Understanding the Casts eye collimation instructions

[bluesilver]

Hi,  I am in the process of of using the Castseye collimation tools on a Skywatcher 16 Dobsonian,  the collapsible goto version.

All looks fairly straight forward,  i am just having a few issues with the termination that is being used in the manuals and was hoping someone here might be able to help me out with some of the meanings.

The first one is what they call,  Vertical offset adjustment of the secondary mirror

Under that heading it reads:

For optimal "Axial" positioning of the secondary I highly recommend that you consider first setting the corresponding ( and equal ) vertical offset away from the focuser.

Refer to the offset cross-reference table on the last page and locate the nearest value to you secondary mirror size ( minor axis length ) in the left-hand column: then read across to the f/no of your telescope listed across the top,  The appropriate offset value is in inches.

The goal is to move the secondary geometric centre point to the prescribed distance away from the focuser by the offset amount relative to the primary axis centrline.

It kind of reads like you need to be a lawyer to understand it,  but i am guessing unless you are use to the terminology it is fairly straight forward.

So my questions are: 

With the secondary mirror size ( minor axis length )  is this just simply measuring across the smaller part of the oval shaped mirror?

when they are talking about vertical offset away from the focuser,  are they actually saying that the spider vanes need to be moved?

It won't be much going from the numbers on the sheet they provide,  it will only be around .18" or there abouts,  ( haven't actually measured the secondary mirror yet )

It kind of got a tad confusing when i got to this part of the setup and it is only the first page.

 

Or is it just simply saying that,  find the centre of the secondary mirror and then just lower it down towards the primary mirror by the vales given ( in my case something like .18" )

The rest seams to read ok.

Any information would be greatly appreciated.

Thanks.

 

 

 

 

 

Edited June 21, 2020 by bluesilver

[almcl]

1 hour ago, bluesilver said:

With the secondary mirror size ( minor axis length )  is this just simply measuring across the smaller part of the oval shaped mirror?

Yes.  As shown here https://www.mathopenref.com/ellipseaxes.html

1 hour ago, bluesilver said:

when they are talking about vertical offset away from the focuser,  are they actually saying that the spider vanes need to be moved?

Yes, move the secondary away from the focusser by the specified  (small) amount towards the opposite side of the OTA.  This is to ensure that the secondary catches the whole of the light cone from the primary mirror.

1 hour ago, bluesilver said:

Or is it just simply saying that,  find the centre of the secondary mirror and then just lower it down towards the primary mirror by the vales given ( in my case something like .18" )

No, don't move it towards the primary, that will defeat the object.

1 hour ago, bluesilver said:

 

 

[bluesilver]

Thanks for the reply appreciated.

Thanks for helping clear that up for me,

Just some of the technical terms they use gets a tad confusing.

Appreciated.