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Refractor equivalent of an 8" reflector?

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#101 actionhac

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Posted 21 January 2009 - 09:31 PM

I have compared my 4"f10 Vixen to my vintage no special coatings Meade 2080 8"f10 SCT, side by side, with the same EP's.
The Meade absolutely blows it away. Aperture. Physics is physics.

Robert

#102 alanon

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Posted 21 January 2009 - 09:38 PM

I love that Avatar, Robert. :rofl2:

#103 AlienRatDog

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Posted 21 January 2009 - 11:23 PM

I agree, Physics is Physics...

#104 tkobyrne

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Posted 22 January 2009 - 05:48 AM

I think you mispelled the word. It should be spelled psychology.
Which would the average person more vigorously justify and defend.
The $5,000-$10,000+ "premium" refractor/$3,000-$7,000+ "premium" reflector or the $500-$1500 machine produced Synta ED or Dob.
Yes, by any comparitive test you want to make, there is a difference.
But, under the very best conditions that that any amateur astronomer in the history of the world has ever viewed, the atmosphere has cancelled out at least 50% of the theoretical capabilities of a "perfect" telescope.
A person who has really learned to SEE could with 3" find more then most could with an 8".
I know with the really bad weather everyone has been having everyone is getting antsy but just think, in only 4 months we can all be arguing about who has the biggest mosquitos.
Tom

#105 gsoman

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Posted 08 March 2009 - 10:05 PM

Well i have just purchsed a galileo 203-C 203mm reflector and I will admit, that so far I have been more impressed with the 203mm than my 90mm refractor from celestron. Iam not saying that the refractor isn't a good scope but compared to the sigths that I have seen so far i love the reflector by far. I quess Iam saying that because Iam more into deep sky objects than planets. I have found that the refractor gives a clearer image of planets than the reflector but i think it just matters what you are into.

#106 contrailmaker

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Posted 09 March 2009 - 06:36 AM

I have done the side-by-side during a couple of well attended starparties. My well collimated and optically very good C8 performed extremely similar to a very good Meade 6" APO on Jupter on a night of exeptional seeing. The refractor edged it out with better contrast. No surprise. Compared to a home made 8" Newtonian with a small 20% obstruction the images were almost impossible to tell apart.

So, my answer to the original question is: 6" refractor.

I think we are finally well past the myth of a 4" refractor being "sharper" or "showing more" than an 8" reflector.

cm

#107 Covey32

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Posted 09 March 2009 - 07:24 AM

No we're not, apparently.
This old and tired conversation just seems to drag on and on, using the same old arguments that fly in the face of both common sense and physics.
Use what you like and enjoy it, but don't try to substitute personal opinion that contradicts actual physics.
Drop it...let's all agree that a 6" Apo costing $7000 is a bit better than an 8" F5.9 reflector costing less than $1000.
Which one would a sane person on a budget buy?
And if you're not on a budget, why ask?

#108 JerryWise

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Posted 09 March 2009 - 07:51 AM


It doesn't seem bad to rehash these topics occasionally. There are new people coming in all the time with the same questions and curiosities. Old and tired conversations are only old and tired to those that have heard them. Let's give the newbies a chance to learn and not be embarrassed at having to move over the same learning curve we have. As has been said many times regarding APOs, if you can see the difference and need the difference then spend the difference. Many are are asking for help and trying to identify what that difference is.

#109 Covey32

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Posted 09 March 2009 - 09:30 AM

True, and for that reason this may have been a worthwhile thread.
To me, the way it has shaken out as far as a newbie looking for advice is:
If you're loaded and want to cater to your whims, get a nice expensive optically perfect 6" APO.

If you have to count your money and ask what things cost, get a good 8" Reflector or SCT.

Then forget about all the above and enjoy the hobby, then move whatever you bought outside and look through it because either way whatever you select is going to provide views and technology that Galileo would have killed for.

#110 KaStern

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Posted 09 March 2009 - 09:43 AM

This old and tired conversation just seems to drag on and on, using the same old arguments that fly in the face of both common sense and physics.
Use what you like and enjoy it, but don't try to substitute personal opinion that contradicts actual physics.



Jepp :waytogo: :applause:

#111 actionhac

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Posted 09 March 2009 - 10:56 AM

This has been very useful for me. But there are so many variables. One minute a 5" achro seems equal to a 8" newt and a 6" is more telescope, than conditions are right for the reflector and the newt out performs both. I had this happen recently. I was viewing thru a old Criterion RV-8 newt and I wanted to take it home with me.

Robert

#112 oldtimer

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Posted 09 March 2009 - 11:13 AM

I am currently optimizing a 10" F6.5 newt for hogh power visual planetary observation. It will have a 1.5" secondary and a very very low profile focuser. There will be NO LIGHT LOSS at any power above 125X

#113 Alvin Huey

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Posted 09 March 2009 - 11:28 AM

I didn't plow through this thread...but I think a 6" achro is equal to a 8" SCT or slightly less than a 8" Newt. I based this on one night with my 6" achro with my friend's 8" SCT. The views of DSO's were the same. But kinda tough as we had to use very different eyepieces to achieve the same mag as his SCT focal length is twice as long as my achro.

The SCT has a corrector plate and a larger secondary, so there is a slightly less light ending up at the eyepiece in the SCT versus the Newt, hence why I said slightly less than 8" Newt.

#114 Ziggy943

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Posted 09 March 2009 - 01:26 PM

Recognizing that everyone has their own opinions and preferences, here is my opinion and preference based upon 45 years of telescope ownership which by inventory would include something like about 40+ mounted telescopes and numerous guide scopes that would add about 15 or so more to the mix. I have had all types of scopes including both apo and achro refractors, maksutovs, schmidt-cass, newtonian reflectors (both equatorially mounted and dobsonian). While different telescopes are suited for different purposes, my overall preference is the refractor for a number of reasons, in particular the apo refractor of which I have owned 8 different apos. I also enjoy the classic long focal length achro refractor and have extensive use and ownership experience with Unitron refractors. One of my scopes that has been with me the longest is my 6" f/5 Jaegers achromat which I completed in early 1979, so it is now 30 years old. Yes, chromatic aberration is an issue, but for wide field views and tight stars it is a keeper! I also like maksutovs, much more so than schmidt-cass cats. While newtonians have their place and do offer a lot of bang for the buck, they are not particular favorites of mine, even though I do own and have owned them. Too many issues including collimation and viewing positions.

From my personal experience and in general for most applications my rule of thumb would be that a newtonian would have to "out-aperture" a refractor by about 33% to be roughly equivalent for most applications. Accordingly the statement that a 6" refractor is equivalent to an 8" reflector is for the most part, pretty valid. As refractors tend to thin out after 6", the reflector really comes into it's own after you climb above 10" because then an amateur is hard pressed to find or afford an equivalent refractor. I don't think there are too many of us that can argue that their 18" refractor is better or equivalent to their buddy's 24" newtonian dob, not because there are no 24" newtonian dobs, but because I don't see too many 18" refractors in the hands of amateurs.

Barry Simon


I haven't owned as many scopes but have certainly looked through as many and fully agree with your assessment.

#115 drshr

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Posted 12 March 2009 - 06:37 AM

Also Agree.
Remember a smaller refractor need not be equal in an absolute sense to be more reliable and therefore used more.

#116 Takman

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Posted 12 March 2009 - 01:32 PM

My personal experience is that in practice, a good 4" to 5" inch refractor is the equivalent of a good 8" reflector for planetary and lunar observations. For DSOs, you are looking (at most) at a 2" advantage for the refractor, which means a 6" refractor will appear to be as efficient as a 8" reflector on DSOs. As a rule of thumb (and there are many exceptions), you can subtract the diameter of the central obstruction of the reflector to obtain its equivalent refractor performance. Thus, a 10" reflector with a 2.5" central obstruction will perform as well as a 7.5" refractor.

However, in the premium instrument world, this gap diminishes significantly. Assuming premium optics, I think that DSO performance is much more closely tied to aperture, assuming good seeing and dark skies.

#117 Alvin Huey

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Posted 12 March 2009 - 03:36 PM

My personal experience is that in practice, a good 4" to 5" inch refractor is the equivalent of a good 8" reflector for planetary and lunar observations.


I would politely disagree to this. I looked through an excellent 5" apo (my old Tak TOA-130S) and a very good 8" Newt (8" f/7) at Jupiter and Saturn. My Tak was soundly beaten by the views through the 8" Newt.


For DSOs, you are looking (at most) at a 2" advantage for the refractor, which means a 6" refractor will appear to be as efficient as a 8" reflector on DSOs. As a rule of thumb (and there are many exceptions), you can subtract the diameter of the central obstruction of the reflector to obtain its equivalent refractor performance. Thus, a 10" reflector with a 2.5" central obstruction will perform as well as a 7.5" refractor.

However, in the premium instrument world, this gap diminishes significantly. Assuming premium optics, I think that DSO performance is much more closely tied to aperture, assuming good seeing and dark skies.


I agree with the above statement regarding DSO's. I wondered why...then I thought the overall transmission through a refractor is far higher than the reflector, which makes up only part of the difference. The contrast loss due to the central obstruction adds to the effect.

Here is a little surprise, at least I was. I had a 6" achro and a 5" apo mounted together on several occasions. The 6" soundly beats the 5"apo every time on DSOs. I thought it would be very close due to the quality of the optics (achro versus apo). I'm sure that the transmission of the 5" is better than the Chinese made Antares 6" achro.

#118 BillP

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Posted 12 March 2009 - 04:12 PM

...Here is a little surprise, at least I was. I had a 6" achro and a 5" apo mounted together on several occasions. The 6" soundly beats the 5"apo every time on DSOs. I thought it would be very close due to the quality of the optics (achro versus apo). I'm sure that the transmission of the 5" is better than the Chinese made Antares 6" achro.


FWIW, even if the 6" Achro was uncoated and the 5" APO triplet was fully multicoated, the 6" Achro would still pull in almost 1.26x more light. If the Achro was single coated then it pulls in more than 1.38x more light, multicoated then 1.45x more light. So it's extra inch will allow it to win regardless.

btw, I have the same experience as you...on DSO not much difference visually between an APO or Achro of same aperture, especially at low to mid magnifications. At higher ones if brighter stars are around then you can detect the blue halo.

#119 roadi

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Posted 12 March 2009 - 04:17 PM

Here is a little surprise, at least I was. I had a 6" achro and a 5" apo mounted together on several occasions. The 6" soundly beats the 5"apo every time on DSOs. I thought it would be very close due to the quality of the optics (achro versus apo). I'm sure that the transmission of the 5" is better than the Chinese made Antares 6" achro.


I too have a 6" f5 Chinese achro and it always beats my Tak FS102 easily on deepsky. IMO the "probably" better figure, correction and polish on a smaller premium scope shows it stuff when both scopes are pushed to or near the limit of max power when steadiness of the atmosfere permits. At low power the slightly bigger apature will winn in light gathering.
Your 6" achro has 44% more lightgathering power than your 5" premium scope. No polish highly accurate figuring and coating would make up for that difrence I believe!!
;)

#120 bartine

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Posted 13 March 2009 - 09:20 AM

I have had several refractors over the years and several 8" reflectors.

On the refractor front I had:
a 78mm vintage Tasco f15
a 150mm Celestron and Meade (on lxd55 goto)
a 90mm Meade 395
an 80mm Meade ETX

8" reflectors:
Currently an 8" F8 Edmund
a 8" Orion with the single stalk secondary
an 8" Meade 828 F6

In general, a lot can be said for the useability of a refractor - I found mine were well baffled, and they don't have problems with cooling. But - a 6" refractor is HEAVY, and there was some was some color on the 6".

For the 78mm at f15.4 - no color at all. But - it was very long, and took a solid mount to get rid of the shakes. The focus on the refractors was very forgiving, and it seemed easy to get good images on the planets.

The ETX80 gives good views and is a decent wideview scope. It was cheap and very portable, and I like the goto aspects of it.

I always saw more detail in clusters such as the Double Double or nebulas with any of the 8" reflectors. Planetary views were better than the reflectors under some conditions, but on an average night for seeing, without a ton of time spent on collimation, the views through the refractors were quite pleasing.

I eventually got rid of the refractors except for the 80mm ETX. I got a high end 12.5" (a Portaball which is very simple to use), and eventually got an 8" f8 Edmund to compliment it.

#121 Ziggy943

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Posted 13 March 2009 - 09:46 AM

It doesn't seem bad to rehash these topics occasionally. There are new people coming in all the time with the same questions and curiosities. Old and tired conversations are only old and tired to those that have heard them. Let's give the newbies a chance to learn and not be embarrassed at having to move over the same learning curve we have. As has been said many times regarding APOs, if you can see the difference and need the difference then spend the difference. Many are are asking for help and trying to identify what that difference is.



Wise words. :smirk:

#122 Ziggy943

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Posted 13 March 2009 - 09:57 AM

I didn't plow through this thread...but I think a 6" achro is equal to a 8" SCT or slightly less than a 8" Newt. I based this on one night with my 6" achro with my friend's 8" SCT. The views of DSO's were the same. But kinda tough as we had to use very different eyepieces to achieve the same mag as his SCT focal length is twice as long as my achro.

The SCT has a corrector plate and a larger secondary, so there is a slightly less light ending up at the eyepiece in the SCT versus the Newt, hence why I said slightly less than 8" Newt.


This comparison brings to mind that there are also differences between 6" refractors. I heavily favor the long focus refractors. In the real world I prefer the planets through a long focus 6" refractor to *most* of the views I have had through 8" SCT's and most 8" Newts. Throw in a fast 6" achro and I don't know that I would make that statement.

I think there is as much or more of a difference between a 6" F/5 and a 6" F/15 as there is between a 6" F/15 and an 8" SCT or an 8" Newt.


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