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2" focuser vignetting for imaging newtonian?

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#1 betelgeuse91

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Posted 24 November 2020 - 05:00 PM

I see a lot of refractors coming with 2.5" focusers, but almost all imaging newtonians I see come with 2" ones.

 

I am wondering if 2" focuser on newtonian does not cause vignetting (for an APS-C size sensor).

 

Thanks a lot! 



#2 MitchAlsup

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Posted 24 November 2020 - 06:26 PM

This is dependent on the F/ratio, the size of the secondary, and the distance beyone the "tube" where the primary reaches focus.

 

13MIPSOpticalPathHakan.jpg

 

The above is draw optical arrangemetn of my 13" F/3.

 

Notice how the outer edge of the secondary (even if present) would be vignetted by the entrance aperture of the paracorr.

 

Also notice that if you use a 3" paracorr, you need another whole inch of distance from the secondary because P3 is that much longer than P2.

 

The best thing I can advise you to do is to draw up a scale model of the optical paths, and then choose the focuser that optimizes that optical path.


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#3 silios

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Posted 24 November 2020 - 07:32 PM

I see a lot of refractors coming with 2.5" focusers, but almost all imaging newtonians I see come with 2" ones.

 

I am wondering if 2" focuser on newtonian does not cause vignetting (for an APS-C size sensor).

 

Thanks a lot! 

A 2" focuser will cause vignetting on a APS-C (26.88mm diagonal) sensor on a AP (fast-ish < F5 + Oversized secondary for "optimum" field illumination) Newtonian.
Even at F5, you need to take flats to combat the falloff of light to the edges.

Now to calculate the amount of vignetting on APS-C (26.88mm diagonal) with a 2" focuser you need to know the length of the drawtube (assuming an AP purposed Newtonian with oversized secondary to make things a bit simpler).

Light cone expands at the rate of F ratio. If you know the length of your drawtube you can do the calculations to see if there will be vignetting for a given configuration

 

Example :
With a 26mm diagonal sensor at F4, the light cone will be 10mm (40mm/F4) wider (36mm) at a distance of 40mm away from the sensor, and 20mm wider (46mm) at 80mm away from the sensor.
2" inches are 50.8mm wide, so if the distance from the sensor to end of the drawtube is less than 95-ish mm (highly unlikely tongue2.gif)  you have no vignetting.
The faster the scope the worst the vignetting for the same 2" inch focuser.

 

Those calculations are for scopes without reducers which by design increase the angle at which light approaches the focal plane.
If the focuser and thus the drawtube is too narrow to accept light at this greater angle (depending on the entrance pupil of the reducer) the tube will cut into the light cone and cause vignetting.

 

You can test specs from scopes in the NEWT designer online -> https://stellafane.o...b/newt-web.html
Hope that at least some of it makes sense smile.gif


Edited by silios, 24 November 2020 - 07:35 PM.


#4 betelgeuse91

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Posted 26 November 2020 - 07:15 PM

This was a much more complicated problem than what I initially thought. Your comments gave me a lot of insight and things to learn.

 

Thank you both of you.


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#5 gatorengineer

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Posted 26 November 2020 - 08:42 PM

Mitch,

 

MANY thanks for this.....  It also vignettes for visual as well.  I have had this argument with my optician for 10 years.

 

I write this as I look at the scope in the corner with an ASA 2" .73x reducer corrector, that required in focus.  My ASA is supposedly good to F3 as well, one of the main reasons I bought it over the TSoptics variant.




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