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Setting correct backfocus distance = can't focus

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#1 rottielover

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Posted 16 September 2019 - 08:20 PM

I have a really strange problem that's causing my brain to hurt.  When I set my backfocus correctly then I cannot get focus, but if I remove about 10mm it works (remove the tip/tilt plate):

 

So here's my setup.

 

ES 127ED Triplet with a moonlight 4.5" drawtube focuser.  I have in my image train:  ES 3" Flattener / Reducer, ZWO Tip/Tilt Plate, 20mm spacer, 8slot EFW, ZWO 1600mm-cool.

 

The Tip/Tilt plate should be 10mm, the spacer is 20mm, the filter wheel is 20 mm and the camera has a 6.5 mm spacing.  That should be 56.5 mm of backfocus space (camera says +-5mm is OK).

 

If I set things up this way my focus point seems to be closer to the end of the scope than my focuser can travel (IE it's bottoming out at 1mm of travel).

 

If I remove the tip/tilt plate I can get focus with about 15 or 16 mm of draw tube travel.

 

 

I'm at a loss.



#2 DSOs4Me

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Posted 16 September 2019 - 09:38 PM

I have a Stellarvue with a Flattener / Reducer and it comes with a 10MM spacer attached. I almost made the mistake of setting my back focus from that because it just had the Stellarvue name on it and I didn't realize at first that it was a spacer.


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#3 gilbertgrape87

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Posted 16 September 2019 - 10:14 PM

Do you have the 1.75" extension that came with the focuser installed?

#4 pfile

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Posted 16 September 2019 - 10:14 PM

this is a thing - the reducer not only takes up some physical backfocus depending on whether or not it can fit into your focuser drawtube, and it also takes up some backfocus by virtue of the fact that it's a reducer.

 

it's certainly possible that you have used up the ES127ED's available backfocus. i have this problem with my AT10RC when i try to use the tip-tilt collimation adjuster, my moonlite CSL and an AP focal reducer.

 

do you know the stated amount of backfocus available on the ES127ED? you can subtract everything you have from that number and if the number is negative, it just won't work.

 

rob



#5 rottielover

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Posted 16 September 2019 - 10:29 PM

Gilbert - no, I purchased the adapter for the ES 3" from Moonlight so it's screwed directly to the focuser draw tube.

 

Best I can figure Pfile must be correct the reducer must have some amount of "backfocus to it" or inherent in it.  When I remove that extra 10mm of the tip/tilt plate I focus at around 16mm of travel and I'm getting about 1.3 FWHM



#6 gilbertgrape87

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Posted 16 September 2019 - 10:51 PM

No, that's not what I meant.

 

Your focuser should have come with an extension ring that goes between your OTA and the Moonlite.

 

See attached picture.20190916_204945_compress98.jpg



#7 gilbertgrape87

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Posted 16 September 2019 - 10:52 PM

I'm holding it backwards and the picture is sideways, but you get the idea :)



#8 pfile

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Posted 16 September 2019 - 11:51 PM

you can compute the amount of backfocus eaten by the reducer's optics if you know the focal length of the reducer. the other half of the equation is the spacing to the reducer. the formula is on this page:

 

http://webcache.goog...n&ct=clnk&gl=us

 

i included the google cache link since the original page might be down... it's under the "focal reducers" section.

 

anyway gilbertgrape87 has a point, there may be some spacers you can remove. it looks like this reducer may be too wide to fit into the moonlite's drawtube - but if you can get it in there then you can hide the mechanical length of the reducer and win back that backfocus.

 

rob


Edited by pfile, 16 September 2019 - 11:51 PM.


#9 rottielover

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Posted 17 September 2019 - 08:16 AM

gilbert -  THANKS!   good lord I'm an idiot!



#10 gilbertgrape87

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Posted 17 September 2019 - 08:59 AM

Haha, don't sweat it. I've done plenty worse :)

Glad it was an easy fix.
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#11 rottielover

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Posted 17 September 2019 - 09:45 AM

Haha, don't sweat it. I've done plenty worse smile.gif

Glad it was an easy fix.

 

Hopefully anyway :)   I have to "re-arrange" the image train again but shouldn't be too bad.   This definitely feels like the culprit though.



#12 View2

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Posted 17 September 2019 - 03:40 PM

I had the same strange problem and with an explore scientific 127. I didn't have enough in focus when I use my hydrogen Alpha filter and a i think the focal reducer. I ended up buying a flange to add length. I can't remember if I bought it from explore scientific or from the feather touch folks. I still have to use it occasionally.

Edited by View2, 17 September 2019 - 03:41 PM.


#13 View2

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Posted 17 September 2019 - 03:42 PM

I'm pretty sure I got mine from Starlight instruments because it matches the focuser knobs and everything

Edited by View2, 17 September 2019 - 03:42 PM.



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