Celestron Nexstar 127 mirror mount issues

[Jocksquin (Ron)]

Hi all,
just a quick query to anyone who might help with the following;

I was checking out a "binding" focuser knob on a second hand Nexstar 127 I have just bought. Turned out it was the knob casing binding on the back of the ota so a small adjustment fixed it BUT.......

at the start of the investigation and after removing the knob to check things out,I stood the ota on it's 'end',face down and was horrified to hear a mighty 'clunk" and found the primary mirror assembly had fallen down the tube and bashed into the objective.
I then did some quick research and managed to disassemble the whole ota to find the only real damage other than some scuff marks on the inside walls of the ota, was the secondary mirror plastic shield had been dislodged from the front lens.
I then had to remove the objective lens (using the blunt ends of two 1/16" drills to remove the lens retained ring - they fit perfectly) to ensure a correct positioning when fitting for the secondary mirror shield and to clean out all the "swarf" that had dislodged from the 5 inch threads when removing the ota back(and again after I re-fitted the back). So,with everything re-assembled and a quick course in collimating a Mak, everything is working and I had a good visual session last night and am impressed with the performance when comparing it to my 102/500 refractor.

However,I am absolutely stunned that the only thing holding the complete (and heavy) mirror cell in place was the focuser knob. I don't know if this is 'standard' on this product and would expect at least a shaft retaining circlip or similar to be in situ but there seems no provision for that.

Does anyone know if my unit is missing a retaining clip or similar or have I got the scope "as manufactured" - if the latter,I hope my experience can help anyone else who might stand their ota on end and decide to remove the focuser knob - in a word "DON'T!!!

Rgds
Ron

[MortonH]

Pretty sure there should be some kind of 'stop' to prevent the primary mirror going too far. Maybe someone at the factory was a bit slack one day.

[Jocksquin (Ron)]

Yeah mate,I agree with you,but as i said,doesn't seem to be any provision for one.
I have now written to Celestron support and put the question to them.
Will post the results
Rgds
Ron

[Jocksquin (Ron)]

Hi all,
for anyone who had an interest in this subject,I have just received a response from Celestron confirming that the ONLY thing that holds the main mirror cell in place on my Nexstar 127 Mak is the focuser knob - see below....

Hello Ron,
Thank you for contacting Celestron.

You are definitely correct. Removing the focus knob will unlock the primary holding cell and the mirror will be dislodged from the rear plate.

Regards
Wil Garcia
Technical Support Representative
2835 Columbia Street | Torrance, CA 90503 | www.celestron.com

Unbelieveable but true !! I am not sure if the same applies to other versions of the Nexstar range and as I think the Skywatcher Mak range are made in the same factory,I guess it is the same for Skywatcher Maks.
So if you want to disassemble your Mak,OR just stand it up on the corrector plate/objective to do some adjustments,etc the message is:

DON'T REMOVE THE FOCUSER KNOB!!

Rgds
Ron

[VPAstro (Andrew and Cam)]

Wow, that's interesting.
I wonder how many other people have done the same thing, and dropped the mirror into the corrector plate.
Andrew

[Jocksquin (Ron)]

Hi all,
my last input on the subject.
I sent a further request to Celestron to confirm which other models in the Mak range had the main mirror cell held in place by ONLY the focus knob and got the reply below. It's fairly clear this method of "securing" the mirror cell is common to other models in the range. As for Skywatcher Maks,I have no confirmation their Maks are similarly assembled but as they are made in the same Synta factory,you can bet it is the same for Skywatcher.
So,once again,if you stand the OTA on the corrector plate/objective and remove the focus knob,the mirror cell will dislodge and crash down onto the correctore plate and secondary mirror shield.)
I hope no-one gets into the same predicament as I did - it might cost you a new Corrector plate.
See recent correspondence below;
Rgds
Ron

Hello Ron,

The 90mm , 102mm, and the 127mm Maks have the same design. Although I have not seen the larger models, you are right about the extra weight of the mirrors.

Check the photos from the Skywatcher website. The focus knobs look different on the 6" and 7" models.
www.skywatcher.com


Regards
Wil Garcia
Technical Support Representative
2835 Columbia Street | Torrance, CA 90503 | www.celestron.com

Hi Wil,

I find this quite extraordinary. The natural resting position for a Mak when unmounted,is to rest it on the corrector plate/objective as that is the heaviest(and most stable way) to rest it.

I wonder how many owners know they are just two tiny grub screws (holding the focus knob) from a potential disaster. Surely there should be a retaining clip(circlip) around the shaft to ensure what happened to me does not happen to others. I was lucky, as the only damage was a displaced secondary mirror shield.

I imagine Skywatcher Maks have the same method of retaining the mirror cell as I think they come out of the Synta(?) factory as well?

As I will probably upgrade to a 6 or 8" Mak at some stage after I have finished trying out the current 127 2nd hand unit(I am impressed with performance so far),can you tell me on which models the mirror cell relies on the focusser knob to keep the mirror cell in place? or is it all??

I believe, with the weight of the mirror cell on larger models becoming quite heavy,some other retaining method MUST surely be used?

Looking forward to your response
Brgds
Ron