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Refractor equivalent of an 8" reflector?

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#51 astro_anthro

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Posted 04 August 2008 - 10:02 PM

Maybe when you guys finally settle this one you can solve other pressing issues, like who would win in a fight, Superman or The Hulk? :grin:

#52 Daniel Mounsey

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Posted 04 August 2008 - 10:19 PM

I wasn't suggesting they lack consistency from unit to unit. I am suggesting Newts tend lack consistency from night to night in showing high resolution, high contrast images, at high mag.

I'm not talking about local seeing from night to night. I'm suggesting that if you set up a high end APO and a high end Newt of roughly the same size (maybe a 180mm APO and 8" Newt) side by side over several nights, the APO (refractor) will more often show better high res, high contrast, planetary detail.

Rich

Ah, the refractor vs. reflector discussion. I seem to recall some experiences in this area haha. I've been going through this whole thread and this statement is actually correct and one of the reasons is because refractors are simply dummy proof. A beginner really doesn't need to know anything about the design for the most part. Just set it up and it works. About the only set back that a well made achro or apo has is spherical under correction during cool down, and even then, a well made refractor still performs exceptionally well on the planets. :troll: On the other hand, the reflector is far less consistant in optical planetary performance. BTW, I am a planetary Newt man by heart. Once you've seen it work, it leaves an unforgettable mark in your brain that very few refractors can match, due to their limitations in aperture. For a beginner, there are so many things that can go wrong with a Newtonian, hence, the inconsistancy. Is it collimated? is the observers body heat crossing the optical path? Is the observer even aware of body heat? does it need a fan? Is there a boundary layer? Is the observer even aware of a boundary layer? Is the tube design a dormant one? Is it a truss? is there flexture? Is heat from the ground crawling up the tube? So many things can go wrong.

For the most part, with a refractor, these factors are hardly an issue but the real reason that Rich's statement is true is mainly because of one MAJOR set back a Newtonian reflector has to endure, and that's its self destructive thermal behavior, also known as the boundary layer. It's because of this reason that the Newtonian is so often inconsistant. Just a two degree F. difference between the ambient air and the optic is enough to turn an 1/8th wave instrument onto a 1/2 wave system.

Watch the video's and see for yourself. Bryan did a wonderful job.
http://www.fpi-proto...r/sep2000st.htm

#53 Bob Abraham

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Posted 05 August 2008 - 08:42 AM

For observations of features with low contrast (i.e. planetary observing) my experience is in pretty good agreement with the standard "rule of thumb" of determining the equivalent unobstructed optic by subtracting the minor axis diameter of the secondary from the aperture of the primary. Given my local seeing, it's hard to beat a high-quality 8"-10" reflector around here for planets.

Double stars and wide-field panoramas are my favorite objects for a small refractor. In fact, for things that require high magnification, I'd say that double stars are the only class of objects for which I feel a much smaller refractor consistently gives me qualitatively nicer views than a much larger reflector (although the resolution is poorer of course). I personally really enjoy seeing the clean airy disc of small unobstructed optics. Also, there are so many interesting doubles out there that there is no lack of interesting targets for a small refractor. So, in general, for most double star observing I'd actually prefer a 4" apochromat to an 8" Newtonian.

#54 KaStern

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Posted 05 August 2008 - 11:17 AM

Hello drshr,

But 82% of 82% is still pretty poor. And that 33% approx lost light isn't just lost, it scatters and reduces contrast.



I cannot follow your mathematics :question:

And the lost light does not scatter all, most of it goes through the reflecting
layer of the mirror. You see this when you are in a dark room and put a
light
behind the mirror and look at the mirror from the front side, or put a light
before the front side and look from behind the mirror.

The biggest part of the rest of the lost light is absorbed by the mirror
anf only the smallest part is scattered.
From this scattered light on the part that comes into the image plane
accounts for loss of contrast.


Hello Daniel,

... one MAJOR set back a Newtonian reflector has to endure, and that's its self destructive thermal behavior ...


This is a very serious argument! But you can cure most of it`s destructive effect by:

1) making the tube diameter bigger. Most of the industrially made
8" Newtonian tubes are too narrow to avoid serious thermal effects.
The have only 230mm diameter. By adding another 40mm Diameter most
of the thermal air flows get out of the incoming light path.

2) adding a cooling fan to the back side of the tube. A 90mm fan is
sufficiant for an 8" Newt. If the fan sucks the air effectively out
of the tube most thermal effect are solved.

Clear skies, Karsten

#55 mathteacher

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Posted 05 August 2008 - 11:57 AM

I cannot follow your mathematics


If I may, I think he is saying the primary and secondary both have 82% reflectivity.
That means 82% of incoming light is reflected, then 82% of that actually reaches the eyepiece.

82% of 82% is 67%. (0.82 x 0.82 = 0.67) Hence, 33% of light hitting the primary mirror is lost.

#56 spaceydee

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Posted 05 August 2008 - 12:22 PM

that's assuming 0.82 reflectance for both the primary and secondary. who knows what "reality" is.

#57 galaxyman

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Posted 05 August 2008 - 12:23 PM

I cannot follow your mathematics


If I may, I think he is saying the primary and secondary both have 82% reflectivity.
That means 82% of incoming light is reflected, then 82% of that actually reaches the eyepiece.

82% of 82% is 67%. (0.82 x 0.82 = 0.67) Hence, 33% of light hitting the primary mirror is lost.


That's why my 22" is 98% & 98% :grin:

Anyway, a good 8" f/6 newt with great optics and coatings is a bit better than a 6" refractor (either Apo or achro) for most objects, though it is close.

At this size variation I prefer the refractor just on its aesthetics both in look and the slightly cleaner view at the eyepiece.

For me personally, I like the dobs from 12" on up.

Karl
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Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB 8" F/9 Refractor(The Beast)
Astrozap 6" f/8 Refractor
Orion 4" f/6 Refractor

#58 JakeJ

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Posted 05 August 2008 - 12:45 PM

I personally feel there is no refractor equivalent of an 8" reflector - they are totally different animals. The equivalent of an 8" reflector is an 8" reflector.

In other words, assuming both scopes are 8", the relector shows diffraction spikes, the refractor does not. The reflector most likely can go to a wider field and will have a different focal length. The refractor requires a much larger mount, the reflector will cost less, etc. etc. etc.

#59 Chimaera

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Posted 05 August 2008 - 12:49 PM

"But 82% of 82% is still pretty poor."

Perhaps you are putting reflectors in the worst possible light. Secondary mirrors with over 95% reflectivity are readily available and easy to change. I believe primary mirrors with enhanced coatings have well over 90% reflectivity.

Let us not forget that there is light loss from a mirror in a refractor as well.

Anyways, no optical system is perfect, and it may come down to personal preference. Which poison do you wish to take - diffraction spikes and off axis coma, or the psychodelic colours in a big achro.

#60 KaStern

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Posted 05 August 2008 - 01:42 PM

Which poison do you wish to take - diffraction spikes and off axis coma, or the psychodelic colours in a big achro.



LOL :funny:

#61 peleuba

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Posted 05 August 2008 - 02:09 PM

Compared side by side which size refractor would be similar to an 8 inch f5.9 reflector scope?
For example 80m -> 6in, 100mm -> 8in?

I'm asking this because for my next scope I plan on stepping over to the refractor camp (guiding and Goto).



I routinely compare 6" APO to 7" and 8" Zambuto reflectors - see my sig for specifics. I am a dedicated planetary observer and really enjoy such comparisons.

Basically, I've found that the 8" Z-mirror when cooled will be ever so slightly better then the 6" APO. And the 6" APO will edge out the 7" reflector. The 6" APO, an FS152, is a doublet and does not focus all colors to a common point as a matter of design. The blue is thrown way out of focus so the image has a slightly warmer look to it then that of the reflectors. I have often wondered if a triplet was compared to a the 8" reflector how it would perform. Probably the same as the doublet as resolution is primarily a function of aperture.

The differences are quite minor in good seeing. But when the atmosphere is unsteady, I prefer the views in the FS152 over everything.

#62 galaxyman

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Posted 05 August 2008 - 02:10 PM


Anyways, no optical system is perfect, and it may come down to personal preference. Which poison do you wish to take - diffraction spikes and off axis coma, or the psychodelic colours in a big achro.


I agree, though I think some people on each side take those issues a bit too far.

Both my large reflectors show stars sharp and clear almost to the edge with good eyepieces. My 3 achros do not show this psychedelic color, particularly on 99.9 % of the Universe.


Karl
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Chesmont Astronomical Society
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB 8" F/9 Refractor(The Beast)
Astrozap 6" f/8 Refractor
Orion 4" f/6 Refractor

#63 drshr

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Posted 05 August 2008 - 05:59 PM

Galaxyman: that 22" dob with enhanced coatings will be a stunner for sure. I agree with you, the dobs need to be big to gain the advantage. But they will still be fiddly creatures. That 8"er of yours will also be stunning but fiddle free, reliable and aesthetically delightful. This business about kaleidoscopic colour is truly exaggerated. Most of what we look at has no colour and with the semi-apo filter now available for those odd bright objects the problem is solved and cheaply. Triplets are too expensive, too heavy and take longer to cool down. If I were one of your grandchilden, I would be hoping that you would leave me that 8" refractor!
PS how much better is the 8" over the 6" refractor?

#64 galaxyman

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Posted 05 August 2008 - 10:43 PM

Galaxyman: that 22" dob with enhanced coatings will be a stunner for sure. I agree with you, the dobs need to be big to gain the advantage. But they will still be fiddly creatures. That 8"er of yours will also be stunning but fiddle free, reliable and aesthetically delightful. This business about kaleidoscopic colour is truly exaggerated. Most of what we look at has no colour and with the semi-apo filter now available for those odd bright objects the problem is solved and cheaply. Triplets are too expensive, too heavy and take longer to cool down. If I were one of your grandchilden, I would be hoping that you would leave me that 8" refractor!
PS how much better is the 8" over the 6" refractor?


Yes, the 22” is always a jaw dropper every time out. Oh, and my 12.5” homemade dob though needs to be recoated has 96% & 98%.

You mentioned about the dobs being a bit fiddly or fussy, I agree. Though after a bit of time working with them, it’s easy to keep them in perfect working order.

As for the 8” and the 6” refractors. This past late Spring from Cherry Springs here in Pennsylvania, a friend had his Antares 6” f/6.5 next to my TMB 8” f/9.

For some added info, I use to own a fine Antares 6” f/6.5, and always wanted to know how well it would do next to the larger refractor. My mini-van could not fit both scopes to take to a dark site.

Anyway, though the Antares did well, it certainly was dusted by the 8” refractor. Not only were the objects much brighter and more detailed, but the larger scope’s views were much sharper, particularly from medium to high power.

Now my current Astrozap is a tad better than my former Antares, it still wouldn’t be close. For though the Antares, Astrozap, Celestron, and Skywatcher models are real nice, and many go beyond in performance for the price in the refractor world. These APM/TMB’s are more high-end in the achro world, with the optics and coatings that are superior compared to the less expensive achros.

Yes, my grandchildren should have some nice scopes to own, but hopefully that will be a long time from now :gramps:

Karl
E.O.H.

Chesmont Astronomical Society
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB 8" F/9 Refractor(The Beast)
Astrozap 6" f/8 Refractor
Orion 4" f/6 Refractor

#65 stkoepke

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Posted 06 August 2008 - 12:38 AM

Just my two cents worth:
Instead of asking what size refractor will match what size reflector you may want to consider another question.

"What size refractor will best compliment your (insert size here) reflector?" (or visa-versa)...

I use my 8" Dob. quite often, sometimes as a Dob. and sometimes I mount it on a CG5GT or on my Criterion mount. Depending on wether I'm imaging or not will also determine the size refractor I will also use. A small 70mm to 90mm for autoguiding, (if I am using the CG5). Or for visual I may use a 4" to 6" refractor on a one mount and the Dob. on another (either mounted or in it's original base).

Visual, which I do more often than imaging, I find that both types of scopes are have their pro's and con's, but they compliment each other very well.

Pro vs. Con:
Newt. has no CA, but the Achro does.
Achro has no difraction spikes, but the newt. does.
(There are other pros/cons but these are the biggest to me.)

In any case where one type of scope starts to leave off the other picks up and in the end I have a much better viewing session with both types of scopes available to use.

Both my large reflectors show stars sharp and clear almost to the edge with good eyepieces.



Galaxyman, IMHO you "hit the nail on the head" with this one. One of the most important things to consider when buying/using a scope, either refractor or reflector of at least average or better quality, no matter the size, are the eyepieces.

Wether you have a $250 or a $10,000 scope, if you use "cheap" (quality wise) ep's you will have less than favorable views. I'm not saying that you need to get the most expensive EP's on the market, but you should at least use EP's that compliment the scope that you are using.

Example: A Meade "MA" will show the stars but a Hyperion, Stratus, or at the very least a good Super Plossl will give you a far superior view than the MA.

As I said, Just my two cents worth...

#66 KerryR

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Posted 06 August 2008 - 05:12 PM

I have a nice C6-R with a Chromacorr, and a nice 8" f8 newt with a very fine Raycraft Primary, Protostar secondary, and flocked interior. I can't recall offhand the size of the secondary, but it's scaled for my 2" EP's, so it'e bigger than is optimal for planets.

Aestheticly, I find the refractor better, especially on evenings of dropping temps. I know the refractor has to cool too, but the views seem to be sharper sooner, and I never see tube currents.

The newt does reach deeper, though. But, I generally like to see sharper than deeper for the kinds of things I use the refractor for (planets and clusters). When depth is the concern, I opt for more aperture.

Under ideal conditions, I'm sure my 8" newt will out-perform the refractor. But, conditions are rarely ideal, and so the edge goes to the refractor-- tends towards better sharpness and better contrast due, I think, primarily to the lack of tube currents and cooling time.

Apples to Oranges, though.

Kerry

#67 richardr

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Posted 07 August 2008 - 01:42 PM

Agree here, this is my experience too. A good 4" apo will keep up with an ordinary 8" Newt(store-bought). Even a good 4" achro will surprise you with the sharpness of its views on the planets by comparison to an 8" newt. The ordinary 8" newt will show as much detail or more but, it just won't be as pleasing IMO(YMMV etc. etc.).

DSO's are different, aperture rules, However, clear and crisp refractor views have their appeal as well. Go to a dark site star party and check it out. :jump: :jump:

#68 Daniel Mounsey

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Posted 07 August 2008 - 02:19 PM

I agree and it's mainly due to the thermal behavior.

#69 Jan Owen

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Posted 07 August 2008 - 03:18 PM

Well said!!!

Each of my telescopes were chosen for a specific role within the whole of the group... Each supports (more, or less, at least) and complements the rest...

My only regret is that I've had to shed my larger Dob's to support my BACK, which now has 5 consecutive decimated discs at the lower end...

Jan

#70 dougmc

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Posted 17 January 2009 - 11:17 PM

The larger reflector gives you the opertunity to use an off axis appiture stop. My 8 inch F6 gives an 80mm, while my 10 inch gives 100mm. As good as apo or better on double stars or any thing else. Have your cake and eat it to, with the reflector.

#71 Rich N

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Posted 17 January 2009 - 11:25 PM

I've had my AP180EDT (180mm f/9 APO triplet) along side of my friends fine 8" f/6 Newt a number of times. My refractor is much more consistant in giving excellent images, but on a night when the Newt is performing really well the planetary detail is virtually the same.

Rich

#72 CHASLX200

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Posted 18 January 2009 - 11:58 AM

I will go toe to toe with any 7" or 8" APO with my 8" F8 Newt and my Dan Joyce 8" f/6.7 Newt. In the end i paid very litte for both Newts, and the 8" APO owner has paid big time for the 8" APO and it's mount.

Had my share of 5" to 7" APO's and they just dont cut it for such a high price. They do give that super fine snap to focus image, but my well built 8" Newts can pretty much do the same thing and cost 50x less.

Chas

#73 Arizona-Ken

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Posted 18 January 2009 - 01:12 PM

Karsten, on mirror reflectivity, the aluminum coatings on a mirror oxidize with time. I've read estimates from 1-5% degradation per year depending on the atmosphere where your scope is kept.

Also, there are 2 mirrors on an newtonian, so any loss in reflectivity is compounded. If you have 90% reflectivity on both mirrors, that means 81% of light reaches the eyepiece (0.9x0.9 = 0.81).


Wow, that means on my 27 year old C8, I can only get 0.058 percent of the light grasp of when it was new.

How did I see M108 from my back yard in the Phoenix skies last night?

It must be that corrector lens!!!


:foreheadslap:

Arizona Ken

#74 CHASLX200

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Posted 18 January 2009 - 01:17 PM

Karsten, on mirror reflectivity, the aluminum coatings on a mirror oxidize with time. I've read estimates from 1-5% degradation per year depending on the atmosphere where your scope is kept.

Also, there are 2 mirrors on an newtonian, so any loss in reflectivity is compounded. If you have 90% reflectivity on both mirrors, that means 81% of light reaches the eyepiece (0.9x0.9 = 0.81).


Wow, that means on my 27 year old C8, I can only get 0.058 percent of the light grasp of when it was new.

How did I see M108 from my back yard in the Phoenix skies last night?

It must be that corrector lens!!!


:foreheadslap:

Arizona Ken


Ya must have a magic C8. :bow: :jump:

Chas

#75 TheMenace

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Posted 18 January 2009 - 01:18 PM

6" achro. :jump:


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