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GSO 2 speed crayford on Celestron C6

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#1 HoLun

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Posted 17 September 2019 - 01:29 AM

Decided to put a gso crayford on my C6, was excited when the focuser arrived, I have ordered the linear rail version this time since i have a ball bearing version for my refractor, but the ball bearing ones have quite a bit of draw tube slop.
 
s2QgNlkl.jpg
 
I ran into a few problem during and after install.
first, It does not fit correctly, the adapter ring interferes with the stock focuser retaining ring and mounting screws. Well no big deal, I have a lathe, and with that, there is not such thing as "doesn't fit".
 
5NYFkFol.jpg
 
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f2h39BBl.jpg
 

clears everything now, good to go? well kind of.
 1etJLTCl.jpg
 
well, after I got that sorted out, then I noticed the focuser got some serious play when you wiggle the wheels. 
turns out, there is no support to the shaft on left side of the wheel/shaft. I bought this focuser new from agena astro, so its either assembly error from factory or its a design flaw. if any of you can check to see yours has the same issue and post about it here or link me to a post that would be awesome.
heres a video of the issue.

 

https://www.youtube....h?v=oBKRggM6ekk

 

heres what it look like after I removed the knob, as you can see, there are no support, no bushing, no bearings.

only a plastic spacer and a thrust washer to keep the shaft center.

 

Zc4QTHDl.jpg

 

so back to the lathe, this time I use a piece of delrin I have laying around to make a bushing for it. there is also some mill work to get it clear the inside where it meets the main housing.

7BLu7lLl.jpg

 

the existing spacer goes into the bushing and the whole thing fits into the shaft housing.

lpdPFREl.jpg

 

moved the thrust washer up top between the new bushing and the knob.

sCKkZcEl.jpg

 

and heres the video of after installing the bushing, zero play, still turns as smooth as before. and my telescope is now pretty awesome.

 

https://www.youtube....h?v=Tm6FlmeZcxo

 

uMPbj7Jl.jpg


Edited by HoLun, 17 September 2019 - 01:34 AM.

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#2 mclewis1

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Posted 17 September 2019 - 07:20 AM

How do like your new 1800mm f12 scope? You've increased the physical "back focus" position by at least 100mm and that translates to about a 300mm increase in focal length. That 3:1 increase is an approximate number and due to the moving mirror/magnifying secondary optical design of the SCT. Racking that focuser out to it's maximum position makes the C6 an almost 2000mm f14 scope.

 

Moving the focal point that far away from the optimal point Celestron configures these scopes for (approximately 100mm behind the baffle tube opening) is going to introduce additional optical aberrations.  How much that will negatively affect the optical correction is measurable and likely significant enough that it will be noticeable during critical high magnification work on solar system objects.


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#3 Eddgie

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Posted 17 September 2019 - 08:36 AM

Yeah, while I admire the OPs efforts, it is a lot of back focus for a C6.   

 

The other issue is that the C6 has a particularly small range of back focus that can be used before aperture reduction starts and the OP is very close (but maybe not over) that line.

 

Edz's measurements put the line for the C6 at about 200mm, which is actually better than the C9 and C5, but similar for the C8.  It looks like the OP may be pushing it but likely a bit under it. I would estimate that his light path is 180mm or so.

 

To know for sure, the OP would need to measure it, but yeah, this is pushing it.  A 2" diagonal would put the scope into aperture reduced mode if used in this configuration. 

 

Still, the illumination falloff is probably on the order of 40% to 50% at the edge of the field, but a lot of people are fine with a lot of illumination falloff. 

 

But none of this really matters if the OP is happy with his scope.  I could not live with it, but there are a long list of things that people live with that I could not accept.   I do a lot of things that others would find questionable though (I do all my planetary and solar using very inexpensive zoom eyepieces) so that is the way the world works.

 

It is a lot of back focus for a C6 though. 

 

To me, a much better solution (if the scope is going to be used with a 1.25" diagonal) is using 1.25" diagonal with a helical focuser.   This will eliminate mirror shift when focusing, make the scope smaller and lighter, and preserve the native focal length and give a wider true field for a given 1.25" eyepiece. 


Edited by Eddgie, 17 September 2019 - 08:38 AM.


#4 HoLun

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Posted 17 September 2019 - 01:44 PM

oh man, i didn't think when i bought that, i was still in refractor mode, guess i just wasted $180



#5 Eddgie

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Posted 17 September 2019 - 04:17 PM

oh man, i didn't think when i bought that, i was still in refractor mode, guess i just wasted $180

Well, it is not really an optimal setup, but if you like it, don't let our musings stop you from enjoying it.  



#6 HoLun

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Posted 17 September 2019 - 05:47 PM

I only did a quick look thru the telescope after throwing the focuser on, i was wondering why saturn looks huge.

i did some measuring, its 80mm longer before the diagonal vs the visual back that the c6 came with.

I am thinking this might be ok if I just not use a diagonal? its a 1-1/4 diagonal. 

 

guess I will have to do some comparison and see, and its cloudy tonight, go figures



#7 mclewis1

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Posted 17 September 2019 - 09:10 PM

You did nice work with fitting the focuser. My comments are about the numbers involved, and not a particular indictment. As mentioned and to paraphrase Ed if you are happy with it that's all that really matters.

 

Something we haven't talked about are the benefits of your upgrade. The obvious one comes from the external focusing (no image shift) but that longer focal length also means that your planetary work can be done with longer focal length (and presumably more comfortable) eyepieces. 

 

The reason for my comments in my earlier post is that most folks tend to want to go in the other direction - less focal length, faster f ratio for a wider field of view. I find it's not that commonly understood what the impact is when you push the focal position of an SCT back beyond where it usually is ("usually", with common diagonals and adapters). 




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