1st of 3 questions Not achieving perfect focus

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uplooker
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1st of 3 questions Not achieving perfect focus

Post by uplooker »

Hi All,
This is the first of three posts. They are all concerned with different aspects of the TAL 100RS PST Stage 2 Mod that I have. I am making them as separate posts to try and ensure that any answers are as concise as possible.

Not achieving perfect focus
With the setup shown in the photo I found that I could only just achieve focus with the Borg 7315 focuser fully wound in. I shortened the rear etalon adaptor by 5mm so that I had focus adjustment either side of the point of focus. (Note: The Borg focuser only has 10mm of travel). I found that I was able to achieve focus slightly better, but not perfectly. I have posted previously about collimating the standard TAL focuser with the objective. The collimation is marginally out, I have just not had sufficient time to correct it.

Question:
Would this be the main cause of me not achieving perfect focus??



brianb11213

Re: 1st of 3 questions Not achieving perfect focus

Post by brianb11213 »

I suggest you try a low profile 2" to 1.25" adapter.

A "marginal" collimation error will not stop you achieving something like a good focus, even a gross collimation error will let you see where the best focus is - it's just that the sharpness will not be what it should be at the best position.

Too much length in the optical train will cause more issues with a C mount camera like your DMK31 than with most eyepieces. If you haven't already done so, remove the C-CS adapter ring from the camera to shorten the optical path by 5mm. And try a shoulderless C mount nosepiece.


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Re: 1st of 3 questions Not achieving perfect focus

Post by Merlin66 »

Can you dump the diagonal and make a straight through extension - one less variable.

You should also check that the front and rear lenses in the PST etalon assembly haven't become loose...this has happened on a couple of mods - re-tightening fixed the problem.


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Re: 1st of 3 questions Not achieving perfect focus

Post by marktownley »

have you shortened the tube on the tal? I think looking at the picture you have way too much distance on the ep side of the etalon.


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uplooker
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Re: 1st of 3 questions Not achieving perfect focus

Post by uplooker »

Thank you all for your replies.

[quote=marktownley post=77811}have you shortened the tube on the tal? I think looking at the picture you have way too much distance on the ep side of the etalon.

Yes I have shortened the TAL tube. I struggle to understand how I have too much distance on the EP side of the etalon. This would mean that I would not be able to achieve focus. I can get focus of sorts and I am able to go either side of said focus. It is just that it is not as sharp as I would like.

I understand that the light / image comes to focus 200mm to the rear of the etalon, measured from the rear of the knurled rubber band on the tuning ring. Where does an eyepiece need to be in relation to this 200mm point to get focus.

Ian


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Re: 1st of 3 questions Not achieving perfect focus

Post by marktownley »

How do you ensure the PST front lens in front of the etalon is 200mm inside the focal point of the Tal?


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Re: 1st of 3 questions Not achieving perfect focus

Post by Merlin66 »

Ian,
the focal plane/aperture stop of the eyepiece will be at the 200mm spacing....


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