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Borg Fluorite Scopes: 71Fl, 72FL, 90FL, 106FL, 107FL, ...

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#1 Rich Caruana

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Posted 21 July 2020 - 11:46 PM

Picked up a Borg 71FL on AM earlier in the year and am very happy with it. It outperforms my Tak FS-60CB/CQ, yet is just as small, lightweight and packable. My TV76 outperforms the 71FL, but the 76 is a bit heavy. So I bought a 90FL in the hopes of having my cake and eating it too, i.e., a scope that's lighter than the TV76 and breaks into small pieces for travel, but with even better performance.

 

Have only had the 90FL out a few times, but it looks good. In side-by-side tests the 90FL is sharper than the TV76 on Jupiter, Saturn and the moon. And it sees deeper because it collects 40% more light. In mediocre seeing the 90FL even compares well to a 92mm Stowaway, but in excellent seeing the AP92 pulls away. The Stowaway is a f/6.6 triplet, while the Borg is a faster 5.6 (fluorite) doublet, so maybe it's not a fair comparison. The Stowaway is also longer, heavier and much less packable.

 

So I'm pretty happy with the Borg 90FL. It falls between the TV76 and AP Stowaway in performance, yet is lighter and more packable than the TV76. Now I'm wondering if a 107FL would outperform the Stowaway but in a package comparable to the TV76 in weight and packability. I'm mainly a visual observer and this would be for use as a travel scope.

 

Any thoughts?

 

-Rich.


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#2 Tyson M

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Posted 22 July 2020 - 01:18 AM

If you enjoy it Rich it sounds like a great travel scope.  I have been tempted myself on this one but a few well known CN members bought some and had less than  favorable opinions on it.  

 

The 107 would be very cool indeed. These scopes are expensive though with their modular design



#3 marcus_z

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Posted 22 July 2020 - 05:51 AM

It highly depends on your expectations about these scopes. The Borg 90FL and 107FL are not color-free, even though they use fluorite lenses. What I find remarkable is that the 107FL is not that far behind the 90FL in color correction, or even at the same level. At some point I will have to compare these side-by-side to tease that out. Also the 107FL has a sliding tube mechanism that makes its storage length about as short as the 90FL. Actually, the 90FL is so short that it fits in a small case without even disassembling it. The only shortcoming of the 107FL is that the balance point is in front of the 80mm wide tube section part, which requires extenders for the tube rings and a longer dovetail. That made me order a 115mm tube and custom FTF30xx adapters for my 107FL. Unfortunately, I'm still waiting for Starlight Instruments for them to manufacture the adapters. There's also the FTF32xx option, but that's a little bit too much in my eyes for a visual setup.

 

So will the 107FL outperform the Stoway 92? It will depend highly on the targets and user perspective! I didn't have the Stoway in my hands, but I expect a perfect color correction and straylight control, but (due to aperture) less details. I can already see the resolution advantage of the 107FL over a 100mm scope, so 107mm vs. 92mm (15mm difference) will be easy to see. What the 107FL in the 80mm tube lacks a bit is straylight control. I expect a higher contrast of the Stoway on brighter objects like the moon. For deep-sky this will not be significant. In terms of mount requirements, the scopes have similar weight, but I prefer to make sure that weights with dovetails and tube rings are compared. I'm not sure if the weight specifications of AP and Hutech include which accessories, but they might be close.


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#4 bobhen

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Posted 22 July 2020 - 06:39 AM

I used a Borg 90mm Fluorite for a month and the optics (optical quality independent of size) did not compare with my Tak TSA 120. So as some others have said, it will depend on your use.

 

For critical high power observing I would not choose the Borgs. For wide field deep sky observing, night and day imaging, as astrographs, for extreme portability, even perfect airline portability and “casual” (rather than critical) lunar and planetary observing is where I think the Borgs come to play.

 

Although lightweight, the quality of the mechanics is really nice.

 

In the end, my feeling is the Borgs have pushed the fast doublet design (even with Fluorite) just a little too far to be the perfect solution for all things.

 

Bob


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#5 Rich Caruana

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Posted 28 July 2020 - 01:20 AM

@Tyson --- I think Marcus got it right in #3: it depends on your expectations.  My sense is that Borg FLs punch well above their weight, but somewhat below their aperture.  That might be the optimal trade-off for a travel scope, but suboptimal for use at home where size and weight are less critical.

 

@Marcus --- thanks for the detailed info about the size and balance of the 107FL.  Completely agree weight comparisons need to include rings and dovetail.  Question about the balance of the 107FL: do you think if you used 80mm rings on the series 80 tube, and put the rings on the back half of an 8" dovetail with the front 4" of the dovetail extending under the lens assembly (with risers to lift the scope high enough so the dovetail fits under the lens), that it might balance with the mount attached on the dovetail under the lens?  Probably not optimal for stability, but for a travel scope it might be acceptable if not doing AP.

 

@Marcus --- as you suggest, the Stowaway has remarkable color correction and contrast.  When traveling I'm happy with 20X-150X, and don't find color in the 90FL to be an issue at 150X, but it probably reduces fine detail contrast compared to the AP92.  I'm wondering if the 107FL would be loafing so much at 150X compared to 90mm scopes that this might improve MTF enough that the 107FL would have contrast and sharpness equal to or better than the Stowaway, but in a lighter package that's easier to pack?

 

@Bob --- completely agree that the Borg FLs aren't the right scopes for critical high power viewing, particularly for use at home where saving a pound or two isn't so important.  But pound-for-pound, and packable inch-for-inch, the Borgs may actually be the right answer, even for high power viewing.  Maybe the extra aperture will allow a 107FL to show more detail than an AP92 even though the AP92 is essentially perfect and the 107FL isn't?  Did you get to compare your 90FL to excellent smaller scopes to get an idea where it fits, i.e., did the Borg perform like a perfect scope with 10% less aperture?  I wonder how my 90FL would compare to a TV-85, or a stopped-down Stowaway?  I already know it easily outperforms my TV-76.

 

Thanks for all the replies.  -Rich.



#6 bobhen

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Posted 28 July 2020 - 07:02 AM

 

 

@Bob --- completely agree that the Borg FLs aren't the right scopes for critical high power viewing, particularly for use at home where saving a pound or two isn't so important.  But pound-for-pound, and packable inch-for-inch, the Borgs may actually be the right answer, even for high power viewing.  Maybe the extra aperture will allow a 107FL to show more detail than an AP92 even though the AP92 is essentially perfect and the 107FL isn't?  Did you get to compare your 90FL to excellent smaller scopes to get an idea where it fits, i.e., did the Borg perform like a perfect scope with 10% less aperture?  I wonder how my 90FL would compare to a TV-85, or a stopped-down Stowaway?  I already know it easily outperforms my TV-76.

 

Thanks for all the replies.  -Rich.

I did not compare the Borg 90 FL to anything like a TV 85 or AP Stowaway.  See Ed Ting’s comparison between a Sky 90 and a TMB 92mm F5.5 below

 

But judging by the star test, I would be surprised if the Borg 90 could compete with a Stowaway for high power observing.  I think the Borg 89 (do they still make that scope?) would be the better choice for a small Borg scope when used for high power observing.

 

I thought the Borg 90 would give me high power capability and low power deep sky visual use with my image intensifier, and all in a lightweight package. Well it did deliver two out of three. In the end, I decided to keep my inexpensive 102mm F5 achromatic refractor for low power deep sky observing with the intensifier and use the heavier Tak 120 on an alt/az mount (instead of a GEM) for an easier set up for short, high power, lunar/planetary sessions.

 

This is only a guess but it’s just that the Borg “very fast” doubles just get softer than triplets of the same speed and especially triplets that are slower.

 

Takahashi also abandoned their Sky 90 fast doublets probably for the same reasons: Their slower doublets just deliver better image quality. In addition, the cell design for these fast doublets also has to be robust and exact or (because of the wider air-gap in order to get better color correction) you can get lens de-centering and/or are easier to knock out of collimation. 

 

Ed Ting did a comparison between a Sky 90 and a TMB 92mm F5.5 Triplet. Here is his comment regarding “high power” use…

“The two scopes looked similar on Jupiter until we got to about 100X, when the TMB 92 SS started pulling away.  Detail in the Sky 90 looked impressive, until we looked through the TMB.  At 179X, the TMB was sharper.  There is a question as to whether you'd want to go this high on a small 90 mm refractor, as it's pushing 50X per inch.  But the TMB didn't have a problem performing at these powers, and even seemed to be asking for more.  With the TeleVue binoviewer, Jupiter looked fantastic at 179X with 9 mm Naglers.  Edge: TMB 92.

 

Bob



#7 chemisted

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Posted 28 July 2020 - 02:09 PM

Bob has brought up the Sky 90 but there are a few caveats.  While the designs of it and the Borg are similar the lens spacing is not identical.  Regarding Ed Ting's reviews of the Sky 90 (he did 3 he was so enamored with the scope) all three were bought used and so they had unknown histories regarding handling.  Finally, regarding high power viewing, Ed Ting finished the review that Bob has quoted from with the admission that he did not evaluate his Sky 90 with the Extender Q option that removes any residual aberrations.  And this is really why I am making this post.  I am wondering if anyone has used the Borg 90 FL with the Tak Extender Q1.6 and found a similar improvement in high power viewing.  My Sky 90 (SN 18) and the Extender Q provide a perfect star test - even out performing my TV-140 on splitting tight doubles on nights of so-so seeing.



#8 marcus_z

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Posted 28 July 2020 - 03:49 PM

I can understand the temptation to place the sky90 and 90FL at the same level, but they are not! They are equal with respect to aperture, focal length, lens producer, type of the ed element, i.e. fluorite, placement of the ed element in front. However, the sky90 has a large air gap, leading to a heavy lens cell and a design that is vulnerable to decentering. Even though the sky90 has a lightweight aluminum-cast focused, it's weight is at 3.2kg. The same weight as the 92mm Stoway triplet with a heavier CNC-machined focused. The 90FL has a non-collimatable lens cell with supposably smaller air-gap, and weights 1.8kg in the 80 series tube with 2" Feather Touch focuser. I highly doubt that the mating element of sky90 and 90FL are the same! So concluding from one to the other scope should be taken with extreme care.
By the way, Tomytec announced that he mating element of the 107FL is improved with respect to the 90FL, which I believe right away. So even between the BORGs their mating element differs.

#9 marcus_z

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Posted 28 July 2020 - 04:11 PM

@Rich: Yes risers and a dovetail that extends forward under the lens assembly will help. Alternatively, you can use heavy eyepieces or a turret to move the fulcrum to the back 😉

#10 marcus_z

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Posted 28 July 2020 - 04:23 PM

A side note to the Sky90: Roger Vine (http://www.scopeviews.co.uk/) writes in his review of the Sky90: "Push the magnification for doubles or planets and the Sky-90 I tested was just not as sharp as it should be, though I believe this is mainly a collimation/centring issue, not a problem with the optical surfaces."
This issue not what I experience with the 90FL. And as far as I remember, nobody else complained about softness of the 90FL, but rather about color correction.

#11 Simon B

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Posted 28 July 2020 - 06:15 PM

106FL?



#12 Rich Caruana

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Posted 28 July 2020 - 07:49 PM

@Bob --- completely agree with you (and indirectly with Ed Ting) that the 90FL can't compete with the best 90mm scopes such as the Stowaway (or the TMB 92 Ed compared the Sky 90 to).  The question is are the Borg FLs good enough that a 107FL would outperform a 92mm Stowaway given the 16% aperture advantage?  My Tak FS-60CQ has a perfect star test, but the Borg 71FL outperforms it because the extra 18% aperture more than makes up for the 71FL being less perfect and having a worse star test than the FS-60CQ.

 

-Rich.


Edited by Rich Caruana, 28 July 2020 - 07:56 PM.


#13 Rich Caruana

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Posted 28 July 2020 - 08:03 PM

@Marcus --- I guess imperfect color correction might show as softness on fine detail?  Guess one test would be to use a green filter and see if sharpness dramatically improved.

 

-Rich.



#14 Rich Caruana

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Posted 28 July 2020 - 08:21 PM

@Simon --- good question, not sure where I heard of the 106FL, but I can't find any references to it now so I must be mistaken.

 

Thanks.  -Rich.



#15 marcus_z

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Posted 29 July 2020 - 03:13 AM

@Marcus --- I guess imperfect color correction might show as softness on fine detail?  Guess one test would be to use a green filter and see if sharpness dramatically improved.
 
-Rich.

Scotopic vision takes care of that grin.gif



#16 bobhen

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Posted 29 July 2020 - 06:55 AM

@Bob --- completely agree with you (and indirectly with Ed Ting) that the 90FL can't compete with the best 90mm scopes such as the Stowaway (or the TMB 92 Ed compared the Sky 90 to).  The question is are the Borg FLs good enough that a 107FL would outperform a 92mm Stowaway given the 16% aperture advantage?  My Tak FS-60CQ has a perfect star test, but the Borg 71FL outperforms it because the extra 18% aperture more than makes up for the 71FL being less perfect and having a worse star test than the FS-60CQ.

 

-Rich.

When you say "outperform" do you mean…

 

For deep sky? If so, then aperture “can” trump optical quality, as light gathering capability “can” be more important than optical quality.

 

For high power visual? Then that depends on how far apart the 2 scopes in question are in optical quality and size. But “in most cases” the ability to take magnification and revel fine detail and contrast in average to excellent seeing is dependent on the quality of the optic.

 

The Borgs seem to be leaning more and more toward developing imaging astrographs rather than multi-use refractors, like the many other apo triplets out on the market.

 

If one wants a very lightweight, travel refractor for wide field imaging, then the Borgs would be a fine choice. But outside of that, I think there are many other apo triplets on the market that can easily do low AND “very high power” visual as well as imaging and are just the better “all-round” or “general use” refractors.

 

And if one wants an astrograph that is also an excellent visual telescope then the Takahashi FSQ 85mm and 106mm scopes come to mind. So instead of comparing the Borg 107 to a Stowaway, compare it to a Tak FSQ 106.

 

The question is not if this or that Borg can outperform a smaller triplet BUT the question for the buyer is: unless one wants a very lightweight travel astrograph, why purchase something that is pushing the fast doublet technology to the limit (and maybe over the limit) when there are “other proven choices” that can do all things better.

 

Bob



#17 Catalin81

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Posted 29 July 2020 - 11:26 AM

IMO one should always keep in mind what is truly unique to the Borgs. And that is the marvelous engineering that allows for their light weight, and also their modularity. As such, they are premium glass compromises that brilliantly address the portability issue.

 

I am currently waiting on a 90fl while being the proud owner of a LZOS 105 (TMB designed). The latter is an instrument of exquisite optical quality and I bought it as a ‘grab and go all-rounder’, but resistance was futile so-to-say. The Borg 90fl packs shorter and is 3(!!!) times lighter. I could actually fit it in a cabin-sized backpack along with the Rainbow RST135 and all needed accessories either for AP or for visual use (the tripod comes separately). And for this I will happily part with my LZOS eventhough it may be optically superior. 

 

Because of their modular nature, I think the use of the available correctors (reducer, flattener, extender) should be regarded as mandatory for optimal performance. Depending on use case, be it AP or visual, there is a suitable corrector meant to provide you with the desired function. I believe this should be the only way to compare them with triplets or Petzvals. 

 

BTW, did anyone of you guys use a Baader CAF visually with your Borgs?


Edited by Catalin81, 29 July 2020 - 08:42 PM.


#18 Rich Caruana

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Posted 30 July 2020 - 08:45 PM

@Marcus #15 --- do you think Jupiter and Saturn are dark enough in a 90mm scope at 150X (0.6mm exit pupil) for the eye to be Scotopic?  If so, I guess you're right that color correction shouldn't have much impact on fine detail.  Hope it's true!

 

@Bob #16 --- by "outperform" I did mean both detail at reasonably high magnification in addition to deep sky.  Guess my hope is that the Borg 107FL is good enough, that the extra 16% aperture over the Stowaway would help it compete with the Stowaway on resolution, and outperform it on deep sky, but I could believe the AP92 would remain better on fine detail than the 107FL despite the latter's aperture advantage.  Don't know what to do other than try it...  Just what i need, another scope!

 

@Catalin81 #17 --- any idea what correctors would fit a Borg 90FL or 107FL when using Series 80 tubes and 2" FT?  The Q-module significantly improves the FS-60CB.  If there was something similar for the Borg that worked for visual use I'd be happy to try it if it didn't require switching to a helical focuser. 

 

In #7 Chemisted said the Tak Extender Q 1.6 worked perfectly with his Sky 90 and asked if anyone had tried it with the Borg 90FL.  Anyone have any idea how easy it would be to try this?

 

Thanks.  -Rich.



#19 Rich Caruana

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Posted 31 July 2020 - 12:38 AM

@Catalin81 --- what is the Baader CAF?

 

-Rich.



#20 Catalin81

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Posted 31 July 2020 - 12:58 AM


Baader FFC, sorry.
https://www.baader-p...fc)--3x-8x.html

#21 bobhen

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Posted 31 July 2020 - 06:48 AM

 

@Bob #16 --- by "outperform" I did mean both detail at reasonably high magnification in addition to deep sky.  Guess my hope is that the Borg 107FL is good enough, that the extra 16% aperture over the Stowaway would help it compete with the Stowaway on resolution, and outperform it on deep sky, but I could believe the AP92 would remain better on fine detail than the 107FL despite the latter's aperture advantage.  Don't know what to do other than try it...  Just what i need, another scope!

 

 

I’m not saying the Borg would be a bad telescope – not by a long shot. I’m just saying that unless you REALLY NEED its lightweight capability there are better choices for high power and wide field imaging.

 

The Tak FSQs comes to mind
And there are many 4” triplets and some doublets like the Tak 100DZ that WILL best it on high power observing without having to go to the expense of a try out.

 

I can’t say what is “good enough” for you and if you like the lightweight Borg 107 and can afford to give it a try then go for it. You might find it meets all of your needs. I can only say the Borg 90FL that I had did not.

 

Bob



#22 Rich Caruana

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Posted 31 July 2020 - 03:32 PM

@Catalin81 #20:  Thanks for the clarification.  I don't have any experience with the Baader FFC.  Although it looks like a very nice piece of kit, looking at the specs it's not clear that it would improve color correction or reduce aberrations or field curvature the way a matched multiplier like the Tak Q-extenders and Borg flatteners and super reducers do.  Anybody have experience with an FFC and if it helped improve image quality beyond providing magnification?  Also, anyone have experience using an FFC for visual use as opposed to imaging?

 

-Rich.



#23 Rich Caruana

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Posted 31 July 2020 - 04:17 PM

Thanks Bob.  I think most people looking at the Borgs are looking for a scope that's exceptionally lightweight and packable.  At least that's my interest in them.  I guess others might be interested in the Borgs as fast astrographs using Borg's super reducers, but I can't comment on that since I'm mainly a visual observer.

 

-Rich.


Edited by Rich Caruana, 31 July 2020 - 07:57 PM.


#24 Simon B

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Posted 31 July 2020 - 08:12 PM

Borgs are really meant to be astrographs....  you can use them for visual, but they are designed from the drawing board to be very fast. A bit too fast to be completely colour-free at their native focal lengths. But that's where Borg's focal reducers come in, and I think it's actually their reducers that are special, not so much their objective lenses

 

Their focal reducers not only flatten the field and correct edge aberrations, but they also significantly reduce CA - this is very noticeable with my Borg 60ED / 77EDII and 7870 reducer

 

WIthout the reducer, these fast FPL-51 doublets produce pink halos around bright stars, but that basically vanishes with the 7870. It really transforms the scope completely - it's like steroids for the 60ED / 77EDII cool.gif   They had a lot of reducers that were compatible with these scopes, especially the 77EDII, I can think of five off the top of my head;  7870, 7878, 7866, 7704, 7887

 

The 7704 (0.65x ED super reducer) even cost twice as much as the 77EDII lens - such is the quality and effort Borg put on their reducers. Even now, their dedicated 55FL reducer cost as much as the 55FL itself.

 

 

So it seems like they make their objective lenses very fast on purpose, so their reducers can bring them down to even faster focal ratios for astrophotography, yet also correcting for CA. I think after their modularity, this is their second most important design philosophy - make the lens really fast (too fast for visual), then correct the CA with focal reducers afterwards


Edited by Simon B, 31 July 2020 - 08:19 PM.

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#25 Rich Caruana

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Posted Yesterday, 03:09 PM

The benefit of using reducers, flatteners, or extenders with the Borg FLs has been mentioned a few times now.  I'm sure they make a huge difference for imaging.  Anyone have any experience using any of them visually with the Borgs?

 

@Catalin81 --- you said in post #17 that there are suitable reducers, flatteners or extenders for visual use.  Which ones do you think would work visually?

 

Thanks.  -Rich.




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