First images with Lunt60 double stack -- anyone else had a problem this like?

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First images with Lunt60 double stack -- anyone else had a problem this like?

Post by thesmiths »

I recently got a LS60F to double stack on my LS60T (with pressure tuner) with B1200. I've had the LS60T for about 2 years and it's performed really well so I decided to get the LS60F (at rather large expense) to try out double stacking.

On my first attempt, I've noticed that although there is clearly an enhancement in the surface detail (as expected), there is also a strong "sweet spot" effect, not unlike what I recall with my old PST. Have I not tuned something correctly? Or is it likely that there is something misaligned with the front mounted filter? Has anyone else had similar problems with the Lunt60 double stack?

The following is a double stack image taken with a DMK51 using Registax6 and very minor Photoshop. The 2 o'clock direction has a darkening which I just can't get rid of by adjusting the tuner on the front filter:



If I use more Photoshop, I can get rid of most of the gradient and the image looks not half bad:



A little more fiddling with Photoshop results in some nice detail of both the surface and the "oak tree" prominence at 11 o'clock (date was 18 Nov, time was 11:02 UTC):



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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by thesmiths »

The problem is more noticeable if I overexpose to see the prominences better. Now the 9 o'clock position is very overexposed:



In this case, more fiddling with Photoshop can't really fix the problem so well:



I think there is an optical problem as well since the whole surface of the sun's disk is actually not in focus, a problem I never had with the single stack configuration. Perhaps this indicates a misalignment of some element in the front filter?


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by swisswalter »

Hi Douglas

nice shots. I had the same issue not only with a LUNT LS50 also with hte SM90 when double stacking. Have you tried to rotate the second eatlon. that should at least weaken the effect


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by marktownley »

It's the angle and orientation of the external etalon. Like walter says you need to rotate it to find the optimum position...


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by astrodanco »

From recent experience, rotating an external double stack unit can make a big difference.


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by thesmiths »

Thanks for the comments about rotating the front etalon. I can't recall there being any mention of this in the one sheet of instructions that come with the LS60F. I will try this next time there is sun again in London (could be a while).


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by Merlin66 »

Alexandra was experiencing similar focus issues with her double stacked SM40/PST..


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by thesmiths »

I got the following from Kevin LeGore at Woodland Hills (who sold me the LS60F filter): "Usually caused by a tilted etalon. As the others said, try rotating the front 60mm filter a bit. We had to do the same on a LS100 double stack. Once we rotated about 1/4 turn it really poped."


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by marktownley »

Good good! making a gasket from some paper etc to interface between the threads on the LS60F and the scope itself is a good way to find the ideal rotation of the external etalon yet still retain a tight fit on the threads ;)


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by Merlin66 »

A set-up we use on the spectroscope beamsplitters may be of interest....
Ideally we want to align the axis of the beamsplitter to the slit gap, but this is fitted using T threads...guess what...when you tighten it up completely it never sits where you'd like it! The pitch of the T thread is 0.75mm, so a spacer of 0.75/4=0.19mm would reposition the axis by 90 degrees. An easy solution we found was to fit a 1mm O ring over threads and compress this on the joint. It has the benefits of both allowing "fine" rotational adjustment, basically self-locking and also prevents the threads from binding!
Worth considering.


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by marktownley »

Indeed Ken, you describe more eloquently what I was trying to say. This is also what Alexandra needs to do to get the best rotational alignment of the the DS etalon with her PST...


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by Merlin66 »

I get all my bearings and O rings from Simply Bearings in the UK. Excellent service.
http://simplybearings.co.uk/shop/Seals- ... index.html

(I note the 1.5mm O ring goes up to 100mm diameter)


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by JohnC »

I have the same setup,and experience the same issues.Should these filters supplied for double stacking be certified for double stacking by Lunt.Mine dosnt have a certified sticker.Has yours? I have a theory that a single stack was sent by mistake.
Have others received a certified filter when ordering for double stacking?

Regards john.


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by swisswalter »

Hi John

I had two of the LS50Ha and never found a sticker stating certified for double stack


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by Merlin66 »

Hmmmm
The filters should work OK....
I just use two standard SM60 filters (double stacked) on my ED80, no issues, no problems....


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by solarchat »

I meant to respond to this earlier. The Lunt etalons that end with an "F" ie..., LS60F, are all designed to be used as an external double stacking filter. They tune them just slightly higher than the non "F" etalons.

Lunt does not match etalons normally. I am sure that you could send yours back and that they could find the best pair to send back to you but I doubt it would make any difference in the uneven illumination problem. The image you first posted looks about as good as any image I have seen and I suspect that you already have a really good etalon set now.

Any time you put a tilt tuned etalon in a system you get an area that is more on band than the rest. It is much more apparent in a full disk image especially if you use a focal reducer to obtain it. This effect is in every one of my 14 solar scopes from three manufacturers and is just part of it. The effect is minimized with higher focal ratios and more magnification as you tune it so that the entire area that you are focusing is inside the "sweet spot/band".

There are a bunch of different ways to alleviate some of this effect and I generally try to get the brighter area in the center of my full disk shots if possible. I have also found that this effect is caused by blocking filter unevenness as well. I own 5 B3400's or BF30's from Lunt/Coronado and every one of them has a different "best area" on them. It seems to me that the larger the blocking filter, the more unevenness becomes apparent. If I was buying over again I would limit myself to just BF15s or B1800's as they seem to be about the largest size blocking filter you can get and retain uniformity. My absolute best scope uses a 15mm blocking filter.

I would suggest that you rotate the etalon around each time you use it to find the best placement of the sweet band and that you move the scope around to put it into the best part of the blocking filter each time you use it. If you happen to be using a B600 with this scope then there is really nothing that you can do to help the image as you are always right to the edge of its surface area and it will never be uniform at full disk. That's why they recommend the B1220 as a minimum for imaging.


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by JohnC »

I had a 60mm DS filter delivered about a year ago.This one had a certified sticker on and the full disc was on band.
Unfortunately the etalon was scratched,so I returned it to Lunt,who replaced it with a filter without a certified sticker.This one gives this banding issue.Not sure if they still supply these with a sticker.John.


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by swisswalter »

Hi John

welcome to that wonderful TOS free site. Have sun, Have fun


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Second images with Lunt60 double stack -- still a few issues

Post by thesmiths »

On December 2, I managed to do more imaging with my "double stack certified" front etalon. Unfortunately, from London there is a very short window for solar viewing this time of year. I tried the rotation trick mentioned above and this does definitely help. Around 40 degrees rotation seems to optimise things. Of course then the front etalon is slightly loose and could use some spacers or shims to adjust the angle. In fact, I found this angle was one more parameter that could be adjusted for optimisation (but the window for experiments was just too short). Here is a full disk image taken with the DMK51, Registax6 and a little Photoshop:



There is still some uneveness but I could move it to the bottom where there wasn't too much detail anyway. I believe with more sophisticated Photoshop work this could be evened out. Is this about as good as can be achieved with a pressure tuned rear etalon and tilt tuned front etalon?

I then put in a 2x Coronado Barlow and discovered a new problem. There were stripes, presumably Newton rings, all over the image. I guess with the addition of the Barlow there were now lots of almost parallel pieces of glass causing internal reflections:



The stripes are more obvious in real time since the positions do not move as I scanned across the surface. I did not have time to figure out exactly where these reflections were coming from. Any idea how to get rid of them?

This last image is my favourite one so far. Again, there is some uneveness in brightness and the stripes but definitely more detail than I would normally get with single stack (and very little sharpening has been done in any of these images to bring out details):



So overall, I would say that double stacking is a bit of a pain, not as easy as I had expected. There is definitely a learning curve (perhaps I am only at the beginning of the curve). My impression is that for the same amount of money, it might be more worthwhile to go for more aperature rather than go for double stacking (ie a Lunt 80mm single stack PT costs about the same as a double stack 60mm).


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Re: Second images with Lunt60 double stack -- still a few issues

Post by swisswalter »

Hi Douglas

fine you move on. Now the newton rings can be get rid of by tilting the camera very little. See post in this forum solar chat main index


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Re: Second images with Lunt60 double stack -- still a few issues

Post by marktownley »

Hi Douglas,

If you get that spacer / gasket made up and fitted you will find with that 40deg rotation you will improve things still further, the gasket being in place is key. Try dropping the gamma on the camera right down when you are setting up the tilt and orientation of that external etalon. That will help you to see when the field illumination is even.

When you say 'is this as good as can be achieved?' - what are you hoping to achieve? You will find there's less contrast with the 80mm.

I would possibly look at other stacking software too...

Regards the flat fielding I do this with my DS40 http://brierleyhillsolar.blogspot.co.uk ... ields.html

Mark :)


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Re: Second images with Lunt60 double stack -- still a few issues

Post by thesmiths »

Hi Mark,

In fact, I had just watched your video tutorial before my last post (which is why I said "with more sophisticated Photoshop work this could be evened out". I'm not at a very sophisticated stage working with layers, etc., which is why I hope to get the best data as possible! I have taken a quick look at Autostakkert2 but I've been using Registax since version 4 (for planets) so I'm a bit lazy now. But as I think others have pointed out, Registax seems to not do a great job of aligning very large detailed images (solar and lunar). Great idea about the flat frames. That's definitely needed with double stack.

Douglas


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Re: Second images with Lunt60 double stack -- still a few issues

Post by Bob Yoesle »

Hi Douglas,

Lots of issues need to be addressed with double stacking.

Banding can be caused by two issues.

The first issue is inherent in the filter itself, and is beyond your control. It is the parallelism of the filter plates themselves, and this is determined by how accurately the etalon plate spacers were made and applied. If the spacers introduce a slight lack of parallelism between the etalon plates, it will result in some banding no matter what. I would check the filter by itself (i.e. without double stacking) on another scope with a suitable blocking filter. Even though it may have a “high” native tuning, it should present relatively uniform contrast across the field of view. If not, there may be a spacer issue and it should be returned for evaluation.

The other thing that introduces banding is the need to tilt the filter excessively to get rid of the secondary ghost reflections. As the filter is tilted the bandpass across the filter can change, creating a “sweet band” through the center of good contrast. This can be remedied by the aforementioned rotating - otherwise known as “clocking,” which may reduce the amount of tilting required to get rid of the ghosts, and thereby reducing or eliminating the banding due to this cause. Mark's suggestion about using a gasket is good, and inexpensive. I have dealt with this by making a special DS adapter - See:

http://www.cloudynights.com/ubbthreads/ ... PHPSESSID=

For me, double stacking is “the only way to fly”, as the elimination of parasitic photospheric light leakage provides superior results. Single stack 0.7 A FWHM of left, double stack



Note the disk of the photoshere sneaking through in the image on the right, which creates the "double limb" effect. Also note that prominences are just as visible at the narrower bandpass. Both pairs of L-R images used identical processing.

Regarding the last full disk image you posted above, the bright area at the bottom of the disk might be due to an internal etalon sweet spot, and these may be exacerbated by any residual banding from the front etalon... See Andy Lunt's excellent discussion on internal etalons, pressure tuning vs. tilting, etc., here:

http://luntsolarsystems.com/blog/intern ... erformance

Contrast uniformity with double stacking can be as good as a single narrow band filter, it will just take a little more effort to achieve. Here’s a 6-pane SM90/SM90/BF30 full disk mosaic taken on June 15, 2012 using and Orion ED 100 and PGR Chameleon. No flat-fielding was used (lucky me:)




Keep at it,

Bob


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Re: First images with Lunt60 double stack -- anyone else had a problem this like?

Post by JimLafferty »

Douglas
I see similar things with my Lunt 100 (PT)/BF3400 double stacked with a tilt tuned Coronado 90 external etalon (Luntanado). Stephen's explanation and solutions are pretty much my experience too. I mitigate most of the darkening issue (upper right of the solar disk in your image) by adjusting BOTH the PT and the external etalon (rotation and tilt adjustment) slightly. Adjusting just one or the other doesnt do it for me. I also sometimes move the disk slightly on the camera's chip towards one edge of the chip (to the left on your image) so the illumination is more even. You can always copy and paste the disk onto a new canvas in photoshop to recenter it again.
I also found Maxim DL's flattener tool to be effective in mitigating the effect as well. An expensive piece of software to get just for that but if you already have it, give it a shot as well. I have mine left over from my nighttime imaging days.

Jim


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