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Change SW 120ED by TS Photoline doublet SD 125

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#1 Victor Martinez

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Posted 10 February 2021 - 04:11 PM

Hi,
Well, it would be worth changing one tube for another putting € 300, or better to stay with the SW and get a short focuser of 2'5 "? The issue is that the TS (same tube as the Stellarvue SV125A), it is more friendly with the binoviewer, and it has a better focuser and mechanical finish. My question comes with the optics. I honestly have my doubts that the TS will outperform the SW.

#2 photoracer18

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Posted 10 February 2021 - 05:32 PM

Most people forget that early BV ready refractors were normal ones with shorter tubes where the cutoff length was replaced by an extension to the focuser that you could remove to make the scope the perfect length to use the BV. I have had a couple of those (and still have one). Then they got the idea of shortening it a little bit and putting in a focuser with a long focus tube that was barely BV compatible just so mono viewing still worked. I know of some people on the AP refractor list who ordered their scopes with shorter tubes. That is fine if all you do is bino-viewing, otherwise you need to add 4" of extension to get back to mono viewing, give or take a little on the length.

For your issue unless you can get a replacement tube that is the correct length for BV you usually can't fix it with a focuser only as most out there that are replacement focusers are designed to be the same drawtube range as the original one. The scope I have, the original SV102ABV, at around F8 (some years they were F7.8 and some F8.7) had a 4" removeable extension between the focuser and the OTA. That was using an FT2025 focuser. And that focuser has a short body. There is no existing focuser that can be put in the same position with the same height that could be used for everything including mono viewing because it would need a focuser with a 6.5" travel. I was a beta tester for Denkmeier at their beginning. My first refractor attempt was using a Vixen ED114SS at F5.3.  The BV would not reach focus without using a 1.5x 2" Barlow in front of the 2" diagonal (about 2.25x). Would not make it using a 1.25" diagonal either by a small amount. Eyepiece design made a difference but I was not willing to compromise as I was comparing a BV with Plossls verses a single TV Nagler.

As for optical performance I don't think there is much difference between those 2 but my experience is limited of those.


Edited by photoracer18, 10 February 2021 - 05:33 PM.


#3 Victor Martinez

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Posted 16 February 2021 - 03:02 PM

These telescopes that are sold under brand names like TS, Altair, Tecnosky, Stellarvue, etc. have a tube in the focus group that can be removed to accommodate the binoviewer. A vendor and hobbyist here in Spain told me that Sky-Watcher has better quality control than the factory that these tubes come from. In fact, optically Stellarvue I think it gave them problems, as I read in these same forums, and that is why it has finally stopped selling them. If you get decent optics and it is well collimated, it has an excellent value for money. Most SW 120EDs have unbeatable optics for their price, but are mechanically limited



#4 Surfinash

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Posted 22 February 2021 - 12:31 AM

These telescopes that are sold under brand names like TS, Altair, Tecnosky, Stellarvue, etc. have a tube in the focus group that can be removed to accommodate the binoviewer. A vendor and hobbyist here in Spain told me that Sky-Watcher has better quality control than the factory that these tubes come from. In fact, optically Stellarvue I think it gave them problems, as I read in these same forums, and that is why it has finally stopped selling them. If you get decent optics and it is well collimated, it has an excellent value for money. Most SW 120EDs have unbeatable optics for their price, but are mechanically limited

OTAs are actually the lesser of your worries if you plan to use a binoviewer (BV). Most OTAs described here have focal lengths well under 1000mm, and can easily accommodate a glass path corrector (GPC) say 1.3x, or 1.7x from Baader along with the BV (actually, most of these will need a GPC to achieve the focus), and use the OTAs as is. No special "cut piece" OTAs required.

 

If you are serious into using a BV - spend well on the large format 25mm+ prisms (Baader Maxbright II, New TS BV), and pair it with the right diagonal to thread the BV on (no inserts please) and a GPC. You will enjoy with your current OTA without needed any special OTA / replace your existing OTAs.


Edited by Surfinash, 22 February 2021 - 01:44 AM.


#5 Victor Martinez

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Posted 22 February 2021 - 04:14 AM

Apart from the binoviewers, which in fact I am already using a MaxBright II + GPC 2.6X and, on the way 1.7X, with my SW 120ED, I was weighing the change because with the same optical quality between both tubes, the mechanics are superior in the 125ED, and the change came out for little money. But it seems to me, as I have read in different forums, that buying one of these tubes is playing Russian roulette with the quality of optics. Sure, you can get a lemon with the SW, but I haven't seen any reviews to that effect yet, and mine is excellent.



#6 Stopforths

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Posted 22 February 2021 - 02:59 PM

interesting the point optics variable in the 125mm doublets v SW.

 

I had a vixen 115 eds f7.7 which I think was at least the match of the skywatcher but cost a little more.  Never did a direct comparison but pretty sure slightly less CA in the vixen I was ablwe to crank up the power on sirius on that and see no CA in focus.  Could just make it out in the Skywatcher.

 

Image very slightly yellowish to my eye in the vixen.  Very good scope overall. 




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