Quark Chromosphere vs Combo - Blocking Filter Size and Visual Field Stop

[tombardier]

Hi,

I have a Quark Chromosphere, and I love it, for observation primarily.  I use it in an TS Optics f/7 80ED scope (mostly). 

I gather the Combo model which eschews the 4.3 telecentric also has a much larger blocking filter, and my question is this.  Does the Combo model give you a larger sort of field stop visually than the Chromosphere/Prominence models?  If I thought the view would be less restricted, I'd be tempted to get a 4x Powermate and swap my Chromosphere for a Combo.

I don't really understand when the Daystar specifications talk about clear aperture being much larger than the blocking filter size!

Cheers!

[vlaiv]

Depends on how you use it.

What combo model offers is flexibility.

Etalon needs collimated beam for optimum performance.

Both of quarks don't use collimated beam (like front mounted etalons or dedicated solar scopes with special collimation elements) but both use "next best thing" - telecentric lens.

Or rather - regular quark has x4.3 telecentric lens integrated - but for combo - you need to provide your own way of making F/ratio of the telescope slower.

This can be done in one of two ways (or even combination). First is by use of telecentric lens (or barlow, but telecentric performs much better for this application due to light path/angles involved) or second - by use of aperture mask.

You can even combine the two to get results somewhere in between.

Why is this interesting? Well, because you can utilize range of focal lengths and hence magnifications of the sun disk in the focal plane.

If we have fixed size blocking filter, then we can change how much of it is visible in FOV with respect to solar disk - by changing focal length.

With your 80mm F/7 scope - you have several options. That scope is 560mm FL scope, so you can choose to use it as is with say 28mm of aperture mask to give you F/20 system,

With 560mm FL scope, full solar disk will be about 4.9mm in focal plane. That is much smaller than blocking filter and you'll be able to view full solar disk. Only drawback is that you'll be limited to about x50 as far as resolved detail (small aperture).

You can then choose to use x2 telecentric lens to increase your fl to 1120mm. If you want to keep F/20 beam - you'll be using about 56mm of aperture. This will also increase full solar disk to ~9.8mm in diameter. With 12mm blocking filter - there will be much less room around the disk (with larger diameter in combo there will still be plenty of room ...).

Then there is option to use x3 telecentric lens and any sort of aperture mask if one desires or leave the system at F/21. However, solar disk will be about 15mm in this case.

So yes, you can set it up to have more "room"  around the disk - even if combo has the same blocking filter of 12mm - but as far as I gathered it has about 21mm (do keep in mind that blocking filter is not at focal plane and its edges won't be in focus but rather it will create some sort of vignetting, so real usable field will depend on distance of blocking filter to focal plane and will be less than size/aperture of blocking filter).

 

 

[tombardier]

Thanks Vlaiv, this is good information! I can 3D print aperture masks quite easily too, and I already have done with this scope to experiment with different focal ratios.   I use a Denkmeier Binotron with the Powerswitch, and so I can get a .66x reduction from that which will give me full disk views with my normal Quark, but I can't say I can comfortably see the full disk.  I was thinking about buying a ~60mm f/7 scope to scratch that itch.

The main reason for asking is the limited "field stop" generally, and I do use the term loosely, because I'm sure it's not technically correct, but which, as you say appears as more of a vignetting.  I would love to know if the larger BF could improve the size of what I guess is the AFOV?

Cheers!

[vlaiv]

13 minutes ago, tombardier said:

I would love to know if the larger BF could improve the size of what I guess is the AFOV?

Depends on two factors.

First is size of blocking filter, and second is obviously size of the field stop of eyepiece.

The way you are now using your system, and that is 560mm with x4.3 amplification - you get 2408mm of FL.  That makes solar disk 21mm in size. that is quite big.

You need eyepiece with at least 23-24mm field stop - so that would mean plossl of 25 to 32mm of focal length to view the whole disk. That is not limiting factor.

What is limiting factor is size of blocking filter.

It might be counter intuitive how 12mm blocking filter can produce full solar disk - but remember, we are in telecentric beam and blocking filter is away from focal plane.

This is what happens:

right is projected solar disk and left is blocking filter (image is not to scale, but rather a diagram). Central beam will pass as is. Since it is operating at F/30 - if blocking filter is 12mm - it can be 30 x 12mm = 360mm away from focal plane and central beam will still pass at 100% (that is 36cm - I'm sure that blocking filter is much much closer to focal plane - maybe 7-8cm away).

However - edge beams won't pass fully and will be clipped - so image towards the edge of the FOV will darken.

We don't feel things linearly - so even if the light is cut down to only 50% of original - we won't see it as being half the brightness (in fact we don't even notice first ~7% of the drop of intensity).

Note light rays in above diagram - the are not how usually light rays are drawn, but are in fact consequence of telecentric lens.

It turns diverging rays into parallel ones, unlike barlow that spreads them. This of course means that for same image size - barlow would vignette less, but etalon likes perpendicular rays - that is why telecentric lens is used (and is better for this application).

Back to the question - as is with x4.3 telecentric lens - even 21mm blocking filter would vignette somewhat - but it would be much less noticeable. Only way to really get vignette free image is to reduce solar disk size, and only way to do that is to reduce focal length.

 

[tombardier]

Thanks Vlaiv.  This makes a lot of sense!  I wish I could easily just try it out, and see!  I have an F/11 102mm scope as well, which works fine in the Quark actually, but it'd be nice to play with the focal ratio on that to get it dialled in!  I might add the 60mm telescope to my repertoire, and see how I get on with the full disk views before thinking about swapping for a Combo.

Thanks again.

[Highburymark]

If you’ve got a good Quark, and it sounds like you do, there are no guarantees that a new one would be as good - unless you can try it before buying. Consistency does seem to have improved, but each filter is different - and you might get one with a wider bandpass, resolving fewer surface details. Apols if you are already aware of this, but there’s always a risk buying solar Ha etalons, and if you have a really good Quark now, it could be an expensive disappointment.

[tombardier]

@Highburymark that's definitely a concern I have too!  I agonised so much before buying the Quark I have.  I know they're cheap in the grand scheme of things, but I didn't want to shell out £1000 on a lemon!  The Quark anxiety is real!