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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2987
Loc: S. Philadelphia, PA
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I'm hoping someone can explain this to me!
When viewing a target, all light from that target is coming in more or less parallel until it hits the primary, where it is bent into a cone, which bounces off the secondary and into the EP.
I can understand why a wide spider vane that extended down past the secondary toward the primary would eventually cut into the conical light path.
I don't understand how wide vanes that extend forward from the scope will interfere with the parallel light from the target object (assuming the vanes are in-line with the light path).
Please indulge me.  - j
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12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary) 6" f/5 Eero2 ball-scope 6" f/5 Frankenscope Garrett Optical 10x50 binos Edmund 8" yoke-mounted red-tube reflector Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 NexSTar 8i Eeroscope 6" f/5 ball(sacrifice was not in vain) Vixen ED80sf Edmund red-tube 4.25" f/10 Edmund Astroscan
Facts are stubborn things.
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2054
Loc: Massachusetts
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Perfectly parallel vanes will not interfere with one axis objects beyond normal diffraction spikes. It's off axis ojects that the vanes get in the way of.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
One each generic rescue Greyhound (pictured)
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RossSackett
sage
Reged: 08/17/07
Posts: 338
Loc: Memphis, TN
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...and vanes are never perfectly parallel to the optical axis. This doesn't matter much with wires (you just get two wires), but solid vanes will always be slightly twisted and appear thicker than the vane material itself.
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Ross Sackett
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11 scopes currently on the flight line from 4.25 to 18"; 5 pairs of astronomical binocs 35-80mm. My wife suggests that with just one pair of eyes, this might be excessive.
See my scope pix at http://www.flickr.com/photos/8315630@N04/
Carpe noctem!
Amateur astronomer = A mature moon-starer
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Sean Cunneen
professor emeritus
Reged: 08/01/07
Posts: 562
Loc: Blue Island Illinois
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In a perfect world, it wouldn't matter. You would have a much stiffer structure and you wouldn't have to worry about harmonics or your secondary shifting. However and I am speaking strictly for a home-built job here, realsitically, thin vanes make it much easier to get your optical system perfect. With a wide vane, there is much more area for stray light to bounce off and cause flares, They would nee to be coated with a anti-reflective coating or paint, which would add to their thickness. I find it very difficult to keep everything aligned when installing wide vanes. If you were to bend or torque your wide vanes, that wrinkle would have a pronounced effect, especially on bright targets. Adjusting them would have more complexity as well. With a wide vane, you would have multiple in-line mount points that would either have to be dead-on or have some way to adjust them. A vane that mounts with one screw is very easy to reach in and give it a tweak rather than loosen and adjust 2 or four screws. The other problem is being able to figure out if your vanes are parallel or not. That gets much harder the larger the apeture. If you had the capability to drill holes to very tight tolerances on a large bendy tube, then you have half the battle won. Is this a project you are currently undertaking? in my mind there is nothing wrong with the theory, but it's a whole lot more work for a very small benefit.
Sean
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Sean Cunneen
Blue Island IL
8" f/8 Home built Newt, Discovery mirror, curved vane spider, CG5 Motorized head with Intelliscope DSC, Pier
C102hd CG-4
100mm F4 Cedar Tube RF Refractor
2,3,4,5,6mm Televue Plossls
27T3,33T3,44T3
9-22 Ethos Zoom
NO, I'M NOT SELLING!
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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2987
Loc: S. Philadelphia, PA
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I constantly battle streetlights, so my thoughts were that large, wide vanes that extended forward would block more light from entering the focuser. A light shield mounted on the tube would need to extend twice as far to provide equal light protection.
As an extreme example, imagine a 12" dob, but instead of a traditional ring and spider, it has a 9" diameter tube up top with the focuser on one side and the secondary mounted almost flush against the opposite side. The tube would act as a curved spider, light baffle and light shield, maybe extending 15" out past the secondary/focuser, giving the light protection of a 30" light shield.
This is not unlike Ron Ravneberg's "Alice", but pushed to more of an extreme. - j
--------------------
12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary) 6" f/5 Eero2 ball-scope 6" f/5 Frankenscope Garrett Optical 10x50 binos Edmund 8" yoke-mounted red-tube reflector Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 NexSTar 8i Eeroscope 6" f/5 ball(sacrifice was not in vain) Vixen ED80sf Edmund red-tube 4.25" f/10 Edmund Astroscan
Facts are stubborn things.
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2254
Loc: Courtice, Ontario
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I know somebody is using that design on travelscopes on CN, but can't remember who. Id like to hear how that works off axis.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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Joe Cipriano
Entropy Personified
Reged: 09/03/05
Posts: 3830
Loc: Uh... anyone have a GPS?
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I think this is the scope in question.
Jay - you're suggesting extending the baffle/spider up from the secondary?
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In the Land of Eternal Light Pollution & Great Pizza (Chicago)
SN-6, ED80, WO 66SD
Meade 208xt, SBIG ST-4
D70 (modified)
CGE (way modified)
A Wife who understands (unmodified)
Some other stuff...
I may not have gone where I intended to go, but I think I have ended up where I needed to be.
Douglas Adams
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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2987
Loc: S. Philadelphia, PA
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Yes, Joe.
Ron's exquisite "Alice" scope has the spider-baffle going primarily downward, but most of my light enters from the front end. Having it extend forward won't affect the light cone on the front end. I've considered a "U" shape like Ron uses, but also a full tube, both of which would spread the diffraction energy around. - j
(and if you notice, your link was from a thread I started!)
--------------------
12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary) 6" f/5 Eero2 ball-scope 6" f/5 Frankenscope Garrett Optical 10x50 binos Edmund 8" yoke-mounted red-tube reflector Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 NexSTar 8i Eeroscope 6" f/5 ball(sacrifice was not in vain) Vixen ED80sf Edmund red-tube 4.25" f/10 Edmund Astroscan
Facts are stubborn things.
Edited by jayscheuerle (08/07/08 03:21 PM)
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Steven Aggas
sage
Reged: 04/15/08
Posts: 290
Loc: Arizona
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Quote:
When viewing a target, all light from that target is coming in more or less parallel until it hits the primary....
Light from the center of an object is coming in parallel until it hits the primary but light from the edge of the object is entering at that angle to the center, the outermost angle is defined by your true field of view of the scope. I.e., if your tfov is 4 degrees, then light is angled at +/-2 degrees relative to the center. This off-axis light is affected by spider vanes.
Hope this helps.
Steven
--------------------
Mr. Wizard
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
www.DarkSkyObserving.com
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
I use a thin circular baffle over the secondary hanging off the spider body. I make the hole big enough to illuminated the required field and the outside diameter big enough to vignette the far side of the tube from the field. Either 2/3mm Al or an old CD will do. I cover it with black felt as well. It works well with only a small diffraction effect.
Carl Zeiss used a circular baffle extending down from the spider body over the secondary. The front of the baffle was as far down as needed to only see the primary from the field and a hole was machined into the side facing the field. Ideally you need a conical swing of the cutter for the hole. That could be achieved with a router on a overhead pivot. See Patrick Spielman Router Jigs and Techniques for a clue.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2987
Loc: S. Philadelphia, PA
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Quote:
Quote:
When viewing a target, all light from that target is coming in more or less parallel until it hits the primary....
Light from the center of an object is coming in parallel until it hits the primary but light from the edge of the object is entering at that angle to the center, the outermost angle is defined by your true field of view of the scope. I.e., if your tfov is 4 degrees, then light is angled at +/-2 degrees relative to the center. This off-axis light is affected by spider vanes.
Hope this helps.
Steven
How does the light that's not coming in parallel hit the primary in a way that it makes it back to the EP? I thought that's what defines a parabolic mirror (that parallel rays are focused at a single point).
--------------------
12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary) 6" f/5 Eero2 ball-scope 6" f/5 Frankenscope Garrett Optical 10x50 binos Edmund 8" yoke-mounted red-tube reflector Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 NexSTar 8i Eeroscope 6" f/5 ball(sacrifice was not in vain) Vixen ED80sf Edmund red-tube 4.25" f/10 Edmund Astroscan
Facts are stubborn things.
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2054
Loc: Massachusetts
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A parallel bundle of rays from a star will focus to a point. Consider a second bundle of parallel rays from another star in the field of view. That bundle is not parallel to the first bundle. That second bundle will all focus to a different point. That is how an image is created.
--------------------
17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
One each generic rescue Greyhound (pictured)
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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2987
Loc: S. Philadelphia, PA
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Quote:
Hi,
I use a thin circular baffle over the secondary hanging off the spider body. I make the hole big enough to illuminated the required field and the outside diameter big enough to vignette the far side of the tube from the field. Either 2/3mm Al or an old CD will do. I cover it with black felt as well. It works well with only a small diffraction effect.
Carl Zeiss used a circular baffle extending down from the spider body over the secondary. The front of the baffle was as far down as needed to only see the primary from the field and a hole was machined into the side facing the field. Ideally you need a conical swing of the cutter for the hole. That could be achieved with a router on a overhead pivot. See Patrick Spielman Router Jigs and Techniques for a clue.
Cheers. Andrew.
Andrew, I'm having a tough time visualizing this. Do you have any images? - j
--------------------
12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary) 6" f/5 Eero2 ball-scope 6" f/5 Frankenscope Garrett Optical 10x50 binos Edmund 8" yoke-mounted red-tube reflector Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 NexSTar 8i Eeroscope 6" f/5 ball(sacrifice was not in vain) Vixen ED80sf Edmund red-tube 4.25" f/10 Edmund Astroscan
Facts are stubborn things.
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Steven Aggas
sage
Reged: 04/15/08
Posts: 290
Loc: Arizona
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There are parallel rays from the center of the object and parallel rays from the edge of the object and these two areas which enter the scope and eyepiece are not parallel to each other. One area is called 'center' and the other is called 'edge of the tfov'. If you're looking at M42 and seeing the entire complex you are looking at a compilation of angled light that has entered the scope. A nebula that subtends 2 degrees of tfov in the sky is obviously not parallel but differs by 2 degrees edge to edge, or +/- 1 degree from center.... Diagrams are found HERE showing in parenthesis the true fov angle relative to 'center'.
--------------------
Mr. Wizard
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
www.DarkSkyObserving.com
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Quote:
...and vanes are never perfectly parallel to the optical axis. This doesn't matter much with wires (you just get two wires), but solid vanes will always be slightly twisted and appear thicker than the vane material itself.
But how does the thickness of two wires compare with a slightly twisted vane?
--------------------
Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Quote:
However and I am speaking strictly for a home-built job here, realistically, thin vanes make it much easier to get your optical system perfect. With a wide vane, there is much more area for stray light to bounce off and cause flares, They would nee to be coated with a anti-reflective coating or paint, which would add to their thickness. I find it very difficult to keep everything aligned when installing wide vanes.
Doesn't this depend on the size of your secondary? For a larger Dob, with a correspondingly larger secondary mirror, wider vanes are going to provide a stiffer secondary mount, especially if the secondary cage design allows substantial tension on the spider. This enables the scope to hold collimation more effectively. Granted, the wider vane will cause slightly more diffraction, but how important is that compared with having a stiffer secondary mirror that can be more accurately positioned?
My secondary mirror has a minor axis of 4-inches and weighs about 3/4 of a pound not even including the secondary holder. The AstroSystems spider I installed has fairly wide vanes and the result is a scope that can be collimated accurately and holds collimation well.
For smaller scopes, this issue probably isn't as critical.
One thing building a large Dob showed me is that the challenge of creating a large OTA that can be collimated accurately and hold collimation well is not a trivial task. Concentrating on this issue, and simplifying other areas of large Dob design that aren't as critical to optical performance, is likely to result not only in a better performing scope, but one that is easier to maintain in the long run.
--------------------
Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2987
Loc: S. Philadelphia, PA
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Quote:
There are parallel rays from the center of the object and parallel rays from the edge of the object and these two areas which enter the scope and eyepiece are not parallel to each other. One area is called 'center' and the other is called 'edge of the tfov'. If you're looking at M42 and seeing the entire complex you are looking at a compilation of angled light that has entered the scope. A nebula that subtends 2 degrees of tfov in the sky is obviously not parallel but differs by 2 degrees edge to edge, or +/- 1 degree from center.... Diagrams are found HERE showing in parenthesis the true fov angle relative to 'center'.
Thanks for the diagram, Steven.
Sometimes I'm denser than a neutron star... - j
--------------------
12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary) 6" f/5 Eero2 ball-scope 6" f/5 Frankenscope Garrett Optical 10x50 binos Edmund 8" yoke-mounted red-tube reflector Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 NexSTar 8i Eeroscope 6" f/5 ball(sacrifice was not in vain) Vixen ED80sf Edmund red-tube 4.25" f/10 Edmund Astroscan
Facts are stubborn things.
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Steven Aggas
sage
Reged: 04/15/08
Posts: 290
Loc: Arizona
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You're welcome Jay....
Quote:
One thing building a large Dob showed me is that the challenge of creating a large OTA that can be collimated accurately and hold collimation well is not a trivial task. Concentrating on this issue, and simplifying other areas of large Dob design that aren't as critical to optical performance, is likely to result not only in a better performing scope, but one that is easier to maintain in the long run.
I agree with Fiske, and I'll add that form follows function. Too many times when pushing the envelope on telescope design, math goes out the window when it comes to flexure, moment arm, torsion, etc, and things are built empirically. When the image jitters at the eyepiece more than one second after a thump obviously something went wrong but can the source be pinpointed? Sometimes changing an integral component after-the-fact to fix it can have far reaching implications on what all must be change to accommodate the new part to get the jitter under one second and still balance or focus.
Small scopes are great as a test-bed for something new as long as the math is presented too, to substantiate the aspect improved upon and how its also benign to other aspects. I.e., I would not trade vibration and changing collimation from a string spider over the diffraction spikes and solid collimation from a full blade spider. I believe seasoned scope users would notice and comment on image vibration more so than diffraction spikes when viewing through such a scope.
The key elements have to be met first.
Steven
--------------------
Mr. Wizard
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
www.DarkSkyObserving.com
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
I made a 14" F4 with a wooden single vane spider for a club. This was 3/4" at the top and thinner at the bottom. In a thread about parallel truss dobos it was pointed out that one larger diameter pole was stiffer than more thinner ones. By using one vane and having it taper by more than your field in degrees you only have the area of the front of the vane causing diffraction. This may be less in total than four thinner but deep vanes and certainly less than the half circle one above. As it is a single 'clean' obstruction the degradation might be 'smoother'. With a 28mm Pretoria eyepiece I had a very good view of Mars one opposition.
My old Celestron(Vixen) 6" telescope has a single vane as the focusing is by a rack and pinion slider up the tube to give the widest field for photography. Visually you could not see any difference to a standard 4 vane spider.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
A photo with flash.
I am running a smaller better flat than the original so a bit of 'black' paper temporarily taped on the back.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
Edited by HandyAndy (08/12/08 08:35 AM)
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