"Double duo-band"

[hixx]

Ki again,

The thickness of the NB3 also needs the thickness of the 2nd UV/IR blocking filter added. And you need a way to mechanically stack filters, which I do not currently have. But, like Mike, I don't understand why it matters if they are not parfocal with the ALP-T. The NB3 would have separate flats anyway (and Siril has no trouble with that sort of separate calibration).

Yes the IDAS NB-3 would need an additional UV/IR cut filter and so does the NB-2, which why this would probably be the "best" combo. Having said that, you have 2 filters stacked which does not help improving SNR.Usually Transmission is around 95% each, resulting in maybe just about 90% overall transmission for each combo. And the tricky part of the additional glass is not just calibration, but registration. Depending on glass thickness you'll get a slight shift for rays of light depending on the angle, resulting in an overall geometric distortion depending on the position of a star in the frame. And these distortions are almost never identical, especially with 2 filters stacked, because now also the distance between glasses would come into play.
So We are now adding: IDAS NB2 + IDAS NB3 + UV/IR. Let me know whether this actually works without issues, I'd just suspect the better way would be the NB-1 or NB-Z (which both do not need UV/IR) plus a homofocal S-II or better the Antila Duo and S-II (which both have 2 mm glass, but are more costly)
cheers,
Jochen

[hixx]

Hi Stefan,
again, it's not about the convenience of avoiding to refocus, but different geometric distortion like field curvature (even in perfect focus) which might makes it harder to register successfully, at least for larger sensors. Just imagine You're taking images with different flatteners/correctors. This might work for planetary nebulas. I'd say starting with APS-C format, issues will probably become more prominent. Some advanced registration engines may digest this, but they would need to "undistort" regions of the image differently, by fractions of a pixel. This is why I would not mix different vendors. I don't say it doesn't work, but at least I would stay away from spending a few hundred bucks on a trial like this which might produce foreseeable issues (more or less). Saving 100 bucks for a suboptimal combo might make You pay more in the end should You find out it doesnt work. The best You can do for image quality is a properly corrected optical train, dark sky and lots of time. And yes, You'll need flats for each filter separately anyway, but these won't help the distortion, just cater for vignetting and dust.
If Your cam doesn't have an IR cut built in, I'd stay away from the NB-3.

Cheers,
Jochen

[Stefan B]

Hi Jochen,

If Your cam doesn't have an IR cut built in, I'd stay away from the NB-3.

Luckily, mine has. I guess I can cope with the stuff you mention or don't expect it to be big issues. Moreover, the geometry issue should be most relevant for fast optics and my f5 setup isn't one of them. Maybe the NB3 will join me for Christmas

Regards
Stefan

[Mike in Rancho]

Luckily, mine has. I guess I can cope with the stuff you mention or don't expect it to be big issues. Moreover, the geometry issue should be most relevant for fast optics and my f5 setup isn't one of them. Maybe the NB3 will join me for Christmas

Regards
Stefan

f5 is no longer fast? I guess we are all jaded from RASAs and f4 Newts with reducers?

I don't know if differing filter, or even doubling in order to add UV-IR cut, would cause enough geometric differences in light of other flat-fielding or say, tilt, issues that could already exist. But, if somebody tries this out, it would be interesting to see.

All registration requires some goemetric warping, does it not? Even in DSS, which (in)famously mandates identical pixel resolution and field of view. I think there's a whole tab for what interpolation algorithm is used for exactly that kind of alignment. Other stackers, of course, can handle far greater variance in the files, though there are definite caveats and you have to keep an eye on things.

[dx_ron]

I just noticed that Antlia's entry in the "2nd duoband filter" is Sii/Hb (!)

Not sure how that would be used in ST. Not in any hurry to add that to my Antlia Ha/Oiii, so I won't spend much time worrying about it.

[admin]

I just noticed that Antlia's entry in the "2nd duoband filter" is Sii/Hb (!)

Not sure how that would be used in ST. Not in any hurry to add that to my Antlia Ha/Oiii, so I won't spend much time worrying about it.

That's interesting - I hadn't heard of that combo before!
How you want to use the bands depends of course what sort of color mapping you're going for. The Color module may not have presets for S-II/H-beta but you can use the H-alpha/O-III matrix options (all assuming that S-II is in the red channel and H-b is in the blue and green channels).