Centering secondary under focuser confusion

[Craig a]

I’ve just received a new 130pds and have fitted my moonlite focuser to it, to do that I removed the spider from the tube. While I was at it I measured and marked the center point opposite the focuser hole,  and then adjusted the moonlite focuser to point at that mark.when I put the spider back in I centred the secondary by measuring all the spider veins, now when I looked through the Cheshire it’s showing that it’s centred i.e towards/away from the primary but in the towards top/bottom of focuser is saying it’s high in the focuser, so do I adjust the focuser to center the secondary or do I move the secondary moving the spider veins putting the secondary off center in the tube?  But adjusting the focuser will make it point away from the mark I put in opposite? 

[johninderby]

The secondary is supposed to be slightly off centered in the OTA tube. In modern scopes the secondary is usually attached off centre on the holder so the vanes can be simply adjusted to equal lengths.

Edited August 22, 2020 by johninderby

[Craig a]

Thank for replying, I’m not looking at any reflections I’m looking at the actual secondary mirror itself being up towards the the top of the focuser even though it’s centred in the tube 

[Craig a]

[CraigT82]

The moolite should have tilt adjustment screws i think, so should be easy to adjust until the secondary is centered 

[Craig a]

14 minutes ago, CraigT82 said:

The moolite should have tilt adjustment screws i think, so should be easy to adjust until the secondary is centered 

Yes the moonlight has tilt adjustment, I just thought that would defeat the object of getting it square to the tube by marking the opposite center point on the tube , as I’m going to be imaging with it would a tilted focuser just make the sensor tilted with it? 

[CraigT82]

How do you know it's square to the tube? If the secondary is definitely centred in the tube, but is not centred in the view through the focuser, that would indicate that the focuser is not square to the tubes axis? 

[Ricochet]

1 hour ago, Craig a said:

do I adjust the focuser to center the secondary or do I move the secondary moving the spider veins putting the secondary off center in the tube?

You should attempt to get the secondary in position using the collimation screws on the secondary. If you follow the astrobaby guide there are two steps to secondary collimation. If you cannot get the two steps to coincide then you may need to adjust the spider veins, but we're talking maybe half a turn on the thumbscrews I would think. 

[Craig a]

10 minutes ago, CraigT82 said:

How do you know it's square to the tube? If the secondary is definitely centred in the tube, but is not centred in the view through the focuser, that would indicate that the focuser is not square to the tubes axis? 

I measured where the centre directly opposite the focuser is on the tube then inserted a Cheshire with the crosshairs and adjusted the focuser to get the crosshairs on that mark meaning the focuser is pointing at the same central point opposite 

[Pixies]

Looks OK to me:

[Craig a]

2 minutes ago, Pixies said:

Looks OK to me:

Thanks for that but if you look at the third circle out from the center you can see it’s slightly high do you not think?

[CraigT82]

So then if the spider hub is centred, and the focuser is square,  perhaps the secondary is tilted on it's hub? In which case you can use the secondary collimation screws to bring it into line as @Ricochet says above 

[Craig a]

I think what I’m going to do is start over again something just doesn’t seem right, I’m wondering if my Cheshire or my collimating cap is faulty as when I use the collimating cap it says different to the Cheshire, 🤔

[Pixies]

23 minutes ago, Craig a said:

Thanks for that but if you look at the third circle out from the center you can see it’s slightly high do you not think?

Yes. A wee bit out.

[Pixies]

37 minutes ago, Craig a said:

I think what I’m going to do is start over again something just doesn’t seem right, I’m wondering if my Cheshire or my collimating cap is faulty as when I use the collimating cap it says different to the Cheshire, 🤔

I think you might be putting a little too much faith in the tolerances involved in the manufacture of the secondary assembly! 

Also, are you certain the adjuster screws are EXACTLY level?

[Craig a]

I taken it apart and re checked everything ended up exactly the same, high in the focuser, the only thing I can think of is the secondary mirror itself isn’t on the holder square, looking at it it’s only stuck on with A double sided sticky foam rectangle, being one of skywatchers cheapest telescopes it was never going to be an engineering masterpiece was it 😂, I decided to just adjust the focuser to get the secondary centred I didn’t want to adjust the spider taking it off center inside the tube,  will only know what the outcome is on the next clear night On a test image.

Edited August 22, 2020 by Craig a

[Craig a]

Ok this is what I now have through the Cheshire, I found my problem and it was the mounting plate on the moonlite focuser having abit of play in the holes on the tube so I squared the focuser to the mark opposite with the plate loose then tightened down the plate, hey presto the secondary was centred when I put it back in with no adjustment needed on the focuser itself