UHC or DUO? Which IR Cut?

[Ags]

I just sold my last two camera lenses so I am in a position to complete my fun-sized setup:

CF tripod
AZ-GTi
ZS66
F6.3 Reducer
ASI485MC

I desperately need an IR cut filter, and I am thinking about getting the 1.25" Astronomik L-2, which will go close to the sensor via a ZWO T2-1.25" adapter. Is it worth 20 euros over the budget IR cut filters?

To enhance contrast on nebulae, I can choose between a Baader UHC-S or an Optolong L-Extreme.

In favor of the Baader, I expect it is easier to use as it lets more broadband light through - so easier to focus and frame. And star colors will theoretically be better (not a big fan of narrowband stars). Sky fog would only be reduced by a factor of 3 however.

The L-Extreme would reduce sky fog by a factor of more than 10, but I'm just concerned it would be frustrating to use. Has anyone got experience of both? I could also split the difference and get the ZWO Duo filter (like the Bader in blue and green, like the Optolong in red).

For IR I think I will just stick with my trusty Wratten #29.

Edited December 28, 2021 by Ags

[Ags]

I think I will go for the the ZWO Duo filter after all.

[vlaiv]

5 minutes ago, Ags said:

I think I will go for the the ZWO Duo filter after all.

Have you seen this:

8° is about F/7

( arctan(1/7) = 8.13° )

Anything faster than that will put filter in situation where some of incident light is out of band and will be rejected.

[Ags]

Good catch. But surely this would apply to any interference filter, particularly a narrower one like the Optolong L-Extreme? Why is the ZWO more affected?

[vlaiv]

Actually - scratch above - F/7 will actually have angle of 4° on each side, so that is ok.

It is ~F/3.5 that will produce 8° angles.

[vlaiv]

1 minute ago, Ags said:

Good catch. But surely this would apply to any interference filter, particularly a narrower one like the Optolong L-Extreme? Why is the ZWO more affected?

It will, that is why there are dedicated NB filters for fast optics - or one can use wider band like 35nm Ha filter for faster lens.

Actual effect depends on type and number of interference layers.

Interference layers have certain thickness needed for them to work properly (create reflected wave that interferes with forward wave and effectively cancels it). Wavefront coming in at an angle will see "thicker" dielectric layer - simply because of geometry.

[Ags]

Here is a graph for a broadband versus narrowband filter:

https://www.cloudynights.com/topic/737925-l-extreme-or-l-enhance-for-f4/

Seeing as I am imaging at around f4.5, maybe I will go back to the Baader UHC-S, but it leaks IR...

[Ags]

I think another way of looking at it is: given the above graph, the L-Extreme is passing half the incident Ha at f4 versus f8, but broadband light pollution is not attenuated at all (light that would have been blocked bandshifts into view). So the narrow passband of the L-Extreme is effectively doubled in width, in addition to effective aperture being decreased by a factor of 0.7.

[vlaiv]

1 minute ago, Ags said:

Here is a graph for a broadband versus narrowband filter:

Also note that above graph is for optical axis (meaning 8° is about F/3.5)

Edge of the field will depend on your TFOV.

Say you image 2° wide field (in corners) - then you no longer have 8° and 8° on either side of normal - it is more like 6° and 10° because primary ray is tilted as well.

Here is effect exaggerated - but it shows how different rays end up being at a different angles to normal

[Ags]

So, I suppose my question is: can anyone recommend me the best of the best UHC?

[vlaiv]

1 minute ago, Ags said:

I think another way of looking at it is: given the above graph, the L-Extreme is passing half the incident Ha at f4 versus f8, but broadband light pollution is not attenuated at all (light that would have been blocked bandshifts into view). So the narrow passband of the L-Extreme is effectively doubled in width, in addition to effective aperture being decreased by a factor of 0.7.

Actually no.

Only some of "rays" are at far ends of F/4 - central ray is at F/infinity.

So it is really an integral over "cone" with each line in the converging cone having certain angle - being at particular F/ratio - and hence will have certain transmission.

At F/4 - very few angles are really at F/4 - only  "border" ones.

So total transmission is integral of these partial transmissions over rays at angles.

And each point on sensor will have different integral because angles will change as we move away from principal ray (center of sensor)

[vlaiv]

4 minutes ago, Ags said:

So, I suppose my question is: can anyone recommend me the best of the best UHC?

Looking at it - that ZWO one that you first linked looks like very sensible one to get - it has similar response to L-eNhance and is reasonably priced. It is really UHC type of filter - with narrower Ha part - which is good for imaging but probably does not have much impact for visual.

[Ags]

Yes I see that, but remember by area that the highly angled edges are actually most of the aperture.  And when you change from F8 to F4 you are only adding the big angles... The light pollution still gets in from those more angled rays (there is light pollution of the right frequency) but your signal doesn't get through. So by using faster optics with a narrow fliter, you effectively only add more light pollution and hardly increase the signal (in extreme cases).

Edited December 29, 2021 by Ags

[vlaiv]

3 minutes ago, Ags said:

Yes I see that, but remember by area that the highly angled edges are actually most of the aperture.  And when you cange from F8 to F4 you are only adding the big angles...

Yes, but if you go with ZWO - you'll be ok since you are at F/4.5 at most and it works ok down to F/3.5

According to above graph - it is still 90% effective at F/3.5 (8 degrees) and you won't reach that.

[vlaiv]

This gave me interesting idea.

Quite a lot of people image with Samyang 135 F/2 and stop it down to F/2.8 - and for that they need "fast" Ha filters. At F/2.8 their aperture is actually - 48mm, so instead of stopping it internally (and adding diffraction spikes from blades) - they should be using 2" front mounted Ha filter instead - before light starts to converge - that will make their filter work properly even if it is not "fast" Ha filter.

[Ags]

Yes, I have experimented with front mounting filters. One major disadvantage is the filters are exposed to the elements. But I've sold all my lenses now, so no more such experiments for me

[Ags]

OK, so I have decided on the ZWO Duo filter for narrowband work. I am now thinking to get the Astroomik L-3 filter' as it cuts more red and blue than the L-2. Not sure however if that is overkill for a ZS66+f6.3 reducer combo.