Refractor aperture versus Newtonian

[Dangerous-Dave]

Hi all, does anyone know of a formula to convert obstructed Newtonian aperture to the equivalent unobstructed refractor equivalent?

I know my Vixen R200SS has an aperture of 200mm, 800mm focal length and a 37% obstruction. Using pi-r-squared, and deducting 37 percent for the obstruction, the surface area equates to an unobstructed circle of diameter 160mm. Is it a fair assumption, therefore, that a 160mm f5 refractor would have the same resolving power, light grab, field size and contrast as my 200mm f4 Newtonian (assuming both are perfectly collimated and have equivalent quality optics)? Or are there efficiency differences in the two designs to take into account?

The reason I ask is I am considering trading in my R200SS and switching to an apo quadruplet, but I do not want to sacrifice f speed as the quantum efficiency of my CCD camera is not that great. Would my exposures need to be considerably longer using the same camera on a 100mm/f5.8 refractor?

Thanks

[ronin]

A 4 inch refractor is equal to a 4 inch reflector, same for 1,2,3,4,5,6,7,8,9,etc.

Too many partameters for anything else, and you would likely get very much the same answer of 1:1.

[ChrisLX200]

The question is actually quite complex and I'm probably not competant to offer a comprehensive answer. A quality refractor (unobstructed) concentrates more light of a star image within the airy disk and less within the diffraction rings, and it's that property which gives the characteristic views of pinpoint stars and high contrast. So there there is more than just similar light collecting areas to consider when it comes to assessing image quality. The f-ratio is fixed and unaltered by any obstruction, but in any obstructed systems the shorter the f/l then the larger will be the obstruction caused by a secondary mirror (Newts, cassegrains etc). It is the f-ratio which governs the 'speed' of the optical system, of whatever type, whilst the focal length governs the image scale. Those parameters are not affected whether you use an obstructed or unobstructed system. What does matter though, is the quality of the image data and an unobstructed system (such as the refractor) or one with a very small obstruction (f/8 or f/10 Newt for example) will always produce a better image than one with a large obstruction.

ChrisH

[Scooot]

A 4 inch refractor is equal to a 4 inch reflector, same for 1,2,3,4,5,6,7,8,9,etc.

Too many partameters for anything else, and you would likely get very much the same answer of 1:1.

Hi ronin, just happened to read this and curious. So doesn't the obstruction in the reflector make any difference?

[Chris]

I think you exposures would need to be longer with an f/5.8 frac compared to a f/4 Newt. I've heard from some experienced folk on here that aperture isn't particularly important for imaging but f/ratio is. Visual is a bit different I think, I love both fracs and newts equally but I know that my ED120 out performs a 130p visually.

EDIT: I've read your question a bit more carefully and I see what your saying as the central obstruction isn't considered when calculating f/ratio. I'd also like to know this but my gut feeling is that the Newt will be faster despite the obsruction and this is just based what I've noticed when imaging with a number of newts and fracs, e.g. my old 130pds f/5 seemed to collect about the same signal (almost seemed like more?) than my ED120 reduced down to f/4.5. I think this will be an interesting debate 

Chris

[RikM]

My understanding is that the f/5.8 refractor at 580mm FL would show more sky than your 800mm f/4 reflector but to get the same brightness for extended objects you would need exposures more than twice as long. f/4 --> f/5.6 is full f-stop difference, which would double the exposure time.

[Peter Drew]

Don't forget that when comparing a reflector mirror and a refractor objective for lightgrasp, the reflector has to use two mirrors so the resultant brightness is reduced by the two reflections as well as the vignetting by the secondary profile.     

[StarryEyed]

You must subtract the area of the obstruction for brightness.

Resolution is a factor of the diameter ignore the obstruction foe this. A 160mm refractor would not have the same resolving (resolution) ( spot size).

Other factors impact brightness and contrast such the obstruction and optical quality.

[StarryEyed]

Very messy.

[Alkaid]

This is a very interesting question.

The answer lies in exactly what you happen to be looking at.  The effect of the central obstruction in a Newtonian, SCT or other obstructed design is a reduction in contrast. 

When looking at high contrast objects, such as the moon, the rings of Saturn and the Cassini Division,  or a tightly packed globular cluster,  then a Newtonian will perform the same as refractor of equal aperture.  For high contrast, think black on white.  Or a bright colour on a dark background.   A six inch, perfectly collimated, long (ish) focal length Newtonian will perform as well as a six inch apochromatic refractor under these cicumstances.  This also explains why the moon takes magnification well, in any telescope design.

Where the central obstruction lets one down is when observing LOW contrast objects.  For example details in the cloud belts of Jupiter (brown on red), or the globe of Saturn (light and dark yellows).  The effective aperture of a newtonian or SCT under these circumstances is the full aperture, minus the obstruction.  Do not subtract the areas, just the diameters.  For example, my 150mm Newtonian with a central obstruction of 37mm will equal a 113mm Apo when observing Jupiter.  This also explains why 4" refractors always seem to give just as good a view as the bigger Newtonians when pointing at Jupiter.

The planetary imager Damian Peach did some research on his Celestron C9.25 and came to the same conclusion.  The C9.25 will perform like a 9.25" refractor when observing the moon.  But "only" like a 6" Apo on Jupiter.

You will find the following articles interesting no doubt.  The 2nd article gives a table of effective apertures for low contrast objects, I found it a very good read.

http://www.damianpeach.com/c925review.htm

http://legault.perso.sfr.fr/obstruction.html

Enjoy your telescopes!

[Caldwell14]

Out of interest where does the deterioration in mirror quality (2% a year, is that right) fit in? If that deterioration in quality is correct does that mean that a 5 year old scope has effectively 10% less light gathering ability, I.e. that a 8 inch Newtonian would gather the same light as a 7 inch scope

[Caldwell14]

5% per annum seems to be the agreed average on some of the other forums it seems.

[ollypenrice]

In theory I don't thiink the resolving power is affected by the central obstruction so an obstructed 200mm resolves as an unobstructed 200mm. The loss of light grasp by area seems to me to be as you say. I'll stand corrected on both counts. The central obstruction affects contrast but if you've caught more photons you'll be able to put the contrast back in post processing.

Now for the reality; I think you should buy the fast apo and enjoy gettng on with it (but I'm impossibly biased!!!!)

Olly

[John]

5% per annum seems to be the agreed average on some of the other forums it seems.

Does that apply to mirror diagonals too do you think ? (as used with refractors, SCT's etc).

[Caldwell14]

Yes I would think so, it would have to be wouldn't it? Lucky diagonals are cheap easy to replace.

[Caldwell14]

One thing is sure, helix coatings last no longer than yer average Skywatcher number. The one I had was 8 years old and had a fair few large holes in the coatings, and it had been well looked after by a well known astronomer.

[lensman57]

Hi all, does anyone know of a formula to convert obstructed Newtonian aperture to the equivalent unobstructed refractor equivalent?

I know my Vixen R200SS has an aperture of 200mm, 800mm focal length and a 37% obstruction. Using pi-r-squared, and deducting 37 percent for the obstruction, the surface area equates to an unobstructed circle of diameter 160mm. Is it a fair assumption, therefore, that a 160mm f5 refractor would have the same resolving power, light grab, field size and contrast as my 200mm f4 Newtonian (assuming both are perfectly collimated and have equivalent quality optics)? Or are there efficiency differences in the two designs to take into account?

The reason I ask is I am considering trading in my R200SS and switching to an apo quadruplet, but I do not want to sacrifice f speed as the quantum efficiency of my CCD camera is not that great. Would my exposures need to be considerably longer using the same camera on a 100mm/f5.8 refractor?

Thanks

A 4" Apo will always outperform a 4" newtonian , central obstruction reduces contrast and therefore  resolution. Most of the run of the mill newtonian optics are at best 0.85 on the strehl ratio which qualifies them to be labelled Diffraction limited, Good Apos have Strehl ratios of 0.97 and higher therefore resolve more detail with higher contrast. In their favour newtonians are unrivalled for aperture to pound ratio compensating for loss of contrast and resolution with much larger aperture and often faster F ratios but they also end up having longer focal lengths and not suitable for widefield imaging , there are a few expensive exceptions. Newts are large and clumsy in operation and on the mount and need to be housed in an obsy to weather proof them, most widefield Apos have no such problems. A premium imaging Apo can have a flat field of atleast 45mm Diameter with a newtonian the numare more conservative but at least they do not suffer from CA problems as a whole.  I have both Apos and a cheapie but poular newtonian so I have no axe to grind.

Regards,

A.G

[Caldwell14]

Me neither I have owned a fairly large variety of scopes. There is reasonably denying certain scopes are most suited for certain tasks, small grey smudges of photons are far better imaged with refractors than with any other type other scope. Visually  a very large dob is the very best instrument for observing globular clusters and faint smudges of photons. Visually open clusters, douple stars and complex planetary details are best observed by refractors. I sold my last 10 inch Orion Optics dob as I just couldn't cope with the lack of performance on double stars, in particular, and planetary detail. That said the very best views of Jupiter I ever had were with that scope, but only on a couple of occasions, seeing and Newtions is not the best combinations for regular serious planetary observations

We however are now well off topic

[Dangerous-Dave]

Many thanks to you all for your replies. I get a sense that for image quality inch for inch the apo is a no brainer but the jury is out on the light grab issue.The issue of mirror degradation is an interesting one, are we referring to deterioration of the silvering or general build up of grime?