View Full Version here: : 10" Newt Mirror Pinching ? Pic added ?

13-07-2017, 01:57 PM
Hi all,

Been having some issues with my Black Diamond 10" F4.7 which I have dusted off and started using again recently ,,, 1st issue was obvious, collimating the main mirror with laser tool, but not doing the 2ndary, assuming the 2ndary was ok .. The main mirror reflected the laser, but was not even on the target, - so I corrected it as it was producing 'tear-drop' stars with camera, Lol

So I goes and do the 2ndary, then the main mirror again, Looking good !
But lo & behold, the qhy10 was again producing weird goings on with the stars, : And the attached 'sketch' is demonstrating the resulting situation I now face ? Rather bad 'Astigmatism' in a big way :question:

My 1st thoughts are that the main mirror is being pinched in the mirror cell ?
In saying that, I had taken the mirror out approx 9 months ago to give it a clean, and had quite a problem un-doing some of the Mirror retainer lug screws, which in fact 3 of the 'snapped' :rolleyes:
There are 6 lugs holding the mirror & each lug has two screws - so i just put it back together, but some other screws were quite tight just winding back in, even well before getting to the point of a smidge of pressure on the actual mirror, and I know thats all is required, & some screws are slightly bent by the feel and look , mmm :screwy:
So I assume the mirror has pinched somewhat because of this, some lugs holding firmer than others which have only had one screw in them, dew to the snapped ones not being replaced ? yet, ?

Either way, I am Not 100% sure just yet, as I will be rechecking after some relative maintenance capers as about to mention, and rechecking with just an eyepiece, to quantify that the issue is not related to either the camera/chip, Corrector or LP Filter ?
So, I am getting all new screws, and had to quite forcefully with good pliers & WD40, undone the remaining snapped-off bits, and have cleaned out all the screw recesses they wind into of any filings dirt etc

So, In Saying All of This, QUESTION I am wondering 'if so' a mirror that has been stressed in this fashion for a long (9 months'ish) Would/Could it be possible to be corrected, or should it re;conform to its proper shape with just some resting out of the cell entirely ?

Thoughts, experienced replies appreciated....

13-07-2017, 03:36 PM

others will be more qualified than me to comment on the mirror issue. But for the screws, if you buy a plug tap of the right size, it should clean up the threads quite nicely. It's most likely to be a metric thread, probably about M3 or M4 if I remember from my Bintel 12".
cheers Phil

13-07-2017, 03:46 PM
Glass is VERY elastic, like a gelatin.
So it will immediately go back in original shape when stress is removed.

13-07-2017, 06:22 PM
Very much so, Thanx a bunch Phil :thumbsup: and much like what I have just done, Picked up a pack of M4 screws/bolts, as most the originals were bent & lucky I had one 'nut' amongst the collection jars that wound on perfectly to take to ' H-Hmmm, Cunnings, Lol, just to be sure :D And while there I picked up the matching 'tap' incase the recesses were problematic, but they are not, very straight and still well still threaded, tho I reamed them all 'in & out a few times with the drill anyway, but quite amazing they were still good considering the bolts are steel and the cell is alu,,,, who'd of thunk :screwy:
Cheers for the reply & Input champ, definitely on the 'money' :thumbsup:

:cool: The Best news Ive heard today Bojan :D Probably for the week, ? mm, yup, lets say for week mate :lol:
:thumbsup: :thumbsup:
Will put this thing back together and watch the clouds roll in, , :P & Ill happily take all the blame for that ;)
Cheers :thumbsup:

15-07-2017, 12:36 PM
Thought to add a pic ( actual saved jpg from the QHY10 C) of the latest field since de-stressing the mirror - still has some Diagonal curvature ? Weird hey ?

I am also assuming the Baada MPCC (Multi-purpose-come-corr) is somewhat the wrong distance from the chip, , either under/over correcting ?
I have used this exact system some yrs sgo with excellent results from the corrector, but for the life of me cant remember where was on the image/focal-train it was ?
Is there a web page relating 'spacings for baada correctors' or similar about ?
I may need refreshment, or do a search myself when time permits, I know this type -thing has been discussed a few times, so I should be able to find something without too much drama !

Ohh, yes, some more collimation needed, yes, but want a flat field 1st !

I am also considering taking out the LP Filter, as I haven't used it in this system before as last time use was in Dark Skies, just to help isolate where this problem is coming from, Re very long stars in opposite corners, but hardly any in the others, Hmmmmph :rolleyes:

23-07-2017, 04:54 PM
So out-of-touch with scopes, Lol,
All was just the MPCC :rolleyes:
Needed the 50-60mm space from the chip - had it right on the camera :stupid:
Feels like starting all-over from scratch , , :ashamed:, , , :doh: