Big Barlow in Small Refactor? Is this a No no

[bond19]

I know Tele Vue themselves suggest a small fact requires a Powermate for best performance.

Would using a x 2 Big Barlow (TV) in a 102mm frac really be a bad idea? Would I really notice the difference between the two?

[rwilkey]

The TV Barlow should work well in your telescope, as it does in any telescope, the size of the scope makes little difference, it's the quality of the optics that counts, and certainly your TV 102mm will respond nicely to the x2 Big Barlow when used with the right ep.

[John]

Personally I'd take Tele Vue's advice seriously - they designed the thing.

The TV Big Barlow is one of the few TV products that I have read mixed reports on. I've owned a couple of 2x 2" Powermates and they are absolutely superb. You would really not know they are in the light path and they preserve the full field of view and focal position of even 100 degree eyepieces and worked really well in both my refractors and my newtonian.

[RT65CB-SWL]

+1 for Robin's comment.

TeleVue barlows & powermates are very good (as are their other eyepieces and other optical hardware). I personally use Klee 2.8x which have god reviews.

I also use an AstroEngineering ImageMate 4.0x (for webcam astrophotography when I eventually get round to it) - there have been mixed comments & reviews about it on various other astro forums. Another 'useful' add-on is an AstroEngineering 1.6x MagniMax (these simply screw on to the nosepiece of any 1.25" e/p).

[swamp thing]

+1 for Johns comments 

The TV 2" x2 powermate is superb. Ive said it before and I'll say it again......best thing in my eyepiece case.

[Qualia]

+ 1 John x + 1 Robin = A middle ground suggestion Use your TeleVue x2 Barlow and gradually save up for the Powermate After, you can make a comparison and write your findings for SGL and that would be a double plus for everyone

[bond19]

I may just do that Qualia, setup up gradually and report back.

In the meantime I'm going to see if I can source a used x2 PM.. I sadly don't have access to the used section here just yet.

[John]

The UK Astro Buy & Sell site is the place to look. You can place a "wanted" advert there too:

http://www.astrobuysell.com/uk/

Good luck

[bond19]

Good suggestion John, I've put a wanted ad up there now

[Dave In Vermont]

I normally try not to recommend TeleVue to people asking what EP's are good/best. I avoid howling my praise for TV's as there are plenty of other people out there who already heap praise on them. My main reason for my lacking joining the choir is my knowledge that not everyone can afford the Lamborghini's of the eyepiece world. However.....

I make an exception to my rule regarding Barlows. Here I do indeed heap my praise on the TeleVue Barlows. Both the 2X and 3X are excellent. And when it comes to tossing added glass into the optical-train, I want that which is the closest thing to being invisible - e.g. not dimming the view and/or diminishing the clarity and contrast - as can be found. Enter the TeleVue Barlow for winning the title! If I hadn't put these into the pathway, I wouldn't of really noticed it's being there at all. Almost.

The other day I was examining the contents of one of my plethora of cases of astronomical equipment. I have so many such cases that it's like being a kid at Christmas-time! I'm always finding surprises in these aluminum-boxes. And the other day my "present" was a brand-new TeleVue 2.5X Power-Mate. Ho! Ho! Ho! Thanks, Santa!

I finally remembered why I had tucked this away with some other gear that was destined for an imaging system I didn't have time to complete. So I have been putting the Power-Mate through it's paces. And now I can say that the Power-Mate, too, will get an emphatic cheer from me to anyone looking for my advice on Barlows - and related entities. Did you hear that, TeleVue? I'll expect my commission-cheques made out in US Dollars, please.

Clear & Dark Skies (unless you're into Solar observing/imaging),

Dave

[MoonNut]

This is a great thread!

 I am just trying to decide which to get, a TV Barlow or a TV Powermate, exactly what is the difference between a Barlow and a Powermate?

[RT65CB-SWL]

This is a great thread!

I am just trying to decide which to get, a TV Barlow or a TV Powermate, exactly what is the difference between a Barlow and a Powermate?

Quote from TeleVue*... 'Powermate™ vs. Barlow

To understand the Powermate™, we must first understand the Barlow lens. Barlows amplify the power of a telescope. They can be considered "focal reducers" for eyepieces, or "focal extenders" for objectives. Terence Dickenson, in his Barlow test report in Sky and Telescope, July 1997, says: "Technology has erased the old objections. A modern Barlow will not degrade your telescope's optics. Anyone telling you otherwise is using outdated information. Moreover, the highly regarded Nagler eyepieces and their clones have built-in Barlows, ample evidence that the lens is not some detrimental intruder." Thanks, Terence, for laying the myth of the degrading Barlow to rest.

A high quality Barlow must be properly designed and manufactured in order to avoid compromising a telescope's color and spherical aberration corrections. The "invisibility" of Tele Vue's 2-element Barlows has been noted in test reports in the astronomy magazines. However, by the nature of its negative power lens, a Barlow will do more than just increase magnification, regardless of the number of elements.

The diverging rays leaving the Barlow result in moving the exit pupil further out, thereby extending the eye relief. In short to medium focal length eyepieces the change is not noticeable. However, in the case of long focal length eyepieces, the increase can be significant and not without performance consequences.

Vignetting can occur due to the altered ray path, when the eyepiece's lenses are not large enough in diameter to allow all the rays to make it through. Shorter Barlows, or ones with too much magnification, only exacerbate the problem because the ray path entering the eyepiece is steeper.

We had to go beyond the Barlow concept to achieve the goal of a compact, high power, fully corrected image amplifier. The Powermate™ consists of a negative doublet plus a positive "pupil-correcting" doublet. This 4-element system provides the magnifying function of a Barlow without its limitations by restoring the field rays back to their original direction, as if the Powermate™ were not there. The result is a pure magnification increase.

* www.televue.com

[John]

If you want Powermate type performance and characteristics at a barlow type price the Explore Scientific Focal Extenders are really good as well   

[bond19]

My wife just asked me what I want for my Birthday end of Nov. Can any guess what I said??