Disassembled Coronado CaK 90mm

[piekielrl]

NOTE: This is for educational purposes only! Neither I nor the administrator of Solar Chat assume any responsibility for unsafe use of this info!

OK, now having said that, here's another chapter to the thread I posted a few weeks back on disassembling a Coronado Ha 90. This time, I've disassembled my newly-acquired Coronado CaK 90mm.

According to Steve, there were only 6 of these made. This one was originally custom-made for Gary Palmer. From what I have been able to determine, the internal construction seems to be relatively identical to the "stock" CaK 90s.
Unlike my Ha 90 SMII, this unit was NOT held together with any threadlocker, and hence I could disassemble almost all the parts just by unscrewing the pieces by hand. This scope uses two main CaK filters, plus an additional UV-IR blocking filter in the main focuser tube.
The main, large CaK filter is 73mm in diameter and is the exact same filter used in the Coronado CaK 70, but here it is mounted in the middle of the tube, rather than up by the objective, so it is functioning as a "sub-aperture" filter. You can see this filter in the middle of the photos. If you look closely, you will see that it has a ring of "degradation" extending about 1/4" inward from the edge. This section lets light through freely without blocking it. Fortunately, the light cone coming from the objective is small enough to just fit inside the bad zone, so it still works. I plan on masking the degraded zone, juts to be sure.
Next in line is a filter stack of UV and IR blockers, mounted in a chrome barrel, which is screwed onto the front of the main focuser tube. Lastly, an additional mirrored CaK filter is in the front barrel of the diagonal, so the scope is essentially a double-stacked CaK, just like the 70. That's how they achieve the narrow bandwidth of 2.4A, as stated by Coronado (Partially overlapping the transmission peaks of the two filters).

Bigger aperture means a brighter image, so even people with marginal sensitivity to the CaK line can have a better chance of seeing something visually with these larger-aperture CaK scopes.

Any questions, feel free to ask!

Bob Piekiel

[swisswalter]

Hi Bob

good information. Yes I can see very well the ring of Degradation. I would like to have that "broken" filter for experiments ;-)

[piekielrl]

Hi Walter, I'm going to have to mask out and re-use the filter to get the scope up and running, since commercial replacements (like the Lunt CaK module) use much smaller filters than this big 73mm unit).

I can tell you this - if you shine a red or green laser at the center of the filter, it is completely blocked, but if you aim the beam at the edge, the light comes right through. This is a solid "sandwich" type bandpass filter with the coatings on the two inner surfaces, kind of like a solid etalon.

I think I mentioned in a previous post somewhere that several people I know with Coronado CaK scopes are having similar failings of the filters. It supposedly is due to the hygroscpopic nature of the coatings. They absorb moisture from the air and degrade. I know of one owner who had Lunt re-coat a filter for him. I have also seen a CaK 70 that had it's internal CaK filter replaced with a filter scavenged from a Lunt CaK module (that scope works great, BTW)

Thanks, Bob

[marktownley]

Excellent Bob! I do like your curious nature 8)

Any chance you can take a close up of the different filter elements - those in the blocker and also in the OTA; I would like to see how they compare with those in the CaK PST.

Thanks,

Mark

[piekielrl]

Hi Mark, Yes, I will do this shortly. Here's a few more important points about the scope:
ALL the optical components, including the objective, are mounted in cells that have them tilted a very small amount. I know this is for reduction of internal reflections.

Why this doesn't introduce astigmatism in the final image (tilting of the lenses) is a mystery to me. The rotational orientation is not marked or indexed in any way, and since you can freely rotate the diagonal in all of these scopes, the scopes would never remain in a configuration where tilting one optic would be cancelled by tilting another in the opposite direction. (I've rotated the components on my Ha scopes to see what would happen - nothing does).

Tilting a bandpass filter also changes its transmission curve, so I wonder if the amount of tilting in the filter cells is engineered at the factory to "tune" each filter to the desired wavelength. Like you have said before, it is actually less expensive to produce a tunable filter than one that is fixed precisely at the correct wavelength.

Why Coronado did not make their CaK scopes with tunable filter is a mystery to me as well. It wouldn't be hard to do, since the main CaK filter is mounted in a housing which is identical to the housing of the etalons in the "Rich View" SolarMax II scopes.

If the idea of double-stacking to narrow the band pass involves SLIGHTY overlapping the transmission curves of two filters, it seems that it would be an ideal configuration in all scopes that use two internal filters (like these CaK scopes).

Additional pics to follow...
Thanks, Bob

[piekielrl]

OK, here's the pics with a description:

Pic 1 shows the main objective. It is heavily coated, presumably to serve as a UV-IR block as well. It is mounted at a tilt in the cell.



Pic 2 shows the main 73mm CaK filter, mounted in the middle of the tube, in a collar just like the Ha etalons. It is mounted at a slight tilt also, and is the same filter as used in FRONT (by the objective) of the CaK 70mm scope. If you look through it, you see a very dim, deep purple image.



Pic 3 shows the additional UV-IR cut filter mounted in the focuser tube. This is a thick filter stack. They are FLAT. There are no collimating lenses here. The coatings are a deep purple, but the filter lets quite a bit of light pass through. By itself, it gives objects a yellowish color.



Pic 4 shows the diagonal. The mirror looks like a standard coating, NOT a "cold mirror" as used in the Ha scopes. There is only one additional filter - a small mirrored bandpass filter mounted (at a tilt) in the chrome barrel that goes into the main scope. It seems like an identical filter to the main one in the middle of the scope, only allowing deep purple light to pass through.



Hope this helps, Bob

[marktownley]

Very interesting. This is exactly the same configuration of the CaK PST, same filters just different sizes in a different order.

The same filter 'rusts' in the CaK PST too. The magic filter here in this configuration is the one in pic 3. You could remove all the others and use pic 3 filter in conjunction with a Baader K-line or the Baader U (venus filter) to give excellent views - well, when I say views I wouldn't use this visually, but photographically it will excel.

I've double stacked my CaK filter, and experimented with varying amounts of tilt, it really makes negligible difference compared to the Ha equivalent, the only tilt that is required is that to throw the reflections.

[Andy Devey]

Hi Bob

Very interesting indeed. What serial number have you managed to get hold of?

I have the very first one I think no 950001, Greg Beipol had no 950010 leading me to believe there may have been 10 but I have no way of being certain of this.



Regards

Andy

The solar explorer

[piekielrl]

Hi Andy, Glad to know you as another CaK 90 owner! So you have #1? You are correct about Greg. His was labeled #10 (I just messaged him about that and he confirmed it). Greg sold his a short while back, but I didn't ask to whom it was sol to. Our solar chat "leader" Steve has #s 3 and 5.

The one that I have was "custom made, hand made" by David and Andy Lunt (while they were running Coronado) and hand-presented to Gary Palmer. Supposedly it was the best they could make, but there's no way to tell until I get these filters up and running again. This one was made BEFORE the actual production run started, and has no serial number. It doesn't even have the gold and black Coronado label, just a blue "CAK" sticker on the cone-shaped rear cell. Gary sold it to me "as-is" because of the filter degradation.

Steve has told us that there are 6 of these in existence. His #5 came (I believe) from former Coronado employee Brian Stephens, who stated in his ad on Cloudy Nights when selling it that there were only 5 made, so I would guess that is 5 production models plus the one "custom" job that I have now.

One other detail, mine had an all-black cone-shaped rear cell, not the blue-anodized color of the main production scopes. I wrapped the outer band with blue Monokote, just to give it the "look," and not have to fuss with re-anodizing it blue.

As to Mark's comments about the rusty filters, The first thing I plan to do is replace the rear-most ITF with several commercial CaK filters (Baader, Edmunds, Meier, etc.) and see how each one works. That's the least-expensive way to proceed, at least for now.

Thanks, Bob

[swisswalter]

Hi Bob

thank you for all the Details. As mark said, it is the last filter in the scope who makes the Magic. I have not been able to find a replacement

[marktownley]

Can you mount the rusted filter further down the tube? That way the rusted periphery is not being used? This a large ITF filter and provides the blocking up to +2500nm - this is what makes the scope safe for visual use. A commercial alternative to this would be a large piece of KG3 glass mounted sub aperture. The CaK Heliographe at the Paris Meudon uses this approach...

Alternatively, Oliver at beloptik sells a 85mm tri-band ERF that works at CaK wavelengths, this might be useful, then just use that and the yellow filter in pic 3 in conjunction with a Baader K-line and you would have a working photographic scope, throw in one of olivers UV/IR cut KG3 filters into the optical train and you'd be safe visually too.

[piekielrl]

Hi Mark, Yes, as I said in a previous post here the light cone from the objective "just touches the inside of the degraded zone," so by moving the filter backwards in the tube it will be completely out of the bad zone. All I have to do is screw the filter mount collar in "backwards." (The threads are the same at both ends) Because the filter sits closer to one end of the collar than the other, screwing it on backwards moves it about 1-1.5 inches farther back in the optical path. Perfect for getting the light cone to pass only through the good portion of the filter.

I am looking at Beloptik's website. I see the ERFs, but don't see the 85mm tri-band you are referring to. Can you post the exact page link here?

Thanks!
Bob

[swisswalter]

Hi Bob

the triband filter is not on his website. Just drop him an email