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Loose Secondary on my SCT

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#1 FebStars

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Posted 01 July 2011 - 04:25 PM

I have a Celestron C8 purchased new a bit over two years ago. It has the serial number across the front instead of the brand name. While installing Bob's Knobs recently, I noticed that the secondary turns in the corrector plate, sometimes getting tight and then with some prodding, would go loose again. I'm not sure if this was occuring before my installation of Bob's Knobs or not.

I CAN get it to stay put- nice and tight- in a certain location with enough fiddling. Questions are- Where's the right place and what's going on with my secondary? :lol:

Thanks,

Tom

#2 skybsd

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Posted 01 July 2011 - 05:56 PM

Hi,
Just so I understand what you're saying here..,

Is your corrector plate really turning clockwise and counter-clockwise along the front corrector cell as you turn the secondary?

Regards,

skybsd

#3 woodsman

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Posted 01 July 2011 - 07:07 PM

I don't think he was saying that the corrector plate was turning, just that the secondary mirror housing can spin in the glass of the corrector. My C14 is the same way.

I think that you should be reading the serial number from left to right, and usually the corrector screws are oriented such that if you drew lines connecting the screws, you would form a pyramid. To think of it another way, if you drew a line from each screw to the center of the secondary housing, it would form an upside down 'Y'.

Hope that helps.

#4 FebStars

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Posted 01 July 2011 - 09:09 PM

Thanks, guys. Yes, it's secondary mirror housing itself that is turning, not the corrector plate.

Woodsman, are you saying that if my serial numbers are reading horizontally straight across and left to right (and
perpendicular to the dovetail), my secondary mirror should be correctly aligned with my primary?

Tom

#5 woodsman

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Posted 01 July 2011 - 11:57 PM

Hi Tom,
Well now, that is a good question. All of my SCTs are fork mounted, which is a bit different from the dovetail on a GEM mount I imagine. I'm sorry to say that I'm only familiar with my older Celestrons, the orange tubes that mount in the forks. When I have my scope up (plane of the corrector plate perpendicular to the plane of the ground), with the finder at the top, and I'm in the back of the scope, staring at the rear cell, with the OTA parallel to the ground, perpendicular to the Zenith, pointing straight, the focus knob is directly to the middle rt. or East if I were looking at a map with North straight up.

Now if I move to the front of the scope and stare directly at the secondary holder, the screws on that would form an upright Pyramid, like Giza. Does that make any sense whatsoever? I know its difficult for me to describe because I'm only familiar with my scope design which uses the forks and not a dovetail.

I wish I could be of more help, but I'm probably confusing you even more. Hopefully someone will chime in with some more helpful info. Good luck, Rich

#6 skybsd

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Posted 02 July 2011 - 04:18 AM

Hi Tom,
Gosh, thanks for confirming - my mistake..,

Whew!!!! Not as bad as I had thought..,

Regards,

skybsd

#7 PhilCo126

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Posted 02 July 2011 - 05:45 AM

So the screws should be showing an upside down Mercedes logo ?

#8 woodsman

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Posted 02 July 2011 - 09:34 AM

My experience and all my telescopes in their forks shows a Mercedez logo right side up, if one were to draw a line from each screw to the center of the secondary housing. Taking my telescope, with the OTA in its forks and placing the mount straight up on the tripod, if I tilt the OTA to make it parallel with the Earth, and I look straight at the Corrector plate, I see this if I visulize those lines:
http://www.google.co...cedes benz logo

Since I don't have any reference for the GEM/dovetail mounted OTAs, I'm stuck with just trying to relate my experience.

Hope that helps although I figured that someone out there with a dovetail mounted tube, would by this time have chimed in to assist in your questions.

Good luck,
Rich

#9 jmiele

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Posted 02 July 2011 - 11:39 AM

Rich, Your fork tube is mounted with the lettering readable when in the forks so should be the same. My C-6 has the two screw favoring the top. I say favoring because they are not in a perfect aligning to the tube. If I said that correctly. :)

Joe

#10 FebStars

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Posted 05 July 2011 - 07:40 AM

Hi guys-

Well, last night I had some sucker holes- enough to fix on a star. With the serial numbers running horizontal across the face (right- side up), that is, perpendicular to the Dovetail, I was able to fine tune the unfocused donut with ease.

Then it got cloudy. So I'm not sure if I got it right yet, but we'll see.

Thanks,
Tom

#11 rmollise

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Posted 05 July 2011 - 11:29 AM

Hi guys-

Well, last night I had some sucker holes- enough to fix on a star. With the serial numbers running horizontal across the face (right- side up), that is, perpendicular to the Dovetail, I was able to fine tune the unfocused donut with ease.

Then it got cloudy. So I'm not sure if I got it right yet, but we'll see.

Thanks,
Tom


Corrector/secondary rotation is not super critical. Collimation is. Next time out, just barely defocus, just till you can see the star's diffraction rings, till it looks like a little bullseye. If it is concentric (it needs to be centered) you are done. If you see a donut, you are a little too defocused for good collimation adjustment.

#12 Geo.

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Posted 06 July 2011 - 11:42 AM

I have a Celestron C8 purchased new a bit over two years ago. It has the serial number across the front instead of the brand name.


Sounds like a Fastar secondary. Installing Bob's shouldn't have created this problem. I've seen C8s with thread wound into the interface between the corrector and secondary holder hub to deal with this.

#13 apfever

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Posted 06 July 2011 - 02:07 PM

I have a C8, black tube, starbright coatings, orange button on the secondary holder (and on the front cover), and a SMOOTH back cell not the kind with the ribs.

My secondary holder is loose in the corrector plate as well. The secondary holder spins feely and has a small amount of side to side movement.

I'm good with getting the secondary properly located, no problem. My question is how does it attach to the corrector to be secured. I've read that some are glued on. I need help with the securing process and would like to use the original method. How is the best way to re-secure the secondary FIRMLY so it can be collimated and used in the normal fashion without fear of it getting loose? If glued, how does one re-glue it? IF not feasable to redo the original method, what is the best option?

Any help in the method to re-attach the secondary housing correctly to the corrector is greatly appreciated.

Thanks All,
Neil

#14 Twilight

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Posted 07 July 2011 - 01:00 AM

I am not experienced with this but I am thinking something like an RVT silicon glue might be good for this. I am thinking it is flexable enough that it will still allow some movement to collimation screws. I have and aquarium that holds 95 gallons of water and I think water is 8.4 lbs per gallon which would make it 798lbs of water inside pushing outward!. If it will hold 4 pieces of glass together I would think it would safely hold a small mirror.

Just a thought here for you to consider. I don't know if there would or could be any kind of reaction but surley some on here that build scope can chime in on this.
Phil

#15 rmollise

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Posted 07 July 2011 - 06:42 AM

I am not experienced with this but I am thinking something like an RVT silicon glue might be good for this. I am thinking it is flexable enough that it will still allow some movement to collimation screws. I have and aquarium that holds 95 gallons of water and I think water is 8.4 lbs per gallon which would make it 798lbs of water inside pushing outward!. If it will hold 4 pieces of glass together I would think it would safely hold a small mirror.

Just a thought here for you to consider. I don't know if there would or could be any kind of reaction but surley some on here that build scope can chime in on this.
Phil


Nah. That is not the way it works. The secondary rides on a backing plate, and that is held to the secondary assembly by the collimation screws. The attachement of the whole thing to the corrector is via the baffle on the inside that is screwed onto the secondary assembly that protrudes through the corrector. In other words, you have two halves, the secondary assembly and the baffle. They the baffle is screwed onto the seconday. If the secondary is loose, you must remove the corrector and tighten the two halves. Unfortunately, some baffles are glued as well as screwed. :bawling:

#16 apfever

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Posted 07 July 2011 - 04:24 PM

Thanks Rod,

I'll get the corrector off and see what I can do about the baffle tightening. If the baffle is glued as well then I'll see about getting it off..... somehow.....

Neil

#17 rmollise

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Posted 07 July 2011 - 05:18 PM

Thanks Rod,

I'll get the corrector off and see what I can do about the baffle tightening. If the baffle is glued as well then I'll see about getting it off..... somehow.....

Neil


Just make sure you make note of the rotational orientation of the corrector and don't lose any shims (and put them back in their original places) if they are present.

Last one I did a while back was an '88 Super Polaris. Glued. Several freezing warming cycles in the refrigerator's freezer compartement finally got it. Not much fun. ;)

#18 corax

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Posted 07 July 2011 - 11:14 PM

Unfortunately, some baffles are glued as well as screwed. :bawling:


Even more unfortunately, some are ONLY glued; no threads, just smooth plastic that presses together. This is the boat I'm currently in. I'm starting to hunt for a replacement that actually has threads, 'cause I really don't want to glue it back together. It took a strap wrench to get it apart the first time. :shocked:

#19 Twilight

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Posted 08 July 2011 - 05:20 AM

I am not experienced with this but I am thinking something like an RVT silicon glue might be good for this. I am thinking it is flexable enough that it will still allow some movement to collimation screws. I have and aquarium that holds 95 gallons of water and I think water is 8.4 lbs per gallon which would make it 798lbs of water inside pushing outward!. If it will hold 4 pieces of glass together I would think it would safely hold a small mirror.

Just a thought here for you to consider. I don't know if there would or could be any kind of reaction but surley some on here that build scope can chime in on this.
Phil


Nah. That is not the way it works. The secondary rides on a backing plate, and that is held to the secondary assembly by the collimation screws. The attachement of the whole thing to the corrector is via the baffle on the inside that is screwed onto the secondary assembly that protrudes through the corrector. In other words, you have two halves, the secondary assembly and the baffle. They the baffle is screwed onto the seconday. If the secondary is loose, you must remove the corrector and tighten the two halves. Unfortunately, some baffles are glued as well as screwed. :bawling:


Uncle Rod, I figured you could straight my thoughts out on that if I was misunderstanding how it went together. At least I was thinking! HaHa! I guess your saying my idea wont hold water Huh! HaHa!
Phil

#20 rmollise

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Posted 08 July 2011 - 06:43 AM

Even more unfortunately, some are ONLY glued; no threads, just smooth plastic that presses together. This is the boat I'm currently in. I'm starting to hunt for a replacement that actually has threads, 'cause I really don't want to glue it back together. It took a strap wrench to get it apart the first time. :shocked:


Yep...run across some of these too, including one we have here. Me? I just glued it back together using a glue that ought to be a little easier to break than whatever the Big C used. ;)

#21 corax

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Posted 09 July 2011 - 08:14 AM

I just glued it back together using a glue that ought to be a little easier to break than whatever the Big C used. ;)


What kind of glue would you recommend?

Thanks!

#22 rmollise

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Posted 09 July 2011 - 12:31 PM

I used a small but sufficient amount of superglue.

#23 HUsher

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Posted 10 August 2011 - 06:38 AM

I'm suffering the same problem too, and as I've just started using a hyperstar its a real problem. I dismantled the corrector yesterday, tightened things up and it was secure, so I was rather pleased! But by the middle of the night it was all loose again. I'm assuming there's a temperature element here too. I've just found this thread and so plan to use a small amount of silicon glue. If I tighten it and glue the threads at room temperature will I still have a problem when the temperature changes? Could I dab a bit of silicon on the side face of the central hole in the corrector so that there is a metal/glass attachment to stop any rotation? As the insertion and removal of the hyperstar will put stresses on these connections I'd like to make it all as secure as possible! I'm also hoping it will remove one possible source of misalignment in a very sensitive hyperstar system (which is currently producing odd shaped stars!)

Thanks for your help

Helen

#24 rmollise

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Posted 10 August 2011 - 08:41 AM

If you need to glue it...yes...some sort of glue with a little flex. In retrospect, that's probably what I should have used, but I haven't had any further problems with the secondary on that OTA.


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