Tuning animation - Sweet-spot issue rather than tuning inability with Lunt internal DS?

[H-Alpha]

Dear all,
Happy New Year!

Following precious comments and advice by Bob, George and many others, I kept experimenting and testing with my Lunt130MT DS. I am even considering to buy a Hydrogen Spectrum Tube soon for thorough tests (and fun :-) ).

Before passing to the main issue, I would like to note that I tried to test visually both internal etalons independently, but realized that this is impossible for the (added) DS etalon. The reason is that the DERF is attached in front of the original etalon of the scope and although it can be detached, there is no way to attach it to the DS etalon... If you know anyway to do so, please let me know.

Main issue
Trying to capture a full disk with my ASI1600 which has a large sensor (I use the ASI290 for the pictures I usually post here and for the results I will present below), I realized that this was impossible! No way to find an area of the sensor where the larger part of the sun disk could be more or less homogeneous. With SS I always could make it, perhaps not perfectly, but in a very satisfactory way. Ok, I said, I will have to always make composites with DS... However, even this is almost impossible or very difficult. Please have a look at the photo below and you will understand that the sweet-spot is extremely narrow.

2022-12-30-1239_9-h-alpha-sun h-alpha_lapl8_ap136.jpg (338.63 KiB) Viewed 614 times

Coming back to the tuning efforts, I observed that several times I had a part of the limb that had no difference compared to the disc (no 'double limb'), but in other parts of the limb it was impossible to eliminate the differences... I started thinking that my sweet-spot issue was possibly/probably the reason why I could not tune my DS. This is why I made the following experiment. I first removed the second (DS) etalon and left the original. I tuned the original SS. Then I added the DS etalon and started capturing images at different tuning positions of the pressure knob. I started before the point where I believe is the tuning point and I was turning the knob 1/4 of full turn each time. I reached the best tuning point and kept moving inwards for some more captures, until the image became dark and well off tune.

I created an animation of these nine captures (below). As I suspected, around frame 4, the 'double limb' disappeared in some areas (white arrows), but never went away in others (short grey arrows at the south).

Tuning animation.gif (7.46 MiB) Viewed 614 times

1) Can you please tell me if you agree with my conclusion that my problem is the very narrow and rather irregular sweet-spot rather than an ability to tune correctly?

2) Can you please advise me how to solve the narrow sweet-spot issue (if possible), or if I have to sent one or both etalons back to Lunt?

Thanks a lot in advance,
Alexandros

[george9]

Hi. One question. Is the sweet spot narrow or just off-center? I actually put a homemade copper shim between my primary etalon and the DSII unit to tilt the back, and it probably centers my sweet spot. I originally did it to make the reflections concentric, which in my case seemed to minimize them. Now I use a circular polarizer so the reflections are mostly gone. But I still use the shim.

George

[H-Alpha]

Hi. One question. Is the sweet spot narrow or just off-center? I actually put a homemade copper shim between my primary etalon and the DSII unit to tilt the back, and it probably centers my sweet spot. I originally did it to make the reflections concentric, which in my case seemed to minimize them. Now I use a circular polarizer so the reflections are mostly gone. But I still use the shim.

George

Thanks a lot George!

You are right. It is certainly off-center! It maybe narrow as well.
Can you please show me photos of the copper shim you made to center your sweet spot? I may have to make one...
Hmmmm... Are you still referring to your front DS etalon?

Anyway, tilting etalons may make them concentric apparently. Creating more distance between them can also broaden the sweet spot? (I have no knowledge on the theory or the practice on this issue...)

Should Lunt prevent this from happening in first place, or is it too difficult because each etalon is unique?

Bets wishes,
Alexandros

[Bob Yoesle]

I think you'll find the answer is here.

Christian's treatise explains why a front mounted etalon with a CWL close to being on-band with minimal tilt will almost always produce the most uniform full-disc constrast of any air-spaced etalon arrangement, especially when double stacked.

gallery_3892_8222_83338.jpg (490.03 KiB) Viewed 589 times

Double stacked pre-Meade Coronado front etalons.

It has long be realized a circular polarizer will remove some or all of the retroreflections between double stacked etalons, minimizing the need for tilt.

isoon_optics.png (32.91 KiB) Viewed 589 times

My guess is that George's using a shim is "correcting" the built-in tilt of the secondary etalon (tilt seemingly is being used to offset the retroreflections), and then uses a circular polarizer to eliminate the overlapping retroreflections that have been reintroduced by eliminating the etalon tilt...

So you can get a "sweet ring" for either or both an off-band CWL or the tilt-generated "sweet ring." Use of a circular polarizer can eleiminate retroreflections, and if you then get the etalon(s) on band with the correct amount of air pressure and no tilt, you should be able to achieve a hopefully far better contrast uniformity.

What Lunt could do is to forget the HRG filter for reducing the tilt-offset retroreflections, and instead go to using a CP filter with secondary etalons that are normal to the optical axis, and increase the transmission of the blocking filter to compensate for the transmission reduction from the CP filter.

Bob

[marktownley]

Looks like your making some progress. Time and patience will win out. Just remember a scope like the 130 isn't really designed for full disk images so I would expect a sweetspot here.

[H-Alpha]

I think you'll find the answer is here.

Christian's treatise explains why a front mounted etalon with a CWL close to being on-band with minimal tilt will almost always produce the most uniform full-disc constrast of any air-spaced etalon arrangement, especially when double stacked.

It has long be realized a circular polarizer will remove some or all of the retroreflections between double stacked etalons, minimizing the need for tilt.

So you can get a "sweet ring" for either or both an off-band CWL or the tilt-generated "sweet ring." Use of a circular polarizer can eleiminate retroreflections, and if you then get the etalon(s) on band with the correct amount of air pressure and no tilt, you should be able to achieve a hopefully far better contrast uniformity.

Bob

Thanks a lot Bob. All the theory is front of me now, although I will need time to try to better understand it.

In practice, is there anything that could solve my problem?

It is not clear to me where I can put a circular polarizer in the train? Can you please indicate one that is used in such cases and could help?

"if you then get the etalon(s) on band with the correct amount of air pressure and no tilt"
How can I reduce/eliminate the tilt of my etalons? I theory they should mount with no tilt. Isn't it?

Thanks once more,
Alexandros

[rigel123]

Correct me if I’m wrong but the 130 doesn’t have a front mounted DS unit does it? You are using two internal PT etalons, correct? It seems I read about a number of folks that struggle with this configuration due to the fact that it is a bit cumbersome to set the scope up as a SS, tune the primary etalon, and then have to take the scope apart and add the DS unit to tune that one and it is not tilt but pressure you are using to bring it on band. Or am I totally off base?

[H-Alpha]

Thanks Warren.

It is exactly as you wrote!
Two internal etalons and no front mounted DS unit available for Lun130MT.
It is really difficult if not very risky to remove and put back in place any of the etalons, so doing this every few days is out of question!
Fortunately, thanks to previous advice, I feel confident that I have now understood how to tune with both etalons on (without first removing the DS unit). Nevertheless, the sweet spot issue that I have, is still preventing me from enjoying and successfully using the DS unit.

I hope there is a possible and practical solution and George, Bob or someone else with relevant experience will be able to guide me to practical solution.

Best wishes,
Alexandros

PS: I made some captures of full disks in six different positions on the large sensor of ASI1600, so that I can give you a better idea of the problem. Just finished the captures and hope to complete the stacking and post them in a couple of hours.

[Bob Yoesle]

Hi Alexandros,

If you examine the ISOON diagram above, you’ll see the two 150 mm etalons are normal to the optical axis with a tilted polarizer “reflection suppressor” between them.

Unfortunately, assuming it is tilted - you can’t (easily) de-tilt the etalon inside the collimator PT module, and you will need to eliminate the internal etalon tilt of the double stacking module by tilting the module itself with a shim as George describes - you would get the reflections (which should be out of focus due to the collimator lens optics) centered to know when the tilt between the etalons has been eliminated.

Then you place the circular polarizer between the two etalon modules.

If one is available, I'd get the Lunt HRG filter for your particular telescope, as it’s filter cell is appropriately tilted, and remove the filter element itself and replace it with an appropriately sized B+W Kaesemann CP filter. You'll have to look up or find the actual filter diameters from B+W - it won't be the listed filter size, which is the mechanical spec, not the optical element spec. I have found this filter to be difficult to remove, so I'd buy it from the manufacturer or dealer who can remove it for you. This will likely be the B+W distributor for your country.

If an HRG filter cell isn't available for your particular DS module, you will need to have one made or DIY it. I'd tilt the CP 1-2 degrees. The CP will decrease your imge brightness by about 50%, so I'd replace the blocking filter's BG38 filter with a KG3 filter for long IR suppression.

At this point you will have eliminated the tilt-induced sweet-band and etalon reflection issues. The only issue you will have left is the actual circular sweet spot size, which you will not be able to mitigate, as this is inherent with collimator internal etalons, and dictated by the optics which magnify the field angles passing through the etalon. As Mark noted, the 130 may not be optimized for a full disc contrast uniformity, which in general would dictate the collimator focal length(s) are never smaller than 1/2 of the objective focal length, and therefore an etalon that is also no smaller than 1/2 the objectives diameter.

This is assuming of course that you are again able to pressure tune each etalon to get the CWL on-band in the first place, which also seems to remain a question.

Welcome to the world of ultra narrow band etalon filters and their implementation, and the compromises that are dictated by their physics and the need to make them affordable by mere mortals.

Good luck,

Bob

[H-Alpha]

This is what I got with the large sensor of the ASI1600.
Theoretically, I could have an almost full disk in the sweet spot at the middle right of the sensor. But this is at the extreme right edge...
Anyway, when I use the ASI290 the frame is at the very center where the 1/3 left of the image is off band and very bright.
Any idea to fix this? (I tried with an ASI tilter in front of the camera and gained a few milimeters of sensor, but this is not very helpful.

Composite for sweet spot.jpg (992.09 KiB) Viewed 516 times

Best wishes,
Alexandros

[rigel123]

Can you view a full disk visually? Just curious, as I would not think the sweet spot would be that skewed off center.

[DeepSolar64]

Hmm, You all have me thinking. A dangerous thing I know....

Christian's treatise explains why a front mounted etalon with a CWL close to being on-band with minimal tilt will almost always produce the most uniform full-disc constrast of any air-spaced etalon arrangement, especially when double stacked._Bob Yoesle

So true that is. I have a lot of trouble getting both of my Meade Coronados on band with minimal tilt. I think that is one reason I have trouble with disc uniformity.

But tilt should not be an issue with a Lunt pressure tuned system.

[Bob Yoesle]

The primary PT etalon is most likely not tilted, but it appears the secondary DS PT etalon might in order to move reflections between the etalons.

[george9]

Hi, Alexandros. Sorry I was away a few days. Plus I don't have solar chat set up to ping me so I have to remember to check.

The Lunt 130's primary etalon is big enough that a full disk view should work.

I agree with Bob except one correction. The DSII is not purposely tilted by Lunt to shift the reflections. It is too close to the primary etalon to tilt it enough to remove the reflections (I have tried). I was tilting because I found that for mine it was just a little tilted, putting few reflections on one side but too many reflections on the other side. When I perfectly center it, I have reflections all around but they are less bright and objectionable. Then I added the HRG filter first, and then finally replaced that with the CP.

The problem with the CP that there is a little finickiness. E.g., even when you place it in the correct direction, you can get odd effects in your binoviewer due to polarization. So you need to rotate it just right. If you put it in backwards, then the polarization effects are more extreme. With the HRG, it just works. Not as good suppression of reflections, but no complications.

On the tilt, I am away from my scope so I cannot send a photo right now, but I simply went to the hobby store, bought a sheet of copper, and cut a ring whose internal diameter matched the outer diameter of the DSII dovetail area where the thumbscrews hold it to the primary etalon. I then bent the ring along its length on one side to make it a little thicker on that side. Or you can solder an arc of copper to one side of the ring. Whatever it takes to make one side a little thicker than the other.

So it goes between the two etalons. You can experiment with putting a second shim behind the DSII. One shim between the etalons adjusts the primary etalon, and two shims placed 180 degrees rotated adjusts the DSII etalon.

It seems to me that your sweet spot is shifted over more than I expected. You should be able to loosen the thumbscrews and tilt it by hand to see what happens.

I have never used a front-mounted DS unit on my Lunt. I have only used the internal DSII. (I had an old Coronado ASP60 with two etalons front mounted.)

On my Lunt 80, I tighten and loosen the DSII unit for every view because I store it with the PTs on the same side. So I would not be afraid to take the DSII unit on and off a lot, other than wearing out the thumb screws (which I did after 7 years). However, I have learned to tune both etalons at once, so for me no need to take it off to tune.

George