Ugh, so here it goes. For the record also, I am self taught, and terms like HFD/FWHM and techniques of collimation are all learned by reading threads and forums. Please go easy on me.
I recently bought a used AT14RCT off of astromart for a very good price. The seller didn't want to ship so I got a good deal by picking up the scope in TX and driving it to the observatory in CO. I was very excited to get it installed, but unfortunately the pier was too tall. I had done the measurements but didn't account for the fact that the pier is centered. (I live in Baja California Sur in Mexico), so the planning phase was mostly just sketching to see if it would fit. Anyhow, I tore out the cement pier and got a Ioptron tri-pier 360, and voila, the 14 just barely fits in the 8' exploradome.
Yay! crisis averted! Not so fast. Early star tests showed some problems that I could only assume were collimation. So I borrowed a howie glatter 2" laser with the concentric rings attachment. The original owner stated that he had only adjusted the secondary (using the same laser) slightly. I was getting no lower than HFD of 8 or so, but was still generating V-curves, so it was reaching focus, but just very poorly. Initial thoughts were the CA fires from this past summer were making for very bad seeing. After time went on, it was clear that the problem was in the optics. The observatory is in the very dark (bortle 3) dry high basin of Park County Colorado at 9000 feet. Ive had great success with the prior scope (AT10RC).
Focuser: Moonlight CSL with Stepper and controller
Camera: SBIG STF8300 with FW5/OAG
Guider: Lodestar X2
Mount: Losmandy Titan with steppers/EQDrive controller and EQMOD.
The collimation seemed to go ok, and we were able to adjust the concentric rings so there was no shadowing or out of concentricity.
We used the following technique:
Adjust the base ring (focuser ring tilt tip) so the laser is centered in the secondary (not the moonlight tilt tip, this has never been adjusted)
Adjust the secondary so the conecntrinc rings are concentric
Adjust the primary (not needed)
Still poor HFD. Keep in mind this scope lives in an unheated obsevatory so it tracks pretty well with the ambient temperture but none the less sufficient time was given to allow the mirror to cool. The AT10RC could get below 6 HFD even on the very worst of nights in this location.
The first thing I noticed is that this 2850MM telescope was showing 2820MM after astrometric reduction in MaximDL6. This is roughly 30mm out of spec. Admitedly I don't know how bad this is but I can say from experience the AT10RC was never more than +- 2 mm based on temperature, and that was the steel tube version. I imagine that this carbon fiber truss telescope should be similar or better. Having read just about every thread out there on this topic I decided that a focal length reset was in order. Most people are saying not to adjust the secondary's center screw, so we slowly adjusted the primary by loosening the retention bolts and increasing the focal length by adjusting the larger bolts the exact same number of turns. Then we redid the astrometic reduction. After several iterations, we are now within 4mm. Obviously it could be adjusted more, but I can say this has not improved the ability to achieve good focus. Again the V-curve has a definite bottom, this is not a back focus issue.
We have also reset the focuser base ring collimation so that it is in the factory position. The laser is reading in the center spot of the secondary. I even went as far as to buy a newer Moonlight CSL which has larger bearings. I was worried that the drawtube was out of collimation on my original moonlight as the laser seemed to move in the secondary center spot when racking in and out (or the bearings were worn out)
So the main problem seems to be astigmatism. Please see the video at:
Note the astigmatism changes direction inside and outside focus by roughtly 90 degrees. At the moment really out of focus stars seem to be fairly concentric.
Im at a loss as where to go from here.
Ive considered reseting the whole system using this technique:
but that would require adjusting the center screw of the secondary, but the effect would be the same, get the focal length correct (secondary this time), reset all the collimation points to the unadjusted position.
Maybe my secondary is not centered? or the center spot on the secondary is not centered (Astronomics suggested this might be the case)
Im also concerned that no amount of tightening of the moonlight load screws will keep it from flexing the laser off the center spot of the secondary if I pull gently on the laser. Right now I have it tightend to the point just before where the moonlight stepper will stall, which seems too tight to me.
With so many possibilities for what could be wrong, I have really lost my way. I never had to collimate the AT10RC and the tolerances on the focuser never seemed to present a problem in the images.
Thanks for looking.
Edited by pterodyne, 10 January 2019 - 12:43 PM.