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How important is aperture for double star viewing?

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#1 pugliano

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Posted 24 March 2013 - 12:08 PM

My primary interest is viewing open clusters and double stars. Yep, I know..not very compatible! But if I'm just looking to enjoy maybe the top 100 doubles and don't have much interest in the challenge of splitting really tight doubles or searching out very faint doubles, is there much of an advantage to using something like a 120mm long focus refractor over an 80 or 90mm refractor?

I mean, stars are pinpoints at any magnification and there's no detail to see, so what do I gain by increasing aperture other than being able to see fainter pairs or splitting closer doubles? For the top 100 showpiece doubles (and my open cluster viewing!), wouldn't a 90mm be just as good as a 120mm acro?

And if I go with a 90mm, would an APO be a significant improvement over an acro for double star viewing? Or wouldn't there be a huge difference?

Thanks!

#2 WRAK

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Posted 24 March 2013 - 01:50 PM

Open clusters and double stars are very compatible I think - most open cluster include doubles.
Aperture is irrelevant as long as you are satisfied with the limits of a smaller aperture in terms of resolution (for separation, magnitude delta and faintness). So any small scope with an aperture down to 60mm will do a nice job on doubles within its limits.
An APO would in my opinion be only of interest for somewhat larger apertures as with smaller ones you can handle very well a larger focal ratio like F15 compensating potential color errors.
Wilfried

#3 pugliano

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Posted 24 March 2013 - 02:56 PM

Thanks much.

#4 azure1961p

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Posted 24 March 2013 - 06:10 PM

More light brings you longer lists of doubles to choose from and all the inherent variety. There's contention here but I think a 100mm achromatic of proper focal length is a fine thing. The difference of ten millimeters is negligible between refractors but 20 or more and the difference is more appreciated. It all matters but suffice it to say 100 over 80 is a good edge.

You might prefer a 6" f/8 over either and the big lists of colorful doubles it opens up.

Pete

#5 PJ Anway

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Posted 24 March 2013 - 09:21 PM

I would agree with what others have said. I also feel the sweet spot for double star observing is a 100mm refractor. A relatively portable instrument, but still with enough capability (limits) to keep an observer satisfied for a good long while. All my "showpiece" picks are within its range.

#6 CelestronDaddy

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Posted 24 March 2013 - 09:47 PM

I've always had a fondness for refractors. My 80mm, while not 100mm, still does a decent job on doubles and has nice pinpoint images.

#7 buddyjesus

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Posted 24 March 2013 - 11:42 PM

I like my 4" scope for everything so I am biased.

Here is a short bit from the Cambridge Double Star Atlas:

It has been often stated that the person behind the eyepiece of a telescope is far more important than the size or type or quality of the instrument itself.

#8 azure1961p

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Posted 25 March 2013 - 06:37 AM

I like my 4" scope for everything so I am biased.

Here is a short bit from the Cambridge Double Star Atlas:

It has been often stated that the person behind the eyepiece of a telescope is far more important than the size or type or quality of the instrument itself.


I think that's one of the successes of the 100mm refractor that it can cover so many bases so well for so many.

My 70mm is a little light starved on doubles compared to my 6 and 8.
But they can look do darned perfect.
Pete

#9 blb

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Posted 25 March 2013 - 10:03 AM

...But if I'm just looking to enjoy maybe the top 100 doubles and don't have much interest in the challenge of splitting really tight doubles or searching out very faint doubles, is there much of an advantage to using something like a 120mm long focus refractor over an 80 or 90mm refractor?

NO! Although I too think that a 4-inch, 100mm, refractor is a great choice for anyone.

And if I go with a 90mm, would an APO be a significant improvement over an acro for double star viewing? Or wouldn't there be a huge difference?

There are differances between the two of course but is one of them superior to the other, NO! The purpose of an APO is to focus the different colors of light into a common point like a long focus refractor in a much shorter distance. Most people prefer a short tube telescope over a long tube telescope, hence the APO design came into being. Now you can get wider fields-of-view with an APO than you can with a long focus achromatic refractor, but does that mater for double stars, NO it doesn't. Personaly I think something like my 4-inch TV102 simi APO refractor is one of the best all round small instruments to use but my 2.6-inch AT66 semi APO refractor will work just as well for the top 100 double stars (and is only about 16-inches long) and does a good job with all the Messier open clusters too, as well as many NGC open clusters.

#10 Ed Wiley

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Posted 25 March 2013 - 10:08 PM

Top 100 showcase doubles? Much depends on your budget and future plans. Something as small as my WO66 semi-apo to something like a TV101 (semi-premium to premium). If I were looking for a capable refractor a bit larger than my "quick grab" 66 I might think about an 80-100mm and if I had the budget it would be a triplet or A TV85 or TV101 or even a Tak 90. If I was on a budget a nice 80ED semi-apo. F15? Nothing wrong with a 100mm F15 refractor, especially for doubles and Luna, but they are long devils. (Long focal lengths are good for doubles, my Cass DK is F22.5!). Try to use one before you buy if you are thinking of that route.

Ed

#11 cildastun

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Posted 26 March 2013 - 01:14 PM

Until I got a 5" Mak, I had believed my 4" achro refractor was pretty good for doubles; the Mak beats it hands down though and has split some very close pairs indeed. It's also a lot easier to use than an f13 4" frac!

Chris

#12 pugliano

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Posted 26 March 2013 - 07:17 PM

Thanks to all of you for your input!

Chris, I definitely considered getting the Orion 5" mak, but what held me back was the narrow FOV. I figured the refractor would be better for observing open clusters, which I also like to view.






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