CaK module ERF

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CaK module ERF

Post by Bob Yoesle » Tue Jun 09, 2015 9:57 pm

Hi DIY fans,

I'm currently doing CaK imaging (only) with the DS PST filter kit, which includes a Baader Blue CCD filter up-front as an IR ERF of sorts:
Baader Blue CCD.gif
Baader Blue CCD.gif (35.89 KiB) Viewed 2789 times
A Baader CaK filter comes after this closer to focus, and acts a blocking filter ahead of the PST CaK filters, which are closer still:
CaK DS Module New SM.jpg
CaK DS Module New SM.jpg (331.04 KiB) Viewed 2789 times
However, with a recent investigation I discovered that despite having the Blue CCD filter ahead of everything, a considerable amount of heat can be felt near the focus. This likely comes form the 400-500 nm transmission energy, which is falling on the Baader CaK filter, and I am concerned that it might eventually degrade or otherwise harm this filter. Baader goes out of their way to state the CaK needs to be used with Astrosolar 3.8 or a Herschel wedge, and I'm even more concerned as I plan to start WL and CaK imaging with a 130 f8 EDT vs. the current ED 100 f9 refractor.

At this point I would rather not use a wedge for CaK, and I don't know if the 3.8 material would allow much of the CaK through - I suspect not.

What I am considering then is some sort of short-pass or band-pass filter ~ 50 mm in diameter just after the Baader Blue CCD filter -- e.g. up a ways in the OTA so as to not get too concentrated an amount of energy. There are a couple of candidates.

First, and complimenting the Baader Blue CCD transmission profile would be this hard-coated Edmund 400 nm short pass filter:
Edmund short pass.gif
Edmund short pass.gif (13.46 KiB) Viewed 2789 times
It is listed as having an optical flatness of 1/4 wave, an OD of 4, and a surface quality (scratch-dig) of 40-20.

http://www.edmundoptics.com/optics/opti ... ers/84715/

Additionally, there is this hard coated band-pass filter centered at 390 nm, 1/4 wave flatness, but with sufficient bandpass (40 nm) to cover 393 nm, OD 6, but only a 60-40 surface quality, for about a 3rd more in cost (around the price of a Baader wedge):
Edmund 390nm BP OD6.jpg
Edmund 390nm BP OD6.jpg (82.16 KiB) Viewed 2789 times
As the band-pass is stated to be 40 nm, so the cut off would be 410 nm, vs. 400 nm for the short-pass.

http://www.edmundoptics.com/optics/opti ... ers/86359/

Either seems like it would it would do the job, but I'm curious to see what the consensus might be r/t OD 4 vs. OD 6, short-pass vs. band-pass, 400 nm vs. 410 nm cut-off, scratch-dig, etc.

And if anyone knows of any other suitable filter(s) to consider, please let me know.

Thanks for your help...
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Re: CaK module ERF

Post by p_zetner » Tue Jun 09, 2015 10:38 pm

Hi Bob.

I ordered a uv filter from Bob Johnson at Omega some time ago. I've only used it, so far, for my SHG but intend to use it, at some point with my Lunt CaK module. The specs are: CWL 350nm +-5 nm, HBW 100 nm, Transmission Peak >85%, Attenuation OD 5 Avg 420 to 800nm. (No spec on transmitted wavefront.)
http://www.ebay.ca/itm/Optical-Filter-I ... 486028f772
The version I ordered was 50mm dia but I can't find this one in his store at the moment.

There is another eBay vendor called "uviroptics". They have some interesting filters.
http://www.ebay.ca/sch/uviroptics/m.htm ... 7675.l2562

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Re: CaK module ERF

Post by thesmiths » Wed Jun 10, 2015 1:26 am

Bob, I'm curious why you don't want to use a wedge? I'm using a Lunt wedge (1.25 inch), Baader K-line, followed by the Lunt CaK stack (from my B1200). It works really well (and I hope to eventually add a PST CaK filter #1 to double stack it). My motivation was also to go up in aperture to 150mm, as well as a potential increase in contrast. The exposure time is superior to the Lunt B1200 alone. This approach was of course suggested originally by Mark and I can attest that it is a very good solution. What you're attempting to do is, I think, essentially recreating the long front nose piece on the Lunt CaK module. The issue with that is that it is rated only up to 100m, which was the motivation to use the wedge in the first place.

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Re: CaK module ERF

Post by swisswalter » Wed Jun 10, 2015 4:37 am

Hi Bob

for me works an UV/IR cut on KG 3 upfront the baader B filter
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Re: CaK module ERF

Post by Bob Yoesle » Wed Jun 10, 2015 11:43 am

Hi guys, thanks for helping.

Peter: I'm not convinced the Omega's have the needed optical qualities I'm looking for, as I have from previous experience encountered considerable variability in their performance. The UV/IR optics filters look to be photography filters, and likely would have similar issues. I even have checked at Andover, and their short-pass and band-pass filters, which are much much more expensive, typically are rated at 3 - 5 waves, which is not acceptable.

thesmiths: with using my Herschel wedge for continuum, I have found it a pain due to the image reversal I have to go through, and much prefer straight-through imaging. But you may be right, as this might be the overall best approach. I have purchased a couple of high-quality Thor 40 mm pentaprisms, and anticipate converting one of them to a solar pentaprism, which might be a better solution for continuum -- and CaK:
Pentaprism 1.jpg
Pentaprism 1.jpg (14.36 KiB) Viewed 2750 times
But I would also like to explore better filtering as well...

Walter: I contacted Oliver about the KG3 UV/IR filter, since I'm already using one as an ITF replacement. He advised he thought the excess heat was from the focused 400 - 500 nm passing through the Baader B, and his KG3 would not do much better. But perhaps I will pull it out of my BF30 and give it a try and see if it helps...

Thanks again guys for your input.
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Re: CaK module ERF

Post by thesmiths » Wed Jun 10, 2015 10:18 pm

Bob, if the image reversal bothers you, I believe Firecapture has a toggle to invert the x or y directions!

In my setup, I built a pair of inserts that drop into the Lunt wedge. One is a stack of ND filter, Baader contiuum and IR cut. The other is Baader K-line, Lunt CaK stack and polarisation filter. I can quickly swap from one to the other while keeping the wedge in place. I'll take some photos and do a separate post in the mod section. I can use the same pair of inserts in the solar Newtonian I built (i.e. unsilvered primary mirror) since this essentially acts as a Herschel wedge.

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Re: CaK module ERF

Post by swisswalter » Thu Jun 11, 2015 4:33 am

Hi Bob

the penta prisma works great but it does need a lot of inward travel
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Re: CaK module ERF

Post by marktownley » Thu Jun 11, 2015 5:51 am

Hi Bob,

My DS CaK stack module is as follows from front element to rear element: AR coated KG3 from Thorlabs, Baader K-line, CaK Pst #1, Edmonds Optics 387/11nm OD6 bandpass filter, CaK PST #1. This is all constructed as a 'module' like yours. If I am running with the 40mm scope this is used on it's own. With 60mm and 80mm apertures I am using a Beloptik Tri-band ERF on the full aperture. When I am using at 100mm and above I have a 'sleeve' of extension tubes that is fronted by the Baader Blue that slides up the OTA to make a sub aperture ERF. I've never noticed any issues with heating that have come to light, but saying that our skies are not as blue as yours, and we're only pretty much at sea level in terms of altitude so maybe the power of the sun is less?

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Re: CaK module ERF

Post by Bob Yoesle » Thu Jun 11, 2015 1:46 pm

thesmiths -- thanks for the info, Your systems(s) sound very appealing, and I like the solar Newtonian concept especially for WL & / or CaK. Is the secondary aluminized? I can imagine - given the results being obtained by C14's - that this could be a very practical solution with near APO refractor capabilities given the smaller central obstructions that could be attained. Christian Viladrich is doing this as well: http://www.astrosurf.com/viladrich/astr ... /N300.html

Walter - yep, that's why I went with the 40 mm verses the 60 mm pentaprisms -- much less backfocus issues. My ED 100's have the OTA's shortened for binoviewing, so I don't think I'll have and issues there, and the AP 130 is built similarly...

Mark - thanks for the info. I can't find the tri-band on Oliver's site at this time. I'm kind of attempting the equivalent for the CaK ERF/blocking via a short-pass or band-pass filter.

I don't think it's an altitude or similar issue. The way I noticed the heat issue with the Baader B filter is when I removed the Skybender and just had the light going through the objective and then through the Baader B, and then falling onto my palm - it got quite uncomfortable quickly - so now looking for a solution to protect the Baader CaK, and a short-pass or band-pass hard-coated filter such as the Edmund Optics filters seem to be it.

It looks like the 387 11 nm filter is in the same category as the 390 40 nm I cite above. Is this the one you are using: http://www.edmundoptics.com/optics/opti ... ers/84094/ ? Don't know why I didn't see it instead to begin with?! :?

Is there a reason you are using it between the PST filters as opposed to farther up in the filter chain?

FYI - Brian Stephens at Lunt told me the KG filter glasses are susceptible to UV deterioration, so you possibly might want to use the Tri-band (or DERF / other UV blocking filter) even with your 40 to limit UV exposure to it...
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Re: CaK module ERF

Post by thesmiths » Fri Jun 12, 2015 1:40 am

Bob, the solar Newtonian concept seems to work best with a metalised secondary. For CaK, there is not enough light if both mirrors are uncoated. But even with WL, I've noticed there can be strange ghost reflections from the back on an uncoated secondary. My original idea was that a Newtonian would have minimal aberrations at 400nm (most refractors not being optimised for short wavelengths). In practice, the issue becomes how to collimate the telescope precisely (star testing being difficult during the day).

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Re: CaK module ERF

Post by marktownley » Fri Jun 12, 2015 8:21 am

Bob Yoesle wrote:It looks like the 387 11 nm filter is in the same category as the 390 40 nm I cite above. Is this the one you are using: http://www.edmundoptics.com/optics/opti ... ers/84094/ ? Don't know why I didn't see it instead to begin with?! :?

Is there a reason you are using it between the PST filters as opposed to farther up in the filter chain?

FYI - Brian Stephens at Lunt told me the KG filter glasses are susceptible to UV deterioration, so you possibly might want to use the Tri-band (or DERF / other UV blocking filter) even with your 40 to limit UV exposure to it...
That's the one Bob. Only reason the filters are in this arrangement is that is what seemed to work in terms of suppressing reflections, well, throwing them way beyond the focal point so they are not noticeable. I did know the KG3 deteriorates over time, it's a soda(?) glass I think, and goes cloudy as it breaks down, touch wood don't seem to be having any issues with this to date...
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Re: CaK module ERF

Post by swisswalter » Fri Jun 12, 2015 8:18 pm

Hi Bob and Mark

my KG3 UV/IR cut filter has been serving me now for almost 3 years. No problems so far.
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Re: CaK module ERF

Post by christian viladrich » Sat Jul 04, 2015 2:18 pm

Hi,
Very interesting topic ;-)
Years ago, I used a Baader blue filter in 50 mm diameter as an ERF for my Ca K Barr filter.

Now I use the K line as a ERF filter for the Ca K filter on my TOA 150. Both filters are at the F/30 ratio. Still ... there is a lot of energy there. I am trying to find out a way to take the heat out. For example :
http://www.astrosurf.com/viladrich/astr ... 43mnUT.jpg

A 50 mm K line placed farther from the focus would probably be better (but still does not exist ...). EO have a 394 nm FWHM 10 nm hard coated filter in 50 mm size.

As for the use of an Herschel diagonal for Ca K imaging, the problem is that we are starving for light with the use of long focal ratio necessary for high resolution. For example, the exposure time here is 22 ms (instead of 2 ms without the diagonal) :
http://www.astrosurf.com/viladrich/astr ... 47mnUT.jpg

I have had the idea to have an Herschel diagonal multi-coated to increase the reflectivity of in Ca K. Going up to 30 - 40% would do the job. The project is stll on the shelves ...
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Re: CaK module ERF

Post by Bob Yoesle » Sun Jul 05, 2015 4:36 pm

Wow, fantastic CaK images Christian!

So now I'm really suffering from analysis paralysis! Between short-pass vs. band-pass, OD, prisms, surface quality/flatness, etc. my head may explode.

So starting over with what I wish to accomplish and what I already have...

I'm using a Baader Blue CCD filter as a preliminary UV/near IR ERF of sorts, but still getting a lot of energy passing through to the Baader K-line filter (likely a lot more than it is intended to receive and which may damage it over the long term) -- especially since I intend to up the aperture of the scope from 100 to 130 mm (or more). This excess energy is perhaps coming from the 380 to 510 nm 90% + transmission window:
BAA-blau-G.gif
BAA-blau-G.gif (34.4 KiB) Viewed 2471 times
This is also where a big part of the sun's most intense radiation comes from:

sun_radiation_spectrum.jpg
sun_radiation_spectrum.jpg (119.43 KiB) Viewed 2471 times

Baader recommends using their ND3.8 astrosolar film with this filter, which has the following OD transmission:
AstroSolar-38-log.gif
AstroSolar-38-log.gif (134.9 KiB) Viewed 2471 times
So it can be seen in the 400 - 500 nm range, the OD falls between 3.5 and 4.0 and OD 6 might be a bit of overkill. It also has good IR blocking out to 2500 nm - where a KG filter might be an alternative.

All the filters I'm considering have a lambda/4 flatness spec. on fused silica, except for the 394 - 10 nm OD4 mentioned by Christian, which has no flatness specification and the substrate is "optical glass." http://www.edmundoptics.com/optics/opti ... ers/65192/ -- this leads to some concern about thermal stability and optical quality far ahead in the chain of filters (e.g. farther from the focal point -- therefore any defects will have more impact).

The band-pass filters have a lesser surface quality (80-60 for the 394 - 10 nm OD4, 60 - 40 for the 387 -11 nm, OD6, verses 40 - 20 for the 400 nm short pass filters). They are also more expensive (50 mm versions): 387 - 11 nm OD 6 = $630, 394 - 10 nm OD 4 = $475, and the 400 nm OD 4 short pass = $410.

Edmund also has a 400 nm short pass OD2 with 40 - 20 surface quality for $180, but I'm not sure OD 2 would be sufficient: http://www.edmundoptics.com/optics/opti ... ers/64661/

So at this point, I'm leaning towards the 400 nm OD4 short pass -- closest to the OD recommended by Baader for the K line filter, good surface flatness on fused silica, and best surface quality, all of which compliment the Badder Blue CCD filter characteristics and transmission and will substantially reduce the 400 to 500 nm transmission: http://www.edmundoptics.com/optics/opti ... ers/84715/
Edmund 400 SP OD 4 curve.jpg
Edmund 400 SP OD 4 curve.jpg (132.68 KiB) Viewed 2471 times
Since it is slightly larger than the Baader Blue CCD, it might come first, then the Baader Blue CCD, then I'm thinking perhaps a 25 mm KG filter (for far IR blocking) right in front of the Baader K line, and lastly the rare and most valuable PST CaK No. 1 filters... all slightly tilted of course! :shock:

Got some hair to pull out it looks like... any additional thoughts or comments would be appreciated.
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Re: CaK module ERF

Post by marktownley » Tue Jul 07, 2015 5:50 am

Sounds good to me, you've got it all worked out here Bob, sometimes it is just a case of buying and trying some of these filters and seeing what works best optically.
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Re: CaK module ERF

Post by Bob Yoesle » Thu Jul 09, 2015 2:27 am

Thanks Mark. I'm also looking at the Andover KG filters:

https://www.andovercorp.com/products/co ... bing-type/

in addition to the Thor labs:

https://www.thorlabs.com/newgrouppage9. ... up_id=3695

The Andover KG's seem like they might be a good alternative in certain specifications:

Thor (T) verses Andover (A)

Thickness T = 2 mm A = 3 mm
Surface Quality T = 40-20 A = 80-50
Flatness T = < 1 wave A = 1/4 wave
Wedge T = < 3 arc min A = < 30 arc sec
Cost KG3 25 mm T = $35 A = $52

Getting close I think....
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Re: CaK module ERF

Post by marktownley » Thu Jul 09, 2015 5:40 am

They look very good.
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Re: CaK module ERF

Post by Bob Yoesle » Mon Aug 24, 2015 3:43 pm

Have now received both the 50 mm diameter KG5 from Andover, and the 25 mm diameter 400 nm OD4 short pass from Edmund, both rated at 0.25 lamda, so no real worries about optical quality:
KG5 & 400 nm SP.jpg
However, there's no accounting for price with the Edmund packaging!

Hopefully between this project and the Coronado SM140/90 upgrade I'll be getting some actual results soon...
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Re: CaK module ERF

Post by marktownley » Mon Aug 31, 2015 8:22 pm

Looking forward to hearing how you get on with them Bob :)
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