Secondary Mirror Mods to TS/GSO 8" F4 Newt

[Macavity]

Following on from the excellent(!) "stronger spring mods" to the primary mirror cell:

http://stargazerslounge.com/topic/188976-8-f4-newtonian-mirror-cell-mods/ (Ta muchly again, Laser_Jock!)

I decided to revisit the secondary adjustment. I already had "Bob's knobs" installed.

Basically I wanted to find out if my NEWT had an inbuilt, so-called, "secondary offset"!

To begin, I removed the whole secondary assembly. Taking care not to mark the mirror, I traced around the elliptical housing of the mirror onto a sheet of A4.  I then cut out a paper template and located the centre of the ellipse by folding it symmetrically into four. I made a small "window" at the centre by chopping off the folded corner. I then attached the "sticky part" of  *Post-It* label (non-destructively!) over the approx. centre of the secondary.  I marked the accurate geometric centre with a pencil and through the hole in the paper template. Uhm... "X marks the spot"!

You now see that an Engineer's Square, set up behind the mirror, lines up pretty well with the geometric centre (Faint pencil cross) of the mirror - When placed in contact with the spider vein. Thus I claim that this secondary is set up so that the geometric centre of the secondary is central to the OTA. There is seemingly *NO* inbuilt secondary offset.

This rather confirms some vague and general suspicions I had....A laser collimation that "Never looked quite right"? A slight (but visible) "squint" in the plane of the focusser. (The front ring of the none-to-generous sized OTA could be seen to obstruct the view slightly!) Idem perhaps slightly "astigmatic" stars at periphery when going through focus. 

All minor stuff, certainly! But niggling (perplexing) just the same? If you might CARE too (lol) consult:

http://web.telia.com/~u41105032/kolli/kolli.html

The situation is best illustrated in a figure from the above:

For THIS 8" F4 system the calculated (see ref.) *offset* in both planes is: 4.375mm! Or, more realistically, the optical centre is "about 6mm" from the geometric centre of the secondary! Easy enough to offset the secondary along the length of the OTA by centering normally (collimation cap, whatever) within the focusser. With this done (my primary mirror was removed too!) a laser collimator impinged directly onto the optical (not the geometric) - Uhm... QED?!? 

But already the collimation seems subjectively "easier" ... better? HOWEVER, there still remains the task of fixing the required 4mm *transverse* offset of the secondary mirror as well! But I will have to think a bit about that one. Maybe enlist help from my "mate with lathe" ... to make some new, modified, "tensioning caps" for the vein screws etc.  The simplest way, but not sure what some (potentially) "dihedral" spider veins would do to my nice diffraction patterns? 

Note: An offset is not mandatory. I may yet be WRONG re. the above too! Heheh.

But I'm fairly convinced, so I suspect I will still "have a go" at this mod to see.... 

Always nice to (hopefully) understand things better, get the most from etc. 

[libraryman]

an interesting thread on cloudy nights re: collimation and secondary offset. 

Here....http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4494930/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1