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Lunt LS50THa red ERF to Clear for double stacking.

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#1 ldesign1

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Posted 09 May 2013 - 05:36 PM

Hey Everyone:

I thought I'd share this with you as alternative to using nothing in place of the red ERF or Baader Turbo Film as a replacement optical window. I felt that having to round cut, create a mask, and still keep the turbo film flat without stretching it was too much of a hassle. Also, having nothing protecting the etalon was not an option I wanted to consider.

I decided to go with Tiffen 55mm Ultra Clear Digital HT Filter from B&H Photo for $24.95. I removed the filter from its housing and blackened the edge with a black sharpie. The glass measure 52.5mm wide and is 3mm thick. It fits just right with a little wiggle room side to side. Below are the before and after photos. I'm waiting for a DS certified filter from Lunt so I won't be able to do a comparison with and without for at least 2-3 months.


BEFORE MODIFICATION

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REMOVE RETAINING RING

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REMOVE PLASTIC SPACER

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ERF REMOVED

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Tiffen 55mm Digital HT Ultra Clear Glass Filter

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CLEAR FILTER DISASSEMBLED AND EDGE BLACKENED

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CLEAR FILTER IN PLACE

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COMPLETED MODIFICATION

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#2 brianb11213

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Posted 09 May 2013 - 07:04 PM

Interesting idea but photographic filters are rarely anywhere near 1/4 wave RMS & when placed at the objective end of the scope will usually cause a significant loss of resolution.

Turbo film does not need to be "flat" to work well but should definitely not be under tension.

#3 ldesign1

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Posted 09 May 2013 - 10:21 PM

Interesting idea but photographic filters are rarely anywhere near 1/4 wave RMS & when placed at the objective end of the scope will usually cause a significant loss of resolution.

Turbo film does not need to be "flat" to work well but should definitely not be under tension.


How flat is 1/4 wave? The Tiffen fiter is described as being opticallhy pure Water White glass which is ground flat to a tolerance of 1/10,000th of an inch.

More information on the filter HERE.

Transmission and Reflectance Graph

I don't know if the additional information makes this filter any better or not but IMO, I don't think too many people will be able to see a difference in their image. But when I get back home, I will attach the filter to a lens and my DMK camera and take a video of a stationary object with and without the filter. Then I'll run that video through AutoStakkert! and RegiStak with the same settings. We can then see if there is any noticeable difference.

What about Hoya Clear Filters? They are used as a filter replacement by those who modify their DSLR for full spectrum sensitivity.

#4 brianb11213

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Posted 10 May 2013 - 02:51 AM

How flat is 1/4 wave? The Tiffen fiter is described as being opticallhy pure Water White glass which is ground flat to a tolerance of 1/10,000th of an inch.

The wavelength of (green i.e. mid visible spectrum) light is around 500 nm so 1/4 wave is 125 nm. One inch is 25.4 mm or 25,400,000 nm so 1/10000 inch is 2,540 nm which is about 20 times less accurate than is needed to meet the 1/4 wave criterion.

What about Hoya Clear Filters? They are used as a filter replacement by those who modify their DSLR for full spectrum sensitivity.

Photographic filters made by Hoya (also the German company B+W) tend to be significantly better optically than other brands...

... Anyhow the reason that lower quality filters can be "got away with" is that photographic lenses are usually used well stopped down, so that the 1/4 wave criterion applies only to the small area of glass being used to image the part of the image in critical focus; and the resolution of the photographic imaging system is nowhere near diffraction limited until the lens is well stopped down anyway.

If you want 1/4 wave, or better 1/10 wave, optical glass filters, you can get them, but you pay a heavy price ... add a couple of zeros to the price of a photographic filter the same size.

So far as the alternative ERF application is concerned, "nothing" is probably a very good option indeed; if you really want to keep air away from the "unprotected" etalon, turbo film is as good as anything and will definitely be superior to any "affordable" photographic filter.

Do remember that you MUST have (at least) one ERF somewhere in the optical train - preferably in front of the first etalon - for safety, and also to protect the coatings in the blocking filter from being eroded by high energy density radiation.

#5 ldesign1

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Posted 10 May 2013 - 03:39 AM

Thank's for the clarification Brian.

One other question as it relates to you explanation about stopped down photographic lenses. Since I am using a 50mm front filter on an 80mm objective, does that qualify as stopping down? Or is that not enough?

Also, your explanation is basically saying that if I run a test using a photographic lens attached to my DMK camera, it will not show any discernable difference because the lens is stopped down enough to not be affect by the filter.

#6 ldesign1

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Posted 10 May 2013 - 05:34 AM

For the image quality to be maintained, shouldn't all lens elements and filter glass be at least 1/4 wave or better all the way down to the sensor cover glass? I've been mounting my front filter to a low end achromatic refractor. I have no idea if the objective lens is even the same quality as the glass used in Lunts front filters and blocking filters.

If there is one element of lesser quality already in the image train, will the photographic filter added be any worse?

#7 brianb11213

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Posted 10 May 2013 - 11:02 AM

For the image quality to be maintained, shouldn't all lens elements and filter glass be at least 1/4 wave or better all the way down to the sensor cover glass?

Ideally 1/10 wave or better - but as you get closer to the focal plane, you can "get away with" increasingly lower standards because the smearing of the light has increasingly smaller distances to act over ... not to mention the smaller diameter of the light cone meaning that there is a better chance that the area of the filter used to image a point will be sufficiently smooth.

Filters intended to fit in the eyepiece or camera nosepiece are the same sort of accuracy as photographic filters and are adequate for the job. Cheap photo filters can be inadequate even when used with photo lenses. I once bought a used 300mm f/4.5 lens which was fitted with a cheap "protective" filter; the image quality was horrendous when the lens was used at anything like full aperture, only becoming marginally acceptable at f/16. With the filter removed it was good at f/5.6. The same filter would probably have been OK used with a wide angle lens though I never bothered to find out; it was rapidly put in its rightful place (landfill).

#8 BYoesle

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Posted 10 May 2013 - 02:52 PM

I would suggest using either a Baader Red CCD filter or a Baader IR/UV filter - these are specified as being polished to 1/4 wave: http://www.alpineast...ers/filters.htm

Astrodon filters are also specified as being 1/4 wave on fused silica substrate... again I'd go with the Red CCD or IR/UV: http://www.astrodon....lters/e-series/

#9 ldesign1

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Posted 10 May 2013 - 03:51 PM

I would suggest using either a Baader Red CCD filter or a Baader IR/UV filter - these are specified as being polished to 1/4 wave: http://www.alpineast...ers/filters.htm

The only problem with going that route is the fact that the filters are only 50.8mm unmounted. That's barely enough room to fit in the cell window.

Astrodon filters are also specified as being 1/4 wave on fused silica substrate... again I'd go with the Red CCD or IR/UV: http://www.astrodon....lters/e-series/



#10 ldesign1

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Posted 11 May 2013 - 03:53 AM

I ran some test this evening under a controlled setting.
1. Shot indoors
2. Daylight balanced studio lights as only light source
3. Stationary objects
4. Back to Back shots
5. DMK 21AU04.AS Monochrome camera
6. Orion Express 80 Scope
7. Lunt LS50THa Front Filter (ERF Removed)
8. IC Capture 2.2 (capture software)
9. Matched Exposure/Gain/Durations
10. AutoStakkert!2 (all videos stacked with the same settings)
11. No post processing was done to any of the final stacks. Only converted to Greyscale since it was shot from a monochrome camera.

MY OBSERVATIONS:
1. I did not notice any difference in overall quality.
2. The Tiffen filter was slightly darker than no filter for the dart board shot. That was expected. I used 1/8sec (no filter) and 1/7sec Tiffen to match histogram brightness on the second comparison.
3. No filter had slightly more contrast on the face shot but the Tiffen showed more detail in the mid-tones.

Examine the high resolution stack for yourselves if you like. You can download the 3MB LUNT_FilterTest.zip file from HERE.


COMPARISON #1: 1/8sec, 260gain, 60fps, 1min

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COMPARISON #2: 1/8sec, 260gain, 60fps, 1min VS. 1/7sec, 260gain, 60fps, 1min

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COMPARISON #3: 1/15sec, 260gain, 60fps, 1min

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