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# Refractor equivalent of an 8" reflector?

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### #76 stanislas-jean

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Posted 18 January 2009 - 01:38 PM

If the 8" newtonian is of quality P/V10 and central obstruction of 25% that means on planets to get the D-d so a perfect apo of 150mm for catching same contrast levels that are more sensitive to disapear under seeing levels so at leat the half of the resolution of the 8", means 0.3" acc to Dajon scale and about 1.5-1.6" FWHM.
Applying for high and average to moderate low contrasts levels. Under 10% contrast level the central obstruction is not welcome, 25% starting to handicaping the goal and under the seeings.
Stanislas-Jean

### #77 mathteacher

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Posted 18 January 2009 - 01:41 PM

Ken, how did you do your math?

It depends on how you interpret 1-5% a year. You can look at it as 1-5% of the original reflectivity per year. Let's say it is 5% a year. In 20 years, you would have 0%. I don't think that is reasonable.

The other way is to look at it as 1-5% of your existing reflectivity. Let's say you lose 2% a year, so you end up with 98% of what you have every year. After 27 years, you would have 0.98^27 = 0.58, or 58% left. If your secondary is the same way, you'd have 0.58x0.58= 0.34. 34% of your 8" aperture is approximately equal to a 4" aperture. M108 is possible through binoculars, according to Phil Harrington. Unless you live in the white zone, it's entirely possible through you 27 year old C8.

### #78 Deep13

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Posted 18 January 2009 - 02:57 PM

8" f/6. You'll get a little better contrast with an unobstructed scope and imperceptibly brighter image because of light loss on a reflective surface. You won't have coma on a refractive system, but you won't have the huge amounts of CA with a Newt that you would have with an 8" f/6 refractor.

Aperture, aperture, aperture. That's what matters. A high quality, thermal controlled reflective system will give up very little to a similar size refractor.

### #79 Arizona-Ken

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Posted 18 January 2009 - 11:06 PM

Ken, how did you do your math?

It depends on how you interpret 1-5% a year. You can look at it as 1-5% of the original reflectivity per year. Let's say it is 5% a year. In 20 years, you would have 0%. I don't think that is reasonable.

The other way is to look at it as 1-5% of your existing reflectivity. Let's say you lose 2% a year, so you end up with 98% of what you have every year. After 27 years, you would have 0.98^27 = 0.58, or 58% left. If your secondary is the same way, you'd have 0.58x0.58= 0.34. 34% of your 8" aperture is approximately equal to a 4" aperture. M108 is possible through binoculars, according to Phil Harrington. Unless you live in the white zone, it's entirely possible through you 27 year old C8.

Sorry, Mr Wang, but I used your approach.

At 5% per cent a year, 20 years is not 0%. It is never 0% Too simple there.

Your original approach assumes compound reductions of the primary and secondary mirrors. So 5 percent degradation from both mirrors is .95X.95 = .9025 for total system degradation. So then, 0.9^27= 0.05814974. Clearly this is a ridiculous number. (or I have a magic C8).

Your calculation above for a 2% reduction is correct. However, your approach does not bear out the facts that I can see M108 in the light pollution dome in the Phoenix metro area. Suggesting by calculation that the net equivalency of the old C8 to a net 4 inch aperture is not too convincing either. Actually, the scope performs pretty well.

When taking a look at the results the math gives you, a final question must be asked "is the answer reasonable?"

It is not. Believe me, I would notice a 66% degradation, even over time! The numbers you are calculating are too draconian for my real life situation.

There is no problem with the math; it is actually pretty trivial. I maintain that the math does not accurately describe what is happening.

There is no doubt that 27 year old mirror surfaces have degraded. But the assumption that the degradation is linear and constant is what I call into question. (This is as you stated in your post #2556213 that degradation is 1-5% a year, and of course, the basis of the calculations in question. I would like to see references to your information as I have heard these numbers bandied about in the amateur community before.)

I suggest that mirror surface degradation may be non-linear, and may even level off after time. I also suggest that until the nature of the oxidation or other forms of mirror degradation are understood by us on this forum, simplistic calculations as we have used here should be avoided.

Regards,

Arizona Ken

### #80 Tommy5

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Posted 19 January 2009 - 12:51 AM

Isn't a C-8 a SCT?,the mirror sits in a sealed tube, i never heard of anyone getting an sct mirror recoated, but i have heard of lots of places that recoat newt mirrors. I think a lot depends on where you live, the worst your skies the better the refractor will do vs. a newt since everything,cooling, seeing, etc, effects the newt relatively more, i think a 6" mass produced achro is roughly the same as a mass produced 8" newt, the i believe the lenses on the achro are spherical which are much easier to accurately make by machine than the dreaded parabolic curve on the machine grinded newt,The average 6"f/8 mass produced achro will have a strehl rating in the 70-80% in white light light, however in green light they can have a 90%+ strehl rating ,color filters are a necessity in serious observations of Jupiter or mars, i don't know a single planetary sketch artist that doesn't use colored filters to get a the best views of different planetary detail. I'm sure a Parks or Discovery or Spoooner or Zambuto 8" hand figuresd mirror with great collimation and thermal issues properly controlled will easilly outpace the asian achro in good seeing,but that is a lot of ifs, the refractor is there ready to go whenever you need it, and a refractor like a diamond last forever.

### #81 EJN

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Posted 19 January 2009 - 12:57 AM

There is no problem with the math; it is actually pretty trivial. I maintain that the math does not accurately describe what is happening.

There is no doubt that 27 year old mirror surfaces have degraded. But the assumption that the degradation is linear and constant is what I call into question....

I suggest that mirror surface degradation may be non-linear, and may even level off after time. I also suggest that until the nature of the oxidation or other forms of mirror degradation are understood by us on this forum, simplistic calculations as we have used here should be avoided.

I would also like to see where these numbers come from,
without real-world test data they are just guesses.

The one figure I remember reading is a telescope making
book gave a figure of something like a 1 percent
loss in 5 years. This seems more reasonable
in what I have seen in the real world. Silver
coatings OTOH lose reflectivity fairly rapidly
because they tarnish. Aluminum oxide is transparent.

A few years ago I had 2 8" newtonians set up side-by-side.
Both had Beral coatings. One was freshly coated, the other
was about 8 years old and the coating looked horrible,
like someone sneezed on it, even after cleaning. Yet,
comparing M13 side-by-side, I could see no difference
in the number of stars resolved.

I recently bought a mirror with a coating at least
15 years old. The coating looked poor. After cleaning
it looked a bit better. Then I treated it with a quick
immersion in clear ammonia (which removes all oxidation)
and the coating literally looks brand new.

The only way I can see the large losses quoted here
is if you lived by the ocean and the scope was stored
outdoors.

### #82 Arizona-Ken

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Posted 19 January 2009 - 01:03 AM

Tommy, you're absolutely right! The sealed tube of a SCT certainly helps, but I have heard the dreaded 1-5% yearly degradation figure quoted for SCTs also. What also is a factor is how much dew forms on the mirror (SCT or Newt) during its use over time, (like how acidic is your atmosphere?) and one would think any enhanced coatings applied would also affect mirror performance over time.

Arizona Ken

### #83 Luis.E

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Posted 19 January 2009 - 03:22 AM

For DSO, i think the 8" Newton always win camparing to 6" refractor, the resolving power is not the same no matter how good is the refractor.

Cheers,
Luis E.

### #84 mathteacher

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Posted 19 January 2009 - 04:33 AM

Ken:

Don Pensack's post in this thread is where I got the 5% number from. He's a very well respected guy AFAIK. I've read 1% numerous times. Read Starman1's post

I'm open to the idea of non-linear reflectivity loss. I'm also open to the idea that it may be less than 1%. What I know is based on what I read on these forums and elsewhere. Do I believe these calculations are 100% accurate? No, but I believe they give you an idea of what is going on. Until someone does their PhD thesis on this, we'll just have to keep guessing.

By the way, my original post gave no prediction for losses over time. I merely calculated the loss of light in a 2 mirror system, assuming 90% reflectivity as new (it may be higher but that's beside the point). It was you who made the 27 year calculation based on linear losses.

For large aperture, reflective telescopes are the only way to go. Lots of folks are happy with their Newts and SCTs, and I don't begrudge them. You often hear the mantra "aperture rules" or "an 8 inch scope is 78% brighter than a 6 inch." Rarely is the loss of light due to oxidation and having 2 mirrors mentioned. Those two factors and the prospect of re-coating bug me, but that is my personal quirk. You have a 27 year old SCT that can show you M108 in the middle of Phoenix. More power to you.

### #85 Valdis

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Posted 19 January 2009 - 12:40 PM

Just my 0.02\$ about oxidation of mirror coatings. Nowadays most of the mass produced newts come with Al coatings and we all know that Al oxidizes in the atmosphere.
The thing is, if the mirror surface is left alone, the oxidation process will slow down during time. Contrary to Fe oxides coarse and almost flaky structure Al oxide (Al2O3) is very fine. It builds a very thin (few atoms) but dense layer everywhere where a surface is exposed to air (wont happen in a sealed environment filled with nitrogen or some inert gas). The process that happens here is called Passivation (http://en.wikipedia....iki/Passivation). It effectively prevents the surface from further oxidation. If the surface of a mirror would be perfectly flat, then degradation of it would stop a few seconds after the final polish. Impurities and a non-flat surface allow more oxygen to get to the metal and more oxide builds up on the surface. Moisture also helps the oxidation process, because water is more dense, has still a small molecule, therefore it can transport oxygen past impurities much better than air (only 21% or so of air is oxygen, while water is chemically hydrogen oxide - H2O and most metals like to steal the oxygen ion releasing H2).

To summarize all my ramblings here - at the beginning an Al coated mirror will degrade faster, but after some time it almost wont degrade at all. The function of mirror degradation is exponential and might become very flat after a while (the time depends on impurities in the coating and smoothness of the surface).

Clear skies!
V.

### #86 Tommy5

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Posted 19 January 2009 - 12:54 PM

Yes theorically that is true but in the real world,collimation, thermal boundary issues and surface roughness,and even the rusting of the mirror surfaces can conspire to lessen the resolving power advantage, Suppose you wanted to split a double stasr at the dawes limit in a 8" newt, if the atmosphere doesn't steady to less than 1 arc-second seeing ability for a least a second or two you won't be able to split the double, if there is a thermal boundary layer above the mirror it won't resolve the double, if your collimation is a little off it won't resolve it, if your newt is a hand driven and the star isn't in the middle of the fov when the atmosphere steadies it won't resolve it,the 6" refractor of course will never resolve a double closer than the dawes limit of a 8 inch reflector, but the point is on most nights the performance of the two will be pretty close,Ron Bee owns a 4" T.V. scope and bought a 8" discovery f/7 i think with a smaller secondary for the big 03 mars opposition,the discovery did on some occasions reveal more detail than the T.V., but he found the images pretty close on most nights and the tv was a driven scope so it was so much easier to use for planetary operation.The T.V. view was esthetically nicer.

### #87 mathteacher

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Posted 19 January 2009 - 01:44 PM

I've done a little more reading and found these two pages:

Ongoing long-term study of coatings

It would be nice to know what their findings are.

### #88 werewolf6977

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Posted 19 January 2009 - 03:01 PM

My 6" Achro comes close to what my dearly departed N8 could do. The stars are much nicer (pinpoints in fact), but items like M27, M57 are just not quite so bright as the N8's were.

### #89 Lane

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Posted 19 January 2009 - 03:58 PM

My brother recently built an 8" dobsonian around a Coulter F/7 mirror that I gave him. It works as good as any 8" dob I have ever used and that mirror has never been recoated. I purchased it new for \$60 in 1972 and used it for about 11 years before buying an SCT. Then I took my dob apart and stored the mirror in the attic until recently giving it to him. That mirror looks like it was bought yesterday, so why didn't it deteriorate. What factors cause a mirror to degrade

### #90 davidmcharg

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Posted 19 January 2009 - 04:31 PM

Much depends on what you want to do and observe ? A 4" scope is a nice complement to an 8" scope but it is very difficult to compare reflector vs refractor of different apertures. I sold all my previous scopes and now solely observer with a little TV85 why ? first i am under no illusions that a 85mm scope can compete with a 8"+ reflector. Quite simply for my style of observing and weather conditions a small refractor does the trick, minimal hassle and can be deployed in 5 minutes, no setup time etc. I really don't think you can compare as they each have their pros and cons. On dso's the more aperture the better but then throw a wide field eyepiece into a small refractor and you get a much wider field of view which for some is just as pleasing as seeing an object up close. Likewise on the planets, on nights of average seeing a little refractor will be fairly close to what you can see in the reflector, maybe a little dimmer but then on a really good night the reflector will stomp all over the refractor assuming its cooled and collimated. For me the choice to own a small refractor is quite simply the portability, the build, the optics and the wide field views. I would make sure you really want/need a driven mount and goto etc because it does detract from the portability that a small refractor can offer.

### #91 GlenM

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Posted 19 January 2009 - 05:46 PM

David,

I agree with your post 100%

### #92 Arizona-Ken

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Posted 19 January 2009 - 10:22 PM

Mr Wang:

Thank you for the comments on my long post. You correctly understood the point of taking the assumptions and simple mathematical approach to get absurd results and question the validity of the approach.

Thank you also for doing a bit of research on the subject of oxidation of mirror surfaces. I did a quick read of your references and found them very informative. I also appreciate the input of Vladis, Tommy and EJN regarding their personal experiences and the reference to passivation. The information from everyone was much better than I could Google up last night.

Although we can't put a number or a rate on mirror degradation, it is very apparent from my reading of the references and the personal experience offered by the other posters that mirror degradation is very complex and subject to a number of factors, including the type of coating and its individual non-linear degradation rate, the environmental conditions the scope is routinely used in, and simply, how one takes care of one's equipment. This is very consistent with my own personal experience; ie, my scope is in pretty good shape for its age.

Your original thoughts in this thread were to add some insight as to how to get a handle on the equivalency of refractors and reflectors. This thread discusses compensating for the reflector's central obstruction, considering what type of objects you're looking at, considering coma, etc. These should be considered. Some general arithmetical rules can be useful and can certainly be used for the effects of a reflector's central obstruction, for example.

However, I believe that the best thing that can be said for mirror degradation is that it is a complex subject whose effects can range from making a telescope operate poorly to having no significance for a well made, well cared for piece of equipment.

Clear Skies to You.

Arizona Ken

### #93 jouster

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Posted 21 January 2009 - 08:24 AM

Prof. James Gort, Patrick Moore et al are quite right when the say a 4" refractor is roughly equivalent to a 8" reflector in the real world ie field tested on average equipment.

Not in my experience.

### #94 KaStern

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Posted 21 January 2009 - 09:30 AM

Hello to all,

Prof. James Gort, Patrick Moore et al are quite right when the say a 4" refractor is roughly equivalent to a 8" reflector in the real world ie field tested on average equipment.

seems to depend on what people think is "average"...
Average high-end apochromat vs average decollimated low-end reflector?!

I personally am more oriented towards premium quality equipment.
And there you will need at a bigger than 6" apochromat to equal
my 8" reflector.

CS,Karsten

### #95 bobmcg

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Posted 21 January 2009 - 09:37 AM

Aren't telescope mirrors coated? If so wouldn't the coating seal the aluminized surface from the air?

### #96 BarrySimon615

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Posted 21 January 2009 - 10:26 AM

Hello to all,

Prof. James Gort, Patrick Moore et al are quite right when the say a 4" refractor is roughly equivalent to a 8" reflector in the real world ie field tested on average equipment.

seems to depend on what people think is "average"...
Average high-end apochromat vs average decollimated low-end reflector?!

I personally am more oriented towards premium quality equipment.
And there you will need at a bigger than 6" apochromat to equal
my 8" reflector.

CS,Karsten

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

### #97 RogerRZ

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Posted 21 January 2009 - 11:41 AM

Prof. James Gort, Patrick Moore et al are quite right when the say a 4" refractor is roughly equivalent to a 8" reflector in the real world ie field tested on average equipment.

Not in my experience.

Same here. I once owned a very nice TV102 apo. Cost \$2600. Gave very nice views, for a 4".

Anyone care to guess how nice the view through a well built \$2600, 8" reflector would be?

Anyone think that it wouldn't beat the 4" apo in most any performance comparison?

### #98 AlienRatDog

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Posted 21 January 2009 - 12:43 PM

I am just curious to what everyone is saying, so wouldn't lets say a Discovery 17.5" f/5 with around 20% CO (haha I bring it up since I am interested in that scope, with Servocat, cheaper than most 6" Apos)...wouldnt that just rock most 6-8 inch refractors...even on planetary detail...isn't angular resolution a function of aperture. I see this with my C100ED, it is "sharper" than my 12 inch SCT...but on planets the SCT shows me much more...

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Posted 21 January 2009 - 01:51 PM

I have a 4 inch apo and a Chinese mass produced 8 inch reflector. In the real world the views are similar. Yes the reflector collects more light but that means that most deep sky objects are a brighter gray blob. I do most of my observing in a suburban backyard and under these skies you do not see much detail in most DSO's. Yes you can find some things in the 8 inch that you can not find in the 4 inch. On the moon and planets the view is very similar.
My guess is that a great 5 inch apo refractor would equal an average 8 inch mass produced mirror and it would take a great 6 to 7 inch refractor to equal a great 8 inch reflector. A generic 10 to 12 inch mirror will beat or equal any refractor under 8 inches. These guesses are based on real world observing at most objects. There are always exceptions

### #100 Lane

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Posted 21 January 2009 - 07:51 PM

Prof. James Gort, Patrick Moore et al are quite right when the say a 4" refractor is roughly equivalent to a 8" reflector in the real world ie field tested on average equipment.

Not in my experience.

I haven't done many side by side comparisons of telescopes in my life but one I have done was a 4" Tak to my old Meade 8" sct without UHTC coatings. We compared them from sundown almost until dawn. It was not even a close contest, the 8" was considerably better on everything. Maybe a 6" apo is the equivalent, I doubt it, but I would really like to hear from someone who actually did a side by side comparison. Atmospheric conditions change so much that just having used various scopes at different times really tells you nothing, it has to be a side by side comparison or it is just speculation.

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