Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Explore Scientific 2" 2x Focal Extender causes Vignetting?

  • Please log in to reply
20 replies to this topic

#1 EuropaWill

EuropaWill

    Apollo

  • *****
  • topic starter
  • Posts: 1104
  • Joined: 12 Feb 2009
  • Loc: Lower Hudson Valley NY

Posted 29 November 2014 - 04:50 PM

I'm considering adding the ES 2" Focal Extender but read in the specs that it has only a 27mm clear aperture.  :confused:

 

Won't this cause significant vignetting with widefield 2" ep's such as ES68 40mm, ES 68 34mm, ES 68 28mm, ES82 24mm, etc...?

 

 

 

 



#2 faackanders2

faackanders2

    Fly Me to the Moon

  • *****
  • Posts: 5274
  • Joined: 28 Mar 2011

Posted 29 November 2014 - 06:01 PM

I'm considering adding the ES 2" Focal Extender but read in the specs that it has only a 27mm clear aperture.  :confused:

 

Won't this cause significant vignetting with widefield 2" ep's such as ES68 40mm, ES 68 34mm, ES 68 28mm, ES82 24mm, etc...?

Are you sure that wasn't the 1.25" version.  2" versions should be around 40mm.

 

Telecentric (Powermate equivalents) are supposed to vignette less than reqular barlows.



#3 faackanders2

faackanders2

    Fly Me to the Moon

  • *****
  • Posts: 5274
  • Joined: 28 Mar 2011

Posted 29 November 2014 - 06:25 PM

The ES website obviously incorrect since it lists the 2" version as having a 1.25" OD barrel.  The website creater obviously copied some of the 1.25" into the 2".

2" 2x will work (and work better than a normal 2" 2x barlow since it is telecentric).



#4 EuropaWill

EuropaWill

    Apollo

  • *****
  • topic starter
  • Posts: 1104
  • Joined: 12 Feb 2009
  • Loc: Lower Hudson Valley NY

Posted 29 November 2014 - 09:57 PM

I'm not so sure this is a website mess up. Everything I read suggests this has a 27mm clear aperture. The image below seems to substantiate this. Does anyone have one of these and can confirm? 

 

Also, can someone confirm that their widefield 2" EP's are not vignetting magically despite the small clear aperture?

 

 

Es Fe 2s 3 1


#5 star drop

star drop

    contra contrail

  • *****
  • Moderators
  • Posts: 100229
  • Joined: 02 Feb 2008
  • Loc: Snow Plop, NY

Posted 29 November 2014 - 10:23 PM

I'm considering adding the ES 2" Focal Extender but read in the specs that it has only a 27mm clear aperture.  :confused:

 

Won't this cause significant vignetting with widefield 2" ep's such as ES68 40mm, ES 68 34mm, ES 68 28mm, ES82 24mm, etc...?

Try this thread. Pay particular attention to Starman1's replies. :poke: I just noticed that you posted twice in that same thread. :winky:



#6 CeleNoptic

CeleNoptic

    Viking 1

  • *****
  • Posts: 953
  • Joined: 20 Dec 2007
  • Loc: Cloudy East Coast

Posted 29 November 2014 - 10:27 PM

I have the 1.25" 2x Meade 5000 TeleXtender that I believe is a "parent" of the 1.25" ES FC. I took a caliper and found the actual aperture is around ~19mm, so the 27mm can be realistic for the 2".

 

*P.S. No vignetting for my 20mm ES68 or closest to it 14mm Meade 5K UWA. No vignetting for the 32mm TV Plossl either.


Edited by CeleNoptic, 29 November 2014 - 10:31 PM.

  • EuropaWill likes this

#7 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 37104
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 30 November 2014 - 08:55 PM

The clear aperture of a telecentric barlow is not exactly easy to measure.

The lens the light rays first contact is a negative lens and widens the light cone, diverging it to a larger diameter.

This larger light cone hits a positive lens that once again brings the rays in, making them almost parallel where they hit the 

bottom of the eyepiece.

This is why:

--the magnification does not change with distance from the eyepiece

--the diameter of the bottom lens can be smaller than the actual light entering the eyepiece

--4 lenses are used (2 achromats--one negative for magnification, one positive)

If the compression of the diverged light is just perfect, the eyepiece will also see no induced astigmatism or aberrations

and the only thing noticeable will be an increase in magnification.

 

For a better understanding, see:

http://www.brayebroo.../BarlowLens.pdf

and look at the drawing for a "telecentric amplifier".

As you can see, the clear aperture of the negative lens isn't the important figure.

The clear aperture of the top lens is more important, but, even then, vignetting my result in the eyepiece, not the amplifier.

 

The ES 2" 2X focal extender is not, however, parfocal with the eyepiece, so additional in-travel of the focuser is needed when it is used

I measure ~26mm opening on the bottom lens.

 

For what it's worth, the TeleVue 2" 2X PowerMate, which vignettes not at all with large 2" eyepieces, has a clear aperture

of the negative lens of ~35mm.  It is obviously a different design than the ES.

 

It is probably not a great idea to use either the ES or TeleVue with ones very lowest power eyepiece.  I'd select that for the largest field

rather than its effectiveness barlowed.


  • EuropaWill, Vondragonnoggin and bsavoie like this

#8 faackanders2

faackanders2

    Fly Me to the Moon

  • *****
  • Posts: 5274
  • Joined: 28 Mar 2011

Posted 03 December 2014 - 09:31 PM

The clear aperture of a telecentric barlow is not exactly easy to measure.

The lens the light rays first contact is a negative lens and widens the light cone, diverging it to a larger diameter.

This larger light cone hits a positive lens that once again brings the rays in, making them almost parallel where they hit the 

bottom of the eyepiece.

This is why:

--the magnification does not change with distance from the eyepiece

--the diameter of the bottom lens can be smaller than the actual light entering the eyepiece

--4 lenses are used (2 achromats--one negative for magnification, one positive)

If the compression of the diverged light is just perfect, the eyepiece will also see no induced astigmatism or aberrations

and the only thing noticeable will be an increase in magnification.

 

For a better understanding, see:

http://www.brayebroo.../BarlowLens.pdf

and look at the drawing for a "telecentric amplifier".

As you can see, the clear aperture of the negative lens isn't the important figure.

The clear aperture of the top lens is more important, but, even then, vignetting my result in the eyepiece, not the amplifier.

 

The ES 2" 2X focal extender is not, however, parfocal with the eyepiece, so additional in-travel of the focuser is needed when it is used

I measure ~26mm opening on the bottom lens.

 

For what it's worth, the TeleVue 2" 2X PowerMate, which vignettes not at all with large 2" eyepieces, has a clear aperture

of the negative lens of ~35mm.  It is obviously a different design than the ES.

 

It is probably not a great idea to use either the ES or TeleVue with ones very lowest power eyepiece.  I'd select that for the largest field

rather than its effectiveness barlowed.

Except for ES most 2" eyepieces are low power?

 

If TV 2x 2" Powermate has 35mm clear aperture, and ES 2x 2" has 26mm clear aperture; wouldn't that make the TV 2" superior?



#9 Scanning4Comets

Scanning4Comets

    Markus, a.k.a. Scanning4Comets

  • *****
  • Posts: 17938
  • Joined: 26 Dec 2004

Posted 03 December 2014 - 09:44 PM

I've tried the 2" Meade Tele-Xtender which is the same as the ES focal extender using a 28mm ES 68 and here is what I saw in my 10" F/4.7 reflector:

 

It totally cleaned up the edges in the 28mm ES 68 when used with the Focal X-tender, whereas using the 28mm ES 68 alone shows quite a bit of coma which is inherent from the short FL f/4.7 mirror.

 

As for any vignetting using this configuration~ There was NONE. The field stop was still razor sharp and there was no vignetting AT ALL.

 

:penny: :penny:

 

Cheerz! :grin:


  • EuropaWill likes this

#10 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 37104
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 04 December 2014 - 02:07 AM

 

The clear aperture of a telecentric barlow is not exactly easy to measure.

The lens the light rays first contact is a negative lens and widens the light cone, diverging it to a larger diameter.

This larger light cone hits a positive lens that once again brings the rays in, making them almost parallel where they hit the 

bottom of the eyepiece.

This is why:

--the magnification does not change with distance from the eyepiece

--the diameter of the bottom lens can be smaller than the actual light entering the eyepiece

--4 lenses are used (2 achromats--one negative for magnification, one positive)

If the compression of the diverged light is just perfect, the eyepiece will also see no induced astigmatism or aberrations

and the only thing noticeable will be an increase in magnification.

 

For a better understanding, see:

http://www.brayebroo.../BarlowLens.pdf

and look at the drawing for a "telecentric amplifier".

As you can see, the clear aperture of the negative lens isn't the important figure.

The clear aperture of the top lens is more important, but, even then, vignetting my result in the eyepiece, not the amplifier.

 

The ES 2" 2X focal extender is not, however, parfocal with the eyepiece, so additional in-travel of the focuser is needed when it is used

I measure ~26mm opening on the bottom lens.

 

For what it's worth, the TeleVue 2" 2X PowerMate, which vignettes not at all with large 2" eyepieces, has a clear aperture

of the negative lens of ~35mm.  It is obviously a different design than the ES.

 

It is probably not a great idea to use either the ES or TeleVue with ones very lowest power eyepiece.  I'd select that for the largest field

rather than its effectiveness barlowed.

Except for ES most 2" eyepieces are low power?

 

If TV 2x 2" Powermate has 35mm clear aperture, and ES 2x 2" has 26mm clear aperture; wouldn't that make the TV 2" superior?

 

In general, yes, 2" eyepieces are for wide fields at low power.

The aperture of the bottom lens likely indicates there is a different magnification to ES's lower lens than the bottom lens in the PowerMate.

That, in and of itself, does not imply superiority one way or the other.  



#11 EuropaWill

EuropaWill

    Apollo

  • *****
  • topic starter
  • Posts: 1104
  • Joined: 12 Feb 2009
  • Loc: Lower Hudson Valley NY

Posted 06 December 2014 - 09:40 PM

I've tried the 2" Meade Tele-Xtender which is the same as the ES focal extender using a 28mm ES 68 and here is what I saw in my 10" F/4.7 reflector:

 

It totally cleaned up the edges in the 28mm ES 68 when used with the Focal X-tender, whereas using the 28mm ES 68 alone shows quite a bit of coma which is inherent from the short FL f/4.7 mirror.

 

As for any vignetting using this configuration~ There was NONE. The field stop was still razor sharp and there was no vignetting AT ALL.

 

:penny: :penny:

 

Cheerz! :grin:

This is great input, thank you. I've decided to acquire the same 2" Meade Tele-Xtender so I'm looking forward to trying it with my EP's. I also agree with Don's advice that its probably not recommended to use a largest TFOV EP with this unit or others like it and considering my largest TFOV 2" EP is a Meade 5000 SWA 40, I wouldn't be likely to try that combo as I already have an ES68 20mm.  I will however try it with the Meade 5000 UWA 24 and the ES68 28 and QX36. I'm hoping for a major upgrade compared with my GSO 2" ED barlow which seems to magnify the total system field curvature seen via some of my EP's such as the QX36.


Edited by EuropaWill, 06 December 2014 - 09:49 PM.


#12 Scanning4Comets

Scanning4Comets

    Markus, a.k.a. Scanning4Comets

  • *****
  • Posts: 17938
  • Joined: 26 Dec 2004

Posted 07 December 2014 - 12:46 PM

 

I've tried the 2" Meade Tele-Xtender which is the same as the ES focal extender using a 28mm ES 68 and here is what I saw in my 10" F/4.7 reflector:

 

It totally cleaned up the edges in the 28mm ES 68 when used with the Focal X-tender, whereas using the 28mm ES 68 alone shows quite a bit of coma which is inherent from the short FL f/4.7 mirror.

 

As for any vignetting using this configuration~ There was NONE. The field stop was still razor sharp and there was no vignetting AT ALL.

 

:penny: :penny:

 

Cheerz! :grin:

This is great input, thank you. I've decided to acquire the same 2" Meade Tele-Xtender so I'm looking forward to trying it with my EP's. I also agree with Don's advice that its probably not recommended to use a largest TFOV EP with this unit or others like it and considering my largest TFOV 2" EP is a Meade 5000 SWA 40, I wouldn't be likely to try that combo as I already have an ES68 20mm.  I will however try it with the Meade 5000 UWA 24 and the ES68 28 and QX36. I'm hoping for a major upgrade compared with my GSO 2" ED barlow which seems to magnify the total system field curvature seen via some of my EP's such as the QX36.

 

 

You'll love that Tele-Xtender!!! IMO, it is 200% better than a regular GSO barlow! As I said, when I used my 28mm Es 68 in the 2" version, it completely cleaned the edges right up so that the entire field was flat looking. When I used the 28mm Es 68 alone in my 10" dob, lots of coma was seen from the mirror. The Tele-Xtender cleaned the edges up A LOT!


  • Ratimus likes this

#13 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 37104
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 07 December 2014 - 02:27 PM

You'll love that Tele-Xtender!!! IMO, it is 200% better than a regular GSO barlow! As I said, when I used my 28mm Es 68 in the 2" version, it completely cleaned the edges right up so that the entire field was flat looking. When I used the 28mm Es 68 alone in my 10" dob, lots of coma was seen from the mirror. The Tele-Xtender cleaned the edges up A LOT!

Confusing three different things: coma, astigmatism, and field flatness.

The Focal Extender will:

--reduce astigmatism at the edge of the field.  Because the light cone hitting the eyepiece is effectively longer and rays are more parallel. This helps the ES 68 a lot. Stars get much better focused.

--flatten the field.  This because of the effective doubling of the focal length of the scope.  This also reduces the size of the star images at the edge by better focusing.

The Focal Extender will NOT:

--reduce coma.  Coma is from the mirror, and doubling the image scale just makes the apparent size of the coma at the edge of the field twice as large.  Since the linear size of coma is halved by the doubling of magnification and halving of the field of view, coma appears exactly the same at the edge as the eyepiece by itself.

If the edges cleaned up a lot, it just shows you how small an aberration coma really is.  It's why many people don't get coma correctors at f/5.

 

One suggestion to reduce edge of field visibility of aberrations when using the 28 by itself: focus 1/2 way from center to edge rather than focusing on the center.  You'll find this will clean up the edge some and your eye's natural accommodation will focus the entire field more sharply.


  • bsavoie likes this

#14 Scanning4Comets

Scanning4Comets

    Markus, a.k.a. Scanning4Comets

  • *****
  • Posts: 17938
  • Joined: 26 Dec 2004

Posted 07 December 2014 - 03:36 PM

No confusion here Don.

 

I know the difference between coma, astigmatism, as I have explained lots before. Whatever happened when using the Meade FE wit the 28mm ES 68 made all stars appear to look round across the entire field. Doesn't matter to me what the "tech" terms are, it did clean up the field entirely.

 

This is all that really matters. :grin:



#15 star drop

star drop

    contra contrail

  • *****
  • Moderators
  • Posts: 100229
  • Joined: 02 Feb 2008
  • Loc: Snow Plop, NY

Posted 11 December 2014 - 12:42 AM

I'm considering adding the ES 2" Focal Extender but read in the specs that it has only a 27mm clear aperture.  :confused:

 

Won't this cause significant vignetting with widefield 2" ep's such as ES68 40mm, ES 68 34mm, ES 68 28mm, ES82 24mm, etc...?

One is coming my way, I couldn't resist the 25% off sale.

My primary use will be with short focal length eyepieces but I may experiment with a 41mm Panoptic and a 30mm Meade UWA just for the heck of it.


  • EuropaWill likes this

#16 SteveG

SteveG

    Fly Me to the Moon

  • *****
  • Posts: 6777
  • Joined: 27 Sep 2006
  • Loc: Seattle, WA

Posted 11 December 2014 - 02:28 PM

The clear aperture of a telecentric barlow is not exactly easy to measure.

The lens the light rays first contact is a negative lens and widens the light cone, diverging it to a larger diameter.

This larger light cone hits a positive lens that once again brings the rays in, making them almost parallel where they hit the 

bottom of the eyepiece.

This is why:

--the magnification does not change with distance from the eyepiece

--the diameter of the bottom lens can be smaller than the actual light entering the eyepiece

--4 lenses are used (2 achromats--one negative for magnification, one positive)

If the compression of the diverged light is just perfect, the eyepiece will also see no induced astigmatism or aberrations

and the only thing noticeable will be an increase in magnification.

 

For a better understanding, see:

http://www.brayebroo.../BarlowLens.pdf

and look at the drawing for a "telecentric amplifier".

As you can see, the clear aperture of the negative lens isn't the important figure.

The clear aperture of the top lens is more important, but, even then, vignetting my result in the eyepiece, not the amplifier.

 

The ES 2" 2X focal extender is not, however, parfocal with the eyepiece, so additional in-travel of the focuser is needed when it is used

I measure ~26mm opening on the bottom lens.

 

For what it's worth, the TeleVue 2" 2X PowerMate, which vignettes not at all with large 2" eyepieces, has a clear aperture

of the negative lens of ~35mm.  It is obviously a different design than the ES.

 

It is probably not a great idea to use either the ES or TeleVue with ones very lowest power eyepiece.  I'd select that for the largest field

rather than its effectiveness barlowed.

This is what I determined while testing 3x barlows & telecentrics. My 1.25" telecentric has a very small field lens (12 mm), but there is no vignetting when used with a widefield, 24 mm eyepiece. I have a Meade shorty barlow with a 17mm field lens, that will vignette with a 53 deg 24mm eyepiece.

 

One thing that is incorrect above, is the parfocal issue. This is entirely dependent on the eyepiece used. With my ES 3x focal extender, when I place a Meade 5000 24mm widefield (same as the ES 24/68) it is parfocal - no change to the focus point. My Brandon's, when used with the focal extender, require 3 mm inward travel.



#17 EuropaWill

EuropaWill

    Apollo

  • *****
  • topic starter
  • Posts: 1104
  • Joined: 12 Feb 2009
  • Loc: Lower Hudson Valley NY

Posted 27 December 2014 - 05:18 PM

I thought I'd give everyone an update now that my Meade series 5k 2" 2x TeleXtender arrived and I've had a little bit of time with it. 

 

Vignetting or not: I did some tests with the following 2" EP's: Meade 5k SWA 40mm, Meade QX 36mm, Meade 5k UWA 24mm, ES68 28mm.  Amazingly in my AT106LE there are absolutely no signs of vignetting. Not even in the monster SWA 40mm!  I don't see any darkening at the edge or compromise of the field stop when in use on any of these EP's. 

 

Eye Relief: Although the concept of a power amplifier such as the Meade/ES TeleXtender promises to keep the eye relief of your EP constant in use vs natively, I noticed a reduction of eye relief vs the native eye relief of EP's when in use. My GSO 2" ED barlow increases eye relief. 

 

Image Quality: In the AT106LE it produces excellent image quality and sharpness. I do notice this unit seems to flatten the field of my AT106LE which my GSO 2" ED barlow doesn't seem to do. This is a definite and significant advantage! Any minor field curvature seen in the 68* EP's natively through the scope is virtually eliminated when in use. 

 

Overall I'm very satisfied with this acquisition!


  • Vondragonnoggin likes this

#18 Vondragonnoggin

Vondragonnoggin

    Fly Me to the Moon

  • *****
  • Posts: 7339
  • Joined: 21 Feb 2010
  • Loc: Southern CA, USA

Posted 27 December 2014 - 05:27 PM

It's a keeper. It's my only 2" barlow. I was bummed when Meade changed hands, but luckily ES picked up with the 1.25" focal extenders. They are all made by JOC, but I like the finish of the 5000 TeleXtender the best.

 

I picked up all three 1.25" but under ES brand


Edited by Vondragonnoggin, 27 December 2014 - 05:27 PM.

  • EuropaWill likes this

#19 junomike

junomike

    Voyager 1

  • *****
  • Moderators
  • Posts: 13948
  • Joined: 07 Sep 2009
  • Loc: Ontario

Posted 27 December 2014 - 05:46 PM

I thought I'd give everyone an update now that my Meade series 5k 2" 2x TeleXtender arrived and I've had a little bit of time with it. 

 

Vignetting or not: I did some tests with the following 2" EP's: Meade 5k SWA 40mm, Meade QX 36mm, Meade 5k UWA 24mm, ES68 28mm.  Amazingly in my AT106LE there are absolutely no signs of vignetting. Not even in the monster SWA 40mm!  I don't see any darkening at the edge or compromise of the field stop when in use on any of these EP's. 

 

Eye Relief: Although the concept of a power amplifier such as the Meade/ES TeleXtender promises to keep the eye relief of your EP constant in use vs natively, I noticed a reduction of eye relief vs the native eye relief of EP's when in use. My GSO 2" ED barlow increases eye relief. 

 

Image Quality: In the AT106LE it produces excellent image quality and sharpness. I do notice this unit seems to flatten the field of my AT106LE which my GSO 2" ED barlow doesn't seem to do. This is a definite and significant advantage! Any minor field curvature seen in the 68* EP's natively through the scope is virtually eliminated when in use. 

 

Overall I'm very satisfied with this acquisition!

Mimics my experience also.

 

Mike



#20 faackanders2

faackanders2

    Fly Me to the Moon

  • *****
  • Posts: 5274
  • Joined: 28 Mar 2011

Posted 23 April 2016 - 06:25 PM

 

I'm considering adding the ES 2" Focal Extender but read in the specs that it has only a 27mm clear aperture.  :confused:

 

Won't this cause significant vignetting with widefield 2" ep's such as ES68 40mm, ES 68 34mm, ES 68 28mm, ES82 24mm, etc...?

One is coming my way, I couldn't resist the 25% off sale.

My primary use will be with short focal length eyepieces but I may experiment with a 41mm Panoptic and a 30mm Meade UWA just for the heck of it.

 

I may act on the 2" ES Telecentric Focal Extender sale before April 30 end.

 

With 27mm clear aperture would it work with my widest TFOV 2" eyepieces:

40mm 70 AFOV University Optics MK70 Koenig

30mm 82 AFOV Meade 5000

20mm 100 AFOV ES

9mm 120 AFOV ES

 

I assume there would be no issues with wide 1.25" eyepieces:

24mm 68 AFOV TV Panoptics

18mm 82 AFOV Meade 5000



#21 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 37104
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 25 April 2016 - 07:13 PM

Clear aperture on a barlow is not related to vignetting.

It is a by-product of focal length and magnification of the lens.

See the above posts.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics