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Secondary Mirror Obstruction?

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#26 Jon Isaacs

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Posted 23 May 2019 - 03:08 AM

Yes, imaging systems typically use a larger secondary because they usually want a larger fully illuminated field of view that that typically used for visual use since the effects of a smaller one show up on photos but usually isn't noticed in visual use.  A 70mm minor axis secondary is about a 34% obstruction, 20 to 25% is more typical for visual use, but opinions vary with some using sizes above and below that range.  You can find lots of threads and opinions on CN regarding secondary obstruction size and its effects on contrast.

 

The contrast is affected on a fine scale visible at high magnifications when viewing bright objects like the planets.  In this case, the contrast is deep sky contrast and not affected.  

 

The challenge here is that the scope has a lot of back focus for use with a camera so a more normal sized secondary is probably not doable.  If the scope really does have the focal plane 5 inches from the tube, that would mean the focal plane was about 9.5 inches from the secondary's center.  

 

Using Mel Bartel's Diagonal Off-Axis Illumination calculator, a 61mm secondary would barely illuminate the focal plane.  If the distance were 8.5 inches, then the 62mm would be sufficient for visual but in my opinion not worth the effort for such a small gain.  

 

https://www.bbastrod...om/diagonal.htm

 

To achieve a 20-25% CO, the scope would need to be rebuilt with a longer tube and a lower profile focuser. 

 

Jon


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#27 Eddgie

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Posted 23 May 2019 - 07:37 AM

Depends on what you want to use your scope for.  For planetary the smaller the better, for imaging deep sky you can get away with a larger central obstruction.  As a general rule, as the central obstruction increases, resolution of a given aperture decreases. 

 

Steve

There is resolution, and there is contrast transfer.  Resolution is not affected by secondary obstruction size, but contrast transfer is. 

 

This is the effect of contrast transfer:  When contrast transfer falls, a white line on a black background grows wider, and a black line on a white background grows thinner.  This means that a bright feature on a dark background would bleed some of its light to nearby darker features and they would appear smaller than they are.   

 

A good example of this would be a shadow transit on Jupiter.  The scope with better contrast transfer would show that shadow as being larger and more black.   On the scope with worse contrast transfer, the shadow would appear slightly smaller and not quite as black.

 

In both cases, the shadow would be resolved. 

 

The standard test target for contrast transfer is black and white sinusoidal lines of different frequencies (widths - which the above graphs are actually depicting) viewed at he focal plane.  Note that in all of the graphs, all of the apertures actually resolve all of the lines!!! What differs (which is what the slope show) is how grey the lines get as they get to a higher frequency (narrower).   As the line slopes down, it is showing that the white lines get slightly more grey, and the black lines get slightly more white.  The bottom right side of the chart indicates the points where the black lines have become 50% grey and the white lines have become 50% grey, and can no longer be resolved from one another.

 

The frequency at which the lines cannot be resolved is a function of focal ratio and had noting to do with obstruction.  As the above graphs show, all of the scopes resolve all of the lines down to the point where the black and white lines have reached 50% grey and can no longer be resolved by any of the instruments (and the frequency at which the lines can no longer be resolve is a function of focal ratio and independent of aperture).


Edited by Eddgie, 23 May 2019 - 07:58 AM.


#28 Asbytec

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Posted 08 July 2019 - 05:52 AM

How much is too much in obstruction, and what are the effects of it? 

Another way to look at the D - co approximation is by the simple equation (2 - a)a where 'a' is the spatial cutoff frequency of a smaller aperture (D - co) normalized to the larger aperture D. For example for a 200mm with 0.33D obstruction, 'a' is the aperture ratio (200mm - 66mm)/200mm = 0.67. This is the cutoff (actually the maximum) spatial frequency of a smaller (200 - 66) = 134mm unobstructed aperture. 

 

The cutoff spatial frequency can be used to approximate the image degradation of a perfect unobstructed aperture Dmm over the range of the smaller one from 0 to 'a'. So, (2 - 0.67)* 0.67 ~ 0.89. This means the larger obstructed aperture is still putting a Strehl-like approximation 0.89 of the maximum 0.84 light into the central disc over the range of spatial frequencies from 0 to 'a' (which a = 0.67 in this case). So, normalized to 1, that's just below the diffraction limit at 0.89 * 0.84 ~ 0.75 over the range of he smaller aperture. It approximates the larger obstructed one (200mm) over the range of the smaller unobstructed one (134mm). 

 

Importantly, bright low contrast cutoff has a Strehl-like number, too, over the same range of the smaller aperture approximated by 1/(1.43 - 0.43a) ~ 0.88 relative to Strehl 1 for a hypothetical perfect 200mm unobstructed APO. It's about the equivelent of 0.06 waves RMS. I am not sure who would scoff at that value or the contrast transfer of a 134mm (5.3") APO (others rave about) even if you have an 8" Dob. Plus, the 8" Dob has more to offer to the right of 0.67 cutoff spatial frequency, anyway, as well as any detail above the MTF curve of the smaller aperture (i.e., not visible in the smaller aperture, but resolved by the larger one). As Tom said above (if he'll forgive the quote), the 8" is definitely capable of showing more. 

 

All of this is derived from the D - co rule of thumb using the areas under approximated MTF curves of the resulting apertures. It does not look so bad when you see it this way. Makes me feel the obstruction, by itself, is a bit overblown. Once you can begin to approximate the MTF effects of the CO in a quantifiable way, you might see the CO in a different light. "Oh, that's bad. No, that's good!" https://www.youtube....h?v=7fMnQAvz0ek

 

Great read: https://www.telescop...aberrations.htm


Edited by Asbytec, 08 July 2019 - 06:21 AM.


#29 Eddgie

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Posted 08 July 2019 - 08:33 AM

I would guess that for people that are not familiar with MTF plots that a simple basic concept may escape them.

 

This concept is that while we typically think of the top left corner as being 100% contrast, with contrast dropping to zero in the bottom right corner, the reality is that the top left corner can represent any starting contrast.

 

Let's say for example that the top left corner represents a starting contrast of 20%.  Let us also assume that the observer has a scotopic contrast sensitivity threshold of maybe 10% for large detail, and 5% for small detail.  

This would mean that once the line dips to or below the observer's contrast sensitivity detail, the observer would no longer be able to differentiate that detail from its background.

The plot I attached is for a 30% obstructed instrument and a perfect aperture of the same size (though no such instrument exists).

This plot has been "stretched" to show kind of show a starting contrast in the upper left corner of 20%.   This would mean that the .5 on the Z axis would represent 10% contrast and the purple line representing the observer's scotopic contrast sensitivity threshold starts here.  

 

As can be seen, by about .4 of the maximum spatial frequency, a detail with 20% contrast on the target will have lost enough contrast that is at the contrast sensitivity threshold of the observer, and the observer will not be able to pick this detail out from the background.    

 

The "perfect" aperture though does not loose enough contrast to hit the observer's contrast sensitivity threshold until it crosses the .62. line.

 

Starting contrast and constrast sensitivity threshold.jpg

 

Note that also, at no point does the slight recovery in the obstructed instrument ever cross back above the observer's contrast sensitivity threshold.

 

I like Figure #74 on this link:https://telescope-op...romatic_psf.htm  as a good example where the author has included the "Bright low contrast detail cutoff" to show how this comes into play with different size apertures.  A bigger aperture might have more contrast loss at a given frequency, but since that frequency might be higher (smaller detail) in the bigger scope, then even though that scope losses more contrast at that frequency, the observer would still have a better chance of seeing that detail in the larger scope.

 

And this vitally important point. The MTF plot is expressed for the "perfect" detail, which is a straight sinusoidal  line. This will represent the best possible kind of feature for resolution (Cassini Division, while not straight, is in relative terms, ans some length, and this is why it is so easy to resolve in even very small telescopes). In other words, this chart (in my opinion) overstates the actually size of the detail the observer might detect.  An irregular feature like a very low contrast small oval on Jupiter would be much harder than a straight line becuase while it might be as wide, it has rounded ends, making it harder to pick out from the background. 

 

 



#30 Richard Whalen

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Posted 08 July 2019 - 09:52 AM

From practical experience I have found optical quality, coating quality, proper baffling and eyepiece used more important to contrast than CO size once its below around 30%. Why small APO's out perform slightly larger obstructed scopes is usually NOT due to being un obstructed but optical quality, mechanical quality and other factors. A smaller CO is nice, but can limit your fully illuminated field and eyepiece choice. Theory is great, but assumes everything is equal which it seldom is.

 

The biggest enemy of contrast is scatter, stray light and optical quality if you have a reasonable size CO.


Edited by Richard Whalen, 08 July 2019 - 11:43 AM.

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#31 Mike Lockwood

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Posted 08 July 2019 - 10:28 AM

A six-inch scope with a 30% diameter obstruction resolves far better than an unobstructed five-incher. Just generate the non-normalized point-spreads and MTFs to see that in action!

 

PS: This is why a (good) modest-sized Dobsonian will always blow the socks off a good smaller refractor (any smaller refractor!) for both light-gathering and resolution!

 

But, gota admit... refractors make fine finder scopes on big Newtonian reflectors...    Tom

Every time I see yet another thread about secondary mirror sizing and central obstruction (particularly when the MTF graphs start appearing), I say what Tom said above - just use a slightly larger telescope and don't worry about it.  (And those little refractors do make very nice finder scopes.)

 

However, I will also add something else - if you undersize the secondary or size it to only fully illuminate the very center of the field, then you are:

 1) using the part of the secondary that is most likely to have a defect,

 2) using the part of the secondary that might roll off due to cooling,

 3) using the part of the secondary that is often left out of the interferometric analysis, and

 4) forcing yourself into very precise placement of the secondary in order to get something close to a fully and symmetrically illuminated field (in other words, making it very hard on yourself for very little gain).

 

My method to size secondaries for most telescopes is simple - add 4" to half the mirror's diameter to get the intercept distance.  Then divide by f/#.  Then go up one flat size if the calculation yields a size that is close to a standard flat size.

 

So, if I calculate that a 3.1" or 3.2" flat is needed, I go to 3.5".  At 3.4" - 3.5", go up to 4.0".

 

The 4" added to half the mirror's diameter just allows the use of a filter slide underneath a properly placed SIPS or Paracorr 2.  For a little more breathing room, use 4.5" in the calculation.

 

Try this on various commercial Newtonians and you'll find that some have secondaries that are too small.....


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#32 Starman1

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Posted 08 July 2019 - 10:31 AM

See Ed's excellent thread here for a good explanation of MTF:

https://www.cloudyni...t/#entry4023727



#33 Starman1

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Posted 08 July 2019 - 10:37 AM

Every time I see yet another thread about secondary mirror sizing and central obstruction (particularly when the MTF graphs start appearing), I say what Tom said above - just use a slightly larger telescope and don't worry about it.  (And those little refractors do make very nice finder scopes.)

 

However, I will also add something else - if you undersize the secondary or size it to only fully illuminate the very center of the field, then you are:

 1) using the part of the secondary that is most likely to have a defect,

 2) using the part of the secondary that might roll off due to cooling,

 3) using the part of the secondary that is often left out of the interferometric analysis, and

 4) forcing yourself into very precise placement of the secondary in order to get something close to a fully and symmetrically illuminated field (in other words, making it very hard on yourself for very little gain).

 

My method to size secondaries for most telescopes is simple - add 4" to half the mirror's diameter to get the intercept distance.  Then divide by f/#.  Then go up one flat size if the calculation yields a size that is close to a standard flat size.

 

So, if I calculate that a 3.1" or 3.2" flat is needed, I go to 3.5".  At 3.4" - 3.5", go up to 4.0".

 

The 4" added to half the mirror's diameter just allows the use of a filter slide underneath a properly placed SIPS or Paracorr 2.  For a little more breathing room, use 4.5" in the calculation.

 

Try this on various commercial Newtonians and you'll find that some have secondaries that are too small.....

I just tried your rule of thumb (using the 4.5" figure) and compared it to Bartels' calculator when illuminating the eyepiece field stop likely to be used for the f/ratio.

(I tried on 3 apertures), and it works!  Each time, it came up with the secondary size that didn't vignette excessively.

I'll have to play with it a little more, but it seems to be a simple, yet accurate, rule.

Kudos.



#34 Eddgie

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Posted 08 July 2019 - 11:55 AM

Actually, I messed up my MTF chart.  

The contrast sensitivity line should have been lower on the left than on the right and not the other way around, but I am going to cop out and not fix it.

 

The contrast sensitivity would be better for the larger image, so the left end of the purple line should have been at 5% at the left side (it is easier to see a bigger 20% contrast feather than a small 20% contrast feature) and should have risen to 10% on the left side.   The basic premise is the same though.  Once the contrast line crosses the contrast sensitivity of the observer, then the feature cannot be seen any longer.

 

Also, it is important to remember that the human eye has relatively poor contrast sensitivity threshold as compared to modern camera equipment, where the noise of the system sets the contrast sensitivity threshold.  This means that for imaging, a bigger aperture with even a very large obstruction will have a great advantage over a meaningfully smaller aperture with no obstruction. That is why the size of the obstruction is not so important for imaging systems.  The noise of the sensor can be managed by exposure time and stacking, and processing can enhance contrast.  For imaging, a big secondary is not of much concern.

For the human eye though, that cannot resolve detail smaller than about 1.6 arc minutes of field (probably a fair mesopic value) most of the visual observation would be on detail size that would be represented by the left two thirds of the MTF plot.  For high contrast subjects (double stars) it of course goes further to the right because a star typically will have somewhat high contrast against the background (as much as 90% or more depending on SQM). 



#35 TOMDEY

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Posted 08 July 2019 - 02:09 PM

Every time I see yet another thread about secondary mirror sizing and central obstruction (particularly when the MTF graphs start appearing), I say what Tom said above - just use a slightly larger telescope and don't worry about it.  (And those little refractors do make very nice finder scopes.)

 

However, I will also add something else - if you undersize the secondary or size it to only fully illuminate the very center of the field, then you are:

 1) using the part of the secondary that is most likely to have a defect,

 2) using the part of the secondary that might roll off due to cooling,

 3) using the part of the secondary that is often left out of the interferometric analysis, and

 4) forcing yourself into very precise placement of the secondary in order to get something close to a fully and symmetrically illuminated field (in other words, making it very hard on yourself for very little gain).

 

My method to size secondaries for most telescopes is simple - add 4" to half the mirror's diameter to get the intercept distance.  Then divide by f/#.  Then go up one flat size if the calculation yields a size that is close to a standard flat size.

 

So, if I calculate that a 3.1" or 3.2" flat is needed, I go to 3.5".  At 3.4" - 3.5", go up to 4.0".

 

The 4" added to half the mirror's diameter just allows the use of a filter slide underneath a properly placed SIPS or Paracorr 2.  For a little more breathing room, use 4.5" in the calculation.

 

Try this on various commercial Newtonians and you'll find that some have secondaries that are too small.....

Whew! for my 36-inch F/3.75... that comes out to (18+4)/3.75 = 5.9" ... and mine is 6.25", with a nice wavefront! And, frankly... even a tad bigger than that might be prudent. I just happened to already have the 6.25 and characterized the wavefront at work... figured a known good one would keep the project hustling along!  I then teased the focuser as close in as possible... reducing that four inches to about three. When I focus my farthest-innie eyepiece... only have a few mm to spare!  Tom



#36 tommm

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Posted 10 July 2019 - 10:43 AM

For my 22.4" f/3.61 it gives (22.4/2 + 4)/3.61 = 4.2", rounded up to 4.5" gives 20% CO.  I am using a 5" with 23% CO.  The percent fully illuminated fov will depend on focuser height and other factors that effect the necessary prime focus to secondary distance.  My prime focus to secondary distance is 15.6", so 4.5" secondary has a small fifov.



#37 Starman1

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Posted 10 July 2019 - 01:12 PM

I used your figures in Mel's calculator and found, assuming no Paracorr (for illumination purposes) and a 21mm Ethos for low power:

4.5" secondary illuminates a 0.2" field fully and a 0.4" field nearly fully and only rolls off the edge 0.2 magnitudes.

A 5" provides more illumination of course, but it seems the 4.5" secondary provided plenty of illumination for visual use.

 

Now, if the holder covers the edge of the mirror and the 5" has really a 4.8" exposed surface, the situation changes,

but it seems a 4.5" would be adequate in that scope, at least given my assumption about the low power eyepiece.



#38 tommm

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Posted 10 July 2019 - 11:32 PM

Yes, I have an Astrosystems holder so the clear aperture of the 5" is 4.75".



#39 Jon Isaacs

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Posted 11 July 2019 - 04:02 AM

However, I will also add something else - if you undersize the secondary or size it to only fully illuminate the very center of the field, then you are:

1) using the part of the secondary that is most likely to have a defect,

2) using the part of the secondary that might roll off due to cooling,

3) using the part of the secondary that is often left out of the interferometric analysis, and

4) forcing yourself into very precise placement of the secondary in order to get something close to a fully and symmetrically illuminated field (in other words, making it very hard on yourself for very little gain).

 

:waytogo:

 

I agree on all counts, particularly #4.  The 25 inch F/5 Obsession I had, used a 3.5 inch secondary and it was marginal in terms of off-axis illumination.  My current 22 inch F/4.4 Starplitter also came with a 3.5 inch secondary and it was marginal in terms of off-axis illumination.  Positioning the secondary was critical. 

 

I just replaced it with a 4 inch secondary for all the reasons Mike listed though primarily number 4.  The CO went from 16% to 18%, plenty small enough for a scope that is primarily used as a deep space scope.

 

Regarding #4:

 

Collimation consists of 2 issues:  Positioning/placing the secondary and aligning the optical axes.  Of the two, aligning the optical axes is by far the most important.  In discussions of collimation, most of the difficulty and confusion is in the placement of the secondary the one that is of less importance.  

 

I also like Mike's sizing technique but it does assume a low profile focuser. 

 

"My method to size secondaries for most telescopes is simple - add 4" to half the mirror's diameter to get the intercept distance.  Then divide by f/#.  Then go up one flat size if the calculation yields a size that is close to a standard flat size."

 

Half the diameter gets to you to the mirror's edge.  Another inch plus a little gets you to the base of the focuser.  An inch and a half gets you to the top of the foucser, the remaining inch and a half gets you the back focus and illuminated field.  These are pretty standard for most Dobs.

 

My 16 inch doesn't fit that mold.  The focal plane is nearly 15 inches from center of the secondary.  Mike's system would assume it would be maybe 11-11.5 inches, less than 12 inches.  That 15 inches reduces the height of the eyepiece at the zenith by about 3 inches which is a critical 3 inches, the difference between tippy toes over a good part of the sky and comfortable viewing.  

 

Jon


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#40 Kunama

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Posted 11 July 2019 - 04:09 AM

I have two Astrosystems holders, the 4.5" one has a clear aperture of 4.4" and the 4" holder has 3.9" clear aperture.



#41 Starman1

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Posted 11 July 2019 - 08:43 AM

waytogo.gif

 

I agree on all counts, particularly #4.  The 25 inch F/5 Obsession I had, used a 3.5 inch secondary and it was marginal in terms of off-axis illumination.  My current 22 inch F/4.4 Starplitter also came with a 3.5 inch secondary and it was marginal in terms of off-axis illumination.  Positioning the secondary was critical. 

 

I just replaced it with a 4 inch secondary for all the reasons Mike listed though primarily number 4.  The CO went from 16% to 18%, plenty small enough for a scope that is primarily used as a deep space scope.

 

Regarding #4:

 

Collimation consists of 2 issues:  Positioning/placing the secondary and aligning the optical axes.  Of the two, aligning the optical axes is by far the most important.  In discussions of collimation, most of the difficulty and confusion is in the placement of the secondary the one that is of less importance.  

 

I also like Mike's sizing technique but it does assume a low profile focuser. 

 

"My method to size secondaries for most telescopes is simple - add 4" to half the mirror's diameter to get the intercept distance.  Then divide by f/#.  Then go up one flat size if the calculation yields a size that is close to a standard flat size."

 

Half the diameter gets to you to the mirror's edge.  Another inch plus a little gets you to the base of the focuser.  An inch and a half gets you to the top of the foucser, the remaining inch and a half gets you the back focus and illuminated field.  These are pretty standard for most Dobs.

 

My 16 inch doesn't fit that mold.  The focal plane is nearly 15 inches from center of the secondary.  Mike's system would assume it would be maybe 11-11.5 inches, less than 12 inches.  That 15 inches reduces the height of the eyepiece at the zenith by about 3 inches which is a critical 3 inches, the difference between tippy toes over a good part of the sky and comfortable viewing.  

 

Jon

Mike's note would assume 12" to 12.5" for the intercept distance on a 16" scope (he said radius + 4-4.5"), which would be common, as you note, with a typical low-profile focuser.

Probably the reason for that is to keep the secondary smaller.

If you don't mind a larger secondary, there's nothing wrong with your approach, although it is uncommon in scopes.

But how do you achieve that long a distance with currently-available components?  

A large mounting block under the focuser?


Edited by Starman1, 11 July 2019 - 08:44 AM.

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#42 Mike Lockwood

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Posted 11 July 2019 - 10:40 AM

Of course if you know that the intercept distance is larger, you would need to adjust for that.  There is also increasing offset as the telescope becomes larger, so for very large instruments I may also calculate using 5" instead of 4.5".  4" is where I start for a first calculation for smaller to medium-sized telescopes, say 16" - 28".

 

My goal is usually to help people select optics, and the builders that I work with, such as StarStructure, Tom Osypowski, etc., will give me the intercept, and then we discuss what we think is best based on what the client will be doing with the instrument.  We may go up a size for an instrument that may be used for imaging in the future, for example.

 

The key is not to undersize the secondary, for the reasons I gave in the previous post, and also due to the lip of the secondary holder possibly being larger than anticipated.

 

I get asked regularly about how to set up a telescope properly for Paracorr and SIPS use, so I wrote this article:

  http://www.loptics.c...SIPS_facts.html

 

This goes together with choosing the proper secondary size - note that the positions of the focal plane with and without the Paracorr/SIPS are given, and this allows intercept to be calculated for a proposed design.  (I realize that I ought to add dimensions in inches, too, in order to make it simpler for people, but I'll get to that eventually.)


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#43 tommm

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Posted 11 July 2019 - 11:05 AM

I have two Astrosystems holders, the 4.5" one has a clear aperture of 4.4" and the 4" holder has 3.9" clear aperture.

I measured mine with a vernier calipers with the mirror installed. I expected about 1/10" wide lip, but it is larger. My 4.5" holder measures 4.22" clear aperture.  The 5" was purchased last year. The 4.5" was purchased in 2002. So you have only about 0.05" overlap of the lip and the mirror on yours?


Edited by tommm, 11 July 2019 - 11:06 AM.


#44 Starman1

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Posted 11 July 2019 - 11:48 AM

My 2.6" secondary measures 2.42" clear aperture, which is a 0.09" (2.3mm) lip.

I would think it very unusual that the lip might be smaller than that on larger mirrors. 

Tommm's results are about what I'd expect--a 0.14" lip to hold a heavy mirror.

It can always be made thinner with a file, of course, commensurate with safety.

The interferometry scan on my secondary excludes about 5mm from the mirror dimensions in the scan (2.5mm on each side), i.e. they only measured 2.4", so it works out fine.



#45 Jon Isaacs

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Posted 11 July 2019 - 12:08 PM

Mike's note would assume 12" to 12.5" for the intercept distance on a 16" scope (he said radius + 4-4.5"), which would be common, as you note, with a typical low-profile focuser.

Probably the reason for that is to keep the secondary smaller.

If you don't mind a larger secondary, there's nothing wrong with your approach, although it is uncommon in scopes.

But how do you achieve that long a distance with currently-available components?  

A large mounting block under the focuser?

 

Don:

 

The scope is strut scope with a single ring upper cage. The focuser mounts on a 1 inch thick oak board that is attached to the ring with heavy angle aluminum. The upper ring is reinforced in the span that carries the focuser.

 

It all works but with the tensioning rods to triangulate the struts and everything, it's quite a chore to setup. Not a big problem since it mostly stays assembled in Boulevard.

 

But I think the classic pole truss is a better design and I am seriously considering building a new structure based on one Danny's designs.

 

Jon



#46 ed_turco

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Posted 17 July 2019 - 11:12 AM

My article on the Definitive Newtonian Reflector still has relevance to this discussion.  I wrote it in 2015.

 

Egads!  Has it been so long?



#47 Kunama

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Posted 17 July 2019 - 03:55 PM

I measured mine with a vernier calipers with the mirror installed. I expected about 1/10" wide lip, but it is larger. My 4.5" holder measures 4.22" clear aperture.  The 5" was purchased last year. The 4.5" was purchased in 2002. So you have only about 0.05" overlap of the lip and the mirror on yours?

 

 

My 2.6" secondary measures 2.42" clear aperture, which is a 0.09" (2.3mm) lip.

I would think it very unusual that the lip might be smaller than that on larger mirrors. 

Tommm's results are about what I'd expect--a 0.14" lip to hold a heavy mirror.

It can always be made thinner with a file, of course, commensurate with safety.

The interferometry scan on my secondary excludes about 5mm from the mirror dimensions in the scan (2.5mm on each side), i.e. they only measured 2.4", so it works out fine.

My secondary mirror is 4.51", the outside of the holder measures 4.64", the clear aperture (across the minor axis) measures 4.41",

so yes the actual part of the lip covering the edge of mirror is only 0.05". There are no signs of it having been altered, filed etc.


Edited by Kunama, 17 July 2019 - 04:00 PM.


#48 Starman1

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Posted 17 July 2019 - 05:07 PM

Hand-made and obviously variable.

My own had a variable lip width around the mirror, which I filed to even up.

Part of the limited production of telescope parts, for sure.



#49 Jon Isaacs

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Posted 17 July 2019 - 09:07 PM

Hand-made and obviously variable.

My own had a variable lip width around the mirror, which I filed to even up.

Part of the limited production of telescope parts, for sure.

In line with Mike's reasons to use a generously sized secondary is the fact that a wider lip on the secondary holder can be more secure.  The secondary holder is dependent on tightness of the screws and the positioning of the screws holding the shroud to secure the shroud.  

 

In my opinion a wider lip is better even if it does reduce the illuminated field slightly.

 

Jon



#50 Kunama

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Posted 17 July 2019 - 09:37 PM

In line with Mike's reasons to use a generously sized secondary is the fact that a wider lip on the secondary holder can be more secure.  The secondary holder is dependent on tightness of the screws and the positioning of the screws holding the shroud to secure the shroud.  

 

In my opinion a wider lip is better even if it does reduce the illuminated field slightly.

 

Jon

Agreed, though I feel confident that mine is secure enough, my mirror has about 3/8" of dacron wadding behind it sandwiched between the mirror and a foam plug cut at 45ยบ so there is no way for the mirror to move, tilt or fall out. I find this is better than some setups which tend to induce astigmatism by having too much pressure on the back of the mirror.




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