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Secondary Movement

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#1 TonyMan

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Posted 21 November 2019 - 01:19 PM

I had a post about this some time ago. I had determined then that it was only the outside ring moving and everything was fine. Now I find that the whole secondary spins. If you look at the pics I’ve added I marked one up as:

 

A - outer ring

B - secondary

C - looks like some kind of gasket

I also know from the last time that there is a set screw under the outer ring. 
 

The whole thing is spinning and the only way I can get the outer ring off is to get my fingers to grip the portion of the secondary (B). A friend has lent me a Hotech SCA collimator to check collimation while the weather is bad. I do not want to proceed until I understand if this is a major issue or not. Here is what I’d like to understand:

 

1. Is it a problem for this to be able to spin?

2. If so, how do I tighten it? Is that what the set screw is for?
3. If I need to tighten it, is there a certain pattern the screws should be in, or does the text get set as horizontal?

 

A side issue is that the Hotech video on YouTube has a step where they say to remove the secondary. This is not indicated in the Hotech written instructions so I’m wondering if this step is considered necessary? If so, Is this accomplished by loosing the set screw? 

Any info to help me understand this is appreciated.

 

Thanks

 

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#2 TonyMan

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Posted 21 November 2019 - 01:20 PM

Sorry, I forgot to mention it is an Evolution 8”.



#3 photoracer18

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Posted 21 November 2019 - 03:18 PM

Likely had the A part tightened down too tight. B and C need to likely be re-aligned as they could be off the Celestron alignment marks. Assuming the Evo 8 has the same type of alignment marks as the older OTAs. A does not have to be super tight as that is the part you take out to use it in Fastar mode.



#4 Michael_Swanson

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Posted 21 November 2019 - 11:45 PM

You can only tighten this down by removing the corrector plate.  On this page of my website:

https://www.nexstars...dsEvolution.htm

refer to the article "Fastar Secondary Turns When Tightening the Retaining Ring" and the further article it links to.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com



#5 TonyMan

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Posted 22 November 2019 - 06:43 PM

Thanks Michael,

 

You mentioned that in my original post, had given it a cursory glance, but since I thought the issue was corrected I never went back to it. I just read through it, and am getting scared that this is kinda hairy. I’m gonna read it a few more times and will post back with any questions I have.



#6 Michael_Swanson

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Posted 23 November 2019 - 01:26 AM

The apprehension is much greater than the actual event :-)

 

Just be sure you have a quiet place (no pets, no small kids) with plenty of light and set aside 2 solid hours in case you decide you want to clean the inside of the corrector plate while you have it off.

 

Best regards,
Mike Swanson
Author of "The NexStar User's Guide II"
Author of "The NexStar User's Guide"
Author of "NexStar Observer List"
https://www.NexStarSite.com



#7 TonyMan

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Posted 23 November 2019 - 05:43 PM

I've read through this a couple of times and am still a little confused by a couple of things.

 

On the note about Celestron SCA' made after 2006 it says, "use a permanent marker to make corresponding marks on the edges of the corrector, secondary and primary mirrors (if you actually remove the primary mirror - not described here) and the edge of the corrector cell so that you can reassemble the optics in the correct orientation." I take it you are saying mark the surface of the corrector and what I would call the black housing connected to the scope to insure it get put back in the same position.

 

My Evolution has 6 screws, not eight, but I'm thinking that is just a change Celestron made. These are the screws facing me when looking at the corrector.

 

Once I get the corrector off, I take it I should get the outside ring off, tighten the secondary paying attention to the orientation of the secondary key. Hopefully I can find the serial number. Once done put the ring back on, but not too tight so I don't disrupt the alignment. Then put the corrector plate back in the scope. When installed should the "Celestron Fastar Compatible" printing be horizontal and the secondary alignment screws in a certain pattern?

 

Otherwise, I'm feeling semi-comfortable with proceeding. I really appreciate the help.



#8 Michael_Swanson

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Posted 25 November 2019 - 12:18 AM

Hi Tony,

 

Mark the edge of the corrector out where the retaining ring will cover the mark and then mark the black housing (outer ring) at that position.  Just tiny marks to make it easy to return everything to the same orientation.  I would recommend you place the marks at the same orientation as the focus knob as that would be where the corrector's etched serial number would be if it exists.  And, that would be the traditional orientation of the Fastar secondary's slot and key (though newer SCTs might not adhere to this convention).  I haven't seen enough Evolutions to guarantee that will put the Fastar label in an upright position.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com



#9 TonyMan

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Posted 25 November 2019 - 01:01 PM

Thanks again Michael. I’m going to try to get on this later today. I’m pretty much over my apprehension thinking that I installed a dual focuser on this by just taking it easy and not rushing.  Will report back when done.


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#10 TonyMan

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Posted 25 November 2019 - 09:49 PM

Well, I got started and am now a little concerned about the next step. I took out the six screws around the front of the corrector and removed the thin ring and two piece washer under it, looks like its fiber of some sort. The corrector is still in there tight and I am not sure what to loosen next. Here is what I see:

 

There is no serial number or anything printed on the corrector. I marked it to line up with the bottom pin for the cover.

 

There are three flat head screws at 12, 4, and 8 o'clock. I can see that they hold the outer black ring to the OTA because they go into nuts inside the OTA. I cannot tell if those are just nuts or if they are welded to the OTA so was afraid to touch them in case they fell into the OTA.

 

There is also a round head screw at 9 o'clock.I don't see anything inside the OTA it connects to. It looks like it could be directly in line with the corrector. Could this be something like a locking screw for the corrector?

 

There are four set screws which look like adjustments for the corrector. These seem to go to a small piece of white plastic that touches the edge of the corrector. I'm wondering if I loosened these a little, would that allow the corrector to be removed. Also concerned that these may not be glued in place and could fall out if loosened.

 

I also looked at some older threads that referred to the gasket sold by Starizona. From the remarks there this seemed like a good solution to this not happening again. Since we have Thanksgiving in three days this may go on hold until the gasket arrives (hopefully Wednesday) and I'll continue on Friday or Saturday.

 

I'm sorry to be such a pest with this, but don't want to wind up screwing things up.

 

Thanks again for all the info.



#11 Michael_Swanson

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Posted 25 November 2019 - 11:32 PM

I believe the 4 set screws you are referring to are a new addition that are used to center the corrector in the cell ("outer black ring").  You should be able to loosen them just a bit (half a turn might be enough) to prevent them from holding the corrector in place.  It is possible those small pieces of white plastic will fall out but that should be easy to deal with when you are reassembling .  (as long as you don't loose them!)

 

If you find there is already a rubber gasket between the secondary holder cell and the corrector plate, then Celestron is already including that in the standard configuration (item C in your picture at the top of this string).

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com



#12 TonyMan

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Posted 03 December 2019 - 02:30 PM

OK, looks like things are pretty much back together and I can start collimating soon. I found an old email from Celestron where they said the Fastar logo should be horizontal for the correct orientation of the secondary, so that made things pretty simple. I wrote to them last week and they said that yes, the four set screws need to be loosened carefully to remove the corrector.I turned two of them about 1/2 turn and that was enough to allow the corrector to come out. When I tightened the secondary, I had some problems with it still turning off from where it should be. I was going to use the new gasket from Starizona, since they were sticky on both sides,but ran into another issue. The gasket on the secondary seems to be tightly glued to the holder. I didn't feel like scraping it off and possibly not getting it all. Instead, I used an tiny drop of clear silicone sealant at four points of the secondary holder, pressed it to the corrector, and let it sit overnight. Today it was holding tight so I aligned and screwed everything together.The corrector is back in the scope and tightened down by the set screws. Now the fear fun comes! I can't believe Celestron used a two piece gasket under the corrector front plate. I have the scope vertical and it's a real challenge getting the front plate past the two small studs that the front cover locks to. Every time I start pushing it in, a finger hits one of the gaskets and moves it. I put a couple of tiny, tiny dabs of silicon on the gaskets, pressed them in place. And am going to let it sit for a bit to see if it adheres (and take a few deep breathes before fighting the cover plate again). Hoping to finish off this afternoon and collimate then.

 

Celestron also told me that the screw at 9 o'clock is actually for attaching a second dovetail.

 

Michael, I am grateful for all your help with this. I have the second edition of your book,The Nexstar Users Guide II and have found it to be valuable. I'd like to suggest that in the next edition you could add some of the info like loosening the set screws to remove the corrector, and the Fastar logo being horizontal for correct secondary orientation.

 

Thanks again for the help.

 

Tony



#13 Michael_Swanson

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Posted 03 December 2019 - 06:05 PM

Looks like you are nearly back in business Tony.  I'll definitely add the set screw info to the article at my website as well as additional tips for orientation of the corrector and secondary on the newer versions of Celestron SCTs.  I will say though that there have been some iterations where the Fastar logo is not horizontal (depends on the mount type) so that isn't a surefire tip.  

 

I'm glad you've found the book useful.  I haven't included removal of the corrector in my books.  Perhaps in the future but the article will continue to be available on my website.

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com




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