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Refractor vs. Reflector ?

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#76 Daniel Mounsey

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Posted 09 March 2005 - 09:18 PM

A couple of thoughts/questions about curved spiders:

1. They do eliminate the spikes because they spread the diffraction over the entire image so it is essentially invisable, though still there. Over the surface of a planet, they must also provide a similar "smearing" as normal vanes, though probably smoother. The fact that the sky is uniformly dark is aesthetically pleasing but I am curious about how the image itself is affected.

2. Thin vanes are good from an optical standpoint. From a mechanical standpoint, thin vanes in tension have the capability of being far more stable than thin curved vanes. I have heard that good success can be had with a thin wire support for the secondary, I would imagine that one could use piano wire as thin as 0.006 inches. I wonder how this compares to the curved spider.

jon


Hi Jon,
These are valid points however, I'd have to say that if anyone had the opportunity to see the differences side by side, the odds would be zero that the dispersal of the diffraction this way, hinders the image in anyway. If I was primarily doing deep sky, I'd probably go with a straight vane because the spikes look pretty on brighter stars, in fact refractor owners are adding spikes to their astrophoto images for that reason by using strings. I can only say the the images of planets are completely unhindered by Grissoms curves and are only enhanced. It's much cleaner looking and the surface detail is incredible! Even the glow around the planet is hardly evident because of the spider in fact if you look at the image with a standard straight vane, there's still a halo around planets as the spike is thicker at the limbs and gets thinner on its way out.

I wish I could gather all you guys over for a star party at Wilson. I guarantee the odds would be 10-0 in favor of the curve no matter what physics appear to say.

Pons has used very thin piano wire and no matter how thin, there's still a spike. You can test it over a refractor and see this for yourself. I understand that these curves sound crazy. My team and I are the most arrogant swines (haha) when it comes to good optics. We wouldn't be using them if we thought otherwise. I hope someday to share it with others. I really don't feel there's any reason to go with thinner wires, Grissoms curves are already excellent and they are plenty rigid. I'd be a perfectionist to a fault if I did anything else. :)

#77 Daniel Mounsey

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Posted 09 March 2005 - 09:46 PM

Thanks for the interesting post. You raised some good points which I'd like to chat more about. I'll get back to this forum on Friday to answer and listen to more thoughts on these matters if they continue. Thanks everyone and no matter what, you're all a great group of friends to chat with. My fingers are worn to a pulp from typing today and I've gotta take a break. Till Friday :cool: By the way, here's pick of some of the gang. From left to right, Ed Grissom, June Trejano, Ron Hines and myself. Where are your refractors and Newts guys? let's see those pics.

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#78 Tommy5

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Posted 09 March 2005 - 09:53 PM

Mirrors?, spider vanes?, truss dob?,conical mirror?,cooling fans? boundary layers?smooth mirrors?central obstructions? shrouds?, I thought this was the REFRACTOR FORUM? We came here to get away from all that tedious stuff, we just point and look . LOL,actually this discusion has been very informative,I'm almost ready to get a mirror scope, NOT. LOL. :lol:

#79 Ron B[ee]

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Posted 09 March 2005 - 10:07 PM

Mirrors?, spider vanes?, truss dob?,conical mirror?,cooling fans? boundary layers?smooth mirrors?central obstructions? shrouds?, I thought this was the REFRACTOR FORUM? We came here to get away from all that tedious stuff, we just point and look . LOL,actually this discusion has been very informative,I'm almost ready to get a mirror scope, NOT. LOL. :lol:


Mirror - I use a diagonal with my refractor ;)
Spider vanes - Saw diffraction spikes once and I found a spider web inside my old C102HD OTA one day ;)
truss - the tripod aluminum legs of my old CG4 looks similar ;)
cooling fans - definitely on a hot night ;)
boundary layers - well my refractor needs to cooldown too ;)
smooth mirror - yup dielectric diagonal for sure ;)
central obstruction - I made one and snap it over my dewshield just to get the experience ;)
shroud - definitely putting it over my head when doing DSO ;)

LOL :roflmao: :roflmao: :roflmao: Tommy.

Ron B[ee]

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Posted 10 March 2005 - 12:10 AM

Hi Daniel,

Hear hear now :smash: ! This is THE most exciting post I've seen all year :applause:

I'm sure many CN'ers would agree! Bravo!

#81 celestial_search

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Posted 10 March 2005 - 12:47 AM

I've rather enjoyed it and learned some things too. :)

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Posted 10 March 2005 - 02:42 AM

Daniel, please post here when you hear more about the availability of these curved spiders. (I would just buy a full set i.e. every dimension, and get the scopes later ;-)

-- William

#83 NHRob

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Posted 10 March 2005 - 06:32 AM

Dan,
Also waiting to hear about Ed's curved spiders!

Another thought ... is there any real advantage/difference in an oversized sonotube newt with forced air vs. a truss newt with oversized mirror box and shroud and same forced air?? Just curious since I have a mirror box available (14.5x14.5") I could use for my 10" f/6 project. Although, now I'm leaning towards a sonotube.

Any discussion of mirror cell designs?

As for spider designs, I favor a 3-vane straight over 4 vanes any day. It will throw out 6 spikes but, each spike will have < 1/2 the energy of 4-vane spikes.
I've made a curved spider before (D-shaped) from stovepipe steel. It does create a diffused diffraction haze but, my design had problems with alignment so it was not optimal.

Rob

PS: Dan, that's a pretty motley crew there!

#84 NHRob

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Posted 10 March 2005 - 06:33 AM

Da,
forgot to mention .... best thread all year, IMHO.
Rob

#85 Jarad

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Posted 10 March 2005 - 09:08 AM

To be fair, if you'd do the math, that's still "good enough" for those apertures, unless your secondary happens to be very, very small (see my other post - (1/8")/12.5" is exactly 1%...).

So there must be something else going on that spreads light out of the Airy disc - very probably just a differently sized central obstruction, or a mirror that doesn't have the same Strehl ratio.



As an FYI, the 14.5" Portaball has a 2.6" CO, the 12.5" has a 2.14" CO (both are pretty minimal, <18%). They use the same Zambuto primary and Protostar secondary as the scopes Daniel is using, so I doubt that surface roughness is the issue. I am not sure I am convinced it is the spider vane thickness either, but then I haven't had a chance to compare side by side with one of Daniel's, so I will reserve judgement. It could be, I just don't know.

The two biggest sources of planet glow around here are atmospheric haziness (we get a lot of that in the humid southeast - I see a pretty substantial glow around Jupiter and Saturn in my ED-80 most nights, no CO and no spider), or condensation on the secondary (which the Portaballs are prone too, especially if you don't have the secondary heater installed). I'm not saying that's necessarily what Daniel saw, just that those happen a lot down here.

Jarad

#86 Don W

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Posted 10 March 2005 - 08:41 PM

For more info about curved spiders, cooling fans etc., stay tuned to the REFLECTOR forum please. This is the REFRACTOR forum. This has been an interesting thread, but let's stay on topic please. Personally, I think this whole thing belongs in the Equipment forum since it has had little to do with refractors.

#87 sixela

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Posted 11 March 2005 - 05:28 AM

So if the "smearing" of the planetary image is similar,

It isn't. When you have diffration spikes, you have some orientations for low contrast edges that are really impacted nastily. When you don't, you don't ;).

though probably greater

What Daniel and I have tried to convey is:
-it's not "greater". With the set up discussed, it's only 5% greater.
-the width of the vanes is much more important than the length, at least talking about differences in length that small.

other than the fact that there are no diffraction spikes, what are the advantages of curved vane spiders?

You also get rid of the anisoptropic loss of contrast (inside the planetary image, there are all these 'spikes you can't see' emanating from each Airy disc corresponding to a given point of the object. If you integrate all of them, you lose contrast on features precisely aligned with respect to the vanes!)

What I've tried to show (numerically) is that, if only the spider vanes are not longer than necessary to spread the light and thing enough, once the diffracted light is spread, the amount of light diffracted is entirely negligible: the spikes are your only problem, aside from the effect of the central obstruction. Once oyu get rid of them, you get rid of the problem.

#88 sixela

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Posted 11 March 2005 - 05:30 AM

I wish I could gather all you guys over for a star party at Wilson. I guarantee the odds would be 10-0 in favor of the curve no matter what physics appear to say.

The physics favour those vanes as well, though ;).

#89 sixela

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Posted 11 March 2005 - 05:37 AM

I am not sure I am convinced it is the spider vane thickness either, but then I haven't had a chance to compare side by side with one of Daniel's, so I will reserve judgement. It could be, I just don't know.


Of course, I don't mean to say 'good enough' is optimal. There's no reason to make spider vanes 1/8" thick if you can get away with 1/32", of course, just as there's no need to have three 180° vanes when you can get away with three 60° vanes.

[I'd have to add that three 60° vanes are barely any longer than two 90° vanes (equivalent to one 180° vane), if they can be made a lot thinner, better]

#90 Jon Isaacs

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Posted 11 March 2005 - 08:06 AM

>>>It isn't. When you have diffration spikes, yu have some orientations for low contrast edges that are really impacted nastily. When you don't, you don't .
----

On the other hand, that implies there must be orientations that are less impacted than with curved spiders.

>>> What Daniel and I have tried to convey is:
-it's not "greater". With the set up discussed, it's only 5% greater.
-the width of the vanes is much more important than the length, at least talking about differences in length that small.
---

If it is indeed the width that is important, then it would seem that a properly designed straight vane can be much thinner than a curved vane simply because a spider must be stiff in axial loading and a straight vane spider can be loaded in pure tension where as the the curved nature requires some bending stiffness.

>>>>
You also get rid of the anisoptropic loss of contrast (inside the planetary image, there are all these 'spikes you can't see' emanating from each Airy disc corresponding to a given point of the object. If you integrate all of them, you lose contrast on features precisely aligned with respect to the vanes!)

What I've tried to show (numerically) is that, if only the spider vanes are not longer than necessary to spread the light and thing enough, once the diffracted light is spread, the amount of light diffracted is entirely negligible: the spikes are your only problem, aside from the effect of the central obstruction. Once oyu get rid of them, you get rid of the problem.
----

It seems to me that the problem with spikes is simply that they are lined up so that the sum of the diffracted light overlays and sums up into a line from a point source.

A curved spider spreads this light from a single point source so that it is not apparent. But it is still diffracting light from each point source of an equal magnitude as with a straight vane.

However a planetary image is the sum of a series of point sources over the area of the planet. It would seem therefore that the curved vane does indeed have the same issues with loss of detail, it is just summed up over the surface in a different way. I agree that there would be orientations that are worse with straight vanes but it would also seems that there would be orientations that are preferred, just as when splitting unequal double stars the problem arises when the mate lies exactly on the spike of the primary.
======

Jon

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Posted 11 March 2005 - 09:02 AM

I agree that there would be orientations that are worse with straight vanes but it would also seems that there would be orientations that are preferred, just as when splitting unequal double stars the problem arises when the mate lies exactly on the spike of the primary.


I got around this problem by having another focussing position at 120 degrees round the tube from my normal observing position. Not really used this facility much though, double stars are not my favourite. Similar idea was introduced into the Teleport design.

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Posted 11 March 2005 - 09:19 AM

Of course all this talk of spider vanes/thicknesses can be put on the back burner if an optical window is introduced. This is OK for apertures up to 12" - 14", above this, it starts to get a bit tricky to keep a thin optical window free from astigmatism.
A piece of optically flat glass, multi-coated, will work well and look cool. It has two advantages - a.) Holds the secondary without the need for spider vanes, b.) Creates a more stable image as it closes the tube off. Disadvantages - a.) Slight loss of light to the focussed image from two air-glass boundaries (only negligable as they are multi-coated), b.) tendency to dew up, c.) higher cost compared to vanes.
Some might argue that adding another optical component may reduce contrast by adding to total wavefront error of the system thereby reducing the Strehl. It may be true to a tiny amount, but for 99% of observers from their backyards, the gain in image stability may be more than any noticable difference in a .96 Strehl instrument going to a .94 Strehl instrument.
Just an idea.

#93 sixela

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Posted 11 March 2005 - 09:23 AM

On the other hand, that implies there must be orientations that are less impacted than with curved spiders.

Oh yes. You may gain negligible contrast in directions between spikes, and you'll ruin it in the directions perpendicular to the spikes.

I also have an issue with something that introduces bogus anisotropy in images in se: it introduces visual artefacts. You're seeing things which aren't there. It's worse than a small blurring.

As I've said: the central obstruction scatters so much more light out of the Airy disc that the *only* problems of diffracted light coming from vanes occur when it's concentrated.

What would you rather have: a watch that was always running 1 minute late, or a watch that gives the exact time twice a day? ;)

If it is indeed the width that is important, then it would seem that a properly designed straight vane can be much thinner than a curved vane simply because a spider must be stiff in axial loading and a straight vane spider can be loaded in pure tension where as the the curved nature requires some bending stiffness.

Well, you can keep it curved, make the vanes longer, and make sure they can be aligned to appear thing from the primary mirror - exactly what Ed's spiders do.

And again: once the vanes are as thin as Ed's vanes, making them thinner won't matter.

But yes, many people have experimented with tensioned strings to attach secondaries. Not easy to collimate well, though, and it's usually harder to dampen vibrations with such a setup.

It seems to me that the problem with spikes is simply that they are lined up so that the sum of the diffracted light overlays and sums up into a line from a point source.

A curved spider spreads this light from a single point source so that it is not apparent. But it is still diffracting light from each point source of an equal magnitude as with a straight vane.

Exactly. But given the fact that the light spread is an order of magnitude less than than spread by even the most minimalistic central obstruction, it's *only* a problem when it's concentrated.

It would seem therefore that the curved vane does indeed have the same issues with loss of detail, it is just summed up over the surface in a different way.


If the light is spread, it is no longer the primary issue *anywhere* in the field of view, at least not with a setup that is as good as Ed's.

If it is spread, your main issue is getting a smaller secondary, until it is absurdly small (i.e., yields a fully illuminated field that is essentially just the on-axis point) - great for planets, so-so for larger fields of view.

That also means getting a low profile focuser and moving the newtonian focus closer to the centre of the tube will be more helpful than trying to improve on Ed's spider design (warning: if you design the tube too small instead, you'll bump into other issues Daniel pointed out!).

#94 sixela

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Posted 11 March 2005 - 09:30 AM

b.) Creates a more stable image as it closes the tube off.


Not necessarily true. Sadly for CAT owners, they too have to deal with tube currents, and CAT scopes aren't exactly known for eliminating thermal issues and fast cool-down, and those with a Lymax cooler aren't observing with the Lymax cooler in place ;).

The optical window, BTW, isn't neutral and generates its own set of tube currents, given that it's an excellent radiator of energy in to the sky (as CAT owners with dew problems will tell you).

Unless you start building a setup where you temperature control everything (and cool the mirror with a Peltier, heat the optical window, with sensors galore,...).

#95 Jarad

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Posted 11 March 2005 - 09:54 AM

Disadvantages - a.) Slight loss of light to the focussed image from two air-glass boundaries (only negligable as they are multi-coated), b.) tendency to dew up, c.) higher cost compared to vanes.



And d.) a lot heavier than spider vanes. This may not be a big deal for an equatorial mount where you can move the tube in the rings to balance it, but for a dob design adding 10 lbs or so to the UTA to top of the tube is going to mean you have to either add a lot of counterweight to the bottom, or redesign the rocker box and bearings to raise the center of balance.

Jarad

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Posted 11 March 2005 - 10:04 AM

Not necessarily true. Sadly for CAT owners, they too have to deal with tube currents, and CAT scopes aren't exactly known for eliminating thermal issues and fast cool-down, and those with a Lymax cooler aren't observing with the Lymax cooler in place .

The optical window, BTW, isn't neutral and generates its own set of tube currents, given that it's an excellent radiator of energy in to the sky (as CAT owners with dew problems will tell you).



OK. Take for instance the Maksutov Newtonian. It requires a greater cooldown time because of the Meniscus. However, they produce stable images almost from the word 'go', the system is closed and unless you introduce a 'current' as you call it, via a fan or passive cooling holes, the image remains more stable than a Newtonian during cooldown. I have seen this on several occasions.
The benefit of the optical window is that it is thin and hence cools rapidly. It may have issues regarding adding a tiny amount to intial cooling problems via radiatition, but this is small compared to helping to eliminate the chimney effect of an open tube Newtonian. I have used on a few occasions, large reflecting binoculars with optical windows. The windows were not multi-coated, dewed up quite quickly, but gave stable images, very noticable against single Newtonians from the same site at the same time. Once everything had cooled, there was little difference. These were made in the 1980's and were simple optical wndows that did not hold the secondaries.
By the way, not familiar with a Lymax cooler, please explain.

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Posted 11 March 2005 - 10:12 AM

Even a 14" optical window is fairly lightweight, less than or approx. the weight of a 31mm Nagler (2lb). A thin cell adds less than 1 lb. The set up is much lighter than you may think.

#98 Daniel Mounsey

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Posted 11 March 2005 - 10:52 AM

I have been following this thread with interest. I too am a great believer in medium and larger aperture Newtonians being superior planetary performers. I have, over two decades, seen large Newtonians give startling planetary images. There are one or two points that I would like to make and a question or two.
Firstly, the subject of Sonotubes. Have not tried this material so can not comment (perhaps someone in the UK can tell me what we call it over here).
Second. The shallow curved spider vanes. Again, not tried these (in competition) so will leave the commenting to those that have, although I can well believe that shallow curves are a worth while upgrade when it comes to the average thick Newtonian spiders or those strange 'figure 8' types I sometimes see. My own 308mm f/6 has a 4 vane secondary with very thin arms. They seem to work fine, with little annoying effects on planetary images once the system has cooled. I have noticed in the past that any diffraction spikes appear to be worse a.) When the instrument is not cooled properly, b.) When the atmospheric seeing is turbulent. I have noticed that pre-dawn planetary images (and bright star images for that matter) show much cleaner, and less noticable spikes than during the night or early morning.
Mt own Newtonian has a 15" tube diameter (inner diameter) with a 308mm primary. It also has three 90mm holes in the rear cell, which I can use for pushing air through when needed. I also have my tube in two halves. When cooling prior to observation, I leave the two halves seperated. this appears to help with cool down speed.
I get the impression from this thread that the subject of oversized tubes to minimize the forcing of air into the optical path, and strong rear fans are somewhat of a revelation. I find this a little puzzling. This is all old knowledge. Am I missing something here, or have I misread? I have seen many medium size (8" to 14") Newtonians over the last couple of decades with large tubes and rear fans, and (it has to be said) a few with shallow curved spider arms, although as I said, I have not taken the time to study differences. Much has been spoken about these points for many years.
I think it is important here to perhaps suggest that a combination of all these factors arrives at a superior optical performance, and that perhaps one important criteria mentioned, on its own, is not enough to claim a real difference. Perhaps this is one of those occasions when the whole is greater than the sum of its parts.
A friend of mine once told me that he had on a few occasions, read or been told, that a small improvement in one particular area of the optical performance of a telescope, e.g. ensuring the use of a high quality diagonal when using a refractor or Mak may not make a worthwhile difference, or that purchasing an instrument with a wavefront error of 1/10th over an instrument of 1/8th wavefront error would not be worth the effort, or that using an eyepiece like a Monocentric with the average telescope would not bring rewarding observing experience in brightness or contrast improvement, above and beyond that of a competent design like an Orthoscopic or such like. However, if you take your 1/8th wavefront telescope, with its average star diagonal and a Plossl, and compare it to a 1/10th wavefront error version of the same telescope, with a high quality diagonal, and a Monocentric (for example), the results will be there to see. This is similar to the 'Wobbly Stack' analogy mentioned in Suiters book.
So, adding up small merits results in a worthwhile and substantial gain in performance, just like I think this situation shows. (Never use a few words when an essay is available).
I think it important to mention one very crucial issue. The quality of the primary and flat. There are very few opticians in our hobby who can truly and consistently turn out high quality reflecting surfaces. In the US, the more famous ones (as far as we are concerned over here in the UK) are Carl Zambuto and Bob Royce. In the UK we have Optical Surfaces (the best and most consistent I have ever come across, although very very expensive, and they are not really keen on making mirrors for amateur telescopes, because it is not profitable for them). There is one lucky guy in the UK with the only 14.5" Teleport in existence with a Zerodur OS primary with a 'genuine' Strehl of .997. A quartz Protostar flat, and both rear firing and side firing fans. Lucky *bleep*
What I am getting at is that the real juicy bit of the planetary performance of a large Newtonian, is the surface of the primary (and the flat aswell). All of these extra 'tweaks' are servants to the primary mirror, and without the finest surfaces routinely appearing in more and more Newtonians, large tubes, rear fans and curved spiders will make little impact on the perceived notion of a Newtonian as little more than a deep-sky instrument in the minds of the great planet observing community. A good friend of mine (as some seem to enjoy name dropping) Tom Noe at Teleport, whom I have observed with in his back garden in Wylie, in the UK and at the TSP, always said that his telescope design served to allow the Zambuto surfaces to work to their best. The telescope is the servant, the optics the master. I guess that this situation, talking about spiders, tubes and fans is on a similar path. But let us not get carried away with these upgrade issues, otherwise they can quickly replace the need for high performance optics in a Newtonian. I agree with these improvements in the way they allow the Newtonian optical system to work better, but my preference is to concentrate on urging the need for better more consistent mirrors (as the conduit for winning the hearts and minds of the refractor lovers).
One question......Why is a discussion about improving Newtonian performance appearing in a refractor forum? :question:


I'm a bit behind on all the comments and questions, so I'll try to keep up with everyone in order. Yes, it true that these are old techniques but it's not the same anymore. First of all, if you look at the earlier Cave days for example and the Parks Newtonians for example, they did and still have design issues. Many used fiberglass, which in our case has proven not to be as thermally stable in the field. Also, the the clearance around the primary was still not up to proper criteria. Not only that, but the curved spider makes a big differnce when seeing is bad, because there's no bleeding off the limbs. Todays Chinese Newtonians are way off proper designing techniques and now the industry is filled with them. Although they have helped thousands of people get into the hobby for a low price, it still does not change the fact these design flaws are an issue. This old knowledge is no longer being executed, not no mention that many mirrors of the old days were too thick. As Bryan Greer stated, "thin is in" and I couldn't agree more. The key is to get up an running as soon as possible.

I have nothing but the highest regard for Tom Noe, Carl Zambuto and Steve Kennedy who is now making huge mirrors at electrifying level of perfection. I currently own a 10" F-5 Teleport and absolutely love it to this day, in fact I wont part with it and Tom knows this. I also agree with you that the optics are of huge importance. I feel sorry for todays opticians breaking their backs to make good mirrors and many people not having the seeing conditions to appreciate the beautiful work or the know how on the thermal aspects. Good optics though are still not enough. The thermal issues are of the utmost importance and make a huge difference. If you have 1/8th or 1/10th optics or a Strehl of .993, it all looks great on paper, but it's not telling the full story. Even Steve Kennedy will attest to this. Imagine a final wavefront of 1/6 wave going to waste because of a thermal issue which was ignored by the ATM or observer. Now your perfect optics are barely 1/4 wave and in many situations, only 1/2 wave. The thermal issue is huge and is the biggest reason I believe observers get negative results in most cases and are content with their refractors. Thank you for your very constructive post.

#99 Daniel Mounsey

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Posted 11 March 2005 - 11:10 AM

In Mag1 Instruments 12.5" and 14.5" models, both use three 60 degree arcs except the vanes are a whopping 1/8" thick.

To be fair, if you'd do the math, that's still "good enough" for those apertures, unless your secondary happens to be very, very small (see my other post - (1/8")/12.5" is exactly 1%...).

So there must be something else going on that spreads light out of the Airy disc - very probably just a differently sized central obstruction, or a mirror that doesn't have the same Strehl ratio.


I remember trying Starsplitters new compact scope. The vanes were about 1/16th thick which was not too bad, but the "V" shape just killed the image. There was an ugly and distracting diffraction at one side on planets. The shape and thickness of a spider still has a trmendous visual impact in the eyepiece. I realize the numbers dictated in the Portaball seem small however, there's still a large visual difference seen at the eyepiece. Both the telescopes were 12.5" F-5's using Zambutos and the same secondary with the same 17% obstruction. I star tested both scopes and both were the same. Math is a great tool, but sometimes it's hard for it to illistrate what's really going on in the eyepiece. Perhaps a software program would help where we could simulate the differences. I'd also like to say that I have high regard for your posts and math knowledge. Thank you for sharing.

#100 Daniel Mounsey

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Posted 11 March 2005 - 11:39 AM

Hi Ron,
When I compared the 10" with the straight spider to the 12.5" using Grissoms curve and noticed even less halo in the 12.5", that's saying a lot. More so than if I were comparing it to a 12.5" with a straight vane. Since the 10" was producing a dimmer image, it gave the 10" all the advantage it needed, and that is a dimmer image to help cut down the halo at nearly the same magnification, but it was still more aesthetic in the 12.5". As I metioned before, if we study the way straight spiders look, they bleed off the limbs and then they start to get thinner, so there's still light glow all the way around the limbs in a straight vane spider. Another valid issue here is the study of Saturn's "A" ring. If we observe the Keeler division, which is now wrongly known as Encke's division, the straight vane in many cases bleeds right at that ansa, making it harder to distinguish the contrast of this crucial area. All I know, is what I'm seeing. I couldn't be more happy than I am with Grissoms curves, they're awsome! The airy disc looks nicer, cleaner and less disturbed.


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