I Haven't Yet Mastered Collimation with the Edge HD 8 Celestron EdgeHD 8" · Jerry Gerber · ... · 37 · 2145 · 31

I have been enjoying various telescopes for 26 years. My first telescope was a Soligor brand Newton that I still have. I learned to collimate this Newton when laser collimators, computer programs, digital cameras, tribahthinov masks or special Knobs did not exist and it has never taken more than 5 minutes to get it perfect.
A few years ago I acquired a celestron C8 EDGE, a wonderful telescope but with complicated collimation, especially for beginners.
When I started collimating the C8 I noticed a big difference compared to the Newton. The screws were looser, they did not feel firm and most importantly on the C8 when I tightened a screw and loosened it again or did not notice the mirror return to its initial position, which forced me a multitude of extra corrections compared to the Newton .
I disassembled both secondary mirrors, to see the differences. I attach photos.
Newton:

Celestron C8 EDGE. No tension.


The two huge differences are, firstly, that the Newton has a spring that tensions the secondary mirror and secondly, the mirror is held by a tilting screw.This has enormous advantages. The first, when under tension the screws are firmer and greater control is obtained when tightening and loosening. The second and most important, when a screw is loosened, the spring pushes the mirror, so we have total control without the need to adjust other screws. Finally, the presence of the tilting screw prevents accidents, since if it were not there, as happens in the Celestron C8, if we loosen the 3 screws too much the secondary mirror would fall on the primary one.
To obtain the advantages of the secondary mirror that appears in many Newton telescopes, I propose two improvements that are very economical.
The first is to place a spring 7 to 10 mm wide by 5 mm high on the axis with a blunt tip where the mirror tilts. It is an easy spring to acquire and find in a multitude of components that we have at home. With this we will have the tension that will help us in collimation. The second improvement is to change the screws for about 10 mm longer, which provides greater security.


Sometimes when the collimation is achieved, the screws are not firmly secured, so they decollimate when changing position. It is not for nothing that many users collimate with Jupiter's moons before pointing at Jupiter, so as not to move the telescope too much. This problem is eliminated with spring tension. 

I have made these improvements and I can calibrate perfectly in less than 10 minutes, without stress, without problems. When I collimated without the central spring I spent 2 hours on it and never obtained the desired collimation. Of course experience helps and celestron owners acquire skill and achieve perfect collimations in minutes, even more so with the new accessories that are available to help.
I hope that helps.

Leonardo Ruiz:
To obtain the advantages of the secondary mirror that appears in many Newton telescopes, I propose two improvements that are very economical.
The first is to place a spring 7 to 10 mm wide by 5 mm high on the axis with a blunt tip where the mirror tilts. It is an easy spring to acquire and find in a multitude of components that we have at home. With this we will have the tension that will help us in collimation. The second improvement is to change the screws for about 10 mm longer, which provides greater security.

Hi, Leonardo.
This is excellent.  I wondered why Celestron did not have any spring or other such mechanism for maintaining tension, but being fairly new to this sport, figured they know more than me.  But 1.5-2 hrs and a lot of hair pulling is no fun (and nobody warns you ).  10mins sounds damn good!  So your central spring improvement is something I am going to try.


How stiff should the spring be?  Any example devices that would have a comparable spring, or tips on what other toys to pull apart to get a suitable one?
Cheers.
Paul

Thanks, Leonardo.
Super helpful!  And I expect many others will find it so as well.
Cheers.
Paul

an important thing to consider is that the mechanical center is not always the right alignment, as the optical center may differ. I used my OCAL (which i used successfully to collimate my RC and my newtonians) on my C11, it looked way out of alignment from the OCAL although the star test actually looked pretty good. I got everything aligned perfectly thinking i would finally get an amazing collimation but the result once i checked using a star was a disaster, scope was completely miscollimated. Had to redo the collimation using the defocused star, i checked the day after using the OCAL, and it looked misaligned again. That's just how my scope needs to be.

Leonardo Ruiz:

Ian Dixon:
Hi Leonardo,

Thanks for your help and suggestions, this is a nice idea.  I may have missed it, but could you please append an image of the new screws used side by side with the original?  I noted the compression spring height.  Thanks again for this solution. 

Kind regards,
Ian

Hi, Ian.
I show you an image showing the secondary mirror measurements of my Celestron C8 EDGE.



The factory screw is 12 mm. 5 mm remain inside the upper support, 4 mm is the original spacing that I have not modified to avoid variations in the Back focus, approximately 3 mm is what remains pinked and is what maintains the secondary mirror.
The total thread of the lower support is approximately 8 mm, so if we want to have more security and support and take advantage of the 8 mm thread, the screw would have to measure 5mm+4mm+8mm= 17 mm long. I have used the 16mm screw which gives you 8mm clamping, which threads the entire clamping body and I can see when it goes through it. When it appears just half a mm from the screw at the bottom of the fastener, I know I'm at 4mm of spacing.



On the other hand, I measured how much the screw rose or fell when turning it one full turn and it is approximately 0.5 mm.
I adjust, with the mirror in hand, each screw until I have a spacing of exactly 4mm.
Once you have the 4 mm spacing on each screw, the mirror is level and when collimating it is already very close to the desired point, with less than one turn on each screw you reach the desired point, which maintains 8mm of support and together with the tension of the screw it maintains collimation.

I have my C8 in an observatory at my house and I don't take it on trips, so I can't tell you if it holds up to collimation when you take it by car.
I am Spanish, living in Mexico and a week ago the rainy season began, that will last until October. When I have a clear sky I will send you collimation data using CD Inspector, a Tri-Bahtinov mask and some tilt correction tests of the camera chip.

Good luck

Thanks Leonardo, your response is super precise and useful!! 
Kind regards,  Ian

Emilio Frangella:
an important thing to consider is that the mechanical center is not always the right alignment, as the optical center may differ. I used my OCAL (which i used successfully to collimate my RC and my newtonians) on my C11, it looked way out of alignment from the OCAL although the star test actually looked pretty good. I got everything aligned perfectly thinking i would finally get an amazing collimation but the result once i checked using a star was a disaster, scope was completely miscollimated. Had to redo the collimation using the defocused star, i checked the day after using the OCAL, and it looked misaligned again. That's just how my scope needs to be.

This is exactly the problem the OP is having. @Jerry Gerber you are not doing anything wrong. The Ocal is simply the wrong tool for this OTA. I have two Ocal collimators. They are useless for collimating my SCT / C11. I would get perfect mechanical alignment but optical alignment is way off.

With those OTAs, your best bet is to use the defocused star method initially to get the CO roughly in the middle, and then using a tribhatinov mask to dial it in some more. This most often is enough to get you good collimation for long exposure astrophotography.

Now if you are into high resolution planetary imaging, this is not good enough. You will need to collimate using something like Metaguide to really dial it in.