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Focuser for Edge 8

Celestron EdgeHD 8" Focuser
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Jerry Gerber avatar
Jerry Gerber avatar
I don't want to use the stock focuser anymore on my edge 8 or deal with mirror flop.  I am looking at either this focuser:

https://agenaastro.com/baader-2-diamond-steeltrack-sct-focuser-2957035.html

or this one:

https://agenaastro.com/gso-2-linear-bearing-crayford-focuser-for-scts-dual-speed.html

My concern is backfocus.  What if I want to use it with (or without) the .7 reducer? 

Is that even possible?

My usual setup is Edge-reducer-OAG-filter holder-asi2600mc   (total BF = 105.3mm)

or

Edge-OAG-28mm adapter-filter holder-asi2600mc  (total BF = 133.3mm)

Thanks!
Jerry
GalacticRAVE avatar
GalacticRAVE avatar
Hi Jerry,

I don't think it will work for the Edge 8, certainly not for the 105mm bf (the focusers will use up already 90 or 115mm of BF, respectively). 
I had the steel track for the Edge 11, and ran into BF problems  (146mm), as you have basically no room for playing around (e.g. adding
a rotator). 

I found that I could well manage mirror flop by ensuring that the last motion of the focuser was CCW (pushing against gravity), which can
be ensured with the backlash setting. A nuisance is mirror shift (e.g. when changing filters), but surprisingly the crayford did not really help.

you may want to look into some of the low profile focusers such as the Esatto LP from primalucelab, which eats only 34mm of BF
 (I have it now, and am quite happy with it, but still would consider it a non-essential luxury item).

Matthias
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Tommi Liinalampi avatar
Tommi Liinalampi avatar
I have also thought to buy a new focuser for my EdgeHD 8" but realized same facts mentioned above. I think that you have to choose between better guiding (with OAG) and steady mirror (with external focuser). I would choose the first one and try to prevent meridian flips during my photographing sessions.
Jerry Gerber avatar
Jerry Gerber avatar
Thanks Matthias  and Tommi,

I'm glad I didn't click on "Purchase" for this focuser as I suspected that backfocus would be an issue.  

I wish there were a new type of scope that combines the mirrors/corrector plate of the SCT with the focusers available for the RC.  
I don't care for the SCT focuser, there's no focus lock, no markings to show where the drawtube is situated and no fine focusing mechanism.
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Ryan Faulkner avatar
Ryan Faulkner avatar
Have you considered adding the Celestron focus motor (or any other EAF)? This provides you with focus index values, so it’s really easy to get close to focus with every setup. The motor also helps stabilize the focuser.
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Jerry Gerber avatar
Jerry Gerber avatar
Ryan Faulkner:
Have you considered adding the Celestron focus motor (or any other EAF)? This provides you with focus index values, so it’s really easy to get close to focus with every setup. The motor also helps stabilize the focuser.


Yes, I have a ZWO EAF attached to the Edge and it works OK, but it's very slow.  What I'd prefer is to lock down the mirror and use an independent focuser that doesn't even use the stock focuser.  But the only one that seems to be made for the Edge 8 is the Moonlight focuser and it's motorized.  I'd have to stop using the ASIAIR Plus as it won't support any focus motors other than the ZWO EAF.
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