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I am thinking on grinding my own lightweight mirror (first f4 16", later f4 24"). The lightest and cheapest option is to get a thin blank and slump it in a decent kiln.
Anyone has longer term experiene with slumped mirrors? Overall doesn't seem to be more work than a normal (not pregenerated) blank.
Do I have to grind the backside as in case of normal flat back mirrors to avoid astigmatism? How do people support a convex back while grinding? Does it make sense to grind a hole in the middle for additional support? I am remotely considering a convertible Newtonian/Cassegrain system anyway...
Looking for a 12" DOB (or larger) which can be transported to Prestwick, Ayrshire. Any brand, as long as the telescope is optically sound and in good working order. I'm a reliable buyer who has bought and sold on this site before.
I'm trying to balance my AZ-EQ5 as perfectly as I can, all was going well (all 3 axes) until I added the Polemaster. As this is off-axis it applies torque to the RA causing the axis to rotate until the Polemaster is hanging at the lowest point.
I need to couterbalance this, any ideas how?
Do I need to counterbalance this? Am I being too picky?
I have considered taking the arm off and gluing the Polemaster onto the on-axis point of the top cover, but again, is this overkill?
Looking to upgrade from my current 4.5" EQ reflector to an 8" Dob.
Not particularly concerned with brand and don't mind used condition provided all is in good working order.
Bundled accessories are a bonus (EPs and filters especially) though not a must.
Based in North Wiltshire and happy to drive to collect within reasonable distance (<1hr30).
Hope someone with more experience than I, which basically means anyone that has successfully collimated a Newtonian, can answer a couple of compound questions I have based on my first and only attempt at secondary collimation of my SkyWatcher Flextube 250.
1) All three of my secondary collimation screws were extremely snug before I did anything and I was only able to comfortably turn them counter-clockwise. Is this normal? Do I need to loosen all three screws first before I can properly start collimation? Should I be turning any screw beyond "snug"?
2) Before collimating, I placed a yellow sheet inside my OTA opposite my focuser tube and I placed a red sheet between my secondary and primary. The view this gave through my focuser tube was of a red circle surrounded by a partial yellow ring (the secondary mirror stalk blocking a portion of this yellow annulus). While independently turning each of the secondary collimation screws counter-clockwise I looked down the focuser tube (both with and without a sight tube installed) expecting to see some change in the shape of the red area (more or less circular) and/or the yellow area (less or more even thickness). I turned the screws no more that 2 complete revolutions. I did not perceive any appreciable difference in what I saw and I turned each screw back (clockwise) to their original tightness before working with another of the screws. Does it make sense that I didn't perceive any change? Should I have turned the screws more revolutions? Should I have loosened more than one at a time?
Very confused and looking for your help. Thanks