Re: ATM 3 or 4 vane spider

["Christopher J.R. Lord" <chrislord@brayebrook.demon.co.uk>]

Doug Kniffen wrote:
> 
> One thing that hasn't been mentioned on this subject(recently?) is where
> the spikes are.  Remember the last ring plane crossing of Saturn? Preceding
> the event the rings became ever more difficult to see due to the presense
> of the spikes. So the day before the crossing I re-mounted my 4 vane spider
> at an angle that allowed viewing the rings between the diffraction spikes.
> The result was definitely worth the effort. It is also much easier to work
> around four spikes than six spikes.

If you offset the four support vanes by roughly half the radius of the
secondary holder, you will see, instead of four very bright spikes, four
slightly less bright primary spikes. This is a far more noticeable effect
that the difference between 3 or 4 intersecting vanes. I have no optical
explanation, only my experience and that of several other English ATM's.

> Should one try observing double stars, a 3 vane spider is 50%
> more likely to hide a faint companion star under a spike. After
> experimenting with welding rod diffraction on my SCT, I decided that
> decreased spike location is more important than slightly increased spider
> diffraction.

I use a 10-inch f/10 Newtonian originally made by Geo Calver. It has a
rotating tube, and I am primarily a double star measurer. The reason I
ditched the original three vaned spider for an offset Hargreaves four vane,
is precisely the above problem. However I found collimation easier with the
Hargreaves, and the diffraction pattern in practice to be far tidier, and
this wasn't merely a matter of aesthetics either.

Chris Lord