Ca-K Lunt B1200 - possible improvement

[Catalin Fus]

Hi,

I've read a couple of times the threads regarding Ca-K scopes. I've seen also the link provided by Mark through his website where Andy Dodson modification of PST Ca-K module is easy to understand.

The simple question that I have is :
- same rules of 'improvement' apply to Ca-K module from Lunt?

Detailed :

- can I replace the front filter seen in first photo with a Baader Blue CCD/HEUIBAR filter? (any other combo that rejects IR and passes only blue spectrum)
- can I replace the back filter (12mm OD), with a Baader U filter/Ca-K filter or Omega Ca-K filter? that will allow me unvigneted field in bigger chip cameras

I already own some of these filters and it would be great to know if somebody did something alike before I screw things up. Maybe Lunt modules are not so suitable for modding....

[swisswalter]

Hi Catalin

Lunt moduls are prefect for modding :whistle:

The secret of the CaK wedge is in the last filter stack, that little tiny square filter. See our posts in the mod section. No way to get a decent CaK pic with only a K-line filter, the Baader U-filter is not really helping to get to the goal

[Catalin Fus]

Thanks for the input Walter.

The lower part of the module - in the black barrel from first picture - can be removed from imaging train?
(It has only an IR Rejection role?)

[swisswalter]

Hi Catalin

do you mean the mirror the red arrow points to ?

[marktownley]

Hi Catalin.

The important filter in the CaK wedge is the little 12mm filter in the eyepiece end of the module. This could be used to great effect with your Baader U-filter. I think you will need an effective ERF though with this; my first choice would be a Beloptik tri-band, or secondly a solar wedge without any ND filtration in it.

Mark

[Catalin Fus]

Hi,

@ Walter : yes, that mirror/filter that sits at the end of the black body
@ Mark : I have a Herschel wedge . I'll give it a try with the Baader U-filter + IR reject like the UIBAR or another stack Baader U-Filter + Dark blue filter - 435nm bandpass, that I have lying around. Stack of Baader U + 435nm bandpass might isolate pretty good the 350 - 400nm region.
Have tried to buy a tri-band ERF, with no success. Hopefully Oliver will make another batch alike. Would have been good also for the next project -> h-alpha one.

Thanks a lot for your answers!!

[swisswalter]

Hi Catalin

that front filter is a hot mirror. In fact, in my lunt CaK wedge are sitting two of them: I would not recommend to take them out of the light train without a very good replacement. I did it and have burned a K-line filter and a Beloptik UV/IR cut :whistle:

We did a lot of fiddeling around with the Baader U-filter and the 435 nm bandpass with no success. Plots can be found in this section

[Catalin Fus]

Thanks a lot for clarifying these aspects!!

Do not get me wrong, I do understand why that mirror/filter is there and that I have to replace it if I want to take it off !!
My 'quest' is for improving overall transmission but I guess, this is impossible as the small square filter transmits only some percents of incoming light, so any improvement before it, might come with no practical result.

[swisswalter]

Hi Catalin

No. Your point is very good. We lack of light doing HIRES with the standard Equipment. That's the reason we are so happy about the triband filter. Maybe you are able to design and make for very low $$$ a triband filter of 160 mm diameter :whistle:

[Catalin Fus]

Hi Walter,

well, this part eludes me....if the 'square filter' gives only 8% or so transmission, what is the point of improving everything before it?
I've read your thread 'A surprise....' and when you say 'Lunt Ca-K filter stack' I guess you are telling about the square filter only and not that whole assembly gives 8% transmission.

I've lived with the impression that the whole assembly can be improved in regards to transmission but if what I have wrote above is correct it will be nice to let me know more clearly where I'm doing the wrong 'math'.

Thanks a lot!!

[marktownley]

Hi Catalin, some, but not all of the Lunt wedges suffer from reflections that cause a hot spotting when imaging, a result I believe of the pair of ERF filters that sit in the nosepiece. Doing away with these and replacing with alternatives can be a better option. Looking at curves you posted the Baader U would work but you will find the other filter will leak light as the 12mmm Lunt filter starts to pass wavelengths above ~ 500nm. The Baader U also has a higher transmission that the K-line, so more light and shorter exposure for imaging.

[swisswalter]

Hi Catalin

you are not doing a wrong math, not at all. In order to get only light below 400 nm to the lunt filter you have to make sure you cut of everything else above and below the 393-393,5 nm. that is the funktion of the hot mirrors.


Yes you are right. When I spoke of the LUNT stack, I meant that little square filterstack near the eyepiece.

Now there comes your part. Get rid off all the energy above 394 nm and below 393 nm. the rest is taken care of from the lunt stack. although we would like to have a replacement of the LUNT CaK filterstack and the PST CaK filter ;-)

[Catalin Fus]

Hi,

@ mark - yes, makes sense but I wasn't intending of using them separately, but in a stack, to cut in half the bandwidth of the U-filter (don't know if it'll help or not in any way, but I'll try). The coating of the U-filter shows that it blocks IR. My only 'fear' is about energy concentration but I'll figure that out somehow

@ walter - that is food for thought

Anyway, thanks guys for all your support! I'll gladly wait for first chance to do some imaging and I'll post here some tests. Hopefully I won't do something stupid!!