Bob's Knobs too tight

[matt]

Anyone able to tell me what sort of effect having Bob's knobs done up too tight on an SCT would have on star tests/ ronchi test?

Would this impact on the corrector? I'm guessing it would. Most likely the secondary would be "stressed".

I was told it wasn't possible to "overtighten" Bob's knobs????

Thanks

[Dennis]

Hi Matt

Now there is a coincidence!

A couple of nights ago, night I was messing around with collimation using the C9.25, both at prime focus and using an x2.5 Barlow with the ToUcam, watching the diffraction rings on the notebook screen in K3CCDTools.

The rings were slightly non concentric, but I also thought that I noticed a very slight elliptical shape to the overall image. It was difficult to really say this for sure, as the seeing was not sufficiently steady for me to measure circularity against the 8 ring reticule pattern that K3CCDTools can overlay on the image.

I tried correcting for the non-concentricity via Bob's knobs, but discovered that to make the necessary centring movement, I was being forced to tighten one knob too tightly. So, I backed off each knob in turn, in ever so small increments and kept on re-centring the image in the frame until I felt they were not too tight, nor too slack. Then, I tweaked the knobs to make the diffraction rings concentric and this time the knobs had what I judged to be a not-so-tight feel to them. It is soooo difficult to describe or quantify this tactile feeling.

After I let all the thermal eddies settle down, I felt that the overall image was now more circular, so maybe I did have one of them over tightened and maybe this did result in a ever so slight elliptical shape? Really, really difficult to say with any certainty.

The next morning, I took the image of Sunspot 904 which was my best yet, and the same evening I imaged Uranus and 4 of his moons, so the collimation I performed seemed to have worked.

The reason I have been messing around with collimation is that had I fitted a carry handle to the aluminium bar that I have running along the top of the ‘scope, so I could carry the ‘scope. After fitting this handle I felt the ‘scopes performance suffered, so I began to wonder if lifting the OTA by this handle was somehow stressing the mechanical relationship between the two anchor points of the aluminium bar; the corrector cell and the mirror cell?

Cheers

Dennis

[gbeal]

Matt,
while I have no experience with Bob's Knobs, or anyone elses knobs for that matter, I would guess at overtightening as being detrimental to your system. At worst if will cause something to break, or the thread may strip. On some secondaries there exists the possibility of "warping" the part that the screw threads fit into, and if the secondary is mounted directly, maybe it can "distort" this as well. Much like a pinched primary.
I would suggest tight, but not excessive.