In this case, once you have the replacement screws I'd lift the whole secondary and diagonal out before you replace the screws; it would be a tragedy if something untoward happened (eg the secondary falling off and hitting the primary. Personally I would use stainless allen-head grub screws (see accu.co.uk) not "Bobs Knobs".
You also need:
- a $2 plastic hand-spray bottle, a litre of 100% pure isopropyl alcohol (industrial solvent suppliers, its cheap) and a bottle of 100% demineralised water. Fill the hand-spray with a 50/50 mix ready for the final clean during reassembly. You'll also need half a dozen Q-tips or the cotton pads women use to remove makeup.
- a bottle of ladies nail-polish (colour not important),
- a business card, a pair of scissors to cut it, tweezers and/or a toothpick (you'll see why at step 6).
1. Place scope on a bench with something soft covering the work area, eg a thick plush towel in case you drop something expensive. Put a clean well-washed tea towel over that so small screws etc wont get lost in the towel, and the scope on that. I have done this on a carpeted floor, that way there's nowhere for anything to fall.
2. The front ring with all the holes is cosmetic, serves no real purpose. Remove 3 screws and lift off.
3. Under the ring you should find three half-moon metal tabs with small retaining screws holding the corrector in place. Before you undo these, place a piece of tape on the cell in line with the focuser assembly, and put a pencil mark on the ground edge of the corrector (not the polished surface !!) so you can replace it in the same orientation.
If you study the corrector carefully you should find three small shims between the glass and the metal of the cell, these are crucial - they allow for differential expansion/contraction of the cell without crimping the corrector - localised force of metal on glass is a good way to produce a clamshell fracture or worse, break the corrector.
Loosen the 3 screws (they should only be finger-tight), turn the half-moons and lift out the corrector and put it aside. Put the shims somewhere safe too. if one is not looking so good you can make a replacement by cutting up a business card.
Park the OTA somewhere safe.
4. Now deal with the secondary mirror assembly, aiming to replace the screws and put the diagonal back together in the same orientation. Study the assembly; the ones I saw have a knurled ring around the outside of the secondary mounting which, if loosened, allows you to rotate the secondary with respect to the corrector, and the three little screws are for adjusting tip/tilt. The centre bolt adjusts its position along the optical axis. It is advisable to measure the space between the back of the cell and the mounting in the corrector with callipers so you can get this correct again when re-assembling.
5. Clean the optics - including primary mirror - by spraying hard with the water/alcohol mix - not wiping or scrubbing with anything - and allow to drain. The last few drops can be removed by touching each drop with a Q-tip or corner of a cotton pad, if you can do this without touching the glass there will be no lint left behind.
6. Reassemble and place in a warm place - in the sun on a hot day is good, or near a radiator - to drive out the last of the water/alcohol mix. Don't worry if the glass fogs on the inside, it will evaporate later. Don't use a fan - this will blow lint onto the optics.
When replacing the corrector in its cell:
A. make sure the little card shims are in between the glass and the mirror - both under the bottom edge, as well as the side, so glass does not touch metal. This can be quite fiddly and you may need tweezers or a toothpick to manipulate the shims.
B. there are no adjustments to centre the corrector laterally in the cell - the shims take care of that.
C. turn the little half-moon tabs so they are over the edge of the corrector, securing it. Tighten the little screws to hold these in place just finger-tight. if you over tighten these, you will strain the corrector which can have catastrophic effects on image quality. To stop these screws working loose, apply a dab of nailpolish over the screw, it is easily chipped off again if you need to redo all of this.
7. Collimation. This works as per any newtonian, ie rotate the cell to get the primary centred on the secondary and then adjust the tip/tilt screws.
The last part - given the focusser was replaced - is to establish whether the focusser is co-axial with the secondary mirror, it may have to be moved along the OTA. You'll need a cheshire or laser collimator to check this.
8. When all done, point the OTA down somewhat (stops water running inside) and spray the outside of the corrector again to take off any fingerprints, dust or lint.
9. Replace the outer ring with all the holes.
10. After several nights use when you are thoroughly satisfied with the collimation, apply a dab of nail polish to the secondary adjustment screws to stop them moving inadvertently.
With practice it is possible to clean the surfaces as free of lint and dust as Intes made them, and it's spectacular in the "flashlight test".
Edited by luxo II, 06 August 2021 - 06:29 AM.