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Filling in the focuser gap ?

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#1 vkhastro1

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Posted 26 March 2020 - 06:36 PM

I have a Celestron C6 f/5 Newtonian OTA.

Optically very nice images up to 140X. Not really looking to push the magnification but more to optimize it for wide field/low power deepsky viewing.

 

It originally came with a 1.25" Rack and Pinion focuser of dubious quality.

I am in the process of doing a major make over.

 

Removed the original focuser, Vixen style finder bracket, spider/secondary holder with attached

2° mirror.

I enlargened the focuser hole to 2.4" to accept a 2" JMI NGF-2 single rate Crayford focuser with 2"-1.25 adapter. Drilled 4 holes to match the 2.75" square hole pattern of the new focuser.

 

Flocked (Protostar) the first 7" circumference from the front of the optical tube. Renistalled the finder bracket holder. Installed and levelled the new JMI 2" Crayford focuser. Reinstalled the spider/secondary holder/2° mirror. Recollimated the scope. Rechecked daytime focus - requires a 2""extension tube to reach focus with a 2" eyepiece. Have not a chance to access nighttime deepsky viewing.

 

Just one problem with the focuser:

The C6N tube has an outside diameter of ~ 7" but the curvature on the JMI 2" Crayford is more like 12". There is an ~ 1/2" gap running the length of the focuser on the 2 sides parallel to the length of the OTA. There is also a a progressively increasing gap on the other 2 sides. 

When you look into the OTA from the front on an oblique angle, ambient light is visible through the 4 "gaps". 

What would be a simple (easily removable, if necessary) fix to fill in the focuser "gap" and still look nice? Paintable caulking comes to mind. Other options?

 



#2 Jon Isaacs

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Posted 26 March 2020 - 06:45 PM

My first concern is whether you used washers under the bolts so that the focuser is supported by the tube in the area of the bolts.  If it resting on the center of the tube, that's not good. 

 

Otherwise, you want to fill the gap to prevent light from entering?

 

Jon



#3 vkhastro1

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Posted 26 March 2020 - 06:52 PM

Hi Jon,

 

The 2” JMI Crayford has holes that accepted the 8/32-34” bolts.

On the inside of the tube the bolts are attached with a washer and a nut.

Just inside of each of these 4 corner bolts are 4 separate Allan Key screws that are threaded downward to make contact with the metal tube to further stalklike the 2” Crayford focuser.

 

I want to fill in the gap to prevent light leakage.

 

Thx, Gary 



#4 Jon Isaacs

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Posted 27 March 2020 - 04:39 AM

Hi Jon,

 

The 2” JMI Crayford has holes that accepted the 8/32-34” bolts.

On the inside of the tube the bolts are attached with a washer and a nut.

Just inside of each of these 4 corner bolts are 4 separate Allan Key screws that are threaded downward to make contact with the metal tube to further stalklike the 2” Crayford focuser.

 

I want to fill in the gap to prevent light leakage.

 

Thx, Gary 

Gary:

 

I have a couple of DX1's,  As I recall, those are some pretty small bolts. I would be worried that they would dent if the focuser got bumped. I have used washers under the focuser to spread the load.

 

Jon



#5 macdonjh

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Posted 27 March 2020 - 09:04 AM

MoonLite makes extenders for their focusers, perhaps one will fit your scope:

http://focuser.com/products.php

 

One thing to keep in mind: now that you have a larger focuser you might be far enough from your scope's focal plane that in-focus may be problematic.  You may need to move your primary mirror closer to your secondary (since you've already installed your secondary and focuser you probably don't want to move those).



#6 vkhastro1

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Posted 27 March 2020 - 09:40 AM

MoonLite makes extenders for their focusers, perhaps one will fit your scope:

http://focuser.com/products.php

 

One thing to keep in mind: now that you have a larger focuser you might be far enough from your scope's focal plane that in-focus may be problematic.  You may need to move your primary mirror closer to your secondary (since you've already installed your secondary and focuser you probably don't want to move those).

 

I have no issue reaching focus with 2” eyepieces using the 2” NGF focuser up to its most outward focus with a 2” extension tube (GSO Type 1.5” length).
The Moonlite 2” compression ring extenders with 3 set screws are the best (I have one dedicated for a different scope).

No need to move the mirror position. 
The 2ndary mirror size is 44mm (30% by diameter).

I expect there will be minimal light loss at the edges because the 2ndary mirror is quite large.

 

Any thoughts on closing the focuser gap?



#7 havasman

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Posted 27 March 2020 - 10:08 AM

The darkest of the silicone caulks/sealers might work well, apply easily and provide just a slight bit of extra stability too.


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#8 kfiscus

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Posted 27 March 2020 - 12:06 PM

I'm inclined to think that caulk is the solution.  If the gap is too big for the caulk to bridge, consider using caulk filler rope.  It's springy, closed-cell foam in a rope shape.  I have a variety of diameters on hand for home repairs.  Shoot me a PM if you need a few inches.  Yours, for the price of the postage.



#9 KidOrion

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Posted 27 March 2020 - 12:40 PM

Use a couple of layers of black craft foam.

 

https://www.amazon.c...,aps,325&sr=8-9




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