Luminance filter vs Wratten #12

Using SharpCap for other Astro Imaging such as all sky cameras and meteor detection
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oopfan
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Luminance filter vs Wratten #12

Post by oopfan » Tue Nov 10, 2020 3:37 pm

Don't be misled by deceptive marketing that promotes a refractor as "APO-like." I fell for it because I am always looking for a good deal. Well, this deal didn't pay off. In essence, I purchased an Achromat with ED glass. Frankly, I don't know how much of an improvement the glass makes, but I recommend paying the price for an actual APO. I have known for quite some time that my refractor cannot focus blue. Red is excellent, green is OK, but blue is a mess.

Over two consecutive nights, I imaged M34. The left-hand image uses my standard Optolong luminance filter. The right-hand image uses a Wratten #12 (minus blue.) Technically, I piggy-backed the Wratten #12 filter on top of the luminance filter. I did that so that I didn't have to open my filter wheel to replace the luminance filter.

The comparison is stunning. Now all I need to do is figure out how to capture color!

Brian
M34 Luminance vs Wratten #12.jpg
M34 Luminance vs Wratten #12.jpg (273.48 KiB) Viewed 660 times

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Re: Luminance filter vs Wratten #12

Post by admin » Tue Nov 10, 2020 6:48 pm

Hi Brian,

what do the stars look like if you use a blue filter and focus specifically for the blue? Presumably it should be possible to get them sharp when doing that. If that's possible then you could capture R/G/B channels and luminance in RG (presuming a mono camera).

cheers,

Robin.

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Re: Luminance filter vs Wratten #12

Post by oopfan » Tue Nov 10, 2020 8:03 pm

Hi Robin,

You would think, but blue just won't focus.

Here is a test I performed recently while capturing NGC 188 with LRGB filters. With each filter change, I adjusted the focus. You can see how blue is a nightmare, and since the luminance passes blue it is also affected:
NGC 188 LRGB stacks side-by-side.png
NGC 188 LRGB stacks side-by-side.png (210.93 KiB) Viewed 652 times
The best I can figure, the focus error rapidly changes from 400nm to 500nm, while red and green slowly change. Here is a graphic I found at Wikipedia:
Wikipedia Focus Error.jpg
Wikipedia Focus Error.jpg (67.15 KiB) Viewed 652 times
Other people have complained about this refractor from William Optics. I think it is the ED glass, but I think "ED" stands for something else :-)

Brian

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Re: Luminance filter vs Wratten #12

Post by oopfan » Wed Nov 11, 2020 12:48 pm

Hi Robin,

Next opportunity, I will image M34 again, and capture a stack of W12, R, G, and B.

First, I'll use APP to try to create a color image using W12 as L. Hopefully that will give acceptable results.
Second, I'll borrow a technique from narrowband to create a bi-color image from two channels:

100% of W12 => L
67% of R => R
33% of R + 33% of G => G
67% of G => B

Brian

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Re: Luminance filter vs Wratten #12

Post by admin » Wed Nov 11, 2020 7:52 pm

Hi Brian,

I see what you mean in the different colour channels, and I think you're right about the speed of the focus point change with wavelength in the blue channel being the cause of the problem. Perhaps if you can get a narrower bandwidth blue filter it might help?

Cheers, Robin

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Re: Luminance filter vs Wratten #12

Post by oopfan » Wed Nov 11, 2020 10:02 pm

Hi Robin,

I will look into a narrow blue filter, but the downside is the increased integration time needed for the blue stack.

Thanks for your help,
Brian

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Re: Luminance filter vs Wratten #12

Post by oopfan » Thu Nov 12, 2020 4:12 pm

Robin,

This filter would probably be helpful, but I would have to 3D-print a custom holder to fit my filter wheel.

Brian
Edmund Blue Bandpass 475CW 50FWHM .jpg
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Re: Luminance filter vs Wratten #12

Post by admin » Thu Nov 12, 2020 7:21 pm

Hi,

yes, tricky to work out whether it's worth spending money on something like that or putting the same money towards an eventual upgrade to an APO.

Cheers, Robin

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Re: Luminance filter vs Wratten #12

Post by oopfan » Thu Nov 12, 2020 8:43 pm

Exactly my thoughts!

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