Help with ghost image / internal reflection

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uplooker
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Help with ghost image / internal reflection

Post by uplooker »

Hi All,
I have found that my TAL 100RS / PST Stage 2 mod has a ghost image / internal reflection. It is more obvious when I attach my DMK31.
The strange thing is that it disappears when I tune the etalon. I am sure someone has had a similar problem but I am unable to find the thread.

I would be grateful to anyone who may be able to provide advice as to how I can eliminate this. Unfortunate i did not manage to get an image of it.

On a slightly different note - I am shaving another 5mm off the rear etalon adaptor in an effort to get a camera to come to focus.

I look forward to any input.

Ian


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Re: Help with ghost image / internal reflection

Post by Merlin66 »

The ghosting can be caused by reflections between the rear of the etalon and the IFT front surface. Tuning the etalon slightly changes the tilt and hence the ghosting.
Mark suggested to me earlier to use a CP (polarising) filter to reduce the ghosting.
This worked for me.....

What's the current back focus between the rear of the etalon and the camera?
(I found that although the DMK specs say it has a 17.5mm backfocus body, that this applies to the added C thread adaptor and that the distance between the fixed shallow "ring" on the front of the cube and the CCD plane is actually 12.5mm)


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Re: Help with ghost image / internal reflection

Post by marktownley »

Have you tried tilting the ERF a bit more?


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Re: Help with ghost image / internal reflection

Post by peter drew »

I found that rotating the ITF holder could move some reflections out of the field, rotating the diagonal to different posiions produced a similar effect, both possibilities are independent of etalon tuning. I've also noticed a dim circular reflection that varies with eyepiece to eye distance suggesting a reflection from the eyeball. :)


colinsk

Re: Help with ghost image / internal reflection

Post by colinsk »

I don't want to state the obvious here but just incase someone is searching this thread later:

All flat optics create a ghost image. This image either needs to be tilted off the optical axis or removed with polarization filters. Air spaced etalons use the former, solid the later.

Since there are so many flat optics a method of rotating the tilt angles in relation to each other can be helpful. This is called clocking. Clocking is done by starting at the front of the telescope and rotating each reflection till non are in the field of view from the eyepiece.

Using a eyepiece that is not larger than .75 deg real field of view can help get ghosts outside the field of view since the sun is only .5 deg.

The polizeration trick loses about 40% of the incoming light (This should read only keeps 40%, whoops). A diagram can be found in the solid etalon article in the link in my signature.


uplooker
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Re: Help with ghost image / internal reflection

Post by uplooker »

Thank you very much for the replies. I certainly have a few options to look at.
I had it in my head that it was the rear of etalon that was causing it, as the reflection disappeared when I tuned the etalon, an d had no idea how to eliminate it.

I will try my best (clouds permitting) to try out peoples suggestions.

Again, thank you all very much for you responses.

Ian


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Re: Help with ghost image / internal reflection

Post by Allcart »

I think you are using a 75mm sub aperture ERF. Try front mounting it in a diy cell and see if the reflection goes away. I had a horrendous double reflection when I mounted the ERf sub aperture in one of my scopes. When I front mount it on any scope, I have no problems. I never actually front mounted it on the same scope I had problems with, as there were many problems with that one, so I moved on to another scope.


Allan
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Re: Help with ghost image / internal reflection

Post by uplooker »

Hi Allan,
I understand your suggestion and yes you are right I am using a 75mm internal ERF,but I am not keen to limit the apperture to 75mm by front mounting the ERF.

On another note, I have seen your posts here and elsewhere regarding your 152mm Lunt mod. What a beast :rockon:

It certainly looks to be doing the business, judging by the solar images you have posted. Well done Allan

Ian


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Re: Help with ghost image / internal reflection

Post by Merlin66 »

Ian,
I'm sure the suggestion was made to allow evaluation of the outcome, not necessarily as a permanent fix.
If the ghosting is significantly reduced then you know the sub-diameter positioning/ tilt is contributing to the problem.


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Re: Help with ghost image / internal reflection

Post by Allcart »

Thanks Ken, that is exactly what I meant. Its just a process of elimination to find the cause.

@Ian, if I can get my scope to produce images of the quality of Viljo's scope, I will be extremely happy.


Allan
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