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Bumping the base of my SCT fork mount

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#1 Cosmo100


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Posted 25 February 2021 - 06:32 AM

I have a Celestron Evolution 9.25" on the standard fork mount.  How can I prevent my star diagonal or camera from bumping into the base of my fork mount when slewing to a high altitude?  To prevent bumping the objective/camera into the mount base, I have had to turn off the scope, but then I have to realign and sometimes give up on viewing the desired object. It would be nice to be able to cancel a "go to" command if I see I am about to bump the mount.  I suspect there is a simple solution to this problem, I just haven't figured it out.  Can you help?  - Thanks



#2 Bean614


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Posted 25 February 2021 - 08:02 AM

2 Ways:  Loosen the Bolt on the saddle, that holds the Dovetail Bar attached to the scope.  Slide the Scope further forward until the Camera/Diagonal clears the base at Zenith. Tighten the Bolt.

Or, go into the Menu on the Hand Controller, and adjust the 'Slew Limits' in 'Altitude', to a degree that prevents the scope from pointing any higher than that 'degree' limit you set.  For example, set the Altitude Limit to 70-Degrees.

Edited by Bean614, 25 February 2021 - 08:03 AM.

#3 Supernova74


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Posted 25 February 2021 - 08:30 AM

Unbalanced scope not so much with fork mount scopes can cause strain on the motor drives also goto Acuracy can be reduced.are you useing an DLSR or imaging camera of some kind!?so I can imagine the OTA the time you got camera and accessories attached it’s go to be back heavy.so by all means adjust further up the dovetail bar so you have plenty of clearance when the scope points towards the zenith.as a alternative you could maybe make or puchase some small counter weights for the OTA a well balanced scope is a better performance telescope.

#4 GoFish



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Posted 25 February 2021 - 08:33 AM

Pressing any arrow button during a GoTo will stop the slew. 

#5 Noah4x4



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Posted 25 February 2021 - 09:42 AM

This is a common problem with Alt-Az fork mounts. As stated, one can set altitude limits. This post considers what can be done mechanically.


With my 8" OTA fully forward, I can just make it to the zenith, despite the desirable 105mm back focus of a Celestron x6.3 focal reducer with a ZWO NON-cooled camera. See image below.


With any cooled camera, there is no chance of me getting close to this, and I would then have to use a diagonal. But I use a ZWO ASI294mc at f/6.3 for most EAA and Atik Horizon (cooled) on HyperStar f/2 at other times. I found that I don't need cooling for short exposure EAA at my mild climate location hence I deliberately went for the non-cooled ZWO option for use at f/6.3 or f/10. If on a wedge, you will enjoy significantly more clearance, but I hated using my wedge. Too cumbersome and tedious. 


As somebody is bound to ask;.my optical train here is OTA; >  Celestron x 6.3 FR; >  Celestron T-Adapter (part #93633-A); > Baader Varilock (#2956946); > ASI294mc Camera. 


What can be seen below my mount are two plastic ABS Boxes screwed back to back and slid onto my central tripod rod above the leg spreader. I neatly hide excess cables and AC/DC adapters inside these. Affixed to the ABS Boxes are my Intel NUC and MKIT20 Focuser controller. Three cables run to a ring suspended under the centre of my my OTA all neatly inside the white spiral cable tidy depicted; hence camera USB3 cable; Focuser USB cable and power cable (only for Atik Horizon). I can swing the same cables forward or back depending on if I am imaging at f/2 or f/6.3.


I might add that making my OTA quite heavily front end loaded and unbalanced seems to improve my GoTos and tracking. I use the same extreme forward OTA position with HyperStar and the mount copes without any difficulty. 



Edited by Noah4x4, 25 February 2021 - 09:45 AM.

#6 Chris Johnson

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Posted 25 February 2021 - 10:40 AM

I solved my clearance problem with a wedge.

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#7 Cosmo100


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Posted 25 February 2021 - 12:47 PM

Wow,  What an awesome community!  And awesome, speedy feedback!  Thank you all so much for the useful replies.  I will give your suggestions a try.  I just recently found this group and this was my first post, so thank you again the generous and welcoming responses!

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