A Filter Journey

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Bob Yoesle
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A Filter Journey

Post by Bob Yoesle » Thu Feb 05, 2015 6:55 am

The topic of blocking filters, ITFs, and ERFs has received quite a bit of attention on the forum over the past couple of years - so much so that one can become lost in all the permutations -- I did. I have been assiduously trying various things to deal with the complex "stew" of filter components and their effects, and believe I'm finally on the verge of some success.

This story begins with the Coronado SM90 and BF30 I purchased in 2003, which replaced a DayStar filter system obtained in 1976. The SM90 delivered wonderful views, and the image if anything was a bit too bright at low full-disk powers.

In 2004 I was exposed to the benefits of double stacking - spectacular views - and purchased a second SM90 etalon. The image was of course less bright due to the reduced overall transmission, which was now "just right" for full disk viewing.

Then the curse of ITF "rust" struck, and the BF30 was returned to Coronado (Tucson) for repair under the 5 Year Warranty. Returned good to go, until less than 2 years later and the ITF failed again. Back again -- now to Meade Coronado -- and the returned BF30 not only had a dimmer - almost too dim - view when double stacked -- the trim filter itself was scratched from poor assembly technique. So back it went, and returned with a pristine trim filter, but the filter still had a less bright DS view. It seemed that Meade decided to reduce the BF transmission to suit single etalon use exclusively. To me this was dumb, as one can always reduce the image brightness with additional filtering.

So at this point I embarked on an inquiry as to how to 1) avoid any further ITF deterioration(s), and 2) increase overall image brightness. I discovered the root cause of ITF deterioration is thermal loading and cycling causing peripheral epoxy sealant breaching, and also that the RG630 ERF supplied by Coronado has no IR blocking, which contributes to the problem:
Cor-SM60-log.gif
Cor-SM60-log.gif (132.51 KiB) Viewed 2591 times

I then discovered the new Baader C and DERF's do have IR blocking, and are specifically designed to reduce thermal loading and cycling - perfect! It has both IR-UV dielectric filtering on an optical glass substrate (BK7) polished on both sides to 1/10 lambda for very good optical performance.
Baader-D-ERF.jpg
Baader-D-ERF.jpg (415.65 KiB) Viewed 2591 times
Lunt also provided an ERF with IR blocking.

I removed first one of the Coronado ERF's to attain greater image brightness (having two is redundant), then removed the second and used the DERF 110 instead, which you can see via reflective double pass provides a significant improvement in overall transmission:
DERF SM90ERF.jpg
DERF SM90ERF.jpg (227.18 KiB) Viewed 2591 times
SM90-90-DERF.jpg
SM90-90-DERF.jpg (95.47 KiB) Viewed 2581 times
This was much better, but the image is still not as bright as I remember the original BF30s image, and I could use the brightness for when higher magnifications were used, and for imaging purposes to gain shorter exposure times. It does seem the DERF has prevented any repeat ITF failures, although I also have taken to storing my BF30 in a kitchen vacuum seal food bag, and have developed a Argon-flushed BF filter holder as well.
BF30CaKvacuum.jpg
BF30CaKvacuum.jpg (119.79 KiB) Viewed 2581 times
Imgp1097SM.jpg
Imgp1097SM.jpg (191.71 KiB) Viewed 2581 times
I also noticed Solar Scope provides two Blocking filters with their DS filter systems, for the reason to improve image brightness with DSing via a higher transmission BF. Recently I found a second BF30 for sale, and purchased it with the hope it might provide a brighter image, and I would have the same filter retinue as Solar Scope provides. The filter arrived and indeed had a significantly better image brightness! Unfortunately it also had the beginnings of peripheral ITF deterioration (on the right below):
BF30 old - new.jpg
BF30 old - new.jpg (198.45 KiB) Viewed 2591 times
Note the increased transmission visible via the internal reflections in the ITF with "rust."

So I now set out to find a suitable replacement. I called Meade to see if they could just sell me a replacement ITF, as I already know a bit about these things and can remove and replace the ITF myself:
Rusted ITF.jpg
Rusted ITF.jpg (232.25 KiB) Viewed 2591 times
Of course, it is not possible to do this through Meade, and I'd rather not risk another scratched trim filter. Meade also won't reveal their OEM supplier of ITFs, as this is "proprietary."

So I contacted Oliver Smie at BelOptik about his "ITF replacement" -- a KG3 with additional IR-UV cut coatings discussed on the forum many times. This filter has the extended IR suppression of an ITF, and a high H alpha transmission that holds the promise of a brighter final image:
UV_IR_cut_on_KG3_Spektrum_2.jpg
UV_IR_cut_on_KG3_Spektrum_2.jpg (212.85 KiB) Viewed 2591 times
The BelOptik web site also advised the KG3/IR-UV cut filter needs to be used in conjunction with an "ERF filter" - got my DERF already in place. Oliver even turned down the 2 inch filter to 34 mm in order to fit the BF30 ITF cell for a modest extra charge - an advantage as the BF30 ITF cell incorporates tilt to avoid reflections. Total filter cost was less than a the minimum replacement ITF quoted from Meade Coronado. So I ordered the KG3/IR-UV cut filter via Oliver's eBay store and received it very promptly - A+ service! And a perfect fit for the BF30 (left KG3, right ITF):
IMGP3522.JPG
IMGP3522.JPG (185.22 KiB) Viewed 2591 times
Now the test -- !!! Blindingly bright, geenish continuum view --
IMGP3354.JPG
IMGP3354.JPG (40.2 KiB) Viewed 2591 times
(Very verrrry short exposure).

I contacted Oliver, who ever so helpfully explained the need to use an RG filter element with the ITF replacement filter. In hindsight I neglected to remember or review the postings here mentioning using the KG3/IR-UV cut filter with another such as the Baader 35 nm H alpha, which uses RG610 as a substrate. Oliver recommended a number 29, or RG630 such as the original Coronado ERF or the Lunt ERF, as being the best options.

Looking back at the various transmission curves one can now see where the excessive brightness and green wavelengths are coming from. Since I just so happened to still have my original RG630 Coronado ERF's, this would be no problem. Only its the middle of winter in the Pacific Northwest and I may not have an opportunity to see the sun for another month or two! Since I'm not one to wait that long, and short of a trip to California (where I grew up and visit frequently), I decided to make a test of a different nature.

Today I used a incandescent flood light as a simulated sun:
IMGP3511.JPG
IMGP3511.JPG (42.05 KiB) Viewed 2591 times
Between this "sun" and my camera lens I placed the BF30 with the KG3/IR-UV cut filter, but no etalons:
IMGP3512.JPG
IMGP3512.JPG (36.34 KiB) Viewed 2591 times
Note the bright flare and broad dark filaments! This rendition indicates the simulation might just work to show the actual results.

For the next image I used the BF30 with the good ITF (which by the way is applied to a RG630 substrate as well) and DERF:
IMGP3518.JPG
IMGP3518.JPG (34.81 KiB) Viewed 2591 times
The image is a little dim but shows good "detail." Keeping an identical exposure with subsequent filter arrangements should give a good simulation of the changes that occur. Here's the BF30 with the KG3/IR-UV cut, used with the Coronado RG630 ERF alone, and again no etalons:
IMGP3519.JPG
IMGP3519.JPG (34.35 KiB) Viewed 2591 times
A very positive result with significant image brightness improvement! Next, both the DERF and RG ERF:
IMGP3520.JPG
IMGP3520.JPG (33.33 KiB) Viewed 2591 times
Quite acceptable, with minimal brightness loss over the RG630 alone, and still a vast improvement over the BF30/ITF:
Compare Image1.jpg
Compare Image1.jpg (65.91 KiB) Viewed 2591 times
I'm hoping with some real sun to verify these simulated results.

I plan to use the original BF30/ITF with my primary single etalon using the DERF alone, where the decreased image brightness will be perfectly suited to single stack / DERF use. And I have added the original RG630 ERF to my second double stacking etalon for use when double stacking and using the BF30 KG3/IR-UV cut blocking filter, also with the DERF. Since both single and double stack etalon systems will always be used with the DERF, hopefully neither blocking filter should encounter any issues with thermal loading and cycling.

Stay tuned for some real comparison images when the sun reappears...
Last edited by Bob Yoesle on Thu Feb 05, 2015 4:10 pm, edited 9 times in total.
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Re: A Filter Journey

Post by marktownley » Thu Feb 05, 2015 7:06 am

A great story there Bob, I look forward to the images :)
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Re: A Filter Journey

Post by Derek Klepp » Thu Feb 05, 2015 7:51 am

Thanks Bob it will be good to see the final Solar view.

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Re: A Filter Journey

Post by Merlin66 » Thu Feb 05, 2015 8:00 am

I know the Baader 35nm CCD filter has been used very successfully as an ITF replacement in a few Coronado BF's (There's a BF30 and a couple BF15 modded here in Oz)
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Re: A Filter Journey

Post by Bob Yoesle » Thu Feb 05, 2015 3:51 pm

Thanks guys, and hi Ken,

Most if not all of the ERF's, whether on BK7 with IR coatings or RG630 with coatings, don't have IR blocking to the 2500 - 3000 nm range. Here's bare RG630:
RG630 - 1.jpg
RG630 - 1.jpg (169.97 KiB) Viewed 2554 times
From what I remember reading most references state that the Baader 35 nm H alpha filter has a similar coating and transmission as the DERF:
ofil-bp-fhal-1-2.jpg
ofil-bp-fhal-1-2.jpg (73.1 KiB) Viewed 2554 times
Baader-D-ERF.jpg
Baader-D-ERF.jpg (415.65 KiB) Viewed 2554 times
Therefore it would appear to have little blocking beyond about 1500 nm. An ITF on the other hand, will typically block IR out to 2500 nm and beyond,
Cor-BF151-lin.gif
Cor-BF151-lin.gif (131.86 KiB) Viewed 2554 times
as does a KG3 filter glass:
KG1-KG5.gif
KG1-KG5.gif (66.5 KiB) Viewed 2554 times
Being over cautions perhaps, and wanting to avoid issues of injury an liability, most solar filter manufacturers incorporate blocking out to 3000 nm or more via an ITF (Coronado), KG3 (Solarscope), or a combination of other glasses and coatings (Lunt, et.al.) I know opinions vary on this, but there might be some concern over IR B and C with only using the Baader 35 nm as an ITF replacement, even with a good ERF with typical IR blocking.
eye-injuries.jpg
eye-injuries.jpg (85.75 KiB) Viewed 2554 times
So while the Baader 35 nm filter might be safe to use (depending on ones opinion of the eye's susceptibility to IR B & C), and therefore might be used in place of an ITF, it can not be said that the Badder 35 nm filter is an ITF replacement, which may be a subtle but important distinction.

************************************************************

I'm hoping to get back to some real-world testing, but it might be awhile!
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Re: A Filter Journey

Post by Merlin66 » Thu Feb 05, 2015 8:43 pm

The combination of the ITF and the final blocking/ sort filter should also be considered.
There's a write -up in the files area re. INCIRP safety review which should also be reviewed.
I'm not going to discuss NIR here. The exposure required to cause possible injury is well in excess of anything an amateur is likely to incur.
The silicon CCD chips can't record anything beyond 1100nm.
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Re: A Filter Journey

Post by swisswalter » Fri Feb 06, 2015 5:34 am

Hi Bob

thanks for the story and the load of information
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Re: A Filter Journey

Post by Valery » Fri Feb 06, 2015 7:28 pm

Hi Bob,

Thanks a lot for your information. Very useful.


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Re: A Filter Journey

Post by Bob Yoesle » Sat Feb 07, 2015 5:55 pm

Hi Ken, I looked but I couldn't find that document in the Reference Library , can you point me via a link?
The combination of the ITF and the final blocking/ sort filter should also be considered.
The only transmission profile I ca find for the "trim" or "final blocking/sort filter" is this one:
Cor-BF15Ha-lin.gif
Cor-BF15Ha-lin.gif (56.41 KiB) Viewed 2476 times
There appears to be not only significant IR B and C transmission, but IR A as well, which does explain why at least some sort of additional blocking out to 1500 nm is required, if not beyond.
Normen.gif
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Re: A Filter Journey

Post by Merlin66 » Sat Feb 07, 2015 9:37 pm

"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: A Filter Journey

Post by Bob Yoesle » Sat Feb 07, 2015 9:51 pm

Thanks Ken
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Re: A Filter Journey

Post by swisswalter » Sun Feb 08, 2015 1:04 am

Hi Ken

thank you very much. It is so good to have our own Wikopedia ;)
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Re: A Filter Journey

Post by Bob Yoesle » Fri Feb 13, 2015 4:19 am

Some sun finally came out this afternoon, had to get a little sunshine in my eyes!
IMGP3571 adj sm.jpg
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Re: A Filter Journey

Post by Bob Yoesle » Fri Feb 13, 2015 4:20 am

And this also gave me the opportunity with the DSLR to capture "real deal" results -- seemingly identical to the simulated results:
ITF KG3 compare.jpg
For imaging the system using KG3/IR-UV cut from Oliver is over one stop brighter than my ITF BF30 system!
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Re: A Filter Journey

Post by Bob Yoesle » Fri Feb 13, 2015 4:21 am

The visual with the brighter image provided by the KG3/IR-UV cut equiped BF30 is more impressive than I would have imagined - I've never had a view through any H alpha filtered scope - let alone double stacked, that offered as much of a pleasing view. It is superior to the Solarscope DSF100 with the dedicated double-stacking BF in terms of brightness, and hence prominence detail. Can't wait to do some bino-viewing with this arrangement.

Below a 1/30 sec overexposure has leakage into the green and blue bayer layer pixels, preserving details that normally would be lost in the overexposed red pixels. This produces an image that is very similar to that perceived visually:
IMGP3550 crp adj sm.jpg
I think I may have fallen into solar nerdvana...
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Re: A Filter Journey

Post by swisswalter » Fri Feb 13, 2015 4:37 am

Hi Bob

impressive results
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Re: A Filter Journey

Post by MapleRidge » Sat Feb 14, 2015 1:29 am

Quite the project Bob, but nice to see the positive results...well done :bow

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Re: A Filter Journey

Post by marktownley » Sat Feb 14, 2015 7:59 am

Interesting to hear how much more transmission with the Beloptik over the ITF. Most definitely food for thought...
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Re: A Filter Journey

Post by Bob Yoesle » Sat Feb 14, 2015 2:35 pm

Thank you kindly gentlemen.

The BelOptik KG3/IR-UV cut filter modification definitely has a place for those who double stack and experience the image to be of insufficient brightness, and may make triple stacking a realistic possibility.

Moreover, it appears to be a legitimate replacement for a deteriorated ITF as long as there is an RG element elsewhere in the filter system - preferably ahead of the KG3 and trim filter elements. For single stacking, it could be used with additional filtering to accomplish optimum brightness, or help reduce exposures times to achieve optimum imaging parameters.

Very pleased Oliver Smie has made this option a possibility...
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Re: A Filter Journey

Post by Bob Yoesle » Tue Feb 24, 2015 7:57 am

Got my first "real" imaging done today with the new KG3/IR-UV cut ITF replacement filter in mediocre seeing, yet an exposure time of 2.17 ms was able to yield significant detail. Again, very pleased with the results!

DERF 110, RG630, SM90/90, Orion ED100/900, KG3/BF30, PGR Cahmeleon, 02.23.20015 @ 19:33 UTC:
R_193335 SM90 DS jpg crp adj comp.jpg
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Re: A Filter Journey

Post by swisswalter » Tue Feb 24, 2015 5:15 pm

Hi Bob

an impressive result
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Re: A Filter Journey

Post by marktownley » Tue Feb 24, 2015 7:46 pm

Very good Bob. I can see me contacting Oliver at Beloptik before too long...
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Re: A Filter Journey

Post by Bob Yoesle » Thu Feb 26, 2015 3:36 am

Thanks Walter and Mark.

Depending on the substrate of the ITF which you are replacing, YMMV, but in my case the improvement in image brightness is dramatic. The filter also presents no other optical issues, which I have encountered with other ITF replacements.

Dealing with Oliver was easy, and the fact that he can turn-down the filter to allow mounting in an existing filter cell is great -- I think this would be problematic with a "standard" ITF.
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