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question on determining if I have vignetting

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#1 jprideaux

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Posted 17 September 2019 - 08:21 PM

I'm doing some experimentation with using refractors with extension tubes for some terrestrial close-focus work (like looking at bugs and spiders at around 7 feet away).

I have a set-up that works fairly well with a ED66mm 400mm FL refractor with adding some eyepiece extension tubes along with a diagonal to push the focus back.  My focuser, extension tubes, and diagonal are all 1.25" and I'm concerned that it might be vignetting with loosing a sizable portion of the light cone but I'm not really sure how to determine if that is the case.  I'm wondering if I should get a 2" focuser and use 2" diameter extension tubes instead.  

Are there any tricks or tests that one can do to tell if a system is experiencing vignetting?  Remember, I'm not operating at infinity with this particular application.  Thin lens equation applies.

 

For eyepieces, I'm using either a 20mm or a 14mm.

 

For reference, my system looks like the following:

 

IMG_4207.JPG


Edited by jprideaux, 17 September 2019 - 08:24 PM.


#2 WilburTWildcat

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Posted 17 September 2019 - 08:35 PM

I asked a similar question to a wise friend. His answer? Draw it out. Buy a roll of cheap wrapping or butcher paper and draw a 1:1 scale model of the light cone. See if any of the parts get in the way.ee550e65a74cf27e4e7d85809dd1b1a2.jpg093ca3d0cb607c7a5d70c227ff8920fc.jpg
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#3 jprideaux

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Posted 17 September 2019 - 09:13 PM

WilburTWildcat, thanks.  good idea!  I'll do that when I have time.

 

Here is a picture (unmodified except for cutting out a region just using my old iPhone6s+ through my 14mm eyepiece.

 

I had the phone positioned on purpose a bit too far away from the eyepiece just in case that might give some indication of vignetting.  It I were to have the phone closer to the eyepiece, then the field would be completely filled with the white background.  There is something white behind the spider.

 

Image right out of iPhone

 

fullISpiderIage.jpg

 

Showing just the spider. 

 

spider.jpg

 

It looks a lot better through the eyepiece.  

 

I also plan on getting something like a Canon T7i and try some prime focus pictures so there isn't a bunch of glass in the way.

 

The streaks through the bottom image are out-of-focus spider web silk threads.


Edited by jprideaux, 17 September 2019 - 09:23 PM.

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#4 Jon Isaacs

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Posted 18 September 2019 - 02:58 AM

What I do is measure the effective aperture with a laser and eyepiece.  This projects a beam whose diameter can then be measured. Normally focus is at infinity so the beam projected from the objective is a collimated beam.  

 

Another technique is to place a pinhole at the focal plane and then look at a scale or tape placed across the objective. If the objective has three spacers between the lenses, then seeing the spacers in full is a good measure. 

 

A simple calculation is to figure backwards from the focal plane, assuming that the light cone converges at 1/F so it diverges from the focal plane at 1/F.  If the scope is F/6, and the clear aperture of the focuser at the objective end is 1.25 inches, then the beam expands to the 1.25 inch diameter in 1.25 x 6= 7.5 inches. 

 

In this case I think you would need to use the focal ratio calculated using the thin lens equation.   For a focal length of 400mm and a objective distance at 7 feet, I calculate the effective focal length at 492mm or F/7.45

 

7.45 x 1.25" =  9.3 inches.  Figure 3 inches for the diagonal, that's pretty normal for what I have measured. That leaves you with 6.3 inches for the focuser draw tube and the extensions.  I am thinking you probably have more than that. 

 

If it is further than 7.5 inches to the objective end of the focuser, then the drawtube will cut into the effective aperture.  Your drawtube may have a clear aperture at the objective end greater than 1.25 inches.  

 

All this only addresses the on-axis effective aperture, vignetting is off-axis and is more stringent.  To try to optimize illuminated field, you should have the focuser racked out as far as possible and use the shortest possible extensions.

 

Jon 


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#5 jprideaux

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Posted 18 September 2019 - 05:35 AM

Thank Jon. That was all very useful information. I will first do the simple thing of removing one extension tube (I had 2) and rack the focuser most of the way out. I would then just need to move my tripod a few inches farther away from the subject. I will then do some of the other suggestions to determine under what distances I have vignetting. I guess a little won’t matter all that much. At least with this close focus application, I can make the subject as well illuminated as I want (bright light shining on it).
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#6 AndresEsteban

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Posted 18 September 2019 - 05:39 PM

I asked a similar question to a wise friend. His answer? Draw it out. Buy a roll of cheap wrapping or butcher paper and draw a 1:1 scale model of the light cone. See if any of the parts get in the way.ee550e65a74cf27e4e7d85809dd1b1a2.jpg093ca3d0cb607c7a5d70c227ff8920fc.jpg

Wilbur, sorry to tell you but there are several misconceptions in your drawing. First, the light cone does not end in a focal point but ends in a focal image. In fact you'll have the focal plane at the point where the focus apparently ends in a point. At this focal plane you'll have a a focal area that's determined by the eyepiece barrel inner diameter. 17mm for 0.965" eyeoieces, 27 mm for 1.25" eps and 46 mm for 2" eps. So in order to draw your cone, on the objective side you have the inner objective diameter and on the other side, you have the focal plane. On this on tyhe focal area with the diameter previosly desired.
Upon this cone you'll find the position of the baffles that will not generate vignetting.
Baffles' position could be determined graphically or better still mathematically, it's very easy. 
The nexty link will let you download my apaper as how to calculate baffles, its position and diameter, hope you'll enjoy"

https://mega.nz/#!xZ...JQoXEPOoJ-sHNFU

clear skies for us all!
Andy
 


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