Secondary mirror position under focuser

[Peter Reader]

Setting up a Skywatcher 200PDS and concerned about secondary mirror position under focus tube (see image).

Through sight tube it seems the secondary is too close to the primary (needs to move left in the image), however the central screw for the secondary mirror holder is tightened all the way in and there is no more travel.

Can anyone advise?

[bottletopburly]

No that’s the offset , invest in a 2” concentre eyepiece that really accurate for setting up secondary mirror.

[Spile]

4 hours ago, Peter Reader said:

Setting up a Skywatcher 200PDS and concerned about secondary mirror position under focus tube (see image).

Through sight tube it seems the secondary is too close to the primary (needs to move left in the image), however the central screw for the secondary mirror holder is tightened all the way in and there is no more travel.

Can anyone advise?

Yes you have offset error a>b so as per 

https://astro.catshill.com/collimation-guide/

loosen the three outer bolts and central screw to bring the secondary away from the primary.

[Peter Reader]

38 minutes ago, Spile said:

Yes you have offset error a>b so as per 

https://astro.catshill.com/collimation-guide/

loosen the three outer bolts and central screw to bring the secondary away from the primary.

My issue is that even with all 3 bolts fully loosened and the central screw fully tightened, the secondary will move no further away from the primmary...

[Peter Reader]

1 hour ago, bottletopburly said:

No that’s the offset , invest in a 2” concentre eyepiece that really accurate for setting up secondary mirror.

I was under the impression that even offset mirrors should appear round and central under the focus tube...?

[bottletopburly]

Only the concentre will tell mind that’s not to say the focuser is drilled in wrong place my 200pds was similar to you looking through the concentre mine too needed pulling away from primary only a mm or so so maybe yours is the same , I would invest in a 2” concentre eyepiece  ( you could have borrowed mine if you lived closer) and be prepared to move the focuser towards primary, eventually I replaced the focuser with a steeltack from Baader and whilst fitting noticed that the focuser wasn’t square to the Ota so I’d check that too ,run a long metal ruler along spider vane and eye up the edge against focuser body , mine was way off being square to focuser how I never noticed before just assumed the tube was truly round .

[Ricochet]

Check if the focuser is square to the tube. Perhaps there are some adjustments on the connection plate or the connections aren't done up correctly. If the focuser is square I think you may have to remove the secondary and spider in order to drill a new set of holes in the top of the tube to hold the spider further away from the primary. If you're lucky there may be enough play in all the existing holes to shift the secondary a few mm away from the primary and the focuser a few mm towards the primary. If you do remove the secondary and spider check to see if the secondary holder is being pulled tight against the spider. If it isn't you may be able to fit a shorter central bolt rather than having to drill new mounting holes. 

[Peter Reader]

2 hours ago, Ricochet said:

Check if the focuser is square to the tube. Perhaps there are some adjustments on the connection plate or the connections aren't done up correctly. If the focuser is square I think you may have to remove the secondary and spider in order to drill a new set of holes in the top of the tube to hold the spider further away from the primary. If you're lucky there may be enough play in all the existing holes to shift the secondary a few mm away from the primary and the focuser a few mm towards the primary. If you do remove the secondary and spider check to see if the secondary holder is being pulled tight against the spider. If it isn't you may be able to fit a shorter central bolt rather than having to drill new mounting holes. 

Is the offset spider or focuser holes likely? Are Skywatcher scopes known to be assembled with this sort of variation? I would have thought a robot drilled these so they would always be in a consistent position.

 

Will check for play as you suggest but I need like 2-3mm I think which is a lot...

[Ricochet]

I don't recall seeing anyone report this problem before but there are only a few options; the squareness of the focuser, the travel on the collimation screw, or the mounting holes. 

[Spile]

I’ve read someone had their spider vanes incorrectly fitted. Have you taken the spider out and checked that it has been correctly mounted? I am up the M40/M42 if you want to bring it up to Worcestershire so we can compare telescopes? Or maybe we could do this over a zoom call?

[Peter Reader]

Very kind of you to offer. I'd be very interested to meet and compare. I take it your 200PDS is performing well? 

[PeterStudz]

Interesting, I have a similar problem (could be the same) on a similar telescope, a neglected Skywatcher Skyliner 200p Dob which I’ve been restoring.

However hard I tried and whatever I measured I could not get the secondary far enough away from the primary. a>b in @Spile excellent article. I had the whole secondary plus spider out at least half a dozen times. For me the centre spring was maxing out (fully compressed) before the secondary holder physically came into contact with the spider hub. The work around was to cut down the spring, just a little, to ensure a bit more travel. I actually have a spare spring but that’s another story. 

In the picture you can see how little wriggle room I have left. This was after I cut the spring down. At this point it was just roughly collimated, paying attention to getting a=b. I’ve also added the milk bottle mod yet there’s barely room for that. It’s now fully collimated but with the room/movement I have after cutting the spring down I can only just do it. Somethings  not quite right but it now works.

[Peter Reader]

2 hours ago, PeterStudz said:

Interesting, I have a similar problem (could be the same) on a similar telescope, a neglected Skywatcher Skyliner 200p Dob which I’ve been restoring.

However hard I tried and whatever I measured I could not get the secondary far enough away from the primary. a>b in @Spile excellent article. I had the whole secondary plus spider out at least half a dozen times. For me the centre spring was maxing out (fully compressed) before the secondary holder physically came into contact with the spider hub. The work around was to cut down the spring, just a little, to ensure a bit more travel. I actually have a spare spring but that’s another story. 

In the picture you can see how little wriggle room I have left. This was after I cut the spring down. At this point it was just roughly collimated, paying attention to getting a=b. I’ve also added the milk bottle mod yet there’s barely room for that. It’s now fully collimated but with the room/movement I have after cutting the spring down I can only just do it. Somethings  not quite right but it now works.

Very informative, thank you. I also have the spider out at the moment and can see the spring is fully compressed yet there is a few mm available before the holder hits the spider... will cut the spring and see how it looks. As you say it will be close...

[bottletopburly]

That’s the beauty with the baader Steeltack they engineered a way of moving focuser up and down OTA a few mm should it be required, 

[Spile]

13 hours ago, Peter Reader said:

Very kind of you to offer. I'd be very interested to meet and compare. I take it your 200PDS is performing well? 

I have the 200P so as I am aware it is the same spider assembly. Drop me a DM and we can see if we can fix something up. That is unless the other suggestions you get here lead to a solution.