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Help with the function of a focal reducer and WO focuser with a 11 sct

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#1 TonyCaf

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Posted 17 September 2019 - 05:00 AM

Hi everyone.

 

It seems like the aperture bug bit me and I have upgraded from my Ultima 8 to an 11 sct mounted on an Orion Atlas! What a big difference (and heavy). Anyways, the person i bought the OTA from had previously installed a WO focuser. I decided I wanted to experiment with a focal reducer to get a "faster" scope. First night out I used it with the focuser and eyepiece in a straight through configuration and found that I only had to move the primary focus on the OTA a few turns to achieve focus. All is well and good with the world. Although it looks funny behind the WO focuser I decided to leave it on. NEXT NIGHT i put a right angle on and find that I had to CRANK the focuser on the OTA Wayyyyyyyy (in/out not sure) to achieve focus. The WO focuser had the effect of blurring the stars even more out of focus as I cranked it out. It almost seems like the focal length wants to be SHORTER.  I didn't like that I had to crank the mirror the entire way (out/in?) to get focus with aa right angle.. Is there a way I can just keep the reducer on and have it be convenient for both visual and astrophotography work? Can someone explain if I should ditch the WO focuser in favor of a simple visual back and rely on the OTA focuser only. Would that be a better solution to this convenience  issue. Or said in another way would it be better just to put a standard 1.25 visual back on it and be done with the WO focuser? Those things are heavy and LONG! Hope I am making sense here. Any suggestions would be appreciated.



#2 mclewis1

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Posted 18 September 2019 - 07:40 AM

Focal reducers require a specific spacing between the reducer and the eyepiece/camera sensor to achieve their specified reduction. Many folks don't use the full spacing and thus often experience a little less reduction. The more aggressive the focal reduction (the higher the amount of reduction) the more inward the focal point moves.

 

So it makes sense that when you added the diagonal you likely increased the spacing between the reducer and your eyepiece. This increased the amount of focal reduction which moved the focal point inwards (requiring substantially more turns of your SCT's internal focusing knob to get into the general area of focus).

 

I would mount the focal reducer after the focuser and use a diagonal as normal, keeping the amount of spacing as fixed as possible. Different eyepieces have a different position for the field stop which determines the amount of spacing with the focal reducer so there can be a slight increase in the amount of focusing movement required, but it shouldn't be a large amount.

 

Putting the reducer in front of the focuser can be easier mounting wise but when you adjust the focus using the external focuser it then varies the spacing dramatically which means moving the focusing all over the place and this often increases the spacing over what the reducer is designed for.

 

If you want to measure the focusing distances involved (and the potential differences in focal reduction) try this online calculator - http://www.wilmslowa...rmulae.htm#FR_b


Edited by mclewis1, 18 September 2019 - 07:42 AM.


#3 TonyCaf

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Posted 18 October 2019 - 04:22 AM

Sorry for the late reply to this topic. I just wanted to update what happened. I considered the suggestion of mounting the reducer after the focuser but there did not seem to be any physical way to screw on the reducer to the end of the focuser.  SO... I did have a 1 1/4 visual back on my 8 inch sct. I just took off the focuser and mounted the visual back after the reducer and all seems well. Now I can swap between visual and photographic use with a few turns of the original focuser. The good part is that I have the ability to swap back and forth between my eypiece and camera with just a few turns the bad is that I have lost the ability to fine focus. By the way, it seems this is the type of configuration that the reducer was meant for as per the instructions that came with the reducer.. I have since sold the focuser to a good home and will consider updating the OTA tube's original focuser with something else..



#4 Chris Johnson

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Posted Yesterday, 09:09 AM

Moonlight makes a focuser that mounts the focal reducer in the focuser.




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