A testphoto for ghosting in Lunt LS100PT with DS

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Gert
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A testphoto for ghosting in Lunt LS100PT with DS

Post by Gert » Sun Nov 09, 2014 4:01 am

Hi All,

I always wanted to do some more tests on the ghost images in my new LS100PT with the DS unit.
To try this out I took a baseline image without the DS unit and no other auxiliary optics like barlows etc.
I used the Canon DSLR which is non-typical for H-A solar work, but it gave me the biggest sensor I could get.
The exposure gives a reasonable representation of the view in a low magnification eyepiece for full disc viewing.
Next I added the DS unit (PT model) and took same exposures as baseline with the DS unit in 3 different rotations.
Primary and DS PT unit were of course adjusted for best H-A.

See the attached photo. There is a surprisingly strong ghosting which does not rotate with the DS unit but is absent in the baseline. So it must be some reflection from a surface in the base unit that gets illuminated from the DS unit.

I would like to call out to the user community if folks are seeing similar ghosting in LS100 with PT-DS.
(Not sure if LS80 with PT-DS is a valid equivalent)

Image

Clear Skies,
Gert
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by swisswalter » Sun Nov 09, 2014 4:34 am

Hi Gert

that looks heavy
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by GUS » Sun Nov 09, 2014 10:11 am

Hi Gert, did you try rotating the BF to see if it's a reflection from the back of the DS Etalon of the BF.


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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by OTangen » Sun Nov 09, 2014 8:59 pm

Hi Gerd!

I have the same equipment here in Norway and would like to do the same tests as you have done. Unfortunately the weather forecast is not very good for the next 10 days. But I will be on alert and see what I can do. My BF is mounted at the entrance of the of the focuser, so it can be rotated easily. I have a Ha modified EOS40D that I can use (I also have an unmodified EOS20D if the Ha sensitivity is to large on the EOS40D.
I know that I have some ghosting, but I see it mainly when there is slightly overcast, so this is not caused by internal reflections. I do also notice that your images are highly overexposed and with normal exposures, are you sure that the internal ghosting will be such a big problem for you?
As said earlier, I will try to perform tests when the weather permits. One issue though, is that since the sun now is only 17 deg above the horizon at noon, there will be a fair amount of haze that will cause the earlier described ghosting in my images.

Best regards
Oivind Tangen, 60N, 10E

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Gert » Mon Nov 10, 2014 8:03 am

Hi Gus,
GUS wrote:Hi Gert, did you try rotating the BF to see if it's a reflection from the back of the DS Etalon of the BF.
No, I haven't thought of that yet.
But that sounds like a good plan for next weekend.

Clear Skies,
Gert
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Gert » Mon Nov 10, 2014 8:05 am

Hi Pedro,
pedro wrote:Try using other BFs.
I only have 1 BF. Maybe I'll ask Lunt if I can borrow one from them.
(I had shared my tests with them but never got a reply)
(Not sure if they're monitoring this forum)

Clear Skies,
Gert
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by GUS » Mon Nov 10, 2014 9:26 am

Hi Gert, I also want to double stack my LS100 as well and will follow this thread with interest.

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by GUS » Tue Apr 21, 2015 10:22 am

Might be of interest, there is a thread over at Cloudy Nights(about NEAF 2015)in the Solar section regarding a new anti reflection filter for the DS11 units for the Lunt scopes, inserted just behind the main Etalon, hopefully it might solve some of the glare and reflection problems some are experiencing with these units.

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Carbon60 » Tue Apr 21, 2015 11:28 am

A circular polarizing filter could help here. I inserted one between the two etalons of my modified DS Lunt35 (with 150mm objective and collimating lens feeding the Lunt). This can significantly reduce unwanted reflections. I just obtained a photographic grade glass filter and inserted it 'facing backwards' so that reflected light from the rearmost etalon becomes circularly polarized as it reflects forwards, through the filter and onto the front etalon before reflecting back towards the filter again and through the second etalon towards the eyepiece. The action of the filter kills this second reflection from the rear face of the front etalon. Hope this makes sense.

I don't know if this is practical in your case because of larger etalon diameters, but it probably describes the same principles of the 'anti-reflection' filter Gus speaks about.

Stu.
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Bob Yoesle » Tue Apr 21, 2015 2:36 pm

A quarter wave plate and linear polarizer = circular polarizer. These are used routinely in the mica etalons ( http://www.designerinlight.com/solar/Et ... clePt2.pdf ) but become very expensive if sourced for the typical optical supplier in the larger sizes. A good quality photographic CP will be good for smaller sizes. Hopefully the new Lunt CP's will be backwards compatible with the existing DS units -- this will be the best solution for these retro-reflections between internal etalons.
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Gert » Sat Apr 25, 2015 8:36 pm

Hello All,

My apologies for the long delay updating the material from my initial posting.

One commentator asked to identify the ghosting source by rotating the BF and keep the DS unit stationary.

Remember in my first report I rotated the DS unit and then counter rotated the BF with camera back to initial orientation. So only the DS unit could cause the effect. It is interesting to note that the ghost image did not move during that rotation experiment. This opens up the question which optical surface might cause the ghost.

So the new experiment was to keep the DS unit stationary, rotate the BF unit and accordingly counter rotate the camera. The results are below. What surprised me beyond my understanding is that again the orientation of the ghost is not moving. It would be wonderful if the forum experts could help answer that mystery.

Image

Clear Skies,
Gert
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by swisswalter » Sun Apr 26, 2015 5:43 am

Hi Gert

that looks like the DS unit needs some tilt to get rid of the ghost
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Rybak » Sun Apr 26, 2015 6:34 am

Hi All

I see ghost like this one when I put SM90 on the lens cell of my Lunt-100THa (making "Luntonado"). But when I tilt SM90 this ghost disappears from the field of view.
Aleksey

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by DSobserver » Sun Apr 26, 2015 11:17 am

Fully agree : looks you need some tilt!

With my SM90 DS without tilting the entrance SM90 I get same reflexion that couldn't be solve adjusting the internal FP.

But tilting the first one, reflexion can 100% disappear!

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Gert » Sun Apr 26, 2015 7:09 pm

Hello swisswalter and DSobserver,

Thank you for the info. Could you please elaborate if DS tilt is a user adjustment (and how it is done) or if this can only be done at the factory?

Thanks & Clear Skies,
Gert
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by swisswalter » Mon Apr 27, 2015 4:23 am

Hi Gert

you need a tilt adapter between the first and the second etalon in order to tilt the second etalon. That can be done by yourselve if the threads allow for a standardt tilt adapter (T2)
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Gert » Tue Apr 28, 2015 2:43 am

Hi swisswalter,

There is no T-thread connection between the etalons in the LS100. The DS unit is clamped into the tube via a circular dovetail and 3 thumbscrews. Let me get in touch with Lunt to check options.

Clear Skies,
Gert
Last edited by Gert on Wed Apr 29, 2015 1:51 am, edited 1 time in total.
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by OTangen » Tue Apr 28, 2015 9:30 pm

Hi group!

For general interest I can mention that Lunt had a new Heat Rejection filter at NEAF (the red filter in front of the the DS unit) that takes care of the glare when stacking the two filters. It almost reduces the glare to zero. According to Bryan Stephens, chief engineer at Lunt, is reduces the pass band throughput by 40%. That is negligible for visual work and can easily be compensated for doing imaging. I looked through it at NEAF before and after replacing the new filter and it reduced the glare very effectively. I bought one and it works beautifully in my scope too. I paid $ 160 for it. Definitely worth it in my opinion. That might be something for you Gert.

Best regards
Oivind Tangen, 60N, 10E

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Gert » Thu Apr 30, 2015 2:49 am

Hi All,

Talking to Lunt tech support they suggested a test to rotate the camera, ony to exclude the double reflex causing the ghosting is coming from it. They said the new filter from NEAF is an improved stray light filter and will not improve on my ghosting.

Proposal by Carbon60 above to employ circular polarizer look more and more appealing.
Would this one work :

http://www.amazon.com/Tiffen-52CP-52mm- ... zer+filter

Clear Skies,
Gert
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Marco Michelassi » Sun May 10, 2015 5:42 pm

Hi Gert ,
Though not so severe,but I'm experiencing the same problem
that affects your equipment,which is the same as mine.
Perfect without DS unit,ghosting with it.
Not so disturbing for visual use,I'de like so much to eliminate it :
Improved contrast and details are addicting !
I use my scope for observation,and I've never tried imaging with it.
Curiously,the ghosting is almost in the same position.
(I use a diagonal prism,of course).
Considerong that you've already done it,I don't know if it would be worth contacting
Lunt itself,although I've been very favourably impressed by their customer aasistance.
(I bought an used scope,and they replaced my rusted blue filter immediately).
Please let us know of every new-hopefully good ones!
Thanks !
OTA : Lunt 100 DS BF 3400
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Marco Michelassi » Sun May 17, 2015 10:11 am

Stop !
Believe it or not,I solved my problem :hamster:
turning the DS module of about 180°,
So that now it's in the opposite position respect to the other PT knob.
Optical mysteries...
OTA : Lunt 100 DS BF 3400
Mount : Skywatcher Az-Eq6 custom
ZWO-ASI174MM
Visual setup includes Baader Mark V Binoviewer-Zeiss prism and several pairs of Orthos

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Geremia Forino » Sun May 17, 2015 12:12 pm

why a mystery? this demonstrated that your etalon was not perfectly on plane, turning it, has reached a perfect optical plane.
Rather, it seems strange that it has not a tilting system

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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Marco Michelassi » Sun May 17, 2015 4:24 pm

Geremia,
I was ironic... ;)
It's not strange that the Lunt's PT system doesn't have a tilt option,
It's just the way it's made : the pressure tuning replaces the
tilting,and the etalons are fixed.
As explained by Lunt themselves,the concept behind this system is to
avoid the sweet spot issues,mostly deriving from the tilting system which causes
the rays not to come into focus at the same level.
In PT system the etalons are always parallel.
The drawback is that the sky around the sun disc is not dark black
as in tilting system,but shows a diffuse,dim halo in DS mode.
Having owned both,I strongly prefer the PT solution,
mostly for my personal taste and demands.
OTA : Lunt 100 DS BF 3400
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Re: A testphoto for ghosting in Lunt LS100PT with DS

Post by Geremia Forino » Sun May 17, 2015 5:24 pm

Marco Michelassi wrote: It's just the way it's made : the pressure tuning replaces the
tilting,and the etalons are fixed.
:oops: sorry Marco, I had not realized that you had the pressure tuning system, agree with you ;)

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