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Is This Where I ask How To Remove My Primary,,,,,,,?

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#1 Mike W.

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Posted 02 June 2018 - 12:17 PM

Hi all !

I have an older SCT, Nexstar 8i and I'm wanting to remove the primary mirror so that I can do some up-grades to the OTA, want to add a fan to disrupt the boundary layer on the primary.

 

I have had the corrector plate off, marked for orientation, then ran the mirror as far forward as I could, I removed the retaining clip/ travel stop clip at the top of the center tube but the mirror won't come free of the tube,

Do I need to remove the focuser so that the drive is disengaged from the back of the mirror?

 

Thanks 



#2 Mike W.

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Posted 02 June 2018 - 12:26 PM

Well, found a Youtube video on how to do it, thanks anyway!wink.gif


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#3 gfstallin

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Posted 02 June 2018 - 01:41 PM

Do you have the link? 


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#4 Mike W.

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Posted 02 June 2018 - 01:58 PM

yes, I gave it to another guy that's about to take his apart for a re-coating.



#5 gnowellsct

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Posted 02 June 2018 - 10:19 PM

https://www.astromar...sct-childs-play


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#6 gnowellsct

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Posted 02 June 2018 - 10:20 PM

He should not take it apart and send it out for recoating, because he is highly unlikely to ever get it together again.  This is not like a Newt.  The orientation of the primary is difficult.  So far as I know, none of the people who say they will recoat an SCT mirror actually know how to get it back together again.  Been there, done that (had to buy new optics).  GN



#7 Bean614

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Posted 02 June 2018 - 10:27 PM

Just wrap the thing in Reflectix and forget about thermal issues, removing mirrors, and whatever else you may encounter.



#8 gfstallin

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Posted 03 June 2018 - 04:35 AM

He should not take it apart and send it out for recoating, because he is highly unlikely to ever get it together again.  This is not like a Newt.  The orientation of the primary is difficult.  So far as I know, none of the people who say they will recoat an SCT mirror actually know how to get it back together again.  Been there, done that (had to buy new optics).  GN

Is it the problem of removing the primary mirror and reorienting correctly once placing back within the tube assembly or is the recoating process that makes proper orientation difficult? 

 

George



#9 rmollise

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Posted 03 June 2018 - 08:20 AM

He should not take it apart and send it out for recoating, because he is highly unlikely to ever get it together again.  This is not like a Newt.  The orientation of the primary is difficult.  So far as I know, none of the people who say they will recoat an SCT mirror actually know how to get it back together again.  Been there, done that (had to buy new optics).  GN

I agree 1000%. I hope I'm wrong....but I see another hangar queen SCT on the way.

 

For most people a CAT Cooler type fan, the commercial product or a homemade version works every bit as well as drilling big holes in the rear cell. It also preserves the OTA for possible resale. If the primary doesn't have to be removed from the holder, this is a less dicey procedure, but still not for somebody to undertake after having looked at a freaking web page. ;)



#10 rmollise

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Posted 03 June 2018 - 08:21 AM

Is it the problem of removing the primary mirror and reorienting correctly once placing back within the tube assembly or is the recoating process that makes proper orientation difficult? 

 

George

 

All of the above, but mostly getting the primary properly RTVed back into its holder.


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#11 Mike W.

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Posted 03 June 2018 - 08:58 AM

Gentlemen, I'm not quite sure how to take some of the replies, I'm not an idiot.

 

Thank you for your time.


Edited by Mike W., 03 June 2018 - 08:59 AM.


#12 E Sully

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Posted 03 June 2018 - 09:41 AM

Gentlemen, I'm not quite sure how to take some of the replies, I'm not an idiot.

 

Thank you for your time.

I hope you are not taking it the wrong way.  Yes, you can remove the primary with carrier, do your mods, and re-assemble, then collimate it.

 

yes, I gave it to another guy that's about to take his apart for a re-coating.

This is the problem, re-coating of the mirror.  To do this requires breaking the RTV seal for the primary and it's carrier.  Careful alignment must be done when bonding the mirror back to it's carrier.  You cannot just RTV the mirror back onto the carrier, it requires very precise alignment that cannot be done without the proper equipment.


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#13 gfstallin

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Posted 03 June 2018 - 11:26 AM

Gentlemen, I'm not quite sure how to take some of the replies, I'm not an idiot.

 

Thank you for your time.

You're in better shape than I am. The only times in life I have felt like an idiot with any regularity have consistently involved telescopes. 

 

By the numbers: 

Number of times I've said aloud "Lefty loosey, righty tighty" after the age of 10 in real life: 1

Number of times I've said aloud "Lefty loosey, right tighty" after the age of 10 around telescopes: 490. Once last week. 

 

Number of times I've removed the OTA before removing the counterweight: 6

Number of times I've done that in the past year: 2

Number of times I've left my equipment in a public park while I sprinted a 1/2-mile or full mile back to my old apartment to get a Bahtinov mask or dew shield: 3

 

Number of times I've imaged with a Bahtinov mask still on: 3 

Maximum amount of time it took me to realize the Bahtinov mask was still on: all night ("Why are these [lunar] images so dim and fuzzy? Maybe I should just pack it up.")

 

Number of times in real life I've accidentally placed a finger inside the outlet hole of a power strip and received a mild shock: 0 

Number of times around telescopes I've accidentally placed a finger inside the outlet of a power strip and received a mild shock: 1st time last week (I'd never had mains power around a telescope before last March - what a joy provided I do not electrocute myself - so this number will likely increase)

 

Number of times in any situation I've had to think about which direction counterclockwise is when the object is in front of me (electric focus motor) and facing in the same direction (toward the focuser) I am facing: 1 time this week. "Wait, counterclockwise is...the second hand goes from left to right. No mirror image, so..." I then made an L-shape with my thumbs and index fingers to confirm left and right. 

 

I'm ashamed to say these are all actually true stories; however, I can guarantee that every person on here, if she/he has not made these same completely idiotic mistakes, leaps of logic, or misjudgments, has made others. 

 

I doubt anyone here actually intended to sound the wrong way. Forums such as these are horrible when interpreting a person's tone, even with emoticons. 

 

 

George


Edited by gfstallin, 03 June 2018 - 11:32 AM.


#14 Mike W.

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Posted 03 June 2018 - 11:38 AM

Morning again gentleman, maybe I did get a ruffed, sorry, you were just trying to help and when exchanging communication via text rather than face to face misunderstanding's occur. crazy.gif

 

gfstallin, that explains it, silicon glue to secure the primary to the carrier, makes since.

for my project I wouldn't need to separate that but the other guy that's recoating this would be a deal breaker, 

I'm going to contact him and get this straightened out that I was wrong about it being easy to have it recoated.

 

Thanks!  cool.gif



#15 gnowellsct

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Posted 03 June 2018 - 11:49 AM

Morning again gentleman, maybe I did get a ruffed, sorry, you were just trying to help and when exchanging communication via text rather than face to face misunderstanding's occur. crazy.gif

 

gfstallin, that explains it, silicon glue to secure the primary to the carrier, makes since.

for my project I wouldn't need to separate that but the other guy that's recoating this would be a deal breaker, 

I'm going to contact him and get this straightened out that I was wrong about it being easy to have it recoated.

 

Thanks!  cool.gif

Yes getting it correctly RTV'd in place is not just a thing, it is EVERYTHING.  If this were easy MOST of those decades old SCTs would have been through the recoat factory.   I sometimes wish Celestron would just provide a recoat-and-tune service.  (for a charge) That way the older models that, perhaps, have not done the company's reputation much good would be upgraded and we wouldn't have so many people blah blah blahing about what a bad view they had through an SCT.

 

The symptom that you will get of a poorly mounted mirror is that you collimate the scope on one side of focus--and find out it is uncollimated on the other side of focus.  If you collimate on that side, it is un-collimated on the other.  There's no way to get it right because it is mounted wrong.   And the scope will seem "sick" when you try to use it.    It will be the way detractors like to describe the way an SCT is, rather than the way an SCT actually is (or should be).

 

Greg N


Edited by gnowellsct, 03 June 2018 - 12:25 PM.


#16 YAOG

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Posted 09 June 2018 - 08:59 PM

Hi Mike W.,

 

It is not hard to correctly realign an SCT optical system if the OTA has not been messed with and the glass parts still have the correct orientation marks. On a Celestron the mirrors are glued in which is a PITA if you need to reposition it to point primary at the right spot. This is different from Meade practice but Celestron primaries are much less often aimed at the wrong place. Meade relied on masking or paper tape to hold the primaries centered on the baffle tube primary mount so they get out of alignment very easily. But by the same token this makes it easy to reposition them and as I have done, modify the primary mirror mount and add adjusters to the primary mirror mount to allow the primary mirror to be precisely centered and optically aligned with the baffle tube and secondary which makes for really good images at the eyepiece or camera sensor. 

 

Your buddy can send his SCT to Optic Wave Laboratory in Gold River, California. OWL can disassemble, recoat and optically realign and optimize your SCT. It is not inexpensive but the OTA will likely perform better than new. I think the OWL shop is moving to a bigger location but you can still call. 

 

Chip



#17 RussD

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Posted 10 June 2018 - 02:31 PM

HI Mark,

I did this about 11 years ago on an old C14 at a local high school. The mirror coating was about half gone so there was little risk of making matters worse.  Tearing down the OTA is no problem as was outlined in the post you had there.  The process is tough but not impossible to do with some common sense and basic tools.

 

Once you have the mirror out and removed the retaining ring from the front of the mirror you can see the difficulty of the job ahead.  The mirror mirror is RTVd through the central hole to the mirror support assembly.

 

http://www.skyimager...of-mirror-1.jpg

 

I used a guitar string to saw through the RTV as best I could.  It takes a lot of patience to get through it all to the center.  Then it is a matter of twisting the mirror away from the mirror support.  This took a lot more time and force than I would have thought.  Dont worry about thumb prints on the mirror.  A rubber mallet helps to whack the mirror support.  But it does eventually give way as celestron  does not back fill the gap between the mirror and the central baffle.  

 

The mirror can then be removed and cleaned up for recoating.  That is the easy part.

 

The more difficult part is putting it all back together.  Take the time to verify that your central baffle mechanically centered and attached to the OTA and it is square to the back casting.  A large caliper is helpful for this.  They are available inexpensively through harbor freight etc.  I also used a square and other tools just to verify things were not completely out of whack.  The OTA I had was ok but not perfect, I dont remember the tolerance,  but my thought was I cold not have done better in the machine shop where I worked, so I left it.  If I recall the central baffle looked like it was brazed or welded to the back casting.  It looked like a lot of work and not worth the risk.

 

Remounting the mirror took two tries to do right. The first time I just RTVd the mirror to the mirror support and reinstalled.  The result was a system that did not collimate well and the star test verified the problem.

 

I realized I had to be much more careful.  I MAY HAVE JUST GOT LUCKY .The tube must be removed from the back casting to do this.  But what I did was install the mirror support AND THE FOCUSER to the mirror support. I propped the back casting of the OTA on blocks so that the open casting was facing upward so that the focus knob could be accessed from below.  First place a three small cork tabs on the mirror side of the support for the mirror to rest upon.  Then I placed the mirror on the mirror support and used the focuser knob to raise the mirror so that the edge of the mirror just reached the height of the edge of the casting.  I carefully measured that height and placed shims under the mirror support to hold the mirror support and the mirror square to the casting.  Then I removed the mirror and verified the mirror support seemed square with the casting and was supported.  I then placed a bead of RTV on the mirror support and placed the mirror back, again using the edge of the casting as a way to square the mirror.  I eyeballed the outside edge of the mirror to verify it was flush with the top edge of the back casting.   I cannot recall, but I probably placed a straight edge along the top of the casting and carefully shimmed the mirror support until the mirror was perfectly square to the casting. You cannot depend on pressure from the focuser at this point or you will put a tilt into the mirror.  It took a lot of messing around but I finally had it as close as I could get.

 

I let it set for a week and reassembled the scope.  I carefully reentered the secondary and verified I had the corrector in the right spot.  The result was a excellent.  The scope worked extremely well.  I am not sure I would want to do this again.  But it was a pretty fun project. 

 

Here is a little story the local newspaper did on our project...

 

https://www.twinciti...pe-observatory/

Regards,

 

Russ

 


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#18 AxelB

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Posted 10 June 2018 - 02:37 PM

Just buy a new OTA. Get the Edge version so you can easily add Tempest Fans.

Sell the old ota to finance the Tempest fans.

Edited by AxelB, 10 June 2018 - 02:38 PM.


#19 YAOG

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Posted 11 June 2018 - 02:49 PM

Just buy a new OTA. Get the Edge version so you can easily add Tempest Fans.

Sell the old ota to finance the Tempest fans.

 

Not everyone has the money to buy a new or even used EdgeHD OTA and for many of us the EdgeHD optics are a disappointment. I had a nice new EdgeHD-8 OTA the DSP TEMP-est system installed. The DSP TEMP-est fans helped the EdgeHD to cool sooner but it always seems to be chasing thermal equilibrium when even my NexStar-11 SCT OTA was already stabilized. There was also a slight loss of center field sharpness and overall contrast but the stars were round to the edges as promised while imaging with SCT bloat unless using the reducer which improved this significantly. For visual use the EdgeHD-8 never seemed to perform as well as the 8" Meade LX200 ACF OTA I had at the same time but I stupidly sold the 8" ACF OTA thinking I was going to image with the EdgeHD-8 which did not happen as I had a couple of very good apos for imaging. After months of testing and side by side comparisons using other EdgeHD-8 OTAs I sold my EdgeHD-8 OTA. I have not missed it in the least. 

 

Chip 



#20 AxelB

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Posted 11 June 2018 - 04:23 PM

I must say after 3 years of testing my regular C8, I decided to keep it instead of "upgrading" to an Edge version. The reason is I got a good tube showing perfect star test. I also aquired a 5" doublet for wide field applications so I now have no problem dedicating that C8 to med and high power applications where I won’t care too much about the curved focal plane.

I’ll add active cooling and other upgrades to my existing ota, knowing the mirrors are worth it.

#21 gnowellsct

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Posted 11 June 2018 - 04:31 PM

Hi Mike W.,

 

 

Your buddy can send his SCT to Optic Wave Laboratory in Gold River, California. OWL can disassemble, recoat and optically realign and optimize your SCT. It is not inexpensive but the OTA will likely perform better than new. I think the OWL shop is moving to a bigger location but you can still call. 

 

Chip

Does OWL have a web page where they guarantee they can do the job and get it back together right?  I have often thought there would be no money in this kind of operation--too much work.  And I did try it (sent it out) and it was a fiasco.  Greg N



#22 YAOG

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Posted 11 June 2018 - 04:47 PM

Does OWL have a web page where they guarantee they can do the job and get it back together right?  I have often thought there would be no money in this kind of operation--too much work.  And I did try it (sent it out) and it was a fiasco.  Greg N

The OWL web site is closed during a relocation to new digs. OWL can do a lot of things and one of their retail services is disassembly recoating reassembly and optical alignment of SCTs. OWL is a well known optical lab with a good reputation, many have used them and swear by their work. The service is not cheap, about $500 plus upgrades but they know what they are doing and if your optics are known to be good it beats taking your chances with a new unknown OTA. Of course the new Celestron SCTs coming out of China these days are consistently excellent optically. I've star tested several of the CPC/Tri-Lobe SCTs in 8" and 11" and they are amazingly consistent with smooth balanced star test results. It's very hard to complain about them really, much more consistent than the USA meade Meade and Celestron SCTs I have tested over the years. 

 

Where did you send your SCT and what did you need done? 

 

Chip 



#23 YAOG

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Posted 11 June 2018 - 05:24 PM

What I forgot to say is that I HAVE missed the spectacular views through that 8" ACF - but ACFs are hard to find used. So I gave up looking and just bought the next best thing, a used Chinese made Celestron C8-A SCT for the lightweight SVP mount or to mount SBS with an apo for more the additional reach they have. 

 

Chip



#24 gnowellsct

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Posted 12 June 2018 - 09:21 AM

The OWL web site is closed during a relocation to new digs. OWL can do a lot of things and one of their retail services is disassembly recoating reassembly and optical alignment of SCTs. OWL is a well known optical lab with a good reputation, many have used them and swear by their work. The service is not cheap, about $500 plus upgrades but they know what they are doing and if your optics are known to be good it beats taking your chances with a new unknown OTA. Of course the new Celestron SCTs coming out of China these days are consistently excellent optically. I've star tested several of the CPC/Tri-Lobe SCTs in 8" and 11" and they are amazingly consistent with smooth balanced star test results. It's very hard to complain about them really, much more consistent than the USA meade Meade and Celestron SCTs I have tested over the years. 

 

Where did you send your SCT and what did you need done? 

 

Chip 

Well they did the work for free because the idea was that if it worked out they would get follow on.  For example my C14 and many others.  But they didn't do the job right and I'm honoring the deal that if it didn't work out I would refrain from badmouthing them.

 

The c8 in question eventually went back to Celestron and got Synta optics in 2015.  Now mind you....this was a "black ribbed" c8 and I had got it cheap and kept it for experiments so I wasn't really that upset that it was out of commish.  (Plus I have about six other scopes to keep me entertained).  But I had to decide whether to fix it or throw it out or sell it for parts.  The recoating company had completely RTV'd the mirror including the central core full of RTV so to be adjusted at all the mirror needed to be freed.  After several hours working with wire and razor blades I was unable to cut the mirror free--was hoping to do the "spinning" myself.  Or at a minimum was hoping to send it back to Celestron to be spun.  

 

But what I got instead was a mess.  RTV is *really hard* to remove and there are no shortage of internet groups that talk about it and I have yet to see an easy solution.  After three hours of work that mirror was on as hard as ever.  

 

So I just packed the whole thing off and sent it to Celestron with the history I just gave and asked them what if anything could be done.  They offered to replace the optics for $.  I figured I would just resell it when it came back and either lose nothing or not too much.    It was that or throw it out.  I told them to replace the optics so this 80s early 90s ish OTA comes back to me with Synta green-coating  XLT fastar optics installed and WOW! that's a good deal.  It's now one of my favorite scopes, no more medical experiments.  Had it out last night.  Delightful.  Put a 3 inch apo on top and you really don't want anything else....as long as you have a good mount.  I will not be selling this c8 as I had thought I would.

 

If Synta could hit quality like this all the time it would I think put a halt to much of the trash talking.  

 

If there's one improvement I would like to see on c8s (maybe 9.25s too) it's that the focuser mechanism needs to be rethought.  At 1mm and smaller exit pupils (200x and higher) it is noticeably harder to focus than a C14 (which hits 1mm exit pupil at 400x, which one hardly ever uses).   I understand the Celestron Pacific SCTs had a much nicer focus mechanism and if there's one thing that would be worth an upgrade it would be a superior focus mechanism.    It would be interesting to try a threaded arm that had finer threads just to see how that worked out though I think it would be a PITA for gross focus adjustments.  But Newts have dual focus and refractors have dual focus and this is a frontier that should be crossed.  The FT doesn't really change anything.

 

Greg N


Edited by gnowellsct, 12 June 2018 - 09:22 AM.


#25 YAOG

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Posted 12 June 2018 - 10:55 AM

Sad story but who is "they"?

Chip


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