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Ships and Stars

Effect of 2x barlow on EP exit pupil and coma?

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Posted (edited)

Hi all, quite a basic question here.

If I 2x barlow an eyepiece, what effect does it have on exit pupil and coma?

In other words, if an EP's exit pupil is 10mm in a fast scope (say f4) and produces some coma around the edges and I want to reduce it, would adding a 2x barlow reduce exit pupil size by 50% and would it reduce coma as a result by making the effective aperture f8?

Just thinking out loud here regarding some ep/barlow combinations!

 

Cheers all.

Edited by Ships and Stars
replaced EP with exit pupil to clarify...

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The exit pupil does reduce as you suggest.

I believe that coma is not actually changed because thats already in the optical system from the primary mirror which is still F/4 but sometimes it will appear a little improved because of the narrower field that the shorter effective focal length eyepiece delivers.

Barlow lenses increase eye relief a bit too.

There are one or two barlow designs around that claim to reduce coma (eg: the klee barlow) but I've not seen overwhelming evidence that they are effective.

I've always thought that a coma corrector is either essential or very, very good to have if well corrected wider angle eyepieces are to be used to full effect in an F/4 newtonian.

 

 

 

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31 minutes ago, John said:

The exit pupil does reduce as you suggest.

I believe that coma is not actually changed because thats already in the optical system from the primary mirror which is still F/4 but sometimes it will appear a little improved because of the narrower field that the shorter effective focal length eyepiece delivers.

Barlow lenses increase eye relief a bit too.

Thanks very much John! My two lowest-mag eyepieces have relatively huge exit pupils in an f4 scope, don't want to miss any photons, but lunar views with these two EPs are incredible. 

Cheers

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15 hours ago, Ships and Stars said:

Thanks very much John! My two lowest-mag eyepieces have relatively huge exit pupils in an f4 scope, don't want to miss any photons, but lunar views with these two EPs are incredible. 

Cheers

Be aware that barlowing low power eyepieces can lead to vignetting, sometimes difficult to hold exit pupils due to extending eye relief, and sometimes adds spherical aberration of the exit pupil.  Televue's Panoptic Barlow Interface (PBI) was created to use with TV's Big Barlow to eliminate these issues.  I can verify it also works well with the GSO 2" 2X ED barlow.  Alternatively, telecentric magnifiers like the TV Powermate and ES Focal Extender work in the same way but as a single unit.

16 hours ago, Ships and Stars said:

If I 2x barlow an eyepiece, what effect does it have on exit pupil and coma?

In other words, if an EP's exit pupil is 10mm in a fast scope (say f4) and produces some coma around the edges and I want to reduce it, would adding a 2x barlow reduce exit pupil size by 50% and would it reduce coma as a result by making the effective aperture f8?

Anecdotally, I notice much reduced coma when using a 2x barlow with or without the PBI.  I have to use it without the coma corrector because I can't reach focus with both at the same time.  It's not reduced astigmatism because there wasn't any to start with when using the coma corrector alone.

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Just wondering if that difference is noticed with a barlow that is parfocal with same brand EPs’ ?? For example i have a set of Celestron Ultimas in 18mm , 10mm and 7.5mm . My barlow is also the Ultima and is parfocal with the EPs’ .  Wouldn’t it effect the FOV rather than the eye pupil exit ? However all barlows reduce light by 75% . But in reducing FOV will help remove or reduce coma at the edges of the FOV . 

Edited by celestron8g8
corrected question about parfocal , not parabolic

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RonL,

Not sure where the "" However all barlows reduce light by 75% "" comes from.

I could measure the actual transmission with the spectrograph, but I'm sure with today's coatings it's much better than 75%

 

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18 hours ago, Merlin66 said:

RonL,

Not sure where the "" However all barlows reduce light by 75% "" comes from.

I could measure the actual transmission with the spectrograph, but I'm sure with today's coatings it's much better than 75%

 

I think he means by doubling the power, the brightness is quartered, that's all.

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Hmmmm

Only for extended objects.....

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18 hours ago, celestron8g8 said:

Just wondering if that difference is noticed with a barlow that is parabolic with same brand EPs’ ?? For example i have a set of Celestron Ultimas in 18mm , 10mm and 7.5mm . My barlow is also the Ultima and is parabolic with the EPs’ .  Wouldn’t it effect the FOV rather than the eye pupil exit ? However all barlows reduce light by 75% . But in reducing FOV will help remove or reduce coma at the edges of the FOV . 

Not a clue what you mean by a barlow being parabolic with same brand EPs.  Do you mean parfocal?  Mirrors can be parabolic, but I've never heard of a parabolic barlow or eyepiece.  Parabolic describes a curve, not a relationship between two optical items.

Yes, TFOV is halved with a 2x barlow.  AFOV remains the same, assuming no vignetting as described above.

The exit pupil is also halved with the doubling of the power.

10 minutes ago, Merlin66 said:

Hmmmm

Only for extended objects.....

Yes, for extended objects and the sky background brightness (which is technically a huge extended object).  As I recall, point sources will also start to dim as well once the magnification resolves the Airy disk.

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3 hours ago, Louis D said:

Not a clue what you mean by a barlow being parabolic with same brand EPs.  Do you mean parfocal?  Mirrors can be parabolic, but I've never heard of a parabolic barlow or eyepiece.  Parabolic describes a curve, not a relationship between two optical items.

 

Oh sorry , parfocal is what i meant . Meaning that very little if any refocus is needed when changing different EPs’ with the same barlow . My bad :( 

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On 09/10/2019 at 22:51, celestron8g8 said:

But in reducing FOV will help remove or reduce coma at the edges of the FOV .

Remember, coma does not decrease with increasing power.  As long as the AFOV remains the same, coma at the edge will remain the same apparent size.  Coma increases linearly center to edge, so it is more noticeable in ultrawide field eyepieces.  Going to a shorter focal length will double the magnification at the edge, bringing the apparent size of the coma right back to where it was at the lower power's edge of field.

That said, I have noticed that eyepieces with built in Smyth lenses (a matched Barlow in essence) tend to decrease the visible coma.  For instance, I see no difference in visible coma with my 10mm Delos in an f/6 Newt with or without my GSO coma corrector (that is to say, none is visible either way).  The difference with and without is quite noticeable in my 9mm Kellner, which has no Smyth component, despite its much narrower AFOV.

10 hours ago, celestron8g8 said:

Oh sorry , parfocal is what i meant . Meaning that very little if any refocus is needed when changing different EPs’ with the same barlow . My bad :( 

No problem.  I've never heard of parfocalness having any effect good or bad with respect to barlows.  Conjecturing here, it might reduce pushing the eye relief outward, and as a result, not introduce any added SAEP.

I did not know that the Celestron Ultima Barlow (the Japanese made shorty, I presume) was parfocal with the Ultima eyepieces.  You're saying that little to no refocusing is necessary when adding that Barlow to the optical chain?  That would be very useful for refractors in particular.

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1 hour ago, Louis D said:

Remember, coma does not decrease with increasing power.  As long as the AFOV remains the same, coma at the edge will remain the same apparent size.  Coma increases linearly center to edge, so it is more noticeable in ultrawide field eyepieces.  Going to a shorter focal length will double the magnification at the edge, bringing the apparent size of the coma right back to where it was at the lower power's edge of field.

That said, I have noticed that eyepieces with built in Smyth lenses (a matched Barlow in essence) tend to decrease the visible coma.  For instance, I see no difference in visible coma with my 10mm Delos in an f/6 Newt with or without my GSO coma corrector (that is to say, none is visible either way).  The difference with and without is quite noticeable in my 9mm Kellner, which has no Smyth component, despite its much narrower AFOV.

No problem.  I've never heard of parfocalness having any effect good or bad with respect to barlows.  Conjecturing here, it might reduce pushing the eye relief outward, and as a result, not introduce any added SAEP.

I did not know that the Celestron Ultima Barlow (the Japanese made shorty, I presume) was parfocal with the Ultima eyepieces.  You're saying that little to no refocusing is necessary when adding that Barlow to the optical chain?  That would be very useful for refractors in particular.

Here’s a very nice review on the Ultima 2x barlow . It explains the Parfocal system not only with Ultima EPs’ but several other popular EPs’ . 
 

https://www.chuckhawks.com/celestron_ultima_barlow.htm

 

here’s Chucks other reviews on all kinds of astro equip :  https://www.chuckhawks.com/index1.photography.htm

Edited by celestron8g8

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RonL,

He states but doesn’t explain.

As mentioned above the FOV brightness will diminish due to the higher magnifcation being achieved - for the same eyepiece used without the barlow.

You would see the same effect using an eyepiece with half the focal length without the Barlow.

 

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14 hours ago, Merlin66 said:

RonL,

He states but doesn’t explain.

As mentioned above the FOV brightness will diminish due to the higher magnifcation being achieved - for the same eyepiece used without the barlow.

You would see the same effect using an eyepiece with half the focal length without the Barlow.

 

When you add a barlow yes light incoming is reduced . Also since you are doubling your EP power  for example if using a 20mm EP with a 2x barlow as you know you are viewing the same as if you were using a 10mm EP without a 2x barlow . That only makes since . Any time you  add more glass  to your line of viewing it will reduce incoming light .  When referring to FOV  it all depends on the FOV of the EP , but when you add a 2x barlow you are doubling all calculations . Maybe this link can explain to you what i'm saying .  Read all the way down to the "Exit Pupil" topic and he shows the calculations . SInce I don't ever really worry about these calculations I don't remember  everything all the time but these explainations usually are in most Owners Manuals , or least I know they are in my OM for my Celestron 8" SCT I've had since I bought my scope . Some times I wonder why the original question from the OP is even that much of a concern unless a person is a teacher at a school for astronomy or science explaining how optics work or is just curious and asking questions ? But reguardless here's the link :   https://www.chuckhawks.com/telescope_formulas.htm

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The article is poorly worded (or the author is decieving on purpose?). Doubling the magnification will reduce the brightness by a factor of 4 (or by 75%). It doesn't matter if it is done by using a barlow or a shorter focal length eyepiece. The article makes it sound like a barlow robs you of 75 percent of light that would be there if you just used an eyepiece, which is false. Quality barlow with modern coatings will only loose few percent of light.

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A decent quality barlow lens, on average, reduces light transmission by 1-3%.

That is less than the variation in light transmission that is found across different eyepiece designs / brands without a barlow lens in play.

 

 

 

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21 hours ago, celestron8g8 said:

Any time you  add more glass  to your line of viewing it will reduce incoming light .

Unless that glass is increasing the magnification, and assuming that glass is properly multicoated, the light lost to additional glass elements in the path is minimal and below the threshold of human perception.  In fact, if that added glass is a focal reducer, it actually increases the amount of light on a per area basis.

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49 minutes ago, Louis D said:

Unless that glass is increasing the magnification, and assuming that glass is properly multicoated, the light lost to additional glass elements in the path is minimal and below the threshold of human perception.  In fact, if that added glass is a focal reducer, it actually increases the amount of light on a per area basis.

Sorry but i disagree . Even with cameras and lens when you add a teleconverter be it 1.4x or 2x  it doubles your f/stop which limits incoming light . Scopes with EPs and barlows are really nothing more than a big lens and principles  works same for both . Disagree if you wish but I’m through  discussing this topic cause it seems it’s going nowhere on agreements . Have a nice day .

Edited by celestron8g8

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There is a confusion between two things in this thread.

First - decrease in  image brightness as a result of increased magnification/decreased exit pupil. Everyone agrees on that, but this effect is not unique to Barlow. If you just use eyepieces with different focal lengths the effect will be the same - dimmer image at higher magnification.

Second is reduced light transmission due to extra glass added. With good quality Barlow this effect is too small for human eye to detect. It is unrelated to all the principles described above and this is what people mean when they say Barlow doesn't decrease brightness.

Lets say you use 20mm eyepiece. Then you add 2x Barlow. The image got 4 times dimmer because of increased magnification and say extra 3 percent dimmer because of light transmission loss. Now you just use 10mm eyepiece. The image is still 4 times dimmer compared to 20mm just without the extra 3 percent loss. If you compare 10mm to 20mm+2x Barlow you won't be able to tell the difference in brightness. So we can say Barlow doesn't decrease brightness, now that we have taken the effects of magnification out of the equation.

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