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C8 / SCT Focusing

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#1 sailracer_98

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Posted 19 June 2022 - 01:23 PM

I have a C8 OTA that I bought used a few years back.  It's the only SCT I've used and focusing it precisely is very hard.  There seems to be lots of lash and uncontrolled movement making it hard to achieve precise focus.  It seems to take ~3/4 of a full rotation of the focus knob when changing direction before the mirror starts to respond, and when it does start to move it seems to take a bigger than expected jump.  Is this normal for C8s or SCTs in general or is there likely a problem with mine I should investigate?



#2 jimr2

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Posted 19 June 2022 - 01:48 PM

Is definitely a problem with your C-8, but I don't know enough about them to tell you how to fix--hopefully, someone will chime in here who does know....good luck!



#3 LukaszLu

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Posted 19 June 2022 - 05:06 PM

Hi, start by thoroughly distributing the grease by moving the mirror several times back and forward as far as it will go.


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#4 ccwemyss

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Posted 19 June 2022 - 09:03 PM

Is there significant image shift when it does move (e.g., arc-minutes instead of seconds)? 

 

Chip W. 



#5 sailracer_98

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Posted 20 June 2022 - 06:53 AM

Is there significant image shift when it does move (e.g., arc-minutes instead of seconds)? 

 

Chip W. 

I'll have to go double check this.  From memory, if I'm looking at Saturn at about 200X the planet might shift around by two or three diameters when adjusting the focus knob.  So that is maybe 30"?



#6 sailracer_98

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Posted 20 June 2022 - 08:13 AM

Hi, start by thoroughly distributing the grease by moving the mirror several times back and forward as far as it will go.

This may have worked!  I've had nothing but cloudy nights since I got the telescope back out, but the moon is out right now so I went and did some testing on it.  I moved the focuser through its entire range (way more turns than I would've expected) and now it is behaving much better and seems to be reacting immediately when reversing directions.  Thanks for the suggestion waytogo.gif


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#7 ccwemyss

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Posted 20 June 2022 - 08:14 AM

That's not bad shift. As others have mentioned, it could just be stiff grease, and it sounds like you have it fixed. 

 

A large shift could indicate that the primary is (partly) detached from its mount (which is glued to the back of the mirror at three points). When that happens, it doesn't move directly with the focus screw. Instead, the retaining ring that's on the slider moves it, and the ring has to be slightly loose to avoid pinching. So the mirror can just tilt for a bit before it starts moving. A check for this is to center a star near the meridian in the view (using a reticle) and in the finder, then flip the OTA and re-center in the finder. If it's far from the center in the view (or not even in the view) then the mirror is tilting more than the flop that is normal for the slider. 

 

Chip W. 


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#8 LukaszLu

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Posted 20 June 2022 - 05:45 PM

This may have worked!  I've had nothing but cloudy nights since I got the telescope back out, but the moon is out right now so I went and did some testing on it.  I moved the focuser through its entire range (way more turns than I would've expected) and now it is behaving much better and seems to be reacting immediately when reversing directions.  Thanks for the suggestion waytogo.gif

Great! I'm glad I could help :-)


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#9 gnowellsct

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Posted 24 June 2022 - 03:25 PM

This may have worked!  I've had nothing but cloudy nights since I got the telescope back out, but the moon is out right now so I went and did some testing on it.  I moved the focuser through its entire range (way more turns than I would've expected) and now it is behaving much better and seems to be reacting immediately when reversing directions.  Thanks for the suggestion waytogo.gif

You can pull the corrector, pull the mirror, clean off the old grease and put on new.  Then distribute it by moving the mirror up and down.  Pull the mirror and wipe off any excess on the butt or on the bottom of the c8 tube to make that grease blob doesn't migrate in hot weather.

 

Here's a procedure:  https://astromart.co...sct-childs-play

 

Greg N



#10 gnowellsct

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Posted 24 June 2022 - 03:41 PM

I will add that it is objectively a PITA to focus a c8 at about 200x or higher.  It's one of the things I always notice when moving back and forth from my c8 to my c14.  The c14 is much easier to focus at that magnification.

 

High quality focusers on refractors and Newtonians finesse this issue with precision machining and dual focus reduction knobs.  

 

There's no precision machining on the c8 that I can see.  There are three or four dozen hacks to increase fine control that work "kinda."  You can buy the focus wheel from scopestuff.com.

 

I got this hack in the plumbing department at Lowe's.    The FT fine focus does nothing to give you actual fine focus because the mechanism on the other side of the casting remains crude.

 

If you really really really really want fine focus control you need to get an aftermarket crayford focuser that fits on the behind of the SCT.  You tighten the mirror down and then just use the aftermarket focuser.  I've never gone that route because you lose some field of view by moving further out from the tube and SCTs are already starved for field of view.  Some day I might try it out of boredom.  

 

I did shell out $300 or whatever it was for the FT fine focus for my C14 and that was a colossal waste of money.  After about 8 years I took it off.  I was just lazy, once it was on, it focused, but really, no better than OEM.  No disrespect for FT.  I have FT on three of my refractors and it's primo.  But the SCT stuff is a bust.

 

Here's an example of a hack.  Notice how elegant it is, and just adds that je-ne-sais-quoi to the overall sleek appearance and quality of the scope.

 

c8 focuser hack.jpg

 

 

 

 

 

 


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#11 davidmcgo

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Posted 24 June 2022 - 06:27 PM

One trick with the original slip fit diagonals Roger Tuthill told me way way back was to spin the eyepiece a bit with up or down pressure as a pseudo helical.  Doesn’t work with heavy eyepieces or undercuts.

 

What I really like now is my Baader T2 prism diagonal with the focusing eyepiece holder.  That really lets me dial it in.  One of my C11 tubes came with a Feathertouch Micro on it and even that doesn’t help as much as you would think since once you want to reverse direction there is a lot of movement of the fine knob to take up the slack and then easy to overshoot again.  So either a separate focuser on the rear threads or a focusing diagonal work best for me.

 

Dave


Edited by davidmcgo, 24 June 2022 - 06:29 PM.

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#12 Bomber Bob

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Posted 24 June 2022 - 06:35 PM

+1 on the Baader Diagonal.




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