Post by Chicago Astronomer - Astro Joe on Jun 6, 2011 6:51:10 GMT -6
Collimating the C11...
After the major rehab and mod project on the C11, ( astronomer.proboards.com/index.cgi?board=ATM&action=display&thread=2760 ), I knew I had to collimate the scope. Taking it out for it's first raw test outside of my garage, it showed that the views were decent, but not perfect.
Later on that evening, Chicago Astronomers Bill , Steve and I put it through some primary testing, and found we could nudge it better....and then - it got away from us.
astronomer.proboards.com/index.cgi?action=display&board=StarParty&thread=3219&page=1#12510
Hell, we all have done successful collimations before, but nothing we adjusted and observed could get it back to pin point stars that we have been enjoying for the past few years. Stars were all "comet-like" flared like no bodies business. Using Vega as our test star, for it's angle and initial brightness, perfect focus evaded us big time.
What is going on?
It's just three little screws...!
In researching this gross mis-alignment condition, I'm not alone in this.
Some basics:
What I learned form "Bob's Knobs".....
Celestron secondaries are sticky taped to a flat round metal plate...
The plate looks something similar to the pic above, (Bob's Knobs).
(Which by the way, I finally gave in and ordered me some "Bob's Knobs" for the C11 - as I am certain to take the beast apart again sometime in the future and this will help expedite the collimation. )
There is no spring in these Celestron secondaries and ergo...no "Push & Pull" physics going on. It's a simple teeter-toter thing happening....and why it's vital to loosen one & tighten others in the adjustments of the screws when doing the collimation.
- Something that I did not know before and VITAL.....
There are (3) three optical alignment markers on SCTs.
One is on back of the primary mirror....
There, that fat magic marker line on my primary at 3 O'clock - just below the serial number of "5967". An easy landmark and across from the focusing screw at 9 O'clock - as we face the mirror C.P. side.
Second the serial number engraved on the Corrector Plate...
And third...
There is a marker line somewhere on the secondary baffle that aligns with the Primary mirror and the Corrector Plate....and they should all line up at about the 3 O'clock position together - very simple now....
Now, I have been careful to mark and keep the optic train in alignment when dis-assembling, but things happen and perhaps were not placed back properly. And also...the secondary housing is easily rotatable and might have turned.
The three screws on the secondary should also from a triangle with the apex or point "up", with the lettering on the outside of the secondary housing - right side up.
So, first thing is to check the alignments of the three....an easy procedure.
Then, if good...there's that annoying coma stuff on stars and bright points of light. Not only did we use stars to work on the collimation at our session, but bright lights on buildings in Chicago's magnificent skyline. All had comet-like features.
It's not just a little mis-aligned now...but way off!
The point is to tighten the screw opposite the head of the comet. In the middle of our session and with visitors to attend to, sometimes it was foggy to remember which way to adjust. And to compound the situation the secondary shadow kept "circling" the center, not centering.
Damn it!....
And important...if your star is at the edge of the FOV, (even flared), and it goes out of the FOV while making an adjustment...you are going the wrong way. Stop and do the opposite of whatever you did.
The problem with large aperture scopes, is that our arms aren't long enough to observe the star/point of light - and make the adjustments ourselves. We need a loyal astro bud to assist. But...I'm thinking of using a webcam to make it a one man procedure. Using a laptop, I can stand away from the eyepiece and use the screen to guide me as i adjust the screws. Now, all I need is a USB Cam.
More as we dig deeper into this and start the airy-disk testing again after I check the physical components for factory alignments.
But, here are two great Collimation pieces on Astromart:
www.astromart.com/articles/article.asp?article_id=718
www.astromart.com/articles/article.asp?article_id=548
