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HOW TO: make your own Video suitable Focal Reducer

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#1 Dragon Man

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Posted 26 January 2014 - 04:54 AM

A nice easy project to have a play with.
It's cheap, so if it doesn't suit your use, then you haven't lost much :lol:

I made this Video Tutorial of how to make your own Focal Reducer for a 2" Focuser using a Binocular lens and a 1.25" to 2" EP Adaptor. :)

For the Video Tutorial CLICK HERE

Note: If you decide to attempt using a 50mm Binocular lens be aware that 50mm Binocs may actually in some cases have 52mm diameter Primary lenses and can't be used.

Attempt to use a max of 48mm Binocs, they should have a 50mm lens. Any Binocs betweem 32mm and 48mm should be fine.
Also Note that I do state 'Astronomical' Binocs, not those brightly coloured Anti-Glare Horse-racing & nature Binocs.


#2 ccs_hello

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Posted 26 January 2014 - 11:28 AM

Ken,

Great video tutorial.
Just a comment, I do not recommend to use superglue inside the optical system because of the fume. (Remember high school lab or TV CSI shows using superglue fume to lift the finger print :) :)) Or let it dry thoroughly.
For mounting lenses, I'd recommend using an adhesive which is more appropriate for optical use.

Clear Skies!

ccs_hello

#3 ccs_hello

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Posted 26 January 2014 - 11:30 AM

Ken,

Also if you don't mind, I'd like to share this old DIY thread with some pictures.

Clear Skies!

ccs_hello

#4 mclewis1

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Posted 26 January 2014 - 07:24 PM

Ah the old pill bottle mounting setup ... thanks CCS.

Ken, great video. Now I'm looking around at my and my friend's old binoculars in a whole different way.

#5 GlennLeDrew

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Posted 26 January 2014 - 07:44 PM

You'd be surprised to learn how many commercial reducers use what are the very same doublets found in binos. For example, the Lumicon Easy Guider and Giant Easy Guider used 50mm and 80mm bino objectives, respectively. Just to name two specific, relatively 'high end' units. The ubiquitous 1.25", so-called 0.5X reducers use 25mm f/4 bino objectives.

#6 tadpoletoo

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Posted 29 January 2014 - 10:06 AM

Thanks guys! this is the kind of stuff that keeps me coming back. Dave

#7 JimT

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Posted 30 January 2014 - 07:24 AM

Ken

Nice work.

:waytogo: :waytogo:

:bow: :bow:

#8 ccs_hello

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Posted 30 January 2014 - 09:42 PM

Just like to add an important information:

High focal reduction has a steep light cone. To fully illuminate an image sensor, you'd like the objective lens (e.g., donated from a 7x35 bino) as large as possible.
You might find I try to use a larger lens (to avoid optical vignetting), then the medicine bottle (in the shape of reducing profile gradually) to avoid physical vignetting.

Clear Skies!

ccs_hello

#9 rmollise

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Posted 31 January 2014 - 07:38 AM

Just like to add an important information:

High focal reduction has a steep light cone. To fully illuminate an image sensor, you'd like the objective lens (e.g., donated from a 7x35 bino) as large as possible.
You might find I try to use a larger lens (to avoid optical vignetting), then the medicine bottle (in the shape of reducing profile gradually) to avoid physical vignetting.

Clear Skies!

ccs_hello


True, but you would need a huge amount of vignetting before a 1/3-inch or 1/2-inch video chip is affected. ;)

#10 ccs_hello

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Posted 31 January 2014 - 08:44 AM

Unk Rod,

Using a smaller focal reducer
with just 23.5mm lens diameter and a type-1/2" CCD, at 0.34x focal reducing ratio, the image sensor is just 10% fully illuminated <-- by on-axis light (cell B29.)

Clear Skies!

ccs_hello

Attached Files



#11 StarStuff1

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Posted 31 January 2014 - 03:42 PM

Good video, very Informative.

Years ago I bought a grab bag of small lenses from JerryC0 (American Science and Surplus). Using a couple of the achromats and some color filter housings I made both a .6 and a .5 focal reducers. Worked pretty well and I am still using them.

#12 GlennLeDrew

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Posted 31 January 2014 - 04:09 PM

The more aggressive the desired reduction, the faster must be the reducer. No matter how large the lens. For our little video chips, a 50mm f/4 bino objective is really no better than a 25mm f/4.

#13 Dom543

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Posted 31 January 2014 - 05:05 PM

css_hello,
Is your spreadsheet publicly available? I mean the worksheet with the formulas, not just the screen shot.

Glenn,
Why is that? Can't I make reduction more aggressive by increasing the distance between the reducer and the sensor?

Thank you,
--Dom

#14 ccs_hello

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Posted 31 January 2014 - 06:41 PM

Dom,

The spreadsheet came from one of the Y! group and is not mine. I've asked the author in the past and I did not get approval. (I've also asked him to join CN.) May be in the next long holiday I'll re-write it. It is not difficult at all. The on-axis beam coverage is the special one but is not really difficult (get the new image circle size then divide by the sensor's image area.)

Clear Skies!

ccs_hello

#15 GlennLeDrew

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Posted 31 January 2014 - 11:46 PM

css_hello,
Is your spreadsheet publicly available? I mean the worksheet with the formulas, not just the screen shot.

Glenn,
Why is that? Can't I make reduction more aggressive by increasing the distance between the reducer and the sensor?

Thank you,
--Dom


The increase in separation must be supported by the available reducer lens aperture. When moving the camera farther back, you are effectively pushing the reducer farther up into the light cone, where the beam is wider. The more reduction you demand, the faster must be the reducer if vignetting--or worse yet, aperture reduction--is to be avoided.






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