Folks,
I recently "invested" in a fast newt (SW Quattro 8") to supplement my 4" Esprit for imaging, going in eyes open, taking on a challenge for those lovely clear nights while my Esprit merrily collects photons from around the night sky.
However, having done lots of reading and practiced collimation on the bench, my attempts so far appear to have been awry, as my test images look pretty horrid.
The secondary collimation screws on the Quattro are an exercise in futility and I recognise I will have to tackle them separately to make using them reproducible. I accept that this is just one area where an inexpensive scope needs work, and I may later discover other mechanical issues I need to tackle. I feel it's a bit of a project!
On the bench, I adjusted the secondary to be as circular as possible and concentric with the focuser tube, using a collimating cap (the Rigel one).
I then inserted the laser collimator and made the adjustments to get the beam in the centre of the primary. I made the assumption that, even if the primary is out of collimation, the beam should strike the centre spot if the secondary is collimated. Mistake number 1?
With the beam in the centre of the primary, I rotated the laser in the focuser and it doesn't move, so it appeared to me that the laser was collimated. I then racked the focuser in and out and, again, saw no shift in the position of the laser beam on the primary. I interpreted this to indicate that the focuser is correctly positioned with respect to the tube of the scope. Mistake number 2?
Then, I adjusted the primary to get the reflected spot centred in the reflective face of the laser collimator.
With the coma corrector (GPU) inserted and my usual imaging train added (ASI1600 + mini EFW), I pointed up at the night sky...and well, it was a disaster
...and this isn't the first time.
Having been happily collimating my SCTs for some time, I figured I'd approach with a similar strategy...centre a bright star, defocus, and adjust the collimation of the primary to centre the secondary shadow, as that was off-centre. With every adjustment, I re-centred the star, and repeated until I could collapse concentrically down to a focused point.
Regardless of the above strategies, the images don't lie and round stars are eluding me.
I've attached an image of my test target from last night...the Keyhole area of Eta Carinae nebula, cropped down to 1:1, so this is the central 1200x900 of 4656x3520 pixels. Further out from centre, the aberrations appear to get worse and larger. Just looking at the centre, it shows promise from a detail point of view, but obviously isn't performing its best given the issues there.
Is there any significance to the way the left-hand diffraction spike of Eta Carinae itself diverges, whereas it doesn't on the right (and up, down) ?
I don't have the experience with a newt and feel stumped at this point...if anyone has any clues as to where to start tackling this, either from scratch or by deconstructing the (likely multiple) issues in this image or others that I can post, I'd be very grateful. This wasn't first light for this scope and I'm beginning to lose hope with it... I suspected it wouldn't be easy, but there comes a point it needs to earn its keep.
Cheers,
Dunk