--------------------
Meade Lightbridge 305mm F/5 Truss Tube Dobsonian
Meade / Bresser 127mm F/9.4 Achro Refractor
Vixen ED102SS 102mm F/6.5 ED Refractor
Celestron CG5 Mount with dual axis drive system
Naglers and an Ethos



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GlennLeDrew
Pooh-Bah
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Reged: 06/18/08
Posts: 1276
Loc: Ottawa, Ontario, Canada
Re: No field stop? new [Re: John Huntley]
      #3428006 - 11/03/09 06:32 PM

Ed,
If you remove an eyepiece's field stop, its AFoV cannot increase to arbitrarily large size. Most eyepiece designs have field stops not very much smaller than the inherent field of illumination as seen from the eye point. For example, a Plossl having 50 degrees AFoV with its field stop removed may have a slightly larger, though necessarily fuzzy-edged AFoV of perhaps 55 or *maybe* 60 degrees. And that's even if any possibly limiting barrel or other aperture/stop is not present.

--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces

My Gallery

Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.


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deSitter
Carpal Tunnel


Reged: 12/09/04
Posts: 2926
Re: No field stop? new [Re: John Huntley]
      #3428541 - 11/04/09 12:46 AM

Quote:

Quote:

Quote:

Sorry if this has been mentioned before but in the Meade (revelation) eyepiece kit I just bought the 15mm eyepiece does not have a defined field stop. I noticed this on a Celestron kit I bought a couple of years ago too - is this right? Surely they arnt designed this way and something is left out at the factory? To me this particular eyepiece is useless.




What is the (revelation) kit ?




It's a set of re-branded GSO plossls, filter, case etc. GSO kit is often sold under the Revelation name over here.

John
SW England




Whoa, the 15mm Series 4000 Plossl (same as GSO Plossls) is the total star of a good lineup - one of my favorite eyepieces. You must have a defective one. The field stop on mine is razor sharp with no colored fringe, like a good Ortho.

-drl


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XTlegend
member


Reged: 01/20/07
Posts: 51
Loc: U.K 51 25 40N , 0 34 16W
Re: No field stop? new [Re: Mike Hosea]
      #3428676 - 11/04/09 05:01 AM Attachment (13 downloads)

OK just took these 2 photos looking into the eyepieces.

The camera was focused at infinity. As you can see the 20mm has a crisp and clear field stop whereas the other?

Is it expecting too much for a clear field stop??

--------------------
Celestron ED80 AS-GT.
Celestron CR4 Refractor.
Skywatcher 114mm f5 reflector.
Celestron Nexstar 60 SLT.
DSI Colour
Various modified webcams.


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XTlegend
member


Reged: 01/20/07
Posts: 51
Loc: U.K 51 25 40N , 0 34 16W
Re: No field stop? new [Re: XTlegend]
      #3428677 - 11/04/09 05:02 AM Attachment (12 downloads)

And here's the 15mm photo:

--------------------
Celestron ED80 AS-GT.
Celestron CR4 Refractor.
Skywatcher 114mm f5 reflector.
Celestron Nexstar 60 SLT.
DSI Colour
Various modified webcams.


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Lawrence Sayre
Abbe Normal
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Reged: 10/16/04
Posts: 4747
Loc: N.E. Ohio
Re: No field stop? new [Re: XTlegend]
      #3428707 - 11/04/09 05:52 AM

The problem here appears to be that the field stop is not properly positioned at the focal plane of the eyepiece.

--------------------
My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')



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deSitter
Carpal Tunnel


Reged: 12/09/04
Posts: 2926
Re: No field stop? new [Re: Lawrence Sayre]
      #3428761 - 11/04/09 07:45 AM

Lawrence is probably right, or it could just be missing! Remove the chrome barrel and take a picture of the innards (at the optics, not through them).

-drl


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XTlegend
member


Reged: 01/20/07
Posts: 51
Loc: U.K 51 25 40N , 0 34 16W
Re: No field stop? new [Re: deSitter]
      #3428951 - 11/04/09 10:39 AM

I took the eyepiece apart and yes the field stop is at least 10mm to close to the rear lens!! And its not a missing spacer either as the threaded bore isn't long enough to unscrew the retaining ring far enough out. I can only think that its the wrong retaining ring/cylinder.

--------------------
Celestron ED80 AS-GT.
Celestron CR4 Refractor.
Skywatcher 114mm f5 reflector.
Celestron Nexstar 60 SLT.
DSI Colour
Various modified webcams.


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EdZ
Professor EdZ
*****

Reged: 02/15/02
Posts: 14731
Loc: Cumberland, R I , USA42N71.4W
Re: No field stop? new [Re: XTlegend]
      #3429038 - 11/04/09 11:30 AM

Is the field stop in the chrome sleeve? many of them are. if so, is the chrome sleeve screwed onto the eyepiece barrel up-side-down?

edz

--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21


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deSitter
Carpal Tunnel


Reged: 12/09/04
Posts: 2926
Re: No field stop? new [Re: XTlegend]
      #3429424 - 11/04/09 03:31 PM

Quote:

I took the eyepiece apart and yes the field stop is at least 10mm to close to the rear lens!! And its not a missing spacer either as the threaded bore isn't long enough to unscrew the retaining ring far enough out. I can only think that its the wrong retaining ring/cylinder.




Back it goes, that's just wrong for your dealer to sell that.

-drl


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Luigi
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Reged: 07/03/07
Posts: 4940
Loc: MA
Re: No field stop? new [Re: deSitter]
      #3430511 - 11/05/09 07:42 AM

Are you checking the field stop with the EP in the scope or looking through the EP alone? If you haven't already, I suggest trying it in a scope to see what it look like there.

--------------------
17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins


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TonyTowe
member


Reged: 07/21/09
Posts: 36
Loc: North Georgia, USA
Re: No field stop? new [Re: XTlegend]
      #3430537 - 11/05/09 08:03 AM

Not trying to hi-jack this thread, but since XTlegend posted a photo of the vignetting he is seeing with his 15mm plossl, I thought I would interject this here. In a recent thread "TV plossls vs the rest", I posted a comment that all of my TV plossls appear to vignette in my f/13 Mak. I have a 15mm, 13mm, and 11mm, and recently sold a 20mm. All of them vignette. Mike Hosea offered this explanation:

(Quote)

I think the following contribute:

1. The lenses on the TV Plossls are slightly under-sized. This is not intended to be a cost-saving thing. Each lens has a double convex lens cemented to a double-concave lens. The double convex lens only has a thin edge as it is, so it can't be made wider without also being made thicker, which would change the design (to one with less eye relief).

2. Given #1, a slow f-ratio makes it more likely that the light for certain point near the edge is lost entirely rather in part. You might not notice it if you only lost half the light for a given point, but with the slow f-ratio it's closer to all-or-nothing for each given point.

3. Your scope has a negatively powered secondary mirror. This probably introduces a little divergence into the light cone, or at least there is less convergence than for an equivalent focal-length refractor. The divergence introduced by the scope changes the angle of the edge-of-field light cones, making it more likely that they will run afoul of the edges of the under-sized lenses.

(Un-quote)


I've been at a quandry as to what to do. The TV plossls are extremely sharp, and offer exceptional glare/anti-reflection control. I have some older Meades (4 and 5 element designs), and I think they are just as sharp as the TVs, but I have problems with these with both internal reflections and with light reflecting off of my eyes when viewing bright objects (Jupiter, the moon, Vega...). Of course the TVs are preferred for planets, but the vignetting is aesthetically displeasing when viewing the moon at high power, or when viewing DSOs or starfields. I actually prefer the Meades for all other viewing besides planets for the consistent sky background with a sharply defined field stop. I have some duplication between my TV set and my Meade set. So my question is, do I keep the TVs to use for planetary work and use the Meades for all other viewing, or do I find a good compromise with something else? I've been considering UO HD's, thinking I may get good glare control with the HD coatings, and possibly have a sharply defined field stop as well. I know the AFOV will be more restricted, but I am actually probably only getting 40-45 degrees with my TVs anyway because of the vignetting. Any suggestions? Some might say get another scope, but to be honest with you I love it. It is only 4" in aperture, but I have put in more hours with it in the first six months that I have had it than in the entire 8 years that I owned my "white elephant", a 10 inch Meade Starfinder dob. The best scope is the one that you will use most!!!

--------------------
Tony T.


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