CaK filter stack - solutions for narrowing the bandpass

[Catalin Fus]

I've bought a CaK filter from Bjomejag (ebay) and I was thinking of a solution for narrowing further the band. The one I bought has BF10 which should mean around 100Angstroms, right?.

My question to you is, to narrow further the passband , should I take this filter

http://tinyurl.com/cg7h585

or this one, just as suggested here

http://tinyurl.com/bsdd2bo

and apply a tilt?

I have attached a XLSX sheet with a formula that gives you the central wavelength of a filter, after a tilt...can someone please correct it, if need it. The refraction index and other numbers, are ones given by Marcello Lugli for the Cromixsun - H'alpha sets, like one from ebay. Can be modified for any filter as long as you know central wavelength and refractive index of the material at that wavelength.

refractive index for a specific wavelength, founded here :

http://refractiveindex.info/?group=SCHO ... rial=N-BK7

choose the glass material of the filter, set the wavelength and you'll find out the number.

Any more ideas are wholeheartedly welcomed!!

Regards
Cata

Attached files FilterTilt.zip (7.1 KB)

[Catalin Fus]

Rusty, thanks for the ideas!

I can handle the mechanics for the tilt and I want some confirmation that what I have in mind is the right path, of course, if someone has already been there :-)!!
You know, is good to learn from other's mistakes :whistle: !!

What filters did you use in your stack and what's the final bandwidth?

10nm I will have standard on the CaK , I just have to make it narrower...

Regards
Cata

[Catalin Fus]

Rusty, there are two ways that one can use for a tilting option.

First one is without any springs, just two concentric rings with a space between them that allows rotation in one axis.
The inner one is holding the filter and has two threaded holes, 180 degr apart. The outer one also has two holes at 180 degr that allow a rotating screw and the pivot screw to be fixed into the inner ring. Mechanically this assembly can be done in such a way that smooth movement can be achieved and modules like these can be stackable.

The second option is something like the Coronado etalon design. There are two concentric rings linked using screws (for centering and movement) and springs. By rotating a thumb screw, you can have the tilt. This option is nice but in case of small filters, I think is bulky. Also I have to think a way of stacking something like this.

When I'll finish my designs, I'll gladly post them here.

Regards,
Cata

[Catalin Fus]

RustyV,

the giro concept is good if you think carefully the thread depth in each piece and also you make the screws 'snug - fit' into the outer ring, especially the pivot. The one that contains the turning screw can be made to move smooth but firm so it will NOT lose position.

Regarding the etalon one, it contains 2 springs and also two centering screws that don't allow tilt above an angle. It is a good design but bulky in case of filters.

My case , with 25mm filters in 1.25'' casings might be different from the 2'' one so I can not simply comment about a bigger version.

Regards
Cata

[colinsk]

How about if you make a ring and part it off axis. Then you have two rings that can either be perfectly parallel or off axis depending on the rotation.

Like this sheet metal part:

http://www.imperialgroup.ca/product/SF_ ... _small.jpg

[Catalin Fus]

Colin,

I have thought about such a design but I prefer a 'fully controllable' tilt option, for both filters. It is important in my opinion to think very well the tolerances between pieces so the movement is smooth but very firm , just like I said in previous post. Also rotation between filters, in any position of tilt, might be important.....I'm not 100% sure but I must have all options available.

I will do a giro piece in the next weeks and I'll try it to see where it needs improvement.

Regards
Cata

[sullij1]

Like this? Mine is done. I will post assembly later. Tuning experiments now. Not quite tuned pic accompanies post. Pic is Groski filters. Not quite tuned but working on it.


IMG_7550 by Sullij1, on Flickr


IMG_7549 by Sullij1, on Flickr


IMG_7551 by Sullij1, on Flickr


combo by Sullij1, on Flickr

Seems to be working, bad reflections on the test rig but I am working on it.

Setpins allow the Ha stack to be poped out and the Calks to be inserted. Also allows for rotation of the filter as recommended by Bob Johnson of Omega.

[Catalin Fus]

Hey, thanks!! I would like to do something very alike your setup!! How many filters do you have in stack?

For reflections, I think you should check that each filter in the stack has the reflective side pointed to the objective. If the reflective faces are facing each other, you will have unwanted reflections.

When you have your first light, visual or imaging, please let me know your opinion.

Do you have the graphs of the filters?

Thanks!
Cata

[sullij1]

That is the groski set of 2 filters plus the double pass filter, 3 filters stacked.

[Catalin Fus]

You should get 0.6A out of those 3.
Good luck in testing and keep us posted on results!!

Regards
Cata

[sullij1]

Thanks for the advice on the stops. I have one forward pre nose piece. Maybe a stop in between each cell is what’s needed. I also was using 2 piece un-blacked white poster board, sandwiching the ERF together for the test. I thought the reflection might be due to the filters reflecting off the white poster board holding the ERF. I used the 5" F5 for this run and had it stopped down to ~19mm to get this beam telecentric.

Can the filters be backwards? These come unmounted and no front or back marked. Both sides seem to have a mirror coating. How do I tell the difference?

[sullij1]

BTW, to keep it on the CalK topic, Other than the mounted 3nm CalK. Is there a Front and back to these as well and how do I tell?

[Catalin Fus]

All the filters that I have, TiO, HeII and CaK are used with the mirror-like face towards the objective. The problem is that, if I put them AFTER the barlow, directly on the nose of the camera, they will give reflection due to the mirror like surface reflecting in the lens of the barlow.

I think you should try both ways, most reflective face towards objective and towards ccd/eyepiece....and no two high reflective faces opposing each other in the stack.

It's a trial and error I guess and in my case, I can't give more advices based on my experience than the above ones.

Regards
Cata

[marktownley]

All the filters that I have, TiO, HeII and CaK are used with the mirror-like face towards the objective. The problem is that, if I put them AFTER the barlow, directly on the nose of the camera, they will give reflection due to the mirror like surface reflecting in the lens of the barlow.

I think you should try both ways, most reflective face towards objective and towards ccd/eyepiece....and no two high reflective faces opposing each other in the stack.

It's a trial and error I guess and in my case, I can't give more advices based on my experience than the above ones.

Regards
Cata

You need to put them on the barlow nosepeice not the camera nosepeice. Reflective side must be towards the objective....

[Catalin Fus]

Yes Yes Mark!! I do it only in this way .
I was trying to explain in my poor English that is a 'no no' with optical surfaces close to the reflective part of the filter as this might give reflections.

Thank you for the advice anyway!! ..appreciate it : !!

Cata

[sullij1]

Thanks to all, Good to know about the Barlow. Have not gone that route yet. Was hoping to get away from trial and error but sometimes, "you do what ya gotta do". I appreciate the information I am able to gleen from here. Good community, second to none!

[colinsk]

Don't forget that a 1/4 wave plate and a polarizer will block a back reflection if needed.

[sullij1]

Normaly I would not forget but it's difficult to forget somthing I didn't know. :blink:

No disrespect intended Colin but you may assume some may know as much as you. Not so. I am just getting started on this adventure, thus I will need some tutoring. :unsure:

Can you give an example of 1/4 wave plate? I understand polarizer.

Where in the optic train would both be? Forward or rear of the main
Ha stack( and prior to the CCD.)
? Thanks

[colinsk]

Sorry, this might help:

http://www.designerinlight.com/solar/Et ... clePt2.pdf




The circular polarizers are very important for contrast; however circular polarizers cut a
fair amount of the on band light. When the randomly polarized light from the sun passes
through the circular polarizer it is reduced in intensity by approximately 50%. It is also
converted to circular polarization. Circular polarization is better thought of as a
corkscrew and can either be right handed or left handed.
For example if the first pass of the circular polarizer makes the corkscrew right handed,
when light is reflected off the reflective surface it changes handedness from right-hand to
left-hand. When the left-hand polarization returns to the circular polarizer it is converted
to linear polarization but at 90 degrees to the polarizer and is absorbed. This is done to
eliminate the ghost reflections from the blocking filters and etalon mirrors without having
to tilt the stack.

[colinsk]

In practice on a laser bench I can reduce 95% of a back reflection. Past that we need Faraday Rotator technology and it is more than an etalon in cost. From my holography website:

http://holowiki.org/index.php?title=Faraday_Rotator

[sullij1]

AHHHH, Ok. thank you for the explaination. The diagram gives food for thought. Perhaps somthing can be rigged up. JEEZ lazer guys! just kidding and thanks.

[Catalin Fus]

So, here it is the image taken today with an 394nmBP10 (100nm).I feel there is an improvement in resolution over the TiO filter, using the same setup (127mm refractor + TeleXtender 3x).
I'm saying that because the stack now was of 200frames and the sky conditions were very bad with cirrus clouds, some of them rolling over the sun when imaging. In perfect clear conditions and with another filter, to narrow further the band, the results can be even better...
Also, I haven't found best option regarding the variable polarizer position in order to cut exposure time, gamma and gain to reduce the noise.

Regards,
Cata

[Catalin Fus]

RustyV, how could I have done that?
It was used after a Herschel wedge + variable ND and before the TeleXtender. Only the Meade TeleXtender can cause this UV cut...at 394nm, otherwise...dunno!

I haven't tried it at the eyepiece...but photo, this is it

Regards
Cata

[Catalin Fus]

..and if it makes any difference, here it is the picture obtained without the TeleXtender from Meade, directly with camera in focus.



regards
Cata

[sullij1]

I like the ND idea. That may be what I need on my 394NB4nm. I cannot get the light level low enough to image. Even with the Blue ERF. I have Gamma and exposure nearly off. setting for exposure is like 1 and gamma at 2 and still can't get a useful image. Perhaps the Variable ND wil get the light level low enough to start getting some gain and gamma adjustment.

Apollo, have you had any luck with yours?

The wedge seems to be helping in Cata's case (for that NB10)filter. Black and white wider field looks good Cata.
Apollo you also have the cut filters in your NB10 right? That may be the difference.

[Catalin Fus]

I have no choice but to remove the usually permanent ND3.0 at the base of the wedge and replace it with a variable ND (1-40%, 2'' from Orion USA) in order to increase the amount of light that the 394nmBP10 filter receives and go low with gamma and gain (for noise reasons).

[Catalin Fus]

RustyV, I guess you are right somehow !!

First of all, I remembered today that while focusing, I had two steps of focus....which I found it weird but I didn't gave to much attention. It might be the case that I focused on the IR spectrum leak...which should be significant if the result is what it is.
On the ebay it says that this filter has OD6 up to 1500nm+....so it makes no sense, but I have to suppose 'something' exists.

In order to resolve this issue, all that I can do is to use a blue ccd filter instead of the variable ND and see if this comes with an improvement.

Thanks for the idea.
Cata

[Catalin Fus]

Small question.

Does anybody uses this filter as ERF :






I find it very appropriate and far less expensive than a CCD dedicated one. As all CCD/CMOS chips have less than 40% QE under 400nm, the 50% transmission is more than enough and also it can act as a replacement for the ND3.0 in a Herschel prism assembly.

Any opinions?
Cata

[marktownley]

I don't use one but are certain it would work...