Are central obstructions so bad?

[Peter Drew]

Nice one Brian! .

[Ad Astra]

How many people on here have done an honest evaluation of 'scopes of the two types side-by side, under the same conditions?

It may be that if you tend to specialise in, say, planetary or double-star observation, then the difference is significant, but I have a suspicion that 'large central obstruction = poorer contast etc' is just received wisdom, rather than based on a more 'scientific' evaluation. Or does it depend on how experienced an observer you are, so that the majority of hobby astronomers wouldn't really notice much (if any) difference?

I shall stand back and wait for a possibly heated debate!!

Hi there Tomjo,

I've had the opportunity to do just that both as a sales guy for a major scope chain here in the States where I worked for several years, and as an astronomy instructor who teaches 'observational' astronomy - meaning that I have a significant lab (40% of your grade) that is focused on making you a competent observer with dob, EQ refractor, and an SCT to the point that you can set one up, read a map, choose a target, know what the heck you are looking for (and at), and choose the right eyepiece, point the scope, find the object and enjoy the view. I start everyone on 150mm dobs, then we go on to refractors (80 & 100mm Achromat & 120mm Apochromat scopes), and then NexStar 6 GoTo. We also have a CGEM-1100, various binoculars from 7x50's up to a pair of RB-10 bins (30-170 x 250mm), the college has a classic C8, a Meade LX-50 10", and a 13" Coulter dob.

I get to use a lot of gear on a regular basis, I also have to keep it all clean, collimated, and ready for observing at all times.

Having said all this, my personal scope is a gorgeous 4-element 'Super-Apochromat' (mfr's description) refractor; it is a 133mm f/12 monster with a massive 4" focuser that 'stops down' to 2" or 1.25" format; all two meters and 25 kilos of it... and that's just the OTA. Last night, we had the RB-10's, the C-11, and the Apomax (my scope) out on the field. The refractor delivered the best views, hands down. Not by my word, but from the mouths of some 80 people there who looked at all three. You can read my report on working with this scope and the views of Saturn by clicking <HERE>.

I will say that the C-11 gets 90% of the way there, and it is a lot easier to tote and set up... but I'm not in it for 90%. Also, lunar and planetary observing is what I like best - and this scope beats virtually every other machine I've ever used for image quality, resolution, contrast and detail.

To be fair, I did use one reflector that put my Apomax to shame. I got to be a guest observer for an evening at Mt. Wilson Observatory up in the mountains above Pasadena, California some years ago. We put the 60" scope on Jupiter, and put an eyepiece as big as a porthole in. (over 3" clear aperture, I could look in with both eyes!) You had to climb up a 6-ft ladder, and sort of lay your body down along the tube to get your eye to the eyepiece, so I just sort of rode the telescope, and peered in.... Jupiter seemed to be the size of a soccer ball, it was easy to see rotary structure in the Great Red Spot and to see a series of four spiral storms that had spun off the great hurricane. Rich colors, festoons swirling across the bands... I was breathless, and had to be dragged off the scope for the next fellow's turn.

Anyway, that was the best reflector view ever, but my refractor fits in my SUV, so I guess I'll keep it awhile yet!

Dan

[david o]

... one reflector ... Mt. Wilson ... 60" scope ... eyepiece as big as a porthole ... over 3" clear aperture ...

'strewth Dan

Everything about that seems designed to spark incurable aperture fever !

[great_bear]

Sorry, but I just don't buy the "central obstruction reduces contrast" argument.

Take out the eyepiece, point the scope at the moon, and peer into it.

Refractor

The walls of the OTA are plunged into dark shadow by the baffling. The only light reaching the eyepiece is from the objective: No wonder the views are so contrasty!

Maksutov

There's reflected light coming off the sides of the baffle tube. The insides of the tube are designed to reduce this effect, but it's clear it will have some impact on the view.

Newtonian

Whoah! Not only have you got moonshine on the tube wall directly visible opposite the focuser, you've also got illumination on the rearmost side of the inside of the focuser drawtube from light directly coming from the OTA's main opening. You've also got a reflection in the secondary of the tube wall from the very same direction in which that tube wall is being illuminated by light entering the tube. At the bottom, the reflected image of the primary is not only showing additional tube wall length from the primary's point of view, but the primary itself is illuminating the tube wall from that same direction by reflecting incoming light up onto the inner tube surface.

Compared to these major design characteristics, any measurable effect of central obstruction beyond pure light loss must be vanishingly small.

So if you're gonna compare this stuff, you've got to eliminate these design characteristics. That much is a given. To do so, surely you just stick a 60mm diameter dot on the middle of the lens of a C100ED and see if it performs any worse than a C80ED?

[david o]

In other words, test just one variable at a time ?

[great_bear]

In other words, test just one variable at a time ?

Exactly - I'd be very surprised indeed if sticking a dot onto a refractor dropped the contrast performance to a level approaching even the best similarly-obstructed Newt.

[david o]

Further, what measurements should be taken to make it an objective (groan ), not subjective, test ?

[great_bear]

My gut feel is that when it comes to contrast, even a subjective test will clearly demonstrate that scope design overshadows any CO contribution by a wide margin - but I'll leave it to someone with optical measurement experience to state what objective data is useful in determining this.

[Ad Astra]

Yes, the smallest obstruction can cause loss of contrast.

Evidence: spiders on a secondary causing diffraction spikes, ie: light that has been smeared out where no light aught to be - hence, dark areas are less dark, bright areas less bright (where do you think the stray light comes from? - the Airy disk, of course.)

A refractor should be able to show and excellent Airy disk (concentric interference pattern of light and dark bands) inside and outside focus - demonstrating the wave nature of light, by the way. Add a central obstruction and you change the diffraction pattern, smearing it out - more light where it aught not be, taken from the area where it should be. -- ie: loss of contrast.

Physically measurable and demonstrable - just photograph a star out of focus and record the Airy disk before and after the addition of the "dot" on the lens and you can analyze the difference for yourself. With a bit of calculus and knowledge of Fourier transforms, you can even do the quantitative analysis.

"Gut" has nothing to do with it, this is pure physics. The beauty of that is, no one has to take anyone else's word. Do the experiment and the math.

That said, there is nothing wrong with a reflector! Reflectors solve chromatic problems that have always been chronic with refractor designs - Newton himself thought that the chromatic problem was insoluble, and said so - stopping research on lens designs for most of a century. However, due to the wave nature of light, everything you do in optics is a compromise, optically, mechanically, visually. There is no "perfect scope". We all choose according to our desires and our needs and our budget - and there is always somebody else with a nifty machine that will do something your scope will not. Even if you have charge of the Hubble, somebody else's scope does radio waves or infra red better than yours does! In the case of my personal scope, the image is amazing, but the compromise is that I have a 4-element, oil-filled lens that clocks in at almost 10kg, making the entire OTA two meters long and 22 kg. It's a total monster for a scope of that limited aperture (just 133mm!) in both size and cost.

I compromised with my wallet and my back, instead of my eyes.

Dan

[great_bear]

With a bit of calculus and knowledge of Fourier transforms, you can even do the quantitative analysis. "Gut" has nothing to do with it, this is pure physics. The beauty of that is, no one has to take anyone else's word. Do the experiment and the math.

There's plenty of mathematical analysis on the effects of central obstruction here <click> however my central point is this: Is the CO effect less than the contrast losses caused by other, more fundamental design characteristics of the scope?

[Ad Astra]

There's plenty of mathematical analysis on the effects of central obstruction here <click> however my central point is this: Is the CO effect less than the contrast losses caused by other, more fundamental design characteristics of the scope?

That, my friend, is a much more difficult question to answer and I suspect that it varies greatly from instrument to instrument. My suspicion would be that yes, CO contrast loss is minimal compared with a poor mirror figure, ineffective, inconsistent, or inefficient mirror coatings, and scattered light off poorly baffled optical tubes, etc.

I have seen "mak-newt" hybrid scopes with smaller CO, and they perform brilliantly. I've also seen very fast scopes that deliver wonderful images with absolutely huge secondary mirrors... the Parks 8" H.I.T. with its f/3.3 primary and a gigantic 3" secondary comes to mind! Matter of fact, the worst case of CO madness I've ever seen is the HyperStar lens with a Canon camera attached - here the entire camera body becomes an obstruction - and not a nice round one at that - and the images are brilliant.

It all comes down to personal preference, price, and portability for most of us. I'm just a nut who threw out the last two criterion because of the way I use my scopes (mostly planetary and lunar).

I will also say that I didn't start out looking for a refractor like this, I had a chance to buy one and jumped on it - it was a mad idea at the time, but I've never regretted it since, and I didn't begrudge the $5K that it took to mount the darn thing properly. For what I do, it is a wonderful machine with genius-level performance. Absolutely amazing scope!

Dan

[swamp thing]

the masks fared better compared to the unmasked view when the aperture was larger than 200 mm and the seeing was turbulent.Dan

I for one find that interesting for while stepping down my 16" reflector is of great benefit on planets when the seeing is bad. I found it of no benefit on my 10", the gains in seeing and contrast offset against loss of resolution and detail due to the smaller aperture.

I will make another aperture mask for the 10" and have another go at it (Disappointed with past results I binned my old one).

It is always good fun trying out stuff like this anyway.

Regards Steve

[Ad Astra]

I will make another aperture mask for the 10" and have another go at it.... It is always good fun trying out stuff like this anyway.

Regards Steve

That is the spirit of scientific enlightenment speaking! We are scientists, we think, we test, we believe the data and act on it.

Let us know how it works, Steve. I suspect that the difference in our local environments (altitude, humidity, air turbulence) may have a profound effect on the performance of an aperture mask in given conditions. I don't know if anyone has done a comprehensive study on this or not.

Dan

[great_bear]

I suspect that the difference in our local environments (altitude, humidity, air turbulence) may have a profound effect on the performance of an aperture mask in given conditions. I don't know if anyone has done a comprehensive study on this or not.

Bearing in mind that there's essentially no difference between a stopped-down scope with an aperture mask, and a smaller but slower scope, Telescope Optics dot-net has a lot of answers here <click>

[rfdesigner]

Sorry, but I just don't buy the "central obstruction reduces contrast" argument.

...

Newtonian

Whoah! Not only have you got moonshine on the tube wall directly visible opposite the focuser, you've also got illumination on the rearmost side of the inside of the focuser drawtube from light directly coming from the OTA's main opening. You've also got a reflection in the secondary of the tube wall from the very same direction in which that tube wall is being illuminated by light entering the tube. At the bottom, the reflected image of the primary is not only showing additional tube wall length from the primary's point of view, but the primary itself is illuminating the tube wall from that same direction by reflecting incoming light up onto the inner tube surface.

Compared to these major design characteristics, any measurable effect of central obstruction beyond pure light loss must be vanishingly small.

So if you're gonna compare this stuff, you've got to eliminate these design characteristics. That much is a given. To do so, surely you just stick a 60mm diameter dot on the middle of the lens of a C100ED and see if it performs any worse than a C80ED?

I can back this up. I baffled my Tal1M with many many rings cut from cardboard and slipped into the tube and glued in place. I designed them (position and internal diameter) so that no area of the siewall could be illuminated by either the mirror or light from the top of the tube, and none of them stopped down the aperature). The effect was startling, I could see almost pitch black sky when pointing just a bit away from a full moon rather than the normal washed out view. (I will repeat the effort on my 12" when everything else is working right.) It did make finding bright objects harder as you couldn't tell they were just beyond the FOV any more.

Derek