Baader cmos optimized 6,5 nm or 3,5 nm - Forums

I have a 200/800 TS Optics newtonian and my camera is the qhy294MPro with LRGB baader filters set mounted in a qhy3FW. I will buy narrowband filters…
¿What bandwith do you recomend to me?
¿6,5 or 3,5?
¿What is the difference with highspeed?
I live in a bortle 6.
¿Would you recommend another brand of filters?

Thanks,
Joaquín

Hi Joaquin,

In general, narrow band is always better. Gives you better contrast. Especially so in light polluted areas.

But, when using a reflector telescope, the light comes into the system only at an angle, as the secondary mirror obstructs the direct path.

Therefore, the light shifts towards blue. Faster the scope the more noticeable the shift becomes. To compensate for this, filters for fast systems are "preshifted" respectively. Narrower the flter bandwith, the more precise this preshift needs to be. Baader makes ultrafast F2 filters with such preshift especially for the fast scopes like yours (F4 or better).

But, Cuiv the Lazy Geek (look him up on the youtube, if you don't know who he is), discovered that some of the Baader OIII filters were preshifted incorrectly. It is unclear why this happened and by when Baader will update their production process. Therefore, it is much safer to buy a wider bandwith filter, just to be on the safe side.

Hope this helped.

D

David Nozadze:
But, when using a reflector telescope, the light comes into the system only at an angle, as the secondary mirror obstructs the direct path.

It does not. Refractor or reflector, central obstruction or not, the light enters the inlet pupil all parallel to the optical axis and all converge at an angle to the focal plane independently of the type of optical system.

I've gone with cheaper 7nm narrowband filters just to get used to doing narrowband processing. I'm sure the 3.5nm filters will be better eventually, but their cost is such that there are many other upgrades to my setup that would be more cost-effective (e.g. a better mount)

andrea tasselli:
It does not. Refractor or reflector, central obstruction or not, the light enters the inlet pupil all parallel to the optical axis and all converge at an angle to the focal plane independently of the type of optical system.

Maybe it is not perfectly parallel? If it were, would we not have distortion-free image edge to edge? And, also why would Baader or any other manufacturer produce a preshifted filter specifically for fast optics?

No. They are perfectly parallel when entering the optical system as they originate at infinity (or large enough distance compared to the focal length of the system). The "distortion" you refer to comes from the fact that these optical systems create a convergent system of "rays" to create a real or virtual image (of the object at a distance/infinity) at the focal plane since their main scope of being is to concentrate light from a source at distance to make it more visibile to sensor/eye as well as magnify its appearance. Given that these facts are rather indipendent from the specific of the optical system in question we can rule out the central obstruction being the issue at hand. The issue at hand is due to the wavelength dependancy on the incidence angle from one medium to another, wiz.

Lambda.theta=Lambda.normal*(Square root(1-(n.0/n.filter*sin(theta))^2)

Where Lambda.theta is the effective wavelength at any given incidence angle, Lambda.normal is the original wavelength at zero incidence, n.o is the refractive index of the medium before the filter (e.g. air) and n.filter is the effective refractive index of the filter and finally theta is the angle of incidence of the light. Note that this angle depends on the field size and so does the "blue shifting" which adds another layer of complexity in calculating the effective wavelength shift in dichroic filters.

andrea tasselli:
these optical systems create a convergent system of "rays" to create a real or virtual image (of the object at a distance/infinity) at the focal plane

In other words, when a ray can go exactly through the center of the objective/aperture, it does not need to be "converged", it just goes straight through. That is impossible in reflectors. Rays hit the secondary at an angle and reflect at an angle. Thus they also can't hit the filter/sensor at the right angle. This is, as I understand, particularly true for fast systems like newtonians, RASA etc.

I very much doubt that 1 ray makes any difference AT ALL. It's all the others that make the difference and the angle is only driven by the f/ratio. Secondarily your assumption that reflectors need a central obstruction is wrong. Schiefspiegler comes to my mind.

andrea tasselli:
I very much doubt that 1 ray makes any difference AT ALL. It's all the others that make the difference and the angle is only driven by the f/ratio. Secondarily your assumption that reflectors need a central obstruction is wrong. Schiefspiegler comes to my mind.

As I see, you do like strong statements

Yes it does make all the difference. The closer the ray is to the center of the optical train, easier it is to handle. That is why refractors have so much better contrast and sharpness. If there was no technological limit to the size, weight and cost of the main objective lenses, everybody would be using those instead of reflectors.

Also, my assumption about the reflectors and central obsrtuction is not wrong. How may amateur astrophotographers use tilted reflectors? Even the scientists use them for very niche applications. So, as far as the main question of this thread is concerned, I don't think that Schiefspiegler design is relevant.

Our friend here was interested, whether Baader ultrafast 3nm filters are any good. I use exactly these for my newtonian and already experienced some issues with them, as many others like me. Actually, one of the filters came from Baader with a physical defect, not to mention incorrect amount of preshifting. That is why I recommended to buy a wider bandwith version instead, which all Baader users recognize to be a much better choice.

What is your experience with Baader filters?

I have the UNB and also suffer from problems (not high speed for systems slower F5).

1. coating defects on surface
2. central wavelengths shift at the O3 (at least it looks like)
3. extreme reflections on O3, less at Ha and S2

I have contacted Baader and wait for reply.

CS
Rüdiger

David Nozadze:
Yes it does make all the difference. The closer the ray is to the center of the optical train, easier it is to handle. That is why refractors have so much better contrast and sharpness. If there was no technological limit to the size, weight and cost of the main objective lenses, everybody would be using those instead of reflectors.

It does not in the slightest, as far as wavelength shift is concerned, see the formula I gave a couple of replays ago, the rest is just a matetr of knowing the basic of optical design. Refractors have NOT better contrast and sharpness than reflectors. Small very good quality super-apochromatic refractors do compared to similar or slighter bigger catadioptric/reflectors. And that where it ends, as anyone who tried to use even very big APOs where contrast and sharpness matters a lot (planetary viewing/imaging) can testify if they are honest with themselves. Snell law rules.

As for NB Baader stuff I can say they are good value for what they offer but I wouldn't use the 3nm ones anyway in a fast system.

andrea tasselli:
It does not in the slightest

Then why invent an obstruction-less tilted reflector?

To prove the point, I suspect. And because that's what humans do, invent things. Even pointless ones. For those of curious mind it might be interesting to know that the very first newtonian reflector, invented and built by Sir Isaac Newton (obviously!) was a tilted reflector with no secondary mirror. Reason wasn't because of the effects of CO on contrast (which he didn't even suspect) but because speculum reflectivity is rather poor and with 5 cm of primary mirror diameter there was no light to waste. But apparently it was still operating at the diffraction limit.

andrea tasselli:
Refractors have NOT better contrast and sharpness than reflectors.

Telescopes map the light from a point source (star) into a diffraction pattern consisting of a central airy disk and concentric rings around, known as the airy pattern.

Adding obstructions into the optical path will always change the diffraction pattern somehow. Anyone who have used a Duncan mask or Bahtinov mask knows that. When it comes to the secondary mirror in a reflector telescope, this central obstruction will alter the diffraction pattern such that a larger portion of the light will contribute to the airy pattern and a smaller portion ending up in the central airy disk, compared to a refractor with no obstruction. In practice, this will be experienced as a less sharp star and an overall reduction in contrast for the reflector, as shown in the image below.

If I am not mistaken, there is a rule of thumb saying that you can subtract the diameter of the central obstruction in a reflector telescope from the diameter of the aperture to get the aperture needed to produce the same contrast without the obstruction. Thus, a 6 inch SCT with a 2 inch secondary mirror will provide similar contrast to a 4 inch refractor.

Joaquín,

there have been quite some discussions about quality issues with Baader narrowband filters recently, for example see here:

https://www.youtube.com/watch?v=VkSvpOLlD2Y&t=612s

https://www.cloudynights.com/topic/783245-baaders-new-cmos-optimized-filters-any-good/

I for myself also received a defective (scratched surface) Baader CMOS filter (Ha 6.5nm CMOS optimized, standard f-ratio) that was kindly replaced by the German seller. And the replacement Ha-filter does show halos on even 5mag stars....
Even worse than the defects apparent to the naked eye are some inconsistent bandpass windows in these Baader filters that a Polish guy with access to a quality spectrometer unveiled. This is what Cuiv's YT-video is all about - you don't know what you'll get and you cannot test it for yourself.

As for bandpass width - narrower should result in higher contrast but are more expensive and are more prone to quality issues.
The best quality filters - Astrodon and Chroma - are not that expensive for nothing..

CS
Chris

PS: I will probably sell my Baader CMOS Ha and OIII filters and use an Antlia ALP-T dual narrowband filter instead (I use a OSC-camera).
I will need to test the Antlia first in direct comparison to the Ha / OIII Baader filters - weather permitting soon, hopefully.

Hi
Just to make some things clearer: for the bandshift of a narrow bandwidth filter it does not matter if you use a refractor or a reflecting scope. The only parameter which is important is the F-number of your scope, almost …. This number determines the angle of the CONE of rays converging behind the lens or the focussing mirror.
The position (on a wavelength scale) of the transmission curve of a narrow bandwidth filter shifts with the angle of incidence because these filters rely on interference. The refractive index of the involved materials plays only a minor role.
So rays entering the scope in the outer edges of the lens or mirror pass the filter at the outer edges of the cone … and therefore under a very different angle as the central rays. In a Newton or SCT the most central stars are however blocked. The resultant diminished transmission of a filter at low F is a combination of all rays. And for a Newtonian or any reflector scope, the central rays hitting the filter perpendicularly are missing, because they are blocked by the secondary mirror. THAT makes the difference between a refractor and a mirror scope. SO, yes there is a difference, but this is small. So a slightly larger preshift would be better for a SCT than for a refractor.

And concerning the refractive index of the filter: the only relevant refractive index is the index of the material of the thin layer which is deposited on one side of the filter to build up the highly reflecting coatings to generate the interference. Its thickness is in the order of the wavelength of the transmitted light. The refractive index of the glass substrate is completely irrelevant for the bandwidth shift.

Lots of varying views and opinions on filters here. For those of you on a budget (like me), I've just started using the bog standard ZWO 7nm narrowband filters on my budget skywatcher 200PDS newtonian and am more than happy with the results (see

Quick and dirty Crescent
)

No obvious halos and starting to get the benefits of using filters rather than a one shot colour camera.

I know I'm just a beginner, but wanted to help other budget-conscious astrophotographers to not get sucked into expensive filters on day 1.

From a bortle 6 zone a 6.5 nm filter will swamp the RN in about 300s. For a 3.5nm filter you will need around 600s subs. In that case I would go with the 3.5nm for max contrast (S2 and O3, keep the 6.5 for Ha). If you were in a bortle 3 or 4 then no point for ultra narrow filters.

Jonathan Young:
From a bortle 6 zone a 6.5 nm filter will swamp the RN in about 300s. For a 3.5nm filter you will need around 600s subs. In that case I would go with the 3.5nm for max contrast (S2 and O3, keep the 6.5 for Ha). If you were in a bortle 3 or 4 then no point for ultra narrow filters.

Haha .. I live between a bortle 4 and bortle 5 sky, so I guess I am on the cusp. I'll stick with the wider filters for now.

Jonathan Young:
From a bortle 6 zone a 6.5 nm filter will swamp the RN in about 300s. For a 3.5nm filter you will need around 600s subs. In that case I would go with the 3.5nm for max contrast (S2 and O3, keep the 6.5 for Ha). If you were in a bortle 3 or 4 then no point for ultra narrow filters.

Do you recomend to me Ha 3.5 nm and OIII 6.5 and SII 6.5 nm? Is it correct?

Astroneck:
Jonathan Young:
From a bortle 6 zone a 6.5 nm filter will swamp the RN in about 300s. For a 3.5nm filter you will need around 600s subs. In that case I would go with the 3.5nm for max contrast (S2 and O3, keep the 6.5 for Ha). If you were in a bortle 3 or 4 then no point for ultra narrow filters.

Do you recomend to me Ha 3.5 nm and OIII 6.5 and SII 6.5 nm? Is it correct?

Hello Astroneck,
Reversed. Use the 6.5nm for Ha (and to keep the 658 N line) and then 3.5nm for O3 and 3.5 nm for S2 as those signals are generally much weaker than Ha and will benefit from the additional contrast while still allowing a reasonable exposure length.

"If I am not mistaken, there is a rule of thumb saying that you can subtract the diameter of the central obstruction in a reflector telescope from the diameter of the aperture to get the aperture needed to produce the same contrast without the obstruction. Thus, a 6 inch SCT with a 2 inch secondary mirror will provide similar contrast to a 4 inch refractor."

This doesn't sound right to me from a mathmatical viewpoint. The uncovered area of a 6 inch SCT with a 2 inch secondary mirror is (6*6 - 2*2) * PI = 32 square inch while the area of a 4 inch refractor is 4*4* PI = 16 square inch, so just half the area of the SCT. The 6 inch SCT should be close to a 5.6 inch refractor in performance.

The rule of the thumb, as it were, regards, with all its defects, only the difference between diameters not areas so the quote is right. What is wrong is that such rule is based on misconceptions about performance with extended objects relative the distribution of light in the Airy disk. Much more realistic and accurate would be comparing MTFs between the refractors and the reflectors. There we would see that the supposed advantage between obstructed and unobstructed apertures (let's for the sake of the argument ignore that refractors are NOT pure unobstructed apertures but rather less performing in polychromatic light that the nulling condition would otherwise suggests) is only valid within a certain range of frequencies.

At higher frequencies the obstructed aperture have actually better resolution than the unobstructed one which, however, offer a better contrast at the middle frequencies. At lower frequencies both perform about the same. This is still valid somewhat even now where the main detectors are silicon-based rather than Mark I eyeballs yet as we all know it doesn't take a lot to improve contrast in an image if the noise floor is low, which brings the advantage back to reflectors with their superior light gathering capabilities. Fact is that the best amateur images of the solar system have been taken, by and large, by lowly SCTs and Newts.