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Moonlite focuser backfocus problem

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#1 highkamp

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Posted 26 September 2020 - 06:02 AM

I am using 12 inch lx200 SCT with a moonlite focuser that allows me to insert my .63 FR inside the focuser. Problem is that I cannot then add an OAG and filter wheel without exceeding the recommended 105mm spacing between asi1600 camera and the FR. I have tried using the thinner OAGs but I'm still 10 or so mm too much (around 115 or 120 mm backfocus). Is it OK to have a bit more spacing than required? I'm fine going from a .63 to a .50 focal reduction if it means I can keep my filter wheel and OAG. Thanks

#2 sg6

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Posted 26 September 2020 - 06:24 AM

You can have more but the image will not be correct, Suppose you may not get an image but would expect one to be achieved somewhere that you could put the sensor.

 

Is the FR supposed to be used with a focuser? Would have more expected it to be attached at the flip mirror and so "bypass" a focuser and the additional path introduced, then focus by moving the SCT intarnals around.

 

The actual reduction, 0.63 or 0.5, may be slightly irrelevant as it depends where the reduced image is created not how big it is.

 

I would rather unfortunately suspect that the focuser is using up too much of your available back focus separation and there just isn't sufficent left for the items you want to include


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#3 ChrisWhite

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Posted 26 September 2020 - 06:46 AM

You should test this before drawing any conclusions. 

 

With a small sensor like the 1600, you might be okay.  On my Edge 925 I was 6mm too far with spacing due to my focuser/imaging train and stars were still perfect on a 1" size sensor.  I doubt it would have been fine with full frame, but with a small sensor I had no problem at all.  This was with the .7x reducer as well as native f10. 

 

Because I recently upgraded to a full frame sensor I decided to swap out the focuser so I could free up more backspacing.  Went with the Optec LEO. 


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#4 drd715

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Posted 26 September 2020 - 06:56 AM

The distance between the Focal reducer flattener to the sensor is a design parameter for that specific focal reducer flattener. It may not be exactly the length specified, but it will be close. The flat field created only happens at one spacing between the FR and the sensor. You will have to experiment with the spacing to find the exact correct distance for flat even focus across the entire sensor. It will be specific and very slightly different for each telescope. You may want to find the perfect distance with spacers only and then see if you can fit the filter wheel and OAG to this defined spacing. The actual reduction number for a reducer is a design factor for that specific reducer and not adjustable- it is what it is.

For focusing using an inline focuser and the FR inside the focuser you should be able to reach focus, but if not the primary mirror to secondary mirror distance can be slightly reduced BUT the secondary mirror has to be large enough in diameter to reflect the full illumination of the light cone and the FR has to be large enough in diameter to pass the full diameter of the light cone needed to cover the size of the sensor you are using. Larger sensors are more demanding both in getting a larger flat portion of the light circle and in recieving an un-vignetted fully illuminated light circle. Getting the whole optical system to work together can be challenging.

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#5 Jim Davis

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Posted 26 September 2020 - 08:26 AM

If you can't get a good focus, you can contact Moonlite and see if they can make you a shorter draw tube. We have had them customize their focusers for certain applications.


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#6 ChrisWhite

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Posted 26 September 2020 - 08:28 AM

If you can't get a good focus, you can contact Moonlite and see if they can make you a shorter draw tube. We have had them customize their focusers for certain applications.

It's not the drawtube that is the problem, its that the focuser body takes up too much of the backspacing.  Even their CHL/Litecrawler body with the low-profile takes up a lot of backspacing.    Why I had to switch to a LEO, which only takes up 1.35" of backspacing with the drawtube at center extension. 


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#7 Jim Davis

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Posted 26 September 2020 - 08:49 AM

It's not the drawtube that is the problem, its that the focuser body takes up too much of the backspacing.  Even their CHL/Litecrawler body with the low-profile takes up a lot of backspacing.    Why I had to switch to a LEO, which only takes up 1.35" of backspacing with the drawtube at center extension. 

I use the same focuser as the OP, as far as I can tell. There is plenty of room to shorten the tube. You are using the Edge version, not the 2" SCT version. In the image it is move all the way in, but in use it can be racked out pretty far. The reducer is attached inside on the other end of the draw tube.

 

MVIMG_20190727_203238.jpg


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#8 ChrisWhite

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Posted 26 September 2020 - 08:53 AM

I use the same focuser as the OP, as far as I can tell. There is plenty of room to shorten the tube. You are using the Edge version, not the 2" SCT version. In the image it is move all the way in, but in use it can be racked out pretty far. The reducer is attached inside on the other end of the draw tube.

 

attachicon.gifMVIMG_20190727_203238.jpg

Same concept as mine though.  The OP stated he cant get a short enough backfocus with the focuser and his imaging train.  Are you saying that the reducer is attached directly to the drawtube on the scope side of the drawtube?  If so, then a shorter drawtube ABSOLUTELY would make a big difference. 


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#9 AZ Maverick

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Posted 26 September 2020 - 09:23 AM

I have no answer, but I do have the exact same problem on my classic Celestron 9.25" SCT with the Moonlite SCT focuser.

But I knew before I ordered it that there was no way I was going to make 105mm of back focus with my ZWO OAG, ZWO EFW, and ASI1600mm Pro - the closest I can get is 124mm.

I don't know if there is a solution, for now I'm going back to a separate guide scope even though that isn't the ideal guiding solution.


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#10 Jim Davis

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Posted 26 September 2020 - 12:33 PM

Same concept as mine though.  The OP stated he cant get a short enough backfocus with the focuser and his imaging train.  Are you saying that the reducer is attached directly to the drawtube on the scope side of the drawtube?  If so, then a shorter drawtube ABSOLUTELY would make a big difference. 

Yes, it is attached to the drawtube. Image of it here: https://focuser.com/...s/FR-spec-n.jpg


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#11 highkamp

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Posted 27 September 2020 - 01:15 PM

Thanks for all the suggestions and input.  I guess like was mentioned above I can do some tests with blank spacers (no OAG, FW) and see what the images look like then decide from there whether its worth pursuing further.  I know Starizona makes a large format FR for bigger SCTs that has plenty of backfocus for all the other parts of imaging train,  but it will not fit inside the 2 inch Moonlite focuser that I have,  I can contact Ron about whether or not it would fit in the 2.5 inch focuser that Moonlite offers, but again, more complexity and cost for questionable returns.  We can send a man to the moon, but cannot seem to allow SCTs to have FR, Crawford, FW, and OAG all play nicely together. At least I still have a reliable refractor that plays nicely! CS All!


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