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  #1  
Old 09-07-2013, 11:33 AM
PeterHA (Peter)
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120mm ED latest wisdom

All,
What is the latest wisdom in regards of the Chinese Skywatcher and similar 120mm f 7.5 ED doublets for VISUAL ASTRONOMY especially planets, double starts and moon?
There is a lot of information about them for AP use but I am only interested in VA and I am not certain how they are for that application.
How do they compare to a premium doublet like the Takahashi TSA-120 or a AP Star12ED (long discontinued but still around).
I have a Swiss AYO alt az mount which will carry easily up to 8 kg.
Cheers,
Peter
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Old 09-07-2013, 01:40 PM
Profiler (Profiler)
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They are generally regarded to be excellent bang for your buck however there are, naturally, some differences with premium brand refractors. For VA it is fantastic however it might be worth also looking at the latest 100mm Tak doublet FC-100DC which is roughly the same price

http://www.astronomy-electronics-centre.com.au/

Last edited by Profiler; 13-07-2013 at 04:48 PM.
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  #3  
Old 09-07-2013, 03:54 PM
PeterHA (Peter)
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100mm TAK

Hi Profiler,
With 100m the Tak should collect quite a bit less light and from experience I think 120mm at f below f9 is managable, so I would go for 120mm to get max resolution and light gathering for a refractor.
Cheers
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Old 09-07-2013, 04:08 PM
Profiler (Profiler)
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I think there is something to be said about quality of optics in the comparatively small refractor sizes. The Tak I mentoned is a 100mm real fluorite crystal doublet whereas the SW is a massed produced fluorite substitute. For VA it might be surprising.
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Old 09-07-2013, 04:20 PM
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A 120mm scope collects 44% more light than a 100mm . Taks are always given mystical properties that they can transcend the laws of physics - they can't .

In these sizes for visual you do not need flourite glass - my Orion 120mm F7.5 Epoch shows absolutely no visual colour . It is not really grab,n,go as you need to move it around in the case which is about 13kg all up.

All that being said 120mm is not enough light gathering for any level of satisfying visual observing , bar a look at the Moon or a bright star cluster . Put and off axis- 120mm hole in front of your 12" dob and see how fast you lose interest !
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Old 09-07-2013, 05:00 PM
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Traditional wisdom also suggests that whether the refractor is a doublet v triplet is another important variable for contrast in VA hence why the new Tak is specifically a doublet Fluorite crystal primarily intended as a VA instrument.

Additionally, its design makes it very light, portable, easily mounted/balanced and has a super fast cool down period.

Better quality optics (glass) will allow greater magnifications without breakdown in comparison to lesser quality optics. This issue is not simply relevant to looking at bright objects such as planets but also important for viewing dim objects such as DSO's - The current issue of Oz S&T explains this point in nice simple terms as well.

There is something to be said for the old expression quality over quantity. I have personally seen plenty of examples where a much smaller aperture very high quality refractor beats the clappers out of a much larger aperture but lower quality scope.

Last edited by Profiler; 10-07-2013 at 09:24 AM.
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Old 09-07-2013, 06:50 PM
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Quote:
Originally Posted by Kunama View Post
I don't really agree with the 12" dob requirement for visual, they're obviously better at gathering light but nobody can really tell me they are easily portable.
He already owns a 12" Dob according to his sig, which is why I suggested to use an off axis stop too simulate what he would see through an unobstructed 120mm aperture .

In my experience 8" aperture is where the utility starts for a scope on all different kinds of visual objects. Resolution is obviously far superior with a large well collimated quality optics.
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Old 09-07-2013, 08:24 PM
clive milne
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Quote:
Originally Posted by Satchmo View Post
Put an off axis- 120mm hole in front of your 12" dob and see how fast you lose interest !
Never mind 120mm, I lose interest at 18 inches of aperture (unless there is a pair of em)
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Old 09-07-2013, 08:26 PM
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Quote:
Originally Posted by PeterHA View Post
All,
What is the latest wisdom in regards of the Chinese Skywatcher and similar 120mm f 7.5 ED doublets for VISUAL ASTRONOMY especially planets, double starts and moon?
There is a lot of information about them for AP use but I am only interested in VA and I am not certain how they are for that application.
How do they compare to a premium doublet like the Takahashi TSA-120 or a AP Star12ED (long discontinued but still around).
I have a Swiss AYO alt az mount which will carry easily up to 8 kg.
Cheers,
Peter
I'm curious - wouldn't your 150mm Mak be good for visual observation of the objects you mention? If it's f/10 I'm guessing it might be an Intes Micro, and they're excellent!
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Old 10-07-2013, 08:23 AM
PeterHA (Peter)
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Mak and Dobson

All,
My Doboson is a Bintel and so the optics are OK but not stellar. I take the beast out if I have a lot of time.
My Mak is the one I contemplate to replace, it is optically great it is a early Intes MK-65 with the helical start diagonal focuser unit, a bit hard to get focus for all my EPs and the cool down period is quite long. Before I got the Mak (1992) I had a Vixen 102mm f10 Achromat and the Intes was a step up in resolution.
Now I am contemplating to get something which cools down faster, is still as portable as the Mak is and shows at least what the Mak showed on Planets and double starts. I observe mostly from my backyard.
Cheers.

Last edited by PeterHA; 10-07-2013 at 04:33 PM.
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Old 10-07-2013, 09:20 AM
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Peter - A 120mm Refractor would definitely be too large and heavy if you are looking to replace a 6" Mak , and not that much gain from your triplet 100mm APO . Have you considered something like a 6" or 8" SCT OTA ? I've looked through a Celestron 6" OTA and thought it was very good.
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Old 10-07-2013, 09:26 AM
issdaol (Phil)
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Hi Peter,

The TSA120 is actually a triplet APO not a doublet.

I have had a chance to compare our TSA120 side by side with quite a few scopes at some star parties which had amongst other scopes Skywatchers (one was the Black ED) and Televue and on the same targets there is a difference that you can notice visually.

However you have to remember there is a big difference in price as well so it comes down to if that difference is worth the extra dollars to YOU.

For me personally, when it comes to visual observation then I want the best optical performance combined with best build then I look at the dollars.

For me personally 120mm is the smallest I would go for a APO. If you want quality visuals at a higher diameter reflector design then the Mewlon 250 and 300 is very good.

Hope this helps

Cheers
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Old 10-07-2013, 09:31 AM
Profiler (Profiler)
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Hi Peter

If you are:

1) In a light polluted area

2) Want to use something conveniently in your backyard (which is light polluted)

3) Want something easily portable

4) Want something that cools down superfast

5) Want something that is primarily for VA only

6) Want something primarily for looking at planets and double-stars

I would recommend a good quality doublet refractor. You wouldn't even really need a 120mm refractor. When you want a light-bucket use your Dob

In line with Phils comments the TSA-120 has be considered by a few Tak experts I have met as probably the best refractor Tak has made all variables considered. However, it is expensive and is certainly designed for AP purposes - that is - to go well beyond VA and also be superb for AP purposes as well. If money is not an issue then simply buy a top shelf refractor such as a TSA. However, if you can specify your variables about exactly what you want then you can certainly get something else (i.e. smaller) which will certainly do the job and cost less. For the variables you have mentioned a good quality doublet around 100mm would give you the best bang for your buck and images through the EP.

Last edited by Profiler; 10-07-2013 at 02:28 PM.
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  #14  
Old 10-07-2013, 12:51 PM
PeterHA (Peter)
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So we are getting somewhere

Thanks Phil and Profiler,

That is where these forums shine: making experiance accessible!

Based on what you guys have written, and the very good description of my scenario in Profiler's last message, my path forward takes shape:

If I have time I take my 12" dobo and thats it, especially for nebula, GCs, PNs.

If I have not much time and want to look at planets, double starts and other similar objects I should consider a doublet, on my AYO with DSC this would be a nice VA setup.

I see the following options here:
Skywatcher 120mm f 900 ED, that size/ weight is still OK on both my mounts, is not longer and not much heavier than my old Vixen 102M achromat.

Takahashi FC-100D, that is very light, good optics but only 100mm, would be great if they would make a FS-120

A used Star 12ED which is 120mm f 1020 (I know someone selling one)
Question is: will the Skywatcher or the FC-100 be on par with the 150mm Mak. If taking the central obstruction and difraction effects into account the Mak resembles something like 120-130.
If I get close with any of the above, the faster cooling down and better focuser of a refractor would probably do it for me.

If anyone wonders about the 100mm achromat if have in my signature, this is a kit which was sold 2000 in Germany by BW optics. It's a 3 element Achromat f6 (intended for military Binos) which only works up to 100x and also only because I have 110mm glass path before the focus. Without a glass path anything above 60X looks not good. Great for travel only 400mm long and enough size for some fun on holidays on an alt az mount. This is no planet machine..far away from it.
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Old 10-07-2013, 01:30 PM
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Last weekend at Katoomba I had my SV110ED and Skywatcher 150mm Mak (f/12) set up side by side on my dual-scope mount. I had a 22mm Panoptic in the Mak (82x) and a Pentax 10mm in the SV (77x).

In general, I was surprised at how similar the views were. In other words, while the Mak was a bit brighter the contrast in the SV made up for a lot of it. However, I should qualify this by admitting that there was no dew shield on the Mak, which might have improved the contrast.

Steffen had his Intes MK-67 there and it also showed more contrast than the Skywatcher Mak, which was no surprise. I'd assume that your MK65 performs similarly to the MK-67.

So, extrapolating my setup to your question, I might think that a good 120-130mm refractor would perform similarly to your Mak on many objects, and also allow low magnification views that the Mak can't achieve. I think the 100mm would be quite a bit dimmer.

However, the question is how do you define "good" in relation to the quality of your Mak? I've never looked through a 120ED but I imagine the contrast would be a bit below the Tak and AP scopes you mention.

Don't know if this helps or not

Good luck!
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Old 10-07-2013, 02:34 PM
Profiler (Profiler)
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I am sorry Peter I can't help with this question other than to concur with you that I likewise wish Tak's new doublet flourite refractor came in a 120mm or larger size.

Suffice to say the discontinued Tak FS-128 has an absolute cult following which might explain why Tak is gradually and somewhat tentatively going back to flourite doublets.
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Old 10-07-2013, 03:06 PM
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JUST MY 2 CENTS I CONCUR WITH MORTON AND SATCHMO I cant understand?, youve got a quite portable excellent mak already! FOR WHAT ITS WORTH .IVE HAD refrs. A 4" UNITRON REFR from astro many moons ago for instance, and though great on planets, dsos where unresolved and dim.for portability ,optics, brightness, resolution , a 6 sct from celestron easily beat a 6' gold sw mak i had also in every facet and it was no slouch! the bd mak may be better, as fresh coatings, schott glass etc as would be a new intes.there seems to be very little to be gained going for a cumbersome refr of that length! anything bigger and it will be unportable
ps: everything needs some time to cool down optically, some longer than others ,theres NO free lunch! cheers
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Old 12-07-2013, 04:26 AM
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Peter, you already have a 150mm f/10 Mak. A smaller refractor will do absolutely nothing of use for you, IMHO, unless you want to use it as a very big finder, or guide scope.

FWIW I have a 102mm ED doublet APO refractor, an Alter M703 (7" f/10) Mak, and a Skywatcher 180mm f/15 Mak.

My refractor does produce a good diffraction pattern with only the barest hint of colour - surprisingly better than I expected, however f/7 is too short to be much good on planets (the combination of f/15, cassegrain optics and longer eyepieces is much better) and for DSO's disappointing except in very very dark skies. Katoomba airstrip is ok for fun, but IMHO its not that dark compared to really good sites.

My refractor doesn't get much use, frankly because the two maks do significantly better on DSO's (aperture), planets (where high agnification resolution counts) yet small enough to be dead easy easy up. When I do take I out, it usually ends up serving asa very big finder 4 degree field) for the f/15 Mak, which once aligned, can stay at medium to high power. An expensive luxury, perhaps.


A 120mm f/7 won't be noticeably better - having looked through a couple.

Last edited by Wavytone; 12-07-2013 at 04:39 AM.
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Old 12-07-2013, 09:03 AM
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The range of high quality short focal length , long eye relief eyepieces, available today make an F7 ratio telescope perfectly suitable for planetary observation .
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  #20  
Old 12-07-2013, 10:07 AM
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I can't comment on comparisons of one non-descript refractor can supposedly do against another.

However, in my personal experience I have been able to compare a f7 TV-85 against a Vixen 102M which, IMO, was an excellent F/10 achro refractor as well as a Vixen 140 and a host of Skywatcher big achros up to 150mm. On every occasion there was simply no contest, with or without any combination of VR filters etc the TV-85 beat the clappers out of the others in terms of the crisp, clear and detailed image rendered of Saturn (for example).

However, two important variables (both of which Wavy mentions) to keep in mind. First, and most important is LP and thus your location. In a suburban light polluted back-yard I couldn't see any difference in DSOs between a 140/152mm refractor and the TV-85.

Second, quality of eyepiece does make a big difference. You need to be consistent the whole way through with the optical path. Probably the absolute best eyepiece for the 'specific' parameter of light pass/contrast are the Pentax XWs. There are many other excellent eyepieces which are superior on other attributes but for this specific variable in viewing DSOs through a refractor I found the XWs to be best.
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