Glatter Sling

[CraigT82]

I'm considering installing a glatter sling in my 12" f/6 dob.  Looking ta the actual Howie Glatter product it comes with two posts and sliders that the ends of the cable attached to, which I believe is to allow the cable to move with the mirror during collimation, so that the cable always stays perfectly parallel with the flat plane of the mirror as it tilts.

On my dob the posts for attaching the cable ends to could be attached to the base board of the cell, and the whole lot would move during collimation, hence negating the need for the sliders on the posts? Am I right in thinking that?

Edited December 7, 2020 by CraigT82

[markse68]

Hi Craig, I’d have thought it’d be better to put the posts on the main mirror box so it takes all the weight of the mirror. If you do it like you’re thinking all the weight of the mirror would cause a lateral force and bending moment on those collimation studs which could cause lateral movement of the mirror as the scope moves in alt axis.

Mark

[CraigT82]

Hi Mark, thanks for you input, I should have known it wasn't going to be that easy! 

[CraigT82]

For anyone interested found some really good info on Howie Glatter slings in Piero's thread about his Lukehurst dob...

 

[Piero]

19 hours ago, CraigT82 said:

For anyone interested found some really good info on Howie Glatter slings in Piero's thread about his Lukehurst dob...

 

 

Just to clarify, David Lukehurst installed a sling cable which is not the Glatter's design. 

David's sling works ONLY IF it is installed on the same plane of the mirror and if the cable is mounted parallel to the COG of the mirror. If these conditions are not satisfied, astigmatism will be visible. 

In the telescope in that thread, the first condition was present, whereas the second was not (the cable was placed at half mirror height..). I reached the second condition by changing the back mirror supports, effectively raising the mirror height as much as necessary. The other issue that I found is that David installed 3 strong edge supports (unnecessary really as the sling is meant to be the mirror edge support). These additional edge supports tightened the mirror, causing astigmatism and spherical overcorrection as they also affected the way the mirror was supported by the triangles... The description and solution is in that thread. Thankfully it works well now.

 

 

[Piero]

On 07/12/2020 at 09:05, CraigT82 said:

I'm considering installing a glatter sling in my 12" f/6 dob.  Looking ta the actual Howie Glatter product it comes with two posts and sliders that the ends of the cable attached to, which I believe is to allow the cable to move with the mirror during collimation, so that the cable always stays perfectly parallel with the flat plane of the mirror as it tilts.

On my dob the posts for attaching the cable ends to could be attached to the base board of the cell, and the whole lot would move during collimation, hence negating the need for the sliders on the posts? Am I right in thinking that?

Yes, you can do that. You could install a metal bar on the that mirror wood panel to provide a support for the two shafts where the sling cable can be installed. On a 12" mirror, a 1/16" cable is fine. 

The difficulty is that you need to make sure that the height of the sling cable is at the COG of the mirror. 

The bearings in the Howie Glatter's design are there for this reason and because they are installed on the mirror cell of a Kriege's design, which is different from the David's design. 

Here is a tool for calculating the mirror COG: https://www.cruxis.com/scope/mirroredgecalculator.htm .

 

On 07/12/2020 at 09:40, markse68 said:

Hi Craig, I’d have thought it’d be better to put the posts on the main mirror box so it takes all the weight of the mirror. If you do it like you’re thinking all the weight of the mirror would cause a lateral force and bending moment on those collimation studs which could cause lateral movement of the mirror as the scope moves in alt axis.

Mark

That's a reason why I don't particularly like the collimation of the mirror panel, instead of the mirror itself. 

In any case the problem you mention is not caused by the sling. As it is right now, the problem is present. Said this, if the bolts are large enough and remain firm when threaded, it is fine. My dobson (another Lukehurst) is very similar and retains collimation well (tested this with Glatter's laser at 2x and Catseyes tools).

[CraigT82]

I must say that the springs David uses over the collimation bolts are superbly strong, I think they are valve springs  from a fairly large engine! Impossible to compress in the hand.