SharpCap Forums

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

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.

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:
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:
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

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

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

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

Robin,

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

Brian

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

Exactly my thoughts!