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Refractor vs. Reflector ?

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#51 Daniel Mounsey

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Posted 09 March 2005 - 12:56 PM

Notice the side fans in this picture. The rear fans were left out for the photo. I like the side fans for speeding up the cool down, but they are always turned off during visual work while the rear is left on at all times.

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#52 Jon Isaacs

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Posted 09 March 2005 - 01:00 PM

>>>I'm glad you're continuing this thread, Daniel. My Discovery Dob is a sonotube and I really like it. One small thing I would add: the sonotube is gentler on the hand while nudging than the cold numbing metal tube (not to mention the heat transfer you already talked about).
-----------

Interesting stuff. I know that on a cool day just touching a metal tube DOB in order to move it can set a tube current that lasts a minute or more. Gloves help.

jon

#53 Daniel Mounsey

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Posted 09 March 2005 - 01:10 PM

Next, I'll talk about curved spiders. Here's a picture of Ed's. Notice how razor thin it is, which is equivilent to about the thickness of five sheets of paper. Are they rigid? Heck yea! I'll tell you how he does it and why no one has produced one better. Great news as well. I've just gotten word from Grissom that he's going start selling them. His shop is just about up and running. Prices have not been mentioned yet, but I'll find out soon.

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#54 Daniel Mounsey

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Posted 09 March 2005 - 01:22 PM

In this photo, I had him design one for my 12.5" Starmaster. The upper cage was rebuilt to accomidate three struts instead of four.

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#55 Daniel Mounsey

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Posted 09 March 2005 - 01:35 PM

A close up picture from the side. Notice the width of the vane material is 1.5" here. This is also what helps keep it rigid. They can also be tilted for the thinnest possible setting.

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#56 Daniel Mounsey

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Posted 09 March 2005 - 01:41 PM

A close up face on.

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#57 Ron B[ee]

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Posted 09 March 2005 - 02:15 PM

I hope you'll discuss (or let us have it ;)) the curve spider vane next, right Daniel?

Thanks,
Ron B[ee]

#58 NHRob

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Posted 09 March 2005 - 02:17 PM

Dan,
Great! What material does Ed use for the vanes and how thick is it?

So .... in this example there are three 4" fans for a 13.5" scope. I have a 10" scope I'm building. three fans would prob. be overkill. You mention thermostat control of the fan speeds ... can you elaborate? Does Ed use automatic fan speed control with thermostats?
Rob

#59 NHRob

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Posted 09 March 2005 - 02:34 PM

Another thought .... might it be a good investment to get a conical mirror, like a Royce? It's a tad more money but thermal mass would be lower and it should cool more easily.
A few hundred $$ more for this is a wise investment considering a Nagler would easily cost as much and, the benefits would be seen with every eyepiece you ever use!!

Rob

#60 photonovore

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Posted 09 March 2005 - 02:59 PM

I always thought the popularity and optical quality reputation of large refractors in their heyday was due to the poor quality of the reflective coatings available during that time period more than anything else. Modern highly reflective coatings (especially those in high end mirrors) make the former advantage of similar large apertured refractors, due to their otherwise superior transmission of light, rather moot today. And the heat dissipation rates inherent in large mirror masses are problematic as well. So I don't doubt your comparative obervation reports at all with scopes of this size, it makes perfect sense.

However, i'd be wary of translating this to smaller aperture scopes, especially applied to consumer grade newts, with less than optimal coating reflectivity rates, and similarly apertured modern refractors. Refractor users are aware of the difference the 10% light you gain thru using a modern coated 2" diagonal provides. The same type of effect, in reverse, applies to an average consumer grade newt where the coatings of the two reflective surfaces (typically in the high 80%/low 90% range) combined result in a far greater light (and contrast) loss at the eyepiece, even compared to the effect of using a poor star diagonal alone in a modern refractor with an otherwise 95%+ light transmission rate. (Commonly used modern lens coatings usually achieve >1% light loss per air/glass surface; exotic coatings can achieve rates even higher.) There remains a significant & observable difference in these cases, apart from mirror induced tube current issues.

It's interesting to see body heat raised as a significant issue and the tube material or lack thereof in connection. I raised this body heat issue in another thread about tube materials but it wasn't taken up. Obviously it remains an issue; the only questions remaining seem quantatative.

My only comment here is whether the overriding positive effect observed is due to the high induced airflow; and how much effect, in light of this, tube material actually has. Have your friends ever tried the same high volume forced ventilation system with an aluminum tube? It would be interesting to see if there is any discernable difference between body heat transmission into the tube air column, under such high levels of positive forced air ventilation, between sonotube and aluminum tube materials. As the body is an IR radiant heat source, the type of tube coating, in this particular case, may have an effect as well, especially as concerns aluminum.

If the vibration issues are handled (as it apprears your friends have achieved) this seems a more efficient & simplier single solution to tube current issues due to large mirror masses than active mirror cooling alone would provide. Obviously, the high volume of air interchanged has a key effect in the degree of mitigation provided. Perhaps the ultimate solution would be to combine the two methods, active mirror cooling with high volume optical column air exchange--just as is commonly done with large reflecting instruments in most modern professional observatories?

#61 sixela

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Posted 09 March 2005 - 04:03 PM

I had a 10" truss dob and I found that, while it avoided tube currents, I had problems of body heat causing thermal currents drigting across the optical path. This was in cold weather, of course ... which is a good percentage up here in NH.

I know why the focuser of my Dob is on the left side. Observe to the south, dominant winds from the west...

#62 Daniel Mounsey

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Posted 09 March 2005 - 04:59 PM

Why do people love refractors, myself included? It's because they produce the most beautiful images in the eyepiece, not to mention many other fantastic issues they excell at. It's like you're not even looking through a telescope. There's a lot of controversy regarding curved spiders. I think most if not all examples of curves is what has actually given them a bad reputation. I challenge any expert in the world to debate this with me in open forum. The curve is not only smarter, but superior in numerous ways.

If people see nothing but bad examples of curved spider (which most have), then the majority of people who see them will not be sold on them. People have just grown accustom to the straight four vane spiders and they're practically lost in them. I invite anyone to look through a properly made curve and state the the straight spider is better.

There are numerous ways we can look at this. We can say that because the vane is curved, it's using more material. In Grissom's case, this is not true at all. Consider the original vane from my Starmaster. The material was twice as thick, not to mention that it had one extra vane. If we were using a straight, three vane spider while compared to a curve like Grissoms using the same thickness, then adding a curve of 60 degrees for each vane adds so little vane material that it's completely irrelevant to the issue.

I mean we're talking tiny percentages here. From a scientific point of view, people try to argue. These issues are quite simple and NOTHING matters more than taking the scopes out and testing them side by side in the field where it really counts.

There are numerous curves on the market and there are some which come installed. For example, Obsession telescopes appears to be using an older Novak design which has been around for decades on their new 12.5". It utilizes two half circles, each being 180 degrees of arc. In Mag1 Instruments 12.5" and 14.5" models, both use three 60 degree arcs except the vanes are a whopping 1/8" thick. I'll show you guys some pictures of it if you like.

I compared this Protaball 12.5" F-5 to my Starmaster 12.5" F-5 which was using a straight four vane spider at the time. I actually liked the straight one better in this case, because the Mag1 image had a big fat halo around the planet. The actual limb contrast of the straight vane was still superior between the spikes, producing an overall nicer image.

This is going to be the same issue with the Novak design because there is a redundancy of arcs which keep adding more unwanted diffraction. Now let's look at the 1800 Destiny design. I'm at least glad they're trying to bring curved spiders back to life, but a couple of things still hinder them. The vanes, once again are too thick and there is a redundancy of 180 degree curves here as well, three all together. Even though they're smaller curves, you really only need one 180 degree arc to completely disperse the diffraction evenly.

In Grissom's design, which is by far the best, there are only three very thin 60 degree arcs, BUT they total 180 degrees of arc, which is all that you need to properly disperse the diffraction. The key is to minimize thickness and too many curves, otherwise you're just stacking more diffraction over the image and hindering the contrast.

At the present, Bryan Greer is the only one who is actually making curves fundamentally correct on the market as far as I'm concerened and I have a test unit which he sent me and I am quite impressed with it. It's only 180 degrees of arc and the material is thin. The advantage to Grissom's is that they work very well in larger apertures because they're stable and they look pretty, not to mention that they're more easy to install.

How thick are Grissom's curves? 1/32".
What are they made of? stainless and galvinized sheet metals.

Overall, when I compare my 12.5" Starmaster using the Grissom curve to my 10" Teleport which has a standard four vane straight, the image actually looks sharper and cleaner at the limbs in the 12.5" even though it's collecting more light. Anytime brighter light is being carried along the spikes at the limbs, the image will bleed more, producing a less pleasing view when compared to a good curve.

#63 Ron B[ee]

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Posted 09 March 2005 - 05:20 PM

There are numerous curves on the market and there are some which come installed. For example, Obsession telescopes appears to be using an older Novak design which has been around for decades on their new 12.5". It utilizes two half circles, each being 180 degrees of arc. In Mag1 Instruments 12.5" and 14.5" models, both use three 60 degree arcs except the vanes are a whopping 1/8" thick. I'll show you guys some pictures of it if you like.

I compared this Protaball 12.5" F-5 to my Starmaster 12.5" F-5 which was using a straight four vane spider at the time. I actually liked the straight one better in this case, because the Mag1 image had a big fat halo around the planet. The actual limb contrast of the straight vane was still superior between the spikes, producing an overall nicer image.


This is very interesting info you've shared with us indeed, Daniel! Though I've never look through a Newt with curved spider, I've read about the halo with curve vanes, which for planets I would definitely not want. (I don't even like the "4 diffraction rays" from my 4-vane spider but I've learned to accept and live with it ;)).

Overall, when I compare my 12.5" Starmaster using the Grissom curve to my 10" Teleport which has a standard four vane straight, the image actually looks sharper and cleaner at the limbs in the 12.5" even though it's collecting more light. Anytime brighter light is being carried along the spikes at the limbs, the image will bleed more, producing a less pleasing view when compared to a good curve.


Now I'd summit that this is an orange and grapefruit comparison. I'd have been better if you can find a 10" Dob to compare to the Teleport instead, in my opinion, because the extra 2.5" could have factor in favor of the Starmaster ;).

BTW, what do you think about the Protostar curve vanes?

Thanks,
Ron B[ee]

#64 Daniel Mounsey

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Posted 09 March 2005 - 05:38 PM

Dan,
Great! What material does Ed use for the vanes and how thick is it?

So .... in this example there are three 4" fans for a 13.5" scope. I have a 10" scope I'm building. three fans would prob. be overkill. You mention thermostat control of the fan speeds ... can you elaborate? Does Ed use automatic fan speed control with thermostats?
Rob

Hi Rob
Yes, that is a bit much but you could go with something like this photo. This is what Ed uses for his 10" and 8" Newts. Yes, he uses a thrmostat for all his fans. He can turn the fans up to the desired speed based on the best image quality.

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#65 Daniel Mounsey

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Posted 09 March 2005 - 05:44 PM

Another thought .... might it be a good investment to get a conical mirror, like a Royce? It's a tad more money but thermal mass would be lower and it should cool more easily.
A few hundred $$ more for this is a wise investment considering a Nagler would easily cost as much and, the benefits would be seen with every eyepiece you ever use!!

Rob


I couldn't say anything about the conical design because we've never tested it. I'm not saying it doesn't work but it would be nice to test it first. Seems as if the fans would blow beautifully behind the cone shaped back side of the mirror.

#66 Jon Isaacs

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Posted 09 March 2005 - 05:52 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

#67 Daniel Mounsey

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Posted 09 March 2005 - 06:00 PM

I had a 10" truss dob and I found that, while it avoided tube currents, I had problems of body heat causing thermal currents drigting across the optical path. This was in cold weather, of course ... which is a good percentage up here in NH.

I know why the focuser of my Dob is on the left side. Observe to the south, dominant winds from the west...


This is a very valid issue. Both my Starmaster and Teleport have the focusers placed on the left side. In other words if you are looking down the tube, the focuser is on the right. This keeps my body down-wind with the proper laminar air flow from the ocean, which produces excellent seeing while the planets rise in the east and even as they pass zenith. The body heat issue is of major importance. This is also the advantage of the tube, while the truss is far more vulnerable to body currents crossing the optical path. We've conducted numerous star tests to confirm this fact. In Pons and Grissom's tubes, they can be rotated at a moments notice, which is even better. You can also orient Saturn's rings to the lateral position.

#68 sixela

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Posted 09 March 2005 - 06:56 PM

planet, they must also provide a similar "smearing" as normal vanes, though probably smoother.

Of course. But given their thickness, if the diffracted light isn't concentrated, the effect is going to be vanishingly small compared to that of the central obstruction (and very similar) - a wee bit of extra energy into the first few diffraction rings instead of the Airy disc for each point source. With this spider, and vanes that thin, I doubt you could even measure it if you're using a central obstruction, even one of 14%, and even on relatively small apertures. On large apertures, you *certainly* won't be able to experience anything, given that the fraction of energy thrown out of the Airy disc is a power function of the relative thickness of the vanes with respect to the aperture.

From what I remember reading,
[rummages through bookmarks - ah: page 2 of this]
if you have a 10% linear central obstruction (small, eh?), infinitesimally small vanes would lead 82% of the energy into the Airy disc (that 18% being diffracted into the rings by the central obstruction).

Assuming (roughly) that Grissom setup's total length for the vanes is that of a classical four vane, in this scenario, vanes of a thickness of 1% of the aperture would cause "only" 80% instead of 82% of the energy to be directed into the Airy disc...

...and they look *MUCH* thinner than 1% of the aperture to me, and most central obstructions are larger than 10%.

#69 sixela

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Posted 09 March 2005 - 07:06 PM

If we were using a straight, three vane spider while compared to a curve like Grissoms using the same thickness, then adding a curve of 60 degrees for each vane adds so little vane material that it's completely irrelevant to the issue.

The difference is 5% (trigonometry - x/sin(x) is still pretty close to 1 at pi/6).

And the effect on diffracted light intensity of lengthening the spikes is *linear*. The effect on diffracted light intensity of making them thicker or thinner, on the other hand, is a square function - so that 5% extra length is the least of your worries...

#70 sixela

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Posted 09 March 2005 - 07:13 PM

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.

#71 gazerjim

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Posted 09 March 2005 - 07:45 PM

I've seen a lot written about the thickness of vane materials, but not much about aligning each vane so that it presents a minimum aspect to the optical path. I wonder if there is a better method than merely sighting down the tube?

Jim

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Posted 09 March 2005 - 08:15 PM

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:

#73 Daniel Mounsey

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Posted 09 March 2005 - 08:37 PM

I always thought the popularity and optical quality reputation of large refractors in their heyday was due to the poor quality of the reflective coatings available during that time period more than anything else. Modern highly reflective coatings (especially those in high end mirrors) make the former advantage of similar large apertured refractors, due to their otherwise superior transmission of light, rather moot today. And the heat dissipation rates inherent in large mirror masses are problematic as well. So I don't doubt your comparative obervation reports at all with scopes of this size, it makes perfect sense.

However, i'd be wary of translating this to smaller aperture scopes, especially applied to consumer grade newts, with less than optimal coating reflectivity rates, and similarly apertured modern refractors. Refractor users are aware of the difference the 10% light you gain thru using a modern coated 2" diagonal provides. The same type of effect, in reverse, applies to an average consumer grade newt where the coatings of the two reflective surfaces (typically in the high 80%/low 90% range) combined result in a far greater light (and contrast) loss at the eyepiece, even compared to the effect of using a poor star diagonal alone in a modern refractor with an otherwise 95%+ light transmission rate. (Commonly used modern lens coatings usually achieve >1% light loss per air/glass surface; exotic coatings can achieve rates even higher.) There remains a significant & observable difference in these cases, apart from mirror induced tube current issues.

It's interesting to see body heat raised as a significant issue and the tube material or lack thereof in connection. I raised this body heat issue in another thread about tube materials but it wasn't taken up. Obviously it remains an issue; the only questions remaining seem quantatative.

My only comment here is whether the overriding positive effect observed is due to the high induced airflow; and how much effect, in light of this, tube material actually has. Have your friends ever tried the same high volume forced ventilation system with an aluminum tube? It would be interesting to see if there is any discernable difference between body heat transmission into the tube air column, under such high levels of positive forced air ventilation, between sonotube and aluminum tube materials. As the body is an IR radiant heat source, the type of tube coating, in this particular case, may have an effect as well, especially as concerns aluminum.

If the vibration issues are handled (as it apprears your friends have achieved) this seems a more efficient & simplier single solution to tube current issues due to large mirror masses than active mirror cooling alone would provide. Obviously, the high volume of air interchanged has a key effect in the degree of mitigation provided. Perhaps the ultimate solution would be to combine the two methods, active mirror cooling with high volume optical column air exchange--just as is commonly done with large reflecting instruments in most modern professional observatories?


I think what's actually important here is not the "%" of reflectivity. A 10" scope for example is still collecting a good amount of light. I believe what's more important is that the surface of the primary and secondary is very smooth and the light that is making it to your eye is harnessed as efficiently as possible without degridation. For example I have a 10" Zambuto optic with a standard IAD coating. If I compare it to a mass produced Newtonian from China on deep sky, there isn't all that much difference, in fact the cheaper Newt will probably give a slightly brighter image since the IAD has a coffee tint, but when you magnify both scopes, the real truth comes out. There's a big difference. I actually don't recommend enhanced coatings on any planetary scope. A simple, standard IAD coating is best, nothing fancy that could scatter more light. Or, perhaps I'm not understanding your statements correctly.

As far as the seeing conditions allowing our Newts to perform better, sure that's true, but it's not the main reson. As long as reasonable aperture is kept under proper thermal and optical control, it should still win out.

Yes, we have used fiberglass and metal tubes and they do not appear to perform as well, even with all the fans and enhancements. I'd also like to state that the issues you raised regarding body heat are very true and I agree with you.

#74 Daniel Mounsey

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Posted 09 March 2005 - 08:44 PM

Hi Ron,
You may have just missed the comments I made regarding Bryan's curves. I think they're excellent.

#75 Jon Isaacs

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Posted 09 March 2005 - 09:14 PM

Of course. But given their thickness, if the diffracted light isn't concentrated, the effect is going to be vanishingly small compared to that of the central obstruction (and very similar) - a wee bit of extra energy into the first few diffraction rings instead of the Airy disc for each point source. With this spider, and vanes that thin, I doubt you could even measure it if you're using a central obstruction, even one of 14%, and even on relatively small apertures. On large apertures, you *certainly* won't be able to experience anything, given that the fraction of energy thrown out of the Airy disc is a power function of the relative thickness of the vanes with respect to the aperture.
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So if the "smearing" of the planetary image is similar, though probably greater but smoother with a curved vane spider, other than the fact that there are no diffraction spikes, what are the advantages of curved vane spiders?

jon


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