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Flip Mirror Issues


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Hi everyone.  I wondered if anyone could comment on the following problem I have getting focus on a flip mirror diagonal.

I have both a 8" Newtonian and a 5" Short tube refractor (600mm) and both are f/5.   Using a Samsung SCB2000 with a C-mount to 1.25" adapter with a screw on IR filter on the front, I am able to get

the camera  to focus with no issues using a standard 90 degree diagnonal.   So I purchased a parfocal 90 degree flip mirror diagonal to attach both visual EP's and camera

at the same time.    

Problem is with the camera attached to the straight through port I cannot get the camera anywhere near focus, while if attached it to the 90 degree port, I can get focus (but only just) with no room for fine adjustment.   This is the same issue on both scopes.

I am wondering if the problem is because the flip mirror diagonal is parfocal?  

I am puzzled too why the camera should (just) come to focus in the deflected path but is nowhere near it when attached straight through even though the distances involved between the sensor frame and the the focuser seem to be roughly the same . 

Any words of wisdom greatfully received !   :smiley:

Steve 

SkySpy

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Sounds like you have a back focus issue, looks like there is a bit of difference in the length that the light path travels between the 90 degree position and straight to the back... for some reason the back must be slightly longer.

All I can suggest is for you to remove any unnecessary spacers or bits that cause your ccd to be further back, such as your your IR cut filter, place it at the front of the flip mirror rather than on the camera body just before your sensor. That small amount might be just enough for you to have you eyepiece and ccd parfocal.

I had a similar issue with my refractor back focus when I used the filter wheel and the OAG, the back focus was literally just slightly off, what fixed the issue was removing the adjustment ring off the filter wheel which is 3-4mm thick, that was enough for me to have focus when the tube was about 0.5mm from completely retracted.

Hope that helps.

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I had the same issue with my old refactor which was a 100mm f6. The standard 90 deg diagonals usually add quite a bit of length to the optical train. Find out the optical length of your 90 deg diagonal and the flip mirror diagonal from the manufacturer. You will need a spacer for the difference to compensate or if possible get a different flip mirror with longer tubes - that will likely be the best solution.

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Steve,

Which flip mirror are you using?

Thanks to everyone for your comments.. particularly MarsG76, the suggestion to move the IR filter to the front of the diagonal draw tube seems kind of obvious now...  but I didn't think of that one!

Its got to make a difference, though one I have yet to try out.

Merlin66 you asked which flip-mirror am I using... truth be told... I have no idea!

Its a budget buy, although bought from a respected on-line seller of astro specials, he described it simply as a par-focal flip mirror which was best just used for polar alignment as the mirror itself did not give a

full reflection (it is offset),  I have tried to show this in the photos below.   As I was only going to be using this for basic alignment of the visual and imaging fields I did not think this would be a limitation.

Wether the diagonal was designed that way or was a manufacturing fault I dont know, but it resulted in the diagonal being pretty cheap in comparison to most others.  It fitted my budget at the time, but maybe it

was false economy (..I think most of us have been there!).

post-44227-0-88269000-1448972221.jpg

post-44227-0-02379900-1448972229.jpg

I will try the set up again with the IR filter moved (if I ever get some clear sky!) but failing that I may have to invest in another flip diagonal, perhaps one where I can attach the sammy directly to the diagonal casing and do away with the C-mount to 1.25" adapter (...if that is possible).  Again any suggestion or recommendations, thanks in advance  :smiley:

Steve

SkySpy

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Maybe try a flip mirror like this: http://www.bhphotovideo.com/c/product/385680-USA/Vixen_Optics_2680_Flip_Mirror_System.html

Honestly after that first experiment with a flip mirror I never used one again. Much easier to use a bhatinov mask and focus. I cannot see anything visually anyways from my light polluted location.

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