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Old 03-10-2019, 03:27 PM
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150mm Verse 152mm refractor

Is there a visual performance difference between achro refractors of 150mm & 152mm aperture ' both equal optical quality & same focal length ?

I guess theoretically the 152mm should outperform the 150mm but is it noticeable .

Cheers
Bobby
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Old 03-10-2019, 05:29 PM
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Hi Bobby,

Short answer is: "No", assuming equal optical quality.

Best,

L.
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Old 03-10-2019, 06:30 PM
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Quote:
Originally Posted by ngcles View Post
Hi Bobby,

Short answer is: "No", assuming equal optical quality.

Best,

L.
Thanks L ,

May I ask what size difference would be needed to see a difference in your opinion

Cheers & Beers
Bobby .
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Old 03-10-2019, 08:31 PM
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Hi Bobby,

I'd think about 15% larger by surface area of the lens or mirror, so approximately therefore, about 165mm aperture before there is a genuine, apparent difference.

Best,

L.
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Old 03-10-2019, 09:35 PM
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Quote:
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Hi Bobby,

I'd think about 15% larger by surface area of the lens or mirror, so approximately therefore, about 165mm aperture before there is a genuine, apparent difference.

Best,

L.
Gee ' I would of thought less but trust your opinion more

What I greatly appreciated was you gave a figure , so much better then getting a sit on the fence nebulous reply from those robot type thinkers

Regards
Bobby
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Old 04-10-2019, 05:46 PM
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I think Les’s estimation is very close. Of course individual eyes may detect slight variations lower than this. But this will be very subjective.
I have spent time comparing the views through 140 and 152mm refractors and could detect only a very slight increase in brightness in the 152mm scope. Here the difference is 18% and one had to spend time looking for the difference.

Last edited by Tinderboxsky; 08-10-2019 at 07:35 PM. Reason: Correction
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Old 04-10-2019, 10:23 PM
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I think Lesís estimation is very close. Of course individual eyes may detect slight variations lower than this. But this will be very subjective.
I have spent time comparing the views through 140 and 152mm refractors and could detract only a very slight increase in brightness in the 152mm scope. Here the difference is 18% and one had to spend time looking for the difference.
Thanks for your input Steve , splitting close doubles could show the difference a bit clearer ? maybe

Cheers
Bobby.
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Old 05-10-2019, 12:19 AM
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2/3rds of 5/8ths of ...

Hi Bobby,


Quote:
Originally Posted by Bobbyoutback View Post
Thanks for your input Steve , splitting close doubles could show the difference a bit clearer ? maybe

Cheers
Bobby.
In terms of splitting pairs (this also applies to all other visual applications), the difference between 150mm -v-152mm is about 2/3rds of 5/8ths of ...

The Dawes limit (approximation) difference between say 150mm and 165mm is 0.77 arc seconds -v- 0.70 arc seconds. In relatively small apertures, a small increase gives a pretty small gain. 150 -v- 152 is a minuscule difference.

Dawes limit approximation formula: R=11.6/D where D is the diameter of the aperture in centimetres.

So for example. with my 46cm f/4.9 Newtonian the Dawes limit is 0.25 arc-seconds. With the 63.5cm f/5 Newtonian, the Dawes limit is 0.18 arc seconds. Obviously enough, the seeing (virtually) never permits large apertures to approach their Dawes limit. It's a very, very rare night when the seeing is running at about 0.5 arc seconds or marginally better than that. 0.2 arc-seconds is probably something you might see in the Atacama Desert, maybe once every year or few.

I think I've seen seeing better (marginally) than 0.5 arc seconds three times in my observing career of nearly 50 years in NSW. It is a bit better here where I live now than Sydney (where I used to live) that normally has mediocre seeing and rarely good or very good. I think generally, the seeing in Broken Hill is going to be better than either where I live, or Sydney, but still only rarely approach 0.5 arc-seconds.

Best,

L.

Last edited by ngcles; 05-10-2019 at 10:33 AM.
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Old 05-10-2019, 11:41 AM
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Quote:
Originally Posted by ngcles View Post
Hi Bobby,




In terms of splitting pairs (this also applies to all other visual applications), the difference between 150mm -v-152mm is about 2/3rds of 5/8ths of ...

The Dawes limit (approximation) difference between say 150mm and 165mm is 0.77 arc seconds -v- 0.70 arc seconds. In relatively small apertures, a small increase gives a pretty small gain. 150 -v- 152 is a minuscule difference.

Dawes limit approximation formula: R=11.6/D where D is the diameter of the aperture in centimetres.

So for example. with my 46cm f/4.9 Newtonian the Dawes limit is 0.25 arc-seconds. With the 63.5cm f/5 Newtonian, the Dawes limit is 0.18 arc seconds. Obviously enough, the seeing (virtually) never permits large apertures to approach their Dawes limit. It's a very, very rare night when the seeing is running at about 0.5 arc seconds or marginally better than that. 0.2 arc-seconds is probably something you might see in the Atacama Desert, maybe once every year or few.

I think I've seen seeing better (marginally) than 0.5 arc seconds three times in my observing career of nearly 50 years in NSW. It is a bit better here where I live now than Sydney (where I used to live) that normally has mediocre seeing and rarely good or very good. I think generally, the seeing in Broken Hill is going to be better than either where I live, or Sydney, but still only rarely approach 0.5 arc-seconds.

Best,

L.
Hi again Les ,

Top information , I do now understand there is stuff all visual advantage between apertures of small difference


Although i was familiar with Dawes limit what I didn't appreciate was how rare 0.5 arc-second seeing is .

I'm somewhat spoilt with so many nights of excellent seeing out this way ( Broken Hill area ) .

Many years back I was living & working in a tiny town called Silverton pop about 35 .
One night I was lucky to have still been outside in the early morning hours when the seeing went from very good to off the scale ''' I'll call it exquisite , extraordinarily magnificent viewing !
How good did it get ? I've asked myself Answer is ~ it only ever happened that once .

Cheers & Beers
Bobby
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Old 08-10-2019, 11:44 AM
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I was wondering if the optics were less than perfect (technically impossible to achieve) wouldn't the 152mm give increased chromatic aberations and so measurably worse viewing (still assuming both are "same quality" optics)? Not noticable enough to typical eyes but maybe enough to make splitting doubles muddier. Does any manufacturer actually produce two OTAs really identical in performance and only slightly different size?
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