36 replies to this topic

[skypilgrim]

I have to admit I don’t fully grasp Barlow functionality.

 

 I believe the distance from the eyepiece to the Barlow lens at the bottom of the barrel can affect how much magnification that combination produces. That potentially, a 2x Barlow might actually be a 1.9x or 2.1x depending on what eyepiece is inserted.

 

Of course I could be all wet too.

 

Anyway, I want to find a good Barlow, ie, sharp optics, no vignetting, and a true 2x magnification.

 

I am considering the TV 1.25” Barlow, assuming it scores on the first two concerns, but am not sure of the true 2x. Especially using non-TV eyepieces.

If not that one, a better recommendation?

 

Any guidance?

Sam

[SeattleScott]

Correct the magnification can vary 10% or so depending on the eyepiece used. I believe a TV barlow is designed to give stated magnification with TV eyepieces, which are mostly parfocal in the 1.25” models.

I have the TV 3x barlow which is very sharp. I don’t feel like it degrades the image at all, although I know it must. Of course be careful using it in a 1.25” diagonal, but otherwise you are fine.

Scott

[ngc7319_20]

You are right about the magnification depending on the distance from the Barlow lens element to the eyepiece focal plane.  I presume the TeleVue 2x Barlow is designed to give 2x with the TeleVue eyepieces.  Most of their 1.25" eyepieces have the focal plane 0.25" below the 1.25" seat.  If your non-TeleVue eyepieces also meet that criteria, they will probably also give 2x.

 

If you are really concerned about having an accurate magnification, you should instead consider one of the TeleVue PowerMate amplifiers.  With those the magnification is nearly independent of eyepiece. They come in 1.25" both 2.5x and 5x, and the 2" comes in 2x and 4x.

 

TeleVue gives their eyepiece specs here:

 

https://www.televue....page.asp?id=214

 

Column "F" gives the location of the focal plane below the upper barrel seat (in inches).

 

There's a chart of TeleVue PowerMate magnifications here:

 

https://www.televue....b=_app#MagChart

 

If you are just dropping in an eyepiece (no extension tubes, etc.), you will be near the "0" position on the horizontal scale.   10mm or so either way in the focal plane position won't change the magnification very much.

Edited by ngc7319_20, 23 August 2022 - 02:33 PM.

[SoCalPaul]

As I understand it, the TV Powermates uses a "telecentric" design, so the amplification factor does not change with regard to distance between eyepiece and barlow lens.

 

See this: https://www.televue....ate Concept.pdf

 

Clear skies,

Paul

[vtornado]

Think of a barlow as making a diverging cone of light in back of the  barlow optics.

As the view is moved away from the optic, the image gets larger.

 

The barlow factor depends upon how far eyepiece focal plane is from the barlow lens

assembly.  I know all TV plossls are par focal so the barlow will give the exact same

barlow factor for all plossls.  (I don't know if this is truly 2x for this line, but it would be

the same factor).

 

I don't know if TV eyepieces are parfocal across product lines.  (Plossl vs Delite vs ...)

 

You can solve this problem by using parfocal rings on your eyepieces to get the

focal plane in all the same point, if getting exactly 2x is important to you.

 

Another solution is to get a telecentric.  A telecentric intercepts the diverging light

cone and straightens it out when it reaches 2x, so any eyepiece no matter how far away gets 2x magnified. This is a pricey option. 

 

This is where my knowledge is a bit sketchy, but I think??? all 1.25 inch barlows will vignette with wide field 1.25 inch eyepieces (e.g. 32mm plossl).   Wait

for someone to correct me, if I am wrong.  This may be why some folks go to a 2 inch

barlow ???  

 

Barlows also extend the eye relief of an eyepiece.  Desired for short focal length plossls.

Undesired for long eye relief eyepieces.

[TOMDEY]

It's easy to compute... I can derive that if you like. But yeah, going closer than nominal (with the eyepiece's field stop) reduces the amplification and farther increases it. I've designed and built numerous (very weak) Barlows... and the execution and use are reliable. The manufacturer's nominal amplification (e.g. 2x) is exact, provided you put your (make and model) eyepiece's field stop in the right place. This rarely coincides with "pushed in all the way" because there are no standards for field stop location relative to external structure.

 

So... if your Barlow is not operating exactly at the mfg stated amplification, that's actually your fault... not theirs!   Tom

[skypilgrim]

All helpful comments. Thanks!

[Starman1]

I have to admit I don’t fully grasp Barlow functionality.

 

 I believe the distance from the eyepiece to the Barlow lens at the bottom of the barrel can affect how much magnification that combination produces. That potentially, a 2x Barlow might actually be a 1.9x or 2.1x depending on what eyepiece is inserted.

 

Of course I could be all wet too.

 

Anyway, I want to find a good Barlow, ie, sharp optics, no vignetting, and a true 2x magnification.

 

I am considering the TV 1.25” Barlow, assuming it scores on the first two concerns, but am not sure of the true 2x. Especially using non-TV eyepieces.

If not that one, a better recommendation?

 

Any guidance?

Sam

TeleVue eyepieces in 1.25" have their focal planes about 0.25" below the shoulder.

A TeleVue 2X Barlow is 2.0x with a TeleVue 1.25" eyepiece with that focal plane position.

So if the focal plane of your eyepiece is at the shoulder, the 2X Barlow will be a bit more than 2.0x, maybe 2.05x or 2.1x.

I don't have the focal length for this Barlow, so I cannot tell you what it will be, but the chart on the TeleVue site:

https://www.televue....d=52&Tab=_photo

implies the magnification increase is about 0.014x per mm of distance.

That would mean an increase of 0.09x at the opening of the Barlow = 2.09x.  TeleVue's chart can be read as 2.1x at that level, so, rounded off, 2.1x at the shoulder, and 2.0 times 6.35mm into the tube.

 

I suspect you will have similar findings on most Barlows, i.e. that they are not exactly as claimed at the opening of the Barlow.

And if your eyepiece has its focal plane at a location other than the shoulder, the odds are not high the Barlow will be exactly as claimed.

You don't have to read many Barlow threads here on CN before you realize that the magnification of most Barlows is +/- from the rated power.

 

Exceptions would include the 2x, 2.5x, 4x PowerMates:

https://www.televue....?id=53&Tab=_app

which change only minutely with small changes in eyepiece focal plane positions.

 

 

One way to gauge the exact magnification of a Barlow requires a field test:

Time 1: time the passage of any star across the field from edge to edge through the center

Time 2: add barlow and repeat the timing on the same star.

Time 1 ÷ Time 2 = magnification factor.

 

Here is info on the Baader VIP Barlow:

https://www.baader-p..._vip_barlow.pdf

[skypilgrim]

I guess the next question is, how much do I care?

 

If a Barlow is over/under less than 5%, maybe I don’t care. But how much beyond 5% do I start to care? Hmm, not sure. Will need to chew on that a bit.

[TOMDEY]

I guess the next question is, how much do I care?

 

If a Barlow is over/under less than 5%, maybe I don’t care. But how much beyond 5% do I start to care? Hmm, not sure. Will need to chew on that a bit.

Yes indeed! There are very few applications where some precise magnification is needed... visual is quite forgiving and nearly all imagery also forgiving. And you can figure out the image's scale after the fact, if anyone wants to know. It's the quality of the image... not its exact size. I'm also guessing that the high-end manufacturers (e.g. Televue) don't get overly frenetic regarding exact amplification discussions. That may just confuse and cause unnecessary angst among most (nearly all) customers, fretting over stuff that percolating in the background. If you're e.g. measuring double stars etc. then it's really up to you to independently certify image scale as it presents in your scope. Arcane stuff like that. Otherwise --- just enjoy!

 

[Ummm... Eyepiece pairs for binoscopes/binoviewers. With those, prudent to get a pair from the same dealer, same make and model, same lot/shipment.]     Tom

[Astro-Master]

I guess the next question is, how much do I care?

 

If a Barlow is over/under less than 5%, maybe I don’t care. But how much beyond 5% do I start to care? Hmm, not sure. Will need to chew on that a bit.

That is my question also, why do you care?  I like the fact that a barlow's power can be changed by the spacing between the eyepiece and the barlow lens and use it to my advantage.

 

Using my 13 Ethos (160x with the 18" Dob), and my 2" Astro-Physics barlow, the eyepiece slides down farther in the barlow used as a 2" rather than if I used an 1.25" adapter and gives the barlow a power of 1.61x with a power of 258x which changes the 13 Ethos into an 8mm Ethos with no vignetting, which makes me very happy, I don't need to buy an 8mm Ethos.  

[havasman]

I guess the next question is, how much do I care?

 

If a Barlow is over/under less than 5%, maybe I don’t care. But how much beyond 5% do I start to care? Hmm, not sure. Will need to chew on that a bit.

I have an 8 or 10 pound sledgehammer I get out now & then when I really need to bash something up. It's got a lot of whup marks and divots and a couple of gashes along the edge where my swing was just a bit off so it probably doesn't actually weight 8 or 10 pounds anymore if it ever did. But I've never thought about that until now 'cause when I need to really bash something up it's the ticket.

 

Now my TV2x is certainly a tool of a different feather. But it's similar in that when I get it out to use with the 9mm Tak Abbe Ortho to get an effective 4.5mm focal length from a TAO but maintain the eye relief of the 9mm it just works a trick on that. It's a very good Barlow. I leave the fretting over some little matter to somebody else. 

Edited by havasman, 23 August 2022 - 08:51 PM.

[SteveG]

I have to admit I don’t fully grasp Barlow functionality.

 

 I believe the distance from the eyepiece to the Barlow lens at the bottom of the barrel can affect how much magnification that combination produces. That potentially, a 2x Barlow might actually be a 1.9x or 2.1x depending on what eyepiece is inserted.

 

Of course I could be all wet too.

 

Anyway, I want to find a good Barlow, ie, sharp optics, no vignetting, and a true 2x magnification.

 

I am considering the TV 1.25” Barlow, assuming it scores on the first two concerns, but am not sure of the true 2x. Especially using non-TV eyepieces.

If not that one, a better recommendation?

 

Any guidance?

Sam

Based on your criteria, you might consider a Seibert 2x 1.25" Telecentric. I would first confirm with him that the Telecentric won't vignette a 24mm wide field.

 

Perhaps Don knows if the TV 2x barlow vignettes low-power widefields. If it doesn't, it is probably the closest to what you describe.

 

If this is for your 80 mm, I would want a shorty barlow.

[f18dad]

https://www.cloudyni...arlow-lens-125/

[Bill Weir]

I guess the next question is, how much do I care?

 

If a Barlow is over/under less than 5%, maybe I don’t care. But how much beyond 5% do I start to care? Hmm, not sure. Will need to chew on that a bit.

I’m glad you asked instead of me and risked appearing rude. Really being exact would only matter if you were planning on writing a paper or something. I have a couple of Powermates that I use on occasion when the single eyepieces I have won’t get me more usable magnification. I actually only have this problem with my NP101. 

 

Then again this is the Cloudy Nights eyepiece forum where splitting hairs as many times as one can possibly split a hair is the norm. A discussion might even break out as to whether one of them a  even real split of just a partial split. Charts, graphs and many equations will be trotted out. 

 

Bill

[CeleNoptic]

Anyway, I want to find a good Barlow, ie, sharp optics, no vignetting, and a true 2x magnification.
 
I am considering the TV 1.25” Barlow, assuming it scores on the first two concerns, but am not sure of the true 2x. Especially using non-TV eyepieces.
If not that one, a better recommendation?
 
Any guidance?
Sam

 
What about 1.25" 2x ES Focal Extender. It meets all your criteria and is under $100 now (on Amazon), you won't find cheaper even in CN Classifieds, I suspect. Sold by Astroshop (EU) and the price is low probably because of the Euro rate exchange drop. I have a Meade TeleXtender and it's pretty good, same glass as the ES FE made by the same manufacturer (JOC), just a rebrand. They are telecentrics. My TeleXtender works at 2.0 - 2.03x with all eyepieces (TV or other brands) I've tested. It doesn't vignette my 24mm ES68 and it would be strange if it does since telecentrics were designed specifically to address vignetting in wide angle eyepieces. A stable 2x magnification independent on the focal plane position is just a pleasant bonus. Also, doesn't affect eye relief. 

[russell23]

I have to admit I don’t fully grasp Barlow functionality.

 

 I believe the distance from the eyepiece to the Barlow lens at the bottom of the barrel can affect how much magnification that combination produces. That potentially, a 2x Barlow might actually be a 1.9x or 2.1x depending on what eyepiece is inserted.

 

Of course I could be all wet too.

 

Anyway, I want to find a good Barlow, ie, sharp optics, no vignetting, and a true 2x magnification.

 

I am considering the TV 1.25” Barlow, assuming it scores on the first two concerns, but am not sure of the true 2x. Especially using non-TV eyepieces.

If not that one, a better recommendation?

 

Any guidance?

Sam

What I have found over the years measuring barlow-eyepiece magnification factors is that most non-TV eyepieces give a little higher magnification factor with any given barlow than TV eyepieces.

 

For example, with the 3x TV barlow I found TV eyepieces usually give 3.05-3.08x magnification.  Non-TV eyepieces typically give 3.17-3.22x magnification.  When you add a 1” extension tube to the 3x TV barlow it becomes 3.65x with TV and 3.75x with non-TV eyepieces

 

With the Nikon 1.6x barlow TV eyepieces give 1.6x and non-TV eyepieces give 1.62-1.65x.

 

With the 2.5x GSO APO barlow TV eyepieces give 2.0x and non-TV eyepieces give 2.1x.  That is an example of a barlow that has a very different magnification factor from what the barrel says.

 

I don’t think I ever measured the 2x TV barlow so I can’t help you with that one specifically.  I can only say that non-TV eyepieces will provide a little more magnification factor with it than TV eyepieces.  It has to do with how the field stop is positioned relative to the position that seats the eyepiece into the focuser. TV eyepieces must seat a little closer so the distance between the field stop and the barlow is a little less.

[f18dad]

What I have found over the years measuring barlow-eyepiece magnification factors is that most non-TV eyepieces give a little higher magnification factor with any given barlow than TV eyepieces.

 

For example, with the 3x TV barlow I found TV eyepieces usually give 3.05-3.08x magnification.  Non-TV eyepieces typically give 3.17-3.22x magnification.  When you add a 1” extension tube to the 3x TV barlow it becomes 3.65x with TV and 3.75x with non-TV eyepieces

 

With the Nikon 1.6x barlow TV eyepieces give 1.6x and non-TV eyepieces give 1.62-1.65x.

 

With the 2.5x GSO APO barlow TV eyepieces give 2.0x and non-TV eyepieces give 2.1x.  That is an example of a barlow that has a very different magnification factor from what the barrel says.

 

I don’t think I ever measured the 2x TV barlow so I can’t help you with that one specifically.  I can only say that non-TV eyepieces will provide a little more magnification factor with it than TV eyepieces.  It has to do with how the field stop is positioned relative to the position that seats the eyepiece into the focuser. TV eyepieces must seat a little closer so the distance between the field stop and the barlow is a little less.

 

This is awesome granular information from russell23.  Obviously GSO needs to re-label its 2.5x APO. It's beyond misleading and seemingly without purpose. Perhaps chosen to position the product differently in the market or to not compete with its own 2x Shorty.

 

This said, the facts presented corroborate that this barlow should indeed be compared to the TV 2x as Russ S. has suggested in his first post on a different related thread: 

 

https://www.cloudyni...r-tv-2x-barlow/

 

At almost 1/3 the price ($48) the GSO APO does appear to be an excellent good barlow alternative to the TV in a much smaller form factor.

 

I am quite disappointed that GSO has their barlow mis-labeled. Thanks to russell23 for advising the community. GSO should change their labeling. Until they do, sellers should edit their descriptions and technical details to reflect the reality.

Edited by f18dad, 24 August 2022 - 07:29 AM.

[Jon Isaacs]

A couple of tidbits:

 

- The Tele Vue 1.25" 2X barlow has a focal length of -74.4mm, the Tele Vue 1.25" 3X barlow has a focal length of -52.3mm, the Tele Vue 2" 2X big barlow has a focal length of -90.4mm.

 

- The Amplification factor for a Barlow is:

 

A = D/F + 1 Where D is the distance from the Barlow to the focal plane of the eyepiece and F is the focal length of the Barlow. 

 

- By inspection, if D = F then it's a 2X Barlow, if D= 2F,  it's a 3 X Barlow. 

 

- If the TV 2x Barlow is setup for 1.25 inch TV eyepieces with the focal plane forward of the body by 0.25 inches, then for an eyepiece with the focal plane at the body-Barrel, the magnification will be 2.08x.

 

- If you pull a TeleVue 1.25 inch eyepiece back 15 mm, the magnification factor will be 2.20x, a 10% zoom factor that can come in handy.

 

- For the 3x TV Barlow, the focal plane at the barrel-body increases the magnification factor to 3.12x.

 

If you pull the eyepiece back 15 mm, the magnification factor for a 1.25 inch TeleVue eyepiece will be 3.29x, again a 10% zoom that can be useful.

 

Jon

[Starman1]

What I have found over the years measuring barlow-eyepiece magnification factors is that most non-TV eyepieces give a little higher magnification factor with any given barlow than TV eyepieces.

 

For example, with the 3x TV barlow I found TV eyepieces usually give 3.05-3.08x magnification.  Non-TV eyepieces typically give 3.17-3.22x magnification.  When you add a 1” extension tube to the 3x TV barlow it becomes 3.65x with TV and 3.75x with non-TV eyepieces

 

With the Nikon 1.6x barlow TV eyepieces give 1.6x and non-TV eyepieces give 1.62-1.65x.

 

With the 2.5x GSO APO barlow TV eyepieces give 2.0x and non-TV eyepieces give 2.1x.  That is an example of a barlow that has a very different magnification factor from what the barrel says.

 

I don’t think I ever measured the 2x TV barlow so I can’t help you with that one specifically.  I can only say that non-TV eyepieces will provide a little more magnification factor with it than TV eyepieces.  It has to do with how the field stop is positioned relative to the position that seats the eyepiece into the focuser. TV eyepieces must seat a little closer so the distance between the field stop and the barlow is a little less.

Correct.  See post #8.

[Starman1]

A couple of tidbits:

 

- The Tele Vue 1.25" 2X barlow has a focal length of -74.4mm, the Tele Vue 1.25" 3X barlow has a focal length of -52.3mm, the Tele Vue 2" 2X big barlow has a focal length of -90.4mm.

 

- The Amplification factor for a Barlow is:

 

A = D/F + 1 Where D is the distance from the Barlow to the focal plane of the eyepiece and F is the focal length of the Barlow. 

 

- By inspection, if D = F then it's a 2X Barlow, if D= 2F,  it's a 3 X Barlow. 

 

- If the TV 2x Barlow is setup for 1.25 inch TV eyepieces with the focal plane forward of the body by 0.25 inches, then for an eyepiece with the focal plane at the body-Barrel, the magnification will be 2.08x.

 

- If you pull a TeleVue 1.25 inch eyepiece back 15 mm, the magnification factor will be 2.20x, a 10% zoom factor that can come in handy.

 

- For the 3x TV Barlow, the focal plane at the barrel-body increases the magnification factor to 3.12x.

 

If you pull the eyepiece back 15 mm, the magnification factor for a 1.25 inch TeleVue eyepiece will be 3.29x, again a 10% zoom that can be useful.

 

Jon

Pulling the eyepiece slowly out of the Barlow--the "poor man's zoom", albeit a minimal one.

Increasing that distance a lot (maybe several inches), however, leads to visible vignetting.

[Jon Isaacs]

Pulling the eyepiece slowly out of the Barlow--the "poor man's zoom", albeit a minimal one.

Increasing that distance a lot (maybe several inches), however, leads to visible vignetting.

 

A 10% increase in magnification can be worthwhile.  A 4 mm eyepiece + Paracorr + 2x TV Barlow provides 720x in my 10 inch. I use that for close doubles. If I pull the eyepiece out, it covers the 720x-800x range where the 3.5 mm takes over at 820x.

 

It won't be a poor man's zoom until the develop an 82 degree zoom...

 

Bruce (Astromaster) uses longer extensions with the 2x 2 inch TV Big Barlow without vignetting with both the 13 mm and 17 mm Ethos.  

 

There's a lot that goes into the equation.

 

Jon

[skypilgrim]

For those of you picking apart my pickiness,  ……

 

In my defense, I don’t really know how far off Barlows can be. As I mentioned, at 5% I don’t think I care. But there is some point that I, and I suspect many of you, do care.

 

Many thanks to Russell23 for sharing great details.

 

And then we find that 2.5x GSO being 25% off….. 

Sam

[csrlice12]

Use a TV Big Barlow.....if someone tells you, "that really ain't  2X with that eyepiece" you can show them that it doubles as a club.

[Astro-Master]

Use a TV Big Barlow.....if someone tells you, "that really ain't  2X with that eyepiece" you can show them that it doubles as a club.