Skywatcher Quattro 250 f4 Secondary mirror offset

[ThadeusB]

I have a Quattro 250 f4 which is out of colimation. I bought this scope as a used item which I think has been fiddled with. I know that this scope must have secondary mirror offset. However, I am having trouble finding out whether the mirror is mounted offset on the stem, or, whether the offset is set in the spider vanes. If the spider should be offset, does anyone know exactly how much this should be? I have tried contacting OVL but have had no response from them.

If you have this scope yourself, could I please ask you if you could measure the spider to see if the secondary carrier is offset away from the focuser, and by how much.

I hope someone can help.

My thanks in advance.

[alacant]

Hi

We don't have the Quattro, but I beileve our ES f3.9 is the same idea.

Center the secondary in both the tube -using the spider vanes- and the focuser -using the adjusting screws on the secondary support-. Due to the position that the secondary is attached to the support, the offset is then set correctly. 

Fine tune to perfection using a Cheshire sight tube. One with cross hairs, even better. 

Cheers

[Craig a]

Yes I have the Quattro 250cf and the offset is already built into it I believe, just centre the spider in the tube and center the secondary under the focuser and when it’s collimated it will look offset naturally looking through the Cheshire or collimation cap 

[ThadeusB]

Thank you alicant and Craig. I have precisely centred the spider and re-colimated using a colimated laser, colimation cap and a long cheshire. A session last night appears to show that the scope is now properly colimated, although the seeing was not optimal. At least I could see that a de-focussed star was symetrical. You are right Craig, the offset is built into the secondary; it's 5mm +/- a bit of measurement inaccuracy.

I also checked that the focuser is orthogonal to the tube axis.

So, I think all is good now. If the seeing is good enough tonight I may do some  imaging; that should prove the colimation.

I'll post again with the results.

Cheers

 

Phil

[alacant]

30 minutes ago, ThadeusB said:

focuser is orthogonal to the tube axis.

Hi

It doesn't need to be. To save loadsa time whilst collimating, be sure to read the collimation myths.

Cheers

Edited February 28, 2021 by alacant

[ThadeusB]

On 28/02/2021 at 17:35, alacant said:

It doesn't need to be. To save loadsa time whilst collimating, be sure to read the collimation myths.

Yes, that's interesting. It begs a question though. The skywatcher focuser fitted to the Quattro has adjustment screws for setting it orthogonal to the tube axis. Surely they wouldn't do this if they didn't consider it necessary.

[ThadeusB]

All appears to be well now that I have found that the secondary offset is built into the mirror itself, thus the spider is set dead centre in the OTA. A full fresh colimation shows that a star de-focussed has a symetrical pattern.

 

So, solved guys.

[alacant]

On 03/03/2021 at 16:27, ThadeusB said:

if they didn't consider it necessary.

Hi

It's Telia's first myth: you have to square the focuser very accurately. Seronik takes this slant upon the same. 

A guy at our local bar astro group got me to draw the ray diagram. To scale. Until I did, I couldn't understand either. 

**EDIT: three things I forgot...

- so long as they remain fixed relative to each other, the mechanics don't matter. By collimating -especially using a Cheshire tube with cross hairs- any tilt of the focuser is neutralised.

- the secondary mirror has no optical properties other than reflection.

- draw the ray diagram with the focuser at obviously differing angles to the tube. This one really does drive it home: all you do is tilt the secondary. Remember, it's just a flat mirror.

Cheers

 

Edited March 5, 2021 by alacant

[ThadeusB]

Yes, alacant, this is what I found too. Mine is spot on orthogonal.

[iapa]

I did a full strip down of my Quattro 10CF a few years ago and posted some of the steps if it helps: