33 replies to this topic

[lamplight]

Been reading about the explore scientific "focal extender" .. Considering buying one or a 1.25" 2.5x powermate. Is the ES as good (ie: transparent) as the powermates are? I had a 2" powermate and it was great, just heavy. Any Barlow I've used since the powermate I find not that great.. Of course I'm still deciding if I should fill in my eyepiece gaps or get one of these but that's another issue... I kind of prefer just swapping eyepieces instead of barlowing, but its sooo much cheaper. Just curious if anyone's done any side by side testing (brightness of image, eye relief, contrast loss, etc..) I know I can count on CN to tell me what to buy

[T1R2]

I love my ES 1.25" 2x Fx, Televue says theirs will control some C/A and it does, but so does the ES, specially on Saturn that doesn't have hardly any C/A (with my scopes anyway), I havn't done side by side, hoping to try it on Jupiter after morning cloud sickness goes away, but I'm sure you will be satisfied with the savings and the performance of the ES

[SkyGibbon]

Love my ES 2x. Very nice for a fraction the price

[Jon Isaacs]

I love my ES 1.25" 2x Fx, Televue says theirs will control some C/A and it does, but so does the ES, specially on Saturn that doesn't have hardly any C/A (with my scopes anyway), I havn't done side by side, hoping to try it on Jupiter after morning cloud sickness goes away, but I'm sure you will be satisfied with the savings and the performance of the ES

I have never seen any mention by TeleVue that Powermates control Chromatic Aberration. Powermates don't add any but chromatic aberration is defocused light at the ends of the spectrum, red and blue. To correct it, that light would have to be refocused. If those colors are refocused by a Powermate or something else, then that would result in chromatic aberration in color free telescopes.

Jon

[Gert K A]

Jon a question:

I think I read something about the fl of longer refractors reduce CA (in Neil English’s book)
and if a barlow or a focal extender indeed extends the fl of the scope rather than increase the power of a eyepiece,
does it then not stand to reason that also CA will be reduced somewhat with the use of any Barlow?

[MikeBOKC]

I have the 1.25 2.5X Powermate and the two inch 2X ES focal extender and I don't see any appreciable difference between them in performance. Both work well with any of my eyepieces.

[T1R2]

maybe its because of the mag. increase that the brightness is reduced and therefore suppresses the C/A, but I see no C/A at all when viewing Saturn with the AR127 and the FX and a 9mm raised to a 4.5mm at 183X, and there was really only a ghostly trace that takes the shape of the planet and rings without the FX and a 5mm(165x), but Jupiter will be the biggest test when I get a chance to try it for the first time, I guess I just thought I read that and spoke to soon without checking my facts, that could happen to anyone, it was in the wee morning hrs. that I was viewing the TV website.

[Robert Cook]

I think I read something about the fl of longer refractors reduce CA (in Neil English’s book)
and if a barlow or a focal extender indeed extends the fl of the scope rather than increase the power of a eyepiece,
does it then not stand to reason that also CA will be reduced somewhat with the use of any Barlow?

Actually, it does not (aside from any effects of increased magnification or reduced CA of the eyepiece itself). Look at it this way: aberrations can be introduced, corrected, or passed along by any optical element in the system. If CA is introduced (or not fully corrected) by the objective lens of a telescope, then the damage is already done, and all a Barlow is designed to do is resize the image without introducing CA and other aberrations of its own--it just passes along the CA from the objective that is already there. As Jon had pointed out using different words, to actually correct the CA introduced by the objective, the Barlow would have to be designed to introduce reverse CA, in which case it would introduce its own CA when used with a Newtonian, for example, which has exactly zero CA--that would be unacceptable, and clearly this is not the case with properly-designed, general-purpose Barlows and other types of focal extenders. The same goes for other aberrations such as the coma of a "fast" Newtonian, which can be corrected to a meaningful degree using a special correcting lens (e.g. Tele Vue Paracorr) that is very different from a focal extender, which happily passes along coma just like it does CA (hopefully adding none of its own).

I think that people often get confused because of the positive effect that focal extenders can have on eyepiece performance. Eyepieces, just like any other optical element, can potentially introduce their own aberrations, which they try to correct by design before the light exits the eye lens. Generally speaking the "slower" the focal ratio of the light cone entering an eyepiece is, the easier it is for the eyepiece to correct its own aberrations. So while a Barlow/focal extender that is placed before an eyepiece can help the eyepiece itself perform better (sort of like preventative medicine), it cannot help the objective lens, which is placed before it anyway, perform better--all it could hope to do is correct the aberrations already introduced by the objective lens, and as we know by now it is not designed in any way to do this.

[Jon Isaacs]

Jon a question:

I think I read something about the fl of longer refractors reduce CA (in Neil English’s book)
and if a barlow or a focal extender indeed extends the fl of the scope rather than increase the power of a eyepiece,
does it then not stand to reason that also CA will be reduced somewhat with the use of any Barlow?

Gert:

The chromatic aberration in an achromat does depend on both the focal ratio and the aperture, with slower focal ratios, there is a reduction in CA and with smaller apertures there is a reduction in CA. The ratio between the focal ratio and the aperture (in inches) is sometimes called the Chromatic Ratio and can be used to judge the level of chromatic aberration.

There are two general guidelines, the 3x rule and the 5x rule. An achromat with a CR of 3 will have noticeable chromatic aberration but the effect on the high magnification image quality is acceptable. A 4 inch F/12, has a CR of the 3.0, the popular 80mm F/11s have a CR of about 3.8.

The 5x rule is more stringent and achromats at 5x have very little false color and what little there is, does not significantly affect the view. Achromats with a CR of 5 are almost all small, an 60mm F/11, a 3 inch F/16.. a 4 inch that fit this rule would be F/20 and 80 inches long...

That said...

A Barlow/Powermate/focal extender does not actually change the focal ratio of the telescope objective, it does not change the objective's aberrations, coma, chromatic aberration, it cannot correct those. When thinking about the aberrations of the objective, it is best to think of a Barlow as an "eyepiece focal length reducer", it is really part of the eyepiece and not the objective. A 2x barlow divides the focal length of the eyepiece by two, it does not actually increase the focal length or focal ratio of the telescope.

Jon

[Gert K A]

it does not actually increase the focal length or focal ratio of the telescope.

Jon

It really does not, does it, this makes alot of sense
(Oh ya "focal ratio" do I ever get my formulations straight!!?)

Thank you so much both of you for taking the time to set me straight on this.
It makes a lot of sense that a Barlow cannot correct damage already done.

Now I just have to rearrange the knowledge in my brain (not an easy task lol)
The Barlow is part of the optical train but still just an eyepiece after all.. Maybe better yet "part of the eyepiece".
Any improvement I see (short of the maintained eyerelief) could be done with a ½ fl eyepiece
and has if any probably to be ascribed to the dimming effect of spreading the light.


Sigh.. thank you

[lamplight]

Thanks everyone. Astronomics has the ES too

[Gert K A]

Ay Matt did I just hijack your tread? I got caught up in the interesting topic
Sry mate

[lamplight]

Not at all I learned from your question as well!

[T1R2]

Well I wanted to see if the increase in mag would suppress ant C/A so tonight I got the chance to test on Jupiter under very good stable sky, on Saturn this spring the ghostly C/A that was present was all but gone with the FX, on Jupiter tonight there was no visible change, I guess because its still very bright at 183X, but one things for sure, it did not add any.

[Gert K A]

@ Robert and Jon

I just reread your eloquent and precise replies
and I cannot thank you guys enough for your valuable contributions to CN as a whole


As to this question: it is now crystal clear for me that to correct CA in a lens the correcting lens must be tailor made
hence all the fuzz about double and triple APO refractors

[michaeldurban]

this was my first focal extender (barlow..)

on my SW 10" this little 1.25" piece gives me near pinpoint
stars to the edge...
I was impressed, to say the least..

so it still remains my first..and last focal extender..

(mind you.. I'm a barlow virgin, I just like the heft of the
ES range..)

[penguinx64]

The ES focal extenders are a bit heavy.  I have the 1.25 inch 5x version and it weighs over half a pound!  It's heavier than any of my eyepieces.

[GeneT]

 Is the ES as good (ie: transparent) as the powermates are? I had a 2" powermate and it was great, just heavy.

 

As good? No. Good enough? Yes. If you want the best, go with the Powermate.

[gunfighter48]

I have both the 1.25" and 2" ES focal extenders and they are great. I've looked thru the Televue Powermates a few times (don't own one) and I can't say the view was any better thru the Powermates, at least with my eyesight. They are on the heavy side but the ES focal extenders are all I use now. For the price they are excellent.

Edited by gunfighter48, 13 August 2014 - 07:54 PM.

[Starman1]

Jon a question:

I think I read something about the fl of longer refractors reduce CA (in Neil English’s book)
and if a barlow or a focal extender indeed extends the fl of the scope rather than increase the power of a eyepiece,
does it then not stand to reason that also CA will be reduced somewhat with the use of any Barlow?

The barlow doesn't change the image from the main objective.  The EYEPIECE sees a longer f/ratio, and so internal aberrations caused by a short light cone are reduced in the eyepiece, but chromatic aberration and coma and other ills from the objective are still there in the magnified image.

In order to correct the CA from a refractor objective, it would have to do so either in it's internal construction (which Jon pointed out wouldn't make sense) or BEFORE the objective (Hmmm....like a 3rd lens, perhaps? LOL)

[Starman1]

One last comment: the ES focal extenders require more in-travel of the focuser than the comparable PowerMate.

[gdd]

Jon a question:

I think I read something about the fl of longer refractors reduce CA (in Neil English’s book)
and if a barlow or a focal extender indeed extends the fl of the scope rather than increase the power of a eyepiece,
does it then not stand to reason that also CA will be reduced somewhat with the use of any Barlow?

As others pointed out, no that is not the case. But a long refractor can have a focal reducer (or petzal rear element) to reduce the focal length and give you a long but fast refractor with lower CA than a short refractor with no reducer and that same fast focal ratio.

 

Gale

[howard929]

 

 

As others pointed out, no that is not the case. But a long refractor can have a focal reducer (or petzal rear element) to reduce the focal length and give you a long but fast refractor with lower CA than a short refractor with no reducer and that same fast focal ratio.

 

Gale

 

Would that increase the max FOV?

[Starman1]

Of course.  Maximum field of view is inversely related with focal length (i.e. shorter FL = wider fields, and lower powers).

 

My 12.5", with a focal length of 1825mm, maxes out at 1.3 degrees, whereas my 4" f/5.4 apo maxes out at nearly 5 degrees!

If you enjoy wide fields, shorter focal lengths are the way to go--you can always obtain high powers with a barlow (and eyepieces

even go down to 2mm these days).

 

But, remember that if it's an achromatic refractor, you want the f/ratio to be at LEAST 3X the size in inches (i.e. 4" f/12, 6" f/18) to

keep chromatic aberration in check (and preferably 5).  A 2-element ED "semi-apo" refractor can be a little shorter, but only triplets and quadruplets

can be made in really short f/ratios and get away with it.  Fortunately, some are affordable these days.

 

If the refractor is to be used for primarily DSO viewing (not Moon or planets), then an achromat can be short, too.

I've viewed extensively with a 5" f/5 and a 6" f/5 and both did fine on DSOs (though both were HORRIBLE on the Moon and planets).

 

All that explains the popularity of newtonian reflectors--no chromatic aberration.

I used to own a 6" f/5 newt, and I loved that scope.  It burned up in a fire, though.

[howard929]

Thank you Don. Being a card carrying member of the Dob Mob I never venture to the alien shores of the Refractor forum (frightening!!) here on CN but that is good to know.