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Refractor equivalent of an 8" reflector?

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#1 ThunderRock

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Posted 01 August 2008 - 12:22 AM

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).

#2 mathteacher

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Posted 01 August 2008 - 12:38 AM

I think your examples are too optimistic. From what is repeated on this forum, aperture wins for DSO. Images are brighter in a larger reflector. Maybe the increased contrast of the refractor can make up for a dimmer image, I don't know. For lunar and planetary, I'd say these comparisons might hold up. I'm on the fence regarding the 6" refractor or 8" reflector myself.

I'd say 80mm ~ 4.5", 100mm ~ 6" and 150mm ~ 8". Reflector folks will be irritated by this kind of talk.

#3 Stelios

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Posted 01 August 2008 - 02:14 AM

Similar how? For deep sky, the reflector (even considering not the latest coatings) should be at least the match of a 6" refractor. (We are assuming good optics for the reflector).

For lunar/planetary? 5" APO would be my guess.

Obviously refractors have other potential advantages. A 4" APO would make a far better grab-n-go scope.

Is there a reason to choose one over the other? I.e. suitability for AP, tracking, etc?

#4 leonard

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Posted 01 August 2008 - 07:56 AM

Hello ,

Stelios wrote :
>>>>(We are assuming good optics for the reflector).

For lunar/planetary? 5" APO would be my guess.<<<<

Not close enough , a fine 8 inch F 7 reflector would bury it .
Leonard

#5 Luigi

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Posted 01 August 2008 - 08:48 AM

On DSOs, an 8" refractor or thereabouts (allowing for transmission and reflection coefficients). Also, the best view of planets will generally be had through larger reflectors. Put your 8" OTA on an Atlas mount for guiding and go to.

#6 Rich N

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Posted 01 August 2008 - 08:59 AM

A good 7.1" refractor is very close to a good 8" reflector on M13. For planetary resolution most of the time the 7.1" refractor beats the good 8" reflector. But, they can be very close on a good night.

Rich

#7 NorthCoast

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Posted 01 August 2008 - 09:26 AM

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


Rather than stepping over to the other camp, perhaps another perspective would be: "What would compliment an 8" Dobsonian?"

You will find that a lot of refractor owners have (sometimes in secret) other, larger scopes to compliment a smaller (often premium) refractor.

I will admit to owning an 8" SCT with GOTO... It shows me much more than my 90mm or 110mm refractors. They compliment each other very well! :) The 90mm gets used most often because it is grab-n-go...

#8 Eddgie

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Posted 01 August 2008 - 11:20 AM

There are many different factors, so it is impossible to make this comparison.

For example, APERTURE determines resolving power. To match the resolving power of an 8" reflector, you would need an 8" refractor. If you do a lot of double star work, or want to see how big you can make the ring gaps appear, then the scopes would have to esentially be the same size.

A slighly smaller refractor (optical quality being equal) would have similar light gathering, but it would have to be something like a 7.5".

A refractor slightly smaller than this would perhaps match the reflector on low contrast targets (Jupiter). Maybe 7" or so.. While the contrast will be a bit better than the 8" reflector, the superrior resolving power of the bigger scope will start to give it an edge on the high contrast detail if you go much lower (Small bright ovals against dark bands).

So to me, if you want to match the general performance, you will need a refractor very close in size to the reflector.

But then there is quality and design... I have put a 6" APO up against a mass market 8" f/6 reflector and I can tell you that the 6" APO overall is a better performer. It is sharper everywhere in the field, had better planetary contrast, and came SURPRISINLY close in terms of deep sky (Globular and Galaxy) performance.

But this didn't have much to do with the fact that it was a refractor vs a reflector, but rather more to do with the fact that is is a virtually PERFECT refractor up against a telescope with optics that were only "Fairly good" optically.

But.. IF you were to put a TOP QUALITY mirror in your scope, along with the highest quality diagonal you could find, you would find that on axis, it would indeed take refractor very close in size to yours to give a better visual image at the center of the field.

So, no simple answer here.

Aperture is aperture, and all of the quibbling aside, aperture is generally the single biggest factor in how well a telescope is capable of doing. Quality is the next factor, and optical design is the LAST factor.

Take ANY two 8" telecopes with perfect optics, and the difference in performance would be more in the off axis behavior than in the center of the field.

Regards.

#9 galaxyman

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Posted 01 August 2008 - 11:44 AM

Yes, I have been able to see side by side a mass produced 8" f/6 (good optics) with my 6" f/8 refractor on DSO's.

It's very close, and I do prefer the overall image quality of the 6" refractor here.

I would say my friend's 7.1" f/6 is a closer match for a higher quality 8" reflector on DSO's.


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

#10 KaStern

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Posted 01 August 2008 - 12:05 PM

Hello ThunderRock,

Compared side by side which size refractor would be similar to an 8 inch f5.9 reflector scope?



depends on in wich case you want them to be similar. If you have an 8" f/6
high quality Newt you will need a 7" or almost 7" big apochromatic refractor
to get equal results for planetary viewing. The 8" Newt has about the same
light gathering ability as the 8" Newt. On-axis both perform equally good
at deep sky objects like globular clusters. For extended objects the newt
will need a quality coma corrector.

Sun is different again, I would prefer the apochromatic refractor with
Herschel wedge over the Newt.

Clear skies, Karsten

#11 Jacques

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Posted 01 August 2008 - 01:27 PM

I'd say a 6" mass produced achro will come close to the performance of an 8" mass produced reflector (both with good optics) on deep sky. The refractor will make up somewhat for its lower aperture by better light throughput efficiency. The 8" reflector will still have the edge in resolving power and will go somewhat deeper though. I chose to jump to a 10" reflector (mass produced with good optics) to see a significant difference with the 6" frac. I'm not disappointed. That said, I value an 8" dob very much and would recommended an 8" dob to get into the hobby in a heartbeat. As a matter of fact I did and talked friends of mine into an XT8 to get started. For me however an 8" dob would be to close in performance with my 6 inch frac. The latter I give also very high ratings for satisfying deep sky work but that's another discussion.

#12 skypilgrim

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Posted 01 August 2008 - 04:02 PM

"Reflector folks will be irritated by this kind of talk." MT

:) Yes, that's true!

I like how some of the responses have been cautious and have been couched as depending on what it is you will be viewing. I'd like to take it a step further. Within the "DSO" category you have quite a variety of objects, some of which increased aperture is the only route to take while others are not so straightforward.

I'd put galaxies and nebulae in the 8" = 8" section, but open clusters and even double stars could see quite a bit of difference in aperture depending on what you are going for. Generally speaking, the more concentrated the light source the less of a difference you'll see in increased aperture.

Finally, I'd put globular clusters along with SSO's (Solar System Objects) somewhere in-between.

There, I've violated my own rule of never again getting in on these kinds of discussions. I believe one of the reasons is the premise that you can actually do a one-for-one comparison. I believe it is possible but it's much more involved than, "can you see it in this scope, can you see it in that...". Breaking down your reasons into very specific categories helps.

Sam :)

#13 ThunderRock

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Posted 01 August 2008 - 06:37 PM

Wow, thanks for all the info and discussion.
The reason I first asked was that I'm looking for a new scope to become an addition to my current 8" reflector.
The number one feature I want it to have is the ability to track objects (Eq. mount) and GOTO functionality.
One option I toyed with was just buying an atlas mount and setting my current scope up with some ring tubes, but then I would still only have one scope and it wouldn't be as easy to set up.
So that leaves me thinking if I get a refractor that will compare to the 8" in optics I can use that to experiment with AP and still have my trusty 8" for visual.

Then of course that leads to either a Celstron C6-RGT or keep saving for a C14 cat or quality APO on an atlas mount.
I'm just worried that if I buy a C6 it won't be the best for AP work and it will just be the same visually as my 8", feeling that I "wasted" money that could have been better spent on a 12" dob, or saved for a quality APO/C14 cat.

:help:

#14 mathteacher

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Posted 01 August 2008 - 06:49 PM

ThunderRock, I'm a little confused. Why does your AP scope have to be a visual equivalent of your 8" reflector?

You can pick up an 80mm ED scope for imaging, and it will be a good compliment to your 8" dob.

#15 dvb

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Posted 01 August 2008 - 07:28 PM

The 8" Newt has about the same
light gathering ability as the 8" Newt.
Clear skies, Karsten [/quote]

Finally, something on this topic that is not controversial!

:poke: ;)

#16 39.1N84.5W

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Posted 01 August 2008 - 07:55 PM

:gotpopcorn:
CN is so much better than Law and Order.

#17 BillP

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Posted 01 August 2008 - 08:34 PM

In terms of light gathering, perceptually given all the differences probably something near a 6" Refractor will give you the same impression as a 8" Reflector IMO.

In terms of resolving power....well a refractor of equal aperture or perhaps a little less when accounting for impacts from secondary diffraction. So perhaps a 7+" refractor.

In terms of perceived sharpness of plantary and lunar targets on the average to very good evening, a 4" APO will "seem" sharper IMO. I've had my 4" APO now for several months and observe most every evening, and have yet to have an evening where I prefer the planetary image in the 10" DOb to the 4" APO. Moving to galaxies and nebula and Globs of course it is no contest that the 10" wins hands down.

I would hazzard that an 80mm APO would be a great compliment to an 8" reflector...and feel from experience that a 4" APO is simply a wonderful compliment to a 10" Dob as they are far enough apart that they do not seem to replicate each other's strengths, and yet each seems to have very distinct and seperate strengths that the other cannot replicate.

#18 drshr

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Posted 01 August 2008 - 09:25 PM

Several factors are always forgotten in these discussions.
The main one being the degrading mirrors of the reflectors. Newtonians degrade at 10% per year. Add in poor collimation and your average 8" field reflector is struggling to compete with a 4" refractor.

The refractor will perform at 100% of its capabilities yr after yr, a Newtonian will not.

Newtonians also have other difficulties. There optical quality depends on only one surface where as even a achromat refractor has 4 surfaces to even out errors. And because the light bends only a few degrees in the refractor as apposed to 300+ deg in a reflector, errors are less. These errors are magnified by the degree of miscollimation. Also forgotten is the loss of light from the secondary. Yes people calculate the area of the secondary and subtract that from the area of the primary. This is not enough. Diffraction losses from the secondary are about 10%. Plus this diffraction loss is not lost, it is worse than lost as it degrades the contrast and resolution of the final image. Same applies to the spider. Add in coma, some air turbulance in the reflectors tube (because the tube is open and the light has to travel through it twice) and the final image is not anywhere near as good as a refractor of near size. Image brightness is of little use if contrast is lost. Even Sirius is invisible during the day!

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. (Always measuring from optimised reflectors with brand new coatings is not real world.)

In my own case I have a 14" Dob that needs its mirrors re -coating Again! and a 6" achromat refractor. Currently the refractor is out performing the reflector. It is reliable and fiddle free. Requires little to no cool down time or collimation.

The balance will change somewhat in the near future once pure dielectric coatings are used on all Newtonian surfaces. Contrast will be much better and degradation of the mirrors negligible. Also with 98% reflectivity on both mirrors the light loss will more approximate a refractor.

#19 Tommy5

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Posted 02 August 2008 - 12:49 AM

Good points drshr,remember to that the reflector even a 8"f/6 has only about a 1/3 of its field in the center that is coma free, especially on a non-tracking dob the planet is less than diffraction limited viewing most of its dance through the eyepiece,the degradation of the primary mirror is often ignored as is the roughness on machine grinded mirrors from asia.My 6" achro continuously displays better jup images than the 8" reflectors, and 8" sct's in my observing club,on rare nights when everything is working for the mirror scopes they can have a advantage but night in night out the 6" achro will give sharper more contrasty jup images then the 8" reflector scopes .

#20 KaStern

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Posted 02 August 2008 - 12:38 PM

Hello drshr,

The main one being the degrading mirrors of the reflectors. Newtonians degrade at 10% per year.



how do you come to this strange thought?

There optical quality depends on only one surface where as even a achromat refractor has 4 surfaces to even out errors.



Again I can only oppose to that. The surface errors tend addd up.

And because the light bends only a few degrees in the refractor as apposed to 300+ deg in a reflector, errors are less.



Agian strange. 300° reflecting angle? Please take a sheet of paper
and try to figure out the correct angles.

One thing should be mentioned:
A surface error x on a lens will lead to a wavefront error of approximately x/2
A surface error x on a mirror will lead to a wavefront error of 2x.

Therefore the mirror surface has to be really good.

On the other hand the 4 surface errors of the 2-lens refractor add up
so in the real world you can find poor refractors or superb refractors,
poor Newts and superb Newts, depending on their build-quality.

A real world comparison betweeen a well made Newt and a well made apochromatic refractor
took part at the 2005 ITV in Germany. The 90% reflectivity primary mirror
of my ATM Newt was 4,5 Jears old, the high quality US made 94% reflectivity
39mm secondary mirror was 2 Years old. I estimate that the 7" TMB Apo was
6 Years ols, and the 94% reflectivity Televue diagonal was 4 Years old.

Seeing varied from 6/10 to 8/10 inthat night and we compared both scopes
using high mags from 230x up to 300x. Objects were Jupiter and M13.
There was no discernable difference concening M13 resolution
(maybe a very slight advantage of the Apo in terms of star crispness
in the outer parts of the cluster), colour fidelity on Jupiter`s surface
(maybe a very slight advantage of the Newt at the cloudy belts of Jupiter)
surface detail on Jupiter. Views were very much the same, the TMB owner
confirmed this.

Add in poor collimation ...



and every sort of telescope will give bad views. A comparison has to
be a side by side comparison, same objects, some magnification range.

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.



You can find such outdated books in german language too. For example:

G.D.Roth: Planeten beobachten (= planetary viewing)

Strange sort of literature. Maybe that thread can lighten up things a little bit:

http://www.cloudynig.../Number/360923/

Clear skies, Karsten

#21 mathteacher

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Posted 02 August 2008 - 12:51 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).

#22 Davey

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Posted 02 August 2008 - 03:46 PM

Hi Karsten,
In your comparison between the 7" TMB apo and Newt, you didn't mention the aperture of your Newt, but by your signature I see a 2oomm (8 inch) Dobsonian and am assuming that was the scope used. If so, we can see that between near eqivalent aperture/quality Newts and apo refractors, they have more in common than not, visually speaking. For transportation and mounting I'd favor the Newt. (o;

Happy observing,
Davey

#23 Rich N

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Posted 02 August 2008 - 04:28 PM

As I mentioned earlier in this thread my experience is that a 7.1" refractor is very close to an 8" Newt.

I've had my AP 180EDT f/9 APO side by side with a friend's well made 8" Newt a number of times. My refractor is more consistant in giving high contrast, high resolution images but on the right night that Newt can give some fine planetary images. For deep sky the views are very similar.

Rich

#24 drshr

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Posted 02 August 2008 - 06:10 PM

Have a look at Patric Moore's 15" mirror. This scope was his pride and joy yet the optics still turned to custard. And not everyone lives close to a coating establishment making the do I or don't I a bit of a juggle. And fairly expensive too outside of the US, with risk to mirrors unavoidable. Last time I had my mirror re-coated the mirror came back with a chip in it(on the outer rim so not so bad in my case).
It is a factor that people need to consider and is rarely mentioned.

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#25 Rich N

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Posted 02 August 2008 - 06:26 PM

Seems a bit off topic on this thread.


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