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Backfocus of Skywatcher Equinox 120?

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#1 Ava

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Posted 23 October 2013 - 10:42 AM

Hi,

I just bought a second hand Skywatcher Equinox 120, that should arrive late next week :jump:

Anyway, does anyone know the available backfocus with the stock focuser? I'm hoping I'll reach focus without OCS using my binoviewers and 25mm Zeiss Pl 10x-25 for some fantastic wide field views (they both allow focusing so I can gain almost 10mm by racking them all the way in...).

#2 Ava

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Posted 24 October 2013 - 01:52 AM

Anyone?

#3 skyward_eyes

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Posted 24 October 2013 - 08:29 AM

Trying to obtain that information for you.

#4 junomike

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Posted 24 October 2013 - 01:47 PM

I've help test a SW 120ED and It was approx. 4.5" (115mm). It's one of only a few Refractors that doesn't have enough in-focus for Binoviewers when used with the Siebert non-OCS Prism .
The other Refractor is an Orion 120ST (F5).

Mike

#5 Ava

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Posted 25 October 2013 - 11:17 AM

Thanks for the info guys. The Equinox is different from the ED scopes so may not have the same back focus (it doesn't even have the same focuser). Well, if nobody knows I'll measure it when I get the scope in a week or so and post the results here for future reference.

#6 Ava

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Posted 04 November 2013 - 03:14 PM

I thought I'd post an update, I have received the scope and tried it out very briefly a few nights ago between the inevitable clouds. The available backfocus seems to be at least 145mm, maybe a few mms more.

This would bring a Baader Maxbright bino (110mm optical path length) + T2 prism diagonal (38mm), total 148mm, to focus with my Zeiss Pl orthos, since their field stop can be adjusted to below the eyepiece shoulder. With a 1.25x GPC (which may be a good idea anyway in a refractor, see Edgies posts about this) I would even come to focus with my T2 Maxbright mirror (53mm)!

#7 Eddgie

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Posted 05 November 2013 - 04:22 PM

Have you actually tried it with the binoviewers?

It sounds like you will be short without the GPC.

You need about 160mm of in travel (which is really the correct term though we all seem to say back focus) for the binoviewers.

If the Binoviewer is 110mm, and the T2 Prism is 38, then that is 148mm.

If you only have 145mm of in-travel from the focal plane, then that would mean that you are 3mm short.

If you did not focus at infinity, you may find that you cannot reach focus when you do so without the GPC.

Focusing at a terrestrial object makes you have to rack out further than for infinity.

So, if you really measrured 145mm of in-travel, but were using the binoviewer on a terrestrial object, you won't really know until you focus on something in the sky.

I had several people tell me that this scope would not reach focus with a binoveiwer, so will be interesting to hear if it does or not in your case.

Even if you have to use the GPC though, I bet it is still a great telescope.

I went with the 110 because people said that it would not reach focus with the Mark V, and it was to expensive to take a risk in my case.

But I like the 2 degrees of the 110, though there is a part of me that regrets not going bigger.

#8 Ava

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Posted 06 November 2013 - 11:46 AM

Hi Eddgie,

The focus measurement was for infinity. I have not actually tried it with binoviewers since I don't have a prism diagonal, only the T2 mirror. Also, I needed to tighten the tension of the focuser which was very loose when I got it, so binoviewers was not an option. Now it holds very well but clouds have been abundant ever since.

However, the Zeiss Pl orthos I'm using have focus adjustment built in, and previous measurements for those eyepieces placed the field stop at 10mm below the shoulder of the eyepiece when adjusted all the way in. So, 110 + 38 - 10 = 138 so that should work. Even many TV eyepieces should work since many have the field stop at 6mm below the shoulder. With the Mark V adding 15mm for the quick coupler you would not make it. As I said, a 1.25x GPC might be advisable in any case, making it possible to achieve focus even with the Mark V or T2 mirror and other eyepieces.

#9 Eddgie

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Posted 06 November 2013 - 05:47 PM

Well, at f/9 the GPC may not make as much difference, but you have the option of putting it in for planets if you need it, and if you can reach focus without it, then you have the best of both worlds.

Again, I was told by more than one person that the 120ED would not reach focus with binoviewwers, but my fingers are crossed for you.

For general viewing though, I am running without the GPC and see no damage to the image at any power.

On the sun though, I could see fringing without it, but I am also f/7 and perhaps not as color free a glass setup. The GPC though fixed it.

But I would not use if for anything else, and in fact, I felt that the view is slightly vignette with the GPC in place using the lowest power eyepieces. My sky is kind of bright though, so it really shows this kind of flaw more easily than for most people. And the exit pupil being bigger makes it even more obvious than it might be for you.

#10 Jan Owen

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Posted 06 November 2013 - 06:37 PM

Any data on whether the cousin of this scope, the EON 120, is able to accept a binoviewer without an OCS?

#11 Ava

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Posted 07 November 2013 - 01:54 AM

Eddgie, the Equinox 120 is f/7.5. Anyway, these Zeiss orthos also vignette unfortunately, the field stop is just a little bit too wide, but it's acceptable given their performance. I'll let you know about actual experiences with binoviewer once the sky clears.

Jan Owen, I thought the EON was just a rebrand of the Equinox series but I have never seen one so I'm not sure if they are or if they share the same focuser and tube length etc.

#12 Eddgie

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Posted 07 November 2013 - 10:10 AM

Well, I am using f/7 and for solar white light, the GPC is indeed improving the image.

But for general observing, I put in a bino with no GPC and I really can't see any change to the image quality. Everytying looks fine, and even with ES 2468s, there is not a hint of vignetting (27.2mm feild stop).

So, fingers crossed for you. If you can use it without a GPC and get a really big field of view, that would be great.

Again, I really wanted one of these, but two difference people told me that it would not reach focus.

Was possible that it was a different scope they were using (different focuser, but stock focuser in both cases).

One person said that he researched it and decided that the focuser tube would project into the light path if the tube was shortened.

So, watchin this carefully because this was the scope I really wanted to use.

I doubt that I would get one now. I like the SV110ED well enough and love the 2 degree field, but for me, bigger is almost always better and this scope does not weigh any more than the Stellarvue I think and weight was a concern as well.






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