Dream come true!

[Solar B]

All good

Brian

[Bob Yoesle]

Getting a little OCD now... More waiting on some last minute machining.

One of the minor quibbles over the Coronado external etalons is the open T-Max tuner design, which can allow contaminants through the gap when open:

Addressing this in the SM90's, I used a "sprung" internal thin plastic ring to cover the gap and block this from happening - but lost about a mm or two of aperture. In the SM140, I instead decided to use some black elastic fabric material around the periphery, and secured it at both ends under the hinge plate. The OEM steel hinge plate bolts were corroded with rust, so these were replaced with stainless steel.

Hoping I will have some good weather with steady seeing in the next weeks as "first light" approaches.

[Derek Klepp]

I look foward to your first light reports Bob.

[Bob Yoesle]

I guess its time for an update to my saga - as you will see, things have not gone as expected...

After a great deal of effort, all the necessary parts were made or modified, and assembled. I took particular care to make sure optical and mechanical alignments were as best as I could accomplish.

Here focuser and objective alignments are optimized:

The SM140 tilt-tuning rod is installed:

September 27, 2015 first light of the competed SM140/90 CR6 OTA:

[Bob Yoesle]

My first views were very disappointing -- off-band and considerably non-uniform. To get any detail I had to open the SM140 T-Max all the way.

SM140/90 @ native CR6 EFL ~ 1200 mm.

The good news was the resolution appeared good given the low altitude and generally average to poor seeing conditions.

I decided to do my usual side-by-side comparison with with the "reference" DS SM90/90 system, and that's when the true magnitude of the off-band issue became evident.

Comparison configuration:

Using a custom high-quality focal reducer to get similar EFL's, both the visual and imaging results leave little doubt as to the off-band issue:

EFL's ~ 900 mm

[Bob Yoesle]

Over the next several weeks of despondency, I contacted Markus Ludes, who was at a loss to understand why I might be having such difficulties. He advised contacting Andy Lunt at Lunt Solar Systems, who I shared the above imagery with.

Andy asked me to call him, where he advised me that the oft-cited theory of double stacked etalons needing to be off-band to either side of the H-alpha line was completely erroneous. Andy stated that each etalon should be able to come exactly on-band in order to give best performance, and asked me to obtain images with each filter individually. For the front SM140, Andy asked me to obtain images with the filter tilt tuner closed (no tilt), mid-tilt, and maximum tilt. It took several weeks to get any clear weather, and when I did the seeing was very poor, but adequate to reveal the needed information:

No tilt:

Frnt no tilt.jpg (118.4 KiB) Viewed 7840 times

Mid tilt:

Frnt mid tilt.jpg (117.1 KiB) Viewed 7840 times

Max tilt:

Frnt max tilt.jpg (130.96 KiB) Viewed 7840 times

This behavior is quite different from every other front etalon I have used, where the optimum position is mid to near closed tilt to be on-band, and I have never observed a continuum image as seen in the SM140 with no tilt. This indicates the SM140 CWL is tuned quite high, and just begins to come into the red wing of H alpha with the maximum tilt available.

[Derek Klepp]

Thanks Bob does that mean you need a bit more tilt or that's it as far as being on band?

[Bob Yoesle]

Next, for the internal SM90, Andy advised using the DERF ahead of the etalon to reduce incipient IR, as well as the RG630 ERF which is installed just ahead of the collimator lens:

Andy requested that I try several different internal module placements to see if any change produced an improvement. No significant change was observed compared to the theoretical best placement based on the measured FL of the collimator lens:

Theoretical best placement:

777.jpg (136.18 KiB) Viewed 7839 times

Placed 90 mm farther away from the objective:

867.jpg (155.93 KiB) Viewed 7839 times

Placed 110 mm closer to the objective:

667.jpg (143.5 KiB) Viewed 7839 times

The changes in seeing affect the quality of the images, but the overall contrast and contrast uniformity remains essentially unchanged, with little change in the peripheral band-pass as well. This is a testament to the relatively large size of the internal etalon and the long focal length of the collimator lens, which results in minimal field angle magnifications.

But once again we can see this etalon too is not on-band, and therefore the combination of etalons could not be on-band either. My suspicion was that the internal SM90 filter might be a standard production SM90, which again would be tuned hi -- but under "normal" circumstances it would be "tiltable" to bring it on band. In the case of the internal module, no provision for tilt tuning was provided -- it remains completely fixed in the etalon cell and thus off-band Hi.

[Bob Yoesle]

I also had the opportunity to contact John Hunter of Light Machinery Etalons in Canada. John advised me to obtain a Hydrogen laboratory spectrum tube in order to perform an etalon fringe analysis. So I headed back to Yakima Valley Community College and my friend Dr. Rodrigue was happy to assist.

The etalon fringe test is what is used at Lunt as well for determining etalon quality and specs:

Test crp SM.jpg (57.47 KiB) Viewed 7839 times

I felt I had a pretty good handle on how to perform this test. Note a diffuser screen is used to make the fringes a bit more visible. I brought the lab equipment home and made a similar simple test diffuser set up:

When viewing the etalon fringes, one sees concentric circles, and the spacing of the circles is the FSR -- free spectral range. The tightness or "crispness" of the fringe indicates the finesse - sharp fringes with no intervening light between the fringes indicate excellent finesse. If there is a central spot dead-center the etalon is on-band. If the central spot expands, or becomes a circle, the etalon is showing it is off band, and the larger the central circle the father off-band the etalon is.

So first up was a reference SM90 etalon. This is my primary DS etalon, tuned hi, but requiring the least amount of tilt to come on band at my elevation of 610 m:

SM90 Objective.jpg (157.45 KiB) Viewed 7839 times

Next up is the internal SM90 of the new telescope system:

SM90 Internal.jpg (111.73 KiB) Viewed 7839 times

And here's the SM140 external:

SM140 External.jpg (147.87 KiB) Viewed 7839 times

These results seemed to confirm my suspicions. The internal SM90 looks almost identical to my reference SM90 front etalon, tuned perhaps just a bit higher. The external SM140 has a much larger central spot-circle, indicating it is tuned to a much higher CWL. Mr. Hunter and associates indicated that the DS system would not come on-band unless used at about 2000 m or greater elevation.

So again, there's no way this system could perform anywhere near on-band at the altitudes I typically observe at -- 0 to 1200 m elevation. The good news is that both etalons appear to have absolutely superb finesse.

[Bob Yoesle]

So great, now what --

the scream.jpg (158.94 KiB) Viewed 7831 times

my dream turned to nightmare solar telescope is too heavy for a paperweight, and not heavy enough for a boat anchor.

Well the story is not over. On seeing these results, Andy has graciously agreed to evaluate the etalons and refurbish them to be able to perform as intended at the altitudes I will use them at.


This will likely require re-tuning the SM140 CWL via new spacers. The internal etalon will need to be tuned as well, as pointed out on the Lunt web page http://luntsolarsystems.com/etalons-ext ... -internal/

The evolution of internal etalons has taken several steps. The original systems had internal fixed etalons that sat behind a re-collimating lens set (these lenses work to bring the light rays back to parallel). The light from the etalon was then re-focused back to the image plane. These systems cannot compensate for changes in altitude and air pressure. Taking this system from sea level to 8,000ft would not work...

The next step is the introduction of some tilt. This works similar to the tilt on the front of a system. However, due to both off axis light rays that could not be re-collimated with a simple lens, and the angles created by the Sun’s size, the etalon can only be tilted thru a fraction of a front mounted system... The removal of the tilting from the internal system is providing stunning results... This [Doppler True (pressure) tuning] technology has certainly improved the dedicated system...(Empahsis added)

Therefore I anticipate the internal etalon will require vacuum tuning, or need retuning of the CWL to allow for pressure tuning.

To be continued, and holding my breath to hear what lies ahead...

[marktownley]

It's a shame to hear the journey isn't as you would have liked Bob, but you have most certainly been thorough in your approach. The only thing I can think to suggest is for Lunt to change the etalon spacers?

[marktownley]

Lol, you beat me to the post! That's good news!

[Bob Yoesle]

Thanks Bob does that mean you need a bit more tilt or that's it as far as being on band?

Hi Derek

That's the limit for tilt, which is already too much as it introduces significant contrast non-uniformity of the off-band image. As the rest of the story indicates, the SM 140 CWL is too high for use at my altitude, and this will need to be addressed directly.

Thanks Mark, keeping my fingers (and toes) crossed ;-)

[Valery]

Bob,

Why not ask Mr. John Hunter to determinate the necessary spacers thickness, to make them and to assemble the etaion
at the LM laboratory?


Valery

[Montana]

Oh dear Bob, I hope they fix it soon and the dream will come true

Alexandra

[Solar-Cologne]

Hello Bob,

Your problem is very interesting.
I read all your posts here, but i never find the words:
the two etalons must be rotated against each other to find the best h-alpha

Excuse me, if I have overlooked it.

but you had make two mistakes. I phoned with W.Lille and Oliver about your problem.
I ts a great mistake to use an ED or an APO-refraktor for watching h-alpha, especially a TAK with a fluorite lens.
An ed/apo is corrected for the green wavelength and there are allways speciall glasses in it, not good for h-alpha.
And a fluorite lens too, is not focus-stable in h-alpha, like a Zeiss-APQ
I have made many experiments with ed, and see it often, when i may have a look through an apo-self-made h-alpha-scope.

Use a asphäric-chromatic single lens, like a W.Lille h-alpha-chromat-scope:

http://sonnenfernrohr.de/2015/03/lille- ... fertigung/

or use a Fraunhofer-refraktor, which is corrected at the red wavelenght.

Please do some experiments and share your results here in the forum.


Sorry, the SM140 is not the "Holy-Grail", because there are worldwide 4 pieces of a Lunt 160 frontetalon....

sunny Greetings
Achim

[Bob Yoesle]

Hi Valery

Why not ask Mr. John Hunter to determinate the necessary spacers thickness, to make them and to assemble the etaion at the LM laboratory?

I did get a "ball-park" estimate from John:

Code: Select all

Typically we charge about US$20,000 for large aperture etalons, I expect the repair would be about half of that (and may not be successful).

Moreover, this estimate did not include the internal SM90, nor the need to adapt the internal module (or make a new one) to be vacuum/pressure tuned.

The SM140 has 12 spacers around the periphery in addition to the central spacer (the industry standard is three to six spacers on the periphery -- the SM90 has 9 + 1 central)... Bill Dean -- who worked at Coronado when the etlon was made -- told me that the SM140 etalon is one of the trickiest ever made, and the only people he would trust to pull off the repair would be Andy or Brian Stephens. I believe Andy (and/or Brian) have a sincere interest in the project as they were the OEM. Additionally, Andy has very generously stated he will only charge for materials; he is donating the labor to our Friends group for solar outreach. The new scope will therefore be generously adorned with Lunt Engineering decals following successful repair.

Oh dear Bob, I hope they fix it soon and the dream will come true

Thanks Alexandra -- me too! And if not, I have vowed to give up solar astronomy and take up stamp collecting ;-)

the two etalons must be rotated against each other to find the best h-alpha ...or a Fraunhofer-refraktor, which is corrected at the red wavelenght... Sorry, the SM140 is not the "Holy-Grail", because there are worldwide 4 pieces of a Lunt 160 frontetalon....

Hi Achim,

Clocking is not an issue and would not solve the root issue of the too hi a filter CWL.

I did indeed know about the problems with fluorite APO's from discussions with Mike Jones. That's why I specifically bought only the filters from Markus, and adapted them to the CR6, which is a well corrected fraunhofer achromatic refractor.

I didn't know about the 160's -- any details? But part of the "Holy Grail" status for me is the large internal etalon which allows for the most optimum field angles (contrast uniformity) for the double stacking. I doubt any 160's are double stacked with a 100 mm internal etalon, however ;-)

Thank you all for your interest and concern -- I will keep you posted.

2x04-tb_or_not_tb179.jpg (44.42 KiB) Viewed 7790 times

[Solar-Cologne]

Hi Bob,

thanks, i am allways interested in h-alpha and the problems that may arise.

About the SM160, Wolfgang Lille, knows Andy Lunt personally, and he tell me about this very rare front-etalons.
I never seen one of them.
With using google, i found this links:

http://luntsolarsystems.com/etalons-ext ... -internal/
http://www.khanscope.com/productdetails ... uctID=2020
https://www.google.de/search?q=lunt+160 ... I4ChCwBAgk

sunny greetings
Achim

[Solar B]

Fingers / toes crossed here to Bob for a resolution of your fine SM140/90

I'd be surprised if there are any 160s around ? (would love to see one though)

Markus obviously uses a 230mm with rear mounted etalon (but doesn't everyone now)

Brian

[Bob Yoesle]

Hi Achim and Brian,

I was aware of the reference to the LS160F from the time of the original "external-internal" blog post a few years ago, but didn't know any were actually made (and the front etalon pressure tuning likewise apparently was abandoned). I have never seen them mentioned or discussed since. Would be great to know where they ended up -- nothing shows up on my google search.

The images I see on the Kahn site for the 160 are actually the discontinued LS75F "root 3" etalon...

[mdwmark]

Hi Bob,
Andy will fix you up. Its his system, he is the best person to deal with the problem.
Mark W.

[Bob Yoesle]

Thank you Mark -- have a great Thanksgiving all!

[Bob Yoesle]

Got an update on the etalon refurbishment, and things are progressing. Last week I asked Andy if he had a chance to evaluate them yet, and I received the following:

Hi Bob,

Yes we have.. : )

The front mounted etalon was completely reworked. We took the etalon apart and re-assembled using new spacers. The bad news is it took 10 attempts, the good news is that we were able to get the etalon to a uniform and excellent tune. Only a very slight tilt will be required.

The internal etalon is very close to be ideal being just slightly high.
To compensate for this we are going to re-house the etalon and change it's tuning point slight lower.

I imagine that this will be completed by end of next week.

I did test the IR ERF and found the surface to be very good.

Let me know if you have any questions.

Best Regards,

Andy Lunt

Hoping to have additional updates in the coming weeks. Again -- much appreciation to Andy for his efforts... Two thumbs up!

[marktownley]

Really good news Bob, i'm excited for the project

[Bob Yoesle]

Finally had a break from snow and rain this past weekend, which presented an opportunity to check things out through the "new" filters. Although seeing and transparency were very poor, temps rose to ~ 10 C, and I did manage to verify much better performance!

The front 140 etalon comes on band with minimal tilt, and by rotating ("clocking") the etalon cover with the Baader DERF160 I can shift the ERF reflection off-disc so that additional tilt is minimized. There is another out-of-focus internal reflection, which I think is caused by the SM90 RG630 ERF I added to the 90 mm internal etalon -- I may have to give it a bit more tilt after I upgrade it to a Lunt RG630 ERF 105 with IR blocking... or I may forego the internal ERF for a Baader H alpha RG substrate filter placed on my BF30 with the KG3 ITF replacement.

Both etalons are now essentially on-band, and the full disc uniformity is excellent!

Shown above: Milky skies hampered contrast, but the uniformity and CWLs are about right-on, with just a hint of a double-limb.

As noted by Andy above, the internal etalon remains fixed, and has had it's CWL blue-shifted slightly by mechanical pressure-tuning with the addition of a foam ring, which can be compressed by the collimator lens cell. The tuning can be adjusted using the hydorgen spectrum tube, and getting the inner ring down to being just barely a "donut," with only a hint of a central hole.

Looking forward to a better opportunity to check the system out when atmospherics are better. Ultimately I may consider a vacuum tuning system for the internal etalon, but this will require some additional thought, as the mechanical implementation will require a major engineering effort or new etalon housing and/or OTA. It may be unnecessary with additional fine-tuning and better seeing/transparency.

A special thank-you goes to Andy Lunt and Brian Stephens for their outstanding efforts to support, inform, and "make it right."