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While researching Newtonian design online I came across this very useful software:[6]
Link: Newt for the Web
Here was my first attempt before I had bought a (used) reflector style focuser to actually measure:
Newtonian
Metric: 254mm F/8
47mm m.a. (minor axis of the elliptical secondary)
Imperial: 10"
F/8 1.85"m.a.
Primary
Mirror Diameter...................254.000
Focal
Length..................................2032.000
Focal
Ratio..........................................8.000
Tube
Inside Diameter.......................302.000
Tube
Thickness...................................4.000
Focuser
Minimum Height...................50.000 *
Focuser
Inside Diameter....................50.000
Focuser
Extra Travel.........................12.000
Diagonal
Minor Axis..........................47.000
Diagonal
Offset..................................1.469
100% Illumination Diameter..............22.251
75%
Illumination Diameter................35.424
Front
Aperture Diameter..................287.206
Mirror
Face to Focuser Hole..........1815.000
Focuser to
Front End of Tube..........100.000
Mirror
Face to Back of Tube...........100.000
Tube
Length................................2015.000
This would represent an ultra-low focuser height.
And again, but now with an actual measured focuser depth of 100mm. (4")
254mm
f8 47mm(m.a.) 100mm
focuser height
Primary
Mirror Diameter................. 254.000
Focal
Length.................................2032.000
Focal
Ratio.........................................8.000
Tube
Inside Diameter......................302.000
Tube
Thickness..................................4.000
Focuser
Minimum Height................100.000 *
Focuser
Inside Diameter ..................50.000
Focuser
Extra Travel........................12.000
Diagonal
Minor Axis.........................47.000
Diagonal
Offset.................................1.469
100%
Illumination Diameter.............15.686
75%
Illumination Diameter...............32.852
Front
Aperture Diameter ...............283.985
Mirror
Face to Focuser Hole........1765.000
Focuser
to Front End of Tube .......100.000
Mirror
Face to Back of Tube..........100.000
Tube
Length...............................1965.000
Note how increasing the height of the focuser to 100mm has shortened the tube. The diameter of full illumination has also shrunk in comparison with the ultra-low focuser. Even so, no vignetting is reported by the Newt-Web software.
Careful choice of focuser height affects the necessary size of the flat, secondary, elliptical mirror. The higher the focuser the larger the required secondary mirror. The larger the mirror the greater the obstruction in the light path to the primary. Causing even greater diffraction effects from the secondary obstruction.
Those seeking the highest optical quality from their Newtonian telescope will choose a lower focuser. Which would immediately allow a smaller diagonal mirror to be used. Which in turn reduces the obstruction and minimises diffraction effects. The spider vanes add their own diffraction effects but not nearly as much as the secondary mirror itself.
There is no free lunch though. The smaller secondary mirror may/will not illuminate the entire field. The light loss around the edges of the field may become noticeable. No great problem with small planetary objects centred in the field of view. Not so good for deep sky observation or photography.
Now a range of eyepiece choices, when used with a telescope of 2032mm or 80" focal length. Magnification is easily found by dividing the focal length of the objective by the labelled focal length of the eyepiece.
A very simple example would be a 1" eyepiece used with an 80" focal length mirror giving 80x magnification. (or power)
80/1 = 80. Or, in metric terms: 2032mm/25.4mm = 80x One inch is 25.4mm.
The higher the magnification the smaller the field of view. Useful powers on the planets run from about 120x upwards. This will vary with the object being observed and it's orbital position relative to the Earth. A smaller but sharper image is far more desirable than a large fuzzy one. Though higher powers can sometimes be used to examine a particular feature to confirm some detail. The seeing conditions usually place the limit on magnification. When the observed object is boiling from atmospheric disturbance it is a complete waste of time pushing powers too high.
The table below shows the eyepieces from my own inexpensive collection of secondhand Meade 4000 and no name Asian Plossls. Some of the higher powers employ a 2x Barlow lens. This is a negative lens in a housing which simply fits into the drawtube like an eyepiece. While simultaneously providing a matching socket for normal eyepieces to fit into. This doubles the effective magnification of any eyepiece fitted into the Barlow lens. There are higher power Barlow lenses available but I don't own any. Maximum power for a high quality mirror of 10" aperture is 500x and only possible in very good/perfect seeing conditions.
Careful choice of focuser height affects the necessary size of the flat, secondary, elliptical mirror. The higher the focuser the larger the required secondary mirror. The larger the mirror the greater the obstruction in the light path to the primary. Causing even greater diffraction effects from the secondary obstruction.
Those seeking the highest optical quality from their Newtonian telescope will choose a lower focuser. Which would immediately allow a smaller diagonal mirror to be used. Which in turn reduces the obstruction and minimises diffraction effects. The spider vanes add their own diffraction effects but not nearly as much as the secondary mirror itself.
There is no free lunch though. The smaller secondary mirror may/will not illuminate the entire field. The light loss around the edges of the field may become noticeable. No great problem with small planetary objects centred in the field of view. Not so good for deep sky observation or photography.
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Now a range of eyepiece choices, when used with a telescope of 2032mm or 80" focal length. Magnification is easily found by dividing the focal length of the objective by the labelled focal length of the eyepiece.
A very simple example would be a 1" eyepiece used with an 80" focal length mirror giving 80x magnification. (or power)
80/1 = 80. Or, in metric terms: 2032mm/25.4mm = 80x One inch is 25.4mm.
The higher the magnification the smaller the field of view. Useful powers on the planets run from about 120x upwards. This will vary with the object being observed and it's orbital position relative to the Earth. A smaller but sharper image is far more desirable than a large fuzzy one. Though higher powers can sometimes be used to examine a particular feature to confirm some detail. The seeing conditions usually place the limit on magnification. When the observed object is boiling from atmospheric disturbance it is a complete waste of time pushing powers too high.
The table below shows the eyepieces from my own inexpensive collection of secondhand Meade 4000 and no name Asian Plossls. Some of the higher powers employ a 2x Barlow lens. This is a negative lens in a housing which simply fits into the drawtube like an eyepiece. While simultaneously providing a matching socket for normal eyepieces to fit into. This doubles the effective magnification of any eyepiece fitted into the Barlow lens. There are higher power Barlow lenses available but I don't own any. Maximum power for a high quality mirror of 10" aperture is 500x and only possible in very good/perfect seeing conditions.
Eyepieces:
Focal
Length, Power, Exit Pupil, True Field
32mm............63.5x.......4.00mm......0.551°
26mm............78.2x.......3.25mm......0.640°
20mm..........101.6x.......2.50mm......0.492°
15mm..........135.5x.......1.88mm......0.369°
10 mm.........203.2x.......1.25mm......0.246°
7.5mm.........270.9x.......0.94mm......0.185°
6.4mm.........317.5x.......0.80mm......0.157°
5mm............406.4x.......0.63mm......0.123°
3.2mm.........635.0x.......0.40mm......0.079°
With a focal length of 80" = 6' 8" (2032mm) the magnification soon rises. An 80" f/l is equivalent to a 6" f/13. Or 8" f/10. Even a 12" would have an f/ratio of 6.6 at this focal length.
My apologies for the awful formatting of the tables above. It is something to to do with the blog template software. The tables look far worse in the editing mode! I have spent half an hour fiddling with the number of full stops and still it looks completely askew. If I leave the full stops out everything shifts to the left. Completely Ignoring the spaces I carefully inserted between the columns. Another problem is that each digit and letter has its own allotted width. The software cannot automatically arrange numbers or letters in neat columns vertically above each other.
With a focal length of 80" = 6' 8" (2032mm) the magnification soon rises. An 80" f/l is equivalent to a 6" f/13. Or 8" f/10. Even a 12" would have an f/ratio of 6.6 at this focal length.
My apologies for the awful formatting of the tables above. It is something to to do with the blog template software. The tables look far worse in the editing mode! I have spent half an hour fiddling with the number of full stops and still it looks completely askew. If I leave the full stops out everything shifts to the left. Completely Ignoring the spaces I carefully inserted between the columns. Another problem is that each digit and letter has its own allotted width. The software cannot automatically arrange numbers or letters in neat columns vertically above each other.
Click on any image for an enlargement.
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