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Advice regarding eyepieces and barlows\powermates

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#26 Peter Besenbruch

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Posted 07 January 2015 - 02:27 PM

And if you want to save some money, the Explore Scientific Focal Extenders are the same basic design of the Powermate at a much lower price.

 

And my "Focal Extender" was actually advertised as a Barlow. I appreciated the lack of marketing speak.



#27 Peter Besenbruch

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Posted 07 January 2015 - 03:19 PM


Well, this isn't quite accurate.  The Smyth lens is not a Barlow lens at all: it is a field flattener.

 

A very good article by Rodger Gordon on Smyth lenses and their role as a field flattener in very wide angle eyepieces can be found here.  You and I will have to disagree over whether Smyth lenses are or aren't Barlows, or Barlow like in function. In some eyepieces (such as my 3.8mm Ultrascopic, it is very much a Barlow. If you remove the Smyth lens from a Hyperion, you still get a functional, lower power lens, although the edge definition isn't as good. I wouldn't want to remove a Smyth lens from a Nagler, or anything wider.

 

On a different example, I have a Baader zoom that is somewhat soft at the edge of the field in a 600mm refractor. It appears to be caused by field curvature, as you can focus the edge of the field separately from the center. Both an Ultima 2x, 3 element Barlow and the ES 3x, 4 element Barlows cleaned that up.



#28 Peter Besenbruch

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Posted 07 January 2015 - 03:35 PM

Tele Vue does make very good Barlows (and they actually *call* them "Barlows").  I highly recommend them.  However, as for the Tele Vue Powermates, they are NOT Barlows.  A Barlow is a two (or sometimes three) element negative lens set designed as an "image amplifier" to produce higher magnification with a given eyepiece.  Barlows not only amplify the telescope's on-axis light cone making it appear much shallower with a longer effective focal length, but also amplify the angles that some of the near-marginal light rays have when exiting the Barlow lens.  This can introduce problems in the form of vignetting on some wider-field eyepieces, as some of the outer field light is diverted out beyond the edges of the eyepiece's field stop.

 

Here we will have to disagree: A Barlow is an overall negative lens set designed as an image amplifier. It can range from 1 element on up. Give credit to Televue, and possibly Aaron Levin for making a better Barlow, but a Barlow it remains.

 

I found your description of what the 4 element design does especially helpful when discussing H-Alpha solar viewing.



#29 SteveG

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Posted 08 January 2015 - 01:22 PM

 

And if you want to save some money, the Explore Scientific Focal Extenders are the same basic design of the Powermate at a much lower price.

 

And my "Focal Extender" was actually advertised as a Barlow. I appreciated the lack of marketing speak.

 

Isn't it a barlow regardless?

 

Powermate, ES Focal Extender, and Meade TeleXtender are all telecentric barlows (w/4 lenses), as opposed to telenegitive barlows (2 & 3 element).



#30 David Knisely

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Posted 08 January 2015 - 01:46 PM

Might as well add this image (again) so everyone knows the difference between Barlow and PowerMate::

 

Actually, in the case of the Powermates, the forward negative lens is a little smaller than the positive achromat behind it.  Clear skies to you.



#31 David Knisely

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Posted 08 January 2015 - 02:04 PM



 



Tele Vue does make very good Barlows (and they actually *call* them "Barlows").  I highly recommend them.  However, as for the Tele Vue Powermates, they are NOT Barlows.  A Barlow is a two (or sometimes three) element negative lens set designed as an "image amplifier" to produce higher magnification with a given eyepiece.  Barlows not only amplify the telescope's on-axis light cone making it appear much shallower with a longer effective focal length, but also amplify the angles that some of the near-marginal light rays have when exiting the Barlow lens.  This can introduce problems in the form of vignetting on some wider-field eyepieces, as some of the outer field light is diverted out beyond the edges of the eyepiece's field stop.

 

Here we will have to disagree: A Barlow is an overall negative lens set designed as an image amplifier. It can range from 1 element on up. Give credit to Televue, and possibly Aaron Levin for making a better Barlow, but a Barlow it remains.

 

I found your description of what the 4 element design does especially helpful when discussing H-Alpha solar viewing.

 

 

Sorry, but the fact is, the regular original Barlow design (originated by Peter Barlow, in collaboration with George Dolland in 1833) is indeed just a negative lens set (forming a tight group with a negative focal length) diverging image amplifier system that will extend the effective focal length of the telescope.  However, as I noted, the Barlow has the potential for inducing vignetting due to the fact that some of the light rays for objects towards the edge of the field have the potential of not reaching the eyepiece.   Some Barlows stick in a cemented 3rd element to create a so-called "Apochromatic Barlow" (Tele Vue's high-quality Barlows use only two elements), but the angles of the light coming out for marginal rays still can be diverging.  The light cones from the Barlow do *not* accurately resemble that of a longer focal length telescope, but those of the Telecentric Powermates more closely do.  The other two elements (an achromatic doublet actually) are well separated from the negative diverging elements, are somewhat larger in diameter, and do not show the same degree of marginal ray divergence that a Barlow can, so the design is definitely different from that of a regular Barlow.   Indeed, the effects the more telecentric Powermates have when used for achieving the longer focal lengths needed for some solar H-alpha filters demonstrates that difference vividly.  With a 2x, 2.5x, or 4x Powermate, the H-alpha detail can extend across the entire field of view, but with a Barlow, it does not (forms either a thick ring of H-alpha detail or just a small "sweet spot" of detail with the rest of the field looking like a red version of white light filtering systems).  A Powermate can be considered a focal length extender or image amplifier but is not really a Barlow.  Clear skies to you.     


Edited by David Knisely, 09 January 2015 - 02:31 AM.


#32 Starman1

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Posted 08 January 2015 - 05:47 PM

Hello,

I need and advice, especially somebody who used powermates "known to be exponentially" better than barlows.

Due to lack of money I intend to buy those 2x and 4x powermates from TV so that I won't need to buy smaller eyepieces... this way the 12mm would become 6mm and 3mm, the 16 would become 8mm and 4mm and so on.

How much quality\light would I sacrifice by using those barlows? Should I better wait more and buy smaller eyepieces (in mm) and use no barlows?

 

And second question, if I want to go for planet photo, would the 2x or 4x powermate be too much magnification (some say that the best planet photos need no barlows).At the moment I have no t2 adapter so that I can test, so I need your experience

I own an 8" DOB / 1200 FL / Nikon D40.

 

Thank you,

No worries.  It's a good way to get high powers with the eye relief of low powers.

It's definitely cost effective, too.

Buy eyepieces that yield 50x, 100x, 150x for your 8" (24mm, 12mm, 8mm)

Use the 2X barlow to yield 200x and 300X, mabe add a 2.5X for 250X and 375X.

You might use the 300x or 375x on the Moon, planets, double stars.

Unless the seeing conditions and optical quality, allow 400X+ (the result of a 4x PowerMate).  Unlikely.



#33 Dyptorden

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Posted 09 January 2015 - 02:33 AM

Thank you Starman1.

What should happen to the image if I go for a too big magnification when the seeing conditions aren't that good? Does it become blurry or something?

 

Thanks,



#34 Hesiod

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Posted 09 January 2015 - 04:07 AM

Bad seeing "erases" details: if you gaze at high power, the image will appear blurry, and will "boil". If you gaze at lower power, maybe the blurring will be less noticeable but the details still be lacking (if you look at the Moon at 50x the image will be almost always steady, but I doubt you will see a lot of the small features larger magnification could show).

 

When doing a film through a camera however, things will be rather different. The camera shots at a certain framerate, let's say 30 fps: so it "randomly" should grab instants of steady seeing. Better the "visual seeing", higher the chances of grabbing a good frame.

When you select and stack the frames, more the good ones, easier the job: because more frames you will able to stack, lower the noise.

Even in the worst night amongs a couple thousands frames there will be a bunch of good ones: but a stack of 20-30 frames usually yelds rather poor results; while on good night you can stack even 3/4 or more of the farmed frames, with a far greater signal to noise ratio.



#35 Starman1

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Posted 09 January 2015 - 11:50 AM

Thank you Starman1.

What should happen to the image if I go for a too big magnification when the seeing conditions aren't that good? Does it become blurry or something?

 

Thanks,

Hesiod's answer is apropos.

Yes, basically too much magnification makes the focus difficult and makes the images appear to have fewer moments of sharp focus.

And details get lost.

Look at stars.  If they appear like blobs, try lowering the magnification until they appear more normal.

If they're tiny pinpoints at high powers, then you probably have good seeing conditions--a night for planets, Moon, double stars, and any target

requiring better resolution.

Once you see superb images in your scope, you know the optics are good.  From that point on, poor image quality is due to seeing,

not the optics (properly cooled and collimated, of course).



#36 Peter Besenbruch

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Posted 09 January 2015 - 09:15 PM

 

 

And if you want to save some money, the Explore Scientific Focal Extenders are the same basic design of the Powermate at a much lower price.

 

And my "Focal Extender" was actually advertised as a Barlow. I appreciated the lack of marketing speak.

 

Isn't it a barlow regardless?

 

Powermate, ES Focal Extender, and Meade TeleXtender are all telecentric barlows (w/4 lenses), as opposed to telenegitive barlows (2 & 3 element).

 

Yes, I have been arguing it's a Barlow, regardless. I'll add that your labels of telecentric and telenegative Barlows are spot on.

 

Barlow purveyors have been avoiding the name Barlow for decades. Barlows have a certain "reputation," I suppose. Meade called them "telenegative amplifiers." That was when they were trying to distinguish them from the cheap, single element variety so common at the time. It's all marketing speak.



#37 SteveG

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Posted 10 January 2015 - 04:02 PM

 

 

 

And if you want to save some money, the Explore Scientific Focal Extenders are the same basic design of the Powermate at a much lower price.

 

And my "Focal Extender" was actually advertised as a Barlow. I appreciated the lack of marketing speak.

 

Isn't it a barlow regardless?

 

Powermate, ES Focal Extender, and Meade TeleXtender are all telecentric barlows (w/4 lenses), as opposed to telenegitive barlows (2 & 3 element).

 

Yes, I have been arguing it's a Barlow, regardless. I'll add that your labels of telecentric and telenegative Barlows are spot on.

 

Barlow purveyors have been avoiding the name Barlow for decades. Barlows have a certain "reputation," I suppose. Meade called them "telenegative amplifiers." That was when they were trying to distinguish them from the cheap, single element variety so common at the time. It's all marketing speak.

 

Funny - I did some reading on this topic yesterday and found some web sites that say the telecentric is not a Barlow, but others stating it is. Oh well. I know what they do and how they work, good enough I suppose!



#38 Starman1

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Posted 10 January 2015 - 04:25 PM

A Barlow, as invented by Peter Barlow is a negative lens added to a system to increase the magnification (i.e. expand the image).

A telecentric lens, which is similar to a Bravais lens, expands the image scale but keeps rays parallel.

In the first case, rays are diverging, in the second they are brought back together to the same angle.

Parallel rays entering a Barlow lens are diverging after passage through the lens.

Parallel rays entering a telecentric lens are still parallel after passage through the lens, merely larger in scale.

 

In telescope terms, an f/5 light cone, after passage through a 2X Barlow lens, has the same angles as an f/10 light cone.

An f/5 light cone, after passing through a 2X telecentric lens, still has the same angles as an f/5 light cone, only with an expanded image scale.

 

The first turns the instrument from an f/5 to an f/10 with the same aperture.

The second turns the instrument from an f/5 to an f/5 with twice the focal length.

A subtle difference given that the magnification change is the same.

 

In a Barlow, the magnification keeps increasing the farther you are away from the lens.

In a telecentric lens, the magnification would be the same at all distances.

 

In practice, many commercial "telecentric" lenses are not pure and do have a very slightly varying magnification with distance from the lens,

but they are closer to that than any standard Barlow lens.

 

A standard Barlow can have its focal length changed by adding a third lens.  This is often done to put the Barlow into a shorter tube.



#39 Peter Besenbruch

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Posted 10 January 2015 - 10:11 PM

... The first turns the instrument from an f/5 to an f/10 with the same aperture.

The second turns the instrument from an f/5 to an f/5 with twice the focal length.

A subtle difference given that the magnification change is the same. ...


A standard Barlow can have its focal length changed by adding a third lens.  This is often done to put the Barlow into a shorter tube.

 

Thanks for a simple explanation of the differences. I have had all three types, and currently use 3 and 4 lens versions. Their use is identical. I place them in my diagonal, and add an eyepiece. The primary use is to allow me to use eyepieces at high power while enjoying increased eye relief. It also helps me increase the range of a zoom that I own.

 

I also own another Barlow that I don't use much now, the Televue Big Barlow with Panoptic Interface Lens, as it was called back then. It's a telenegative! It's a telecentric! It's a Barlow! It's quite good!

 

I'll close with the following recommendation: This is Cloudy Nights, a community that specializes in acronyms. I propose TNB and TCB. These can join BillP's hoped for TAOs in the pantheon of abbreviations.



#40 NHRob

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Posted 11 January 2015 - 08:50 AM

Don,

 

  This means that with a telecentric, the eyepiece still sees a light cone similar to the native f-ratio of the objective.

Given this, a telecentric would not help clean up edge performance of eyepieces which are not well corrected ... like a barlow does.

Is this correct?

  For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

Rob



#41 russell23

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Posted 11 January 2015 - 09:19 AM

Don,

 

  This means that with a telecentric, the eyepiece still sees a light cone similar to the native f-ratio of the objective.

Given this, a telecentric would not help clean up edge performance of eyepieces which are not well corrected ... like a barlow does.

Is this correct?

  For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

Rob

 

Rob,

 

That is not my experience.  I have used the 2.5x Powermate and the 2x and 3x ES focal extenders.  In all three the edge performance of Brandons has improved with the use of the Powermate and focal extenders. 

 

What I do find is that sometimes an eyepiece vignettes in a PM/FE but not in a barlow, so there is an advantage to having both.  For example, the 35mm Ultrascopic did not vignette in a regular barlow but vignettes in the 2x and 3x FE.  The 32mm Brandon is the opposite - vignettes in a barlow, but not in the 2x FE.

 

Anyway, your Konigs should still show improvement with the focal extenders and powermates.

 

Dave



#42 Dyptorden

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Posted 11 January 2015 - 11:00 AM

 

Don,

 

  This means that with a telecentric, the eyepiece still sees a light cone similar to the native f-ratio of the objective.

Given this, a telecentric would not help clean up edge performance of eyepieces which are not well corrected ... like a barlow does.

Is this correct?

  For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

Rob

 

Rob,

 

That is not my experience.  I have used the 2.5x Powermate and the 2x and 3x ES focal extenders.  In all three the edge performance of Brandons has improved with the use of the Powermate and focal extenders. 

 

What I do find is that sometimes an eyepiece vignettes in a PM/FE but not in a barlow, so there is an advantage to having both.  For example, the 35mm Ultrascopic did not vignette in a regular barlow but vignettes in the 2x and 3x FE.  The 32mm Brandon is the opposite - vignettes in a barlow, but not in the 2x FE.

 

Anyway, your Konigs should still show improvement with the focal extenders and powermates.

 

Dave

 

Hi Dave,

As I've started this topic "...if I want to go for planet photo, would the 2x or 4x powermate..." both of these powermates are 2".You've said about a 2.5x wich is 1.25". The 1.25" can vignette your pictures, but the 2" that I speak about... shouldn't



#43 Starman1

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Posted 11 January 2015 - 11:34 AM

Don,

 

  This means that with a telecentric, the eyepiece still sees a light cone similar to the native f-ratio of the objective.

Given this, a telecentric would not help clean up edge performance of eyepieces which are not well corrected ... like a barlow does.

Is this correct?

  For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

Rob

I haven't ever run into this question before, and I don't know.  It sounds logical, but it might not work that way.

More research is required.



#44 russell23

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Posted 11 January 2015 - 12:24 PM

 

 

Don,

 

  This means that with a telecentric, the eyepiece still sees a light cone similar to the native f-ratio of the objective.

Given this, a telecentric would not help clean up edge performance of eyepieces which are not well corrected ... like a barlow does.

Is this correct?

  For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

Rob

 

Rob,

 

That is not my experience.  I have used the 2.5x Powermate and the 2x and 3x ES focal extenders.  In all three the edge performance of Brandons has improved with the use of the Powermate and focal extenders. 

 

What I do find is that sometimes an eyepiece vignettes in a PM/FE but not in a barlow, so there is an advantage to having both.  For example, the 35mm Ultrascopic did not vignette in a regular barlow but vignettes in the 2x and 3x FE.  The 32mm Brandon is the opposite - vignettes in a barlow, but not in the 2x FE.

 

Anyway, your Konigs should still show improvement with the focal extenders and powermates.

 

Dave

 

Hi Dave,

As I've started this topic "...if I want to go for planet photo, would the 2x or 4x powermate..." both of these powermates are 2".You've said about a 2.5x wich is 1.25". The 1.25" can vignette your pictures, but the 2" that I speak about... shouldn't

 

 

Hello,

 

I'm talking visual observations - with eyepieces - and it does depend upon the specific eyepiece barlow combination as to whether or not there will be vignetting.   There is not firm rule that all eyepieces do this or that with barlows vs telecentrics where vignetting is concerned.  It is a case by case basis and even with the 2" ES focal extender I found there was vignetting with some eyepieces. 

 

Dave



#45 Hesiod

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Posted 11 January 2015 - 01:12 PM

IMHO the vignetting is badly overstated.

 

You have an autotracking telescope, so when gazing at planets, really need a very little field of view (a 5mm eyepiece with less than 20° afov should be more than adequate to frame all planets and their immediate "surroundings"); even on Moon, to "read" the small details only a very small portion of the eyepiece's field is employed (take note that the "hi-res" field of your eye is quite small, both in linear and angular size, far smaller than the overall field of view), exactly the central one.

What happens near the edges is totally worthless.

 

Even for planetary astrophoto, it is highly unlikely that you will ever experience vignetting (I dare to say it is impossible: you have to employ an insane sampling with a 8" to have a planetary disk so large to show vignetting), while this could happen on the Moon. Even here there it is not so a big trouble (with large sensors edges are rather poor after the stacking, so could be a good idea to cut away them in any case), and you can employ flat fields to keep the pic balanced.

 

I am not writing that Powermates are useless; au contraire, these have great advantage either with small sensor/ field stops (the constant amplification) and with large sensor (lack of vignetting when this is needed). Furthermore, their top craftmanship assures excellent results.



#46 MitchAlsup

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Posted 11 January 2015 - 02:48 PM

The first turns the instrument from an f/5 to an f/10 with the same aperture.

The second turns the instrument from an f/5 to an f/5 with twice the focal length.

A subtle difference given that the magnification change is the same.

 

Err: no. F/5 light cone of twice the focal length would have a conservation of energy violation unless the aperture were also doubled.

 

The PowerMate changes an F/5 light cone into an F/10 light cone from the same original aperture.

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#47 Starman1

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Posted 11 January 2015 - 04:51 PM

My understanding is that the f/4, converging, light cone is diverged by the negative lens in a barlow to be an f/8 converging light cone.

In essence, the f/4 light cone, with steep angles at the edge rays, becomes an f/8 light cone with less steep angles at the edge rays.

 

On the other hand, if you look at parallel rays hitting the negative lens, they'd be diverging on the other side.  That means the image will be enlarged less

near the lens and more farther away from the lens.  Indeed, that is how a barlow works.  At one particular distance from the lens,

the image scale will be doubled.

 

In a PowerMate, if you look at parallel rays hitting the negative lens, they diverge, but are brought back to parallel after the positive lens,

but the image has been enlarged.  This also explains why distance from the lens system doesn't have an effect on magnification.

 

But if you look at it from the standpoint of a converging light cone, the f/4 light cone will be diverged into a f/? converging light cone by the negative lens

If the negative lens comes first. The positive lens' effect would be to refocus the diverging light cone back into a converging light cone.

I presumed a light cone twice as wide but with the same angle to the lateral rays as before the PowerMate.  But I see the light cone is at

the angle of a scope with twice the focal length--more parallel than the light cone hitting the fist lens. Since that's the case, then the first lens has

more divergence than a standard Barlow lens since convergence takes place after passage through the positive lens.

 

I am assuming that the PowerMate is a reverse Bravais lens, with the magnifying element first, and the converging lens second.

Careful design could make it parfocal with the image before the PowerMate is placed in the optical train.



#48 Peter Besenbruch

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Posted 11 January 2015 - 09:23 PM

For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

I'm going to tentatively agree with Dave. The 3x ES telecentric sharpened the edge of a Baader zoom.



#49 David Knisely

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Posted 12 January 2015 - 01:26 AM

Don,

 

  This means that with a telecentric, the eyepiece still sees a light cone similar to the native f-ratio of the objective.

Given this, a telecentric would not help clean up edge performance of eyepieces which are not well corrected ... like a barlow does.

Is this correct?

  For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

Rob

 

No, a longer effective focal length would help in general, whether it is done via a Barlow, a Telecentric image amplifier, or just a telescope that intrinsically has a longer focal length.  The light rays in these "optically-extended" cases strike the elements of the eyepiece at a somewhat shallower angle to the optic axis, which helps the eyepiece deal with these rays more effectively (the eyepiece has less "work" to do to form the image properly).  Steeper angles from shorter focal length systems are harder for simpler eyepieces to deal with, as aberrations (particularly astigmatism and field curvature) become more pronounced.  I remember when as a kid, I put my old 6mm HM eyepiece from my little 2.4 inch f/11.7 refractor in my brand new home-built 8 inch f/7 Newtonian.  At f/11.7, that eyepiece worked just fine, but at f/7, the little 6mm HM demonstrated horrid field curvature.  When I stuck in a low-power negative lens just ahead of the eyepiece, the performance improved, as it was now running at a considerably longer effective focal length and thus the field curvature generated by the eyepiece was notably smaller than at f/7.  Clear skies to you.      


Edited by David Knisely, 12 January 2015 - 01:34 AM.


#50 CeleNoptic

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Posted 12 January 2015 - 03:54 AM

 

  For example, my Konigs  are not sharp at the edge with my f/7.5 refractor.  A barlow would help with edge performance but, it sounds like a telecentric would not.

 

Rob

Rob,

 

This is not my experience either. I have the 18mm RO Koenig which is sharp ~50% on-axis in my f/6 Dob and ~70% in my f/9 refractor. The 2x Meade TeleXtender which is a telecentric cleans up the FOV to the edges in the refractor and sharpen FOV significantly in the Dob. My telenegative GSO Barlow  does the similar things but introduces vignetting since it's a 2-element "shorty" Barlow.




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