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Equipment Discussions >> Eyepieces

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russell23
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Loc: Upstate NY
Siebert regular or telecentric barlow? new
      #3571215 - 01/19/10 07:43 PM

I'm looking at getting a 1.25" 3.5x Siebert barlow. I have a 2" 2x Siebert Telecentric that is very good. My question is whether or not I would see a noticeable difference in performance between the regular barlow and the telecentric. I'm aware that the telecentrics preserve the performance characteristics (eye relief etc.) better than the regular barlows. Setting that aside would I notice any difference in image quality?

Dave


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Mike B
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Siebert regular or telecentric barlow? new [Re: russell23]
      #3571291 - 01/19/10 08:10 PM

Hi Dave-

I've used a Siebert 1-1/4" "Modular" Barlow with excellent results... i'm sure the fixed FL Barlows would work equally well. Also picked up his 2.5x Telecentric Barlow (2") used & it works wonderfully, as well! Both seem invisible in actual use, so i guess it'd be hard to say one is "better"

Funny thing is, with all the flexibility the "modular" one provides, i've ended up using it at its 1.7x configuration almost exclusively... so i probably overpaid for the use i'm getting. But afaic its a top-drawer piece of glass, so its all good!
mike b


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dscarpa
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Re: Siebert regular or telecentric barlow? new [Re: russell23]
      #3574638 - 01/21/10 11:23 AM

I haven't used a Siebert telecentric but I do have a 1 1/4" 1 1/2X barlow and I'd rate it optically as good as my TV Powermate and barlows. Light transmission and sharpness are excellent, it's like it's not there. David

Edited by dscarpa (01/21/10 11:33 AM)


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russell23
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Reged: 05/31/09

Loc: Upstate NY
Re: Siebert regular or telecentric barlow? new [Re: dscarpa]
      #3574741 - 01/21/10 12:20 PM

Quote:

I haven't used a Siebert telecentric but I do have a 1 1/4" 1 1/2X barlow and I'd rate it optically as good as my TV Powermate and barlows. Light transmission and sharpness are excellent, it's like it's not there. David




Thanks! That's what I was wondering. If it is as good as a powermate then I would think it is as good as a telecentric and they cost a lot less. My 2.4x Dakin barlow is excellent and also very transparent, but I'd like some other magnification options and Siebert seems to have an excellent barlow reputation.

Dave


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Mike B
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Loc: shake, rattle, & roll, CA
Re: Siebert regular or telecentric barlow? new [Re: russell23]
      #3574771 - 01/21/10 12:34 PM

Dave-
I'd recommend calling him up to discuss options- he'll make one to whatever spec you want. You may even want to consider a mudular or variable type design? That he absolutely loves what he does, even fanatically so, comes thru VERY clearly!


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BillP
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Reged: 11/26/06

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Re: Siebert regular or telecentric barlow? new [Re: russell23]
      #3574952 - 01/21/10 01:59 PM

There will be a difference (physics) as the telecentric design is 4 elements in 2 groups whereas the barlow design is 2 elements in 1 group. So very very slightly better transmission and scatter characteristics with a Barlow. However, as real as these differences may be, from a practical standpoint they would more than likely be either below the visual threshold or would only rear their head as being noticable under very narrow circumstances.

I also have both a Siebert Barlow and Telecentric. Like them both and both very sharp and transparent. However, unless you are looking to use them on a longer focal length wide field type of eyepiece, the Telecentric/Powermate options I feel are of little use. I generally tend to use these add-ons for eyepieces of 20mm or shorter focal length and in this range I have found that Telecentrics have little if any advantage, so don't need the extra glass or the extra costs.


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Mike B
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Siebert regular or telecentric barlow? new [Re: BillP]
      #3575020 - 01/21/10 02:33 PM

Quote:

I have found that Telecentrics have little if any advantage, so don't need the extra glass or the extra costs.



In my case i wasn't even planning on a 2.5x amplifier, but spotted the 2" Siebert Telecentric for an amazing *used* price!

But now that i have it, its come in VERY handy for larger 2" EPs, and has given some amazing hi-power views when coupled with a 12mm T4 Nagler in my Dob (361x). Couldn't really say as to its performance compared to a std. 2" 2.5x barlow, as i've never tried one- would no doubt take a direct A-B comparo to ferret out any differences.

My point, tho, is if one were interested in such a varmint, i'd recommend watching the *used* ads here & on AM... they DO pop up now & then, so with a little patient watching...


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russell23
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Re: Siebert regular or telecentric barlow? new [Re: BillP]
      #3579031 - 01/23/10 02:39 PM

Quote:

There will be a difference (physics) as the telecentric design is 4 elements in 2 groups whereas the barlow design is 2 elements in 1 group. So very very slightly better transmission and scatter characteristics with a Barlow. However, as real as these differences may be, from a practical standpoint they would more than likely be either below the visual threshold or would only rear their head as being noticable under very narrow circumstances.

I also have both a Siebert Barlow and Telecentric. Like them both and both very sharp and transparent. However, unless you are looking to use them on a longer focal length wide field type of eyepiece, the Telecentric/Powermate options I feel are of little use. I generally tend to use these add-ons for eyepieces of 20mm or shorter focal length and in this range I have found that Telecentrics have little if any advantage, so don't need the extra glass or the extra costs.




That sounds like excellent advice (as always) Bill. I currently have a 2x 2" Siebert Telecentric that I picked up used and a 2.4x Vernonscope Dakin barlow. I'm thinking about getting a 3.5x Siebert so that I can push magnification over 200x for lunar observing. It sounds like the telecentric would not be needed for that. I can stack my 2.4x Dakin inside the 2x Siebert and get 300x with my 16mm eyepiece. Those 2 barlows happen to stack very nicely -- but I'm adding 6 elements to the optical path when I do that. And at 300x the exit pupil is less than 0.5mm and any dust on the eyelense of my eyepieces becomes obvious.

You guys have settled the issue for me. When I'm ready I'll go for the regular barlow.

Dave


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russell23
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Re: Siebert regular or telecentric barlow? new [Re: Mike B]
      #3579038 - 01/23/10 02:43 PM

Quote:

Quote:

I have found that Telecentrics have little if any advantage, so don't need the extra glass or the extra costs.



In my case i wasn't even planning on a 2.5x amplifier, but spotted the 2" Siebert Telecentric for an amazing *used* price!

But now that i have it, its come in VERY handy for larger 2" EPs, and has given some amazing hi-power views when coupled with a 12mm T4 Nagler in my Dob (361x).




I have a couple of 2" eyepieces that show some astigmatism in the outer FOV. I've noticed that when I put them in my 2x Telecentric barlow the edge performance improves dramatically. For that reason alone I think I'll be keeping the telecentric barlow and just adding a regular 1.25".


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russell23
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Loc: Upstate NY
Re: Siebert regular or telecentric barlow? new [Re: russell23]
      #3579069 - 01/23/10 02:58 PM

I'm considering picking up a Meade 8.8mm 5000 UWA. Would the field of this eyepiece experience some vignetting in a 1.25" barlow?

Dave


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BillP
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Re: Siebert regular or telecentric barlow? new [Re: russell23]
      #3579154 - 01/23/10 03:53 PM

I get no obvious light loss at edge of field or vignetting when using the 8.8 with any of my Barlows.

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stratocaster
sage
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Reged: 10/27/11

Re: Siebert regular or telecentric barlow? [Re: BillP]
      #5558133 - 12/06/12 07:27 PM

I thought I'd resurrect this thread.

I've been pondering a 2" barlow for quite a while. I read a comment on another thread about the ES four element tele-extender. I'm assuming this would be a telecentric barlow.

I like ES products a lot. The problem is that the ES barlow is heavy at 23 oz. This may be a big deal as I'm already having balance concerns with an ES30 82 and ES 20 100 in my dob. Throw in a paracorr and a heavy barlow and we have issues.

In light of my recent reading of the ES barlow I thought I'd compare the specs of the ES to the powermate and the Siebert. The powermate is around 16 oz without the 1.25" adapter. Much better than the ES. But the 2" Seibert telecentric is only 8.5 oz?! In light of the high quality I've read about Siebert optics this almost makes it a slam dunk for me.

But now there's that question of standard barlow or telecentric. I've read the telecentrics act as a field flattener. This would be useful when using my 100 degree eyepieces in my refractor, as I've noticed field curvature using the 100 degree eyepiece (though one would think that would induce FC in my dob, so I'm not sure I understand that).

I've also read that telecentric barlows may circumvent the need for a paracorr, but I'm not sure this is true. Can anyone confirm? If so, then at least if I barlow I don't need my paracorr, which further addresses my balance issues and would steer me towards the telecentric.

I was also leaning towards a telecentric initially due to some long focal length eyepieces I have - a Pentax XW 5mm and 7mm.

Bill, I believe you have some XWs and both the telecentric and standard Siebert barlows. So I'm curious as to your thoughts here, as well as any others who may have input on telecentrics flattening the field and eliminating the need for a paracorr.


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Mike B
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Re: Siebert regular or telecentric barlow? new [Re: stratocaster]
      #5559264 - 12/07/12 01:01 PM

Quote:

...input on telecentrics flattening the field and eliminating the need for a paracorr.



Couldn't say re: field flattening, but i'm pretty sure that NO amplifier, telecentric or other, corrects for coma. Otherwise, they'd be inducing coma in a non-coma scope!

Possibly what is being suggested, however, is that image amplification itself may reduce coma, or its appearance- and i believe this is true. As i understand it, you're magnifying the coma "free" zone to a larger degree, so more of the field seen will be "free" of (ie. less affected by?) coma. And this is what my eyes tell me, too.

So many times, when using an amplifier for achieving ~300x or more, i'll skip the ParaCorr.

Interestingly, the Siebert 2.5x telecentric i mentioned earlier in this thread (some time ago, too!) i subsequently *field-measured* as actually producing about 2.0x... so possibly had been mislabeled? But it was eventually replaced with a std. 2" Barlow, one of the older U.O. lonnnng versions... i found it just a wee bit sharper when used with my Naglers, and now with my Ethos & ES100. I see OPT has one of these same Barlows in their "used" section.


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catboat
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Reged: 12/01/09

Loc: Maine
Re: Siebert regular or telecentric barlow? new [Re: Mike B]
      #5559661 - 12/07/12 05:39 PM

Quote:

I see OPT has one of these same Barlows in their "used" section.




Hmmm… $99? Those UO 2-inchers sold for $55 new in the late 90's. They pop up now and again on AM for about $50. It's a good, clean barlow -- but big, as you say. I've rarely used mine because I've never had much in the way of 2" eps. I may never own an Ethos, but at least I have the right barlow for it!

Edited by catboat (12/07/12 05:41 PM)


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Mike B
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Loc: shake, rattle, & roll, CA
Re: Siebert regular or telecentric barlow? new [Re: catboat]
      #5559780 - 12/07/12 06:36 PM

Quote:

Those UO 2-inchers sold for $55 new in the late 90's. They pop up now and again on AM for about $50.




Really? I don't think i've seen hardly any Amart versions, and the ones i *have* seen were like $75 or so... do you have a link to old UO ad copy showing one of these? I'd be really curious to see.


Even so, if sold "new" today, i've gotta figure they'd be quite a bit more than $55... probably a notch more than $99, too.


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catboat
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Re: Siebert regular or telecentric barlow? new [Re: Mike B]
      #5559862 - 12/07/12 07:40 PM

Here's the only link I have in my "archives" and I'm not certain of the date. It's 1998 or later, judging from the web copyright. My memory of the price when I bought (1996?) could be off. I remembered it as $55 or perhaps $59. But the brain is the first thing to go… Could it have been $79? Sure.

UO catalogue link

On AM, I'd noted the prices were more like $50. Seeing those prices, made me think that no one was much interested in these big 2" barlows that weren't TV and didn't have detachable lens units like the GSOs. Anyway, I figured (with a sigh) that if it ever came to selling mine, it would probably go for cheap.

So I was surprised to learn from your post that this overlooked UO could be the "go to" barlow for the mighty Ethoi. Makes me feel prepared! The OPT price was also a surprise, but perhaps it's not out of line.

Hope I haven't gone too far off topic from Dave's original query. I never see these UO 2" barlows mentioned, so I jumped in.


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Mike B
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Re: Siebert regular or telecentric barlow? new [Re: catboat]
      #5560153 - 12/07/12 11:25 PM

Hey- thanks, Ed!

Yup, there she is- down near the bottom... $90 new in 12/98.

When i first picked this one up *used*, i did quite a bit of web searching... wasn't much on it, but what WAS there gave it high marks! So i took a chance; have not been disappointed.

I also have an AP 'Barcon', which works at 1.5-1.6x, depending on the EP. The UO cranks at 2.0-2.1x, again, depending. These two are my BIG amplifiers, and they are just 'bout invisible in use. They give both my 13E & 9ES 100's extra leverage, with no bad behavior exhibited.


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