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Disassembling 1.25" filters without notched internal ring.

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#1 happylimpet

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Posted 17 February 2019 - 05:39 AM

Hi folks.

 

I want to put two Astronomik (each 1mm thick) filters into a single housing, to make a 2mm filter. Ive done this before successfully, but the new filters Ive just bought dont have any kind of notches on the internal threaded ring which holds the filter in place, so I cant unscrew them.

 

Does anyone have any tricks for getting the filters out?

 

This pic of a different filter shows the notches I mean, absent on my filters....

 

s-1f.jpg

 

FWIW, its a 642-842 proplanet and an L3 (UV/IR cut).

 

Cheers!



#2 TOMDEY

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Posted 17 February 2019 - 07:49 AM

Interesting! I hope someone knows.

 

I've done that sort of thing, over the years. Bought cheap used filters just to get the mounts with traditional retaining rings. Recall some cheap filters are seemingly just bonded in there, must destroy the mount trying to get the glass out?

 

While we're at it, combining two absorption filters goes as expected. The transmitted spectrum is the simple scalar product of the two alone. But if either or (especially if) both are dielectric interference filters... the combination is most definitively not the scalar product of the two! I worked out the math and wrote a paper on that, long time ago. [The generalized mechanism is a combination of coherent summations over all past possible histories and incoherent summations between the two (or more) filters.] We then applied it to products (the customer was Xerox). Fascinating theory and immediate pragmatic applications... quantum mechanics in action!

 

Here's a couple of my cartoons and the (most important) summary, for limiting cases of the two-filter case. >>>    Tom

 

Anyway, my point is... combining interference filters may give disappointing, even terrible results, manifesting as light-leakage, fringing, and degraded wavefront. Stuff like that.   Tom

Attached Thumbnails

  • 12.1 95 Toms Filter Paper Excerpt 1.jpg
  • 12.2 95 Toms Filter Paper Excerpt 2.jpg
  • 12.3 95 Toms Filter Paper Excerpt 3.jpg
  • 12.4 95 Toms Filter Paper Excerpt 4.jpg
  • 12.5 95 Toms Summary Table.jpg

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#3 happylimpet

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Posted 17 February 2019 - 08:16 AM

Part 1: A scary episode with a dremel and an abrasive tool and i managed to create some notches, and then used a micrometer (nice sharp adjustable points) to undo them! So success.

 

Part2: Aha, intriguing Tom. I will take a look.  Not overly worried about wavefront as this is for DSOs, not planetary, however I hope the transmitted spectrum is reasonably close to the expected product of transmissions. 



#4 happylimpet

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Posted 17 February 2019 - 08:21 AM

Taking a look at a white light through a diffraction grating certainly appears as I hoped!!! The approximately correct section of spectrum appears bright and isolated.

 

The idea, by the way, is to make an H-alpha filter for redshifted sources. The combo transmits from approx. 650 to 678 nm, giving a sensible range of redshifts. Its long annoyed me that there isnt a commercial h alpha filter which allows for redshifted sources - even the wides ones, such as the Astronomik 35nm, is mostly on the blue side of the rest wavelength.



#5 TOMDEY

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Posted 17 February 2019 - 12:05 PM

Part 1: A scary episode with a dremel and an abrasive tool and i managed to create some notches, and then used a micrometer (nice sharp adjustable points) to undo them! So success.

 

Part2: Aha, intriguing Tom. I will take a look.  Not overly worried about wavefront as this is for DSOs, not planetary, however I hope the transmitted spectrum is reasonably close to the expected product of transmissions

Taking a look at a white light through a diffraction grating certainly appears as I hoped!!! The approximately correct section of spectrum appears bright and isolated.

 

The idea, by the way, is to make an H-alpha filter for redshifted sources. The combo transmits from approx. 650 to 678 nm, giving a sensible range of redshifts. Its long annoyed me that there isnt a commercial h alpha filter which allows for redshifted sources - even the wides ones, such as the Astronomik 35nm, is mostly on the blue side of the rest wavelength.

Interesting application! I have often computed that red shift (e.g. solar rotation, radial surface motion, stars and nebulae...) and thought an ultra-narrowband tunable redshift filter would be cool to experiment with.

 

BTW, the ~correct~ equation [for all-dielectric two filters] is that last sheet upper right:

 

Tnet = T1T2 / (T1 + T2 -T1T2)  You apply that at each wavelength and that is the spectrum to expect.

 

As with everything else, I then proceeded to make a bunch of such design-optimized filters, and measure them... and build them into products. That was the Xerox "Blues and Super-Blues" and low pressure sodium street lights (for the European market).    Tom



#6 happylimpet

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Posted 17 February 2019 - 01:39 PM

The two equations (T1T2 and the more complex combination) give the two transmission plots below for quick and dirty test numbers...ie doesnt seem to make much difference!

 

Capture2.PNG

 

How I miss low pressure sodium.


Edited by happylimpet, 17 February 2019 - 01:39 PM.



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