3D Printed 6" Newtonian

[Stub Mandrel]

15 minutes ago, johninderby said:

Glad to see it has worked out. 👍🏻

Flocking or paint?

Probably a combination - I only have enough flock to do about 2/3 of the tube.

[johninderby]

I gues flock the top and bottom and paint the bit in the middle.🤔

[Stub Mandrel]

14 minutes ago, tooth_dr said:

That’s awesome!  Quick question - the focuser seems quite far racked out and the eyepiece quite a distance from the tube.  Is this fixed and determined by the mirrors?

That's deliberate, I set it so the focal plane is about 1/2 from racked all the way out so it will focus all my EPs without intruding into the tube. This means there's 1 1/2" of backfocus available if I want to try it with a camera.

I just had ago, I was able to split Mizar easily, even at low mags, although with the 2.5 and 5mm it was pair of airy discs (more like triangles to be honest, I think that's affected by the spider).

There was no hope of galaxies with moonglow. I found M13 and although it was clearly quite big it was a faint patch of light grey on a pale grey background, so I packed up.

[Stub Mandrel]

Lessons learnt!

It's too easy to nudge the main mirror collimation knobs. This is true of most scopes, but most have lock screws. Also the secondary adjustment screws are menace when they protrude, tehy also mean a bhatinov mask would need a big hole in the centre.

So I'm doing the opposite of 'Bob's Knobs'. I've ordered long M5 grub screws for secondary adjustment. I will also use these to lock the primary adjustment, while adding circular 'guards' around the adjustment nuts that will also act as feet if standing the scope on end.

The joy of 3D printing is that if I had milled out some aluminium shapes it would be very hard to abandon things, but instead I can put £2 worth of PLA in the recycling (it would be nice to have a filament shredder/filament extruder!) and print again.

[Chriske]

There's no way triangular shaped stars are produced by a spider Neil.
Look for tension somewhere in the optics.

[Stub Mandrel]

1 hour ago, Chriske said:

There's no way triangular shaped stars are produced by a spider Neil.
Look for tension somewhere in the optics.

It can't be mirror pinch as it's glued at three points, the 'clips' don't actually grip it, just locate it centrally.

I wonder if it's a result of reflections inside the white tube combined with the shapes I've used.

... time passes ...

I've done some exploring online and the nearest image I could find was caused by secondary mirror clips... now I've glued the secoondary but when I removed it from the old holder under each of the clips was noticeable patch of grey blooming.

Before assuming the primary was distorted by its original mounting (unlikely with a mirror 6" diameter and 1" thick) I think I'll get the secondary recoated as well, it can wait a while as the effect is only going to be an issue if I was trying to split tight doubles.

[Chriske]

In the past I've glued mirrors, as it was explained to us, in those days.
These days I never do. All friends's  telescopes that have odd shaped diffraction images I remove glue/silicone or whatever. 99% of the time problem solved.
All mirrors should be able to move in it's cells, be it very little.

Edited May 4, 2020 by Chriske
spelling

[Stub Mandrel]

Interesting... my 10" is glued and gives a pretty good star test, although collimation was slightly off.

 

Another astrophotographer's reaction on seeing my vanes was to ask 'do you get trinity diffraction patterns' because he had them in the past. It does seem triangular apertures may produce the sort of shapes I saw. I need to repeat the exercise on a brighter star to get a clearer image.