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Secondary Mirror Obstruction?

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#51 Jon Isaacs

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Posted 18 July 2019 - 05:21 AM

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 45º 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.

The real problem is that Astrosystems secondary holders do not use a full circle shroud, they are a sheet that has been formed and have a cut.  This means they have the potential, if everything is not correct, to open up and the secondary fall out. 

 

755107-1.jpg

 

As you might guess, this is a concern of mine because it has happened to me.  A secondary safety cord is a good idea but a wide lip is a plus as well.

 

Jon


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#52 F.Meiresonne

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Posted 18 July 2019 - 02:57 PM

Absolutely true.

My Obsession has a similar holder , maybe it is an astrosystem dunno, but there is a safecord attach at the back of the mirror! Those things in a 18" scope become pretty big allready ....it that hits the primary it is a disaster...


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#53 pkrum

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Posted 29 October 2019 - 10:17 PM

I think secondary obstruction becomes less important the bigger the aperture. With those big 20” plus scopes the diffraction pattern is smaller than the seeing. At what aperture range does co for visual observation become irrelevant?

Please discuss.


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#54 hakann

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Posted 30 October 2019 - 05:58 AM

Well, it’s of course a trade off.

 

It can be simple, but one can complicate this up – if one like ;-)

 

If go bigger and stay on ground it will be around 25% or more ( depends on diameter ) but very big diameter and ‘fast’ can also get down to around 20%.
But go bigger diagonal slightly get a less aperture, so in percent it’s very small, and in mag drop here is not really the issue.
But a to small diagonal will drop easy 0.4 mag or more ( lower powers ) and some say it don’t matter for visual use but that reduce ex a 18” to a 15” aperture.
But all this comes into where one observe and on what targets normally.

 

I has to my 18” Dobson 3 diagonals at ; 3.5”, 4”, 4.5”.
The Antares in Pyrex 3.5” is tested trimmed -5 mm ( 3.4” ) to around 1/20 w, the Antares in Quartz 4” needs to trimmed as test to a 3.6” and it is a 1/10 w and the 4.5” is tested full out to edge (?) at 1/20 w ( will be used as 4.4” )
A lip will be on them all.
Idea was L-distance/focal so that was around 3.3” so a 3.5” should be fine minus a lip at 0.1" ( mag-drop in Bartels program at around 0.3 at E21 mm )
I thought had little to much mag-drop on the 3.3” C-A, so I order a 4”, but I think it went to low in tolerance and it was to little C-A.
The 4.5” is a great diagonal and I has ‘room’ and at zero mag-drop, but will obstruct actually with the holder over 25%.
But I’m not observe planets, just deep-sky and most eye candy.
( Scope do not track and seeing is always a issue )

 

 

On Mike L’s formula.
That one is based from a TV coma corrector.
From end of tube to std focus pt it is 88.5 mm ( data from TV = 90 mm ) but add up a filter at 5.5 mm and a clearance to mirrors edge ( say 18 mm if ex E21 mm ) one get ; 18 + 5.5 + 88.5 + 2 mm extra it is 114 mm = 4.5” ( Mike said 4” or ‘whatever’ one like to use )

 

One idea is CAD up the light cone and check the diagonal distance and add 0.5” at least, and see what one get for diameter and then go up to next std size ( but do see your test report from manufactory ) and a lip will be at least 0.1” for best of made diagonals.

 

In my case I will learn if a 3.5” will gain ’my’ observing targets and sky/mag, vs to the bigger 4.5”.

In my case a 4” would be maybe best, but as if low powers and the use of wide field EP and to be coma free ( Paracorr and at f/4 ) and USE the full aperture - I like to try the 4.5” deal. ( if no gain I use it to another project or sell it )

 

Next thing is also the tolerance on a diagonal and stiff mounted and keep out stray light and keep heat from body and ground.

I had a thread on faint fuzzes and the use of a very small diagonal for 1.5 pupil observing, and in that case ex a FS at 7.5 mm and a full diameter to FS and no coma corrector and the Delites EP and a f/4 or 4.5 parabol and a drilled out fat blank in Zerodur.

 

 

Context was said ; today with the good fast/big ones in plano Pyrex this is a crazy idea I was into as those new will wipe what I was talking about EASY.
-Right or wrong, well...


Edited by hakann, 30 October 2019 - 10:12 AM.


#55 Galicapernistein

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Posted 30 October 2019 - 10:12 AM

Physiologically, the point of a big, fast mirror is to funnel light to the more light sensitive rods that are found in the outer edges of the retina. It’s the rods that make dim objects appear when looked at with averted vision. A slightly bigger or smaller secondary isn’t going to change these kinds of views. If the primary mirror is big enough, enough light will get around the secondary to do the job. A planetary scope effectively funnels light into the eyes cones, which populate the center of the retina.  The cones work at resolving not only detail, but they seem to work very well at bringing out contrast. A small secondary helps, but so does a long focal length, for high power views.   And I have no regrets increasing the size of the secondary by an eighth of an inch when I installed an Astrosystems secondary holder, when my original secondary was chipped at the edge. This is for an 8” scope.                                                                                        
 

Instead of slightly reducing the size of the secondary in a big, fast scope, it seems a better way to get the planet killer experience would be to install a tiny secondary, then stop the scope down until the F ratio is F8 or higher. The area of maximum resolution will be tiny, but so are the planetary details you’re looking for. But if you can only stop the scope down to 6”, you would be better off just buying a six inch scope. From my experience, an 8” F8 is the minimum size to start with. 

 

And at some point, the sheer size of a primary mirror is going to compensate for a big secondary, and show not only more planetary detail, but a much brighter image, but a bright image is not necessarily a good thing when observing planets.


Edited by Galicapernistein, 30 October 2019 - 10:31 AM.


#56 Mike Lockwood

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Posted 30 October 2019 - 10:32 AM

Galicapernistein, I understand what you are thinking, but that is just not how optics work.

 

All areas of the primary, except those blocked by the secondary, contribute to all (fully illuminated) points of an image.  So, a "mixture" of the light from all points on the mirror is going to all parts of your retina.

 

The effect of a central obstruction is a very small amount of lost light and diffraction, and the slightly increased effects of diffraction caused by slightly increasing central obstruction are often greatly exaggerated by those who like to obsess over such things.  Even at f/3.0, central obstruction in larger instruments can be kept at 25% or less.  As Hank describes above, I don't recommend making the secondary as small as one possibly can.

 

On Mike L’s formula.

That one is based from a TV coma corrector.
From end of tube to std focus pt it is 88.5 mm ( data from TV = 90 mm ) but add up a filter at 5.5 mm and a clearance to mirrors edge ( say 18 mm if ex E21 mm ) one get ; 18 + 5.5 + 88.5 + 2 mm extra it is 114 mm = 4.5” ( Mike said 4” or ‘whatever’ one like to use )

I believe that the 1.5mm difference may be due to the difference in the Paracorr 2 housing and the SIPS lens group.

 

I also don't understand your 18mm figure, that clearance is not necessary.  If you use 4", then you have 12mm underneath the SIPS to use for a filter slide.



#57 hakann

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Posted 30 October 2019 - 11:45 AM

If one like to has half low power FS in 'radial clearance' it will be 36.2 / 2 = 18.1 mm radial for not intrude whit the Paracorr tube. ( Ethos 21 mm has the FS of 36.2 mm )

I knew many intrude here and it will not make a difference as told.

I tested this and installed a credit card in the end and I has never seen that.

 

In this picture you see the Paracorr with a filter on. ( this is my P2 to + 9 mm = 88.58 mm )

Red vertical line is mirror OD.

Radial clearance is in the top of UTA 18.1 mm + 1 mm extra if tilt the primary, but where focuser is there is only 14.73 mm needed. ( I has some extra mm to go for nearsited )

Focuser is Starlight's 2" plus the plate.

 

I don't say this half FS clearance is needed.

 

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#58 Galicapernistein

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Posted 30 October 2019 - 12:22 PM

Galicapernistein, I understand what you are thinking, but that is just not how optics work.

 

All areas of the primary, except those blocked by the secondary, contribute to all (fully illuminated) points of an image.  So, a "mixture" of the light from all points on the mirror is going to all parts of your retina.

 

The effect of a central obstruction is a very small amount of lost light and diffraction, and the slightly increased effects of diffraction caused by slightly increasing central obstruction are often greatly exaggerated by those who like to obsess over such things.  Even at f/3.0, central obstruction in larger instruments can be kept at 25% or less.  As Hank describes above, I don't recommend making the secondary as small as one possibly can.I

Mike, I agree with what you’re saying. A fully illuminated field is best. If someone wants a smaller secondary, they should get a slower scope. Stopping down the scope to increase the F ratio, and reducing the size of the secondary, will reduce field distortions, and image brightness. But depth of focus should improve. Even Rob Teeter mentions on his site how his planet killer scopes are comfortable to use all night long. Fast mirrors have narrower depths of focus, and as my eyes age, I find them less comfortable to use. People with more flexible eyes probably won’t care. But again, I agree that there are no shortcuts. You can’t separate secondary size from F ratio, and all the performance factors that go with it. Reducing secondary size without increasing the F ratio won’t gain anything.



#59 Mike Lockwood

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Posted 30 October 2019 - 02:37 PM

Our eyes don't "focus out" defocus when looking through an eyepiece, so that is not what you are experiencing.  Defocus is simply due to the positioning of the eyepiece (assuming that the eyepiece is suitable for use in a fast system).

 

A fast system is indistinguishable from a slower one at focus if you simply change the gearing of the focus knob.  This is why FeatherTouch focusers are used on the vast majority of "fast" telescopes.



#60 Galicapernistein

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Posted 30 October 2019 - 03:12 PM

Our eyes don't "focus out" defocus when looking through an eyepiece, so that is not what you are experiencing.  Defocus is simply due to the positioning of the eyepiece (assuming that the eyepiece is suitable for use in a fast system).

 

A fast system is indistinguishable from a slower one at focus if you simply change the gearing of the focus knob.  This is why FeatherTouch focusers are used on the vast majority of "fast" telescopes.

So depth of focus is nothing but the physical distance an eyepiece has to travel to reach focus?



#61 hakann

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Posted 30 October 2019 - 05:13 PM

I had never had a issue focus on fast Newtons and been observing in Mel's telescopes at sub 3 ( std Moonlite 2" focuser/knobs ) and in Ed's Elvira.

I heard about this issue should be so hard, but I has not experiancet that.

I'm 53 and use reading glasses, and night sight is good ( ex driving a car or in a EP ) but Ok, it's not the same as at 25..

Retina is way harder and pupil is now around 5.5 mm, maybe 6..

One has to trust Ol' experiance down the road, my guess.



#62 Kunama

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Posted 30 October 2019 - 05:50 PM

So depth of focus is nothing but the physical distance an eyepiece has to travel to reach focus?

.......... simply, it is the amount you can move the eyepiece (by focuser movement) while still seeing the image as if it is focused......

 

 

Depth of focus refers to the zone either side of best focus (in scopes, the focal plane) in which the image still appears to be sharp.  This zone is greater in slower optics, thus in an F10 refractor for instance the focuser can be moved back and forth more and still retain an acceptable sharp focus compared to an F3.5 instrument where the tiniest movement of the focuser is seen to affect the image or view.  This has to do with the fact that the human eye and indeed camera sensors to a lesser degree cannot distinguish between an actual point of light and a very small disc (see: Circles of Confusion) 

 

With an F10 scope the light cone is long and gently tapering so it takes about 3 times as much movement from 'focus' before the 'point' becomes a circle of about 0.03mm than an F3.3 optic where the light cone tapers much more steeply.... and even the slightest movement has the point becoming a circle of 0.03mm and after that comes the blurring that we see as 'out of focus'

 

Depth of Field on the other hand refers to the refers target the optics are looking at.  For instance if you focus an optic at a distant tree then it and only other items at the same distance will be in focus but, due the fact that eyes and cameras cannot differentiate between small circles and point sources, there will be zone that will also appear equally sharp.  That zone extends a certain distance (depending on the F ratio) from the best focus toward the optic and double that distance beyond the object.....

 

So what it means is that faster optics have a much tighter tolerance for focusing on the target, so as Mike L. mentioned the best way to still have enough fine control of focusing is to gear our focuser to allow finer movements......  I do this my adding a larger diameter wheel to the FT giving me about a 30:1 ratio fine focuser.....

pictured below...

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Edited by Kunama, 30 October 2019 - 06:08 PM.

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#63 hakann

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Posted 31 October 2019 - 02:24 AM

Matt,
I don’t know the ratio on the Moonlite focuser and knob is tiny.
On Mel Bartels 25” the focal is at 2.6 and he has the Moonlite.
I has both of this type of focusers ( and used both allot on many scops ) and I see it as cheap stuff with allot of sag and clearance vs the type of design.
So I’m not jumping, but they work.
I has bought the jumbo wheel but in Mel’s I don’t see the point.
I read about this and told Mel, but he said one should test.
Before used the 25” I had no idea about this and had use his 13.2” with no issues.

Edited by hakann, 31 October 2019 - 05:46 AM.


#64 tommm

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Posted 31 October 2019 - 12:06 PM

I measured about 44.5 mm focuser tube travel for 3 rotations of the coarse focus knob on my Moonlite, or 14.8 mm per rotation. The fine focus is 1/8 that or about 1.9 mm per rotation.

 

Depth of focus for f/4 is about 48 micron or about 25/1000 of a rotation, 9 deg. F/3 is 26 micron or about 14/1000 of a rotation, 5 deg.

For a knob of radius r = 10 mm, rotated an through an angle = 5 deg or 0.09 radian, the arc length through which the O.D. of the knob moves is s = 0.9 mm. A larger radius knob gives proportionately greater s.



#65 hakann

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Posted 31 October 2019 - 01:27 PM

Tommm,

I has never had a problem get it in focus, but the problem is that most telescopes or sky don't allowed it, so it's often lousy stars.
I tested my new 7 mm Delite last Friday here in Sweden in a 18" at f/4.5 and it was just terrible at 285X.
It's a Moonlite on that scope and electrical and that is something I don't like. ( can't get the idea)
Actually one of the only time I seen stars that got my attention was in Jorge Peters Lomo/Sitall 25" at f/4.5




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