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Classical CC8 owners - I need a bit of help...

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9 replies to this topic

#1 WAskywatcher

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Posted 26 October 2020 - 11:02 PM

Greetings. I think I messed up my CC8. In trying to remove the secondary mirror to check a suspected smudge, I did not document the positioning of the mirror on the threaded attachment that sets the distance between the mirrors and as a result it seems to not be focusing correctly. Collimation was redone and looks fine on a star test, it just won't focus. In reassembling, I screwed the mirror all the way down onto the threaded attachment and locking ring, but from what I remember, I think there should be a gap where the arrow is where the mirror isn't all the way down. Does this distance make any difference in focus quality?

 

Just to be sure, if I could ask a CC8 owner(s) to take a quick measurement for me, that would be very much appreciated. Please be as accurate as you can. I have calipers in metric and digital inches, so I can be very accurate. Even a photo would be helpful. Thank you, thank you, thank you if you can help out. 

  

CC8_secondary.jpg



#2 luxo II

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Posted 27 October 2020 - 12:12 AM

Closing the gap that was there previously implies you have effectively increased the space between the mirrors, which will have pulled the focal plane inwards - and is probably why you can't focus.

 

Changing the spacing by 1mm probably shifts the focal plane by something like 9...20mm depending on the actual geometry of these scopes; so if the gap was several mm, you figure... you can use this to set the backfocus as you prefer.

 

In addition, that gap is probably different in each scope - its there to accommodate minor variations in the actual radii of the mirrors in each set - GSO would adjust it in final assembly to set the backfocus to some nominal value.


Edited by luxo II, 27 October 2020 - 12:37 AM.


#3 glend

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Posted 27 October 2020 - 04:21 AM

Yes, I agree, the CC is very sensitive to changes in distances between the mirrors, resulting in larger focal point changes. There is a big discussion going on in the thread about 6 and 8 inch CCs in this sub forum.  Honestly you should not have tried to remove the secondary, that ' offset' for lack of a better word, is factory set,  it is possible to clean it without removing it. Asking here first would have been a good idea. I will check mine but not sure I can tell you much; as said above each CC set up in the factory maybe slightly different due to subtle differences in mirrors used. The factory fix these in place on an optics bench. 

In future, I suggest you think about using "witness marks" on anything you consider moving or removing. 

 

Here is a link to the other thread currently discussing focal distance factors, read from page 50 onward. There is alot of math thrown around but the important take away is that small changes can result in significant focal point shifts.

 

https://www.cloudyni...segrain/page-51


Edited by glend, 27 October 2020 - 04:52 AM.


#4 glend

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Posted 27 October 2020 - 04:50 AM

Ok, i just had a look at my CC8 and that gap seems to be about 3mm, measured with a small tape measure. My micrometer is in the back shed and its raining here. I will find it tomorrow in the daytime. Again my number may mean nothing by way of comparison. Maybe if you get ten people to measure theirs and take an average you might get close. 

Once you are able to detect focus within the range of your focuser, then you could consider changing the secondary gap to get you back to where focus was originally- provided you have not tampered with the Primary.



#5 jgraham

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Posted 27 October 2020 - 09:27 AM

Ugh, I went through this while rebuilding a 40 year old 6" f/20 Cassegrain. Yep, small changes in the mirror spacing can make _big_ changes in the back focus. I'll take a peek at my GSO CC8 to get a guessimate of what the secondary mounting spacing looks like. As a sanity check you can place the focuser where you think it should focus and then adjust the secondary until it focuses about there. That's how I set the spacing on my 6" f/20 and it seems to work well. Getting the primary/secondary spacing precise should not have a big effect on the image quality as most of the large observatory Cassegrains that I have seen focus by moving the secondary.

 

Food for thought.



#6 WAskywatcher

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Posted 27 October 2020 - 10:33 AM

Thank you all for some great advice. I usually have an "if it ain't really broke, don't fix it" approach to things, so yes, I should have just left it alone. My main concern is if there is a critical mirror distance for these scopes - unlike SCT's that focus by moving the mirror. I can get it to go "through" focus, it's just that the focus point is not that sharp. It's easy enough to play with different mirror distances on a treetop a couple miles away and see what happens, but a hint at what the factory setting might have been is very helpful. I have extra focuser spacing rings and can set backfocus to what it was before I "fixed" it.

 

Thanks again. All part of the adventure.   



#7 jgraham

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Posted 27 October 2020 - 11:26 AM

I just took a peek at mine... the spacing looks to be about 3-4mm...

 

CC8 Secondary-2.jpg

 


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#8 luxo II

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Posted 27 October 2020 - 11:57 PM

I can get it to go "through" focus, it's just that the focus point is not that sharp. 

In that case don't futz with the spacing. Let's see a star test at focus, inside and outside focus; that will tell all.

 

Point it at a bright star, attach camera, you know the rest.


Edited by luxo II, 27 October 2020 - 11:58 PM.


#9 MikeECha

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Posted 28 October 2020 - 01:14 PM

WAskywatcher, on 27 Oct 2020 - 11:33 AM, said:

Thank you all for some great advice. I usually have an "if it ain't really broke, don't fix it" approach to things, so yes, I should have just left it alone. My main concern is if there is a critical mirror distance for these scopes - unlike SCT's that focus by moving the mirror. I can get it to go "through" focus, it's just that the focus point is not that sharp. It's easy enough to play with different mirror distances on a treetop a couple miles away and see what happens, but a hint at what the factory setting might have been is very helpful. I have extra focuser spacing rings and can set backfocus to what it was before I "fixed" it.

Thanks again. All part of the adventure.

If you have an image you took with factory defaults you can blind solve it and see the fl was.

Sorry if the font comes out too big. I am using my phone and I am not sure how this will look.

#10 WAskywatcher

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Posted 12 November 2020 - 08:16 PM

Thanks to all. I managed to get it back into working order. I decided to forget all the fancy laser collimators that were giving conflicting and inconsistent answers, and work just with the Farpoint cheshire. I cranked all the primary mirror adjusters all the way in, and then out one turn in an attempt to "zero" it out, and the primary looked great in the cheshire. The secondary was set to according to jgraham' s photo and just needed a bit of center tweaking and I called it good. In the sky, out-of-focus stars look very good, and although the seeing wasn't great, Mars imaged very close to the first images - before I started messing with it. 

 

So the lesson is - don't mess with things if they're not really broken, and you don't really know what your doing... :^)


Edited by WAskywatcher, 12 November 2020 - 08:18 PM.

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