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Re: IR/UV cutoff

PostPosted: Wed Nov 15, 2017 4:53 pm
by olav
Sam wrote:you can use one 1.25" filter on the 294 with the help of 1.25-T2 adapter

Yeah I think we could, sometimes. Other times we shouldn't. And other times again, we can't.
- Ok if the scope is not faster than f/4 or so, and there's enough room for the relatively high 1.25" cell filter plus adapter, and the sensor is not bigger than 4/3.
- Not ok with fast instruments like a f/2 Hyperstar setup, much more vignetting introduced in a case with relatively bad inherent vignetting.
- Many mFT lenses are of great value and quality, but could only be used, with a 1.25" filter and the 1.25-T2 adapter, if the filter cell above the threads are 4.0mm or less. Have not come across anything lower than 6.0mm.

Sam, can I please have the exact protect window (glass) measurements?
I'd say ZWO is specializing in cameras, wouldn't expect that you have all kinds of filters!
(ASI camera body design housing a protect window based on standard filter sizes, like unmounted 31mm and 36mm, would be great)
It's only EUR 55 extra, plus long wait, to have custom size filters, as long as the thickness matches reasonably.

Re: IR/UV cutoff

PostPosted: Thu Nov 16, 2017 9:37 am
by olav
I ended up with a temporary solution, making a "filter rim extender" for 31mm filters.
I think the mounted 31mm filters, from Astronomik and possibly other manufacturers, were best suited. The rim is thin and low, no threads or profiling. These filters measure 31.08 x 3.62 mm.
I pressed the filter into a 3mm thick plastic disc, making it flush at bottom and sticking out 0.62mm at the top, which is ok even for the tight mFT lenses.

As for the ASI294, I'd say that the lack of an original or accessory IR/UV-block protect window is a glitch.
Due to it's very high dark current (but other great features), the only good AP scenario would be with very fast instruments, making 1.25" filter/holder not such a great idea. (Check out the CN thread if you'd like)

Re: IR/UV cutoff

PostPosted: Sun Feb 11, 2018 10:44 pm
by astrovirus
arbydog wrote:
Sam wrote:
arbydog wrote:If I were to use a 2" UV/IR cutoff filter like the one you sell, where would it fit in the optics train. I have the AS1600mm-cool ==>Filter wheel==>OAG==>focal reducer

I don't see a good location to attach this. Would a 1.25" filter work better, somehow?

why do you need 2“ cut filter if you already have the filter wheel

I am getting significant bloating on the blue and luminance filters even after refocusing. The Baader UV/IR cutoff I plan to use cuts off about 410nm where the ZWO goes down to 380. I am hoping that will help tighten up the starts in the blue and luminance channels.

I use my ASI1600MM-C with the 'new' ZWO LRGB filter set on a TS 80 mm triplet apo refractor with 0.8X reducer.
I also observed notably larger stars with the blue and luminance filters.

M71 channel comparison.PNG
Star bloat with B filter

After much trouble-shooting, I discovered that the telescope optics could correctly focus light down to about 420 nm (deep blue), but the filters in front of my ASI1600MM-C camera passed light all the way down to 385 nm (violet). The light between 385 and 420 nm is always slightly out of focus, even with my triplet apo lens, so this creates colored halos and other artifacts around the stars.

After reading about the UV cutoff, I purchased a 2" Baader UV/IR-cut filter and put this in front of my focal reducer.
This filter should have a 420-680 nm passband according to their transmission curve.
The sharpness of the blue channel improved indeed and my star color is now also more accurate.

M15 channel comparison.PNG
With additional Baader UV/IR filter

Most light pollution suppression filters also have a UV/IR-cut function, so you can probably use one of those instead of the Baader UV/IR-cut filter if you have one of them laying around.

Note that the LRGB filters are not faulty, it is a limitation of refractor telescope optics.
These LRGB filters are probably perfect for reflector telescopes (Newton, RC), since they focus light of all wavelengths at the same point.

Re: IR/UV cutoff

PostPosted: Wed Feb 14, 2018 12:41 am
by DominatorDan
I have the new ASI071 Pro, and I can't get focus with the IR/UV cut filter. I think its adding too much spacing. My image train is ASI071=>supplied 16mm spacer=>ZWO OAG=>IR/UV Cut=>Stellarvue FF. I think I need to get the 8mm to 12mm variable M48 spacer to swap for the 16mm spacer