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2 inch Barlow for 2 inch diagonal


25585

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Having been lucky enough to get a Celestron Ultima Japanese-made 2x Barlow for my eventual bv buy, I am wondering what good 2 inch Barlows there are that would work with a 2 inch diagonal. TV do not recommend their own.

My 2 diagonals are both 2 inch, an older TV one which came with my Genesis, and an Astro Physics one. I don't want to exceed 2x magnification nor have any CA. So what choices are there please?

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Would it be correct to assume that you would prefer a barlow rather than a Powermate, TeleXtender or Focal Extender ?

The best 2" true barlow that I have used is the early version Antares 1.6x which worked very well with Ethos eyepieces. The later versions of this barlow have compression fittings and filter threads but require much more inwards focuser travel than the earlier version so I could not reach focus with the later design, unfortunately.

I did try a GSO 2" ED barlow but found that did introduce some CA and more scatter than I was prepared to tolerate. The AP Barcon barlows have a superb reputation but I've not tried one personally.

I ended up using a Powermate 2" 2x which is the best amplifier that I've used. I think the Meade TeleXtender and ES Focal Extenders come close though. I appreciate that these devices might not be what you are looking for though.

 

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1 hour ago, John said:

"Would it be correct to assume that you would prefer a barlow rather than a Powermate, TeleXtender or Focal Extender ?

The best 2" true barlow that I have used is the early version Antares 1.6x which worked very well with Ethos eyepieces. The later versions of this barlow have compression fittings and filter threads but require much more inwards focuser travel than the earlier version so I could not reach focus with the later design, unfortunately.

I did try a GSO 2" ED barlow but found that did introduce some CA and more scatter than I was prepared to tolerate. The AP Barcon barlows have a superb reputation but I've not tried one personally.

I ended up using a Powermate 2" 2x which is the best amplifier that I've used. I think the Meade TeleXtender and ES Focal Extenders come close though. I appreciate that these devices might not be what you are looking for though."

    

John, i have read elsewhere about the GSO having CA. Ideally a 2 inch for a BV gives extra sturdiness. Extenders are more expensive, I would love the Barcon but to justify its & others cost, I would need to use it a lot. 

For BV use obviously a length modifiable Barlow would be better for different scopes, but as a compromise are there any mid-length quality models around?

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I must say that I don’t really “get” 2” barlows. 2” eyepieces are heavy enough on their own and most people will have 1.25” EPs in the same focal length range as a 2” EP + Barlow would produce. In the 1.25” format barlows make a lot more sense to reach higher powers or fill in gaps. 

The Celestron Ultima barlow is excellent - you did good there. In fact, it is the only part of my original kit from ~20 years ago that is still with me. You may want to replace the set screw with an M3 nylon bolt though as I did. No more worries about scratched EPs! 

Being a “shorty” barlow the magnification varies quite a lot with the distance between the shoulder of the barlow and the eyepiece focal plane. I’ve measured it to be about 2.2+0.02*f, where f is the distance to the focal plane. With most EPs it is therefor closer to a 2.2x barlow, with most TeleVue EPs (f=-6.3mm) it is about 2.07x. Add some extension between the barlow and the eyepiece (or camera) and you can easily adjust the magnification.  

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On 05/01/2018 at 16:21, 25585 said:

Having been lucky enough to get a Celestron Ultima Japanese-made 2x Barlow for my eventual bv buy, I am wondering what good 2 inch Barlows there are that would work with a 2 inch diagonal. TV do not recommend their own.

My 2 diagonals are both 2 inch, an older TV one which came with my Genesis, and an Astro Physics one. I don't want to exceed 2x magnification nor have any CA. So what choices are there please?

Hi,

by "bv", do you mean binoviewing?

if so, then the last thing you need is a 2" Barlow.

which binoviewer are you looking to buy?

you will want to keep the lightpath of your binos as short as possible to give greatest chance of achieving focus with them.

you may need to drop to a 1.25" diagonal (due to the much shorter light path), I know that I did.

if so, then a 1.25" Barlow will be needed for the 1.25" diagonal. My Baader 1.25" T2 diagonal accepts both 1.25" and 2" front (not sure what the part is called) so I can insert it into a 2" focuser.

https://www.teleskop-express.de/shop/product_info.php/info/p605_Baader-T2-diagonal-prism-90----dual-T2-connection---short-back-f.html

they do both prism or mirror diagonal. There is also the more expensive Zeiss variant. You can find them all at teleskop-express (and buy with confidence from them too, they are a superb company in my experience, delivery is super quick from Germany). I have the Baader Zeiss 1.25" prism that I use with my Baader markV binos (they connect direct together) FYI.

https://www.teleskop-express.de/shop/product_info.php/info/p481_Baader--16-Adapter-from-2--to-T2---2--focal-adapter.html

https://www.teleskop-express.de/shop/product_info.php/info/p480_Baader-T2-Adapter-for-1-25--focuser---from-1-25-to-T2-thread.html

I can't speak highly enough of the flexibility of the T2 system and wonder what specifically are you trying to achieve with the 2" Barlow and binoviewer combination?

I read somewhere else that you were going to binoview with an SCT.

if you get the 1.25" Baader T2 diagonal and the 2" T2 front piece then this will go into your 2" visual back. Depending on which binoviewer you have then you need another T2 fitting to connect the bino to the diagonal.

For any other bino, just make sure you get a 2" nosepiece so you can push it into your 2" diagonal. (But the lightpath will be extra 30-40mm because of the 2" diagonal). As the binos only accept 1.25" eyepiece then best to lose the 2" diagonal and save the light path length.

If you don't control the length of the light path then you will impact the focal length of your scope by a greater amount and you will get more and more magnification at the eyepiece.

Do you know which binoviewer you are buying, start with this choice first?

No binoviewers are 2" at the front end, so only a 1.25" barlow size is needed. Many binoviewers have specific barlows (called GPC, glass path corrector) designed specifically for them so a fancy expensive Barlow is ott.

Alan

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I wouldn't disagree with anything Alan has said, optical path length is always a challenge when trying to reach focus with binoviewers. I can't recall how much inwards focus there is with a Genesis, but given that it has a reducer/flattener at the rear it is possibly limited although it does work fine with a 2" diagonal and standard eyepieces obviously.

Regarding 2" barlows, the best I have used, and one I still have, is an AP Barcon. Very good quality and with extention tubes will go up to x4 without degradation. I use mine with the Tak which does have plenty of inwards focus, mainly with a Baader CoolWedge and binoviewers for white light solar. It reaches focus with no problem.

Well worth checking out the backfocus situation on the Genesis though before proceeding. They are wonderful widefield scopes and, of anything, I would be trying to use it natively with the BVs for low power views.

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Hi all, I would like to be able to use bino viewers on all my scopes! Mainly the TV Genesis (Petzval 500mm) and 2nd (other extreme) C8 SCT with 0.63x field flattener & 2 inch visual back. But mainly the Genesis which with binoviewer would be using 20mm eye pieces for 25x magnification, which I hope could be Barlowed 2x to give 50x.

Long ago I chucked away the Celestron diagonal that came with my C8. 

So using a 2 inch diagonal and a bino viewer (say Baader Maxbright)  with or without a Barlow, what would I also need? (TV BV is too expensive for first buy).

 

 

 

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3 hours ago, 25585 said:

Hi all, I would like to be able to use bino viewers on all my scopes! Mainly the TV Genesis (Petzval 500mm) and 2nd (other extreme) C8 SCT with 0.63x field flattener & 2 inch visual back. But mainly the Genesis which with binoviewer would be using 20mm eye pieces for 25x magnification, which I hope could be Barlowed 2x to give 50x.

Long ago I chucked away the Celestron diagonal that came with my C8. 

So using a 2 inch diagonal and a bino viewer (say Baader Maxbright)  with or without a Barlow, what would I also need? (TV BV is too expensive for first buy).

 

 

 

The C8 will have no trouble reaching focus with only a mild increase in focal length (10% maybe?).  The Genesis will probably need a 2x nosepiece from a barlow or an OCA/GPC element to reach focus.  You'll need about 100mm minimum of available back focus to begin to reach focus without some sort of optical help.  Most refractors don't have this much to spare.  Using a T2 connected 1.25" prism diagonal would maximize available back focus if you wanted to try to reach focus natively.

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11 hours ago, JeremyS said:

Anyone any experience of the Celestron Luminos 2.5x 2" Barlow Lens?

And any ideas why they went for 2.5x, rather than the more conventional 2x format?

Jeremy

I have one and love it. No noticeable CA that I can tell, it's a five element lens. It looks huge, but with the 1.25 adapter out and a 2" EP in it, it's fairly short for its girth. Not sure why it's a 2.5X, but they do (Barlows in general) come in quite a few magnifications, from 1.5 to 5X.

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8 hours ago, 25585 said:

Hi all, I would like to be able to use bino viewers on all my scopes! Mainly the TV Genesis (Petzval 500mm) and 2nd (other extreme) C8 SCT with 0.63x field flattener & 2 inch visual back. But mainly the Genesis which with binoviewer would be using 20mm eye pieces for 25x magnification, which I hope could be Barlowed 2x to give 50x.

Long ago I chucked away the Celestron diagonal that came with my C8. 

So using a 2 inch diagonal and a bino viewer (say Baader Maxbright)  with or without a Barlow, what would I also need? (TV BV is too expensive for first buy).

I would start by assuming the 2" diagonal route is not going to work as you want on your refractor. Don't buy anything assuming it will work or you may end up wasting your money. In the end you can just throw bigger and bigger Barlow in and eventually reach focus but with massive magnification.

Best bet is to buy the bino's first then experiment with the barlows, diagonals that you already own to try to reach focus. You will probably fail but you will know if you are close to focus on nowhere near! (Adding the bino just added about 4" of outocus !)

You can test them out with one eyepiece too. You don't need two eyepieces to test if you can reach focus!

If you go Baader then the first step to save light path is to get a 1.25" T2 diagonal and the T2 connections required to mate with the bino and allow it to fit into the focuser. (This step saves about 2" of infocus). They have many variants and wide price range.

Now, to save more light path you need to add a Barlow or glass path corrector. I would experiment with cheap barlows to find what size barlow you need then go buy the appropriate glass path corrector instead (I would not buy a Barlow at all, gpc is much neater).

The glass path correctors screw inside the Baader binos and add nothing to the light path. A huge external barlow is only adding to the light path and reducing its effect.

Here is a link to all the maxbrights accessories 

https://www.teleskop-express.de/shop/index.php/language/en/manufacturers_id/5/filter_id/60/astro/Baader.html

I already posted you links for the T2 diagonal and accessories

Alan

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2 minutes ago, alanjgreen said:

The glass path correctors screw inside the Baader binos and add nothing to the light path. A huge external barlow is only adding to the light path and reducing its effect.

The GPCs also correct for CA induced by the binoviewers so are a ultimately a better route than a barlow anyway as Alan says.

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7 hours ago, Luna-tic said:

I have one and love it. No noticeable CA that I can tell, it's a five element lens. It looks huge, but with the 1.25 adapter out and a 2" EP in it, it's fairly short for its girth. Not sure why it's a 2.5X, but they do (Barlows in general) come in quite a few magnifications, from 1.5 to 5X.

Many thanks Luna-tic. Good to know!

Jeremy

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