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Locking the focus


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#1 handymanCT

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Posted 29 September 2014 - 04:34 PM

Has anyone locked their focus to use an electronic focuser like optec or Finger Lakes?  If I just put the bolt back in that my scope was shipped with is that good enough for locking it? 

 

Thanks

Rich



#2 Mark Sibole

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Posted 30 September 2014 - 09:59 AM

What scope???


Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com

#3 handymanCT

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Posted 30 September 2014 - 10:26 AM

12" ACF



#4 Mark Sibole

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Posted 30 September 2014 - 01:38 PM

OK very helpful.If uou use the seperate focuser than the Meade focuser yes you would lock the focuser when you get to your mid point of focus for your secondary focuser.

I have mine set up with the meade focuser fo i ran the meade electric focuser to the mid point then used the focuser on the OTA to gain focus.Then I locked the mirrow down.Now all of my focus adjustments are made with the electric focuser.Only  when using a seperate focuser you will lock the Mirror and NOT on the electric focusers that mount on the original focus control.

 

 

Regards

 

Mark

 

PS a suggestion when doing this.

check to see what gives you the best image when locking the mirrow down.If it is better in the infocus or out focus position.It does make a bit of difference as the mirrow will slightly shift in each focus direction.


Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com

#5 handymanCT

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Posted 01 October 2014 - 08:02 AM

Thanks for that information  I wish i knew how much back-focus there was in this thing.



#6 cmiller46

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Posted 07 October 2014 - 12:35 PM

Mark:

 

Can you elaborate a bit on locking the mirror of an LX850-ACF OTA using the shipping bolt?  I thought the shipping bolt held the primary mirror in either its fully foward or fully backward position.  You are saying the shipping bolt will lock the mirror in any position?  Can you discuss how the shipping bolt works with the internal crayford focuser?  I have looked down the tube of my 12" LX850 OTA and can see how the crayford bearings work on the central baffel tube but don't see how the shipping bolt works.

 

Thanks:

 

Charlie



#7 cmiller46

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Posted 07 October 2014 - 12:53 PM

HandymanCT:

 

I have never been able to find a Meade published spec on backfocus.  Here is what I can pass on -- I have a 10" LX850-ACF configured with an Optec TCF-Si focuser, an Optec Pyxis LE rotator and an SBIG ST-2000XCM camera.  I recently did a plate solve using Visual Pinpoint and it said my plate scale was 0.78 arcseconds / pixel.  Working backwards, that tells me the effective focal length of the imaging train is 1950 mm or f/7.68.  The 10" ACF f/8 should have a focal length of 2032 mm.  This says my camera's position is inside the focal point the scope would have if the camera were sitting precisely at the f/8 position.  I am not at my scope but the best guess of the distance from the top of the 3.25" threaded back to the chip in the camera is around 5.25 inches.  This would suggest the "optimum" backfocus distance is around 5.5 inches.  My further guess is that the backfocus could be moved to 6 inches without causing any optical degradation.

 

Charlie



#8 handymanCT

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Posted 08 October 2014 - 03:10 PM

Charlie that's great information.  I read some spec some where that said the back focus was 3 or 3.5".  Of course that was in an online document some where and now I can't locate it again.

 

I took the visual back off of my scope and replaced it with an Astro-Physics 27TVPH Telcompressor.  So according to George over at AP I should be at about 0.75x compression...probably 0.78x or thereabouts.  So roughly about f6.3 now.

 

I also know that putting this in the light path changes the back focus more.  So at this point I have no idea what it is. 

 

Thanks

Rich



#9 cmiller46

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Posted 08 October 2014 - 08:22 PM

Rich:

 

I believe there was a consensus that the LX200-ACF OTAs did have an optimal back focus of 3.5 inches.  My speculation is that Meade seriously targeted the imaging market with the LX850-ACF product and knew a longer back focus would be needed.  So the switch to the f/8 optical design was coupled with the longer back focus I measured on my LX850-ACF.

 

I have my OTA on an AP 900GTO mount, so I am familar with the AP focal reducer.  It appears to work differently than the focal reducers that have very critical spacing requirements.  For example, the Optec Lepus reducer for the LX200-ACF requires the camera chip to be at exactly 105mm from the lens element. 

 

The 27TVPH has a reduction value that is variable and depends on the spacing between the reducer lens and the camera chip.  AP has a chart on their web site that shows a graph of spacing versus reduction factor that could be useful in setting up your imaging train.

 

Charlie



#10 handymanCT

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Posted 09 October 2014 - 04:12 AM

Charlie

I didn't know about the graph.  Is this what your talking about?  http://www.astro-phy...67 & 27TVPH.pdf



#11 cmiller46

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Posted 09 October 2014 - 05:48 AM

Rich:

 

Yes, that is the graph I remember looking at.  As you can see, the camera chip needs to be about 6" back from the reducer to start getting any real reduction benefit.  To me the important point is that the exact spacing is not critical unless you are after a precise redcution factor.

 

Charlie






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