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close focus with short FL refractor and binoviewer

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#1 jprideaux

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Posted 29 August 2019 - 08:47 AM

I have an interest in viewing bugs and spiders with two eyes in high magnification at a distance of around 6-10 feet.  I already have some equipment in order to view with 2 eyes at low power at this distance (binoculars) and at high power with one eye (Questar 3.5 but at above 15 feet).  I could pick up a relatively inexpensive 60mm spotting scope that can focus down to 11 feet at its lowest power (15x) but that would be for just one eye.

 

In another posting I received some caution that binoviewers may not work with refractor-like spotting scopes with the way the focusing works in the spotting scope and an astronomical refractor may prove to give more flexibility to do unorthodox things.

 

One combination I am looking at for high-magnification 2-eye close-focus work is the following equipment line-up:

  • Orion ED66 CF refractor (focal-length of 400 mm)
    • includes Orion Deluxe 45-degree Correct-Image Prism Diagonal, 1.25" which has 27mm clear aperture
    • includes 14 mm Wide-field 65-degree eyepiece, 1.25"
  • Williams Optics binoviewer
    • includes optional barlow and two 20 mm eyepieces
    • consumes optical distance of about 100 mm without barlow

I am assuming that the telescope can reach focus at infinity with using the 45-degree Correct-Image prism diagonal with the provided eyepiece

 

The thin lens equation calculator when plugging in 400 mm focal length and 6.6 feet object distance has the image distance at almost exactly 500 mm (100 mm more than at infinity.  Right where the binoviewer would put the image).

 

Is it correct to reason that the system with binoviewer (without using the barlow) would be able to focus at around 6.6 feet +/- some value dependent on the telescope focus travel distance?

 

I'm thinking that this might be a good way for me to get a two-eye, high-power, close-focus system.

 

I could just try a binoviewer with my existing 3.5" Questar but I'm finding that the Questar is producing magnification that is really too high for this particular application.  With one-eye, I was looking at just the head of a dragon-fly the other day with the Questar at about 17 feet way. 

 

I could also just buy the Swarovsky BTX with the 30x65 objective which is supposed to focus down to 7.2 feet but this system would cost around $4800.  The Orion ED66 + Williams Optics binoviewer combo would just cost $650 + $270 = $920.  One-fifth the cost.

 

Any words of wisdom would be appreciated.



#2 db2005

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Posted 29 August 2019 - 09:58 AM

You need outward focus to focus on close objects, but in your case that works in your favor. The binoviewer adds optical path, so adding a binoviewer makes it possible to focus on closer objects, but focusing at distant objects may not be possible without a glasspath-corrector or Barlow. I have focused on objects as close as 5-6 m with a binoviewer in an 81mm APO and I'm pretty sure I could focus on even closer objects.

 

Spherical correction is not going to be optimal at such short distances, but I'm not convinced anyone would notice the difference.



#3 jprideaux

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Posted 29 August 2019 - 07:16 PM

Thanks db2005.  What was the focal-length of that 80 mm that you used?  625 mm?



#4 db2005

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Posted 30 August 2019 - 12:29 AM

Yes, it was a Vixen SD81S. I have never experienced any problems reaching close focus with a bino-viewer in any refractor - and if you do, you just need to add an extension tube to the focuser. The real challenge lies in focusing in distant targets because most refractors I've tried don't have enough inward focus travel. But to focus on near objects, you need outward focus travel.



#5 Mark9473

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Posted 30 August 2019 - 12:57 PM

I have done that with my WO 80/555. Worked great! I was looking at a sparrow about 4-5 m away and it was a superb view. I think I recall I needed a 6" extension tube on the back. With a good quality focuser it shouldn't be difficult to add as much extension as you need.

#6 jprideaux

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Posted 11 September 2019 - 10:10 PM

I have successfully been using a binoviewer on a 66mm (400 mm FL) refractor to look at spiders about 7 feet away.  Sometimes I get lucky as see one eating another bug.   I typically don't use a diagonal and add in an extension tube (to keep the optical path as free as possible from distortion).  The combination of the extension tube and the binoviewer pushes the eyepieces far enough out to reach focus for these near focus targets.  Its fun to use telescopes for other things as well as astronomy.  


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