Is the SN10 focuser intentionally off-axis? (image heavy)

(Please note that this is also posted in Cloudy nights which I'm sure a number of you may frequent so feel free to delete this thread if it's breaking some rule).


I've been battling with the replacement Revelation crayford that came with my 2nd hand SN10. I suspected that there may have been something wrong with the alignment of the focuser so I took the corrector cell off the tube with the aim of checking it's alignment. There is a mark on the inside of the tube approximately directly across from the focuser but, on inserting a Hotech self-centring laser and switching it on, I noted that the laser-equipped focuser is not aligned with this mark.

I decided to be cautious and not trust the mark. I carried out a focuser alignment as described in WarpsCorp's ultimate collimation guide and found that whilst close, the mark wasn't totally accurate.

Adding an alarming 3mm worth of shims to one side of the focuser mounting plate was necessary to align it with my 'correct' point. When I reinserted the corrector and attempted to align the secondary I found that the rotation looks wrong when I adjust the secondary to make it concentric under the focuser.

I tore everything down and triple-checked the focuser alignment but it checks out. Have I missed something? I've been quite careful to follow the SN10 collimation procedure as described by Peter Kennett & Danny Lunsford (which does not include a section on squaring the focuser).

Any assistance is greatly appreciated.


Here are a few illustrative images:

I noticed that the previous owner was a bit lax in drilling accurate holes for the focuser so I started by fixing this.


Paper ring inserted into OTA beneath focuser. Note the laser pointer spot on the paper.


Here's the locations of the various points with an unshimmed focuser:


Shimming needed to get the laser spot to the adjacent point:


End result of shimming:


Secondary rotated using a paper tube and the rib technique to verify:


Ok, here's where it gets odd. Note that the secondary is centred and concentric with the focuser:




My understanding is that one would expect the end of the OTA to be visible and aligned with the long axis of the secondary at the very least by this stage. My only guess (although it does not seem logical) is that the focuser is not intended to be aligned with the axial axis of the OTA.

Can anyone comment?

If I understand the issue you are referring to:

www.skyandtelescope.com/howto/diy/3306996.html

www.fpi-protostar.com/bgreer/collim.htm

Michael

Hi,
Its very difficult to diagnose a collimation problem like this remotely, but to me from the second last image, it looks like that the secondary is rotated with the top of the secondary turned too far away, the secondary doesnt look square with the focuser, that's the way it looks to me going on the orange/brown cut off circle reflection in the secondary mirror.

Thanks guys, I got to the bottom of it; the weight of the focuser was deforming the tube or the tube is not very round. I thought I'd allowed for the first case by viewing the laser point with the focuser in both a horizontal and vertical position. I took the secondary off the corrector and placed the corrector on the tube - sure enough, the laser pointer showed that the focuser had been raised too far on one side.

After fixing it I now have a nice, fairly concentric image of the end of the tube and primary mirror.

Michael: My understanding of the technique you posted is that it's an attempt to maximise the light cone cast by the primary mirror onto the secondary. My issue was that the focuser wasn't square and thus the only way to view the primary was to move the secondary to a point where it was no longer square to the focuser.

Hi Martinus
I had a similar question in relation to my orion optics 10".
One particular method for squaring the focuser is to use an A3 sheet of paper and do various marking out until you can mark a point opposite the focuser. When i was finished i had to shimmy the focuser of the tube by about 3mm to align it to the mark.
I was surprised at how much i had to adjust it, but than i took a look at it again, the tube is not a perfect round tube, it obviously starts out as a flat sheet and is then rolled. There appears to be a section oppposite the focuser that is flatter, not really obvious but i can see it.
Now i am not sure if the method i used to find this point is accurate when the tube is not perfectly round, how did you get over this?

Mike

mlkcampion wrote: Hi Martinus
I had a similar question in relation to my orion optics 10".
One particular method for squaring the focuser is to use an A3 sheet of paper and do various marking out until you can mark a point opposite the focuser. When i was finished i had to shimmy the focuser of the tube by about 3mm to align it to the mark.
I was surprised at how much i had to adjust it, but than i took a look at it again, the tube is not a perfect round tube, it obviously starts out as a flat sheet and is then rolled. There appears to be a section oppposite the focuser that is flatter, not really obvious but i can see it.
Now i am not sure if the method i used to find this point is accurate when the tube is not perfectly round, how did you get over this?

Mike

Thankfully my SN10 has a cell for the schmidt corrector plate. I removed the corrector plate glass and fitted the cell.

I can think of potentially two things you could do to ensuring that the weight of the focuser does not deform the tube:

1: Print out a transparency with a circle of close to the radius of the scope's aperture and lay it over the end to verify if the deformation is occurring whilst the laser is on.

2: Make sure the scope is upright.

Sorry I can't give a more comprehensive answer.