IR/UV cutoff

arbydog

I have a ASI1600-mm cool with the LRGB and narrowband filters I bought about 6 months ago and I have a couple questions.

1) Is is necessary to use a UV/IR cutoff filter in front of the individual RGB or narrowband filters?

2) I am also getting a lot of purple finging when using the 1600mm-cool that I didn't get using my Canon DSLR. The telescope is the same, its an Explore Scientific ES 127 APO. Orion was taken with the 127 APO and a Cannon with M13 was recently done with the ES127 and the ASI1600-mm cool. I do have trouble getting a good focus with the blue filter. Usually with the L, R and G filter I can get around a 2-3 on Half Flux radius but the blue is usually 3-4 even with trying to refocus. Could there be an issue with the blue filter? Thanks.

Sam@ZWO

the RGB and narrow banding filter already cut IR and UV

so you can imaging with R G B L or even without L

about blue problem, maybe just because of seeing

blue light is more easier affected by seeing

arbydog

Sam,

I looked into my blue fringing issues and i do recall in my notes that I can gain a better focus on the green and red filters versus the blue and lum filter. The narrowbands also focus much tighter. How sensitive is the ASI1600 to the IR spectrum? I am thinking that the cutoff point of the IR and Blue filters in the RGB set let just a little too much below 400 nm or such through and starts to bloat the stars. Several weeks ago I spent some time calibrating the focus on each filter to check offsets for autofocus, the ability to get sharper focus for the L and B filters were by far worse than the G and R. The air temp was the same as were the seeing conditions and same time of night. I am thinking an IR/UV cutoff is needed to block light below 400-420 to improve the lower end performance of the camera.

In the end I think my blue fringing is a simple result of fatter stars across the blue and L filter vs the red and green. That would also explain why this fringing shows up on this camera and not my dslr which is unmodified. Thoughts?

merlin66

I use a FULL full modded Canon 1000D in spectroscopy, and find I can record down to around 365nm.

Yes, focusing in the UV is difficult and the focal point can be mm from the green focus.

Try adding a UV-IR filter and see if it makes any difference.

arbydog

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?

Sam@ZWO

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

arbydog

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

Sam,

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.

hansdemoor

I'm getting the exact same thing using a Televue Pronto refractor and the ASI1600MM-cool with the ZWO LRGB filter set. But I don't get it with a Maksutov telescope and a SCT.

The refractor images through the R and G filters have nice small stars - even the bright ones, but the L and B images have stars that are easily twice the size of the R and G. I think the underlying problem is that our refractors are optimized in the visual range of colors and wavelengths below 400nm are not well corrected. The new ZWO filter set for the ASI1600 cuts off at 380 nm. The reason you can't refocus to take care of the blue may be that the telescope may have a different focus point for blue close to 480 nm than blue close to 380 nm.

If you look at other manufacturers that have filters specifically designed to reduce the bloated stars in the blue/UV range they mostly cut off at 420. Even the other ZWO UV cutoff filters cut off at 400nm.

Though it isn't very useful for tri-color imaging I took some test exposures with a Lumicon 2" Deep Sky filter. It allows light to pass from 450nm to about 540nm. The stars were very sharp.

So I wish that ZWO had a version of the protect window that had a transmission like the D25 IR-CUT protect window designed for other camera models. So perhaps a question to Sam would be, could a commonly available sized 31mm filter work in place of the 32 mm filter found on the ASI1600MM?

After the clouds pass I'm going to try a 2" Baader UV/IR cut placed on the telescope side of the reducer/corrector and use the ZWO filters in their normal place between the camera and reducer/corrector and see if that helps. I'll let you know.

Hans

olav

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Sam@ZWO

you can use one 1.25" filter on the 294 with the help of 1.25-T2 adapter

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.

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)

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

Sam,

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

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

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.

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

gaztro

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

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.

Many thanks for this tip. I also have the TS-80 triplet and have been considering the ASI1600MM + filters and wheel but the star halo issue has kept me from pulling the pin. Your post makes sense to me and I'm now ready to take the plunge into mono cmos imaging!

Cheers, Gary.