Shifting primary mirror

I’m not going to ask you why you’re shooting low to the horizon…..:)

With rubber pads one would expect a tiny bit of shift at very low angles just because the rubber will compress a bit but it shouldn’t be any more than that. I would remove the rear cell and adjust things so that you see no movement with change of angle. You might also need to loosen things up a little when the cold weather returns. You could change the rubber pads out for plastic for just a bit less movement.

If you see any effects after that it will probably just be due to side loading the mirror causing potato chipping. I wouldn’t expect that in a 200mm mirror but you never know. Primary mirror cells like yours are design to back load and once you start getting away from that, you’re pretty much out of spec.

Christian Bennich · Mar 6, 2026, 07:30 PM

Hi All
I hope that you have some experience that can help me here.

When I image low on the horizon, my stars are not good📷 image.png
Near Zenith - same star.
📷 image.png
Yesterday I realised that my Primary mirror shifted - and that most likely was the cause of this.
I pulled it out, and it did indeed shift when I tilted the mirror cell.

After tightening, I do however still see slight shifting of the primary mirror. (the first image is after reduing the shifting - it was worse before)

So my question is, should I tighten the supporting rubber pads a bit more (risking pinched stars), should I (if possible) buy a new set - as I have never seen this problem before - and maybe they have been worn or something.
I use the Skywatcher 200 PDS - anybody who knows if the rubbert support pads can be bought anywhere?

Thank you in advance for any input on this.

Yo e comprado una lámina de Goma Eva negra y e sentado el espejo literalmente sobre ella . Los soportes con tornillos solo sirven ahora para que no se salga para adelante. 😁😁 Porque a entrado bien justo . Cortés lo que sobraba y encima puse mi anillo protector de reflejos. Y creo que e solucionado varias cosas a la vez. Las luces traseras , los movimientos laterales y evitar apretar de más los tornillos de momento funciona perfecto . Después de la modificación. . otro punto débil si tienes el enfocador original de los PDS es que ellos tienen un juego en la corredera … hay tonillos que lo ajustan desde fuera. Porque también sino al inclinar el telescopio se mueve. La prueba final es retraer el enfocador con el láser y que jamás se mueva de su punto . Yo compré un QUATTRO 200 F4 y venía todo mal 😔😔 tuve que desarmarlo y ajustarlo todo . En los viajes de fábrica a tienda … todo se afloja . 🤦🏻🤷‍♂️

Seizal Bong · Mar 7, 2026, 12:21 PM

Mirror shift can be such a pain, especially when you're trying to get a perfect focus for imaging. Using an external focuser (like a Crayford) and just locking the primary mirror down really is the way to go. It makes a world of difference in stability.

That’s not his problem exactly. He’s using a Newtonian, not an SCT.

Craig Towell · Mar 7, 2026, 08:50 PM

It’s been an issue with Newtonian pretty much ever since they were invented: Holding the mirror tight enough to prevent any movement but not too tight to cause deformation. It’s a very fine line.

A well-made mirror cell handles this without problem. For a 200mm primary mirror, the lateral clearance between the cell and the mirror edge can be as low as 0.1mm overall to account for differential thermal expansion. It’s really a problem of mechanical design (with known solutions).

andrea tasselli · Mar 8, 2026, 10:12 PM

Much less than that.

it depends on the material used. For an aluminum cell 0.1mm is OK up to -10°C if I remember well, with a carbon cell smaller values are possible.