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>10" aperture on double stars?

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#1 auriga



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Posted 06 July 2013 - 12:37 AM

That's my question. This question applies to double stars of various brightness and separations and also applies to colored double stars.

Is there such a thing as too much aperture for double star observing? I have a 16" scope and am curious.

Thanks for any experiences,


#2 Erik Bakker

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Posted 06 July 2013 - 03:06 AM

A few weeks ago I had a few hours devoted to double stars with my 16" f/5 and BinoVue after first observing the double-double with that combo at 270x and being smitten by the beauty of the image. Delta Cygni will remain edged in my memory. Sooo much separation, such a tiny companion, the colors, the brightness, the intense concentration of starlight, just such a beautiful sight. A huge step up from a 4" apo view, or 7" Questar for that matter. So yes, a 16" is a very useful instrument for double stars in my experience.




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Posted 06 July 2013 - 04:31 AM

Very close and faint doubles are within your reach but seeing will always be a topic.

#4 azure1961p


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Posted 06 July 2013 - 08:14 AM

Well at worst you can use an off axis mask and still have medium aperture! On the better nights its got seriously excellent potential . The problem is often this though...

When you go to that great an aperture many if the colorful doubles that are swell in an 8" tend to get bleached out due to excessive brightness. Reds tend to look orange and oranges tend to look yellow and blue looks like cool white. What becomes of great importance than is choosing doubles that fit that aperture bracket. My 70mm seems to prefer magnitude 4 while my 8" is around 6- 7v . I'd guess you'd find the best color saturation around magnitude 10. Too, the diffraction pattern is clearer due to lack of irradiation. If I had a 16" aperture reflector Id draft up a new list of doubles entirely and bypass the 80mm refractor "gems" . You will likely have more to choose from as well since the list for fainter but appropriate doubles becomes so much longer. At the very least it would seem a variable polarizer or ND filter is in order if you want to observe mag 6 doubles in a 16".

Good luck. I am envious of the wide list of doubles that are available to you and the inevitable jewels that await discovery that probably most people are t aware of simply because they lack enough brightness and color in lesser apertures.


#5 fred1871



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Posted 06 July 2013 - 09:50 AM

Good comments Pete, but I'd think you've taken the magnitude for best colour saturation a bit too faint. With 70mm best around mag 4, you'd expect 8-inch to be mag 6-7 as you found - and if we just go with the amount of light gathering, 16-inch will be 1.5-mags fainter than the 8-inch, so that gives around mag 8 to 8.5 as the likely point for best saturation.

And that was also pretty much what I found years ago with a C14 I used a lot some years back. Around mag 8, rather than near 10, was where colour showed well - not too faint, not too bright. The light gathering difference between 14 and 16 inches is small; less than a half magnitude.

Of course, some folk have better colour vision and see fainter than many of us, so they might find stars a bit fainter still looking good for colour.

And whether or not the colours show at their best, bigger telescopes will show far more doubles as Pete has said, including showing well and easily doubles that are dim or invisible in smaller telescopes. And on the steadier nights, pairs too close for smaller scopes become possible as well.

#6 The Ardent

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Posted 06 July 2013 - 04:51 PM

Last night I split Otto Struve 525 and 44 Bootis with the 18" using 230x. Typical poor conditions: thin cirrus and average seeing. Yeah I use a smallish-big dob for doubles.

#7 coopman



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Posted 06 July 2013 - 08:45 PM

More aperture = more resolution, but the atmosphere has to cooperate too.

#8 azure1961p


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Posted 06 July 2013 - 08:58 PM

Thanks for the clarification Fred. I'm convinced there are big aperture doubles that are stunning but get less press simply because most folks have smaller apertures for any number of reasons. I learned my lesson with color and aperture years ago when I kept spouting off to an observing buddy how rich the reds were in this particular double till one night we are out under the stars and he's got his 17.5" Coulter and he shrugged a bit when he saw it. I was taken aback - till I looked - crimson was now rusty orange!!!

Again thanks for the input!


#9 ggalilei



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Posted 08 July 2013 - 09:54 AM

I find the large aperture (18") great for splitting tight doubles. For instance, I was able to push the magnification all the way up to 1100x to really separate the components of lambda Cygni even if the seeing only allowed 200-250x on Saturn that night.

#10 Nucleophile


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Posted 08 July 2013 - 12:05 PM

I also like having more aperture so that more stars at my disposal to resolve. I find I cannot get as close to the Dawes Limit with the 15 inch (vs my 8 inch), but this doesnt bother me much.

If I want to, I can always stop the aperture down--the attached photo shows a 6 inch off axis mask I made for my 15 inch scope--its ugly, but it works!

As Pete alluded to earlier, the smaller apertures can offer more pleasing views of colorful doubles--I saw this first hand the other night with the 6 inch mask in place--Izar, Cor Caroli, Rasalgethi and many others came alive! I can see why folks love their refractors. :grin:

Interestingly, on this same night of quite poor seeing, Antares B showed better as a disk separated from the primary when using the 6 inch off-axis mask versus unmasked to the full 15 inches. So, there exists practical utility with its use as well.

Attached Files

#11 mikey cee

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Posted 08 July 2013 - 05:59 PM

My 10" F/11 R30 Istar refractor is all I need. Seeing won't allow much more to be seen anyways. Most of the .5"-1.6" doubles are great now seeing permitting. I can tell that the .4" ones are egg shaped at 685x(15 plossl) and 815x(12.5 ortho). I could buy shorter FL eyepieces and go to a 1000x but I'm really satisfied with my present near perfect views now. ;) Mike

#12 Ziggy943



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Posted 10 July 2013 - 07:37 PM

Aperture under the right conditions will help. I was able to compare side by side my 9" refractor with an 18" reflector at about 10,000 feet on Beta Delphinius. I was not able to split it with my 9" but we did with the 18". So yes, aperture helps. Beta Delphinius was under .5" at the time.

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