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SCT Focus Advice

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10 replies to this topic

#1 bowersox1

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Posted 22 April 2021 - 06:34 PM

Sorry for all these questions. How do you folks leave your course focus set on your scopes? I am trying to find a tip or trick to use so i'm not spending lot's of time turning the rough focus knob. I am putting a glow in the dark line on the micro focuser at the half way travel to aid me. Has anyone added a larger dial to the stock meade knob to give easier turning?

 

I am really thinking of upgrading the course focuser assy with the Starlight Feather touch Micro. Will the starlight feather touch be a one to one ratio like the stock assy?

 

Thanks for the help in advance.

 



#2 Bean614

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Posted 22 April 2021 - 07:18 PM

Often overlooked, but very important,  mentioned in every SCT Owner's Manual,  whether you use an external focuser or not:  When using the Internal Focuser,  ALWAYS end by turning Counterclockwise!  You'd be surprised at the resultts.


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#3 Virtus

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Posted 22 April 2021 - 10:11 PM

Often overlooked, but very important, mentioned in every SCT Owner's Manual, whether you use an external focuser or not: When using the Internal Focuser, ALWAYS end by turning Counterclockwise! You'd be surprised at the resultts.


Can you further elaborate on this?

#4 Nippon

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Posted 22 April 2021 - 10:25 PM

Counter clockwise pushes the mirror up against gravity and preloads the screw threads of the focusing rod. It keeps the mirror from slipping back from any play in the threads.


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#5 PJBilotta

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Posted 23 April 2021 - 12:05 AM

Adding a larger dial, knob, or even a clothes pin to your SCT focus knob does wonders. It allows you to make very fine focus adjustments with the lightest touch. Many of us use a peanut butter jar lid with a hole punched in the middle.

A more refined option is a secondary helical focuser that you can insert into the EP holder for making fine adjustments. A few diagonals including Baader's and APMs even have built in ones, and BORG also makes several - as does SVbony at a very low price point.
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#6 Tapio

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Posted 23 April 2021 - 01:02 AM

Or plain old Crayford focuser...

#7 mcnamado

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Posted 23 April 2021 - 03:23 AM

Similar experience as folk above. Increased the diameter of the stock focus knob by adding a section of foam mat rather than the lid option mentioned above (because I never thought of that option). I also have a Borg Helical focuser to tweek things and things really improved with the counter clockwise final move. After years of frustration, I reckon most problems are seeing related (with my C9.25 in my conditions). Following a super observing weather spell recently ( thats two nights without clouds here) I left the focus untouched between the two nights observing (12 mm on a BHZ) . Found and then synced on out of focus alignment stars then next night. Then went DSO hunting with the BHZ at 12 mm and everything was perfect in focus the second night again.

 

Donal

Another Drizzle Dweller 


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#8 Rick-T137

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Posted 23 April 2021 - 07:17 AM

I use a ScopeStuff fine focus knob on my Meade 2080 and I find that helps a lot.

 

http://www.scopestuff.com/ss_foc1.htm

 

I also have a helical focuser on my diagonal similar to the one Baader offers:

 

https://agenaastro.c...1-2456005k.html

 

So, I find I get into the ball park quickly with the fine focus knob (finishing CCW), and then I dial in the focus with the helical on my diagonal. It works very well.

 

Clear skies!

 

Rick



#9 Mike G.

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Posted 23 April 2021 - 12:04 PM

this works well for me...

https://www.cloudyni...9#entry10829480

 

and it looks nice as well.


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#10 GGK

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Posted 23 April 2021 - 02:44 PM

Turning the focus knob counter clockwise by 30 degrees or 1/12 turn takes up the backlash in the threads so there is no settling from gravity to change focus. 

 

A larger focus knob makes it easier to make small adjustments simply because it's harder to accurately turn a small knob just a few degrees to get the focus perfect without overshooting and needing to to go back clockwise and start again.  That's the challenge with having to end in a CCW rotation. 

 

I find the standard focus knob to be fine for open clusters and other larger DSOs, but when viewing in the solar system or on smaller DSOs where I need a finer focus, I use a Baader T-2 micro-focusing eyepiece holder for the fine adjustment.  This leaves the OTA primary mirror in place and just moves the eyepiece in or out a small amount for the final touch.

 

There's a few good alternatives to the basic stock focus knob.

 

Gary


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#11 ayadai

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Posted 23 April 2021 - 03:06 PM

Probably a matter of taste, but the addition of a motorized focuser was a revelation for me. Faster and more accurate in general, and I made a spreadsheet with settings for various eyepieces, binoviewers and cameras both with and without barlows so that I can just plug in a number and the focus is perfect every time.


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