Keithdrengen
sage
Reged: 02/20/08
Posts: 254
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The stock obstruction is 50mm. How do I find out the smallest usable secondary size for visual? For future replacement. I dont know how to "read" the numbers in "Newt".
CJ
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Joe H
member
Reged: 07/19/06
Posts: 72
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Hi CJ,
An old formula I think from sky and telescope was check the distance from the center of tube to the point where your eyepieces focus and divide by the F ration. So if you have a 7" tube you have 3.5" plus the focuser height 3" or so? and divide by 6 (in most situations a 1.3 should be big enough for visual use). This formula shows pretty strongly the difference a low profile focuser makes.
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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You could use a calculator like this one. Click here
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4938
Loc: MA
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>>>You could use a calculator like this one.<<<
Better yet. Use this one. It has more parameters to input and calculates the angular TFOV for varying amounts of field illumination. It also shows the results diagrammatically. Basically, size the secondary by trading off the diameter of the fully illuminated and 75% illuminated TFOV. It depends on the details of your OTA, but IMO, you could probably change out your 2" 33%CO for a 1.5", 25%CO and lose a significant amount of illumination only at the edge of widest TFOV with a 2" EP. 
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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I did a quick look using Newt.exe and found that the design is quite sensitive to focuser height and secondary size. A 3 inch high focuser really reduces the brightness of the image over a large percentage of the eyepiece. Please provide:
Tube thickness
Focuser height
Focuser diameter 2” or 1.25”
With these numbers I can come up with a recommendation.
For my own 6” F/9
2” focuser, 1.65” high in an 8” tube I used a 1.3” secondary.
--------------------
Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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Quote:
>>>You could use a calculator like this one.<<<
Better yet. Use this one. It has more parameters to input and calculates the angular TFOV for varying amounts of field illumination. It also shows the results diagrammatically. Basically, size the secondary by trading off the diameter of the fully illuminated and 75% illuminated TFOV. It depends on the details of your OTA, but IMO, you could probably change out your 2" 33%CO for a 1.5", 25%CO and lose a significant amount of illumination only at the edge of widest TFOV with a 2" EP.
If you read the origial post. The poster said he didn't know how to read the numbers in Newt.
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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Keithdrengen
sage
Reged: 02/20/08
Posts: 254
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Mel Bartels calculator is the easiest for me thanks. Thing is I dont have the tube yet.. so I dont know the exact numbers yet.. Other than that the ota is 7" and I´ll use 1.25" EP´s. But what Luigi said about ~40mm gives me an idea.
Maybe its irellevant for me, but I never understood why a given field is not allways 100% illuminated.
CJ
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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My guess would be a 1.3" minor axis secondary.
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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Keithdrengen
sage
Reged: 02/20/08
Posts: 254
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I know its just a guess, but it sounds promising then.
CJ
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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Yes but it's an educated guess. I have a 6" f-6 with 1-1/4" low profile JMI focuser and a 7" tube and 1.3" was perfect for all around use.
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Given:
Tube Inside diameter 7”
Mirror Diameter 6”
Focal Ratio 6
Focuser diameter 1.25”
Case 1: 1.3” secondary 3” focuser height
100% illuminated diameter .457degrees .19 inches
Case 2: 1.3” secondary 1.6” focuser height
100% illuminated diameter .73 degrees .3 inches
Case 3: 1.5” secondary 1.6” focuser height
100% illuminated diameter 1.1 degrees .69 inches
I recommend case 2. The larger, 1.5” secondary will get you a larger 100% illuminated area but Newt indicates that the tube is too small to use all of the area of the secondary. Using the 1.3” you get a large area with a smaller obstruction. At 22% the loss of contrast is not negligible but it is well within the low impact range. With the 1.5” you would be at the 25% obstruction area where the loss of contrast becomes noticeable to many people.
I also recommend getting a low profile focuser. Case one shows that the extra height of the 3 inch unit will greatly reduce the 100% illuminated area. If you are optimizing for planets you would have to keep the planet within that small area to get maximum contrast. My own 6” F/9 has a 1.3” secondary and is an excellent planetary scope. Due to its longer focal ratio it provides a very wide 100% illuminated area to support my use of 2” wide field eyepieces.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7807
Loc: Franklin, Ohio
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Quote:
I dont know how to "read" the numbers in "Newt".
The help files for Newt are pretty good at explaining what's what, plus they provide a good insight into Newtonian telescope design.
Patrick
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10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Celestron Regal 8x42 Binocular
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount
My Astronomy Pages
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wfj
sage
Reged: 01/10/08
Posts: 259
Loc: California, Santa Cruz County
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Depends on how much you are bothered by vignetting. Some really hate it and insist on larger secondaries. Others like myself aren't bothered by it, even with 2" EPs.
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Keithdrengen
sage
Reged: 02/20/08
Posts: 254
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Quote:
Yes but it's an educated guess. I have a 6" f-6 with 1-1/4" low profile JMI focuser and a 7" tube and 1.3" was perfect for all around use.
I know it was an educated guess, sorry if you got me wrong, and thanks all for the answers and thourough examples.
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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You are also limited to available secondary sizes. 1" would be to small and 1.52 to big for my liking. Below is my chart showing light cutoff for the 1.3" secondary I used in my 6" f-6 with 7" tube and 5.5" distance to focal plane. Very good to about 3/8" fov.
Off-Axis Illum. Light Loss
0.00 in 100.0% 0.00 mag
0.10 in 100.0% 0.00 mag
0.20 in 100.0% 0.00 mag
0.30 in 95.12% 0.05 mag
0.40 in 84.77% 0.17 mag
0.50 in 73.25% 0.33 mag
0.60 in 61.50% 0.52 mag
0.70 in 49.93% 0.75 mag
0.80 in 38.83% 1.02 mag
0.90 in 28.41% 1.36 mag
1.00 in 18.93% 1.80 mag
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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I had a 6" F/6.25 for several years. It had a sled focuser, which allowed the field stop of the eyepiece to be located about 1/4" above the 8" tube's surface. ("A"= 4.25")I had a 1.063" secondary that allowed a fully illuminated area of 3/8"~ at the focal plane. It was an excellent combination. I saw Olympus Mons on Mars with it. I'm not really sure where I got the secondary, but it tested out adequately, and made for a superb combination in that scope. The tube was 4 foot long, so it had a built in glareshield. A pic.
Mark
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So many projects, so little time!
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Keithdrengen
sage
Reged: 02/20/08
Posts: 254
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Is light cut off the same as aperture loss?
CJ
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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Nice looking scope Mark. Any problems holding alignment with the sled focuser?.
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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Not really, I was suprised how well it worked, first time. This was my first sled-focuser scope.
M.
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So many projects, so little time!
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4938
Loc: MA
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>>>If you read the origial post. The poster said he didn't know how to read the numbers in Newt. <<<
Oops, my bad. I forgot to add, " and figure out how to use it."
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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