ERF behind front lens?

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marktownley
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Re: ERF behind front lens?

Post by marktownley »

No reason why it wont work. Robert Arnold has used this technique very well.


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Re: ERF behind front lens?

Post by Merlin66 »

It's tha basis of the LADD/LADS design.
We recommended that the ERF be larger than 50% the aperture and slightly tilted to minimise ghosting.
There have been some users who say the heat build-up does cause some deterioration of the image.....


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Re: ERF behind front lens?

Post by Merlin66 »

With conventional filters they absorb the wavelengths they don't pass and re-radiate the energy absorbed as heat.
The interference type (D-ERF) reflect back the non-pass wavelengths...this, depending on where the ERF is positioned behind the objective can re-focus 99.99% of the incoming energy onto the rear of the lens - NOT a nice situation!
The slight tilt, a couple of degrees, is enough to throw this return beam off centre so subsequent reflections don't end up on the etalon.
BTW I've tried many and various photographic red/ UV-IR filters, and find generally that their thinness and optical accuracy give a significant loss of resolution - just my experience.


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Re: ERF behind front lens?

Post by Merlin66 »

Also regularly check out Surplus Shed.. they sometimes have interesting stuff...


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Re: ERF behind front lens?

Post by Merlin66 »

I'd think the 6mm thick Arriflex would be a nice piece of glass!!
All my failures have been with "standard" +/- 3mm camera filters (Hoya etc).
I did get a couple of thick (8mm?) 4.5" diameter red filters from Surplus Shed a few years ago (Ex-Aerial cameras) and only one of them was usable....
If anyone comes across a good supply source of quality Wratten red filters I'd love to hear about it.


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Re: ERF behind front lens?

Post by Merlin66 »

Hmmm
I did try the combo of the HMC 77mm red and a 2" Baader UV-IR as a pseudo EFR - whether it was tube currents or poor lens...don't know.
Do a star test on the combo...see how it performs.


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Re: ERF behind front lens?

Post by oldfrankland »

This article on the Daystar web site might be of some little interest. It also mentions the reflection, not absorption, of UV/IR by a dielectric coated filter.

url=http://www.daystarfilters.com/inout_art ... Refractors][/url]


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