Curved Spider Vanes

By Daniel Mounsey

INTRODUCTION

I have been a lunar & planetary observer for over ten years and have tested curved spiders extensively. Curved spiders are nothing new and have been around for decades, yet few observers have ever looked through one that is properly designed. Some ATM’s use thick, 180 degree half circles, or two half circles, but they are not the same, nor are they nearly as effective as a thin, three vane curve. Sometimes, badly designed curves can produce images that are worse than a simple straight, four vane spider. About 7 years ago, I had the fortunate opportunity to view through one of Ed Grissom’s spiders and this guy knows exactly how to do them. Not only are his spiders a work of art, they are very thin and very rigid. Ed is part of our lunar & planetary observing group and unfortunately does not sell his spiders publicly.

HOW DO THEY PERFORM?

If you have never seen through a thin, three vane curve, let me first tell you that you have no idea what you’re missing. I would consider myself an optical fanatic, seeking nothing but perfection in visual planetary performance, and these spiders without a doubt, take the cake. The views are so spectacular, that they are practically indistinguishable from the sharpness of an Apo Refractor on steroids.

About six years ago, I set up my FS102 Apo next to an 8” F-6 with one of Ed’s curves and I literally could not stand the site of planets in my 4” after that. You will never know how un-satisfying something is until you compare it to something better. That is when I seriously decided that a well-designed Newtonian was the best way for me to go.
Ed also has a 13 inch F-6 with a curve and optics figured by Steve Kennedy, who also observes with the team. Steve used to work for Celestron and probably even did an SCT that one you may already own. Steve’s Newtonian mirrors are nothing short of spectacular and you may even see them in Osypowski’s new truss scopes. Ed’s 13” is equipped with the curve, which we compared to my partners 12.5 inch Portaball. It houses a Zambuto and a straight, four vane spider The views in the ball are spectacular on steady nights, but man, as soon as you look through Ed’s 13”, images take on a whole new meaning. John Pons is another optical guru and planetary fanatic in the group. He has the most experience because he’s been doing this for 50 years. His scope houses a 12” F-6 Ed Beck mirror with a 60 degree, three vane curve and a 12% secondary obstruction. All of these Newtonian’s are so dialed, that nothing comes close, and I mean nothing.

To put Pons’s Beck Newtonian to the test, I put it head to head with his own 10” Zeiss F-16 apo and took notes over a one month period and the results will be posted in explicit detail in an upcoming review. Each one of these observers uses the curve and is a testament to its credibility. One day John Pons invited me in to view some of his most cherished blue prints on spider vanes and scope design. Each one was a master piece in itself. Print after print, and I was mesmerized just looking at documented work that will probably never be seen.
Unfortunately, I cannot show these prints as Pons prefers to keep them kept away.

DOES THE CURVE GET RID OF THE DIFFRACTION?

No, but don’t worry. This section is directed at any optical expert or Newtonian designers who is skeptical about curves for technical reasons. Here are some examples of what happens. A straight, three vane spider produces six tiny diffraction spikes, while a straight four vane spider produces four longer and brighter diffraction spikes. A curved three vane spider does not eliminate diffraction, but instead, distributes the diffraction evenly against the entire field to the point where it is practically invisible in the eyepiece. This is one of the reasons why some Newtonian designers disagree with a curve, because they feel that this brightens the back ground and diminishes contrast.
This is where I beg to differ. I invite any optical expert to compare images with any four vane scope they have and announce the results in open forum. The results regarding contrast are completely unnoticeable on all deep sky objects and the issues with them hurting contrast on the brighter planets has proven to be absolutely untrue during all my tests.

It amazes me to see how many experienced planetary observers believe they need a jet black back ground to observe planets. I’ve talked to a number of other very dedicated observers who will swear that some of the best planetary images occur from a bright city or even better, twilight or dawn. When have you ever heard that twilight and dawn produce jet black backgrounds?

If you want to get a reasonable representation of a curve and you are using a straight four vane spider with your Newtonian, start observing a planet as soon as the Sun sets. The image will usually look more aesthetic if it’s not too hot out, because the diffraction spikes are being washed out and camouflaged into the grey background and the flaws are not as noticeable. Surface color on planetary surfaces will also be very noticeable during this time of observation. As the back ground gets darker, you will begin to see the diffraction spikes tinge at the limbs of the planets more obviously. This is why some observers prefer looking through an Apo instead, simply because the views look more sharp and aesthetic. If you are lucky enough to spot a grayer background while observing planets through a curved spider, then consider yourself lucky.

If anything, this will help reduce eye irradiation, which is basically an overwhelming contrast that can produce minor eye strain on the brighter planets in larger Newtonian’s against a black back ground and actually reduce color and detail. Larger Newtonian’s do not need pitch dark skies to reveal detail on planets, they already generate plenty of light. What’s more important, is to flush those ugly looking diffraction spikes out of the picture and sharpen the limbs of the planets and make them look the way they do in a nice Apo or Maksutov. Some designers will tell you that diffraction spikes are less noticeable at higher magnifications. That’s what I used to say until I saw through a curve, and what if the seeing isn’t great and you need to lower the magnification? This makes the diffraction spikes brighter, more visible and thus, less aesthetic on planets.

WHY DON’T BIG DOBS HAVE CURVES?

This is because the upper cage assemblies on the most popular Dobsonian’s use four struts to keep them held together. The four straight vanes are then screwed into each one of these struts in order to secure the spider. I really wish medium size DOB designers would come up with a three strut system and evolve into the curve. The other problem is the stability of the secondary with bigger apertures since 20” scopes have heavy secondary’s. So far, Mag One Instruments appears to be the only one using the curved three vane spider with their larger models. I have personally never tried these newer Mag One’s, but if they’re anything like Ed’s spiders, they’ll be a winner.

IF THESE CURVES ARE SO GOOD, WHY DOESN’T EVERYONE USE THEM?

I wish I knew. It’s probably because they are more costly and difficult to manufacture than a typical straight. They are also more difficult to install properly than a straight four vane spider, but I am willing to bet that if 10 people lined up to look through two of the exact same Newtonian’s, one with a straight four vane and one with a 60 degree curve, the odds would be 10 to 0 in favor of the views through the curve. I don’t know a single observer in the team I view with who doesn’t prefer the curve and they’ve been viewing planets for decades.

WHAT DO BRIGHT STARS REVEAL?

Many years ago, Astronomy magazine did an article with some photographs which were intentionally over exposed in order to allow an observer to actually see where the diffraction’s appear through different spider designs. The straight four vane revealed a more bloated star image with four diffraction spikes, while a single curve revealed a bloated and elongated diffraction out of each end. There were also two different types of curved three vane spiders, one at 60 degrees and one at 90 degrees. Before I tell you what these degrees mean, let me first tell you about the image. The curve at 60 degrees (which are the ones I tested) revealed tighter looking stars than any other design except the 90.
A 60 degree arc means that the point where the spider attaches to the tube, arcs at a 60 degree angle from where it originally left the secondary, while a 90 is arced at a 90 degree angle from the secondary to the tube. A 90 physically has a longer and bigger arc and although it looks a bit tighter in the exposures, it still ads a bit more diffraction than a 60, simply because it uses more spider vane material to complete its arc. A 60 is all that’s needed to remove the diffraction spikes, so that’s plenty enough.

A TIP FOR VANE STABILITY

All of Ed’s scope tubes are intentionally over-sized for ventilation. Let’s pretend we are using and 8” mirror with an inside tube diameter of 10”. To keep the curved spider as compact as possible, Ed uses very light pine wood blocks to compensate for that extra 2”. The vanes are then inserted into the blocks. With the blocks inserted against the inside of the tube, they now only need to cover a complete diameter of 8”, which is only the diameter of the mirror. This is done on all the scopes up to 16”, but beyond that, stability can be an issue. Here’s another tip. Unless you are prepared to buy a curve and install one, I recommend you DON’T try one, because once you do, you’ll never want to go back to a four vane again and deep sky views are just heavenly.

Steady Skies!

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