Are Quarks pre band shifted?

[Ags]

Similar to Baader high speed narrowband filters, are Quarks band shifted? Assuming the filter was tuned for light at the edge of a light cone, that would maximise the amount of on band light, and would also imply that Quarks have a maximum f ratio (f8 according to their specs), beyond which slower focal ratios would actually shift the light slightly off band?

[vlaiv]

I don't think it works like that at all.

All etalons should be capable of tuning as environmental factors can pull etalon of band (say atmospheric pressure or temperature expansion).

Quarks operate the best with F/7-F/8 scopes because you need F/30 for etalon to work the best and they have matching x4.2 (or x4.3 - not sure) telecentric lens already integrated into device.

You can use slower scope - and it won't affect operation of etalon - but you'll possibly have issues with exit pupil (like blackouts as our pupil tends to get really small in broad daylight).

Quarks have dial that you use to put it on band / tune exact CWL depending on your conditions.

Btw - you also need to tune exact CWL depending on features that you are observing as there is Doppler shift for things that are moving towards you.

[Ags]

Intuitively one thinks slower is better, but Daystar recommend a focal ratio of f4-f8. Why the maximum?

[vlaiv]

28 minutes ago, Ags said:

Why the maximum?

I think it is down to magnification and exit pupil.

Say you put quark in F/10 scope - like 4" F/10.

Your system will operate on F/43. Imagine now you put 25mm plossl that is often recommended for quark - you are instantly observing at x172 - which might be too much for Ha and seeing. Exit pupil will also be 0.58mm which will be probably uncomfortable for most people in daytime (even at night you'll see floaters and image will be too dim).

Etalon itself will work better on slower beam - but setup as a whole might be too uncomfortable to observe.

There is another possibility - maybe telecentric is optimized for F/4-F/8 entrance beam and does not work the best in slower beams (just guessing).

[Ags]

2 minutes ago, vlaiv said:

There is another possibility - maybe telecentric is optimized for F/4-F/8 entrance beam and does not work the best in slower beams (just guessing).

Maybe I wasn't clear, that is what I was suggesting.

[vlaiv]

1 minute ago, Ags said:

Maybe I wasn't clear, that is what I was suggesting.

I think that is really a long shot. Most optical elements work better in slow beam than in fast.

I've never heard of a barlow or telecentric that is meant only for fast scopes.

[Roy Challen]

1 hour ago, vlaiv said:

I think it is down to magnification and exit pupil.

Say you put quark in F/10 scope - like 4" F/10.

Your system will operate on F/43. Imagine now you put 25mm plossl that is often recommended for quark - you are instantly observing at x172 - which might be too much for Ha and seeing. Exit pupil will also be 0.58mm which will be probably uncomfortable for most people in daytime (even at night you'll see floaters and image will be too dim).

Etalon itself will work better on slower beam - but setup as a whole might be too uncomfortable to observe.

There is another possibility - maybe telecentric is optimized for F/4-F/8 entrance beam and does not work the best in slower beams (just guessing).

I agree with this, and think that 25mm eyepiece is not the best for visual with a quark. My 32mm baader plossl works much better, possibly even longer f/l would work better still, but image would start to be rather small depending on telescope focal length.

[Stu]

16 minutes ago, Roy Challen said:

I agree with this, and think that 25mm eyepiece is not the best for visual with a quark. My 32mm baader plossl works much better, possibly even longer f/l would work better still, but image would start to be rather small depending on telescope focal length.

Yep, I agree. I used to use a 32mm Plossl, sometimes a 40mm just to keep the mag down. I use 40mm in my PST mod for the same reason, even though they don’t offer any more fov.

[Floater]

All of the above fits with my experience using a Quark. 

Also, in my experience(s) with more than one of the little beasties, they can be a bit idiosyncratic. All Quarks are not the same.

Live with yours, work with it and learn with it … as with that old over-worked expression ‘your mileage may differ’. 🤗

[BGazing]

On 10/04/2022 at 14:45, Ags said:

Similar to Baader high speed narrowband filters, are Quarks band shifted? Assuming the filter was tuned for light at the edge of a light cone, that would maximise the amount of on band light, and would also imply that Quarks have a maximum f ratio (f8 according to their specs), beyond which slower focal ratios would actually shift the light slightly off band?

They do not shift it off band but the bandpass becomes wider.

There were some useful calculations on other forums, I cannot find them right now. Around f/30 you get what it advertised, tighter filters are more sensitive to ratio above f/30. Quark has a built-in 4.2x telecentric.

[BGazing]

Found it...

http://www.astrosurf.com/viladrich/astro/instrument/solar/FP.htm

[Ags]

2 hours ago, BGazing said:

Found it...

http://www.astrosurf.com/viladrich/astro/instrument/solar/FP.htm

But but but this is why I expect Quarks to have a shifted CWL. If you know the beam is not collimated, it makes no sense to design for a collimated beam. Better to pick an average focal ratio and design for that. From the article linked above:

[vlaiv]

6 minutes ago, Ags said:

But but but this is why I expect Quarks to have a shifted CWL. If you know the beam is not collimated, it makes no sense to design for a collimated beam. Better to pick an average focal ratio and design for that. From the article linked above:

I'm not sure you fully understand how filters work and why there is a shift.

There is no design for converging beam - only for collimated beam. Nothing changes in etalon itself because light hits it at a different angle.

Every etalon design suffers from this as sun is not point source. There is always some light that hits etalon at an angle - even with front mounted etalons that don't get any sort of instrument angles.

In order to get very narrow FWHM needed Solar Ha - two parallel surfaces are used as source of interference - like single layer of dielectric coating on regular night time Ha filter.

Unlike night time Ha filters where you have designed wavelength that is center and that you can adjust for faster light beam - wavelength of F-P etalon depends on distance between parallel surfaces. This is controlled by tilting of etalon (tilt tuning), or by pressure between plates (pressure tuned) or by temperature expanding mica crystal (mica is used as it can be split on atom boundary and creates perfectly flat surface - this is how quark is tuned).

CWL shift and FWHM widening happen just because some of the rays are not perpendicular to etalon.

You can never have all rays be perpendicular to etalon. Imagine front mounted etalon pointed at the center of sun. Light ray coming from the center of the Sun will be perfectly perpendicular to etalon - but light rays coming from the edge of the disk will be at an angle of 0.25 degrees (sun is about half a degree in diameter, so 0.25 degrees radius).

As soon as light wave is not perpendicular - it no longer presents correct wavelength to filter. Light is still correct wavelength but since it is at an angle - filter sees longer wavelength of light.

(above orange is actual wavelength, but as soon as we tilt wave with respect to vertical - wavelength "seen" by filter becomes - red one which is longer)

Now important thing happens. I'll write larger numbers (around 656nm) as it is simpler - but this actually works on fraction of angstrom when we talk about Solar Ha filter.

If light is perpendicular than any small change in wavelength results in lowering of transmission. Say filter transmits 90% for 656nm and only 70% for 655nm and 657nm

But if we have wavefront that we tilt a bit - this is what happens:

656nm will present to the filter as 657nm and will start transmitting at 70%

655nm will present to the filter as 656nm and will start transmitting at 90%

657nm will present to the filter as 658nm and will start transmitting at even lower - say only 40%

We have shift in CWL!

This happens to front mounted etalon. Although we did nothing to the etalon - as you move from center of the sun to the edge - CWL slowly changes as angle increases.

With converging beam something else happens as well. Not all light that is for same point at focal plane is coming the same angle - here even for central point angle is changing!

Now you can't simply say - transmission is 90%. It is for that central ray - but as soon as you observe other rays - they change wavelength that they present to etalon and their transmission falls.

Total transmission is sum of all these transmissions. Wider the beam (faster F/ratio) - more total transmission is lowered.

This is why there is dip in above diagram:

Not because transmission at particular wavelength changed - but because wider beam contains more rays that are at an angle (and wider angles) so average transmission goes down.

This is also the reason why there is broadening of curve (when you consider other light that is initially not of needed wavelength - but gets shifted to correct wavelength by being at an angle).

You can't design this away.

You can choose one of the three setups:

1. front mounted etalon - that only suffers from CWL shift

2. internal etalon with collimated beam (like Lunt 50 has for example) - it has completely parallel beam as it employs a pair of lenses - one diverging before etalon that will create collimated beam and then one converging that will then bring collimated beam to focus.

This configuration has advantage that etalon can be made smaller (easier to manufacture and less expensive), but has disadvantage that angles associated with sun disk get amplified and are no longer from -0.25 to +0.25 degrees but larger - which again impacts CWL shift more

3. Converging beam - this is what "eyepiece" filters do. They suffer from both CWL shift and FWHM broadening / peak transmission reduction, but can be made the smallest as they are closest to focal plane.

They are in essence the same as point 2 - if you apply pair of negative / positive lenses (before and after the filter) - like barlow and reducer carefully placed. Although, it is best to have lens specially designed for that purpose and optimized for 656nm.

In any case - closer the beam to collimated - less of that broadening will happen. CWL shift will still happen due to the fact that sun has dimension and different parts of the sun will send light at different angles.

 

[Ags]

9 minutes ago, vlaiv said:

I'm not sure you fully understand how filters work and why there is a shift.

Neither am I!