A useful disk on a mirror?

[xelasnave]

Been a busy day inventing a couple of ultimate ota arrangements around this idea of having the primary mirror ride directly above the connection to the mount.☺

I think mounting the mirror there would place less stress and minimize flex and in any event make it controlable.

The way an ota goes the weight of the primary is balanced at the mount to see saw with the camera etc at the other end...also I think mount the camera where the non existant secondary could have been. And using a baffled carbon fiber tube with extenal ribs and hiding in the shadow of the spider holding the camera will be like a web a carbon fiber "wires" adding internal bracing..the camera region particularly supported that way...but there is so much structure you can hide under the spider vanes at the top ...hopefuuly☺

I am trying to get something that cant flex and if it does carbon fiber strings attached to the back of the mirror enable a crude form of addaptive optics to off set any flex...hopefully.

Anyways it occured to me that it would be beneficial on a reflecting scope to place on the primary mirror a disk of the same size as the secondary...reason any light coming from this region cant be good...its a spot that cant reflect parrallel incoming rays so any thing that reaches the chip from that region cant be what we want.

So what do you think?
Alex

[xelasnave]

And what do you think about a gyro scope on a mount and maybe a gear mechanism etc to keep the CSP perfect , via software.
Alex

[LewisM]

Most Cat/Cass designs have the mirror very close to the mount axis - for instance, looking at my VC-200L on it's GPD mount, the mirror is only about 6" back. Most of the weight is very close to the centreline of the dec. axis.

[Wavytone]

Well sure...
- there are string telescopes (Dobs)
- there are scopes with gyro stabilisers in orbit,
- there are scopes with spinning mirrors,
- there are scopes with adaptive optics...
- counterweights and baffles, sure every scope has them...

Put all in a pot, stir thoroughly and see what comes out.. should all be within reach of a DIY effort... it CAN be done...

A fast spinning primary mirror should do nicely for gyro stabilisation, and average out the optical surface errors ... and act as a counterweight. Attach some blades around edge and it will be self-fanning, and should reach equilibrium quickly and keep the dew off. Win-win-win !!!.

One of those fancy Royce conical mirrors should do nicely, I suggest, spinning at 3000 rpm.

Suspend top end on some sticks and string, with each string attached to an electric power winch (like those used in RC model yachts) and rig up a control system to reel in the strings automatically applying the right tension to keep the optics aligned as the scope moves ... should work fine, right ? A 6-channel RC handset could come in handy too for collimation , just twiddle the sticks to tweak the strings like tuning a violin.

[LewisM]

Sounds like a Thunderbirds marionette...

[JA]

Yeah .....
https://www.youtube.com/watch?v=6y2hFqgtjLk

Best
JA

[xelasnave]

Wow Wavy you seem to have a wonderful imagination way past my humble attempts:D

I know you were having a shot..thats ok.

The main thing I was interested to find out was the idea of a spot to cover the middle unuused part of the primary.

alex

[xelasnave]

and forget the strings it was an attention getting step for the main question...the benefit of a spot the same size as the secondary.

alex

[Wavytone]

Ok ... I’ll bite.

Optically a spot the same size would makes matters worse not better. There’s a simple example of why.

Find an SCT and point it at the sky, and look into the back end (no eyepiece). Concentrate on the disk of sky - ie the aperture of the telescope at the corrector. On-axis the obstruction you see in the centre is the outline of the secondary mirror.

But as you move your eye off axis to the edge of the drawtube you’ll see the central tube baffle increasing the size of the central obstruction - and making it non-circular. This is bad, as we all know the central obstruction should be as small as possible - not increased. Secondly a non-circular obstruction scatters more energy into the diffraction defects around point images (stars). In addition the baffles may obstruct some of the perimeter of the aperture causing vignetting. Hence SCTs really do give the best images exactly on axis.

Doing as you suggest to a newtonian would have the same negative effects off axis. By all means put a disk of paper on your primary in daytime and try this for yourself.

Lastly the central portion of the mirror is usually where the polish and figure are best. No reason to hide it.

PS I’m warming to the idea of an RC handset to operate a telescope ... the more I think about it the more I like it. Wired handsets are so last-century.

[xelasnave]

Thanks Wavy most enlightening.
Alex