Lunt LS100 ghosting now with polarizer

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Lunt LS100 ghosting now with polarizer

Post by Gert » Sun Nov 01, 2015 4:19 am

Hello All,

I showed the prev state of improvement of the ghosting issue in the LS100 in this thread:

http://solarchat.natca.net/viewtopic.php?f=10&t=17982

Following user 'Carbon60' recommendation I attached a rear facing circular polarizer (Amazon photo accessory) to the front nose piece of the Lunt DS unit. Just in front of the newly updated red glass with high efficiency coatings. There is enough clearance in the LS100 OTA to insert the DS unit with the polarizer attached.

Visually the image was noticeably darker. Physics allow a 50% max transmission of a polarizer with incident randomly polarized light. Alas the Amazon filter may not be perfect so an additional darkening happens. Visually the ghosting was very much eliminated so I went on to photographic tests. Using the same DSLR attachment and camera settings I went through a test series with 120deg rotations of the DS unit. I estimate the exposure penalty to about 3x. Somewhere between the (expected from physics) 2x and some penalty from the non-perfect transmission of the Amazon filter. When I had prev. 1/4sec to 1/8sec I now need 1/2 sec to 1sec to achieve similar image intensity. Nicely the ghosting is reduced to a non-noticeable level.

Trying to get my head around this I ask myself why Lunt isn't endorsing this kind of fix. (They didn't mention it to me in numerous phone calls and emails) I can only assume that the substantial darkening of the image would bring in more customer complaints than then ghosting does.

Alas, below is the series of images (bottom row to compare)

Clear Skies,
Gert

Image
Last edited by Gert on Sun Nov 01, 2015 4:20 pm, edited 1 time in total.

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Re: Lunt LS100 ghosting now with polarizer

Post by marktownley » Sun Nov 01, 2015 9:05 am

Glad to hear the polariser works, alas at the expense of brightness.
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Re: Lunt LS100 ghosting now with polarizer

Post by Carbon60 » Sun Nov 01, 2015 9:07 am

Hi Gert,

Maybe you could try using an imaging camera rather than DSLR. For example, my DS 60 with acA1920-155um runs at around 6ms. Your 125-1000ms seems excessive!

By the way, the image link doesn't seem to work.

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Re: Lunt LS100 ghosting now with polarizer

Post by zorgdotnl » Sun Nov 01, 2015 5:31 pm

As the gost doesn't turn with the DS module, did you try to test the influence of the BF filter orientations ? We tried several BF orientations on several scopes (Lunt100+BF3400, Lunt100+BF600, Front coronado+BF3000) and there was an optimal orientation for optimizing the resulting bandwidth.

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Re: Lunt LS100 ghosting now with polarizer

Post by Gert » Sun Nov 01, 2015 6:48 pm

Fixed the image link. Should be OK now.
Exposures with the DSLR were longer on purpose to analyze the ghosting.
Observations in mission-mode will use the USB DMK camera.
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Re: Lunt LS100 ghosting now with polarizer

Post by marktownley » Sun Nov 01, 2015 7:11 pm

One thing I think this does show is that the reflection is between the 2 etalon units, shame this can't be sorted by Lunt instead of us band of modders coming up with solutions. It's a darn expensive setup for it to not be working properly.
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Re: Lunt LS100 ghosting now with polarizer

Post by Gert » Sun Nov 01, 2015 7:32 pm

@marktownley
marktownley wrote:One thing I think this does show is that the reflection is between the 2 etalon units, shame this can't be sorted by Lunt instead of us band of modders coming up with solutions. It's a darn expensive setup for it to not be working properly.
Oh well, I could start marketing the Lunt-DS-Ghost-Buster and sell Amazon polarizer filters and maybe a screw-on cell at a monstrous profit. :-)

Cheers,
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Re: Lunt LS100 ghosting now with polarizer

Post by Carbon60 » Sun Nov 01, 2015 8:04 pm

Looking good there, Gert. I'm looking forward to seeing the DMK images.

It does confirm that the ghosting comes from reflections between the two etalons, just as in my own case with my modified Lunt35. Glad the fix has worked, even at the expense of brightness.

Stu.
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Re: Lunt LS100 ghosting now with polarizer

Post by Gert » Mon Nov 02, 2015 4:50 am

@STU,

I am keeping the images in my 2015 solar observing blog.

http://drgert.dyndns.ws:8000/sun_2015/s ... _2015.html

Clear Skies,
Gert
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Re: Lunt LS100 ghosting now with polarizer

Post by Carbon60 » Mon Nov 02, 2015 7:34 am

Nice images, Gert.

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Re: Lunt LS100 ghosting now with polarizer

Post by Bob Yoesle » Mon Nov 02, 2015 11:47 pm

It does confirm that the ghosting comes from reflections between the two etalons, just as in my own case with my modified Lunt35. Glad the fix has worked, even at the expense of brightness.
Great results Gert!

Unfortunately, double stacked systems are complex and finicky, with multiple trade offs. For maintaining the brightness you have without the circular polarizer (linear polarizer and quarter wave plate) and relief from significant ghosting, one might have to resort to a 100 mm front (objective) mounted DS etalon which is meant to be slightly tilted to remove ghosts and also be on-band with the resultant small amount of tilt. Tilting of internal etalons is more problematic, and why pressure tuning is so superior...

I note that the internal reflection between the etalons is blurred, as would be caused by the collimation optics with the internal etalon system. I also observe no change in the out-of-focus ghost image with changing the clock position of the DS unit, probably indicating the reflection originates off the primary etalon (naturally).

With front etalons, no blurring occurs and the retro-reflections are equally focused with the primary image:
DS ghost.jpg
SM90 - SM90 front etalon double stack.

With a little additional tilting, the front DS ghost image is usually easily removed from the field of view, but there is a delicate balance of just enough and too much tilt, which can introduce some contrast non-uniformity, and ultimately "banding" if the DS etalon is tuned too "high," requiring too much tilt to come on band. This was the origin of the "factory matched" DS etalon requirement with front etalons - having a second etalon tuned just slightly higher for the purpose, but not too much higher.

For internal modules George9 on Cloudy Nights has also found a little bit of fiddling with tilt on the LS80DSII has been helpful:
... so I use a few layers of paper as a shim to force a tilt between the spacer and the telescope body. This pushes the reflections to one side or another of the sun.
http://www.cloudynights.com/topic/50198 ... try6621681
... shame this can't be sorted by Lunt instead of us band of modders coming up with solutions. It's a darn expensive setup for it to not be working properly.
The best solution for internal DS systems would be to have the other SS filter components designed for the brightness level that would be encountered when using a circular polarizer to eliminate reflections with the DSII etalon. When used in SS mode, an appropriate ND filter could be used to decrease the image brightness. Then for DSII use, the ND filter is removed/omitted and you still would have a decent level of image brightness, and no reflections due to the presence of the circular polarizer - at least in theory ;-)
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Re: Lunt LS100 ghosting now with polarizer

Post by Gert » Tue Nov 03, 2015 4:16 pm

Hi Bob,

I'm shocked. Lunt asks $5k for a front 100mm double stack unit and then we see a double-star Sun. Wow!
My guess is that Lunt is balancing between building DS unit WITH polarizer and folks complaining about too dark an image and WITHOUT polarizer and complaints about ghosts. Between a rock and a hard place! :-(

I would not want any ND filter in my scopes. For outdoors visual observation with H-A the dark towel is almost a must. Any darker is not desirable.

I am building a threaded receptor for the polarizer filter on the DS unit nose part to selectively mount it when I want it. I might only use it for full disk imaging, and alas I have the LS60 for that. For small field of view imaging the problem is out of sight out of mind.

Thanks & Clear Skies,
Gert
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Re: Lunt LS100 ghosting now with polarizer

Post by Bob Yoesle » Tue Nov 03, 2015 8:58 pm

Sorry Gert, I think you misunderstand my intention with showing the image above, which is to demonstrate that all double stacked etalons produce ghost images - it is inevitable from the optical physics.

The question then becomes how do you eliminate or mitigate them. As I also stated, the front DS etalon is usually chosen - or purposefully intended - to be tilted to remove the ghost image(s) from view, as can be seen in th DSLR image below where they well out of the filed of view:
IMGP3550 crp adj sm.jpg
IMGP3550 crp adj sm.jpg (198.74 KiB) Viewed 5447 times
Moreover, the image does not suffer from the additional 2+ x brightness reduction you have discovered with the use of a circular polarizer. Front etalons will also generally suffer less contrast uniformity issues from small to moderate amounts of tilt, as the field angles are not magnified by collimator optics, and/or experience increased instrument angles. Therefore, front etalons are generally in the location for optimum performance, but also have the highest cost to make per aperture, as the cost goes up proportional to surface area, not diameter.

The LS100 was designed to be an internal SS and external (front objective mounted) DS. As you have discovered, the more recently developed internal DS'd etalons are a bit trickier to implement (but less costly), and the circular polarizer is a good solution for elimination of the retro-reflections (ghost images), as tilting generally is not as good solution with internal etalons. Unfortunately you are implementing it with a SS etalon system that was not originally intended to be used with an internal DS etalon with a circular polarizer, and therefore the image brightness is not what it would have been if the system were purposefully designed for such a DS retro-reflection suppression solution.

See: http://luntsolarsystems.com/etalons-ext ... -internal/
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Re: Lunt LS100 ghosting now with polarizer

Post by Bob Yoesle » Thu Nov 05, 2015 4:16 pm

Hi Gert,

The images you obtained without the circular polarizer got me thinking, and while the HRG filter did reduce the intensity of the ghost, it seems that it remained unchanged in position (overlapping and intruding into the primary image), although I do note a slight degree of variability in the location of the center of the ghost image over multiple posts. Other commentators have noted they have observed the same reflection, but it is farther out of the field once the DSII unit is installed.

I just found the instructions for the LS100 DSII module, which state the following:
The DSII utilizes a unique optical design that works to narrow the band pass while also removing the back-ghosting that would normally occur with 2 highly reflective etalons sitting in such close proximity to one another. This ghost removal would typically be done by tilting one of the etalons with respect to the other. The optical design works to move the “ghost” images to an area outside the normal field of view. However, users who have low magnification eyepieces (~25mm), or eyepieces with a wide field of view may notice this “halo”. Increasing the magnification or using an eyepiece with a narrower field of view will reduce this effect... (Emphasis added.)
http://luntsolarsystems.com/wp-content/ ... DSII1.docx

I'm not an expert on these matters; but I am curious about how this stuff works, and in my reading tilt and circular polarizers are the only methods I've found used to remove the retro-reflections between etalons. Tilt is well know for DS etalons, especially on the front mounted (ahead of the objective) etalons. Circular polarizers are well known to be used with the mica type DayStar and Solar Spectrum filters due to the bifringence of these materials ( http://www.designerinlight.com/solar/Et ... clePt2.pdf ). Circular polarizers are also used professionally between FP etalons in larger telescope systems:
isoon_optics.jpg
isoon_optics.jpg (219.73 KiB) Viewed 5400 times
While the wording is a bit vague, it sounds like Lunt might actually be using some built-in tilt to move the retro-reflexion(s) off the central field of view with the DSII modules. I'm unaware of how else one could "move the 'ghost' images to an area outside the normal field of view." And the wording "move...outside the normal field of view" verses 'eliminate' or 'suppress' seems to suggest use of tilt - just as is done with front etalons.

For a front etalon, it would take about a 0.75 (3/4) degree of tilt to completely remove this ghost image from a 1 degree field of view covering a whole solar disk, as any tilt angle supplied to the DS etalon will be doubled in reflection off of the primary etalon.
DSII ghost tilt req.jpg
DSII ghost tilt req.jpg (49.18 KiB) Viewed 5385 times
It seems from the image of the ghost position with the DSII that the center of the ghost lies about 1/4 a solar diameter off-center (~1/8 of a degree), and therefore the DS etalon itself would be tilted about 1/16 of a degree -- or about 1/12 of what would be required in the foregoing postulation. This seems like it could possibly be out-of-spec (perhaps indicates a mix up with a primary internal etalon cell/housing vs. DSII etalon cell/housing?) Regardless, the ghost does not appear to be "outside the normal field of view."

The perplexing thing here would be that if the DSII etalon is indeed tilted to any degree, I would have thought the ghost would have rotated with the change in module position -- and it did not. This leaves another possibility -- that the ghost is originating from the blocking filter diagonal (BG filter, long-pass dichroic filter mirror, order selection "blocking" filter, RG filter) -- which apparently maintained the same orientation to the camera throughout the imaging-rotation sequence. The mystery deepens -- and again I'm not a expert, and these are essentially guesses on my part. I would be sending images directly to Lunt to try and have this sorted out, as what you are seeing and imaging (without the CP filter) is definitely not what one would expect from the system - as stated by Lunt themselves.

In any event, you and Stu have discovered that one of the two "conventional" ways to eliminate retro-relfections between etalons - a circular polarizer - works very well, but with the resultant reduction in image brightness. My concern is that the optical quality of the Tiffin photographic CP filter might be OK for low power or full-disk use, but might not be too great for hi-res work.* Therefore you might want to also try removing the CP and applying some additional tilt (via shims, etc.), as George9 does with his LS80 DSII, and see what happens. The only caveat is that since you are also tilting the monolithic collimator-etalon-refocus system, other optical aberrations may come into play -- so I would be cautious with the amount of tilt applied so as to only have the ghost moved sufficiently off the main image disk.

* The B&W Kaesemann CP filters would be my choice for the purpose, as they appear to be of a higher quality standard:

http://www.the-digital-picture.com/Revi ... lters.aspx
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Re: Lunt LS100 ghosting now with polarizer

Post by Valery » Fri Nov 06, 2015 4:28 pm

Thanks, Bob, for your great help.


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Re: Lunt LS100 ghosting now with polarizer

Post by Gert » Sun Nov 08, 2015 3:38 am

Hello Bob Yoesle,

Thank you for the very detailed discussion. You write :
The perplexing thing here would be that if the DSII etalon is indeed tilted to any degree, I would have thought the ghost would have rotated with the change in module position -- and it did not.
That is correct. But recall that in prev discussion I showed photos with rotated BF and nothing happened.

See discussion here : http://solarchat.natca.net/viewtopic.php?f=10&t=13805

The most probable hypothesis is that the ghost is create by the interaction of primary vs secondary etalon. If the surface causing the ghost in the secondary has 0 tilt it explains that no rotation effect is observed when turning the DS unit. That leaves a rear surface of the primary etalon as the most likely suspect. It might be tilted by the 1/8th degree that Bob postulates. It is unknown if that is by design or by accident.

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Re: Lunt LS100 ghosting now with polarizer

Post by Bob Yoesle » Mon Nov 09, 2015 2:48 am

Hi Valery & Gert,

The orginal LS100 would by design have an adjustable tilted etalon for tuning, and was intended to be double stacked with the tiltable LS100F front (objective mounted) filter:
LS100 DS.jpg
LS100 DS.jpg (94.34 KiB) Viewed 5253 times
In a pressure tuned telescope, there should be no tilt to the primary etalon. However, there are other plano components that are tilted, such as:

12. The RG630 ERF ("hot mirror" coated) ahead of the etalon,

36. The ITF or an analog for filtering longer IR (now a BG filter),

27. The "long-wave pass" diagonal mirror,

34. The order selection interference filter (on RG630 substrate).
Lunt LS100 schematic.jpg
Any of these might be the cause of a spurious reflection if not tilted the proper amount, given the complexity of adding the DSII into the mix. Both the collimator and refocus lenses also have a plano surface, and perhaps one of these has a missing AR coating...

And again as I pstulated, perhaps the secondary DSII etalon itself needs some additional tilt.

The only reason the primary PT etlon might be angled is that one of the two-part OTA sections was assembled with some misalignment, or there was a bit of sag when the etalon was "potted" with silicone supports in the etalon cell. In any case, these would be repairable conditions that should be covered under warranty.

I would urge you to send your images of the ghost and rigorous testing methodology directly to Lunt via Faye Heyde:

faye(dot)h(at)luntsolarsystems(dot)com

Although you have the circular polarizer fix, hopefully they will have some additional insight and would be able to correct the issue at the source. They are good people and I'm sure will want to see that the scope works as intended and that you are happy with its performance.
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Re: Lunt LS100 ghosting now with polarizer

Post by Derek Klepp » Tue Nov 10, 2015 10:06 am

Thanks Bob for the in depth explanations .I think I will stick with my SS PT Lunt 100. Good luck Gert.

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Re: Lunt LS100 ghosting now with polarizer

Post by Gert » Sat Nov 14, 2015 8:12 pm

Hello Again,

Thanks to all who posted comments and supporting material and illustrations.

I did mention in the prev post that between my post on 'Mon Aug 10, 2015 2:34 am'

here : http://solarchat.natca.net/viewtopic.php?f=10&t=17201

And this post started on 'Sun Nov 01, 2015 4:19 am' I had the entire LS100 shipped to Lunt for lab-analysis and repair.

Their initial discovery was that what was shipped to me as the 'updated HRC Glass' unit was indeed a mis-shippment. What had been picked up from the shelf in the shipping department was indeed a 'clasic style glass'. In the lab-analysis they mounted the true 'updated HRC glass' and realized the improvement it made. That improvement was deemed final and the scope returned to me. Along the way I had positive communication with their tech-staff. They even provided me with a loaner BF1200 to use with my LS60 while the LS100 was with Lunt. (For cost reasons I only purchased the LS100 without BF as I already have one from the LS60 and didn't want to buy two of them without need).

At this time I am working on an aluminum cell to make the polarizer filter in the DS unit by-demand mountable so that I can chose when to use it (darker image without ghost) and when not. (brighter image with some ghosting)


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Gert
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Re: Lunt LS100 ghosting now with polarizer

Post by Gert » Thu Nov 26, 2015 7:31 am

Hello All,

One update. I have collected the texts and photos on a little web blog on my astro-hobby page.

http://drgert.dyndns.ws:8000/lunt_ls100 ... ghost.html

When I have new photos with the polarizer in place I will post them there as well.

Clear Skies,
Gert
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Re: Lunt LS100 ghosting now with polarizer

Post by GUS » Thu Nov 26, 2015 7:56 am

Thanks for sharing your trials with the DSII unit Gert, this will help others.

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