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Bresser 127 Mak, f/15; Two Baffling Qs

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#1 Joe1950

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Posted 27 July 2018 - 09:44 AM

Hola a todos.

 

Not long ago I got a used Bresser 127mm Mak, f/15. It needed a few fixes but is a good scope. The detail I see is more than I expected.

 

Anyway, the secondary aluminized spot imposes a 30% central obstruction. BUT! When you add in the secondary baffle, which flares out, that jumps to about 2" or 40%!

 

So I'm thinking of removing the secondary baffle and see what the views are.

 

Has anyone done this and what were your findings?  I would tend to think that most affected would be daytime views, but I don't use it in the daytime. Also, If i understand correctly, the venerable Questar 3.5 does not have a secondary baffle.

 

Any thoughts or experiences would be appreciated.

 

BTW: The Bresser 127mm 'Messier' Mak is essentially the same scope as the Explore Scientific 127mm, f/15 Mak.

 

____________________

 

 

My second Q is that some have added focusers to the back port of Maks or SCTs to do away with annoying image shift with the regular focusing mechanism. My scope has some, but not too much.

 

But isn't the optimum spacing in the system very important to have the best correction? In other words, if you change the spacing from the mirror to the secondary a significant amount to compensate for the added focuser length, are you possibly changing the level of spherical aberration and thus not getting the best images?

 

 

Thanks. Have to go to work for a couple hours, but I'll Be Back.


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#2 dweller25

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Posted 27 July 2018 - 10:59 AM

Re secondary baffle - I think Asbytec has done this very thing and from memory it did not detract from the views. Try searching for his Mak 150 posts.


Edited by dweller25, 27 July 2018 - 10:59 AM.

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#3 sg6

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Posted 27 July 2018 - 11:09 AM

Obstructuion is not 30%, the obstruction is by area and that is the radaii or diameter squared.

So 30% = 0.3 and 0.32 = 0,09 = 9%

All that the central bit "blocks" is 9%.

The baffle is likely there for a reason and that will be stray light and reflecctions. Even if that raises it to 40% by diameter that comes to 16% by area so you "might" get 5% more - not enough for you to generally distinguish, and you may get internal reflections.

 

When it comes to a scope it is area that is the relevant aspect. A 200mm scope collects 4x the light that a 100mm scope does not 2x. Linear measurement are both misleading and incorrect.


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#4 Asbytec

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Posted 27 July 2018 - 11:31 AM

Re secondary baffle - I think Asbytec has done this very thing and from memory it did not detract from the views. Try searching for his Mak 150 posts.

Thanks for remembering. :)

Joe, I did remove my baffle without negative effects. Long story short. I think I bookmarked the thread a few years ago in DIY forum. Basically, I did so because the baffle system was too tight restricting the scope to 140mm aperture with a properly sized primary mirror (at 162mm).

You may want to check your effective aperture using the flashlight test (recent thread) or at least you have a primary mirror about 4 to 8% larger than the meniscus. Good info deciding to start the project.

I left the foam retaining ring in place in case I had to reverse the mod. Turns out, after 2 years without any trouble other than a less than textbook and slightly scarey star test, as it was pre mod, I finally removed the foam ring. Be prepared for any potential star test results. But, if you're at full aperture already, you should be fine.

I did take care to baffle every reflection I could see and identify near the exit pupil of a low power eyepeice with the scope pointed out a sun lit window. Including flocking the primary baffle interior. I dampened all I could see just in case. Works just fine. In daytime, too, best I can tell.

I toyed with shaving back the primary baffle, too, for more FOV. But I balked at doing it. Dont wanna press my luck. In fact, Orion 150 went off the market for a few years before being reintroduced. I wonder if they changed the baffle system.

Edited by Asbytec, 27 July 2018 - 11:35 AM.

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#5 Joe1950

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Posted 27 July 2018 - 11:41 AM

Thanks, Norme! We did discuss it at some point, but my memory eluded me. I'll check the threads and likely give it a go. Worse is that I'll put it back. But I have a feeling it may be for the better. Thanks.

 

David, SG6, thanks very much.

 

I believe the linear measurement across the face of the scope is used for diffraction purposed, not light gathering, where area is the important ratio.

 

Thanks again to all!


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#6 Redbetter

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Posted 27 July 2018 - 02:46 PM

Obstructuion is not 30%, the obstruction is by area and that is the radaii or diameter squared.

So 30% = 0.3 and 0.32 = 0,09 = 9%

All that the central bit "blocks" is 9%.

The baffle is likely there for a reason and that will be stray light and reflecctions. Even if that raises it to 40% by diameter that comes to 16% by area so you "might" get 5% more - not enough for you to generally distinguish, and you may get internal reflections.

 

When it comes to a scope it is area that is the relevant aspect. A 200mm scope collects 4x the light that a 100mm scope does not 2x. Linear measurement are both misleading and incorrect.

That is not the way obstruction is normally expressed.  Central obstruction is typically given as linear obstruction, not area.  Ironically, area percent obstruction is typically given to mislead the consumer, rather than the other way around. 

 

It is not percentage that the central bit blocks so much as the diffraction impact to the image.  As linear obstruction grows larger the percent of light falling within the airy disk radius declines considerably (on top of the area percent dimming) while at the same time an increasing portion of the light is thrown into ever widening area in outer rings.  

 

This is likely responsible for a portion of the softer views seen with increasingly obstructed scopes.  The linear detail is there, but with reduced contrast.  The resolution is reduced by energy transfer well away from the central image.  How much so?  The theoretical energy recovered out to the 3rd diffraction ring in an unobstructed scope is 95.2%; at 20% obstruction it is still 94.7%; at 30% it has fallen to 92.9%, while at 40% it is down to 90.7%.  That means that the light thrown increasingly widely around the image has gone from a baseline of 4.8%, to 5.3%, then 7.1% and finally 9.3%.  

 

It is even worse if one measures the central disk energy divided by this wide scatter as a form of signal to noise ratio. For the cases above this would be 83.8/4.8 = 17.5, 76.4/5.3 = 14.4, 68.2/7.1 = 9.6, 58.4/9.3 = 6.3.   The result is considerably more glow surrounding bright objects in the eyepiece:  as when trying to see faint moons next to a planet, or faint nebulosity or galaxies near moderately bright stars. 

 

I see some indication of this when looking for Tethys and Dione in the 127 Mak (~40% obstruction) vs. the 110ED.  Side-by-side the 110 has a considerably easier time of it because the surrounding background is darker.  The effective system transmission of the two scopes are nearly identical and the planetary detail is very similar in good conditions, except that the 110ED's fine details are less blurred, so it shows slightly finer subtle detail.  This is most apparent on festoons which run a range of sizes and contrast profiles.  The 110ED might show 3 arcs when the 127 is showing 2 when both scopes are dialed in.     


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#7 Joe1950

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Posted 27 July 2018 - 04:48 PM

Thanks Red for putting numbers to it. I've been getting good images with the 127, and if I can squeeze a little more out of it without impacting something else, it would be all the better.



#8 Asbytec

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Posted 27 July 2018 - 06:21 PM

Joe, here's the thread. I started out trying to calculate how to trim the baffle back. Been a while.

https://www.cloudyni...ary-baffle-mod/

 

Another way of looking at the numbers Red expressed above is using the calculation (1 - co^2)^2. It calculates the light loss in the central disc due to both added diffraction and obscuration by area. For example, a 40% obstruction reduces the central disc by (1 - 0.4^2)^2 ~ 0.71 of it's normalized to 1 peak intensity. This means it is putting 0.71 * 0.84 = 0.6 or 60% of the light in the central disc instead of 84% for a perfect unobstructed aperture.

 

Once you know (or estimate) your optics Strehl, you can find the final working peak intensity of a point source on the focal plane. With a Strehl of 0.95, you get I * S or 0.6 * 0.95 ~ 57% normalized to 1 for a perfect unobstructed aperture. About the maximum obstructed aperture to retain 0.8 (basically diffraction limited performance) for a perfect aperture you can have up to 0.33 obstruction at (1 - 0.33^2)^2 ~ 0.80. Calculate the final peak intensity by multiplying the Strehl, a good scope at 0.95 Strehl will be about 0.95 * 0.8 ~ 0.76. A scope with a good Strehl of 0.95 can get away with something near 30% or a little smaller and still be "good." 

 

You can also approximate similar RMS effects by w = 0.21o, or 0.21 * 0.4 ~ a roughly equivelent RMS of 0.084. That's equivelent to something more than 1/4 PV LSA. At o = 0.3, you get a rough RMS equivelent estimation of about 0.21 * 0.3 ~ 0.63. Remember, 1/4 PV LSA is about RMS 0.074 and the Marechal criteria is RMS 1/14 = 0.071. Just talking equivelent approximations here, the effects on the MTF for a ~ 33% obstruction and 1/4 SA are similar but not exactly the same. 

 

But that you, and I, get good images operating with an obstruction is the basis of my claim the diffraction limit is still pretty good and at least acceptable. Not perfect, but certainly not terrible. My views of Jove in excellent seeing have shown that to me - in the field, as they like to say. It's another reason I love my MCT. It's undercorrected with a strange wavefront (I suspect), but it's smooth and puts up nice images when cooled, collimated, and operating in good to excellent seeing.

 

My silvered spot is about 44mm or about 0.3 obstruction. Did I notice a difference? Maybe, but not immediately. It's about the difference of maybe 8% in effective peak intensity (obstruction down from  ~0.37 to 0.3). Not easy to tell a huge difference. I am mostly comforted by the idea it's operating at full aperture best I can tell. No ill effects and no vignetting on the secondary baffle, so I left it off. I opened up the edge of the spherical(?) primary mirror allowing the edge to contribute to the image. The star test was affected, but not the image in focus. Best I can tell...good as new. :)

 

Great info on obstruction effects here. http://www.telescope...obstruction.htm


Edited by Asbytec, 27 July 2018 - 06:46 PM.

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#9 Joe1950

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Posted 28 July 2018 - 12:13 AM

Wonderful, Norme! As always, thanks for all the help.

joe


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#10 Ed D

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Posted 28 July 2018 - 06:52 AM

Joe, I removed the secondary spot baffle from an older Synta-made 127mm Mak that I regrettably sold.  I used the collimated laser testing method and found the working secondary obstruction to be 36%, down from 40% with the baffle.  Before removing the baffle I had already flocked the inside of the scope tube, as well as the inside of the secondary baffle tube.  After removal of the secondary spot baffle the planetary contrast noticeably increased visually to a refractor-like level, which I did compare side by side.  I also used the Mak for planetary imaging with very good results.  I always used an Astro Zap dew shield to block out my idiot neighbors' floodlights - 29 of them!  Never had an issue with internal reflections, or any other issues after removing the baffle.  BTW, it was Norme's work that inspired me to do the mod.

 

Ed D


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#11 Joe1950

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Posted 28 July 2018 - 08:21 AM

Thanks Ed! That’s great to hear. Sometimes a little effort goes a long way in improvement.

 

Norme has been of immense help to me on many ocassions. He certainly knows his stuff on many, many topics.

 

I have the same floodlight issue here. But not 29 of them! Wow, I thought 2 was bad enough. They are angled horizontally and cover my front and back yards very well. I have an Astro Zap and have to use it for the glare more so than the dew.

 

Thanks again, Ed!



#12 Asbytec

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Posted 28 July 2018 - 09:24 AM

BTW, it was Norme's work that inspired me to do the mod.

Ed D

Gosh, its not a project for the faint of heart. I feel a little responsible if anything would have gone wrong. Being inspiring and all... :)

Glad it worked out. We often hear the word regret in conjunction with selling a Mak. Something I hope to avoid. :)

Joe, I probed mine with a laser pointer with all the enthusiasm and care of an budding intern proctologist. I peeked up and down every angle, on axis and off, to see if a direct ray could find its way into the eyepeice field lens. I'm confident none can.

But any measure to ensure they cannot can't hurt. Flocking, dew shields, and deadening reflections close to the exit pupil. Your primary baffle, the silvered spot, and visual back may well be sufficient.

Look it over closely, you can tell. See pics in my thread.

Edited by Asbytec, 28 July 2018 - 09:27 AM.

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#13 Joe1950

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Posted 28 July 2018 - 11:51 AM

Great Norme! Actually, the less effort the better for me. I will have to read the thread because I'm not familiar with the flashlight or laser techniques.

 

You know, if I had a brand new Tak 102 I wouldn't touch it. But I got this scope for a good price, already fixed a couple things and will still "carefully" make improvements.

 

Maks are unique and seem to, given good optics, squeeze a lot out of the aperture. I'll definitely keep this one. I have a refractor with a great lens, a refugured Newt reflector and a cat that puts up good images. I'm good to go. 


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#14 fcathell

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Posted 29 July 2018 - 01:19 PM

I finally got around to trying Norme's secondary baffle mod to my Orion 127 Mak by removing the baffle completely. I need to try it visually on the planets, but after a brief indoor artificial star test I definitely see some subtle changes to the I/O focus diffraction patterns at 150X.  The thing I noticed most was the slight "flaring" effect on one side of focus that I didn't see before.  Reminds me of a turned edge on a Newt mirror. This certainly can't be the case unless the inner part of the mirror with the cored out hole had issues, and this is doubtful since the retaining nut would mask it.  In any case the central obstruction size went from 50mm down to 40mm and could be smaller if the secondary were trimmed down some (no way will I attempt this!).  This drops the obstruction from 41% down to 33% approximately. If I look through the 90* diagonal at an extreme angle I can see daylight coming straight through but not on axis or even at 50% off axis.  I will be interested in planet testing this. I did notice that the flared baffle I removed is very shiny at grazing angles - not good at all.  If I put it back in it will definitely be flocked. I suspect from reading Norme's extensive work on this that it might not be necessary because I use this scope of planetary and double star work only. 

 

Interesting work Norme!

 

Frank


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#15 Joe1950

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Posted 29 July 2018 - 02:45 PM

Ironically, Frank, I just completed the same.

 

 

The baffle would make a good shot glass if it had a bottom.

 

Interesting, the baffle is thick at the bottom where it attaches to the corrector. I would guess about 3mm or there abouts. And, that thickness covers that much of the secondary spot!

 

Without the baffle I went out in the daylight and had some views of trees and leaves. Really not bad, actually. They were sharp and contrasty, save for a couple spots where I noticed a reflection. It may have been from an intervening group of leaves that were in the field and out of focus.

 

It's gotten like the Amazon River basin in this area with trees and bushes. I don't think there is a 1/4 mile unobstructed view except up.

 

Looking through the eyepiece diagonal w/o an eyepiece, I can see an annulas of light around the secondary. Then there is a black ring and then the secondary reflection. The ring is probably the secondary that is cut off by the primary baffle.

 

If I lengthen the primary baffle a little, the annulas ring may be blocked.

 

 

But before doing anything, I'll use it as is on Jupiter. If there is contrast improvement, I'll leave it as is. I should be able to tell. Also, I'll look at an infocus star and check the Airy disk and ring pattern.

 

Looking at out of focus stars with a Mak does not follow conventional star test results. So I don't bother.

 

 

Anyway, I'll report back. It's clear right now, but twilight is a long way off and who knows what conditions will be like then.


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#16 dweller25

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Posted 29 July 2018 - 03:05 PM

Hi Joe, looking forward to your thoughts when you test your scope on the night sky


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#17 infamousnation

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Posted 29 July 2018 - 03:15 PM

1. The engineer put the baffling there for a reason.

2. Yes, best correction depends on distances. I believe the distance between the 2 mirrors effects correction. I don't know the equations that well though.
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#18 fcathell

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Posted 29 July 2018 - 03:32 PM

If I remember correctly, the Questar doesn't even have a secondary baffle, so if it's "necessity" may be dependent on other factors. 

 

Joe - how did you remove the baffle adhesive?  Actually, I think I remember how I did it on that ETX I had with the slipped baffle.  I used alcohol on a Q-tip!

 

Frank


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#19 Joe1950

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Posted 29 July 2018 - 03:50 PM

I would imagine the baffle would help during the daylight views, but how much it helps looking at Jupiter, Saturn, the Mars sandstorm and some doubles is what I want to find out.

 

As far as the engineer is concerned, if there actually was one involved, anyone who would take a 33% obstruction and make it into a 41% obstruction with a baffle is not in tune with planetary observation.

 

Frank, you are correct about the Questar, at least the 3/5". I know they have an excellent primary baffle and perhaps take care of any light problems that way. I don't think they suffer at all in the contrast department.

 

The glue was like a rubber cement or such. Not RTV. It was brownish and flexible. I didn't think of alcohol (don't have any on hand) but that would work without possible harming the secondary spot.

 

I did it the dangerous way. I put a little pressure on the baffle and where it lifted, I cut with a blade, being careful not to touch the aluminized spot. It worked. There was some adhesive residue that remained but it was easily removed by 'rolling it up' if you know what I mean.

 

It was easier than I thought it would be. Then I cleaned the whole thing and it came out like new. No sleeks, scratches... I was lucky.

 

The coatings on this scope are well done I believe. Both the corrector and the reflective coatings look like new.

 

 

I just don't understand why they would make the secondary spot a certain size and then cover about 6mm of it with the baffle. Whatever the secondary spot is, the baffle reduces the diameter by about 6mm as its thick edge sits on the edge of the spot. It is glued to the aluminized spot, not the glass.

 

 

Also, and I'm sure I'm missing something here, with a refractor you have a lens objective. So as you look through the eyepiece holder on axis, there is no baffling of the background light coming in through the lens.

 

They have off axis baffles along the tube, but nothing like a Mak or SCT have at the secondary position. The entire background light, as would be the case in a daytime view, is there, unblocked. Probably missing something.  shrug.gif

 

 

ADDED: 

 

 

 

Look at that! 3mm on the schnoz!  So the whole baffle sits on the aluminized spot, is glued to the spot, not the glass and reduces the diameter of the spot by 6mm. Don't get it, but what do I know. 

 

 


Edited by Joe1950, 29 July 2018 - 04:06 PM.


#20 fcathell

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Posted 29 July 2018 - 04:18 PM

I took another look at mine and I get a direct light annulus when I look directly into the visual back, but not with the 90* prism diagonal inserted.  With my eye centered in the diagonal eyepiece holder, I only get the reflection of the black end of the primary baffle surrounding the secondary mirror. Moving my eye to either side starts revealing a portion of the light annulus. Your Bresser Mak must have a slightly shorter primary baffle than my Orion. Of course I only have a true 120 mm aperture instead of 127 mm due to the mirror being exactly the same size as the corrector.  I didn't measure it, but that corrector is almost an inch thick.  It surprised me.  One test I'm going to use is looking at Saturn's moons. I have used this scope many times trying to see 4 moons and I almost always get 3, but getting Dione and Tethys at the same time is difficult.  I have heard they have different incremental magnitudes depending which side is facing earth. Someone here on CN said you can usually see one or the other depending on which side of Saturn they are on because of this. Obviously more aperture would do the trick but I like pushing the limits of small scopes. I have actually seen 3 of the moons (Titan and Rhea are the easy ones) frequently in my 90 Mak.  I will post any result comments I have after planet testing it.

 

Frank 


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#21 Joe1950

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Posted 29 July 2018 - 04:32 PM

Very good, Frank.

 

If needed, I'll extend the primary baffle to the point where the annulas is covered. That's probably what the Questar folks do.

 

It's supposed to stay clear, so if I can get a look at Jove, I'll be able to judge. The scope is out on the back deck, shaded from the sun, acclimating to the ambient temp. Actually the best views I get of the planets is during mid-twilight. So, there is a little background light there.

 

If, I notice better contrast on Jupiter during that time, it's a done deal. If it contrast is less, I guess it goes back on, but maybe cut down somewhat. No difference? Don't know what I'll do. Probably leave it off for a while.

 

 

SCTs don't usually have a cone baffle, nor do classical Cassegranians. True, the secondary holder is larger than the mirror and acts as a baffle of sorts. But it doesn't seem to be a great deal.



#22 Redbetter

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Posted 29 July 2018 - 04:45 PM

1. The engineer put the baffling there for a reason.

 

That is a debatable assumption unfortunately.  I have doubts as to whether an actual engineer did many designs or if an original design was just adapted without much engineering analysis.  There often seems to be a lot of design "inertia" where parts are used and reused for situations that no longer strictly apply, rather than being specifically engineered for the design.  The baffle size in some cases appears to have been chosen for another system, and not updated for longer focal length, etc.   

 

Assuming it was actually designed, the cases chosen might not have been appropriate leading to an overly conservative implementation (large baffle.)  Removing the baffle altogether is a big step, particularly with a smaller central mirror spot, because it could allow light bypass directly down the central hole.  However, since the central spot is stopped down by 6mm, this is probably less of a concern.   Looks like the central mirror actually utilized with the baffle in place is only ~24% of the aperture, but with the baffle cone results in ~40% obstruction.  Doesn't sound like optimized engineering to me.   

 

I see more evidence of engineering in redesigns such as increasing the primary mirror diameter to match the stated effective aperture.


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#23 Joe1950

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Posted 29 July 2018 - 04:59 PM

Knowing where and how these scopes are made, Red, I just can't envision an optical engineer ray tracing and optimizing the light path. 

 

Probably an upscale or downsize from another model, or taking one of the competition's models apart and doing some quick measuring.

 

Call me a cynic.  lol.gif



#24 fcathell

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Posted 29 July 2018 - 05:15 PM

You are both correct on this "cloning/copying" propensity when "designing" technical stuff.  I was a circuit designer in the power electronics industry for 45 years and I saw my share of design "cloning" or outright technology theft in some cases. In many instances it would be for specific circuit functions and not necessarily the whole design, and, as such, the cloned circuit was not necessarily optimum for the intended function, but it "got by". The cloning of semiconductor parts is rampant in the East and they even use the true, original manufacturer's name on the part to try and legitimize it!  These part are then sold in quantity to "brokers" who advertise them at discount as over manufactured parts or parts that were liquidated due to a cancelled contract. For this reason I totally agree with Redbetter.  It happens on anything where there is money to be made! I'm a cynic too, but cynicism comes with age and experience!

 

Anyway, good discussion! I can't wait to try the modified Mak out, but we now have those infernal monsoons here in AZ which brings in the T-storms in the evenings.

 

Frank 


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#25 fcathell

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Posted 29 July 2018 - 05:26 PM

Joe - just a side note that I remember.  I had an almost new Meade Star Navigator Mak apart some months back and I noticed that the main mirror baffle was threaded on the end, and had about a 3/4 inch extension tube on it.  You could actually unscrew the extension. I wonder if this was some kind of "fix" for a too short primary baffle.  I also wonder if the Meade Mak is made by the same outfit that makes the Bresser you have.  It certainly wasn't a Synta internal construction.  It had really bad image shift and focus backlash which I was able to improve quite a bit but not eliminate either. The optics were really bad due to massive undercorrection on the mirror. I wound up selling this scope cheap to a bird watcher in which the mirror problem was not an issue.

 

Frank


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