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Refractor equivalent of an 8" reflector?

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#26 mathteacher

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Posted 02 August 2008 - 06:33 PM

drshr, welcome to Cloudy Nights by the way. You sure picked a nice friendly topic to leap into :lol:

I agree with you that reflectivity loss is rarely mentioned. I'm looking for more aperture, and having a hard time deciding between refractor and reflector. I guess a 12"mirror will gather more light than a 6" lens for a long time, but I have difficulty getting over the idea that I'm losing performance over time.

#27 drshr

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Posted 02 August 2008 - 06:56 PM

Sorry if it is, but the point I am trying to make is that in the real world the consistent performance of the refractor means that it will equal that of a reflector of 1.5 to 2x its aperture. Most reflectors out there are not in pristine condition. Reflector users on this site will obviously tend to their equipment more regularly. The question was simple, 4" refractor vs 8" reflector. If I was given that choice I would go with the refractor every time. If it isn't quite as good deep sky as the reflector when both are new it will be in a few years time. It has crisper views with more contrast from the moment you step out of the door with no fuss or fiddle, year after year. And don't forget the wide field views that only the refractor can give. And being smaller it will be used more. I am not anti reflector, in large sizes there is nothing to compare, but to me this question is a slam dunk no brainer. The only strong argument for the reflector that I can see is its cheap cost.

#28 Rich N

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Posted 02 August 2008 - 07:26 PM

Sorry if it is, but the point I am trying to make is that in the real world the consistent performance of the refractor means that it will equal that of a reflector of 1.5 to 2x its aperture. Most reflectors out there are not in pristine condition. Reflector users on this site will obviously tend to their equipment more regularly. The question was simple, 4" refractor vs 8" reflector. If I was given that choice I would go with the refractor every time. If it isn't quite as good deep sky as the reflector when both are new it will be in a few years time. It has crisper views with more contrast from the moment you step out of the door with no fuss or fiddle, year after year. And don't forget the wide field views that only the refractor can give. And being smaller it will be used more. I am not anti reflector, in large sizes there is nothing to compare, but to me this question is a slam dunk no brainer. The only strong argument for the reflector that I can see is its cheap cost.


The question wasn't limited to a 4" refractor vs. an 8" reflector. '

Here is the original post:
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"Here is the original question:
"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)."
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To say a 4" refractor isn't quite as good a deep sky telescope as an 8" reflector is a gross understatement. I love refractors but I also believe my eyes, at least most of the time. I'm sorry you don't have easy access to a mirror recoating facility. An 8" mirror would be pretty dirty if it was giving an image as bright as a 4" refractor.

Rich

#29 spaceydee

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Posted 03 August 2008 - 01:03 AM

I have to agree with Rich. I'd say my reasonably dirty 8" reflector does better than the 4" on most if not all object again depending upon the seeing conditions. What does "reasonably dirty" mean? It means that I was given some good humoured ribbing at a star party for my mirror being so dusty.

#30 Chimaera

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Posted 03 August 2008 - 08:08 AM

When we are comparing the 4" refractor to the 8" reflector, we are talking about almost 4 times greater light gathering ability for the reflector. The mirror coating on the reflector will have to be pretty badly deteriorated for the refractor to match it.

While the loss of reflectivity is compounded by the secondary mirror in the the reflector, light loss through the refractor is compounded by a mirror as well - the diagonal. You can get a durable, high reflectivity dielectric diagonal, but you can also get a dielectric secondary mirror for the reflector.

The hassle and difficulty of collimating the 8" reflector is overblown. At f-6, the optical system is pretty tolerant of minor collimation errors. An 8" scope is most likely a solid tube scope, and if the mirror is mounted on a good cell, collimation once set will need to be re-adjusted very infrequently.

Cool down times for an 8" size mirror is pretty quick, especially with a fan installed. Also, cool down for the refractor is not exactly instantaneous, especially when you are talking about high magnification views.

Don't get me wrong. I love the aesthetic contrasty views in my refractor (SV-90T), but when I want to see something faint, I go to my 8in Royce conical reflector, especially in my light polluted back yard.

#31 Davey

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Posted 03 August 2008 - 02:15 PM

Hi Dee,
It's a simple matter to clean your mirror. I just get a gallon of distilled water and flush off the debris. You'll have a clean mirror in a few minutes and no more ribbing. (o:

Happy observing,
Davey

#32 Jan Owen

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Posted 03 August 2008 - 04:35 PM

I think that's a generally fair reply...

My 8" f/6, with it's stunning Mike spooner primary, and an equally spectacular Anatares Optics certified INSANELY flat and reflective secondary, has been known to shame some well-known APOs in apertures into the 7's, for lunar/planetary, and into at LEAST equal aperture for DSO's...

I have FOUR primary mirrors for this scope, and they are ALL quite nice, including the original, which came from Meade. But I don't know if, in this size range, Mike Spooner can be beat... Tyrannosaurus Wrecks notwithstanding...

Equalled, perhaps... But even that hasn't happened YET...

No. The EON 120 ISN'T gonna' beat it... Haven't tested THAT yet, but I know what Spooner optics can do... And a 120mm ED refractor versus a 200mm Spooner reflector is a foregone conclusion...

Attached Thumbnails

  • 2558026-Jan's Meade Spooner Antares Atlas Beast (Medium).jpg


#33 Guest_**DONOTDELETE**_*

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Posted 03 August 2008 - 05:30 PM

Jan,

I've compared views on an awesome night at Stellafane years ago between an 8" f/7 with Spooner primary and several 4-6" apos from top flight companies. This was mainly on Jupiter when it was really good. The 8" Spooner walked all over the apos. I'm a refractor lover as much as the next guy, but there are some seriously misinformed folks when it comes to comparing good refractors to good reflectors.

Clyde

#34 noisejammer

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Posted 03 August 2008 - 06:10 PM

This question provoked me to make a quick test in poor seeing. I fired up my venerable LX200/12 and had a look at Jupiter through the murk. Frankly, at similar magnification my 4" apo did almost as well.

'Ever heard this... "There is no substitute for cubic inches?" Sadly, I expect the same is largely true of telescope aperture.

Clearest
Bruce, Toronto

#35 Zoomit

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Posted 03 August 2008 - 06:12 PM

I'll second Clyde's opinion. There are many misinformed folks out there!

#36 Tommy5

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Posted 03 August 2008 - 06:30 PM

Yes a well crafted 8" reflector with a spooner f/7 mirror will outperform most 5" or 6" achros or even similiar apos so what? That's not what the original poster was talking about, he has a 8" mass produced Zhummel f/5.9 and wanted to know how big a refractor will give better views.drhsr mentioned how reflectors mirror coating oxidize over time, this isn't a "dirty" mirror but a rusty one it will only get worse over time,of course the spooner scope was mounted on a nice gem, probably cooled and collimated perfectly,sky conditions probably great also,any of these factors change the final result changes,in the real world night in night out all kinds of conditions a 6" achro will put up as good an image of dso and planets as the an off the rack mass produced 8" scope .Their are many missinformed folks out their.

#37 Guest_**DONOTDELETE**_*

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Posted 03 August 2008 - 06:58 PM

No, a well crafted 8" reflector with a Spooner f/7 mirror will totally outclass not only 5 or 6" achros, but 5 or 6" apos as well. And look at the original posters question again. He was wondering if an 80mm refractor would equal a 6" reflector or a 100mm refractor would equal an 8" reflector. No, and it's not really close. And if you can tie your shoe you can collimate a reflector and clean the mirror once a year.

And my 6" $250 Orion 6" f/8 Dob has run totally neck and neck with my buddy's Tak 102 on more than a few nights on the moon and planets. And it's beaten the 4" apo on some nights. Same result with a TV102 and a Vixen Fluorite 102. And it beats my TV85 and his Tak 78 100% of the time. Myths die hard.

#38 Guest_**DONOTDELETE**_*

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Posted 03 August 2008 - 07:00 PM

p.s. The Spooner was a Dob with a 15% central obstruction. My Dob has 19% CO.

#39 Rich N

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Posted 03 August 2008 - 07:17 PM

All this wonderfullness about Newts is fine, to a point. ;)

My experience with them, or side by side with them, is they lack the consistency of a fine refractor at giving high resolution, high contrast images.

For some of you with very small central obstructions, you might check to see how much light loss you are experiencing with an undersized diagonal.

I can afford a very nice Newt and have the mounts to put it on, but I love my refractors. :)

Rich

#40 Guest_**DONOTDELETE**_*

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Posted 03 August 2008 - 07:38 PM

I love my refractors too. But again, that's not what the OP asked. He asked if an 80mm refractor equalled a 6" Newt. No. He asked if a 100mm refractor equalled an 8" Newt. No.

#41 Guest_**DONOTDELETE**_*

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Posted 03 August 2008 - 07:50 PM

"and a 120 ED refractor versus a 200mm Spooner reflector is a foregone conclusion..."

One would think.... but apparently not!

#42 Scott Beith

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Posted 03 August 2008 - 08:53 PM

8" of quality aperture is hard to beat. :bow:

#43 Jan Owen

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Posted 03 August 2008 - 09:01 PM

I wasn't talking about every Newtonian on the planet... I was talking about *my* Mike Spooner Newtonian.

But I'll expand that...

Mike Spooner Newtonians are known quantities... So when you start fabricating generalities around them, chances are, you'll get burned...

Such is the case here...

Side by side, Spooner Newtonians DO NOT lack consistency... They are SUPERB as a class... Certainly up to top craftsmen in other optical venues... And folks who own Spooner optics know that...

#44 Rich N

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Posted 03 August 2008 - 10:29 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

#45 Tommy5

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Posted 04 August 2008 - 12:54 AM

RonBee owes a 102mm T.V. "Lightcup" and a 8" Discovery f-7 dob with an extra small secondary, if you read his analysis of both, he says sometimes in good seeing the dob shows a bit more detail, he even drew sketches of mars using both instruments, the dob did on occasions of good seeing show more detail,they would probably look alike to a novice star gazer, the newt was more difficult to sketch with as it is not driven,Sol Robbins also has done sketches with a 3-inch, and 6 inch refractors and 9.50 reflector where it is easy to see the differences with more aperture.

#46 KWB

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Posted 04 August 2008 - 01:58 AM

8" of quality aperture is hard to beat. :bow:

IMO the most appropriate answer in this thread.However it is obtained,even if financial considerations are the bottom line to ones choice of telescope type,to me that is the bottom line answer.

#47 KaStern

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Posted 04 August 2008 - 07:08 AM

Hello drshr,

coating technologies have improoved over the years.
A german stargazer measured the reflectivity of his over 20 year old
8"f/5 mirror with 88% reflectivity and it came out with 82% refectivity.

Clear skies, Karsten

#48 ranger

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Posted 04 August 2008 - 11:53 AM

Just read all the reviews on this web site where many people have independently compared various large reflectors & cassegrains to refractors and their observations usually keep the refractor in close proximity visually. On paper you would think there would be a wider gap in the comparisons but there doesn't seem to be one. I don't think you can easily chalk it up to inexperienced observers either. It doesn't sound like the views change dramatically when comparing a bigger reflector to a smaller refractor. Also, you need to take into account the amount of obstruction with reflectors and cassegrains which can be as much as 2 inches.

#49 dlapoint

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Posted 04 August 2008 - 12:34 PM

A month or Two ago I had my 8" dob, 5inch achro and 80mm ed apo out for some side by side views. It was a poor night. Seeing was about a 3-4 of ten. The Two targets I used were Jupiter and a 3/4 moon. Both were low in the sky. The moon maybe 25 degrees and Jupiter 15. The dob is well col and only a year old. Now all of the scopes were only outside for 30 mins when I did this test. The 5 inch refractor had the best views of the moon and the 80mm apo won on Jupiter. Now This is a night when the seeing was the limiting factor. On cold dark night viewing dso's or even the planets the 8 inch dob almost always wins hands down. I think a lot of this 8 inch newt= 4" refractor has more to do with the observers location, and sky conditions, than the scopes them selves. I live on the east coast of Canada. Spring and summer sky's have a lot of haze. The fall and winter are much better. Thats why I have more than One type of scope. To the posters question. I would get a 4" apo on a goto eq5 mount. It do everything you need. The 8' newt will bust up the dso's. Your apo will handle the astro pics, let you enjoy the colors of double stars and give great planetary, lunar views. A tracking mount makes a big difference for high power study of your fav objects. Just my 2 cents.

#50 drshr

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Posted 04 August 2008 - 07:22 PM

Thanks for the Welcome Mathteacher. I have been viewing this site for a long time but not posted before. I never felt that I had much to add to comments already posted. On this occasion though I thought I should chip in since I thought a few factors were being missed. I am a Dob owner and I strongly think that a refractor makes a better first purchase for the reasons I expressed above. In summary the image is simply superior. If anything will get you out observing it is the magnificent view from a refractor. Pinpoint stars on a black background, almost 3d. It is how it should look. Even the best reflectors such as the spooner mentioned above have some degradation of image due to light scatter and diffraction effects.
I was out looking at the Saturn nebula last night. Great seeing conditions. But not an easy target. I had my well collimated 14" Dob (with 10yr old mirror coatings) next to my 6" achro. Firstly the view through the refractor was much more pleasant as expected. The Neb was also easier to find as contrast was better and no coma so anything that isn't a star is obvious. Through the reflector the image was brighter, and it had the saturn look. But through the refractor at 200x the extensions on either side of the planatary were revealed also. Why, - better contrast. The eye is a contrast detector.

As for your question about a 12"Dob or 6" refractor. Well thats pretty much what I have. And when I reach for a scope it is always the refractor these days. Just look at the number of happy posts on the celestron 6" scope. Must be the biggest topic on the site! a great scope for a great price.

As for improved coatings, that is true. But 82% of 82% is still pretty poor. And that 33% approx lost light isn't just lost, it scatters and reduces contrast. Then add lost light due to secondary plus diffraction affects on top and viola, more lost contrast, then a few spikes sticking out from every star, plus at least 50% coma around the periphery. Then some dust, grime and mis-collimation to top it off. Crapola! Unless you go BIG with a Dob you are much better off with a refractor. :)


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