CaK PST - Filter sizes

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CaK PST - Filter sizes

Post by Merlin66 »

Can anyone confirm (Mark?) the sizes of the front trimming filter and the final tuning filter in the CaK PST?
I've just thought, as the PST fl is 400mm the final tuning filter (2.2A) may only be 5mm diamter?
If so, then making a "CaK module" for a longer fl ie 1200mm may be problematic.


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Re: CaK PST - Filter sizes

Post by Merlin66 »

Rusty,
Any chance of a link?
(I did a search on CN but didn't find it.)
OK Found it! -It was given in a recent thread by Vanter.
www.cloudynights.com/ubbthreads/showfla ... ost4769421

Bob used an off band Baader D filter and the Baader CaK filter - I can't see a detailed reference to the original PST CaK narrow band filter (or it's size)....


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Re: CaK PST - Filter sizes

Post by marktownley »

Hi Ken,
The front trim filter is 20mm in diameter and the final trim filter is indeed 5mm diameter. All other bits of filter in between are 20mm. However, this is where modders heaven comes into play - the final trim filter is easily replaced with a Baader K-Line or Omega 80a CaK filter, which are >25mm diameter - changing your BF5 into a BF30, which really opens things up in terms of usuable focal length. I image with mine at ~2m focal length with the 127mm frac with no vignetting issues.
HTH,
Mark :)


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Re: CaK PST - Filter sizes

Post by Merlin66 »

Mark,
Thanks for that..
Are you saying that there's more than two filter elements in the "CaK" stack?
I was thinking that the front element was a broadband blue filter (around 50A??) and then a smaller extreme narrow band (2.2A) at the rear...
I'm beginning to get the feeling that the final 2.2A is being achieved by the overlapping/ interference of muliple broad band filters?
Can you help??
(EDIT) I can see, I think what may be happening...The Baader "B" CCD interference has a tight cut-off at around 380nm(??- no detailed transmission curves), combined with the eBay 80A may give a "narrowband" outcome???


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Re: CaK PST - Filter sizes

Post by marktownley »

Most definitely multiple elements. When I was making my module I completely disassembled it optically - just to see what was going on inside. The very front (objective side) element has a yellow / blue reflection to it - I think this has IR cut, if memory serves theres another filter below this, then a final back (eyepeice end) element that is a gold mirrored finish. If you look through this with naked eye at a bit of blank sky the view is a greeny black in colour. The very final bf5 filter has the cak purply / violet colour to it when you look through at peice of blank sky with naked eye.

I originally tried imaging without BF5 in place and you get a soft image similar to that with Baader K-Line or Omega 80a CaK. However when Baader K-Line is put in place of the BF5 the view (via camera) is gobsmacking.

The CaK PST most definitely works by a series of overlapping interference filters which is why I always keep a very close eye on whats for sale on the Omega website, as one of these days are hoping something will come up that will be stackable with the 80a cak to produce something much narrower and very affordable...


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Re: CaK PST - Filter sizes

Post by Merlin66 »

OK, getting the picture...
That being the case is there any real value in investing in the CaK PST as a filter source???
Or just go with the Baader B and the Omega 80a combo????


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Re: CaK PST - Filter sizes

Post by marktownley »

OK, getting the picture...
That being the case is there any real value in investing in the CaK PST as a filter source???
Or just go with the Baader B and the Omega 80a combo????

Crucial thing is going to be whether the ovelap works to go from an passband in the tens of angstrom to one down to a couple of angstroms - this is the trial and error bit. Depends whether you are after a result similar to the K-Line or a CaK PST (but obviouslly with larger aperture).


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Re: CaK PST - Filter sizes

Post by Merlin66 »

Certainly feels worth trying....
(I can't get onto CN at the moment ;-( )
I'd feel uncomfortable upgrading to then only use the 5mm PST filter.
So, it would mean spending 300gbp on a s/h CaK PST just to get two or three of the filter element.....
Has anyone experience of only the Baader K line used after a Baader B CCD filter????


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Re: CaK PST - Filter sizes

Post by marktownley »

Has anyone experience of only the Baader K line used after a Baader B CCD filter????

Yes, just the same as using the K-Line alone...


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Re: CaK PST - Filter sizes

Post by Merlin66 »

OK Understood.
Let's see what other options come together....


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Re: CaK PST - Filter sizes

Post by marktownley »

The trick would be to get tilt tuner that is compatible with a scope that operates on a vernier scale - ie the ability to make really small changes in the angle of tilt... This is the holy grail that will really get the filter thingy going properly...


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Re: CaK PST - Filter sizes

Post by Merlin66 »

Yes.
But the filter(s) being tilted can only go towards the blue..
So, if the Omega is 80A centred pretty well on the CaK line then any "clipping" by say, a tilted Baader B would/could only apply to the region below the 3934A - this still leave the other 40A or so bandpass to the red side of the Omega....
A possible(???) combo could be a UV pass filter+ Omega 80a+Baader B......
(If I can find suitable filters I can use the spectroscope to do the tuning....)


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Re: CaK PST - Filter sizes

Post by marktownley »

Yes.
But the filter(s) being tilted can only go towards the blue..
So, if the Omega is 80A centred pretty well on the CaK line then any "clipping" by say, a tilted Baader B would/could only apply to the region below the 3934A - this still leave the other 40A or so bandpass to the red side of the Omega....
A possible(???) combo could be a UV pass filter+ Omega 80a+Baader B......
(If I can find suitable filters I can use the spectroscope to do the tuning....)

Yup ;)


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Re: CaK PST - Filter sizes

Post by Merlin66 »

Well....
No cheap options so far.
Horiba do a UV 390BP10 - XB63 for 146gbp
Edmund do a NT67-897 - 390nm, 10nm bp for $319!
They also do the cheaper NT65-676 - 394nm, 10nm bp for $169....
http://www.edmundoptics.com/products/di ... c#products
Onwards and Upwards


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Re: CaK PST - Filter sizes

Post by marktownley »

That's a great resource, thanks Joe! :)


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Re: CaK PST - Filter sizes

Post by Merlin66 »

A bit of lateral thinking....
If the PST etalon has a 10A finesse, then down in the UV the pass bandwidth would still be 0.8A and the wavelengths centre around 3933/3943/3953 etc.
If we had a suitable 6-8A blocking filter then we should be able to get a 0.8A narrowband CaK filter?????


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Re: CaK PST - Filter sizes

Post by marktownley »

In theory, yes... :)


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Re: CaK PST - Filter sizes

Post by marktownley »

Well....
No cheap options so far.
Horiba do a UV 390BP10 - XB63 for 146gbp
Edmund do a NT67-897 - 390nm, 10nm bp for $319!
They also do the cheaper NT65-676 - 394nm, 10nm bp for $169....
http://www.edmundoptics.com/products/di ... c#products
Onwards and Upwards

How about one of these...

http://www.ebay.co.uk/itm/Optical-Filte ... 48408e2c0d

[ebay]310320704525[/ebay]

With a bit of tilt, would put it's bandpass exactly where we want ;)


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