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Ags

Is a 70-72mm ED enough for double stars?

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Posted (edited)

I am looking for a small refractor that can get to 3 degrees field of view with 1.25" eyepieces and still split my favorite doubles - Double Double, 61 Cygni, Izar and similar. Would a 72ED do the job? Or should i rather look to 80mm F6 refractors?

There is this - but I think it will show too much false color at high magnifications (I want to see the double stars in their correct colors):

https://www.teleskop-express.de/shop/product_info.php/info/p11436_Long-Perng-90-mm-f-5-5-Apo-with-Doublet-Objective-and-2--RAP-Focuser.html

It is also at double my budget....

Edited by Ags

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Posted (edited)

Hi Agnes. I am surprised at the lack of response to your question. I have tried to split Izar with my SW ED scope without success but have managed the double double split. Looking back at my observation notes I have had poor seeing conditions each time I have tried to split Izar (3 times) . On one of these occasions I also had my ED 120 set up and managed the split with that. I have not used the ED 72 very often so have limited experience with it so I can’t really say how good it is on other close doubles. Hope this helps,

John

Edited by laudropb
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The maximum useful magnification for a 72ED is about 140x. Wide doubles like 61 Cygni, Castor and Algieba should be no problem. With excellent optics, you might be able to split the double double and Izar. I remember I struggled with them using a 3" table dob, but that one had a spherical primary mirror so it's nothing compared to a decent apo, I suppose.

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I could split Epsilon Lyrae with my old Tele Vue Ranger 70mm. I seem to recall that Izar was very "touch and go" though.

 

 

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I would need to double check but fairly sure my TS 72mm will split Izar. It will definitely do the Double Double.

Coincidentally I’ve just been having fun with two tiny scopes splitting these doubles, a Zeiss Telementor (63mm refractor albeit f13.3) and a TAL Alkor 65mm F7.7 Newt. Both split these doubles quite easily.

An 80ED could be picked up fairly cheaply and would have a small resolution advantage, but a decent 72mm should do the job. As said, I’ll check mine out and report back when I get the chance.

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As an experiment I tried splitting Izar tonight with my ST80, barlowed with a cheap barlow. At 120 magnification there are certainly a lot of abberations to look at! The false color was of course expected, but it appears the ST80 needs collimation really badly (or the barlow does) - stars were showing as actual lines. Nevertheless, there was a definite split. The double double was a bit more dicey but one of the pairs split - the other would have split but was oriented badly versus the collimation issues.

Well, if a battered old ST80 can do it, a nice little ED will definitely do it. 

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I quickly checked this out again last night, and can say that the 72ED will split the Double Double (even down low as it still was) and Izar. My scope unfortunately has a slight miscollimation which needs sorting, but even so they were both quite doable. I used a Nagler zoom generally a the 5 and 4mm settings ie x86 to x108 although Izar benefitted from going up to max which was x144. With perfect collimation they would be easier, and the Telementor beside it did look clearer.

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Many (quite a lot) years ago I had a 60mm f15 refractor. That scope was particularly good at splitting doubles. I imagine the 72ED to be much better quality and could manage them quite easily.

I have an 80ED but have never tried high power on it...

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Hi Agnes. I had another go at splitting Izar with my SW ED 72. The seeing was very good and I managed a clean split at 110 x. So yes it is possible.

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