First of all, Hi! Nice to finally find a decent ATM Forum is still out there - seems like the web is devoid of any decent information on the subject, at least I'm having a hard time finding it.

I'm working on my first ATM project - incidentally, also my first woodworking project. It's an old 8" Starhopper dob (My first scope) converted into a truss tube configuration. So far it's going pretty well - I have the mirror box and secondary cage done, except for two little things...

1) Back panel cover/fan (Easy)
2) Secondary Spider  (Hard, apparently)

Here's a pic.

So the old mid-90's Starfinder has a single-vane secondary. I'm not a fan of it; you can't control the secondary's position over the primary, and I want this thing to be one collimatable beast. So I tried to convert the four-vane mount from my C6-NGT into one wide enough to use; essentially I replaced the 3" vanes on the secondary mount to 5" ones. The metal was way too flimsy though.

So I just wanna see any suggestions on how I can mount up a secondary. I need its position to be adjustable, and preferably pretty strong. I understand many ATMs use pre-made spiders; I'd like to avoid using pre-fab parts as much as I can, otherwise I won't learn anything. Anybody got some innovative DIY Spider designs?

Thanks, and once again, nice to see this forum is out there!!!

Here's a couple of sites you might try.

http://www.europa.com/~swayze/spider/spider.html
http://www.atmsite.org/contrib/Carlin/spider/

 

One of those is pretty good, but I already have a nice mirror holder, just the spiders were giving me problems. Thanks anyway, though!

Regardless, I solved the problem, with some eye-bolts. I slowly added tension to the metal (tightened the bolts) and hit them with a torch, softened them, and let the cool into their new shapes. Like a charm.

That's cool, glad you got the problem solved.

Hi!

I was wondering has anyone done any research on curved vs. straight vanes?  it seems curved is all the rage these days.  Supposedly it smears the "line" diffraction over the entire image, resulting in an overall better image.  But it seemed to me that the curved ones would be more wobbly, so I did a full blown-out finite element analysis on several designs and came up with some interesting results. (I'd be happy to post it here, but don't know how and don't have another URL to post it to). 

Seems like it may not be worth it.  The vanes for a "full-curved" type (that is, they wrap around far enough that it presents a flat surface to the tube) have to be many times thicker for the same stiffness, otherwise you will have terrible vibration issues.  A slightly curved vane (in steel rather than aluminum) might be OK at .035", but still not as stiff as an .020 straight aluminum vane.  In the end,  it might be better to go with a very thin straight vanes and just minimize the diffraction rather than losing light and adding more diffraction just to smear it around....

What do you think?

I bought a newt that came with a curved secondary holder a few years ago. Great for visual work, VERY bad for photo work. I repalced the curve with a standard straight. The curve resulted in big messy stars on long (10-15minute) exposures. The straight results in the normal small str w/ spikes.

 Robert

Hi,

 I recently constructed a pretty basic secondary holder, thats completely adjustable. You can find details on my website at the following link. Below is a photo of the completed spider and vane assembly.

 http://www.astronomyasylum.com/ota.html

Good luck with your project,

Bill